Massimo Baroncini, Serena Silvi, Alberto Credi, Margherita Venturi e-mail: massimo.baroncini@unibo.it

Dipartimento di Chimica “G. Ciamician”, via Selmi 2, 40126 Bologna, Italy

Photochemical Nanosciences Laboratory Bologna

Ø Molecular Logic Ø CdSe Quantum Dots Ø Molecular Machines and Devices

Ø Dendrimers Ø Si Quantum Dots Ø Water Splitting

Photochemical Nanosciences Laboratory Bologna

Supervisors…

Ø Molecular Logic Ø CdSe Quantum Dots Ø Molecular Machines and Devices

Ø Dendrimers Ø Si Quantum Dots Ø Water Splitting

Photochemical Nanosciences Laboratory Bologna

Supervisors…

•  IMM: Vittorio Morandi, Luca Ortolani (QDs) •  ISOF: Giovanna Barbarella, Francesca Di Maria (Supramolecular Systems)

Rotaxane

Rotaxanes:  “Molecules   in  which  a  ring  encloses  another,  rod-­‐like  molecule   having   end   groups   too   large   to   pass   through   the   ring  opening,  and  thus  holds   the  rod-­‐like  molecule   in  posi;on  without  covalent  bonding.”    IUPAC.  Compendium  of  Chemical  Terminology,  2nd  ed.  (the  "Gold  Book").  

Rotaxane

Rotaxanes:  “Molecules   in  which  a  ring  encloses  another,  rod-­‐like  molecule   having   end   groups   too   large   to   pass   through   the   ring  opening,  and  thus  holds   the  rod-­‐like  molecule   in  posi;on  without  covalent  bonding.”    IUPAC.  Compendium  of  Chemical  Terminology,  2nd  ed.  (the  "Gold  Book").  

Pseudorotaxanes:   Rotaxane-­‐like  molecular   assembly   in  which   the   threading  component(s)  has(have)  ends  small  enough  to  permit  threading  or  dethreading  of  the  macrocyclic  molecule(s).    Andrey  Yerin,  Edward  S.  Wilks,  Gerad  P.  Moss  and  Akira  Harada  Nomenclature  for  rotaxanes  and  pseudorotaxanes  (IUPAC  Recommenda;ons  2008)  

Pseudorotaxane

Rotaxane

Rotaxanes:  “Molecules   in  which  a  ring  encloses  another,  rod-­‐like  molecule   having   end   groups   too   large   to   pass   through   the   ring  opening,  and  thus  holds   the  rod-­‐like  molecule   in  posi;on  without  covalent  bonding.”    IUPAC.  Compendium  of  Chemical  Terminology,  2nd  ed.  (the  "Gold  Book").  

Pseudorotaxanes:   Rotaxane-­‐like  molecular   assembly   in  which   the   threading  component(s)  has(have)  ends  small  enough  to  permit  threading  or  dethreading  of  the  macrocyclic  molecule(s).    Andrey  Yerin,  Edward  S.  Wilks,  Gerad  P.  Moss  and  Akira  Harada  Nomenclature  for  rotaxanes  and  pseudorotaxanes  (IUPAC  Recommenda;ons  2008)  

Pseudorotaxane

O

O O

O

O

OO

O

+

DBA   DB24C8  

NH2

O

O

O

O

DB24C8  ·∙  DBA-­‐1H+  

NH2

The Beginnings…

The Axle (E,E-1H·PF6) Made of two terminal azobenzene units and a central ammonium station, which is a recognition site for DB24C8.

NH2

N NN N

Me MePF6

The Beginnings…

The Axle (E,E-1H·PF6) Made of two terminal azobenzene units and a central ammonium station, which is a recognition site for DB24C8.

NH2

N NN N

Me MePF6

The Ring (DB24C8) Dibenzo[24]crown-8

O O

O

O

OO

O

O

The Pseudorotaxane 1H NMR (400 MHz, CD3CN, 304 K) 5mM in both components.

NH2

N NN N

Me

O

O

O

O

MePF6

The Beginnings…

The Axle (E,E-1H·PF6) Made of two terminal azobenzene units and a central ammonium station, which is a recognition site for DB24C8.

NH2

N NN N

Me MePF6

The Ring (DB24C8) Dibenzo[24]crown-8

O O

O

O

OO

O

O

Photochemistry

NH2

N NN N

HM

H1H2

H3H4 HN

PF6

NH2

N NN N

HM

H1

H2

H3H4

HN PF6hν

hν’ or Δ

Photochemistry

NH2

N NN N

HM

H1H2

H3H4 HN

PF6

NH2

N NN N

HM

H1

H2

H3H4

HN PF6

δ

HM  HN  H3   H2   H4   H1  

HM  HN  H3   H2   H4   H1  

400 MHz, CD3CN, 304 K

The azobenzene units of the axle-molecule can be almost quantitatively photoisomerized from trans to cis by means of UV-light irradiation. The complete cis > trans thermal conversion takes place in ca. 4 weeks at 20°C.

hν

hν’ or Δ

Slow down

+  

O O

O

O

OO

O

O

NH2

N NN N

Me Me

PF6

Slow down

+  

Slow down

+  

400 MHz, CD3CN, 304 K

k(in)ZZ 2.9 x 10-3 M-1s-1

k(out)ZZ 7.2 x 10-5 s-1

Slow down

+  

Time  (h)  

Concen

tra;

on  (m

M)  

400 MHz, CD3CN, 304 K

k(in)ZZ 2.9 x 10-3 M-1s-1

k(out)ZZ 7.2 x 10-5 s-1

Time  (h)  

Pseudorotaxane and Rotaxane

hν

hν’or Δ

KEE 820 M-1

k(in)EE 37 M-1s-1

k(out)EE 4.5 x 10-2 s-1

Time  (h)  

Pseudorotaxane and Rotaxane

hν

hν’or Δ

KEE 820 M-1

k(in)EE 37 M-1s-1

k(out)EE 4.5 x 10-2 s-1

Time  (h)  

Pseudorotaxane and Rotaxane

KZZ 400 M-1

k(in)ZZ 2.9 x 10-3 M-1s-1

k(out)ZZ 7.2 x 10-5 s-1

hν

hν’or Δ

hν

hν’or Δ

KEE 820 M-1

k(in)EE 37 M-1s-1

k(out)EE 4.5 x 10-2 s-1

Time  (h)  

Pseudorotaxane and Rotaxane

KZZ 400 M-1

k(in)ZZ 2.9 x 10-3 M-1s-1

k(out)ZZ 7.2 x 10-5 s-1

hν

hν’or Δ

hν

hν’or Δ

Photoisomerization of the azobenzene ends of the axle destabilizes the complex and slows down the threading/dethreading processes: both the energy minimum and the energy barriers are raised.

∆G‡threading

E-azoE

∆G

E

Reversible Photoswitching of Rotaxane Character and Interplay ofThermodynamic Stability and Kinetic Lability in a Self-Assembling Ring–

Axle Molecular System

Massimo Baroncini, Serena Silvi, Margherita Venturi, and Alberto Credi*[a]

Dedicated to Professor Luigi Fabbrizzi, recipient of the 2010 International Izatt–Christensen Award in Macrocyclic Chemistry

! 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eur. J. 2010, 16, 11580 – 1158711580

DOI: 10.1002/chem.201001409

Reversible Photoswitching of Rotaxane Character and Interplay ofThermodynamic Stability and Kinetic Lability in a Self-Assembling Ring–

Axle Molecular System

Massimo Baroncini, Serena Silvi, Margherita Venturi, and Alberto Credi*[a]

Dedicated to Professor Luigi Fabbrizzi, recipient of the 2010 International Izatt–Christensen Award in Macrocyclic Chemistry

! 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eur. J. 2010, 16, 11580 – 1158711580

DOI: 10.1002/chem.201001409

Introduction

Pseudorotaxanes are supramolecular complexes minimallycomposed of an axle-like molecule surrounded by a macro-cycle. These complexes can be rendered kinetically inert,that is, transformed into rotaxanes, by attaching bulkygroups at the extremities of the axle to prevent dethread-ing.[1,2] Therefore, the pseudorotaxane or rotaxane behaviorof a given axle–macrocycle pair is determined by the thread-ing–dethreading rate constants that in turn depend on thetemperature and the energy barriers associated with theseprocesses.[3]

Rotaxanes and related species are primarily interestingfor the construction of molecular machines.[4,5] The opera-tion of most rotaxane-type machines is based on classicalswitching processes between thermodynamically stablestates. It has become clear, however, that functional molecu-lar motors will only be realized if the reaction rates betweenstates can also be controlled,[6] thus enabling the implemen-tation of ratchet-type mechanisms.[7,8] In this context, theability of adjusting the threading–dethreading kinetics[9] bymodulating the corresponding energy barriers through exter-nal stimulation is an important goal.

A previously explored approach to convert a pseudoro-taxane into a rotaxane by using light was based on a stilbenederivative as the photoswitchable capping group at one endof the molecular axle.[10,11] Qualitative observations indicatethat the shape change of the stilbene end group broughtabout by E!Z photoisomerization is sufficient to hamperthe dethreading of a dibenzo[24]crown-8 ring from the axle.

However, these experiments were conducted at 0 8C undernitrogen and an external photosensitizer had to be used.Moreover, the E!Z conversion was not complete and re-versibility was not demonstrated; in fact it is known[12] that,besides isomerization, irradiation of stilbene causes cycliza-tion and dimerization. Thus, stilbene derivatives do notappear to be optimally suited for the reversible photochemi-cal control of motion kinetics in threaded and interlockedcompounds. Conversely, E-azobenzene can be converted tothe Z form by direct UVA irradiation with yields exceeding95 % and virtually no side products. The process is fully pho-tochemically or thermally reversible.[13]

Herein we describe a self-assembling system based on theknown[2,3a,14] dialkylammonium–crown ether recognitionmotif, which can be switched between thermodynamicallystable (pseudorotaxane) and kinetically inert (rotaxane)forms by light irradiation owing to the presence of photoiso-merizable azobenzene units at the extremities of the axle.[15]

The structural formulas of the molecular components,namely the bis-azobenzylamine axle EE-1H+ and the diben-zo[24]crown-8 ether ring 2, are shown in Scheme 1. We pres-ent a quantitative investigation into the effect of the isomer-ic state of the azobenzene end groups on both the associa-tion equilibrium constant and the threading–dethreadingrate constants of the molecular components. Furthermore,we report how a change in the ring–axle intercomponent in-teractions affects the kinetic behavior of the complex.

Results and Discussion

The dibenzo[24]crown-8 ring is commercially available. Theaxle was synthesized in good yield by using Mill!s couplingof bis(4-aminobenzyl)amine and 4-nitrosotoluene in aceticacid, followed by anion exchange with NH4PF6 (Scheme 2).The EE-1H·PF6 salt was fully characterized by using 1H(Figure 1c) and 13C NMR, DQF-COSY, ESI-MS, and UV/Vis absorption spectroscopies.[16] The 1H NMR spectrum of

Abstract: We have designed, synthe-sized, and investigated a self-assem-bling system that can be reversibly in-terconverted between thermodynami-cally stable (pseudorotaxane) and ki-netically inert (rotaxane) forms by lightirradiation. The system is composed ofa dibenzo[24]crown-8 ring and an axlecomprised of a dibenzylammonium rec-ognition site and two azobenzene endgroups. The isomeric form of the azo-benzene units of the axle has a little in-fluence on the stability constants of therespective pseudorotaxanes but greatlyaffects the threading–dethreading rate

constants. In fact, equilibration of thering and the axle in its EE isomericform occurs within seconds in acetoni-trile at room temperature, whereas theZZ axle threads–dethreads the ring atleast four orders of magnitude slower.Moreover, we show that a change inthe stability of the complex, achievedby deprotonating the dibenzylammoni-

um recognition site on the axle, affectsits kinetic behavior. We compare theresults of these experiments with thoseobserved upon dethreading the (pseu-do)rotaxane by using a competitiveguest for the ring, an approach whichdoes not inherently destabilize thering–axle interaction. This study out-lines a general strategy for the reversi-ble photochemical control of motionkinetics in threaded and interlockedcompounds and constitutes a startingpoint for the construction of multicom-ponent structures that can behave asphotochemically driven nanomachines.

Keywords: azo compounds · crowncompounds · hydrogen bonds · mo-lecular devices · supramolecularchemistry

[a] Dr. M. Baroncini, Dr. S. Silvi, Prof. M. Venturi, Prof. A. CrediDipartimento di Chimica “G. Ciamician”Universit" di Bologna, Via Selmi 240126 Bologna (Italy)Fax: (+39) 051-2099456E-mail : alberto.credi@unibo.it

Supporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/chem.201001409.

Chem. Eur. J. 2010, 16, 11580 – 11587 # 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemeurj.org 11581

FULL PAPER

The End…

hν

hν’or Δ

A New Beginning

∆G‡threading

EEE

∆GEE

E

∆G‡threading

ZZ

∆GZZ

∆G‡threading

Passive

E

∆G‡threading

E E

∆G‡Threading

Passive

Directionally Controlled Threading/Dethreading

Directionally controlled threading

Photochemical gate locking hν

Directionally controlled dethreading

Gate unlocking Reset hν’ or Δ

E

Fast

∆G‡E-azo

∆G‡P

Slow

E

∆G‡Z-azo

Fast

Slow

∆G‡P

E

E

E

∆G‡E-azo

∆G‡P

Directionally Controlled Threading/Dethreading

Directionally controlled threading

Photochemical gate locking hν

Directionally controlled dethreading

Gate unlocking Reset hν’ or Δ

E

∆G‡Z-azo

Fast

Slow

∆G‡P

E

E

Standing On The Shoulders of Giants

NH2

PF6

P. R. Ashton, I. Baxter, M. C. T. Fyfe, F. M. Raymo, N. Spencer, J. F. Stoddart, A. J. P. White, D. J. Williams, J. Am. Chem. Soc. 1998, 120, 2297-2307.

k(in)C5 1.3 x 10-1 M-1s-1 O

OO

O

O

O O

O+   N

H2

O

O

O

O

PF6

Standing On The Shoulders of Giants

NH2

PF6

P. R. Ashton, I. Baxter, M. C. T. Fyfe, F. M. Raymo, N. Spencer, J. F. Stoddart, A. J. P. White, D. J. Williams, J. Am. Chem. Soc. 1998, 120, 2297-2307.

k(in)C5 1.3 x 10-1 M-1s-1 O

OO

O

O

O O

O+   N

H2

O

O

O

O

PF6

hν

hν’or Δ

NH2

NN

PF6

NH2

NN

PF6E-3H Z-3H

Standing On The Shoulders of Giants

NH2

PF6

P. R. Ashton, I. Baxter, M. C. T. Fyfe, F. M. Raymo, N. Spencer, J. F. Stoddart, A. J. P. White, D. J. Williams, J. Am. Chem. Soc. 1998, 120, 2297-2307.

k(in)C5 1.3 x 10-1 M-1s-1 O

OO

O

O

O O

O+   N

H2

O

O

O

O

PF6

hν

hν’or Δ

2H

[2H⊂R][PF6]

R

NH2

NN

PF6

NH2

NN

PF6E-3H Z-3H

Standing On The Shoulders of Giants

NH2

PF6

P. R. Ashton, I. Baxter, M. C. T. Fyfe, F. M. Raymo, N. Spencer, J. F. Stoddart, A. J. P. White, D. J. Williams, J. Am. Chem. Soc. 1998, 120, 2297-2307.

k(in)C5 1.3 x 10-1 M-1s-1 O

OO

O

O

O O

O+   N

H2

O

O

O

O

PF6

hν

hν’or Δ

Kinetic and thermodynamic data for the self-assembly of the investigated complexes in CD3CN at 298 K.

Complex K (M–1)

–ΔG° (kcal mol–1)

kin

(M–1 s–1) –ΔG#

in (kcal mol–1)

kout

(s–1) –ΔG#

out (kcal mol-1) t½

[EE-1H⊂R][PF6] 820 3.9 37 15 4.5×10–2 19.3 15.4 s [ZZ-1H⊂R][PF6] 400 3.5 2.9×10–3 20.9 7.2×10–6 24.5 27 h [2H⊂R][PF6] ≈30 2 1.3×10–1 18.6 4.4×10–3 20.7 2.6 min [E-3H⊂R][PF6] 225 3.2 22 15.6 0.1 18.8 6.3 s [Z-3H⊂R][PF6] 230 3.2 5.1×10–2 19.2 2.6×10–4 22.3 46 min

2H

[2H⊂R][PF6]

R

hν

hν

K+

hν

hν’or Δ K+

hν

hν’or Δ K+

18C6

[K⊂18C6]+

hν

hν’or Δ K+

18C6

[K⊂18C6]+

Directionally Controlled Transit

Angew. Chem. Int. Ed. 2012 (17) 4223-4226.

E

Fast

∆G‡E-azo

∆G‡P

Slow

E

∆G‡Z-azo

Fast

Slow

∆G‡P

Toward a Molecular Pump

Directionally controlled threading

Photochemical gate locking

hν

Directionally controlled dethreading

Gate unlocking Reset hν’ or Δ

IN

E

E

E

∆G‡Z-azo

Fast

Slow

∆G‡P

Toward a Molecular Pump

Directionally controlled threading

Directionally controlled dethreading

Gate unlocking Reset hν’ or Δ

E

Photochemical gate locking

hν

OUT

E

E

The Group of Photochemistry

Thanks to:

Dr. Elisa Bandini, Dr Massimo Benaglia Lab 506

Dr. Roberto Zamboni CNR-ISOF for the kind hospitality!

Photochemical Nanosciences Laboratory Bologna