ufdcimages.uflib.ufl.edu · 4 acknowledgments first of all, i would like to thank my family,...
TRANSCRIPT
NOVEL Pi-CONJUGATED ARCHITECTURES FOR APPLICATIONS IN SUPRAMOLECULAR CHEMISTRY AND MATERIALS
By
YU ZHU
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
2017
© 2017 Yu Zhu
To my family and my dream of becoming a scientist
4
ACKNOWLEDGMENTS
First of all, I would like to thank my family, especially my mom. Thanks for all the
support you have given to me. I would not make this trip to study abroad without you. I
cannot forget the support I was given while preparing for the standardized exams and in
my undergraduate studies. I cannot thank you enough for what you have done.
I would like to thank my advisor Prof. Ronald K. Castellano. Thank you for
bringing me into your group. Thank you for your guidance over these years. You are
always knowledgeable and helpful. I have learned not just science, but also lessons in
life from you.
I want to thank my groupmates, Dr. Raghida Bou Zerdan and Danielle Fagnani. It
was a wonderful experience to collaborate with you. I have learned a lot from each of
you. I really enjoyed the projects we had. To Will Henderson, it is great to work with you
on both the science and repair works. I also want to thank all the Castellano group
members. Everyone gave me a lot of help. It has been like family with all of you. I thank
my former undergraduate student, Bryce Reeves. Our brief collaboration was fun.
I would like to thank our collaborator, Dr. Nathan Shewmon from Prof. Jiangeng
Xue’s group in University of Florida Department of Materials Science and Engineering.
I want to thank my dissertation committee members: Prof. Ronald K. Castellano,
Prof. Aaron Aponick, Prof. Steven A. Miller, Dr. Benjamin W. Smith, and Prof. Kirk J.
Ziegler. Thank you for all the help given to me. I would like to extend my gratitude to
those who previously served on my committee: Prof. Jiangeng Xue, Prof. Kirk S.
Schanze, Prof. Franky So, Prof. W. David Wei, and Prof. Kenneth B. Wagener.
I would like to thank my labmate Tural Akhmedov. Given the third floor lab is
away from the majority of the group on the second floor, I developed a close friendship
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with you, the man who sat next to me. I cannot forget about the conversations and
laughter we had. To Lei and Asme, we would always have interesting chats on the
second floor. Thanks to Dr. Davita Watkins for all your help including the calculations
and my research proposal. To Renan and Ania, thanks for the discussions on research.
Special thanks to my wife, Yuting Wang, for your company during the past nine
years. Going to a foreign country to pursue a PhD in science is otherwise a lonely
journey. It is also a long way from Tianjin to Gainesville. I have been fortunate to come
here with you. You made it different. No one else would understand me more than you
do.
I would like to thank the University of Florida, Department of Chemistry for giving
me the opportunity to study abroad with my wife, Yuting Wang. It has been my pleasure
to work with many wonderful colleagues from different countries. I have made many
friends.
Finally, I thank the National Science Foundation (NSF) and American Chemical
Society Petroleum Research Fund (ACS PRF) for funding my research.
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TABLE OF CONTENTS page
ACKNOWLEDGMENTS .................................................................................................. 4
LIST OF TABLES ............................................................................................................ 9
LIST OF FIGURES ........................................................................................................ 10
LIST OF SCHEMES ...................................................................................................... 14
LIST OF ABBREVIATIONS ........................................................................................... 16
ABSTRACT ................................................................................................................... 18
CHAPTER
1 INTRODUCTION .................................................................................................... 20
Organic Electronics ................................................................................................. 20 Organic Photovoltaics ............................................................................................. 24 Molecular Switches ................................................................................................. 28
Supramolecular Polymer ......................................................................................... 31 Benzene-1,3,5-tricarboxamide (BTA)...................................................................... 32
[2.2]Paracyclophane Tetraamide (pCpTA) .............................................................. 33 Planar Chirality ....................................................................................................... 37
[3.3]Paracyclophane ............................................................................................... 38 Scope and Organization of the Dissertation ............................................................ 40
2 THE INFLUENCE OF SOLUBILIZING CHAIN STEREOCHEMISTRY ON SMALL MOLECULE PHOTOVOLTAICS ................................................................ 42
Introductory Remarks.............................................................................................. 42
Synthesis ................................................................................................................ 46 Isomeric Composition ............................................................................................. 49 Molecular Absorption Properties ............................................................................. 51
Thermal Properties ................................................................................................. 53 Characterization of the SMDPPEH Compositions in the Solid State ...................... 55
Photovoltaic Device Performance ........................................................................... 62 Conclusion of the Chapter ...................................................................................... 65
Experimental ........................................................................................................... 67 General Methods .............................................................................................. 67 Synthesis of SS-SMDPPEH (2-1SS) ................................................................ 69
Synthesis of RS-SMDPPEH (2-1RS) ............................................................... 74 Characterizations .................................................................................................... 77
Absorption Measurements ................................................................................ 77 Thermal Analysis .............................................................................................. 77
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Thin Film Characterization ................................................................................ 78 Device Fabrication and Characterization .......................................................... 78
3 KETO-ENOL TYPE TAUTOMERICALLY ACTIVE MODULES CONTAINING BENZODIFURAN FOR Pi-CONJUGATED MATERIALS ........................................ 80
Introductory Remarks.............................................................................................. 80 Molecular Design .................................................................................................... 85 Theoretical Calculation ........................................................................................... 88 Synthesis ................................................................................................................ 90
Characterization ...................................................................................................... 99 Development of a Benzodifuran Based Self-Assembled Material ......................... 104
Introduction ..................................................................................................... 104
Synthesis ........................................................................................................ 107 Conclusions and Future Directions ....................................................................... 108 Experimental ......................................................................................................... 108
General Methods ............................................................................................ 108 Synthesis ........................................................................................................ 109
Absorption Measurements .............................................................................. 115 Thermal Analysis ............................................................................................ 116
4 A THERMALLY SWITCHING HEMIACETAL SYSTEM ........................................ 117
Introductory Remarks............................................................................................ 117 Theoretical Calculations ........................................................................................ 121
Synthesis .............................................................................................................. 123
Characterization .................................................................................................... 131
Conclusion and Future Work ................................................................................ 133 Experimental ......................................................................................................... 133
5 [3.3]PARACYCLOPHANE SUPRAMOLECULAR POLYMER .............................. 137
Introductory Remarks............................................................................................ 137 Molecular Modeling ............................................................................................... 139
Synthesis .............................................................................................................. 142 Conclusion and Future Work ................................................................................ 146 Experimental ......................................................................................................... 146
Computations ................................................................................................. 146 Synthesis ........................................................................................................ 147
6 CONCLUSIONS ................................................................................................... 150
The Influence of Solubilizing Chain Stereochemistry on Small Molecule Photovoltaics ..................................................................................................... 150
Keto-enol Type Tautomerically Active Modules Containing Benzodifuran for Pi-Conjugated Materials ......................................................................................... 151
A Thermally Switching Hemiacetal System ........................................................... 152 [3.3]Paracyclophane Supramolecular Polymer ..................................................... 152
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APPENDIX
A NMR SPECTRA OF CHAPTER 3 ......................................................................... 154
B GRAPHS AND NMR SPECTRA OF CHAPTER 4 ................................................ 163
C MISCELLANEOUS INFORMATION OF CHAPTER 5 .......................................... 171
LIST OF REFERENCES ............................................................................................. 180
BIOGRAPHICAL SKETCH .......................................................................................... 189
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LIST OF TABLES
Table page 2-1 Optical properties of 2-1 in CHCl3 ....................................................................... 53
2-2 Thermal properties of 2-1 ................................................................................... 54
2-3 Characteristics for 2-1 photovoltaic devices ....................................................... 63
3-1 Conditions used for protection of 3-5. ................................................................. 91
3-2 Conditions used in amide formation between 3-7 and hexylamine. .................... 92
3-3 Conditions used in cyclization attempts with 3-11. ............................................. 93
3-4 The substitution reaction of 3-3 and 3-12. .......................................................... 94
3-5 Conditions used in coupling reaction of 3-7 with 1-pentylhexylamine 3-16. ........ 95
3-6 Conditions of the ring closure reaction of thiosalicylic acid 3-26. ........................ 98
3-7 Optical properties of 3-1 in DMSO. ................................................................... 102
3-8 Conditions used in coupling reaction of 3-7 phenol. ......................................... 107
4-1 The dependence of the ratio of 4-1 to 4-2 on temperature tracked by IR and NMR.. ............................................................................................................... 119
4-2 Energy differences of compounds 4-1 to 4-8 within each pair. ......................... 122
C-1 Summary of calculation result of monoamides. ................................................ 171
C-2 Summary of calculation result of diamides. ...................................................... 171
C-3 Conditions used in Wolff-Kishner type reactions. ............................................. 179
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LIST OF FIGURES
Figure page 1-1 Conduction band depictions of solids. ................................................................ 21
1-2 OLED devices ..................................................................................................... 22
1-3 A schematic illustration of spin-coating. .............................................................. 23
1-4 Structure of a bilayer organic photovoltaic cell. .................................................. 25
1-5 Functional mechanism of a bilayer organic photovoltaic. ................................... 25
1-6 Structure of a bulk heterojunction organic photovoltaic cell. ............................... 26
1-7 Research cell efficiency records plotted by the National Renewable Energy Laboratory (NREL). ............................................................................................ 27
1-8 Typical examples of photochromic molecules. ................................................... 28
1-9 The isomerism of 1,2-bis(2,5-dimethyl-3-thienyl)perfluorocyclopentene. ............ 30
1-10 General chemical structures of C=O- and N-centered benzene-1,3,5-tricarboxamide (BTA) molecules. ........................................................................ 32
1-11 Schematic representation of benzene-1,3,5-tricarboxamide self-assembly into helical one-dimensional aggregates ............................................................ 33
1-12 Structure of [2.2]paracyclophane and pCpTA. .................................................... 33
1-13 The design of pCps capable of self-assembly through hydrogen bonding. ........ 34
1-14 X-ray crystal structure of pCp-4,7,12,15-tetracarboxamide. ............................... 35
1-15 Solution-phase self-assembly of pCpTA reported by variable-concentration 1H NMR spectroscopy. ....................................................................................... 36
1-16 The planar chirality of (E)-cyclooctane and [2.2]paracyclophane. ...................... 38
1-17 X-ray crystal structure of [2.2]pCp and [3.3]pCp. ................................................ 39
1-18 The cyclophane bridged triarylamine-naphthalene diimide dyad system where [3.3]pCp serves as the bridge. ................................................................. 39
1-19 Boat and chair conformations of [3.3]pCp. ......................................................... 39
2-1 π-Conjugated polymers and small molecules prepared in stereocontrolled fashion with respect to their 2-ethylhexyl side chains. ........................................ 43
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2-2 HPLC analysis of 2-1. ......................................................................................... 50
2-3 Absorption spectra and Beer-Lambert plots in CHCl3. ........................................ 52
2-4 Normalized absorption spectra of 2-1 in CHCl3. ................................................. 53
2-5 TGA and DSC analysis of 2-1 samples. ............................................................. 54
2-6 XRD spectra for spin-coated films. ..................................................................... 55
2-7 AFM images of unannealed spin-coated films. ................................................... 58
2-8 AFM images of annealed spin-coated films. ....................................................... 59
2-9 Absorption spectra for spin-coated films. ............................................................ 61
2-10 Characterization of 2-1 BHJ photovoltaic devices. ............................................. 64
3-1 Common heterocyclic building blocks employed in π-conjugated architectures. ...................................................................................................... 80
3-2 Unconventional π-electron building blocks. ........................................................ 82
3-3 The idea of dynamic tuning. ............................................................................... 82
3-4 Diarylethene molecular switch. ........................................................................... 82
3-5 TAM in extended π-electron system. .................................................................. 83
3-6 Relationship of tautomerism, aromaticity and π-electron delocalization. ............ 83
3-7 The tautomerization of phenol and 9-anthrol. ..................................................... 84
3-8 Examples of CH/OH type tautomerically active molecules. ................................ 85
3-9 Tautomerrically active modules (TAMs) in extended π-electron systems. .......... 86
3-10 TAMs that will be used to explore the interplay of extended π-electron delocalization and tautomerism. ......................................................................... 87
3-11 Target compounds 3-1 and 3-2. ......................................................................... 87
3-12 Energy differences between geometry-minimized tautomers of 3-1 in the gas phase. ................................................................................................................. 88
3-13 Electronic structures, frontier molecular orbital energies, and HOMO-LUMO energy gaps based on gas-phase DFT calculations. .......................................... 89
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3-14 1H NMR spectrum of product of the reaction between 3-21 and ethyl bromoacetate in CDCl3. ...................................................................................... 97
3-15 1H-NMR spectrum of 3-1 in DMSO-d6 at room temperature. .............................. 99
3-16 1H NMR spectrum of 3-5 in DMSO-d6 44 days after the sample was prepared. ......................................................................................................................... 101
3-17 HSQC spectrum of the aged 3-5 sample. ......................................................... 101
3-18 UV-Vis absorption spectrum and Beer-Lambert plot of 3-1 in DMSO. .............. 102
3-19 TGA analysis of 3-1. ......................................................................................... 103
3-20 DSC analysis of 3-1. ......................................................................................... 104
3-21 Examples of self-assembling π-conjugated molecules. .................................... 105
3-22 The self-assembly of NDI-1 and NDI-2.. ........................................................... 106
3-23 The design of a functionalized benzodifuan for self-assembly studies. ............ 106
4-1 Mandelic acid, salicylic acid, 2-hydroxymandelic acid, 4-1, and 4-2. ................ 117
4-2 The thermal conversion of 4-1 to 4-2 and their characteristic protons and carbonyls. ......................................................................................................... 118
4-3 The design of the linear donor-π-acceptor system and methoxy derivative. .... 120
4-4 Naphthalene derivatives. .................................................................................. 121
4-5 FMOs, corresponding energy levels, and HOMO-LUMO energy gaps of 4-3 and 4-4. ............................................................................................................ 122
4-6 Thermal isomerization of 4-1 at 60 °C as tracked by NMR spectra. ................. 132
5-1 Various [3.3]pCp structures. ............................................................................. 137
5-2 The self-assembly of BTA, pCpTA, and [3.3]pCpTA. ....................................... 139
5-3 The structures. .................................................................................................. 141
5-4 1H NMR spectrum of [3.3]pCpTA (5-1) in chloroform-d. ................................... 145
A-1 1H NMR spectrum and 13C NMR spectrum of 3-4. ............................................ 154
A-2 1H NMR spectrum and 13C NMR spectrum of 3-5. ............................................ 155
A-3 1H NMR spectrum and 13C NMR spectrum of 3-6. ............................................ 156
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A-4 1H NMR spectrum and 13C NMR spectrum of 3-7. ............................................ 157
A-5 1H NMR spectrum and 13C NMR spectrum of 3-8. ............................................ 158
A-6 1H NMR spectrum of 3-7. .................................................................................. 159
A-7 1H NMR spectrum and 13C NMR spectrum of 3-22. .......................................... 160
A-8 1H NMR spectrum and 13C NMR spectrum of 3-23. .......................................... 161
A-9 1H NMR spectrum of 3-21. ................................................................................ 162
B-1 Energy minimized structure of 4-1. ................................................................... 163
B-2 Energy minimized structure of 4-2. ................................................................... 163
B-3 Energy minimized structure of 4-5. ................................................................... 164
B-4 Energy minimized structure of 4-6. ................................................................... 164
B-5 Energy minimized structure of 4-7. ................................................................... 165
B-6 Energy minimized structure of 4-7. ................................................................... 165
B-7 1H NMR spectrum and 13C NMR spectrum of 4-18. .......................................... 166
B-8 1H NMR spectrum of 4-19. ................................................................................ 167
B-9 1H NMR spectrum and 13C NMR spectrum of 4-1. ............................................ 168
B-10 1H NMR spectrum and 13C NMR spectrum of 4-19. .......................................... 169
B-11 1H NMR spectrum of 4-3. .................................................................................. 170
C-1 Energy minimized structures of pseudo-ortho diamides. .................................. 173
C-2 Energy minimized structures of pseudo-para diamides. ................................... 175
C-3 Energy minimized structure of [3.3]pCpTA. ...................................................... 176
C-4 1H NMR spectrum and 13C NMR spectrum of (±)-5-6. ...................................... 177
C-5 1H NMR spectrum of (±)-5-7. ............................................................................ 178
C-6 1H NMR spectrum of (±)-5-1. ............................................................................ 178
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LIST OF SCHEMES
Scheme page 2-1 Synthesis of 2-1SS. ............................................................................................ 47
2-2 Synthesis of 2-1RS. ............................................................................................ 48
3-1 Synthesis of 3-1. ................................................................................................. 90
3-2 Treatment of 3-5 with benzyl bromide. ............................................................... 92
3-3 Early installation of the amide. ............................................................................ 93
3-4 Attempted synthesis of 3-13. .............................................................................. 94
3-5 Synthesis of 3-18. ............................................................................................... 95
3-6 Attempted synthesis of 3-2 starting from 3-19. ................................................... 96
3-7 Attempted synthesis of 3-2. ................................................................................ 96
3-8 Cyclization of 4-aminobenzoic acid and attempted cyclization of 3-26. .............. 98
3-9 Synthesis of 3,4,5-tris(dodecyloxy)phenol 3-33. ............................................... 107
4-1 Attempted synthesis of 4-1 and 4-3 via one-pot method. ................................. 124
4-2 Intermediates and reaction types in the one-pot conversion of 4-13 to 4-1. ..... 124
4-3 SeO2 oxidation of 4-11...................................................................................... 124
4-4 Halogenation reactions of 4-13. ........................................................................ 125
4-5 DMSO oxidation of 4-18. .................................................................................. 125
4-6 Oxidation of 4-18 and 4-19 via the corresponding nitrates. .............................. 126
4-7 Nitration and oxidation of 4-18 in stepwise procedure. ..................................... 126
4-8 Attempted synthesis of 4-7 from 1-naphthol (4-21). .......................................... 127
4-9 Attempted halogenation of 4-25........................................................................ 127
4-10 Attempted synthesis of 4-5 through methy protection. ...................................... 128
4-11 Modified synthesis with late stage deprotection................................................ 128
4-12 Test protection reactions. ................................................................................. 129
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4-13 Acetyl protection and deprotection of 4-22. ...................................................... 130
4-14 Bromination of 4-35. ......................................................................................... 130
4-15 Proposed synthesis of 4-5. ............................................................................... 131
5-1 Synthesis of [3.3]pCpTA (5-1). ......................................................................... 142
5-2 Attempted reactions. ......................................................................................... 143
5-3 Attempted synthesis of 5-6 with pre-installation of bromine. ............................. 144
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LIST OF ABBREVIATIONS
1H NMR Proton nuclear magnetic resonance
13C NMR Carbon nuclear magnetic resonance
AFM Atomic force microscopy
AM 1.5 G Air mass 1.5 global
BHJ Bulk heterojunction
BOC tert-Butyloxycarbonyl
BTA Benzene-1,3,5-tricarboxamide
CD Circular dichroism
DART Direct analysis in real time
DCM Dichloromethane (methylene chloride)
DFT Density functional theory
DMSO Dimethylsulfoxide
DPP 2,5-Dihydropyrrolo [3,4-c]pyrrole-1,4-dione
DSC Differential scanning calorimetry
EQE External quantum efficiency
ESI Electrospray ionization
FMO Frontier molecular orbital
HB Hydrogen bonding
HOMO Highest occupied molecular orbital
HPLC High performance liquid chromatography
ITO Indium tin oxide
LUMO Lowest unoccupied molecular orbital
MO Molecular orbital
MS Mass spectrometry
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NMR Nuclear magnetic resonance spectroscopy
NDI Naphthalene Diimide
OFET Organic field effect-transistor
OLED Organic light emitting diode
OPV Organic photovoltaic
PCBM Phenyl-C61-butyric acid methyl ester
PCE Power conversion efficiency
pCp Paracyclophane
SMDPPEH Small molecule diketopyrrolopyrrole ethylhexyl disubstituted
TAM Tautomerically active module
TFA Trifluoroacetic acid
TGA Thermogravimetric analysis
THF Tetrahydrofuran
TLC Thin layer chromatography
TOF Time-of-flight
TosMIC Toluenesulfonylmethyl isocyanide
UV-Vis Ultraviolet-visible spectroscopy
XRD X-ray diffraction
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Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy
NOVEL Pi-CONJUGATED ARCHITECTURES FOR APPLICATIONS IN
SUPRAMOLECULAR CHEMISTRY AND MATERIALS
By
Yu Zhu
August 2017
Chair: Ronald K. Castellano Major: Chemistry
All the stereoisomers (i.e., (R,R), (S,S), and (R,S)) with respect to the 2-
ethylhexyl side chain chirality of a commercially-available, solution processable
diketopyrrolopyrrole-containing oligothiophene (SMDPPEH) have been synthesized.
After confirming isomeric purity by chiral HPLC, the properties of the pure isomers and
two isomeric mixtures (syn-SMDPPEH and com-SMDPPEH) were evaluated in solution
and the solid state. Stereoisomerism has no influence on absorption in dilute solution
but (R,S)-SMDPPEH has a higher melting and crystallization temperature (based on
DSC analysis) than the other four. The SMDPPEH samples were then blended with a
PC61BM acceptor to prepare thin films and bulk heterojunction (BHJ) solar cells. The
thin film morphology of neat and blended films was investigated via X-ray diffraction
(XRD) and atomic force microscopy (AFM). Overall, the chirality of the side chain had
little effect on the amorphous films, but a significant influence on the film morphology
when thermally annealed. The thin film consequences did not considerably affect the
power conversion efficiencies of annealed BHJ solar cell devices.
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Keto-enol type tautomerically active modules were designed and incorporated in
fused polycyclic ring systems. Specifically, two tricyclic fused ring compounds based on
coumaran-3-one with amide substituents in the 2-position were created as probes to
study the interplay between tautomerism and π-delocalization in polycyclic aromatic ring
systems. While the model compounds were not stable enough for the study, significant
synthetic progress was made.
2-Hydroxybenzofuran-3(2H)-one was reported to undergo thermally induced
isomerization to 3-hydroxybenzofuran-2(3H)-one and the ratio of the isomers was
thermally controllable. To follow up on this observation we have designed a simple
donor-π-acceptor system that could show disparate photophysical properties for the two
isomeric states. The model compounds were synthesized. NMR analysis revealed that
the ratio of the isomers was not reversible controlled by temperature. Current work
involves extending the system to naphthalene derivatives to investigate the
consequences of structural isomerism.
Benzene-1,3,5-tricarboxamide (BTA) forms robust and versatile 1-D self-
assemblies. We borrowed the concept and prepared [2.2]pCp-4,7,12,15-
tetracarboxamide (pCpTA). It formed 1-D homochiral rods through intra- and
intermolecular hydrogen bonding. We designed and synthesized [3.3]pCp-5,8,14,17-
tetracarboxamide ([3.3]pCpTA) here for the first time. The self-assembly behavior is
currently being investigated.
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CHAPTER 1 INTRODUCTION
Organic Electronics
Organic electronics is a field of materials science concerning the design,
synthesis, characterization, and application of organic small molecules or polymers that
show desirable electronic properties such as conductivity. Organic (carbon based) small
molecules and polymers are the two main types of organic electronic materials. Unlike
conventional inorganic conductors and semiconductors, organic electronic materials are
prepared from small molecules or monomers using synthetic strategies of organic
chemistry and polymer chemistry.1
Organic electronic materials have several advantages over their inorganic
counterparts. For example, the low cost in large scale manufacturing is an attractive
factor in industry. Their properties can be varied by rational design and engineering of
chemical structure (prior to synthesis) and addition of dopants (post-synthesis, similar to
inorganic materials). They also can display excellent mechanical flexibility and are light
weight compared with conventional inorganic materials such as metals and silicon.
Finally, they have good compatibility with a wide range of substrates as well.1,2
Adequate conductivity is the most important requirement for organic electronic
materials. The conductivities of conventional inorganic materials (for example, metals as
conductors and silicon as a semiconductor) can be explained through conduction band
theory. (Figure 1-1) The molecular orbitals of organic molecules consist of orbitals with
discrete energy levels. Their conductivity is usually explained by molecular orbital theory.
Common organic compounds are mainly constructed from σ-bonds, which have a large
energy gap between σ(bonding) orbitals and σ*(antibonding) orbitals. These orbitals are
21
combined to form molecular orbitals. Such compounds thus have a large energy gap
between their highest occupied molecular orbital (HOMO, which serves as the valence
band in the context of conduction band theory) and lowest unoccupied molecular orbital
(LUMO, which serves as the conduction band) and are often insulators. Π-bonds have
smaller energy gaps between their π(bonding) and π*(antibonding ) orbitals. An
extended π-conjugated system reduces the energy gap further, and enough for the
compound to be semiconductor. Common organic semiconductors used in organic
electronics include fused acenes, π-conjugated oligomers (considered as small
molecules because they are smaller than polymer), and polymers.
Figure 1-1. Conduction band depictions of solids. Adapted from http://hyperphysics.phy-astr.gsu.edu/hbase/Solids/band.html.
The earliest reported semiconductive material, polyaniline, was described by
Henry Letheby in 1862. Since the 1960s, more research on polymeric organic materials
was performed. Conductive plastics and devices saw significant development in the
1980s. The first OPV cell (by Tang in 1986),3 the first OLED (by Tang and Van Slyke in
1987),4 and the first OFET (by Koezuka, Trumura, and Ando in 1987)5 were all prepared
22
during that time. The 2000 Nobel Prize in Chemistry was awarded to Alan J. Heeger,
Alan G. MacDiarmid, and Hideki Shirakawa jointly for their work on conductive polymers.
Various kinds of devices are still early stage prototypes and they need
optimization before their scientific and engineering challenges are overcome and
become real-world products. However, some devices have already been
commercialized and are now widely used. For example, organic light emitting diodes,
organic solar cells, and organic transistors constructed from both small molecules
(oligomers) and polymers are now commercially available. Among them, OLED displays
(Figure 1-2) have received the most commercial success so far given advantages such
as high contrast, high brightness, fast refresh rate, low power consumption, etc.
Figure 1-2. OLED devices: OLED TV (left) and OLED smartphone display (right). (https://www.wired.com/2015/08/long-last-lg-launches-flat-screen-oled-tv/, https://www.oled-info.com/super-amoled)
There are mainly two types of processing methods for organic semiconductive
materials, vacuum deposition and solution processing. Vacuum thermal deposition is a
technique widely used in processing conventional inorganic semiconductors. Small
molecule organic semiconductor processed in the method should be thermally stable
and can be sublimed. In the procedures, the molecules are evaporated from a hot
23
source, transported through vacuum, and deposited onto a substrate, forming a thin film
on the substrate. The technique is expensive.
Solution processing can deal with both soluble polymeric and small molecule
organic semiconductors. This method requires the material to be dissolved in a volatile
solvent, filtered and deposited onto a substrate. Common solution processing methods
include drop casting, spin-coating, doctor-blading, inkjet printing, and screen printing.
Among them, spin-coating (Figure 1-3) is a widely used technique for small area thin
film production. The drawback of the method is material loss. Although the processing
methods vary, both solution processing and vacuum deposition techniques produce
amorphous and polycrystalline films with variable degree of disorder.
Figure 1-3. A schematic illustration of spin-coating. Adapted from http://www.spincoater.com/what-is-spin-coating.php
In order to facilitate solution processing, flexible, saturated hydrocarbon chains
are typically introduced to the periphery of π-conjugated molecules to promote favorable
van der Waals interactions with polarizable organic solvents, hence weakening
intermolecular π–π interactions and enhance their solubility. They can be linear,
branched, or substituted alkyl chains. In extended π-conjugated molecules, the side
chains are the insulating component compared with the aromatic cores. Similar types of
side chains could only moderately influence the molecular properties. However, they
can affect the aggregation of the materials in the solid state. As important components
24
of the molecules, side chain type,6,7 length,8,9 branching,10,11 and placement12,13 have
independently been shown to influence molecular packing, thin film morphology, charge
carrier mobility, and device performance.
Organic Photovoltaics
An organic photovoltaic cell is a device that uses organic electronics for light
absorption and charge transport to produce electricity from sunlight by the photovoltaic
effect. It is based on an organic semiconductor diode.
An OPV device consists of one to several layers of photoactive or charge
transport materials sandwiched between two electrodes. The single layer organic
photovoltaic cell is the simplest cell and it was first reported by Kearns and Calvin in
1958.14 Single layered cells have low efficiency. The two layer cell was first described
by Tang in 1986.3 The structure of two layered cell is close to those developed later. A
typical bilayer organic photovoltaic device is shown in Figure 1-4. Since then, more
complex device structures, for example cells with electron or hole injection (or blocking
layers), tandem cells, and heterojunction cells were described.
In a bilayer cell, sunlight is absorbed in the photoactive layers composed of
electron donor (D) and acceptor (A) semiconductive organic materials. As shown in
Figure 1-5 (light absorption), the donor material harvests photons and is excited by the
photons. An electron from the HOMO (valence band) of the donor is excited into the
LUMO (conduction band), forming an exciton. The excitons diffuse to the
donor/acceptor interface driven by a concentration gradient (exciton diffusion). The
excitons separate into free holes (positive charge carriers) and electrons (negative
charge carriers) at the interface (charge separation). Then the positive charge is
transported through the donor material to the anode while the negative is transported
25
through the acceptor material to the cathode (charge extraction). The electron is
transported through the circuit and recombines with the hole.
Figure 1-4. Structure of a bilayer organic photovoltaic cell. Adapted from http://www.sigmaaldrich.com/materials-science/organic-electronics/opv-tutorial.html
Figure 1-5. Functional mechanism of a bilayer organic photovoltaic. (D = donor, A = acceptor) Adapted from http://www.sigmaaldrich.com/materials-science/organic-electronics/opv-tutorial.html
The design and selection of the donor material is critical for organic photovoltaics.
Typically, the band gap lies in the range of 1-2 eV. Besides featuring an extended π-
conjugated system, the donor material often has donor and acceptor moieties to reduce
the energy gap and facilitate exciton formation. The acceptor material is usually a
26
compound with a low lying HOMO and LUMO, such as phenyl-C61-butyric acid methyl
ester (PCBM). Various electron (or hole) injection or blocking materials with different
charge carrier mobilities have been developed in combination with the photoactive
materials to achieve the best device efficiencies.
Bulk heterojunction (BHJ) solar cells are considered as the state of the art
organic photovoltaic cells. Bulk heterojunctions have an absorption layer consisting of a
nanoscale blend of donor and acceptor materials, commonly created by forming a
solution of the two materials, casting, and then allowing the two phases to separate,
usually facilitated by thermal annealing. (Figure 1-6) The two components will ideally
self-assemble into an interpenetrating network connecting the two electrodes.15 Bulk
heterojunctions have an advantage over layered photoactive structures because they
can be made thick enough for effective photon absorption without the difficult
processing involved in orienting a layered structure while retaining a similar level of
performance.
Figure 1-6. Structure of a bulk heterojunction organic photovoltaic cell. Adapted from https://www.photonics.com/Article.aspx?PID=5&VID=100&IID=592&AID=49904
27
The morphology of bulk heterojunctions are usually difficult to control, but is
critical to photovoltaic performance. The domain sizes of the heterojunctions must be on
the order of nanometers, allowing for excitons to reach an interface and dissociate. The
charges could be trapped in the islands of the blend (domains that have no contact with
electrodes) and recombine. A large enough domain size is required to allow charges to
migrate to the desired electrodes. However, phase segregation could lead to a
decrease of interfacial surface area and efficiency.16
So far, the performance (power conversion efficiency) of single junction organic
photovoltaic cells has reached a record of 11.5%.17 The development of efficiencies of
different types of solar cells is maintained by the National Renewable Energy
Laboratory (NREL) (Figure 1-7). Organic photovoltaic cells remain a hot research topic
in both academia and industry.
Figure 1-7. Research cell efficiency records plotted by the National Renewable Energy Laboratory (NREL). (https://www.nrel.gov/pv/)
28
Molecular Switches
Figure 1-8. Typical examples of photochromic molecules: azobenzene, spiropyran, furyfulgide, and diarylethene.18
A molecular switch is a molecule that can be reversibly shifted between two or
more stable states. The most notable molecular switches are photochromic molecules.
Photochromic molecules can reversibly transform between two isomeric forms having
different absorption spectra induced in one or both directions by photoirradiation. Four
typical examples of photochromic molecules are shown in Figure 1-8. Upon ultraviolet
(UV) irradiation, azobenzene and spiropyran change color from pale yellow and
colorless to orange and blue, respectively. The photogenerated colored isomers are
29
thermally unstable, and their colors disappear in the dark at room temperature.
Furylfulgide and diarylethene convert from colorless to colored forms, too. But the
colored forms are thermally stable and only convert back with visible light. 18
The chemical bond rearrangement during the photoinduced transformation
induces electronic and geometrical structure changes. An example of such changes in a
dirylethene are shown in Figure 1-9 with 1,2-bis(2,5-dimethyl-3-
thienyl)perfluorocyclopentene chosen as the representative compound. The chemical
structures of the open- (left) and closed- (right) forms are shown in Figure 1-9a. The
absorption spectra of the open-(black) and closed-(red) ring forms are shown in Figure
1-9b. In the open-ring isomer, π-conjugation is localized in each thiophene ring and the
thiophene rings are not conjugated through the ethene because they are not co-planar
(refer to crystal structures in Figure 1-9d). On the other hand, greater π-delocalization
exists in the right-side closed-ring isomer. The extended conjugation together with the
donor (sulfur)-acceptor (fluorine moiety) character decreases the HOMO-LUMO gap.
The absorption spectrum shifts to a longer wavelength as a result. At the same time, the
geometrical structure of the molecule also changes. The size of the molecule is
measured by the size of the triangle (depicted in blue dashed lines) with the carbon
atom of the middle difluoromethylene moiety and the two carbon atoms of the outer
methyl groups as the three apexes. The base width decrease from 1.01 nm to 0.90 nm
and the height increases from 0.49 nm to 0.56 nm when the open-ring isomer converts
to the closed-ring isomer. (Figure 1-9c) The side-view of the crystal structure shows
how the two thiophene rings are rotated out of the plane of the ethene in the open-ring
30
form while the molecule is flat in the closed-ring form. The diarylethene molecule
undergoes an anisotropic shape change upon photoisomerization.18
The electronic structure changes can be applied to optical memory media19 and
various photo switching devices.20 The structural change can be applied to various
photonic devices21 and light-driven actuators.22
Figure 1-9. The isomerism of 1,2-bis(2,5-dimethyl-3-thienyl)perfluorocyclopentene (a) chemical structures of the open- (left, black) and closed-ring (right, red) isomers, (b) absorption spectra of the open- (black line) and the closed-ring (red line) isomers, and (c) top- and side-views of the geometrical structures of the isomers in crystals. The two isomers were isolated and independently recrystallized. Reprinted (adapted) with permission from (Irie, M.; Fukaminato, T.; Matsuda, K.; Kobatake, S. Chem. Rev. 2014, 114, 12174–12277.). Copyright (2014) American Chemical Society.
31
Supramolecular Polymer
Known as “chemistry beyond the molecule”, supramolecular chemistry is
concerned with preparing assemblies of molecules using a combination of secondary
chemical interactions rather than covalent bonding. Small molecules (for example,
monomers of supramolecular polymers) are driven to spontaneously form self-
assemblies and are held together via secondary interactions include hydrogen bonds
(H-bonds), metal-coordination sites, and van der Waals forces.23 Sometimes described
as “the science of things that put themselves together,” the previously mentioned term
self-assembly encompasses a diverse range of processes whereby a disordered
system forms an organized structure or pattern as a consequence of local interactions,
without external direction. Self-assembly is an important concept in supramolecular
chemistry. Nature gives numerous examples, such as the DNA helix and other
biological macromolecules. Self-assembly can occur over a broad length scale, from the
molecular scale to macroscopic objects.24
Supramolecular polymers are a type of polymer whose monomeric units are held
together via highly directional and reversible non-covalent interactions.25 Biomolecular
supramolecular polymers, such as DNA and proteins, are also formed through such
interactions. Due to the nature of these interactions, the assembly forms as intended
and the finished assembly may be annealed or undergo self-healing of defects, and
reversibly dynamic changes in structures and function.26 While such dynamic properties
are different from conventionally bonded polymers, supramolecular polymers display
characteristics of macromolecules. Supramolecular polymers show great potential in
optoelectronic device,27 biomedical application,28 such as drug delivery,29 gene
transfer30 and etc.
32
Although hydrogen bonds between neutral organic molecules are not among the
strongest noncovalent interactions, they hold a prominent place in supramolecular
chemistry because of their directionality and versatility. A high degree of polymerization
of supramolecular polymer is often obtained through use of multiple hydrogen bonds or
other noncovalent interactions in addition to hydrogen bonds, or hydrogen bonds
enhanced by liquid crystallinity or phase separation.25
Benzene-1,3,5-tricarboxamide (BTA)
Benzene-1,3,5-tricarboxamide (BTA) has attracted considerable research
interest in the past decades in supramolecular chemistry. The molecules consist of a
benzene core and three amides groups connected to the benzene ring at the 1, 3, and
5-positions. The amide group is attached to the benzene ring via the carbonyl group,
yielding C=O-centered BTAs or via the nitrogen, giving rise to N-centered BTAs. (Figure
1-10) BTAs are versatile since different functionality can be introduced to the R
positions. The R groups in BTAs can be aliphatic or aromatic, polar or apolar, charged
or neutral, chiral or achiral. The three amide groups are capable of hydrogen-bond
formation, and one-dimensional (1D) growth of the monomers into supramolecular
polymers is achieved under selected conditions.31 (Figure 1-11)
Figure 1-10. General chemical structures of C=O- and N-centered benzene-1,3,5-tricarboxamide (BTA) molecules.
33
Figure 1-11. Schematic representation of benzene-1,3,5-tricarboxamide self-assembly into helical one-dimensional aggregates, which are stabilized by threefold intermolecular H-bonding. Reproduced from Cantekin, S.; de Greef, T. F. A.; Palmans, A. R. A. Chem. Soc. Rev. 2012, 41, 6125–6137 with permission from the Royal Society of Chemistry.
[2.2]Paracyclophane Tetraamide (pCpTA)
[2.2]Paracyclophane ([2.2]pCp) has many unique properties due to its “bent and
battered benzene rings”.32 (Figure 1-12) In previous work of our group, the first self-
assembly strategy to promote the stacking of [2.2]paracyclophanes to produce 1D
supramolecular architectures was reported. The design is inspired by BTAs, and amide
groups that facilitate transannular hydrogen bonding are installed on [2.2]pCp to give
pCp-4,7,12,15-tetracarboxamide (pCpTA, (±)-1-1, Figure 1-12) in order to form a
noncovalent supramolecular polymer stereospecifically. The formation of the assembly
has been confirmed in the solid state through single-crystal X-ray analysis, and in
solution by NMR, UV/Vis, and IR spectroscopy.33
Figure 1-12. Structure of [2.2]paracyclophane and pCpTA.
34
Figure 1-13. The design of pCps capable of self-assembly through hydrogen bonding: Intramolecular (transannular) hydrogen bonds (dotted black lines) in a pCp-4,7,12,15-tetracarboxamide (pCpTA) predispose the molecule for self-complementary association with a stacked neighbor. The design draws inspiration from the benzene-1,3,5-tricarboxamide (BTA) assembly motif. Monomer configuration (i.e., Rp for pCpTA) and conformation (i.e., anti for pCpTA) are representative. Dashed colored lines depict the double and triple helical hydrogen-bond lacing for the pCpTAs and BTAs, respectively. Reproduced in part from Fagnani, D. E.; Meese, M. J.; Abboud, K. A.; Castellano, R. K. Angew. Chemie Int. Ed. 2016, 55, 10726–10731 with permission from John Wiley and Sons.
As shown in Figure 1-13, in the BTA system, molecules form a 1D assembly
through threefold intermolecular H-bonding between the amide groups. BTA is achiral
and the assembly not chiral. Chiral assemblies are constructed by introducing chiral
side chains (R groups). pCpTA self-assembles through twofold intermolecular (similar to
BTA, between two pCpTA monomers) and intramolecular (different from BTA, between
the amide group of two decks of a pCpTA monomer) H-bond combination. Because of
the design of pCpTA, the assembly forms under the direction of the alternating (and
continuous) inter-and intramolecular H-bond between the tilted amide groups. That is
similar to the BTA assembly. The consequence is a helical linear stack, similar to duplex
DNA. The intrinsic planar chirality (Rp or Sp) of the monomer is inherited in the
propagation and as a result, the pCpTA linear assembly is homochiral,34 which means
35
each member of a 1D stack shares the same absolute stereochemistry (for example, all
monomers in a column are Rp).33
Figure 1-14. X-ray crystal structure of pCp-4,7,12,15-tetracarboxamide, unit cell containing each enantiomorphic asymmetric unit. The two helical H-bond laces are denoted with differently colored dashed lines.
The single crystal X-ray diffraction data of (±)-1-1a is shown in Figure 1-14.
Details of the conformation (anti and syn) and planar chirality (Rp or Sp) are not
discussed here in favor of bond lengths and angles. The average intramolecular H-bond
(N···C=O) distance is 2.81 Å, and that is close to the average intermolecular H-bond
distance (2.77 Å), suggesting the two types of H-bonds have comparable strength. The
amide torsion angles (Φ1 ≈ 38°; Φ2 ≈ -141°) are in good agreement with computational
36
predictions. The average intramolecular distance is 3.1 Å, almost identical to [2.2]pCp,
while the average intermolecular centroid-to-centroid distance is 3.8 Å. All the data
suggests formation of close 1D stacks of pCpTA.33
Figure 1-15. Solution-phase self-assembly of pCpTA reported by variable-concentration 1H NMR spectroscopy (0.1-30 × 10-3 M in CDCl3 at 25°). Top left: molecular structure of pCpTA with protons labeled. Top right: nonlinear curve fitting of concentration-dependent NH chemical shift data (CDCl3) fit to an isodesmic model. Adapted from Fagnani, D. E.; Meese, M. J.; Abboud, K. A.; Castellano, R. K. Angew. Chemie Int. Ed. 2016, 55, 10726–10731 with permission from John Wiley and Sons.
A variable concentration 1H NMR experiment was performed on (±)-1-1b in
CDCl3 from 0.1-30 mM. (Figure 1-15) The concentration-dependent chemical shift
changes are consistant with the self-assembly shown by X-ray crystallography. The
amide N-H protons are in three different environments: solvent exposed (Ha),
37
intramolecularly H-bonded (Hb), and intermolecularly H-bonded (Hc). They have different
chemical shifts and the ratio of them changes with the association/dissociation of the
monomers, and that is revealed by the amide N-H resonance (the weighed average of
the environments) in the NMR spectra. As the concentration increases, the amide NH
resonance shifts downfield to about 8.1 ppm (Δδ 0.6 ppm), indicating an increasing
percentage of H-bonded protons (sign of association). And the time-averaged aromatic
pCp protons (Hd and He) shift upfield to about 6.6 ppm (Δδ 0.3 ppm). The upfield C-H
shifts presumably arise from the ring current effect induced by -stacking bacause the
effect is enhanced as the chain enlongates. The downfield shift of the amide proton
confirms the formation of an H-bonded assembly, and nonlinear curve fitting of the
concentration and chemical shift data to an isodesmic (equal-K) self-assembly model
provides Kel = 63 ± 5 M-1.33
The self-assembly of pCpTA was also characterized with FT-IR, DOSY NMR,
and UV-Vis. These characterizations were performed in a less polar solvent where
stronger aggregation was observed.33
Planar Chirality
Planar chirality refers to stereoisomerism resulting from the arrangement of out-
of-plane groups with respect to a plane (chirality plane). It is a special kind of chirality
because it applies to a chiral molecule lacking an asymmetric carbon atom, but
possessing two non-coplanar rings that are each dissymmetric and which cannot easily
rotate about the chemical bond connecting them. The chirality of 2,2'-dimethylbiphenyl
is the simplest example, but planar chirality is also exhibited by molecules like (E)-
38
cyclooctene, some di- or poly-substituted metallocenes, and certain monosubstituted
paracyclophanes.35 (Figure 1-16)
Figure 1-16. The planar chirality of (E)-cyclooctane and [2.2]paracyclophane.
[3.3]Paracyclophane
[3.3]Paracyclophane ([3.3]pCp) was first synthesized by Cram and co-workers in
1954.36 Its structure is quite similar to [2.2]paracyclophane ([2.2]pCp). Both compounds
have their phenylene ring banded together by short bridges, dimethylenes and
trimethylenes, respectively. Because of the short side bridges, both compounds have
small transannular centroid-to-centroid arene distances; 3.1 Å for [2.2]pCp and 3.3 Å for
[3.3]pCp.37 (Figure 1-17) The two rings are forced to interact and consequently repel
each other, making the two rings bend to adopt such a geometry. Defining the geometry
are unique through-space interactions.38 For example, a cyclophane bridged
triarylamine-naphthalene diimide dyad system was found to have long lived charge
separated state (indicating a slower charge recombination than the acetylene bridged
comparator) due to a large electronic coupling and a large exchange coupling.38 (Figure
1-18)
Although both molecules are strained, [3.3]pCp is less strained on the ring and
the benzylic position and has some flexibility at the middle methylene of the bridges. In
analogy to cyclohexane, [3.3]pCp displays both chair (trans) and boat (cis)
conformations. The boat conformation is slightly lower in energy. The two quickly
39
interconvert at a rate of 26300 s-1 in CDCl3 at 303.2 K,39 however, only the chair
conformation is present in the solid state.37 (Figure 1-19)
Figure 1-17. X-ray crystal structure of [2.2]pCp and [3.3]pCp.(CCDC ID: 977369, 1229531) Size is shown.32
Figure 1-18. The cyclophane bridged triarylamine-naphthalene diimide dyad system where [3.3]pCp serves as the bridge.38
Figure 1-19. Boat (left) and chair (right) conformations of [3.3]pCp.
Although the high symmetry conformers (the boat conformer is C2v and the chair
conformer is C2h) may be considered to be the structures of lowest energy, the true
energy minimized structures are in lower symmetry. As noted by Bachrach, the high
symmetry was broken first (the boat form was brought down from C2v to Cs point group
40
and the chair form from C2h to Ci, otherwise higher symmetry would be preserved in the
computation), and then geometrically optimized through computation. The lower
symmetry structures led to the minimum according to the computation results.40 The
results are similar to [2.2]pCp, whose two aromatic rings are not perfectly eclipsed.32
Scope and Organization of the Dissertation
With the introduction of π-conjugated systems in electronic devices, achieving a
detailed understanding of the supramolecular interactions between π-conjugated
molecules has become one of the most challenging scientific research areas.30 The
design of novel π-conjugated architectures containing building blocks capable of
controlling supramolecular assembly and tuning optoelectronic properties are solutions
for high performance materials. The goal of this doctoral work is to approach novel π-
conjugated architectures through self-assembly and switching. Three directions were
pursued in four projects.
The influence of side chain stereochemistry on small molecule photovoltaics is
discussed in Chapter 2. SMDPPEH, a small molecule bearing two 2-ethylhexyl chains,
was chosen and all stereoisomers were prepared. Together with racemic mixtures, their
molecular properties, solid state properties and device performance were investigated.
The side chain chirality did not affect the molecular properties, but significantly
influenced the thin film morphology of the annealed thin films of the material. The thin
film consequences did not considerably affect the power conversion efficiencies of
annealed BHJ solar cell devices.
Switching is an attractive functionality. Two types of switching were studied in the
next two projects. Switching through tautomerization (Chapter 3) and thermally
controllable isomerization (Chapter 4) were investigated. Model compounds were
41
designed and synthesized. In Chapter 3, the interplay between tautomerism and π-
delocalization in polycyclic aromatic ring systems was studied. While the model
compounds were not stable enough for the study, significant synthetic progress was
made. In Chapter 4, NMR analysis revealed that the ratio of the isomers of the model
compound was not reversibly controlled by temperature.
Designing new self-assembled system for functional -systems leads to new
architectures and potentially new properties. In previous work, [2.2]pCp-4,7,12,15-
tetracarboxamide (pCpTA) was reported to form 1-D homochiral rods through intra- and
intermolecular hydrogen bonding. In Chapter 5, [3.3]pCp-5,8,14,17-tetracarboxamide
([3.3]pCpTA) was designed and prepared. The self-assembly behavior is currently being
investigated.
42
CHAPTER 2 THE INFLUENCE OF SOLUBILIZING CHAIN STEREOCHEMISTRY ON SMALL
MOLECULE PHOTOVOLTAICS1
Introductory Remarks
The development of organic semiconductive materials is a fast growing field of
research.41 Compared with conventional inorganic semiconductive materials, they enjoy
lower cost,42 lighter weight,43 tunable optoelectronic properties by varying chemical
structure,44 and compatibility with flexible substrates.45 Different types of devices have
been made including organic photovoltaic (OPV) cells,3 organic field effect transistors
(OFETs),5 and organic light emitting diodes (OLEDs).4 This chapter concerns organic
semiconductive materials that are promising for solar energy conversion. Relative to
polymers, small molecules tend to have good batch-to-batch reproducibility and their
purification is easier. Small molecules also lend themselves to deriving structure-
property relationships, and display good crystallinity and therefore typically can achieve
long-range ordering. Finally, small molecules can be processed through both vacuum
deposition and solution processing.46
Thermal vacuum deposition, one traditional materials processing method,
requires high temperature and vacuum and is usually expensive and hard to operate.
Solution processing is a more affordable method for large scale device production at
lower temperature.47 Unfortunately, π-conjugated macromolecules experience strong π-
π interactions that render them hard to dissolve as a consequence of aggregation.
1This work has been published in Advanced Functional Materials, 2014, 24, 5993−6004 Copyright © 2014
John Wiley and Sons.
43
To address the problem, flexible side chains are typically introduced to the periphery of
π-conjugated molecules to enhance their solubility for purification and device
fabrication.48 These can be linear, branched, or substituted alkyl chains where the
additional van der Waals interactions experienced between them and an organic solvent
represents favorable solvation. The caveat is that the vibrational motions of alkyl chains
may disrupt the interactions between π-conjugated systems in the solid state which are
typically required in thin-film devices such as OFETs and OPVs.10
Figure 2-1. π-Conjugated polymers (A49,B50,C51) and small molecules (D52,E) prepared in stereocontrolled fashion with respect to their 2-ethylhexyl side chains. SMDPPEH (2-1) studied in the current work.53
44
2-Ethylhexyl is one of the most popular solubilizing groups.16,7,54 Besides its
branching, the chain also features a chiral center. Conventional wisdom states that the
stereoirregularity should reduce crystallinity and therefore improve solubility. Along
these lines, most molecules used as organic semiconductive materials (e.g., polymers)
have more than one ethylhexyl side chain. The asymmetric carbon atom consequently
introduces isomeric complexity to such molecules given that the absolute configuration
of the alkyl chains can be R or S. That is, molecules prepared from racemic 2-ethylhexyl
reagent precursors would result in a mixture of stereoisomers (both enantiomers and
diastereomers). Although the influence of alkyl side chain size and position has been
thoroughly investigated,9,12,8 the impact of their stereoisomerism was generally ignored
until recent times.
A few research groups have investigated how chiral 2-ethylhexyl chains can
influence the properties of conjugated polymers. Reynolds and co-workers reported the
synthesis and characterization of (PProDOT-((2S)-ethylhexyl)2) (Figure 2-1A) where
enantiomerically pure (S)-2-ethylhexyl was used as the solubilizing group.49 The
polymer has almost identical macroscopic properties, including conductivity, thin film
absorption, and redox potential, with its counterpart prepared from racemic reagents.
However, the aggregates of the polymer exhibited a circular dichroism (CD) response
while the racemic variant was CD inactive. Neher and Scherf reported poly(9,9-bis((R)2-
ethylhexyl)fluorene-2,7-diyl(PF-2/6) (Figure 2-1B) which displayed chiroptical behavior
(absorption and fluorescence)50 because the chiral side chain biased the helical
backbone formation. Most recently, Ikai and Maeda reported thieno[3,4-
b]thiopheneebenzo[1,2-b:4,5-b’]dithiophene-based polymers (PTB5) (Figure 2-1C)
45
bearing optically pure 2-ethylhexyl pendants, named (RR)- and (SS)-PTB5, respectively.
They have similar properties as optically inactive PTB5 in terms of thermal stability,
absorption, frontier orbital energy levels, and organic photovoltaic performance.51
On the other hand, Nguyen and co-workers were the first to report the influence
of chiral 2-ethylhexyl chains on π-conjugated oligomer thin film morphology and OFET
performance. They isolated the three diastereomers of DPP(TBFu)2 (3,6-bis(5-
(benzofuran-2-yl)thiophen-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione)
(Figure 2-1D).52 The (R,R)- and (S,S)- enantiomers have expectedly similar
characteristics while the (R,S)-isomer displayed a shorter interplanar -stacking
distance revealed by single crystal structure x-ray analysis. The authors surmised that
this is a result of the (R,S)-isomer’s centrosymmetric structure that leads to tighter π-π
stacking and a different thin film morphology. When fabricated into a FET device, the
(R,S)-isomer displayed higher mobility than its two diastereomers.
Though various literature example have shown that the chiral center of 2-
ethylhexyl has an impact on the properties of some materials, the isomeric purity of
commercially available building blocks and materials has rarely been considered. Also,
the influence of side chain stereochemistry on small molecule (oligomer) photovoltaic
device performance has not been investigated.
Herein, we have evaluated the functional influence of stereodefined 2-ethylhexyl
solubilizing groups in small molecule photovoltaics. Optically pure versions of 2,5-di-(2-
ethylhexyl)-3,6-bis-(5′′-n-hexyl-[2,2′,5′,2′′]terthiophen-5-yl)-pyrrolo[3,4-c]pyrrole-1,4-dione
(SMDPPEH) (Figure 2-1E)54 were synthesized and the effect of stereoisomerism on
optoelectronic properties in organic photovoltaic devices was studied.
46
Synthesis
Five different SMDPPEH compositions of different stereochemistry were
employed in this study. 2-1com refers to the commercially available material purchased
from Sigma-Aldrich. 2-1syn refers to the material synthesized with racemic 2-ethylhexyl
bromide in our laboratory. 2-1RR, 2-1SS, and 2-1RS refer to (R,R)-, (S,S)- and (R,S)-
SMDPPEH, the enantiomerically pure versions of the molecule prepared with the
appropriate optically pure 2-ethylhexyl bromide (3-(bromomethyl)heptane) as an
alkylating reagent.
Synthesis of enantiomerically pure (S)2-ethylhexyl bromide started from (S)-4-
benzyl-2-oxazolidinone 2-2S. The compound was deprotonated with n-butyl lithium and
acylated with hexanoyl chloride to generate 2-3S.55 2-3S was deprotonated with sodium
bis(trimethylsilyl)amide followed by addition of ethyl iodide to install the ethyl group in a
stereocontrolled fashion to give 2-4S. Attempts to use ethyl tosylate as an alkylating
agent were not successful. 2-4S was reduced with lithium borohydride to yield optically
pure 2-ethylhexanol 2-5S.56 Attempts were made to brominate 2-5S directly but these
gave dibromide side products that were hard to remove, so an indirect route was
pursued. The alcohol 2-5S was tosylated and the resulting intermediate 2-6S was
converted to its corresponding bromide 2-7S.57 2-7S was used to alkylate DPP
derivative 2-9,58 which was synthesized following a literature procedure59 (attempts to
use a stronger base in this step, such as sodium hydride, did not work). The resulting
alkylated DPP derivative 2-10SS was brominated at the thiophene 5-positions to afford
2-11SS.58 2-11SS was cross-coupled with commercial reagent 2-12 to give
enantiomerically pure (S,S)-SMDPPEH 2-1SS (Scheme 2-1).60
47
Scheme 2-1. Synthesis of 2-1SS.
48
Scheme 2-2. Synthesis of 2-1RS.
In order to synthesize the unsymmetrical (R,S)-SMDPPEH 2-1RS, we first tried
to mono-protect 2-9 with a Boc group but the yields obtained were not useful.
Alternatively, mono-alkylation of 2-9 using an excess of 2-9 gave promising results, so
2-1RS was prepared via sequential direct alkylation. The synthesis of (R,S)-SMDPPEH
2-1RS started from mono-alkylation of 2-9. An excess of 2-9 was alkylated with (R)-2-
49
ethylhexyl bromide 2-7R and the product 2-13R was subsequently alkylated with an
excess of 2-7S to give 2-14RS. Bromination of 2-14RS then yielded 2-15RS. 2-15RS
was finally cross-coupled with 2-12 to afford (R,S)-SMDPPEH 2-1RS (Scheme 2-2).
SMDPPEH 2-1syn and (R,R)-SMDPPEH 2-1RR were prepared by a colleague, Dr.
Raghida Bou Zerdan, using a similar approach starting from racemic 2-ethylhexyl
bromide 2-7 for 2-1syn and (R)-4-benzyl-2-oxazolidinone 2-2R for 2-1RR. All five
compounds were successfully analyzed by 1H NMR, 13C NMR, HRMS, and elemental
analysis. Worth noting, 2-1syn and 2-1com showed no evidence by NMR of existing as
isomeric mixtures.
Isomeric Composition
The enantiomeric purity of 2-1RR, 2-1SS, and 2-1RS compounds as well as the
isomeric composition of 2-1syn and 2-1com were evaluated by HPLC (chiral stationary
phase, see general methods) under similar conditions reported by Nguyen.52 We
achieved successful resolution of the SMDPPEH isomers of 2-1syn and 2-1com. Our
assignment were based was the chromatograms of the pure isomers. These results are
in good agreement with Nguyen’s results.52 (Figure 2-2)
The technique offered the retention times of the isomers (2-1SS = 50±1 min; 2-
1RS = 58±1 min, and 2-1RR = 69±1 min) (Figure 2-2). Each isomer boasts an isomeric
purity ≥ 97%. HPLC analysis of the individual isomers confirmed the isomer
assignments within the mixtures.
50
Figure 2-2. HPLC analysis of 2-1 (Chiral stationary phase, 8/92 iPrOH/hexane, elution rate: 0.8 mL/min, detecting wavelength: 350 nm)
As noted by Nguyen and co-workers in their preparation of DPP(TBFu)2 isomers,
synthesis of SMDPPEH from racemic 2-ethylhexyl bromide should yield the (S,S)-,
syn
com
51
(R,S)- and (R,R)-SMDPPEH isomers in a statistically predicted 1:2:1 (25%, 50%, 25%)
ratio, respectively52. Integration of the three peaks detected for 2-1com shows a
24:48:28 composition that is close to prediction. For 2-1syn, the composition deviates
slightly from 2-1com on the two batches tested. Ratios of 28:42:30 (batch #1, one
chromatography column and two recrystallizations) and 26:41:33 (batch #2, one
chromatography column and one recrystallization) were obtained. From this limited data
(more work would need to be done to achieve statistical meaning), it seems that
isomeric composition could depend on preparation and purification conditions, and
therefore not all oligomer syntheses show the typically advertised batch-to-batch
reproducibility.
Molecular Absorption Properties
The optical properties of the SMDPPEH samples were evaluated in dilute
solution (CHCl3, 2.5 × 10-6 M – 30 × 10-6 M). The five UV-Vis absorption spectra (Figure
2-4 and Table 2-1) show that side chain stereoisomerism does not affect the intrinsic
electronic properties of the chromophores or their solution phase conformations. Linear
Beer-Lambert plots indicate all compounds are molecularly dissolved and no
aggregation is observed at the concentrations used (Figure 2-3). There are three
absorption bands in the UV-visible region. The first one, a higher energy band at λ =
384 nm, corresponds to a π-π* transition of the central DPP and the thiophene. The
peaks at λ = 616 nm and λ = 646 nm were previously assigned to an intramolecular
charge transfer transition and aggregation in solution, respectively.61 However,
aggregation is not suggested in dilute solution on the basis of Beer-Lambert analysis.
Consequently, the two peaks in the charge transfer band are assigned to vibrational fine
structure although this has not been further characterized. The molar extinction
52
coefficients are comparable within experimental error. All compounds have an onset
absorption of around 699 nm. This translates to a relatively low optical gap of 1.77 eV
as a result of the compound’s strong donor-acceptor character.
300 400 500 600 700 8000.0
0.5
1.0
1.5
2.0
Ab
so
rban
ce
Wavelength /nm
2.5 M
5.0 M
10 M
15 M
20 M
30 M
200 300 400 500 600 700 800
0.0
0.5
1.0
1.5
2.0
2.5
Ab
so
rban
ce
Wavelength /nm
2.5 M
5.0 M
10 M
15 M
20 M
30 M
300 400 500 600 700 800
0.0
0.5
1.0
1.5
2.0
Ab
so
rba
nc
e
Wavelength /nm
2.5 M
5.0 M
10 M
15 M
20 M
30 M
0 5 10 15 20 25 300.0
0.5
1.0
1.5
2.0
Ab
so
rban
ce
Concentration /M
= 61,590 M-1
.cm-1
log = 4.79
r2 = 0.99885
0 5 10 15 20 25 30
0.0
0.5
1.0
1.5
2.0
2.5
= 73,150 M-1
.cm-1
log= 4.86
r2 = 0.99788
Ab
so
rba
nc
e
Concentration /M 0 5 10 15 20 25 30
0.0
0.5
1.0
1.5
2.0 = 65,010 M-1
.cm-1
log = 4.81
r2 = 0.99477
Ab
so
rban
ce
Concentration M
300 400 500 600 700 8000.0
0.5
1.0
1.5
2.0
Ab
so
rban
ce
Wavelength/ nm
2.5 M
5.0 M
10 M
15 M
20 M
30 M
300 400 500 600 700 800
0.0
0.5
1.0
1.5
2.0
Ab
so
rban
ce
Wavelength /nm
2.5 M
5.0 M
10 M
15 M
20 M
30 M
0 5 10 15 20 25 30
0.0
0.5
1.0
1.5
2.0
= 62,190 M-1.cm
-1
log= 4.79
r2 = 0.99892
Ab
so
rban
ce
Concentration /M
0 5 10 15 20 25 300.0
0.5
1.0
1.5
2.0
= 65,870 M-1
.cm-1
log= 4.82
r2 = 0.99856
Ab
so
rban
ce
Concentration M
Figure 2-3. Absorption spectra and Beer-Lambert plots in CHCl3 (2.5×10-6 M–30×10-6 M) for A) 2-1syn; B) 2-1com; C) 2-1SS; D) 2-1RR; E) 2-1RS.
A) B) C)
D) E)
53
300 400 500 600 700 800
0.0
0.2
0.4
0.6
0.8
1.0
No
rma
lize
d a
bs
ob
an
ce
Wavelength (nm)
SMDPPEH
comSMDPPEH
S,S-SMDPPEH
R,R-SMDPPEH
R,S-SMDPPEH
Figure 2-4. Normalized absorption spectra of 2-1 in CHCl3 (20 × 10-6 M).
Table 2-1. Optical properties of 2-1 in CHCl3 (20 × 10-6 M)
Material λmax [nm] λonset [nm] ɛ ×104 [M-1cm-1] ΔEopt [eV]
2-1syn 385/616/646 700 6.2 ± 0.1 1.77 2-1com 384/616/646 699 7.3 ± 0.2 1.77 2-1SS 384/616/646 699 6.5 ± 0.2 1.77 2-1RR 384/615/646 699 6.2 ± 0.1 1.77 2-1RS 385/615/646 699 6.6 ± 0.1 1.77
Molar extinction coefficient was calculated based on the absorbance at the maximum absorbance wavelength (646 nm).
Thermal Properties
The thermal properties of the SMDPPEH compounds were studied by
thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) to
evaluate the effect of side chain stereoisomerism on the bulk behavior of these
materials. The techniques provided the decomposition temperature, melting
temperature (Tm), and crystallization temperature (Tc) differences among the isomeric
compositions.
All compounds show similar 5% weight loss temperature at around 343 °C and
that is an indication of consistent high thermal stability (Figure 2-5A and Table 2-2).
54
DSC (Figure 2-5B and Table 2-2) results show that 2-1syn, 2-1com, 2-1RR, and 2-1SS,
have melting temperatures of about 160 °C, consistent with Nguyen’s report.54 The two
enantiomers, (R,R)-SMDPPEH, (S,S)-SMDPPEH displayed two additional melting
peaks at 155°C and 156°C, respectively. These arise from different crystal phases. All
four materials crystallize upon cooling (Tc) at about 130 °C. 2-1RS shows a higher Tm
(180 °C) and Tc (154 °C). The result is consistent with Nguyen’s work on DPP(TBFu)2,
where the RS-DPP(TBFu)2 had the highest thermal transition temperature and lowest
solubility among all the isomers. By analogy, centrosymmetric (R,S)-SMDPPEH packs
most efficiently in the solid state and features tighter -stacking interactions.52
0 100 200 300 400 5000
20
40
60
80
100
Weig
ht
/ %
Temperature /C
2-1syn
2-1com
2-1SS
2-1RR
2-1RS
50 100 150 200
-4
-2
0
2
4
6
8
10
He
at
Flo
w /W
/g
Temperature /°C
2-1syn
2-1com
2-1SS
2-1RR
2-1RS
Figure 2-5. A) TGA analysis of 2-1 samples; B) DSC analysis (exothermic peaks down) of 2-1 samples.
Table 2-2. Thermal properties of 2-1
Material 5% Weight Loss [°C] Tm [°C] Tc [°C]
2-1syn 342 158 126 2-1com 341 159 133 2-1SS 343 156/160 132 2-1RR 343 155/160 127 2-1RS 343 180 154
TGA and DSC experiments of 2-1. The 2nd and 3rd heating and cooling scan cycles were employed to determine the thermal transition temperatures, at a scan rate of 10 °C/min, under N
2 atmosphere.
55
Characterization of the SMDPPEH Compositions in the Solid State
All solid state characterizations were conducted by Dr. Nathan Shewmon in Prof.
Jiangeng Xue’s group in the UF Department of Materials Science and Engineering.
X-ray diffraction (XRD) was used in the θ-2θ mode to study the crystallinity of
neat and blended (with PCBM) spin-coated SMDPPEH thin films prepared on silicon
substrates (pre-coated with 25 nm thick PEDOT:PSS films to allow a direct comparison
with films incorporated in photovoltaic devices, vide infra).
Figure 2-6. XRD spectra for spin-coated films of (a,b) neat 2-1 or (c,d) 2-1:PC61BM. Films (a,c) were not annealed, whereas films (b,d) were annealed at 100 °C for 5 minutes.
For neat, unannealed films (Figure 2-6a) a single, weak diffraction peak is
observed for all five materials. The peaks for 2-1syn (2θ = 6.20°; d = 14.2 Å), 2-1com
56
(2θ = 6.16°; d = 14.3 Å), and 2-1RS (2θ = 6.21°; d = 14.2 Å) are centered at similar 2θ
values. Of these three the 2-1RS peak is most intense, supporting the DSC data with
regards to the compound’s higher degree of crystallization. Enantiomers 2-1SS (2θ =
6.66°; d = 13.3 Å) and 2-1RR (2θ = 6.63°; d = 13.3 Å) show expectedly nearly identical
XRD spectra but a slightly tighter (by ~ 1 Å) molecular packing than the other three
materials.
After annealing (at 100 °C for 5 min), an increase in the degree of crystallization
for all of the SMDPPEH materials is observed, accompanied by a 2–3 fold increase in
diffraction intensity (Figure 2-6b). 2-1syn and 2-1com continue to exhibit identical XRD
spectra, with a single peak centered at 2θ = 6.00° (d = 14.7 Å). Apparently, the
differences in isomer composition between these two compounds have little effect on
neat film crystallinity. Neat, annealed films of 2-1SS and 2-1RR again show expectedly
similar behavior, but now with two peaks (2θ = 5.54˚ (d = 15.9 Å) and 2θ = 6.36° (d =
13.9 Å)) observed for each film. Implied are co-existing crystal phases, a result
consistent with the DSC data (vide supra). The difference in relative intensity for the two
peaks between the 2-1SS and 2-1RR samples indicates a different fractional film
coverage for the two phases, with both materials preferentially forming in the d = 15.9 Å
phase while 2-1RR forms slightly more of the d = 13.9 Å phase. Precedent62 leads one
to suspect that such differences might arise from the clockwise substrate rotation during
spin coating that was used here, which could affect the chiral materials differently.
At any rate, it appears that even for enantiomers the film crystallinity can vary
depending on subtle differences in film preparation. Interestingly, unlike the other four
films, the 2-1RS XRD peak shifts about half an angstrom to a smaller d-spacing (2θ =
57
6.46°; d = 13.7 Å) upon annealing. Again, 2-1RS shows the strongest peak intensity,
indicating a higher degree of crystallization in the film.
When blended in a 1:1 weight ratio with PCBM in solution, the spin coated films
of SMDPPEH are completely amorphous with the notable exception of the 2-
1RS:PCBM film, which displays a weak, broad peak centered at 2θ = 6.11° (d = 14.4 Å)
(Figure 2-6c). The presence of PCBM along with rapid drying of the CHCl3 solvent
inhibits crystallization,63 suggesting the effectiveness of the PCBM acceptor in
interfering with SMDPPEH molecular packing.
However, after annealing at 100 °C for 5 minutes a single XRD peak is present
for each material (Figure 2-6d). The peaks for 2-1com and 2-1RS are both centered at
2θ = 6.53° (d = 13.5 Å), while a broader peak is centered at 2θ = 6.36° (d = 13.9 Å) for
2-1syn. The spectral differences here may be assigned to differences in isomer
composition between the 2-1syn and 2-1com materials. For 2-1SS and 2-1RR, a single
peak for each is observed at 2θ = 6.79° (d = 13.0 Å) and 2θ = 6.84° (d = 12.9 Å),
respectively. Again, we would expect identical behavior between the enantiomers,
however small processing differences, for example the clockwise rotation of the spin
coater, may contribute to the differences observed in peak intensity. Comparing the
annealed neat films to the annealed films blended with PCBM, it is clear that PCBM can
have a strong effect on the phase adopted by the different SMDPPEH isomers. Of the
five materials, the neat crystal structure appears to be maintained only by 2-1RS after
blending with PCBM, while the other four materials show a significant shift toward
shorter d-spacing upon PCBM addition.
58
The thin film surface morphologies of the five SMDPPEH materials were
investigated using atomic force microscopy (AFM). Neat, unannealed films (Figure 2-7a-
e) all show a random pattern, with a root-mean-square (RMS) roughness for all five
films in the range of 1.0–1.3 nm. Upon annealing, large crystal formation is observed
(Figure 2-8a-e). While the surfaces of the annealed neat films of 2-1syn and 2-1com
(Figures 2-8a,b) appear nearly identical, the annealed 2-1SS and 2-1RR films (Figure 2-
8c,d) both show noticeably larger domain features, and contain two different phases,
consistent with the XRD data shown in Figure XRD b. The annealed neat film of 2-1RS
(Figure 2-8e) is significantly flatter than the others, with an RMS roughness of 3.3 nm.
The 2-1syn, 2-1com, 2-1SS, and 2-1RR annealed films show RMS roughnesses of 9.3,
8.1, 6.6, and 10.0 nm, respectively.
Figure 2-7. AFM images of unannealed spin-coated films of (a,f) 2-1syn, (b,g) 2-1com, (c,h) 2-1RR, (d,i) 2-1SS, and (e,j) 2-1RS. (a-e) are neat films while (f-j) are
blended 1:1 by weight with PC61BM. All images are 5 5μm.
59
Figure 2-8. AFM images of spin-coated films of (a,f) 2-1syn, (b,g) 2-1com,(c,h) 2-1RS, (d,i) 2-1SS and (e,j) 2-1RR. All films were annealed at 100 for 5 minutes. (a–e) are neat films while (f–j) are blended 1:1 by weight with PCBM. The full height scales (see the color bar shown at the far right of this figure) are 80 nm for (a–e), 10 nm for (f,g,j) and 40 nm for (h,i). The scanning area is 5 × 5 μm for all images.
Without any annealing, films blended with PCBM are quite flat and generally
featureless, with RMS roughness of approximately 0.6 nm for all five materials (Figure
2-7f-j). After annealing, however, two distinct morphologies are observed for the 2-
1syn:PCBM, 2-1com:PCBM, and 2-1RS:PCBM films (Figure 2-8f,g,j) on one hand, and
2-1RR:PCBM and 2-1SS:PCBM films (Figure 2-8h,i) on the other hand. The annealed
blended films were generally flatter than the annealed neat films, although less so for 2-
1RR and 2-1SS isomers, with RMS roughnesses of 1.3, 1.5, 3.3, 7.4 and 1.0 nm for
annealed 2-1syn:PCBM, 2-1com:PCBM, 2-1SS:PCBM, 2-1RR:PCBM, and 2-
1RS:PCBM films, respectively. In summary, very similar surface morphology is
observed for unannealed films of all five isomers; however, upon thermal annealing
significant differences are observed. The observed differences agree well with XRD
data, and the techniques taken together suggest a number of different phases form
depending on the isomer type and processing conditions.
60
Optical absorption spectra of SMDPPEH thin films prepared on glass substrates
(precoated with 25 nm thick PEDOT:PSS films) are presented in Figure 2-9. All films
show three absorption bands centered at 400 nm, 635 nm and 720 nm. The first
absorption peak is attributed to a π–π* transition for both the thiophene and the central
diketopyrrolopyrrole (DPP) units,61 while the second and third peaks are attributed to
intra- and intermolecular charge transfer transitions, respectively. Given that neat
unannealed films of 2-1syn, 2-1com, and 2-1RS show similar absorption spectra
(Figure 2-6a), speaks to the greater ability of 2-1RS to crystallize, thus dominating the
crystal phases of 2-1syn and 2-1com, with disordered regions containing the RR- and
SS-SMDPPEH isomers in between. These observations are supported by the XRD data
(Figure 2-9a). On the other hand, neat unannealed films of 2-1RR and 2-1SS show
slightly blue-shifted absorption peaks, with the shoulder at 600 nm becoming more
prominent. As seen in the XRD analysis above, these films crystallize into a different
phase than the 2-1RS containing films.
After annealing, the spectral shape of the absorption from neat 2-1syn and 2-
1com films is relatively unchanged (Figure 2-9b), but a slight blue shift is observed.
Moreover, the 2-1RS absorption spectrum no longer matches those of the isomeric
mixtures, suggesting that the meso compound adopts a different phase, in agreement
with XRD measurements. Again, after annealing, 2-1SS and 2-1RR films also show
nearly identical absorption spectra. Taking the neat film XRD and absorption data
together, before annealing, the RS–SMDPPEH component of the isomer blends
crystallizes the fastest, and thus dominates the absorption and XRD spectra of
unannealed 2-1syn and 2-1com films. However, after annealing, the isomer blends
61
take on a crystal structure that is different from both 2-1RR and 2-1SS films on the one
hand, and the 2-1RS film on the other. It can be concluded that after annealing the
isomer blends crystallize into a structure that incorporates all three of the isomers.
Figure 2-9. Absorption spectra for spin-coated films of 2-1 on glass/PEDOT:PSS not annealed (a,c) and annealed (at 100 °C for 5 minutes) (b,d) neat (a,b) and blended with PCBM (1:1 by weight) (c,d).
The absorption spectra for unannealed 2-1:PCBM blended films are nearly
identical (Figure 2-9c) as a result of the low crystallinity of these films. After annealing,
however, all of the films are found to crystallize, resulting in differentiation of the
absorption spectra (Figure 2-9d). The most pronounced change is observed for the
annealed 2-1RR and 2-1SS:PCBM films, for which the 600 nm peak becomes dominant,
while the peak at 700 nm is strongly suppressed. In agreement with the AFM and XRD
62
results, the annealed 2-1RS:PCBM film absorption spectrum resembles that of the 2-
1syn and 2-1com:PCBM films, albeit with a reduction in intensity of the 700 nm peak.
This peak has been previously assigned to intermolecular charge transfer in 4-1syn due
to aggregate species,61 and thus a reduction in its intensity may be interpreted as a
reduction in crystallinity. However, such a conclusion is not supported by the AFM and
XRD measurements in this work, which show that the surface morphology and degree
of crystallization of annealed 2-1RS:PCBM, 2-1syn:PCBM, and 2-1com:PCBM films
are quite similar.
Photovoltaic Device Performance
In order to probe the effect of side chain stereochemistry on the optoelectronic
properties of the SMDPPEH materials, photovoltaic devices were fabricated with the
structure: indium tin oxide (ITO)/PEDOT:PSS/2-1:PCBM/Al. Active layer formation was
carried out in an identical fashion to the films analyzed above.
Unannealed devices showed very similar performance with the exception of the
2-1RS:PCBM device (Table 2-3, Figure 2-10a,b). For the other four materials, relatively
low fill factors (FF) of 34–36% presumably result from fast recombination of charge
carriers in the amorphous blends due to a lack of efficient charge extraction pathways.
External quantum efficiencies (EQEs) in all four cases are also very similar (Figure 2-
10b); as the light absorption efficiency was found to be the same, this suggests that the
internal quantum efficiencies are nearly identical. The stereochemical purity of the 2-
ethylhexyl side chains is not expected to significantly affect the electrical properties of
the films when they are not allowed to crystallize. Thus, it is inferred that the more
readily crystallizing 2-1RS material begins to phase segregate from the PCBM without
63
annealing, leading to enhanced charge extraction in the corresponding device and a
significantly higher FF of 45%.
Table 2-3. Characteristics for 2-1 photovoltaic devices
Material Annealed Voc [V] Jsc [mA/cm2] FF [%] PCE [%]
2-1syn no 0.742 ± 0.005 5.4 ± 0.2 35 ± 1 1.4 ± 0.1 2-1com no 0.742 ± 0.006 5.3 ± 0.2 34 ± 1 1.3 ± 0.1 2-1SS no 0.756 ± 0.005 5.5 ± 0.2 36 ± 1 1.5 ± 0.1 2-1RR no 0.752 ± 0.007 5.2 ± 0.4 35 ± 2 1.4 ± 0.2 2-1RS no 0.740 ± 0.005 6.5 ± 0.2 45 ± 2 2.2 ± 0.2 2-1syn yes 0.800 ± 0.005 7.3 ± 0.2 52 ± 1 3.0 ± 0.1 2-1com yes 0.766 ± 0.005 7.5 ± 0.3 50 ± 1 2.9 ± 0.1 2-1SS yes 0.705 ± 0.005 7.8 ± 0.3 52 ± 1 2.9 ± 0.1 2-1RR yes 0.703 ± 0.005 7.6 ± 0.3 50 ± 1 2.7 ± 0.1 2-1RS yes 0.688 ± 0.005 5.1 ± 0.2 55 ± 1 1.9 ± 0.1
For annealed devices (Figure 2-10c,d and Table 2-3) the performance is again
quite similar for all of the devices except those featuring 2-1RS. The most dramatic
change after annealing for the other four materials is the increase in FF to 50–52%.
Additionally, peak EQE increases from 30% for the unannealed devices up to 43–45%
for the annealed devices, resulting in a relative increase in JSC by 35–45%. Clearly, the
crystallization that occurs during the annealing step for the 2-1syn:PCBM, 2-
1com:PCBM, 2-1SS:PCBM, and 2-1RR:PCBM devices results in an improved
photocurrent generation efficiency. For these annealed devices, the EQE spectral
shape matches the film absorption spectra above, with 2-1RR and 2-1SS showing
stronger absorption from the 600 nm peak and 2-1syn and 2-1com showing stronger
absorption from the 700 nm peak. On balance, the spectral shift results in a similar
short-circuit current (JSC) of 7.3–7.8 mA/cm2 for these four annealed devices.
Differences in device performance for 2-1RR and 2-1SS may be related to differences
in the degree of donor crystallization, as observed in the XRD data. A significant
64
reduction in open-circuit voltage (VOC) is observed for 2-1RR and 2-1SS devices (0.703
V and 0.705 V, respectively) relative to 2-1syn and 2-1com devices (0.800 V and 0.766
V, respectively). If these differences in VOC resulted from a change in band gap (Eg), we
would expect to see a red shift of 30–50 nm in the absorption onset of the 2-1RR and 2-
1SS devices relative to the 2-1syn and 2-1com devices. A red shift is indeed observed
in the onset of EQE, but only by approximately 5–10 nm. The discrepancy may arise
from a difference in the relative shift of the optical and transport gaps.64
Figure 2-10. Characterization of 2-1 BHJ photovoltaic devices. (a,c) show current density under 1 sun AM1.5G illumination, (b,d) are EQE spectra. (a,b) were not annealed, while (c,d) were annealed at 100 °C for 5 minutes.
Unlike the other four devices, annealing of the 2-1RS:PCBM device results in a
reduction in EQE, and therefore a reduction in JSC. Differences in overall absorption
65
cannot explain the reduction in EQE observed here, as peak EQE for the 2-1RS:PCBM
device is only 26% (Figure 2-10d). Instead, some internal element of photocurrent
generation efficiency must be negatively affected by the annealed 2-1RS:PCBM film’s
morphology.
Overall, we find that the photovoltaic device performance is only weakly affected
by the differences in morphology between the pure stereoisomers 2-1SS/2-1RR and the
stereoisomeric mixtures 2-1syn/2-1com. Shifted absorption spectra balance out to give
approximately the same coverage of the solar spectrum. Moreover, although very
different surface morphologies are observed with AFM, coincidently the ability of each of
these materials to transport charges is only weakly affected. However, 2-1RS devices
perform much differently from those made with the other four materials. This isomer
crystallizes more rapidly, resulting in differences in optimum film processing. Most
notably, while the PCE of the other devices improved upon mild thermal annealing at
100 °C, the performance of the 2-1RS:PCBM device deteriorated with the same
treatment. It is expected that a more thorough optimization of processing conditions for
this material could bring the overall efficiency closer to that of the other four materials.
Conclusion of the Chapter
The goal of the current work has been to evaluate the consequences of 2-
ethylhexyl solubilizing group chirality, and the isomeric complexity it creates, on small π-
conjugated molecule morphology and bulk heterojunction photovoltaic device
performance. To this end, SMDPPEH, a commercially-available organic semiconductor,
was prepared in isomerically defined form from enantiomerically pure reagents. The
crystallization behavior and optoelectronic properties—neat and as blends with PCBM—
of the three pure isomers (2-1RR, 2-1SS, and 2-1RS) were systematically compared to
66
typical ∼ 1:1:2 isomer mixtures purchased commercially (2-1com) or prepared in the
laboratory (2-1syn).
Expectedly and gratifyingly, the enantiomers (2-1RR, and 2-1SS) showed overall
similar thin film morphology and absorption (neat and as blends; annealed or
unannealed), and consequently bulk heterojunction device performance throughout the
studies. Although different in these respects from the enantiomers, and despite small
differences in isomer composition, 2-1syn and 2-1com showed similar morphologies
and optoelectronic characteristics. All four SMDPPEH compositions were amorphous as
1:1 blends with PCBM without post-deposition thermal annealing, a situation where the
crystallinity, and therefore stereochemical information, was lost. 2-1RS was found to
crystallize most readily of the pure isomers, in the presence and absence of PCBM, and
consequently dominated the absorption and XRD profiles of the neat isomeric mixtures.
Indeed, 2-1RS maintained some crystallinity in the presence of PCBM prior to thermal
annealing resulting in a 50–60% improvement in photovoltaic device performance
relative to the other four compositions.
After thermal annealing, 2-1RR/2-1SS, 2-1syn/2-1com, and 2-1RS revealed
different crystal structures and morphologies suggesting that the isomer mixtures (2-
1syn and 2-1com) adopted a phase that incorporated all three components. Blending
with PCBM had a strong effect on the crystal packing (by XRD), where again the more
strongly crystallizing 2-1RS dominated the profiles of the isomer mixtures. For 2-1syn,
2-1com, 2-1SS, and 2-1RR, a substantial increase in photovoltaic device performance
was observed after thermal annealing, while 2-1RS showed a decrease in device
performance. Overall, the stereocenter affected the morphology of the active layer and
67
the thin film absorption spectra, but had a relatively weak effect on the overall
photovoltaic performance.
The extent to which alkyl side chain stereochemistry should be considered an
important tunable parameter in materials design is most certainly device/application
dependent. To wit, Nguyen and coworkers demonstrated that while the isolated
stereoisomers of DPP(TBFu)2 showed similar morphologies, RS-DPP(TBFu)2 displayed
much higher FET mobility (before and after annealing) as a consequence of its crystal
structure with tighter π–π stacking, as well as a more homogenous domain shape/size
and smaller film roughness (versus RR- and SS-DPP(TBFu)2). The study conveys the
importance of isomeric composition/purity on a small molecule’s carrier mobility, and the
observation appears generally extendable to other π-conjugated systems. Our work
shows that the side chain chirality, while strongly impacting the thin film morphology and
optical properties of SMDPPEH, has modest effects on the photovoltaic performance of
the material as blends with PCBM. While this does not mean that side chain
stereochemistry does not influence blend structure, it does suggest that the structural
consequences lead to fortuitously compensatory optoelectronic effects in this context.
Studies among additional classes of semiconductors will expose whether side chain
chirality can be employed to rationally improve OPV efficiency or whether it can be
safely “ignored”.
Experimental
General Methods
Reagents and solvents were purchased from commercial sources and used
without further purification unless otherwise specified. THF, Et2O, CH2Cl2, and DMF
were degassed in 20 L drums and passed through two sequential purification columns
68
(activated alumina; molecular sieves for DMF) under a positive argon atmosphere. 2-(5'-
Hexyl-[2,2'-bithiophen]-5-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane and [1,1′-
bis(diphenylphosphino)ferrocene]dichloro palladium(II) (complex with dichloromethane,
[Pd(dppf)Cl2•CH2Cl2]) were purchased from Sigma-Aldrich and used as received. Thin
layer chromatography (TLC) was performed on SiO2-60 F254 aluminum plates with
visualization by UV light or staining. Flash column chromatography was performed
using Silica gel technical grade, pore size 60 Å, 230−400 mesh particle size, 40−63 μm
particle size from Sigma-Aldrich. Isomeric purity was determined by HPLC analysis
(Shimadzu) using Chiralpack IA column. Specific Optical rotations were obtained on a
JASCO P-2000 Series Polarimeter (wavelength = 589 nm) and then corresponded to
the literature. 500 (125) MHz 1H (13C) NMR were recorded on an INOVA 500
spectrometer. Chemical shifts (δ) are given in parts per million (ppm) relative to TMS
and referenced to residual protonated solvent purchased from Cambridge Isotope
Laboratories, Inc. (CDCl3: δH 7.26 ppm, δC 77.16 ppm; DMSO-d6: δH 2.50 ppm, δC 39.52
ppm). Abbreviations used are s (singlet), d (doublet), t (triplet), q (quartet), quin (quintet),
hp (heptet), b (broad), and m (multiplet). Refer to Adv. Funct. Mater. 2014, 24, 5993–
6004 to view the 1H and 13C NMR spectra. ESI-TOF-, APCI-TOF-, and DART-TOF-MS
spectra were recorded on an Agilent 6210 TOF spectrometer with MassHunter software.
EI-MS (70 eV) spectra were recorded on a Thermo Scientific DSQ MS after sample
introduction via GC with data processing on Xcalibur software (accurate masses are
calculated with the CernoBioscience MassWorks software). MALDI-TOF-MS was
performed on a AB Sciex TOF/TOF 5800 in reflectron mode while the data is processed
69
with Data Explorer. Samples were prepared by mixing the molecule of interest in
dithranol (DTL) and then applied onto the MALDI plate.
Synthesis of SS-SMDPPEH (2-1SS)
(S)-4-Benzyl-3-hexanoyloxazolidin-2-one (2-3S). (S)-(-)-Benzyl-2-
oxazolidinone 2-2S (5.0 g, 28.2 mmol) was dissolved in THF (56 mL). This mixture was
cooled to -78 °C, then n-butyl lithium (2.5 M solution in hexane, 11.3 mL, 28.2 mmol)
was added dropwise. After stirring for 30 min, n-hexanoyl chloride (3.93 mL, 28.2 mmol)
was added to this mixture and stirred for 1 h. The reaction mixture was poured into
saturated aqueous NaHCO3 and this mixture was extracted with ethyl acetate twice.
The combined organic layer was washed with brine, dried over NaSO4, filtered and
evaporated in vacuo. The crude was purified by column chromatography on silica gel
(10% ethyl acetate in hexanes) to provide (S)-4-benzyl-3-hexanoyloxazolidin-2-one
(7.41 g, 95.4%) as a colorless oil. TLC Rf = 0.53 (20% ethyl acetate in hexanes); [α]D23=
+89.1°(1.2 MeOH); 1H NMR (500 MHz, CDCl3, δ): 7.33 (tt, J = 7.5; 1.5 Hz, 2H), 7.27 (dt,
J = 7.5; 1.5 Hz, 1H), 7.22 (dd, J = 6.5; 2 Hz, 2H), 4.67 (m, 1H), 4.17 (m, 2H), 3.30 (dd,
J = 13.5; 3.5 Hz, 1H), 2.93 (m, 2H), 2.76 (dd, J = 13.5; 9.5 Hz, 1H), 1.70 (m, 2H), 1.37
(m, 4H), 0.92 (t, J = 7.5 Hz, 3H). 13C NMR (125 MHz, CDCl3, δ): 173.6, 153.6, 135.5,
129.6, 129.1, 127.5, 66.3, 55.3, 38.1, 35.6, 31.4, 24.1, 22.6, 14.1; HRMS (ESI) calcd.
(M+Na)+ 298.1414, found 298.1418.
(S)-4-Benzyl-3-((S)-2-ethylhexanoyl)oxazolidin-2-one (2-4S). A solution of 2-
3S (6.63 g, 24.1 mmol) in THF (26.7 mL) was added dropwise over 30 min to sodium
bis(trimethylsilyl)amide (1.0M solution in THF, 36.1 mL, 36.1 mmol) cooled to -78 °C.
After 1 h, ethyl iodide (2.89 mL, 36.1 mmol) was added dropwise, and the resulting
mixture was allowed to warm to -40 °C and was stirred at the same temperature for 16 h.
70
The reaction was quenched by addition of a saturated solution of ammonium chloride at
-30 °C, followed by evaporation of the solvent. The aqueous phase was extracted with
chloroform, the organic extracts were dried over sodium sulfate, and the solvent was
removed. The residue was purified by flash chromatography (5% ethyl acetate in
hexanes) to afford 2-4S as a colorless oil (2.87 g, 40%). TLC Rf=0.63 (20% ethyl
acetate in hexanes); [α]D23= +55.8°(1.0 DCM); 1H NMR (500 MHz, CDCl3, δ): 7.33 (tt, J
= 7.5; 1.5 Hz, 2H), 7.27 (dt, J = 7.5; 1.5 Hz, 1H), 7.24 (m, J = 6.5; 2 Hz, 2H), 4.70 (m,
1H), 4.16 (m, 2H), 3.73 (m, 1H), 3.34 (dd, J = 13.5; 3.5 Hz, 1H), 2.70 (dd, 1H) 1.74 (m,
2H) 1.61 (m, 1H), 1.50 (m, 1H), 1.33-1.22 (m, 4H), 0.96 (t, J = 7 Hz, 3H), 0.88 (t, J = 7
Hz, 3H). 13C NMR (125 MHz, CDCl3, δ): 171.0, 153.3, 135.6, 129.5, 129.1, 127.5, 66.0,
55.7, 44.2, 38.3, 29.7, 25.6, 23.0, 14.1, 11.6; HRMS (ESI) calcd . (M+Na)+ 326.1727
(2M+Na)+ 629.3561, found 326.1719, 629.3567.
(S)-2-Ethylhexan-1-ol (2-5S). To a mixture of 2-4S (3.03 g, 10 mmol) and
ethanol (640 μL, 10.9 mmol) in dry diethylether (32.3 mL), cooled to 0 °C was added
dropwise a 2 M solution of lithium borohydride in THF (5.9 mL, 11.8 mmol). The
resulting mixture was stirred at 0 °C for 3 h and quenched by adding a 1 N solution of
sodium hydroxide. The organic phase was washed with water and dried over sodium
sulfate, and the solvent was removed. The crude product was purified by flash
chromatography (1:3 diethylether/pentane) to afford 2-5S (1.04 g, 80%) as a colorless
liquid. TLC Rf = 0.44 (20% ethyl acetate in hexanes); [α]D23= +6.4°(1.0 DCM), lit
[α]D22= +3.4°(1.0 DCM);65 1H NMR (500 MHz, CDCl3, δ): 3.55 (d, J = 5.5 Hz, 2H), 1.44-
1.22 (m, 9H), 0.89 (t, J = 2 Hz, 6H). 13C NMR (125 MHz, CDCl3, δ): 66.5, 42.1, 30.3,
71
29.3, 23.5, 23.2, 14.2, 11.2; GC-EI-MS found [M-OH]+ 112, fragment pattern matches
with 2-ethyl-1-hexanol.
(S)-2-Ethylhexyl 4-methylbenzenesulfonate (2-6S). TsCl (p-toluenesulfonyl
chloride) (1.55 g, 8.16 mmol) was added to an ice-cooled and stirred solution of 2-5S
(0.85 g, 6.52 mmol) and dry pyridine (2 mL) in CH2Cl2 (12.5 mL). To this mixture was
added DMAP (4-dimethylaminopyridine) (4.3 mg, 0.0352 mmol) and the mixture was
stirred at 4°C for ca. 48 h. To this was added water with ice-cooling. Then it was poured
into 3 M hydrochloric acid and extracted with diethyl ether. The ethereal extract was
washed with 1 M hydrochloric acid, water, a saturated aqueous NaHCO3 solution and
brine, dried with NaSO4 and concentrated in vacuo to give 1.3 g (70%) of 5. [α]D23=
+5.9°(1.0 DCM); 1H NMR (500 MHz, CDCl3, δ): 7.79 (d, J = 8.5 Hz, 2H), 7.34 (d, J = 8.5
Hz, 2H), 3.92 (qd, J = 9.5; 5.5 Hz, 2H), 2.45 (s, 3H), 1.53 (m, J = 6.5 Hz, 1H), 1.36-1.10
(m, 8H), 0.84 (t, J = 7 Hz, 3H), 0.79 (t, J = 7.5 Hz, 3H). 13C NMR (125 MHz, CDCl3, δ):
144.7, 129.9, 128.1, 110.1, 72.7, 39.2, 29.4, 28.8, 23.4, 23.0, 21.8, 14.1, 10.9; HRMS
(ESI) calcd. (M+Na)+ 307.1338 (2M+Na)+ 591.2785, found 307.1348, 591.2789.
(S)-2-Ethylhexyl bromide (2-7S). Lithium bromide (0.887 g, 10.45 mmol) was
added to a solution of 2-6S (1.98 g, 6.97 mmol) in dry acetone (18.6 mL). The mixture
was stirred and heated under 50 °C for ca. 6 h. To this was added water and the solvent
was removed by distillation. The residue was poured into water, and extracted with n-
pentane. The pentane extract was washed with water and brine, dried with NaSO4, and
concentrated in vacuo. The residue was chromatographed on silica gel (n-pentane) and
distilled to give 1.1g (81%) of 2-7S. [α]D23= +5.3°(1.0 DCM); (500 MHz, CDCl3, δ): 3.46
(qd, J = 10; 5 Hz, 2H), 1.53 (m, J = 5.5 Hz, H), 1.46-1.20 (m, 8H), 0.93-0.85 (m, 6H).
72
13C NMR (125 MHz, CDCl3, δ): 44.2, 39.3, 32.1, 29.0, 25.3, 23.0, 14.2, 11.0; GC-EI-MS
found [M-H]+ 191, M+ 192, fragment pattern matched with 3-bromomethylheptane.
3,6-Dithiophen-2-yl-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (2-9).
Potassium tert-butoxide (16.81 g, 149.8 mmol) was added to a round flask with argon
protection. Then a solution of t-amyl alcohol (2-methyl-2-butanol) (89.2 mL) and 2-
thiophenecarbonitrile 2-8 (11.72 g, 10 mL, 107.03 mmol) was injected by a syringe one
portion. The mixture was warmed up to 100-110 °C, and a solution of dimethyl
succinate (5.21 g, 35.67 mmol) in t-amyl alcohol (28.6 mL) was dropped slowly in 1 h.
When the addition was completed, the reaction was kept at the same temperature for
24 h. Then the mixture was cooled to 65 °C, neutralized with acetic acid and reflux for
another 10 min. Then the suspension is filtered and the black filter cake was washed
with hot methanol and water twice each and dried in vacuum to get coarse product and
could be used directly to next step without further purification (8.21 g, 77%). 1H NMR
(300 MHz, DMSO-d6, δ): δ 8.21 (dd, J = 3.9; 1.2 Hz, 2H), 7.96 (dd, J = 4.8; 1.2 Hz, 2H),
7.30 (dd, J = 5.1; 4.2 Hz, 2H). 13C NMR (75 MHz, DMSO-d6, δ): 161.65, 136.19, 132.66,
131.26, 130.82, 128.72, 108.57, 128.72.
2,5-Bis-((S)-2-ethyl-hexyl)-3,6-dithiophen-2-ylpyrrolo[3,4-c]pyrrole-1,4-dione
(2-10SS). In a three-necked, flame-dried round-bottom flask, 3,6-dithiophen-2-yl-2,5-
dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione 2-9 (0.383 g, 1.27 mmol) and anhydrous K2CO3
(4.15 g, 30.0 mmol) were dissolved in 50 mL of anhydrous N,N-dimethylformamide
(DMF) and heated to 120 °C under argon for 1 h. (S)-2-Ethylhexyl bromide 2-7S (0.616
g, 3.19 mmol) was then added dropwise, and the reaction mixture was further stirred
and heated overnight at 130 °C. The reaction mixture was allowed to cool down to room
73
temperature; after that solvent was removed in vacuo. The crude product was purified
by flash chromatography using chloroform as eluent, and the solvent was removed in
vacuo. Product was recrystallized in dichloromethane-hexanes mixture to give a pure
product as red powder (0.164g, 25%). TLC Rf = 0.77 (DCM); [α]D23= -28.2° (0.1 DCM);
1H NMR (500 MHz, CDCl3): δ 8.89 (d, J = 3.5 Hz, 2H), 7.63 (d, J = 4.5 Hz, 2H), 7.27
(dd, J = 5; 4 Hz, 2H), 4.02 (m, 4H), 1.86 (m, 2H), 1.41-1.15 (m, 16H), 0.92-0.78 (m,
12H). 13C NMR (125 MHz, CDCl3, δ): 161.9, 140.6, 135.4, 130.7, 128.6, 108.1, 46.0,
39.2, 30.4, 28.5, 23.7, 23.2, 14.2, 10.6; ESI-DART-MS calcd. [M]+ 524.2526, [M+H]+
525.2604, found 524.2549, 525.2629.
3,6-bis(5-bromothiophen-2-yl)-2,5-bis((S)-2-ethylhexyl)-2,5-
dihydropyrrolo[3,4-c]pyrrole-1,4-dione (2-11SS). In a three-necked, flame-dried
round-bottom flask, compound 2-10SS (160 mg, 0.305 mmol) was dissolved in 10 mL of
anhydrous CHCl3, covered with aluminum foil, and stirred at room temperature under
argon for 15 min. N-bromosuccinimide (0.12 g, 0.671 mmol) was then added, and the
reaction mixture was kept at room temperature with stirring for 48 h. The reaction
mixture was poured into 25 mL of methanol, and the resulting suspension was stirred at
room temperature for 1 h. The solid was then collected by vacuum filtration and washed
with several portions of hot distilled water and hot methanol to obtain pure product as a
shiny dark-purple powder (159 mg, 76.4%). [α]D23= -51.1° (0.1 DCM); 1H NMR (500
MHz, CDCl3): δ 8.63 (d, J = 4 Hz, 2H), 7.21 (d, J = 4 Hz, 2H), 3.92 (m, 4H), 1.83 (m, 2H),
1.41-1.17 (m, 16H), 0.93-0.82 (m, 12H). 13C NMR (125 MHz, CDCl3, δ): 161.5, 139.6,
135.6, 131.6, 131.3, 119.2, 108.1, 46.2, 39.2, 30.3, 28.6, 23.7, 23.3, 14.2, 10.6. ESI-
74
DART-MS calcd. [M+H]+ 681.0814, found 681.0809, [M+H+2]+ calcd 683.0799, found
683.0793, [M+H+4]+ calcd 685.0784, found 685.0776.
2,5-bis((S)-2-ethylhexyl)-3,6-bis(5''-hexyl-[2,2':5',2''-terthiophen]-5-yl)-2,5-
dihydropyrrolo[3,4-c]pyrrole-1,4-dione (2-1SS). Compound 2-11SS (127.6 mg,
0.187mmol), 2-(5'-hexyl-[2,2'-bithiophen]-5-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane
2-12 (210.7 mg, 0.56 mmol), K2CO3 (0.413g, 3mmol) and Pd(dppf)Cl2 ( [1,1'-
bis(diphenylphosphino)ferrocene]dichloropalladium(II) ) (27.32mg, 0.0373 mmol) was
added to a round bottomed flask and degassed. Toluene (5 mL) and water (1.7 mL) was
degassed for 30 min before addition to the reaction flask. Aliquat 336 (2 drops) was
then added. The reaction mixture was stirred overnight at 80 °C, cooled back to room
temperature, precipitated into MeOH and filtered. The crude product was purified by
column chromatography on silica gel, using a mixture of 1:3 to 1:1
dichloromethane/hexanes as the eluent and then the solvent was removed in vacuo
(116 mg, 61 %). TLC Rf = 0.96 (DCM); [α]D24= -57.6° (0.1 DCM); 1H NMR (500 MHz,
CDCl3, δ): 8.94 (d, J = 4 Hz, 2H), 7.24 (d, J = 2.5 Hz, 2H), 7.17 (d, J = 3.5 Hz, 4H), 7.01
(t, J = 4 Hz, 4H), 6.69 (d, J = 3 Hz, 2H), 4.02 (m, 4H), 2.79 (t, J = 7 Hz, 4H), 1.92 (m,
2H), 1.68 (m, 4H), 1.45-1.23 (m, 28H), 0.96-0.88 (m, 18H). 13C NMR (125 MHz, CDCl3,
δ): 161.8, 146.6, 142.7, 139.5, 139.2, 136.8, 134.4, 134.3, 128.3, 126.0, 125.2, 124.6,
124.2, 124.0, 108.7, 46.3, 39.6, 31.7, 31.7, 30.7, 30.4, 28.9, 28.8, 24.0, 23.3, 22.7, 14.2,
10.8; MALDI-DTL-MS calcd. M+ 1021.3952, found 1021.4001; CHN analysis calcd. C
68.192, H 7.104, N 2.742, found C 68.214, H 7.524, N 2.571.
Synthesis of RS-SMDPPEH (2-1RS)
(R)-2-(2-ethylhexyl)-3,6-di(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-
1,4-dione (2-13R). In a three-necked, flame-dried round-bottom flask, 3,6-dithiophen-2-
75
yl-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione 2-9 (16.65 g, 55.45 mmol) and anhydrous
K2CO3 (1.53 g, 11.09 mmol) were dissolved in 300 mL of anhydrous N,N-
dimethylformamide (DMF) and heated to 120 °C under argon for 1 h. (R)-2-Ethylhexyl
bromide 2-7R (1.071 g, 5.545 mmol) (prepared in the same method as 2-7S) was
dissolved in 100 mL of DMF and then the solution was added dropwise. The reaction
mixture was further stirred and heated overnight at 130 °C. Then it was allowed to cool
down to room temperature. After that solvent was removed in vacuo. The mixture was
extracted with excess DCM and the solvent was removed in vacuum to give crude
product. The crude product was purified by flash chromatography (5% ethyl acetate in
DCM), and the solvent was removed in vacuo to give a pure product as a reddish brown
powder (0.376g, 16%). 1H NMR (300 MHz, CDCl3, δ): 9.63 (s, 1H), 8.80 (d, J = 6.5 Hz,
1H), 8.31 (d, J = 6 Hz, 1H), 7.63 (d, J = 8 Hz, 1H), 7.56 (d, J = 8 Hz, 1H), 7.24 (dd, J =
4.8; 4.2 Hz, 1H), 7.17 (t, J = 4.5; 4.2 Hz, 1H), 3.99 (m, 2H), 1.85 (m, 1H), 1.42-1.18 (m,
8H), 0.92-0.82 (m, 6H). 13C NMR (75 MHz, CDCl3, δ): 162.5, 161.8, 141.1, 136.8, 135.6,
132.3, 131.2, 131.1, 130.9, 129.85, 129.1, 128.5, 46.0, 39.2, 30.3, 28.5, 23.6, 23.2, 14.2,
10.6; HRMS(DART) calcd for C22H24N2O2S2 [M+H]+: 413.1352, found: 413.1357.
2-((R)-2-ethylhexyl)-5-((S)-2-ethylhexyl)-3,6-di(thiophen-2-yl)-2,5-
dihydropyrrolo[3,4-c]pyrrole-1,4-dione (2-14RS). In a three-necked, flame-dried
round-bottom flask, (R)-2-(2-ethylhexyl)-3,6-di(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-
c]pyrrole-1,4-dione 2-13R (0.376 g, 0.91 mmol) and anhydrous K2CO3 (0.378 g, 2.73
mmol) were dissolved in 9.1 mL of anhydrous N,N-dimethylformamide (DMF) and
heated to 120 °C under argon for 1 h. (S)-2-Ethylhexyl bromide 2-7S (0.352 g, 1.82
mmol) was then added dropwise, and the reaction mixture was further stirred and
76
heated overnight at 130 °C. The reaction mixture was allowed to cool down to room
temperature; after that solvent was removed in vacuo. The crude product was purified
by flash chromatography using chloroform as eluent, and the solvent was removed in
vacuo. Product was recrystallized in dichloromethane-hexanes mixture to give a pure
product as red powder (0.07g, 15%). 1H NMR (500 MHz, CDCl3, δ): 8.88 (d, J = 3.5 Hz,
2H), 7.62 (d, J = 5 Hz, 2H), 7.27 (dd, J = 5; 4 Hz, 2H), 4.02 (m, 4H), 1.85 (m, 2H), 1.41-
1.19 (m, 16H), 0.90-0.82 (m, 12H). 13C NMR (125 MHz, CDCl3, δ): 161.9, 140.6, 135.4,
130.7, 130.0, 128.8, 108.1, 46.0, 39.2, 30.4, 28.5, 23.7, 23.2, 14.2, 10.6; HRMS(DART)
calcd for C30H40N2NaO2S2 [M+H]+: 525.2604, found: 525.2609.
3,6-bis(5-bromothiophen-2-yl)-2-((R)-2-ethylhexyl)-5-((S)-2-ethylhexyl)-2,5-
dihydropyrrolo[3,4-c]pyrrole-1,4-dione (2-15RS). Compound 2-15RS was prepared
analogously to 2-11SS beginning from 70 mg of 2-14RS with yield of 0.05 g (55%). 1H
NMR (500 MHz, CDCl3, δ): 8.61 (d, J = 5 Hz, 2H), 7.21 (d, J = 4 Hz, 2H), 3.93, (m,4H),
1.83 (m,2H), 1.40-1.20 (m, 16H), 0.91-0.84 (m, 12H). 13C NMR (125 MHz, CDCl3, δ):
161.6, 139.6, 135.5, 131.7, 131.4, 119.1, 108.4, 46.3, 39.4, 30.5, 28.6, 23.9, 23.2, 14.1,
10.7; HRMS(DART) calcd for C30H38Br2N2O2S2 [M+H]+:calcd 681.0814, found 681.0807,
[M+H+2]+: 683.0799, found: 683.0800, [M+H+4]+: calcd 685. 0774, found 685.0781.
2-((R)-2-ethylhexyl)-5-((S)-2-ethylhexyl)-3,6-bis(5''-hexyl-[2,2':5',2''-
terthiophen]-5-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (2-1RS). Compound 2-
1RS was prepared analogously to 2-1SS beginning from 50 mg of 2-15RS with yield of
0.041 g (55%). 1H NMR (500 MHz, CDCl3, δ): 8.91 (br, 2H), 7.26 (neck, 2H), 7.18 (br,
2H), 7.02 (d, J = 4 Hz, 4H), 6.69 (d, J = 3 Hz, 2H), 4.04 (m, 4H), 2.80 (t, J = 7 Hz, 4H),
1.94 (m, 2H), 1.70 (m, 4H), 1.46-1.20 (m, 28H), 0.97-0.82 (m, 18H). 13C NMR (125 MHz,
77
CDCl3, δ): 161.8, 146.6, 142.7, 139.5, 139.2, 136.8, 134.4, 128.2, 126.0, 125.2, 124.6,
124.2, 124.0, 108.7, 46.3, 39.6, 31.7, 31.7, 30.7, 30.4, 28.9, 28.9, 24.1, 23.3, 22.7, 14.2,
10.8; HRMS(MALDI) calcd for C58H72N2O2S6 M+: 1021.3952 found: 1021.3981. CHN
analysis Calcd for C58H72N2O2S6: C 68.19, H 7.10, N 2.74, found: C 68.11, H 7.35, N
2.71. The enantiomeric purity (98%) was determined by HPLC analysis (Chiralpak IA, 8%
i-PrOH in hexanes, 0.8 mL/min, 350 nm).
Characterizations
Absorption Measurements
Absorption spectra were measured for at least six different concentrations (2.5–
30 μM) of SMDPPEH using a Cary 100 Bio spectrophotometer and 1 cm quartz
cuvettes. All solvents were spectrophotometric grade and purchased from Sigma-
Aldrich. The absorbance at λmax was then plotted against the concentration in all cases
to confirm, by linearity, that the compounds followed Beer’s law. Molar extinction
coefficients (ε) were determined from the linear plot for each compound (where A = εbc).
Thermal Analysis
Thermal gravimetric analysis (TGA) was performed on 2-1syn, 2-1com, 2-1SS,
2-1RR, and 2-1RS using a TA Instruments TGA Q5000-0121 V3.8 Build 256 at a
heating rate of 10 °C/min using 1–3 mg of sample in a 100 μL platinum pan (under
nitrogen). The data was analyzed on Universal Analysis 2000 4.4A software.
Differential Scanning Calorimetry (DSC) was performed on 2-1syn, 2-1com, 2-
1SS, 2-1RR, and 2-1RS using a TA Instruments DSC Q1000-0620 V9.9 at a
heating/cooling rate of 10 °C/min using 1–3 mg of sample in a sealed aluminum pan,
with respect to an empty aluminum reference pan. Three cycles of heating and
78
subsequent cooling were performed, however, only the second full cycle is shown. The
data was analyzed on Universal Analysis 2000 4.4A software.
Thin Film Characterization
UV-vis absorption (thin film) measurements were carried out with a calibrated
Newport 818-UV Si photodiode illuminated by a Newport Oriel Apex illuminator and
monochromator system chopped at 400 Hz. The signal was detected using a Stanford
Research Systems SR830-DSP lock-in amplifier.
Atomic force microscopy was carried out using a Veeco Innova AFM in tapping
mode with a silicon tip (radius ∼ 8 nm) at 325 kHz with a force constant of
approximately 40 N/m. X-ray diffraction measurements were performed using a
PANalytical X’Pert Powder diffractometer operated in the θ -2θ mode using Cu Kα
radiation (λ = 1.54 Å).
Device Fabrication and Characterization
Organic photovoltaic devices were fabricated on commercial indium-tin oxide
(ITO) coated glass substrates with a sheet resistance of ∼15 Ω/square, while films for
XRD and AFM analysis were fabricated on Si(100) substrates. The substrates were
sequentially sonicated for 15 minutes in detergent, water, acetone and isopropanol
before UV-ozone treatment for an additional 15 minutes. PEDOT:PSS (Clevios AI4083)
was spin-coated in air at 8000 rpm to form a ∼25 nm thick layer, which was annealed at
150 °C for 30 minutes in air before passing into a nitrogen glovebox (H2O ∼ 1 ppm).
Photovoltaic active layers were spin-coated from a 5:5 mg/mL solution of
SMDPPEH:PC61BM in CHCl3 at 500 rpm to form a ∼110 nm thick layer. Films were then
passed into a vacuum chamber pumped down to 10−6 Torr, and a 100 nm Al cathode
was evaporated through a shadow mask with thicknesses monitored by a quartz crystal
79
monitor. The cross-bar geometry was used to defi ne an active area of 4 mm2 for the
organic photovoltaic cells. After aluminum deposition, some devices were annealed at
100 °C for 5 minutes in a nitrogen glovebox, and then encapsulated with a UV-curable
epoxy layer to prevent degradation from exposure to ambient oxygen and water before
characterization in air.
Devices were characterized under illumination from a 150W Xe-arc lamp solar
simulator with a KG1 filter, calibrated to 1 sun intensity using the AM1.5G spectrum.
External quantum efficiency measurements were carried out with a calibrated Newport
818-UV Si photodiode illuminated by a Newport Oriel Apex illuminator and
monochromator system chopped at 400 Hz. The photocurrent signal was detected
using a Stanford Research Systems SR830-DSP lock-in amplifier.
80
CHAPTER 3 KETO-ENOL TYPE TAUTOMERICALLY ACTIVE MODULES CONTAINING
BENZODIFURAN FOR Pi-CONJUGATED MATERIALS
Introductory Remarks
π-Conjugated organic architectures are among the most promising frameworks
for the design of next generation functional materials. They are typically prepared from
kinetically stable aromatic and heteroaromatic (mainly N-, O-, and S- containing)
building blocks (Figure 3-1) that have appropriate optical and electronic properties to
promote a desired function in solution or in a device. Their applications include
sensors,66 light emitting devices,67 light harvesting and solar energy conversion,68 field
effect transistors,69,70 two photon absorption,71 molecular wires,72 etc.73 The design74
and preparation of π-conjugated constructs have been well adept by the community.
Building and optimizing π-conjugated polymers and small molecules for device
fabrication has become a hot research topic. However, the π-conjugated framework of
these structures remains generally unchanged. More versatile structures would be
attractive.
Figure 3-1. Common heterocyclic building blocks employed in π-conjugated architectures.
81
The main idea of this research project is to consider unconventional π-electron
building blocks. Different methods have been used by chemists across the world
including incorporation of other heteroatoms75–83 within the π-conjugated backbone and
new consideration of aromatic84,85 (or antiaromatic86,87) structures88–90 (Figure 3-2) and
dimensionality.91–93 With the conceptual advances of “static” main-chain π-conjugated
structures in mind, we are considering the dynamic tuning of π-conjugated frameworks.
We seek to expand the tunability of π-conjugated materials hoping they could lead to
novel properties (e.g. control of bulk ordering, responsive material, self-assembled
devices, etc.) and ultimately enhance performance.
Some research has been conducted on dynamic/switchable π-delocalized
structure within molecules (Figure 3-3).94 A number of systems that feature electro- or
photochemically switchable π-conjugated units along the π-conjugated backbone were
constructed.95–98 (Figure 3-4) The “reversible conjugated polymers” of Bielawski and
coworkers is an example.99 The monomers bear N-heterocyclic carbene (NHC) and
isothiocyanate functional groups and the bond formation and cleavage can be controlled
by temperature. We are interested in incorporation of tautomerically active modules
(TAMs) within π-delocalized systems (Figure 3-5), with the intention of adding tunability
to the architectures. These incorporated modules can thus influence the aromaticity and
π-electron delocalization of the molecule. (Figure 3-6) In that way, new optoelectronic,
supramolecular and processing properties can be accessed, which are otherwise not
available in “static” structures.
82
Figure 3-2. Unconventional π-electron building blocks. A) Containing other hetero atoms.78,80 B) aromatic84,85 or antiaromatic structures.90
Figure 3-3. The idea of dynamic tuning.
Figure 3-4. Diarylethene molecular switch.94
83
Figure 3-5. TAM in extended π-electron system.
Figure 3-6. Relationship of tautomerism, aromaticity and π-electron delocalization.
Tautomers are constitutional isomers of organic compounds that readily
interconvert by a chemical reaction called tautomerization. This reaction commonly
results in the formal migration of a hydrogen atom or proton, accompanied by a switch
of a single bond and adjacent double bond. Tautomerism is closely linked with
aromacity and π-electron delocalization, thus influencing optoelectronic properties of π-
electron systems.100–102 An example of enol-keto tautomerization is illustrated in Figure
84
3-7. For phenol, the enol exists exclusively in the ground state (with an aromatic
stabilization energy of over 25 kcal/mol), while 9-anthrol is disfavored (pKT = 2.10 in
aqueous solution) in the anthrone system (prefers the preservation of terminal 6π-
electron rings).100 Tautomeric systems require careful design because the two
tautomeric forms should be close in energy in order to coexist. Many factors
(temperature, pressure, concentration, atomic mutation, solvent, phase, molecular
dipole, hydrogen bonding, or a combination of them) are involved in the
equilibrium,100,103,104 particularly when multiple tautomeric forms co-exist in solution. Not
sufficiently studied are tautomeric equilibria in the context of π-stacked supramolecular
structures and extended π-delocalized systems.
Figure 3-7. The tautomerization of A) phenol and B) 9-anthrol.100
Examples involving CH/OH tautomerization (or prototropy) are important to
review for this work. Benzodifurantrione (Figure 3-8A) can undergo an extended keto-
enol tautomerization (which is a 1,7-proton transfer instead of a 1,3-proton transfer)
across the benzene ring to form a phenylogous enol.105 The equilibrium is quantified.
Interestingly, the loss of aromaticity of the central benzene ring is compensated by the
loss of the high-energy adjacent carbonyl groups, rendering the two structures similar in
energy. 2-(2-(3-Nitrophenyl)-4,5- diphenyl-1H-imidazol-1-yloxy)-1- phenylethanone
(Figure 3-8B) displays solid state photochromic behavior.106 It is capable of enol-keto
tautomerization by light exposure through excited state intramolecular proton transfer
85
(ESIPT) to change color from yellow (keto form) to red (enol form). The red enol form
would convert back to yellow form in the dark. All the historical work indicates that the
introduction of this functionality to the “static” systems would be a great way to create
“dynamic” architectures.
Figure 3-8. Examples of CH/OH type tautomerically active molecules. A) Benzodifurantrione.105 B) 2-(2-(3-nitrophenyl)-4,5- diphenyl-1H-imidazol-1-yloxy)-1- phenylethanone.106
Molecular Design
In order to understand the interplay between tautomerism and π-electron
delocalized molecular and supramolecular structures, fused polycyclic aromatic ring
systems containing TAMs are considered here. Successful synthesis of these
molecules could give access to studies of how size, geometry and aromaticity influence
tautomeric and associated optoelectronic properties. Conditions that can tune the
tautomeric equilibria will be investigated. Then the π-conjugation could be expanded by
added pendent aromatic groups and we can move forward to explore the relationship
86
between tautomerism and linear π-conjugated architectures. Appropriately elaborated
TAMs can be studied in hydrogen bonded and π-stacking environments with respect to
tautomer stability and aggregate optoelectronic properties, elucidating relationships
between tautomerism and supramolecular π-delocalized structure. (Figure 3-9)
Figure 3-9. Tautomerrically active modules (TAMs) in extended π-electron systems. (A) Polycyclic aromatic rings. B)linear π-conjugated architectures. C) supramolecular assemblies.
Initial studies will focus on model compounds. Numerous studies have been
performed on five- and six-membered ring systems regarding their synthesis, tautomeric
interconversion, stability, etc.100,101 We selected the 3-hydroxylfuran/thiophene system
featuring CH/OH tautomerization (Figure 3-10). Significant physical, chemical, and
optoelectronic property change can be expected from tautomeric interconversion of
these modules. Their enolic forms are relevant to known thiophene, furan, and
benzenoid systems that are widely employed in organic π-functional materials. Fused
ring systems based on these modules could be built and studied.
87
Figure 3-10. TAMs that will be used to explore the interplay of extended π-electron delocalization and tautomerism. X = O, S
Tautomerism has been studied in heterocyclic molecules containing a single
TAM. Fused polycyclic ring systems with more than one TAM are good starting points of
research. With leading reference on the model system107,108 and our previous work on
benzotrifuranone system109 in mind, we designed the following target compounds
(Figure 3-11) for initial study. They are fused polycyclic system with two peripheral
TAMs and are our probes to explore relationships between tautomerism and π-
delocalization in such systems. The two TAMs allow two or more accessible tautomeric
states. They can give us access to ‘trapped’ extreme states for characterization. Their
acyl substituents (amide or ester) provide sites for solubilizing groups and chemical
stability. Amide groups would trap both tautomeric forms through intramolecular
hydrogen bonding (refer to Figure 3-12 for hydrogen bonding pattern). The side chain of
the amide could be functionalized. The two phenyl positions could potentially allow
extension of π-conjugated framework and give access to study the relationship between
tautomerism and π-delocalization in linear π-conjugated architectures.
Figure 3-11. Target compounds 3-1 and 3-2.
88
Theoretical Calculation
Theoretical calculation is a powerful method to estimate the properties of the
proposed targets. For initial studies we used the following protocol: a) low energy
structures were obtained by manually positioning groups to form hydrogen bonding and
geometrically minimized by MM2 force field; b) precise geometry minimizations,
electronic structures, and tautomer energies were obtained by DFT calculations;110,111 c)
MOs were visualized using GaussView.112
Figure 3-12. Energy differences between geometry-minimized tautomers of 3-1 in the gas phase.
The gas-phase energy minimized structures of different tautomeric forms of
target compound 1 were computed at the B3LYP/6-31G* level. Hexyl groups were
replaced by methyl groups for simplicity of computation. The gas-phase tautomer
energy differences and equilibrium constants are shown in Figure 3-12. The enol form is
denoted by the letter E while the keto form is denoted by the letter K. The tautomer 3-
1EK is about 1.6 kcal/mol higher in energy than 3-1E2, so the distribution of 3-1EK and
3-1E2 can be calculated as the Boltzmann factor (equation):
.
As calculated, there should be about 93.7% of the compound in the 3-1E2 form
and 6.3% in the 3-1EK form at room temperature. The remaining tautomeric form, 3-
1K2, is about 3.2 kcal/mol higher in energy than 3-1EK and thus it is very minor. The
89
analysis indicates that the first two tautomerization states may be experimentally
accessible.
MOs of the tautomers are shown in Figure 3-13. As shown in the plots, the
energies of the HOMOs and LUMOs are lowered with the introduction of each keto form
in the module. The difference in the HOMO-LUMO energy gap between 3-1E2 and 3-
1EK is small since the first keto group lowers both the HOMO and LUMO similarly. The
energy gap of 3-1K2 drops more significantly as a result of tautomerization since the
second keto group lowers the energy level of the LUMO more than it does to the HOMO.
Overall, tautomerization in the system can modestly tune the frontier molecular orbital
energy levels and consequently the energy gaps.
Figure 3-13. Electronic structures, frontier molecular orbital energies, and HOMO-LUMO energy gaps based on gas-phase DFT calculations.
90
Synthesis
The synthesis of 3-1 (Scheme 3-1) started with commercially available diethyl
2,5-dihydroxyterephthalate 3-3 which underwent a condensation reaction to install a
pendent ester group to give 3-4. Ring-closure of 3-4 in basic conditions formed the furan
unit and yielded 3-5.113 These two steps were originally performed by a former member
of the group, Dr. Yan Li.
Scheme 3-1. Synthesis of 3-1.
We initially proposed to protect the hydroxyl group of 3-5 with methyl by using
dimethyl sulfate, but this reaction gave a low yield which only decreased in larger scale
91
(Table 3-1). Several other protecting groups (including benzyl, silyl, and MOM) were
attempted (Table 3-1) and the best results were obtained using the MOM protecting
group.114 This is probably due to the low reactivity of the anion of 3-5 as the negative
charge is resonance stabilized. MOM and BOM protection groups are frequently used in
protecting phenol hydroxyl groups. Direct aminolysis of 3-5 failed to produce the desired
product 3-1.115
Table 3-1. Conditions used for protection of 3-5.
Reagent Solvent Base Temperature Duration Result (yield)
Me2SO4 acetone KHCO3 56 °C overnight no reaction Me2SO4 DMF K2CO3 60 °C overnight 10 % Me2SO4 DMF K2CO3 60 °C overnight 1 % Me2SO4 DMF t-BuOK 60 °C overnight 10 % BnBr DMF K2CO3 60 °C 12 h 88% TMSCl THF Et3N 0 °C to rt overnight no reaction TESCl DMF Et3N,DMAP 0 °C to rt overnight no reaction MOMCl DMF DIPEA 0 °C to rt 5 h quantitative BnCl DMF K2CO3 60 °C 12 h complicated
mixture
Interestingly, benzyl protection gave both O- and C-reaction products in high
yield instead of the desired bis-O-protection (Scheme 3-2). The outcome of the reaction
confirmed the reactivity of the enolate anion of 3-5; both the O and C atoms are
nucleophilic with the outcome dependent on reaction conditions and electrophile.
Although the C-reaction product could be taken as the “trapped” keto tautomer, the
outcome of the reaction did not provide direct evidence of the accessibility of the keto
form through keto-enol tautomerization.
92
Scheme 3-2. Treatment of 3-5 with benzyl bromide.
Direct condensation of 3-6 with amines (to form the corresponding amides) was
attempted but was unsuccessful. Saponification, however, afforded diacid 3-7 in good
yield where the MOM group was expectedly left intact. Table 3-2 then summarizes
attempts to convert 3-7 to the corresponding hexyl amide. Conversion to the diacid
chloride using either oxalyl chloride or thionyl chloride, to precede amine introduction,
were not successful. Also attempted was diamide formation directly from the diacid
using various dehydrating reagents. Among them, the uronium salt type coupling
reagent HBTU worked to offer 3-8 most efficiently and reproducibly. The acid activation
time was particularly important for the success of this reaction. Finally, 3-8 was
hydrolyzed with aqueous HCl to give 3-1 in moderate to good yield. Other MOM-
cleavage conditions, like TMSCl/NaI, did not work.
Table 3-2. Conditions used in amide formation between 3-7 and hexylamine.
Reagent Base Solvent Temperature Activation time
Reaction time
Result (yield)
CDI THF 66 °C 1 h 3 h no reaction CDI DMF 60 °C 1 h 3 h trace DCC(HOBt) DMF rt 16 h trace DCC DMF rt 16 h no reaction HBTU DIPEA DMF rt 15 min overnight trace HBTU DIPEA DMF rt 10 h overnight 52 % HBTU DIPEA DMF rt 10 h overnight 49 % EDC(NHI) DIPEA DMF rt overnight overnight no reaction
93
Because the deprotection of the MOM groups of 3-8 was not efficient, synthetic
routes that avoided protection were also considered. Along these lines, envisioned was
introduction of the amide before the cyclization (Scheme 3-3). Bromoacetyl chloride 3-9
was reacted with hexylamine to yield the amide 3-10. While subsequent ether formation
with 3-3 gave the product 3-11 in low yield, all attempts to cyclize 3-11 (Table 3-3) were
unsuccessful.
Scheme 3-3. Early installation of the amide.
Table 3-3. Conditions used in cyclization attempts with 3-11.
Base Solvent Temperature Time Result
t-BuOK THF 0 °C 1 h hydrolyzed NaOMe benzene 80 °C 1.5 h no reaction t-BuOK THF 0 °C 15 min hydrolyzed
Also attempted was preparation of the phenyl ester derivative 3-13 (Scheme 3-4),
assuming the ester would provide better reactivity and later conversion to the amide.
While 3-9 could be converted to 3-12 in reasonable yield, reaction between 3-3 and 3-
94
12 failed under the conditions attempted (Table 3-4). Since neither case worked, no
further attempts were made to pre-install the amide or a more activated ester.
Scheme 3-4. Attempted synthesis of 3-13.
Table 3-4. The substitution reaction of 3-3 and 3-12.
Base Catalyst Solvent Temperature Time Result
K2CO3 acetone 56 °C overnight decomposed K2CO3 KI DMF rt 24 h no reaction
More troublesome, while characterizing 3-1 it became clear that its poor solubility
would hinder its broad solution study (e.g., a good 13C NMR could not be obtained). At
this stage we considered introduction of a branched side chain in place of the hexyl
chain to improve the solubility. The 1-pentylhexyl chain was selected since, while
branched, it would not introduce isomeric complexity; synthesis of the new target, 3-14,
is shown in Scheme 3-5. The precursor 3-16 was synthesized from the reductive
amination of 6-undecanone 3-15. It was then attached to the core using a similar
method to 3-1. Initial attempts using uronium salt type coupling reagents were not
successful. After a number of trials (Table 3-5), COMU116 was found to work the best
even though it only afforded the bishexylamide product in 11% yield (the 3-17 product
95
still had unreacted amine impurity). Once 3-17 was obtained, the MOM groups were
hydrolyzed and product 3-18 was afforded, albeit impure. Unreacted 3-16 was hard to
remove even with acid wash. Given the general problems with this route it was
abandoned.
Scheme 3-5. Synthesis of 3-18 (branched side chain version of 3-1).
Table 3-5. Conditions used in coupling reaction of 3-7 with 1-pentylhexylamine 3-16.
Coupling reagent
Base Solvent Temperature Activation time
Reaction time
Result (yield)
TBTU DIPEA DMF rt 15 min overnight No reaction TBTU DIPEA DMF rt 15 min overnight No reaction HBTU DIPEA DMF rt 10 h overnight 1 % HBTU DIPEA DMF rt 10 h overnight 0.6 % COMU DIPEA DMF rt 15 min overnight 0.2 % COMU DIPEA DMF rt 15 min overnight 11 %
96
In parallel, the synthesis of 3-2 was pursued. The original plan was to begin with
1,4-dibromo-2,5-dimethylbenzene 3-19 oxidize it to form 3-20, followed by thiol group
introduction (Scheme 3-6). The approach failed at the substitution step.
Scheme 3-6. Attempted synthesis of 3-2 starting from 3-19.
Scheme 3-7. Attempted synthesis of 3-2.
An alternative route (Scheme 3-7), involving a Newman–Kwart rearrangement,
was attempted to install the thiols (the reaction is a common way to prepare
thiophenols).117 The new route started with 3-3 which was converted to O-
thiocarbamate 3-22. The Newman–Kwart rearrangement of 3-22 was attempted but the
conversion was not satisfying under standard thermal conditions; usage of a microwave
97
reactor helped considerably. The method worked well after some optimization and pure
product 3-23 was obtained.
Although it was originally intended that 3-23 could be alcoholyzed by potassium
ethoxide/ethanol to deprotect the thiophenol while preserving the ester, the major
product from such a reaction turned out to be fully hydrolyzed 3-21. Given this,
KOH/water was used to affect efficient hydrolysis with the intention of later esterifying.
The hydrolysis reaction worked well and 3-21 was obtained in good yield.
Figure 3-14. 1H NMR spectrum of product of the reaction between 3-21 and ethyl bromoacetate in CDCl3.
Intermediate 3-21 bears both thiophenol and carboxylic acid groups, which need
to undergo SN2 substitution and esterification, respectively, to make the desired
intermediate 3-25. The substitution was attempted first. Neither NMR (Figure 3-14) nor
MS confirmed the product. No carboxylic acid proton was identified by the former
98
(suggesting conversion to an ester), and the found mass (539.33 (M+H)+, 561.27
(M+Na)+) did not match the calculated mass of 458.1069. The structure of the isolated
material was not elucidated, and reactivity studies (e.g. reaction with t-BuOK or an acid
chloride) failed to confirm available functionality.
An alternative approach to the systems was conceived to involve a ring-opening
reaction (Scheme 3-8). The specific reaction involved trichloroacetyl chloride coupling
with anthranilic acid and subsequent ring opening with ethanol,118 a well-known way to
esterify anthranilic acid. The thiol version of anthranilic acid, thiosalicylic acid, could
serve as a model compound and give a new solution to the synthesis of 3-2. Two
conditions were tested for the conversion of 3-26 to 3-27 (Table 3-6) but neither worked.
Perhaps the lower reactivity of the thiol could be the reason.
Scheme 3-8. Cyclization of 4-aminobenzoic acid and attempted cyclization of 3-26.
Table 3-6. Conditions of the ring closure reaction of thiosalicylic acid 3-26.
Reagent Solvent Temperature Time Result
trichloroacetyl chloride
dioxane 101 °C 4 h no reaction
Carbonyl diimidazole
benzene 80 °C 3 h no reaction
99
Characterization
The potential structural changes accompanying tautomerism of 3-1 were studied
by NMR. 3-1 was dissolved in DMSO-d6 and kept at room temperature. Its 1H-NMR
spectrum was taken every day to monitor the changes. (Figure 3-15)
Figure 3-15. 1H-NMR spectrum of 3-1 in DMSO-d6 at room temperature. 1-8: spectrum after 0, 5, 10, 15, 20, 25, 30, and 34 days.
New peaks began emerging within the first few days (at the expense of those
from 3-1) leaving a complicated spectrum after 10 days. For example, the hydroxyl
peaks (~ 10.9 ppm) slowly disappeared and new peaks in the aromatic region appeared.
Qualitatively, the spectral changes (e.g., desymmetrization and greater signal number,
disappearance of the hydroxyl protons) seemed consistent with keto-enol
tautomerization. However, if 3-1EK was generated, its methine proton would have a
100
chemical shift of about 5.5 ppm. This and other chemical shift changes suggested that
3-1 underwent an unexpected chemical transformation (perhaps involving keto-enol
tautomerization). A deuterium exchange experiment with 3-1 failed given its poor
solubility in DMSO-d6 (upon the addition of D2O). More data would have to be collected
to determine the fate of 3-1 over time.
Focus was turned to 3-5 because of its simpler structure and easier synthesis
and purification. It has the same tautomerizable groups except ester substitution in
place of the amide groups of 3-1. NMR analysis of 3-5 was performed fresh and after 44
days (after storage at room temperature). (Figure 3-16) The resulting spectrum is
cleaner than in the case of 3-1. Together with Dr. Ion Ghiviriga in the Department of
Chemistry, an HSQC experiment was conducted to give some structural clues.
101
Figure 3-16. 1H NMR spectrum of 3-5 in DMSO-d6 44 days after the sample was prepared.
Figure 3-17. HSQC spectrum of the aged 3-5 sample.
102
It was discovered that all of the new peaks in the 7.0-8.0 ppm range are
connected to aromatic carbons in the 100-120 ppm range (Figure 3-17). The data was
not consistent with an original thought that the three new peaks in the 7.0-8.0 ppm
range arose from the two aromatic peaks and the methine peak of the tautomer 3-5EK.
Consultation with Dr. Ghiviriga led to the conclusion that there are three to four
compounds co-existing in the solution, and separation would benefit further analysis.
Overall this work revealed that compounds such as 3-1 and 3-5 are not stable in
DMSO-d6 solution and undergo unexpected chemical transformations. Their insufficient
solubility in other solvents limits study in other solvents.
300 400 500
0.0
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1.2
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so
rba
nce
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2.5uM
5uM
10uM
15uM
20uM
30uM
0 5 10 15 20 25 30
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Ab
so
rba
nce
Concentration (10-6 M)
Figure 3-18. UV-Vis absorption spectrum and Beer-Lambert plot of 3-1 in DMSO (2.5×10-6 M–30×10-6 M).
Table 3-7. Optical properties of 3-1 in DMSO (20 × 10-6 M).
Material λmax [nm] λonset [nm] ɛ ×104 [M-1cm-1] ΔEopt [eV]
3-1 259/329/342/359 372 3.86 3.33
Molar extinction coefficient was calculated based on the absorbance at the maximum absorbance wavelength (342 nm).
The optical properties of 3-1 were characterized by UV-Vis spectroscopy (Figure
3-18 and Table 3-7). The sample was tested the day it was prepared. The three
maximum absorption peaks at 329, 342, and 359 nm are presumably from different
103
vibrionic energy levels of the lowest excited state. The onset absorption at 372 nm
corresponds to an optical energy gap of 3.33 eV, in good agreement with theoretical
calculations and consistent with the compound’s slight yellow color. A linear Beer-
Lambert plot (R2 = 0.9998) shows that 3-1 is molecularly dissolved in DMSO. The
compound has red-shifted absorption compared with parent benzodifuan (maximum
absorption at 277 nm) 119 due to the functionalization of hydroxyl and amide groups that
produce push-pull effect.
Figure 3-19. TGA analysis of 3-1.
TGA analysis of 3-1 (Figure 3-19) showed a 5% weight loss temperature of
255.7 °C, speaking to good thermal stability. The shape of the TGA curve suggested
stepwise thermal decomposition mechanisms. The phase transition temperatures were
255.67°C 95.00%
0
20
40
60
80
100
120
Weig
ht (%
)
0 100 200 300 400 500 600 700
Temperature (°C)
Sample: YZ2054Size: 3.9700 mgMethod: Ramp
TGAFile: D:...\TAM A2\TGA\YZ2054.001Operator: Yu ZhuRun Date: 28-Mar-2015 15:42Instrument: TGA Q5000 V3.17 Build 265
Universal V4.5A TA Instruments
104
analyzed by DSC. 3-1 has a melting temperature (Tm) of 88.4 °C and crystallization
temperature (Tc) of 84.9 °C. This is lower than benzodifuran (Tm = 111 °C). The side
chains are responsible for lowering the melting temperature.
Figure 3-20. DSC analysis of 3-1.
Development of a Benzodifuran Based Self-Assembled Material
Introduction
The self-assembly of π-conjugated molecules through various types of non-
covalent interactions, such as hydrogen bonding, π-stacking, and van der Waals
interactions to achieve functional 1D nanostructures has been an active area of
research.120 The community is interested in the tunability of the non-covalent
interactions which could allow control over the nanostructure and properties. Different
88.43°C(I)
89.70°C
89.76°C
84.90°C(I)
86.30°C
85.86°C
-0.8
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-50 0 50 100 150 200
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Sample: YZ2054Size: 3.7500 mgMethod: Ramp
DSCFile: D:...\TAM A2\DSC\YZ2054.001Operator: YuRun Date: 22-Apr-2015 13:27Instrument: DSC Q1000 V9.9 Build 303
Exo Down Universal V4.5A TA Instruments
105
chromophores have been studied for their self-assembly and application.121–124 (Figure
3-21) We are interested in our benzodifuran derivative. It is an p-type chromophore
used in organic semiconductor research. We would like to functionalize it for self-
assembly studies.
Figure 3-21. Examples of self-assembling π-conjugated molecules.121,123,124
Naphthalene diimide (NDI) is a well-studied chromophore. Its solubility and
aggregation behavior can be directed by functionalization at the imide positon. Rajdev,
Molla, and Ghosh prepared two NDI derivatives, NDI-1 and NDI-2. (Figure 3-22) NDI-2
has two amide groups that NDI-1 does not have. It was found that the hydrogen
bonding of NDI-2 can direct the self-assembly. Although it weakens the self-assembly of
the chromophore by itself, it forms an intercalated co-assembly with a pyrene derivative.
We are inspired by this research and designed a functionalized benzodifuran for
self-assembly studies. (Figure 3-23)
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Figure 3-22. The self-assembly of NDI-1 and NDI-2. Reprinted (adapted) with permission from (Rajdev, P.; Molla, M. R.; Ghosh, S. Langmuir 2014, 30, 1969–1976.). Copyright (2014) American Chemical Society.
Figure 3-23. The design of a functionalized benzodifuan for self-assembly studies. Reprinted (adapted) with permission from (Rajdev, P.; Molla, M. R.; Ghosh, S. Langmuir 2014, 30, 1969–1976.). Copyright (2014) American Chemical Society.
The cores of the molecules are the benzodifuran, expected to interact through π-
π interactions. The MOM group remains to maintain stability, while the trialkoxybenzene
groups on the periphery, connected to the core with ester (3-28) or amide (3-29) bonds,
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will enhance solubility in less polar solvents. The amide bonds can potentially form
intermolecular hydrogen bonds to further direct 1D self-assembly, so the two would
likely show different self-assembly behavior.
Synthesis
The synthesis of the two derivatives was divided into two parts. The first was the
condensation of 3-7 with phenol or aniline (test nucleophiles), while the second was
synthesis of the appropriate trialkoxyphenol and trialkoxyaniline.
Table 3-8. Conditions used in coupling reaction of 3-7 phenol.
Coupling reagent
Base Solvent Temperature Activation time
Reaction time
Result
TBTU DIPEA DMF rt 15 min overnight no reactoin COMU DIPEA DMF rt 1 h overnight no reactoin DCC DMAP DMF 0 °C to rt 5 min (0 °C) 3 h no reactoin DCC DMAP
(cat.) THF rt 1 day no reactoin
Scheme 3-9. Synthesis of 3,4,5-tris(dodecyloxy)phenol 3-33.
Several attempts were made to condense 3-7 and phenol (Table 3-8) but they
were not successful, probably due the low nucleophilicity of phenol. Attempts to couple
3-7 with aniline or to make the activated ester (not shown) did not work either. However,
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the synthesis of 3,4,5-tris(dodecyloxy)phenol worked well (Scheme 3-9). Following a
literature procedure,125 3,4,5-tris(dodecyloxy)phenol (3-33) was successfully prepared.
Conclusions and Future Directions
In conclusion, keto-enol type tautomerically active module containing
benzodifuran compounds have been designed and synthesized. Theoretical calculation
and modeling in the gas phase showed the intramolecular hydrogen-bonding stabilized
tautomeric forms have small energy differences and are potentially accessible.
Benzodifuran based model compound 3-1 was successfully prepared. It has red-shited
UV-Vis absorption compared with parent benzodifuran and good thermal stability.
However, the low stability of 3-1 in DMSO solution limited further study on its
tautomerization behavior. The synthesis of benzodithiophene base model compound 3-
2 was yet to achieve success.
3-1 was redesigned for the study of the self-assembly of electron rich extended
π-conjugated system. The synthesis of trialkoxyphenol precursor intended to be
installed on the periphery of the molecule was successful. However, test reactions of
functionalization of the core failed.
For the future work, other synthetic methods will be attempted to prepare 3-28
and 3-29. Once the target compound is obtained, its self-assembly behaviors will be
characterized.
Experimental
General Methods
Reagents and solvents were purchased from commercial sources and used
without further purification unless otherwise specified. THF, Et2O, CH2Cl2, and DMF
were degassed in 20 L drums and passed through two sequential purification columns
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(activated alumina; molecular sieves for DMF) under a positive argon atmosphere. Thin
layer chromatography (TLC) was performed on SiO2-60 F254 aluminum plates with
visualization by UV light or staining. Flash column chromatography was performed
using Silica gel technical grade, pore size 60 Å, 230−400 mesh particle size, 40−63 μm
particle size from Sigma-Aldrich. 500 (125) MHz 1H (13C) NMR were recorded on an
INOVA 500 spectrometer. Chemical shifts (δ) are given in parts per million (ppm)
relative to TMS and referenced to residual protonated solvent purchased from
Cambridge Isotope Laboratories, Inc. (CDCl3: δH 7.26 ppm, δC 77.16 ppm; DMSO-d6: δH
2.50 ppm, δC 39.52 ppm). Abbreviations used are s (singlet), d (doublet), t (triplet), q
(quartet), quin (quintet), hp (heptet), b (broad), and m (multiplet). ESI-TOF-, APCI-TOF-,
and DART-TOF-MS spectra were recorded on an Agilent 6210 TOF spectrometer with
MassHunter software. MALDI-TOF-MS was performed on an AB Sciex TOF/TOF 5800
in reflectron mode while the data is processed with Data Explorer. Samples were
prepared by mixing the molecule of interest in dithranol (DTL) and then applied onto the
MALDI plate.
Synthesis
Diethyl 2,5-bis(2-ethoxy-2-oxoethoxy)terephthalate (3-4). To the solution of
starting material diethyl 2,5-dihydroxyterephthalate 3-3 (1.00 g, 3.93 mmol, in 40 mL
acetone) was added K2CO3 (3.25 g, 23.52 mmol), followed by the addition of ethyl
bromoacetate (1.00 mL, 9.02 mmol). The resulting mixture was heated to reflux for 4
hours and then cooled to room temperature. After removal of the solvent, water (100
110
mL) was added and the product was extracted with ethyl acetate (4 × 30 mL). The
organic layers were combined, washed with brine, and dried over Na2SO4. After
evaporation of the solvent, the crude product was yielded (1.68 g, quantitative yield) as
an off-white solid. 1H NMR (500 MHz, CDCl3, δ): 7.38 (s, 2H), 4.68 (s, 4H), 4.39 (q, J =
7.1 Hz, 4H), 4.28 (q, J = 7.1 Hz, 4H), 1.38 (t, J = 7.1 Hz, 6H), 1.29 (t, J = 7.1 Hz, 6H);
13C NMR (125 MHz, CDCl3, δ): 168.4, 165.0, 151.6, 125.9, 118.7, 67.8, 61.7, 61.5, 14.3;
HRMS (ESI) m/z : [M+Na]+ calcd. for C20H26O10: 449.1418, found 449.1439.
Diethyl 3,7-dihydroxybenzo[1,2-b:4,5-b']difuran-2,6-dicarboxylate (3-5). To
the solution of 3-4 (1.68 g, 3.94 mmol) in 60 mL dry THF at 0 °C was added potassium
tert-butoxide (2.21 g, 19.7 mmol) in portions, and then the color of the reaction solution
changed to orange and became cloudy. The solution was kept stirring for 1 hour and
then quenched with NH4Cl solution. The reaction mixture was diluted with water. HCl
(0.1 N) was added to the aqueous solution until the solution was acidic. Near-white
precipitate formed and was collected by filtration and washed with small portions of
water. The product was dried in vacuum and was obtained (972 mg, 74%) as a near-
white solid. 1H NMR (500 MHz, DMSO-d6, δ): 10.86 (br, 2H), 7.97 (s, 2H), 4.33 (q, J =
7.0 Hz, 4H), 1.32 (t, J = 7.0 Hz, 6H); 13C NMR (125 MHz, DMSO-d6, δ): 159.1, 148.6,
147.0, 128.4, 123.3, 103.0, 60.1, 14.4; HRMS (ESI) m/z : [M+Na]+ calcd. for C16H14O8:
357.0581, found 357.0580.
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Diethyl 3,7-bis(methoxymethoxy)benzo[1,2-b:4,5-b']difuran-2,6-
dicarboxylate (3-6). To a stirring solution of 3-5 (0.3253g, 0.973 mmol) and i-Pr2NEt
(0.508 mL, 2.92 mmol) in DMF (6 mL) at 0 °C under argon was added chloromethyl
methyl ether (0.185 mL, 2.43 mmol) and the reaction was stirred at room temperature
for 2 h. Additional i-Pr2NEt (0.508 mL, 2.92 mmol) and chloromethyl methyl ether (0.110
mL, 1.46 mmol) were added and the reaction was stirred for an another 3 h before
quenching with dilute NaHCO3 aqueous solution. The mixture was extracted 3 times
with CH2Cl2, the organic fractions combined, dried over Na2SO4, filtered, and then the
volatiles were removed under reduced pressure. The crude material was purified by
flash chromatography over silica gel eluting with 1:1 hexanes/CH2Cl2 to afford pure
product (0.3492 g, 96 %) as a near-white solid. 1H NMR (500 MHz, CDCl3, δ): 7.89 (s,
2H), 5.41 (s, 4H), 4.45 (q, J = 7.0 Hz, 4H), 3.63 (s, 6H), 1.43 (t, J = 7.0 Hz, 6H); 13C
NMR (125 MHz, CDCl3, δ): 159.1, 149.9, 146.7, 134.4, 124.4, 103.7, 99.5, 61.5, 57.4,
14.5; HRMS (ESI) m/z : [M+H]+ calcd. for C20H22O10: 423.1286, found 423.1299.
3,7-Bis(methoxymethoxy)benzo[1,2-b:4,5-b']difuran-2,6-dicarboxylic acid
(3-7). 3-6 (0.3755 g, 0.89 mmol) was suspended in methanol (10 mL) and potassium
hydroxide (0.499 g, 8.89 mmol) was added. The reaction mixture was heated to reflux
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for 2 h. The reaction mixture was allowed to cool. The methanol was evaporated under
reduced pressure. The resulting solid was washed with DCM. Water was added, and
the solution was acidified with 1M hydrochloric acid. White precipitate formed and was
filtered. The solid was dried in vacuum to afford 3-7 (0.2766 g, 85%) as an off-white
solid. 1H NMR (500 MHz, DMSO-d6, δ): 13.55 (br, 2H), 8.08 (s, 2H), 5.44 (s, 4H), 3.54
(s, 6H); 13C NMR (125 MHz, DMSO-d6, δ): 159.6, 148.9, 145.1, 134.8, 123.8, 103.4,
98.8, 56.8. HRMS (ESI-) m/z : [M-H]- calcd. for C16H14O10: 365.0514, found 365.0494.
N2,N6-Dihexyl-3,7-bis(methoxymethoxy)benzo[1,2-b:4,5-b']difuran-2,6-
dicarboxamide (3-8). 3-7 (0.5394 g, 1.47 mmol) and O-(benzotriazol-1-yl)-N,N,N′,N′-
tetramethyluronium hexafluorophosphate (HBTU) (1.396 g, 3.68 mmol) were dissolved
in anhydrous DMF (10.0 mL). The solution was stirred for 15 min under an atmosphere
of nitrogen after which diisopropylethylamine (0.4489 mL, 3.68 mmol) was added. The
solution was stirred for 10 h at room temperature under argon. Then hexylamine
(0.4864 mL, 3.68 mmol) was added and the solution was stirred overnight at room
temperature under argon. 100 mL of water was added to dilute the reaction mixture. 3 M
HCl solution was added to acidify the aqueous solution. After extraction with DCM, the
organic portion was separated and washed with 1 M HCl solution. The organic portion
was washed again with brine and dried over Na2SO4. The drying agent was filtered and
the solvent was removed in vacuum. The crude product was purified by column
chromatography (1:1 DCM/EtOAc) as eluent to afford 3-8 as a white solid. 1H NMR (500
113
MHz, CDCl3, δ): 7.84 (s, 2H), 6.84 (t, J = 5.2 Hz, 2H), 5.45 (s, 4H), 3.62 (s, 6H), 3.46 (q,
J = 6.5 Hz, 4H), 1.62 (p, J = 7.0 Hz, 4H), 1.33 (m, 12H), 0.88 (t, J = 6.1 Hz, 6H); 13C
NMR (125 MHz, CDCl3, δ): 158.7, 149.2, 142.6, 136.4, 123.1, 103.3, 99.0, 57.5, 39.4,
31.6, 29.8, 26.8, 22.7, 14.1. HRMS (ESI) m/z : [M+H]+ calcd. for C28H40N2O8: 533.2857,
found 533.2833.
N2,N6-Dihexanoyl-3,7-dihydroxybenzo[1,2-b:4,5-b']difuran-2,6-
dicarboxamide (3-1). 3-8 (0.1 g, 0.19 mmol) was dissolved in a mixture of dry
THF/MeOH (2 mL/2 mL) and the solution was stirred at 0 °C. Then concentrated HCl
(1.5 mL) in 1,4-dioxane (5 mL) was added dropwise, and the mixture was allowed to
warm to room temperature. The reaction mixture was stirred overnight, and the
suspension was filtered and washed with MeOH and DCM. The resulting solid was
dried in vacuum to give 3-1 (57 mg, 68%) as a white solid. 1H NMR (500 MHz, DMSO-
d6, δ): 10.60 (br, 2H), 8.05 (t, J = 5.8 Hz, 2H), 7.85 (s, 2H), 3.29 (q, J = 6.5 Hz, 4H), 1.53
(quin, J = 6.8 Hz, 4H), 1.36-1.26 (m, 12H), 0.87 (t, J = 6.5 Hz, 6H). HRMS (ESI-) m/z :
[M-H]- calcd. for C24H32N2O6: 433.2188, found 433.2192.
The synthesis of 3-17 is in similar method of 3-8. 3-18 was prepared with the
same method as 3-1. Both compounds have amine impurities and NMR data are not
reported here for quality issues.
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Diethyl 2,5-bis((dimethylcarbamothioyl)oxy)terephthalate (3-22).117 2,5-
Dihydroxyterephthalic acid diethyl ester 3-3 (0.5 g, 1.97 mmol) and DABCO (0.88 g,
7.87 mmol) were dissolved in dry DMF (5 mL) under a argon atmosphere and cooled to
0 °C in an ice bath. Dimethylthiocarbamoyl chloride (0.87 g, 7.87 mmol) dissolved in dry
DMF (2.5 mL) was added dropwise under nitrogen. The mixture was stirred for 16 h at
room temperature. The off-white precipitate was filtered, washed extensively with water
(30 mL), and dried under vacuum, yielding compound 3-22 (845 mg, 100%). 1H NMR
(500 MHz, CDCl3, δ): 7.71 (s, 2H), 4.30 (q, J = 7.0 Hz, 4H), 3.45 (s, 3H), 3.39 (s, 3H),
1.33 (t, J = 7.0 Hz, 6H); 13C NMR (125 MHz, CDCl3, δ): 187.3, 163.1, 150.6, 128.7,
127.8, 61.6, 43.4, 39.0, 14.2; HRMS (ESI) m/z : [M+Na]+ calcd. for C18H24N2O6S2:
451.0968, found 451.0961.
Diethyl 2,5-bis((dimethylcarbamoyl)thio)terephthalate (3-23).117 Compound
3-22 (0.3 g, 1.47 mmol) was added to a sealed tube and NMP (3 mL) was added. The
tube was filled with nitrogen and sealed with screw cap. It was stirred and heated in a
microwave reactor at 250 °C for 3 h. After the reaction was completed, the reaction
115
mixture was allowed to cool and some product crystallized. The product was collected
with filtration and the solution was diluted with water. White precipitate formed and it
was collected through filtration. The combined products was recrystallized in MeOH and
dried in vacuum to give 3-23 (0.253 g, 84%) as white solid. 1H NMR (500 MHz, CDCl3,
δ): 8.11, (s, 2H), 4.34 (q, J = 7.1 Hz, 4H) 3.12 (s, 6H), 3.01 (s, 6H), 1.36 (t, J = 7.1 Hz,
6H); 13C NMR (125 MHz, CDCl3, δ): 165.4, 165.4, 138.8, 137.2, 131.0, 81.7, 37.2, 14.2.
HRMS (DART) m/z : [M+NH4]+ calcd. for C18H24N2O6S2: 446.1414, found 446.1430.
2,5-Dimercaptoterephthalic acid (3-21).117 Water (5 mL) was degassed and
compound 3-23 (0.1 g, 0.233 mmol) was suspended in the water. KOH (0.233 g, 3.97
mmol) was added and dissolved. The reaction mixture was refluxed under an inert
atmosphere for 2 h. The reaction mixture was cooled and then acidified with
concentrated HCl. A bright yellow precipitate was formed, filtered, and washed
extensively with water, yielding compound 3-21 as a yellow solid (52 mg, 96%). 1H NMR
(500 MHz, CD3OD, δ): 8.04 (s, 2H). HRMS (ESI-) m/z : [M-H]- calcd. for C8H6O4S2:
228.9635, found 228.9642.
Absorption Measurements
Absorption spectra were measured for at least six different concentrations (2.5–
30 μM) using a Cary 100 Bio spectrophotometer and 1 cm quartz cuvettes. All solvents
were spectrophotometric grade and purchased from Sigma-Aldrich. The absorption
intensity at λmax was then plotted against the concentration in all cases to confirm, by
116
linearity, that the compounds followed Beer’s law. Molar extinction coefficients (ε) were
determined from the linear plot for each compound (where A = εbc).
Thermal Analysis
Thermal gravimetric analysis (TGA) was performed on 3-1 using a TA
Instruments TGA Q5000-0121 V3.8 Build 256 at a heating rate of 10 °C/min using 1–3
mg of sample in a 100 μL platinum pan (under nitrogen). The data was analyzed on
Universal Analysis 2000 4.4A software.
Differential Scanning Calorimetry (DSC) was performed on 3-1 using a TA
Instruments DSC Q1000-0620 V9.9 at a heating/cooling rate of 10 °C/min using 1–3 mg
of sample in a sealed aluminum pan, with respect to an empty aluminum reference pan.
Three cycles of heating and subsequent cooling were performed, however, only the
second full cycle is shown. The data was analyzed on Universal Analysis 2000 4.4A
software.
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CHAPTER 4 A THERMALLY SWITCHING HEMIACETAL SYSTEM
Introductory Remarks
Molecular switches have been a hot research topic. They can serve as a building
block and add tunability to larger molecules.126 Systems like diarylethene,18
spiropyran,127 and azobenzene128 have been well studied in constructing
photoresponsive systems. Their applications include photoactuators,129 electronic
devices,130 photoregulated catalysts,131 and photopharmacology.
Figure 4-1. Mandelic acid, salicylic acid, 2-hydroxymandelic acid, 4-1, and 4-2 (2-hydroxymandelic acid lactone).
Mandelic acid was tested and proved to be an effective drug treating urinary
infections.132 Ladenburg and co-workers proposed 2-hydroxymandelic acid (Figure 4-1)
could be more effective than the parent mandelic acid. That is because salicylic acid
could pass the body into urine unchanged.133 They prepared 2-hydroxymandelic acid
(and later turned out it was not the exact compound), 4-hydroxymandelic acid, and a
compound they thought was the lactone of 2-hydroxymandelic acid (2,3-dihydro-3-
hydroxybenzo[b]furan-3-one) 4-2. After their test, these compounds did not have better
performance. However, Howe and co-worker believed the actual structure of the
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compound claimed as 2-hydroxymandelic acid lactone was not correct.134 It should be
2,3-dihydro-2-hydroxybenzo[b]furan-3-one (4-1). Howe also claimed Ladenburg never
made 2-hydroxymandelic acid. They prepared the compound and it was proved to be
not active in treating urinary infections as tested by Howe’s colleague.
While initially investigated as a drug, the switching behavior of 4-1 was later
studied. Sterk and co-workers reported 4-1 would convert to 4-2 upon heating, and
revert by cooling.135 They investigated the phenomenon with variable temperature (VT)
NMR in bromoform-d and IR in bromoform. They tracked the ratio of 4-1 and 4-2 by the
integration of the characteristic peaks. The carbonyl (on carbon 3) of 4-1 has a wave
number (by IR analysis) of 1810 cm-1 and the proton 2 has a chemical shift of 5.80 ppm.
The carbonyl migrates to carbon 2 of 4-2 and the wave number becomes 1720 cm-1.
The proton moved to position 3 and had a chemical shift 5.55 ppm. (Figure 4-2, Table 4-
1) Both changes are caused by the isomerization.
Figure 4-2. The thermal conversion of 4-1 to 4-2 and their characteristic protons and carbonyls. Adapted from Sterk, H.; Kappe, T.; Ziegler, E. Monatshefte für Chemie 1968, 99, 2223–2226.
As shown in the table and as reported in the literature, 4-1 isomerizes to 4-2
upon increasing the temperature. The authors claim the process is controlled by
temperature as the ratios reported. At 100 °C, complete conversion in bromoform is
observed. However, such isomerization is not observed in DMSO.
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Table 4-1. The dependence of the ratio of 4-1 to 4-2 on temperature tracked by IR and NMR. Adapted from Sterk, H.; Kappe, T.; Ziegler, E. Monatshefte für Chemie 1968, 99, 2223–2226.
Temperatue [°C]
IR NMR
4-1 carbonyl 3 1720 K
4-2 carbonyl 2 1810 K
4-1 H2 5.80 ppm
4-2 H3 5.55 ppm
20 100 - 100 0 60 75 25 70 30 90 25 75 20 80 100 5 95 - 100
We are interested in the process. If it works as reported, the molecule could
serve as a tunable module in an extended π-conjugated molecule that responds to an
increase of temperature by reversible isomerization to its isomer. The carbonyl in 4-1
can be taken as an electron acceptor conjugated to the aromatic ring. The compound
isomerizes with the rise of temperature, which is observed as the carbonyl on carbon 3
migrating to the carbon 2 position in 4-2. And the carbonyl on carbon 3 is not
conjugated to the aromatic ring, and that can be taken as the acceptor ‘turned off’. The
central benzene ring serves as a π-spacer. An electron donor could be installed at the
para position of the carbonyl in 4-1, where it is in conjugation with the carbonyl. The
above makes a linear donor-π-acceptor system that could show disparate photophysical
properties for the two states. For example, the methoxy group is a good and simple
electron donor. It can be installed at the para position of the carbonyl of the compound,
making target compound 4-3 and 4-4 (Figure 4-3). 4-3 is a donor-π-acceptor system. 4-
3 isomerizes to 4-4 with an increase of temperature and the acceptor is turned off,
giving the off state. Initial studies would not involve complicated donors, like
oligothiophene, but rather donors commonly seen in discussions of substitutent effects
on benzene rings. The methoxy group was selected to initiate the study.
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Figure 4-3. The designs. A) The linear donor-π-acceptor system. B) The methoxy derivative.
Besides that, we also found naphthalene derivatives 4-5 to 4-8 interesting.
(Figure 4-4A) A recent methodology paper proposed the efficient synthesis of isomers
4-5 and 4-7.136 Careful inspection of the NMR data in the publication revealed a product
carbonyl carbon chemical shift of a ketone for the reaction of 4-9 (so, producing 4-5),
but an ester carbonyl carbon chemical shift of 173.06 ppm for the reaction of 4-10
(Figure 4-4B). The chemical shift suggests 4-8 and not 4-7 was characterized (despite
the authors’ claim). It is possible that 4-7 was generated first and then converted to 4-8
in hot DMSO. We wanted to prepare 4-5 and 4-7 and investigate this phenomenon.
Initial studies started with simple systems, namely synthesis of 4-1 as the parent
compound reported in literature and evaluation of its thermal isomerization behavior. 4-3
as the simplest donor-π-acceptor system would be prepared and studied in parallel.
Finally, we would then entertain 4-5 and 4-7 to learn how the extended conjugation of
and connectivity in the naphthalene systems affected the isomerization properties.
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Figure 4-4. Naphthalene derivatives. A) 4-5 to 4-8 with extended π-conjugation. B) The hemiacetal formation reactions reported in literature with our correction.
Theoretical Calculations
The energy of compounds 4-1 to 4-8 was minimized in the gas phase using
Gaussian 09112 at the B3LYP/6-31G* level. The energy minimized structures were
obtained (Figure 4-5, Appendix B) and the energy differences within each pair of the
hemiacetals and lactones were calculated. (Table 4-2)
All the lactones are lower in energy than the corresponding hemiacetal because
of the stable ester bonds, presumably serving as the driving force for the isomerization
to proceed to the lactone form. By this simple analysis there would be no thermal driving
force for isomerization in the opposite direction, despite literature claims. Through
calculation, addition of a donor group (in 4-3) has a similar effect as extending the π-
conjugation (in 4-5 and 4-7); that is, decreasing the isomer energy difference. Among all
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the compounds, 4-7 and 4-8 have the smallest energy difference. We are interested in
whether the small energy difference contributes to the formation of 4-8 in place of 4-7 in
literature.
Table 4-2. Energy differences of compounds 4-1 to 4-8 within each pair.
Hemiacetal Lactone Energy difference (kcal/mol)
4-1 4-2 -4.71 4-3 4-4 -2.68 4-5 4-6 -2.78 4-7 4-8 -2.15
Figure 4-5. FMOs, corresponding energy levels, and HOMO-LUMO energy gaps of 4-3 and 4-4.
123
Plots of the FMOs of 4-3 and 4-4 are shown in Figure 4-5. The HOMO plots show
that the carbonyl is in conjugation with the benzene ring in 4-3 while the carbonyl is not
interacting with the aromatic ring in 4-4. The reduced HOMO-LUMO gap for 4-3
confirms the donor-π-acceptor design.
Synthesis
The synthesis of 4-3 and attempted synthesis of 4-7 was performed by Bryce
Reeves, an undergraduate student under my supervision.
The synthesis of targets 4-1 and 4-3 was attempted (Scheme 4-1). For 4-3, we
chose to use precursors with the methoxy group pre-installed to keep the procedure
simple. The synthetic method reported in literature,136 which converts o-
hydroxyacetophenone and derivatives to the corresponding hemiacetal, was a fast way
to access the target compounds, so that method was attempted first. The acylation of
phenol (4-11) and m-methoxyphenol (4-14) both gave excellent yields and pure
products. The Fries rearrangement of the acetates gave low to moderate yields of the o-
hydroxyacetophenones 4-13 and 4-16. Conditions using (Lewis) acids other than AlCl3
were tested but none gave better results. The two compounds were then subjected to
the literature oxidative cyclization conditions.136 The desired product was not obtained in
either case. 4-13 did not react and 4-16 suffered electrophilic aromatic iodination.
There are three chemical steps in the published one-pot conversion of 4-13 to 4-
1 (or 4-16 to 4-3) (Scheme 4-2). First is α-iodination of the acetyl group, second is α-
carbon oxidation to the aldehyde, and finally is cyclization (hemiacetal formation). A
stepwise synthetic approach was entertained next.
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Scheme 4-1. Attempted synthesis of 4-1 and 4-3 via one-pot method.
Scheme 4-2. Intermediates and reaction types in the one-pot conversion of 4-13 to 4-1.
Direct α-carbon oxidation was attempted first (Scheme 4-3). The reaction of 4-16
with SeO2 was attempted but it over-oxidized to the carboxylic acid.
Scheme 4-3. SeO2 oxidation of 4-11.
We then retreated to α-halogenation reactions. Several halogenation conditons
were tested (Table 4-2, Scheme 4-4). The iodination that is part of the one-pot reaction
was evaluated first (entry 1) but no reaction took place. Bromination with CuBr2 gave a
125
good yield of mono-brominated product 4-18.137 Iodination with Selectfluor and iodine
gave a complex mixture. We briefly tried converting 4-18 to its corresponding iodide 4-
17 but the reaction gave a low yield. (Table 4-2, Scheme 4-4) The CuBr2 method was
selected.
Table 4-2. Test halogenation reactions of 4-13 and 4-16.
Substrate Reagents Solvent Temperature Time Result
4-13 I2, CuO MeOH 65 °C 2 h no reaction 4-13 CuBr2 CHCl3/EtOAc 77 °C 5 h 55 % 4-16 Selectfluor, I2 MeOH rt 1 day complicated
mixture
Scheme 4-4. Halogenation reactions of 4-13.
With 4-18 in hand, DMSO oxidation was attempted since it was reported in
conjunction with the iodide in the one-pot procedure. Neither attempt afforded the
aldehyde or corresponding hemiacetal. (Scheme 4-5)
Scheme 4-5. DMSO oxidation of 4-18. Conditions: (1) DMSO, 90 °C, overnight; (2) NaHCO3, DMSO, rt, 5 h.
126
The oxidation of 4-18 via its nitrate was tried next. The protocol employs mild
reagents and conditions.138 Despite purification problems, the two-step reaction worked
for both substrates and gave moderate yields of the desired products. (Scheme 4-6)
Scheme 4-6. Oxidation of 4-18 and 4-19 via the corresponding nitrates.
A stepwise version of the oxidation shown in Scheme 4-6 was attempted next.138
(Scheme 4-7) The nitrate 4-20 was isolated to improve the final oxidation and
cyclization step. A cleaner product 4-1 was indeed obtained, but no significant
improvement in overall yield was achieved. In summary, 4-1 and 4-3 were, however,
successfully synthesized via a stepwise oxidation sequence.
Scheme 4-7. Nitration and oxidation of 4-18 in stepwise procedure.
For naphthalene derivatives 4-5 and 4-7, the synthesis of 4-7 was pursued first
using the successful synthesis of 4-1 and 4-3 as a guide. The acylation of 1-naphthol (4-
21) gave its acetate 4-12. 4-22 underwent Fries rearrangement to yield 2-aceto-1-
naphthol 4-23. The bromination of 4-23 did not work well and gave a complicated
mixture based on NMR analysis. (Scheme 4-8) Towards 4-5, intermediate 1-aceto-2-
naphthol 4-25 could be purchased (Scheme 4-9). As in the case of 4-23, the
127
bromination reaction failed and attempted iodination was also not successful. Formation
of a copper complex (with the molecules as ligands) was suspected, so protection of the
hydroxyl group (as its methyl ether) was entertained.
Scheme 4-8. Attempted synthesis of 4-7 from 1-naphthol (4-21).
Scheme 4-9. Attempted halogenation of 4-25.
The methyl protection of 4-25 worked well to yield 4-28 (Scheme 4-10). This time
the bromination led to desired product 4-29. The methyl group was removed with BCl3
successfully to yield 4-30. However, the substitution reaction of 4-30 gave a
128
complicated mixture. (Scheme 4-10) Complexation issues were again potentially to
blame. In a final attempt, deprotection was pursued after intermediate nitrate generation.
The nitrate 4-32 was successfully prepared from 4-29 (Scheme 4-11), but it
decomposed in the deprotection reaction. With this, the methyl protection/deprotection
route hit a dead end.
Scheme 4-10. Attempted synthesis of 4-5 through methy protection.
Scheme 4-11. Modified synthesis with late stage deprotection.
129
Scheme 4-12. Test protection reactions. A) Silyl B) Acetyl.
Given the issues with methyl deprotection, requiring harsh conditions, other
phenol protecting groups that could be removed under more mild conditions were
considered. Silyl protection was considered first as an alternative method given mild
removal using a fluoride source like TBAF. Reaction of 4-25 with TBDPSCl
unfortunately gave poor conversion. (Scheme 4-12A) It is possible that this silyl
protecting group was too sterically bulky, but further silyl group protection was not
pursued. A second choice came in the acetyl group, which can be removed mildly with
sodium perborate. The deprotection reaction was studied first, using 4-22. The
conversion was good but some purification of the product was required. (Scheme 4-12B)
Next the protection and deprotection sequence was attempted using 4-25 (Scheme 4-
13).
The protection reaction offered 4-34 in quantitative yield, while the deprotection
showed evidence of the deprotection product 4-25 (not purified or isolated) by NMR.
The bromination of 4-34 was conducted next (Scheme 4-14), giving a mixture of desired
130
4-35 and its cyclized side product 4-36. The yield was overall low. Further steps may
require the recovery and re-protection of the deprotection product (probably 4-30).
Scheme 4-13. Acetyl protection and deprotection of 4-22.
Scheme 4-14. Bromination of 4-35.
The issues here are that the protecting group needs to survive the harsh
conditions of the bromination reaction yet be removed under mild conditions that do not
decompose the intermediate nitrate ester. A potential solution is to use two different
protecting groups for the bromination and oxidation steps. For example, methyl
protection has been proven successful for the bromination reaction. (Scheme 4-10) 4-30
could then be protected with silyl (TIPS, for example), which would help 4-30 go through
the nitration reaction to give 4-38. Then the silyl group could be removed using TBAF
and the product subsequently cyclized to yield desired target 4-5. (Scheme 4-15) The
proposed synthesis would certainly require testing of the stability of nitrate ester with
fluoride (e.g., TBAF).
131
Scheme 4-15. Proposed synthesis of 4-5.
Characterization
Variable temperature (VT) NMR experiments were conducted to examine the
thermal isomerization behavior of 4-1 and 4-3.
VT NMR experiment was first performed on 4-1 in C2D2Cl4 because bromoform-d
was not available and C2D2Cl4 is also a high boing point solvent. The temperature was
rapidly raised to 60 °C and 90 °C with no obvious change in the proton NMR spectrum.
The same experiment was performed with 4-3 and the same results were obtained. The
thermal isomerization experiment was performed again with 4-1 in bromoform-d. The
temperature was raised to 60 °C, 90 °C, and 100 °C. The peaks shifted slightly but the
structure of the compound did not change.
Next an NMR sample of 4-1 in bromoform-d was prepared and heated at 60 °C
(in a water bath) for an extended period of time. Spectra were recorded before heating
and then after heating for 12 h and then 24 h. Apparent from the spectra was gradual
isomerization of the hemiacetal 4-1 to the lactone 4-2. The transformation could be
tracked by the changes various characteristic peaks. The methine proton has a
132
chemical shift of 5.67 ppm before heating. After heating at 60 °C for 12 h, the intensity
of the peak decreased and the peak shifted moderately to 5.63 ppm. A new peak with a
chemical shift of 5.42 ppm appeared, assigned to the methine proton of the lactone 4-2.
The relative chemical shifts recorded (hemiacetal 5.67 ppm, lactone 5.42 ppm) were
comparable to the ones reported in the original literature (hemiacetal 5.80 ppm, lactone
5.55 ppm). After heating for 24 h, 4-1 mostly isomerized to 4-2 as shown by the
intensities of the characteristic peaks. (Figure 4-6) The results show that the equilibrium
does not lie at 70:30 of 4-1:4-2 as reported in the literature and cannot be controlled by
temperature.
Figure 4-6. Thermal isomerization of 4-1 at 60 °C as tracked by NMR spectra. Spectra taken in bromoform-d. Bottom: before heating; middle: heated for 12 h; top: heated for 24 h.
133
The final sample was cooled to 25 °C and kept for 12 h. Another proton NMR
spectrum was taken and there was no change, naturally proving that 4-2 could not be
reverted to 4-1. Overall, the hemiacetal (the higher energy isomer) isomerized
irreversibly to the lactone (the lower energy isomer). The conclusion contradicts the
conclusions from the original literature but make good thermodynamic sense.
Finally, both acid (4-1 in chloroform-d with TFA added) and base (4-1 in
chloroform-d with triethylamine or DIPEA added) isomerization experiment was also
conducted.
Conclusion and Future Work
In conclusion, we studied the thermal isomerization behavior of a hemiacetal
system. From theoretical calculations, the lactone form is lower in energy than the
hemiacetal form. The isomerization could, in principle, tune the energies of the frontier
molecular orbitals. The hemiacetal compounds 4-1 and 4-3 were prepared and their
isomerization behavior was studied by NMR spectroscopy. We found the isomerization
proceeded over an extended time in solution with heating and the ratio of products was
not temperature controllable; instead the isomerization to the more stable isomer
occurred completely and irreversibly. In future work, we plan to fully characterize the
hemiacetal systems 4-1 and 4-2. Similar experiment will be performed on 4-3 and 4-4.
We will also complete the synthesis of the naphthalene derivatives 4-5 and 4-7. We will
continue to investigate the stability of the compounds and the influence of the extension
of π-conjugation on their properties.
Experimental
Compounds 4-11 through 4-16 are commercially available.
134
2-Bromo-1-(2-hydroxyphenyl)ethan-1-one (4-18).137 To a round bottom flask
fitted with a reflux condenser was added Copper(II) bromide (2.18 g, 9.75 mmol) and
ethyl acetate (5 mL). The suspension was brought to reflux with stirring. 4-13 (0.83 g,
6.09 mmol) was dissolved in chloroform (5 mL) and added to the flask. The resulting
reaction mixture was refluxed with vigorous stirring for 5-6 h and the reaction was
completed as indicated by the disappearance of green color. The reaction mixture was
cooled to room temperature. The white precipitate was removed by filtration and was
washed with chloroform. The filtrates were combined and the solvents were removed in
vacuum. The crude was purified with a Teledyne Isco CombiFlash Rf+ chromatography
system (30%-40% DCM in hexanes) to yield a light yellow oil which solidified as a white
solid (1.05 g, 80%). 1H NMR (500 MHz, CDCl3, δ): 11.73 (s, 1H), 7.75 (d, J = 8.1 Hz,
2H), 7.53 (dd, J1 = 8.5 Hz, J2 = 7.2 Hz, 1H), 7.03 (d, J = 8.5 Hz, 1H), 6.94 (dd, J1 = 8.1
Hz, J2 = 7.2 Hz, 1H), 4.45 (s, 2H); 13C NMR (125 MHz, CDCl3, δ): 197.1, 163.3, 137.6,
130.5, 119.4, 119.1, 117.1, 30.1.
2-(2-Hydroxyphenyl)-2-oxoethyl nitrate (4-20).138 4-18 (1.35 g, 5.5 mmol) was
dissolved in acetonitrile (20 mL) and to this solution silver nitrate (0.936 g, 5.5 mmol)
was added. The flask was covered with aluminum foil to exclude light. After stirring for
135
24 h at room temperature the mixture was filtered, the silver bromide was washed with
ethyl ether, and the combined filtrate and washings were evaporated in vacuum to
remove the solvents. The crude nitrate ester was purified with a Teledyne Isco
CombiFlash Rf+ chromatography system (30%-40% DCM in hexanes) to yield a white
solid (0.48 g, 39%). 1H NMR (500 MHz, CDCl3, δ): 11.39 (s, 1H), 7.60, (d, J = 8.1 Hz,
1H), 7.57 (dd, J1 = 8.6 Hz, J2 = 7.4 Hz, 1H), 7.06 (d, J = 8.4 Hz, 1H), 6.97 (dd, J1 = 8.2
Hz, J2 = 7.4 Hz, 1H), 5.64 (s, 2H).
2-Hydroxybenzofuran-3(2H)-one (4-1).138 The crude nitrate ester 4-20 (0.48 g,
2.43 mol) was dissolved in DMSO (5 mL) and to the stirred solution ground sodium
acetate trihydrate (0.033 g, 0.243 mmol) was added. After 1 h at room temperature the
reaction mixture was poured into water, and was extracted with ethyl acetate twice. The
combined organic solution was washed with water and brine and dried over anhydrous
sodium sulfate. The drying agent was filtered and the solvent was removed in vacuum.
The crude was purified with silica column chromatography (20%-50% ethyl acetate in
hexanes) to give 4-1 (0.28 g, 76%) as a white solid. 1H NMR (500 MHz, DMSO-d6, δ):
8.04 (br, 1H), 7.73 (t, J = 7.8 Hz, 1H), 7.61 (d, J = 7.6 Hz, 1H), 7.17 (d, J = 8.4 Hz, 1H),
7.12 (t, J = 7.4 Hz, 1H), 5.59 (s, 1H); 13C NMR (125 MHz, DMSO-d6, δ): 198.4, 170.6,
139.0, 124.2, 121.9, 119.4, 113.3, 97.7. NMR spectra taken in chloroform-d can be
found in the literature.136
136
2-Bromo-1-(2-hydroxy-4-methoxyphenyl)ethan-1-one (4-19). 4-19 was
prepared in the same method as 4-18. 1H NMR (500 MHz, CDCl3, δ): 12.21 (s, 2H),
7.63 (d, J = 9.0 Hz, 2H), 6.47 (d, J = 9.0 Hz, 2H), 6.44 (s, 1H), 4.35 (s, 2H), 3.85 (s, 3H);
13C NMR (125 MHz, CDCl3, δ): 195.2, 167.1, 166.4, 132.1, 111.3, 108.5, 101.3, 55.8,
29.8.
2-Hydroxy-6-methoxybenzofuran-3(2H)-one (4-3). 4-3 was synthesized using
the same method as that of 4-20 and 4-1 combined from 4-19. The nitrate intermediate
was not purified with chromatography and the crude was used in the second step. 1H
NMR (500 MHz, CDCl3, δ): 8.02 (d, J = 9.3 Hz, 1H), 7.51 (d, J = 8.6 Hz, 1H), 6.71 (s,
1H), 6.67 (d, J = 8.6 Hz, 2H), 5.56 (d, J = 9.3 Hz, 1H), 3.87 (s, 2H).
137
CHAPTER 5 [3.3]PARACYCLOPHANE SUPRAMOLECULAR POLYMER
Introductory Remarks
[3.3]Paracyclophane ([3.3]pCp) (Figure 5-1a) has drawn interest in research for
over six decades.36 Its strained structure consists of “bent and battered benzene rings”
and bent bridges.32 That allows for through-space (transannular π-π) and through-bond
[σ(bridge)-π(deck)] interactions,139 and thus [3.3]pCp has unique chemical, optical, and
electronic properties. Its through-space interaction (Figure 5-1B) has been well studied
in parallel with [2.2]pCp.140,141 [3.3]pCp is a good π-electron donor. Its complexation
with tetracyanoethylene (TCNE) has been well investigated.142–145 [3.3]pCp can form
metal complexes as a result of its electron donating ability.146–148 [3.3]pCp’s host-guest
interaction was also studied.149 [3.3]pCp and its functionalized substituted derivatives
are hard to synthesize, which limits their study.150
Figure 5-1. Various [3.3]pCp structures. A) [3.3]pCp, B) donor-[3.3]pCp-acceptor for the study of through-space interaction,140 and C) four-layered [3.3]pCp.151
Extended π-stacked cyclophanes could have delocalized electronic states.
Shinmyozu and co-workers constructed three- and four layered [3.3]pCp (Figure 5-1C)
138
and showed absorption emission maxima at longer wavelength upon increasing the
number of layers.151 Their charge delocalization was investigated with transient
absorption spectroscopy.152 Electron acceptors could be incorporated to the framework
to give a three-layered donor-donor’-acceptor system and its charge transfer properties
were investigated.153 Higher or functionalized systems are relatively difficult to
construct.154 We are interested in building the robust self-assembly of [3.3]pCp through
non-covalent interactions. We hope to generate predictable and even chiral one-
dimensional (1D) supramolecular ordering in solution and the solid state. [3.3]pCp has
only been studied in metal complexes in this reagard.146–148
Our design of pCp self-assembly borrows the concept of benzene-1,3,5-
tricarboxamides (BTAs). BTAs form 1D assemblies through threefold hydrogen-bonding
between the tilted amides of adjacent π-stacked monomers. (Figure 5-2A) The BTA
assembly has found numerous applications across the materials and biomedical
sciences.31 In our previous work, we combined the hydrogen bonding pattern and the
scaffold of [2.2]pCp to design [2.2]pCp-4,7,12,15-tetracarboxamide (pCpTA) (Figure 5-
2B).33 The pseudo-ortho substitution pattern and the [2.2]pCp bridged structure make
the amides rotate out of the plane of the aromatic rings and the two amide groups on
the same side are close to each other. As a result, intramolecular N-H···O=C hydrogen
bonds between pseudo-ortho-disposed amides are formed. The amides are ‘locked’ in
the conformation and the other side of the amide groups are pointed away from the
center of the monomer, making the monomers pre-organized for intermolecular H-
bonding with two π-stacked neighbors. The 1D arrangements are homochiral, with each
column comprising monomers sharing the same planar-chiral configuration, and
139
helically laced-up by two strands of anti-parallel hydrogen bonds. The arrangement and
persistence has been characterized.
Figure 5-2. The self-assembly of A) BTA, B) pCpTA, and C) [3.3]pCpTA.
We extended the design to [3.3]pCp and came up with [3.3]pCp-5,8,14,17-
tetracarboxamide ([3.3]pCpTA). (Figure 5-2C) [3.3]pCp is very similar to [2.2]pCp in size,
including the distance between the two decks.37 Together with the identical substitution
pattern, we expect the same type of intramolecular and intermolecular H-bonding would
form. Yet there are slight differences between the two platforms. [3.3]pCp is less
strained and has a less distorted phenylene decks.37 [3.3]pCp is also a better electron
donor.140,141 We would like to know whether the flatter, strain-relieved rings of [3.3]pCp
could better accommodate this assembly motif.
Molecular Modeling
Gas-phase calculations (DFT M06-2X/6-31+G*)40,155 were performed. The
modeling will start from [3.3]paracyclophane, the hydrocarbon backbone of [3.3]pCpTA,
and extend to the monoamide (Figure 5-3A), pseudo-para diamide, pseudo-ortho
diamide (which is capable of transannular H-bonding), tetraamide, and tetraamide
assembly.
140
[3.3]pCp boasts two conformations: the chair and the boat. The boat
conformation is calculated to be slightly lower in energy39 but only the chair
conformation exists in the solid state.37 The two conformations interconvert very rapidly
in solution at room temperature. Both conformations were taken into consideration for
our modeling studies. The benzyl hydrogens are in either axial or equatorial positions.39
The equatorial protons are close to the plane of the phenylenes and they can interact
with substitution groups such as amides. (Figure 5-3B)
Methyl groups were used as the R group in the amides for simplicity of
calculation. The monoamide of [3.3]pCp was modeled first. There are four different
types of pCp backbone conformations. We call one “syn”, when the middle methylene of
the closer bridge bends towards the amide group. The “syn” has both chair and boat
conformations. Such is the same for the “anti”, which also has both chair and boat
conformations. Conformational searches on the four sets of backbones were performed.
Twelve low-energy conformations were found, eight with the “syn” conformation (four as
the chair and four as the boat) and four conformers with the “anti” conformation (two as
chairs and two as boats). We found the interaction of the equatorial proton of the closest
benzyl position of the amide with the N-methyl group of the same amide group is
unfavorable when the bridge methylene is “anti” to the amide group. (Figure 5-3C) The
amide group might require the methylene to adopt the “syn” conformation in order to
freely rotate. The conformers are close in gas phase energy (within a range of 1.62
kcal/mol). The torsion angles (defined by the dihedral angel of the C=O and the aryl
planes), Φ1 and Φ2, are similar among the conformers (Φ1≈ 44°~48°, Φ2≈ -137°~-141°).
From the comparison of the conformers with similar torsion angles, the “syn” conformers
141
are lower in energy than the “anti” conformers. In another way, the “syn” interaction is
relatively favorable and the “anti” interaction is unfavorable. However, these energy
differences are relatively small and the sets of conformers can interconvert rapidly.39
The results are summarized in Table C-1 in appendix.
Figure 5-3. The structures. A)monoamide. B) The interaction of the equatorial benzylic proton with the phenyl proton. C) the amide substituent.
Then the pseudo-ortho diamides (four conformers with different chair/boat
conformations) were minimized. The four conformers’ energies are within a range of
4.32 kcal/mol. The reason for the larger energy differences is the favorable or
unfavorable interaction of the benzylic proton with the amide group is doubled in the
extreme cases. The H-bonding may also “lock” the two amide groups in a less
comfortable conformation. The torsion angles (Φ1 ≈ 46°~50°, Φ2 ≈ -137°~-141°) are
similar. Intramolecular H-bonding is formed in all cases as supported by good linearity
and optimized distances (N···C=O ≈ 2.9 Å). (Figure C-1, Table C-2)
Base on the results of the monoamides and the pseudo-ortho diamides, the
pseudo-para diamides were also modeled. They serve as the non-hydrogen bonding
comparators of the pseudo-ortho diamides. The pseudo-para diamides’ energies are
within a range of 1.82 kcal/mol. They are compared with their corresponding pseudo-
ortho diamides and the energy difference in the four pairs ranges from 8 kcal/mol to 11
kcal/mol. The results are similar to that of the [2.2]pCp diamides and shows the energy
142
of intramolecular H-bonding more than compensates for the unfavorable N-methyl and
equatorial hydrogen interaction (1.6 kcal/mol each). The torsion angles (Φ1 ≈ -52°~-55°,
Φ2 ≈ -139°~-142°) are similar to the pseudo-ortho diamides. (Figure C-2, Table C-2)
Synthesis
[3.3]pCpTA (5-1) was synthesized from [3.3]pCp (5-2). The synthesis is shown in
Scheme 5-1.
Scheme 5-1. Synthesis of [3.3]pCpTA (5-1).
To prepare [3.3]pCp, the toluenesulfonylmethyl isocyanide (TosMIC) pathway
was selected because the conditions are mild. Initial attempts to couple 5-3 and TosMIC
in 1:1 ratio to prepare 5-5 directly failed to give the desired product.156 (Scheme 5-2A)
143
The stepwise procedure gave good results.157 The hydrolysis removes the tosyl and the
isocyanate to yield the diketone 5-5.158 (Scheme 5-1) The coupling reaction was also
performed in a DCM/water heterogeneous system,150 (Figure 5-2B) conditions similar to
the synthesis of 5-4. (Figure 5-1) Although a better yield was obtained, the condition
required 5-4, a polar and barely soluble compound, to fully dissolve in DCM. That
limited the reaction from being scaled up and the condition was abandoned.
Scheme 5-2. Attempted reactions. A) one-step condensation of 5-3 and TosMIC. B) coupling of 5-4 and 5-3 in DCM/water system.
Then 5-5 was reduced in a Wolff-Kishner-Huang reduction to yield [3.3]pCp (5-
6).151 At first hydrazine dihydrochloride was not added and potassium hydroxide was
added at the beginning.158 This approach gave low yield. Other related reduction
methods were tried, too. None gave satisfying results. (Table C-3) On the other hand,
the addition of hydrazine dihydrochloride helped increase the yield. It is believed the
proton promoted the formation of the hydrazone. Final modification was to add KOH
144
later to allow longer reaction time for hydrazone formation. The modified procedure
gave moderate to good yield of [3.3]pCp.
The first attempt of bromination of [3.3]pCp followed the procedure of
bromination of [2.2]pCp (8 days in neat Br2).33 The reaction gave benzylic bromination in
addition to the desired aromatic substitution. Because the bromide on the benzylic
position cannot be easily hydrogenated, that over-brominated product could not be
converted to the desired product 5-6 (shorter reaction times were ultimately attempted,
vide infra). Concurrently, a route wherein the bromides were pre-installed on the
aromatic ring was considered. All isomers of dibromo-[3.3]pCp were synthesized with
that method.150 The earlier steps worked well but the cyclization step did not work
despite different conditions being tried. (Scheme 5-3)
Scheme 5-3. Attempted synthesis of 5-6 with pre-installation of bromine.
145
Direct bromination was revisited. The bromination was performed at 0°C with
slow addition of bromine in a dry air atmosphere. After 6 h, TLC was taken and it was
found the reaction was completed. 5-6 was isolated and its structure was characterized.
The better reactivity of [3.3]pCp in bromination (reaction time is 6 h) than that of
[2.2]pCp (reaction time is 8 days) can be explained by the easiness of forming the π-
complex.154 The halogen-lithium exchange reaction followed by quenching with CO2
yielded the tetraacid 5-7. 5-7 was converted to its tetraacid chloride and condensed with
hexylamine to give [3.3]pCpTA (5-1).33 (Scheme 5-1) The reaction still requires
optimization and a better yield is expected.
Figure 5-4. 1H NMR spectrum of [3.3]pCpTA (5-1) in chloroform-d.
146
The 1H NMR spectrum of [3.3]pCpTA (5-1) is shown in Figure 5-4. The amide
proton has a chemical shift in CDCl3 of 7.68 ppm. Compared with our previous work on
[2.2]pCpTA, the chemical shift of the amide peak is similar. That is evidence that
hydrogen bonding is formed.33 Further investigation on the self-assembly behavior will
be conducted.
Conclusion and Future Work
In conclusion, we have designed and synthesized [3.3]pCp-5,8,14,17-
tetracarboxamide ([3.3]pCpTA). The molecule is desigen form intra- and intermolecular
hydrogen bonds and a 1D stacked assembly.
In future work, more [3.3]pCpTA will be synthesized. Its self-assembly behavior
will be characterized (by variable concentration and variable temperature NMR, IR, and
UV-Vis spectroscopy). We will also prepare the propyl version of [3.3]pCpTA and study
its aggregation in the solid state by single crystal X-ray diffraction.
Experimental
Computations
The backbone of [3.3]pCp was adapted from the literature. The amide groups
were installed on the backbone using Maestro 11.1, a Schrödinger software.159
The conformational search of [3.3]pCp monoamide was performed using Maestro
11.1. All carbon and hydrogen atoms were fixed. The amide group was rotated around
the C(aryl)-C(carbonyl) bond. The energy was calculated every 1° and plotted. Either
four or two minima were found. Each of local minimum was geometry optimized using
Gaussian 09 at the M06-2X/6-31+G* level. The torsion angles and energies were
recorded.
147
The amide groups of the pseudo-ortho damides were manually positioned in the
same plane to form hydrogen bonding. The compounds were geometry-optimized using
Gaussian 09 at the M06-2X/6-31+G* level. The corresponding pseudo-para diamides’
torsion angles were set based on the torsion angles of the monoamides. They were
optimized using Gaussian 09 at the M06-2X/6-31+G* level.
Synthesis
(±)-5,8,14,17-Tetrabromo[3.3]paracyclophane ((±)-5-6) Bromine (5 mL) was
slowly added over 3 h via addition funnel to a mixture of [3.3]paracyclophane (61 mg,
0.13 mmol) and iodine (25 mg, 1 crystal) in a round bottom flask covered with aluminum
foil to exclude light in an ice-water bath. Emitted hydrogen bromide gas was trapped in
cold saturated NaHCO3 solution. The reaction mixture was stirred at rt under dry air.
TLC was taken to monitor the progress. After 6 h, the reaction was completed. Residual
bromine was evaporated with agitation from the reaction mixture to leave a red powder.
The powder was dissolved in dichloromethane and washed with 10% sodium thiosulfate
solution and brine. It was dried over Na2SO4 and solvent was removed in vacuum. The
crude product was purified by repeated silica gel column chromatography (100%
hexanes) to yield a white solid (73 mg, 0.26 mmol, 51%). 1H NMR (500 MHz, CDCl3, δ):
7.30 (s, 4H), 2.99 (dt, J1 = 14.4 Hz, J2 = 6.0 Hz, 4H), 2.58 (dt, J1 = 14.4 Hz, J2 = 6.4 Hz,
148
4H), 2.19 (p, J = 6.1 Hz, 4H); 13C NMR (125 MHz, CDCl3, δ): 139.4, 133.5, 124.4, 34.6,
23.9. HRMS (DART) m/z : [M]+ calcd. for C18H16Br4: 551.7940, found 551.7946.
(±)-5,8,14,17-Tetracarboxy[3.3]paracyclophane ((±)-5-7) A 1.9 M solution of t-
butyllithium in hexanes (0.406 mL, 0.77 mmol) was added slowly to a solution of (±)-5-6
(42.6 mg, 0.077 mmol) in THF (3 mL) at -78 °C. The mixture was stirred at -78 °C for 20
min. CO2 (16 g) was bubbled through the solution via a long needle with stirring until all
the gas was consumed as the reaction mixture warmed to room temperature. The
reaction mixture was dissolved into 10% NaHCO3 and then acidified with concentrated
HCl. The white precipitate was collected through filtration and the filtrate was extracted
into ethyl acetate. The organic layer was separated and dried with Na2SO4. The Na2SO4
was filtered, and the solvent was removed under reduced pressure. The resulting solid
was combined with the precipitate as product (29.3 mg, 0.07 mmol). 1H NMR (500 MHz,
DMSO-d6, δ): 12.88 (br, 4H), 7.37 (s, 4H), 3.59 (m, 4H), 2.54 (dt, J1 = 14.1 Hz, J2 = 6.0
Hz, 4H), 2.06 (p, J = 6.1 Hz, 4H).
149
(±)-5,8,14,17-Tetra(n-hexyl)amide[3.3]paracyclophane ((±)-5-1) Oxalyl chloride
(0.048 mL, 0.56 mmol) was added to a solution of (±)-5-7 (0.046 g, 0.11 mmol) in DCM
(5 mL). A catalytic amount of DMF was added and the mixture was stirred at rt for 2 h.
n-Hexylamine (0.15 mL, 1.12 mmol) and N,N-diisopropylethylamine (DIPEA) (0.2 mL,
1.12 mmol) were added dropwise at 0 °C and the reaction mixture was allowed to warm
to rt over 2 h. The reaction mixture was diluted with DCM, and washed with 2 N HCl (2
×), H2O, and brine. The organic layer was separated, dried with Na2SO4, filtered, and
the solvent was removed under reduced pressure. The product was further purified by
silica gel chromatography (40–70% ethyl acetate in hexanes) and was obtained as a
solid (4.6 mg, 5%). 1H NMR (500 MHz, CDCl3, δ): 7.68 (br, 4H), 7.01 (s, 4H), 3.45 (m,
8H), 3.11 (br, 4H), 2.55 (br, 4H), 1.94 (p, J = 5.5 Hz, 4H), 1.68 (m, 4H), 1.44 (m, 4H),
1.38 (m, 8H), 0.92 (t, J = 7.0 Hz, 4H).
150
CHAPTER 6 CONCLUSIONS
This work in the dissertation has explored different types of novel π-conjugated
architectures and their applications in supramolecular chemistry and in materials.
The Influence of Solubilizing Chain Stereochemistry on Small Molecule Photovoltaics
The first project evaluated the consequences of 2-ethylhexyl solubilizing group
chirality, and the isomeric complexity it creates, on small π-conjugated molecule
morphology and bulk heterojunction photovoltaic device performance. To this end,
SMDPPEH, a commercially-available organic semiconductor, was prepared in
isomerically defined form from enantiomerically pure reagents. The optical, thermal,
morphological, and photovoltaic properties of the three pure isomers (2-1RR, 2-1SS,
and 2-1RS) were systematically compared to isomer mixtures purchased commercially
(2-1com) or prepared in the laboratory (2-1syn).
All synthesized isomers had high isomeric purity. The isomeric composition did
not have any effect on photophysical properties in dilute solution. But the side chain
chirality influenced the thermal properties of the materials. Both enantiomers (2-1RR,
and 2-1SS) showed overall similar thin film morphology and absorption (neat and as
blends; annealed or unannealed), and consequently bulk heterojunction device
performance throughout the studies. 2-1syn and 2-1com showed similar morphologies
and optoelectronic characteristics. All SMDPPEH compositions were amorphous as 1:1
blends with PCBM without thermal annealing. 2-1RS was found to crystallize most
readily of the pure isomers, in the presence and absence of PCBM, and consequently
dominated the absorption and XRD profiles of the neat isomeric mixtures, resulting in a
151
50–60% improvement in photovoltaic device performance relative to the other four
compositions.
After thermal annealing, 2-1RR/2-1SS, 2-1syn/2-1com, and 2-1RS revealed
different crystal structures and morphologies. Blending with PCBM had a strong effect
on the crystal packing, where 2-1RS crystallized most readily. For 2-1syn, 2-1com, 2-
1SS, and 2-1RR, a substantial increase in photovoltaic device performance was
observed after thermal annealing, while 2-1RS showed a decrease in device
performance. Overall, the stereocenter affected the morphology of the active layer and
the thin film absorption spectra, but had a relatively weak effect on the overall
photovoltaic performance.
The extent to which alkyl side chain stereochemistry should be considered an
important tunable parameter in materials design is most certainly device/application
dependent. Our work shows that the side chain chirality, while strongly impacting the
thin film morphology and optical properties of SMDPPEH, has modest effects on the
photovoltaic performance of the material as blends with PCBM. This means that side
chain stereochemistry does indeed influence blend structure. It also suggests that the
structural consequences lead to fortuitously compensatory optoelectronic effects in this
context. Studies among additional classes of semiconductors will expose whether side
chain chirality can be employed to rationally improve OPV efficiency or whether it can
be safely “ignored”.
Keto-enol Type Tautomerically Active Modules Containing Benzodifuran for Pi-Conjugated Materials
Keto-enol type tautomerically active modules based on the benzodifuran
framework have been designed and synthesized. Specifically, two tricyclic fused ring
152
compounds based on coumaran-3-one with amide substituents in the 2-position were
created as probes to study the interplay between tautomerism and π-delocalization in
polycyclic aromatic ring systems. Theoretical calculation and modeling in the gas phase
showed the intramolecular hydrogen-bonding stabilized tautomeric forms have small
energy differences and are potentially accessible. Benzodifuran based model
compound 3-1 was successfully prepared. It displays a red-shifted UV-Vis absorption
compared with parent benzodifuran and good thermal stability. However, the low
stability of 3-1 in DMSO solution limited further study of its tautomerization behavior.
The synthesis of benzodithiophene base model compound 3-2 has yet to be successful.
A Thermally Switching Hemiacetal System
The thermally controllable isomerization behavior of a hemiacetal system (2-
hydroxybenzofuran-3(2H)-one) was investigated. A simple donor-π-acceptor system
that could show disparate photophysical properties for the two isomeric states was
designed. From theoretical calculations, the lactone form is lower in energy than the
hemiacetal form. The isomerization could, in principle, tune the energies of the frontier
molecular orbitals. The hemiacetal compounds 4-1 and 4-3 were prepared and their
isomerization behavior was studied by NMR spectroscopy. The isomerization was found
to proceed over an extended time in solution with heating and the ratio of products was
not temperature controllable; instead, the isomerization to the more stable isomer
occurred completely and irreversibly. The system was extended to naphthalene
derivatives to investigate the consequences of structural isomerism.
[3.3]Paracyclophane Supramolecular Polymer
Following our previous work on [2.2]paracyclophane-4,7,12,15-tetracarboxamide
(pCpTA), we have designed [3.3]pCp-5,8,14,17-tetracarboxamide ([3.3]pCpTA). The
153
molecule is designed to form 1-D homochiral rods through intra- and intermolecular
hydrogen bonding. [3.3]pCpTA was successfully synthesized for the first time. The self-
assembly behavior is currently being investigated.
154
APPENDIX A NMR SPECTRA OF CHAPTER 3
Figure A-1. 1H NMR spectrum (CDCl3, 500 MHz) (top) and 13C NMR spectrum (CDCl3, 125 MHz) (bottom) of 3-4.
155
Figure A-2. 1H NMR spectrum (DMSO-d6, 500 MHz) (top) and 13C NMR spectrum
(DMSO-d6, 125 MHz) (bottom) of 3-5.
156
Figure A-3. 1H NMR spectrum (CDCl3, 500 MHz) (top) and 13C NMR spectrum (CDCl3,
125 MHz) (bottom) of 3-6.
157
Figure A-4. 1H NMR spectrum (DMSO-d6, 500 MHz) (top) and 13C NMR spectrum
(DMSO-d6, 125 MHz) (bottom) of 3-7.
158
Figure A-5. 1H NMR spectrum (CDCl3, 500 MHz) (top) and 13C NMR spectrum (CDCl3,
125 MHz) (bottom) of 3-8.
159
Figure A-6. 1H NMR spectrum (DMSO-d6, 500 MHz) of 3-7.
160
Figure A-7. 1H NMR spectrum (CDCl3, 500 MHz) (top) and 13C NMR spectrum (CDCl3,
125 MHz) (bottom) of 3-22.
161
Figure A-8. 1H NMR spectrum (CDCl3, 500 MHz) (top) and 13C NMR spectrum (CDCl3,
125 MHz) (bottom) of 3-23.
162
Figure A-9. 1H NMR spectrum (CD3OD, 500 MHz) of 3-21.
163
APPENDIX B GRAPHS AND NMR SPECTRA OF CHAPTER 4
Figure B-1. Energy minimized structure of 4-1.
Figure B-2. Energy minimized structure of 4-2.
164
Figure B-3. Energy minimized structure of 4-5.
Figure B-4. Energy minimized structure of 4-6.
165
Figure B-5. Energy minimized structure of 4-7.
Figure B-6. Energy minimized structure of 4-7.
166
Figure B-7. 1H NMR spectrum (CDCl3, 500 MHz) (top) and 13C NMR spectrum (CDCl3, 125 MHz) (bottom) of 4-18.
167
Figure B-8. 1H NMR spectrum (CDCl3, 500 MHz) of 4-19.
168
Figure B-9. 1H NMR spectrum (DMSO-d6, 500 MHz) (top) and 13C NMR spectrum (DMSO-d6, 125 MHz) (bottom) of 4-1.
169
Figure B-10. 1H NMR spectrum (CDCl3, 500 MHz) (top) and 13C NMR spectrum (CDCl3, 125 MHz) (bottom) of 4-19.
170
Figure B-11. 1H NMR spectrum (DMSO-d6, 500 MHz) of 4-3.
171
APPENDIX C MISCELLANEOUS INFORMATION OF CHAPTER 5
Table C-1. Summary of calculation result of monoamides.
Backbone conformation
Relative position of the amide and the close methylene
Number Name Dihedral angel(°)
Energy (kcal/mol)
cis anti 1 monocis1-1 45.8 1.16
2 monocis1-2 129.8 0.18
3 monocis1-3 -140.3 1.62
4 monocis1-4 -43.0 4.41
syn 1 monocis2-1 44.4 0.70
2 monocis2-2 -138.9 0
trans anti 1 monotrans1-1 48.2 1.39
2 monotrans1-2 130.8 0.02
3 monotrans1-3 -141.0 1.44
4 monotrans1-4 -43.5 4.40
syn 1 monotrans2-1 44.6 0.62
2 monotrans2-2 -137.5 0.30
The conformers with 4+ kcal/mol energy difference are not counted because their
conformations are not applicable to diamides.
Table C-2. Summary of calculation result of diamides.
Substitution pattern
Backbone conformation
Number Name Dihedral angle 1 (°)
Dihedral angle 2 (°)
Energy (kcal/mol)
Pseudo-ortho
boat 1 ortho1 -142.6 47.6 4.32
chair 2 ortho2 -140.5 50.6 3.14
chair 3 ortho3 -138.9 45.8 3.82
boat 4 ortho4 -138.2 47.1 0
Pseudo-para
boat 1 para1 -140.3 44.5 12.58
chair 2 para2 -137.5 44.6 11.19
chair 3 para3 -141.0 48.2 13.00
boat 4 para4 -138.9 45.8 11.18
172
ortho1
ortho2
173
ortho3
ortho4
Figure C-1. Energy minimized structures of pseudo-ortho diamides.
174
para1
para2
175
para3
para4
Figure C-2. Energy minimized structures of pseudo-para diamides.
176
Figure C-3. Energy minimized structure of [3.3]pCpTA (R = methyl).
177
Figure C-4. 1H NMR spectrum (CDCl3, 500 MHz) (top) and 13C NMR spectrum (CDCl3, 125 MHz) (bottom) of (±)-5-6.
178
Figure C-5. 1H NMR spectrum (DMSO-d6, 500 MHz) of (±)-5-7.
Figure C-6. 1H NMR spectrum (CDCl3, 500 MHz) of (±)-5-1.
179
Table C-3. Conditions used in Wolff-Kishner type reactions.
Hydrazine Base/Reducing reagent
Other reagents
Solvent Temperature Time Results
H2NNHTs NaBH3CN p-TsOH DMF, Sulfoane 110 °C 2 h No raction
H2NNHTs LiAlH4 THF reflux overnight Some product but contaminated by grease
H2NNHTs KOH Diethyleneglycol 200 °C 2 h No reaction
(TBSHN)2 t-BuOK Sc(OTf)3 (cat.)
t-BuOH, DMSO rt overnight 26 %
180
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BIOGRAPHICAL SKETCH
Yu Zhu, known as Bill in the United States, was born and raised in the city of
Shenyang, a populated city known for its heavy industry in northeastern China. He
attended Nankai University in Tianjin majoring chemistry and received B.S. in 2011. He
then moved to the United States to pursue Ph.D. in organic chemistry under the
guidance of Prof. Ronald K. Castellano at the University of Florida in the fall of 2011.
There he became a fan of college football. He received his Ph.D. from the University of
Florida in the summer of 2017.