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August 31, 2016
Research updateRavil R. Petrov
Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow G1 1XL, United Kingdom
Development of new organic materials for solar-cell applications continues to be a subject of
considerable interest.1 Among a wealth of promising targets to pursue in this field, EDST is a molecule of
great value because of its huge potential in a number of optoelectronic applications.2 Incorporation of EDST
into polymers with donor-acceptor (D-A) structure can potentially result in enhanced polymer performance
for solar energy conversion due to stronger nature of intermolecular contacts between Se atoms. This unique
feature can be employed to attain a cross-linking effect without disturbing the polymer architecture. In order
to assess the benefits of using EDST for solar energy harvesting, we wanted to give a closer look to a
corresponding polymer featuring alternating D-A units based on the structure of PTB7-Th3 which
demonstrated the power conversion efficiencies of 10% (Fig. 1).
The synthesis of the target polymer EDST-PTB7-Th (6) was achieved by iterative transformations
involving stannylations and the subsequent Stille condensations, as illustrated in Scheme 1. Procedures for
the preparation of derivatives 1, 2, and 5 have been reported previously.4-7
Scheme 1. Synthesis of polymer EDST-PTB7-Th, reagents and conditions: (i) 2 eq. comp 2, Pd(PPh3)4, toluene, W, 160 °C, 2 h, 66%; (ii) 6.1 eq. (n-Bu)3SnCl/3.1 eq LiTMP/2.4 eq. LDA THF, -78 °C to RT, 73%; (iii) comp 5, Pd(PPh3)4, toluene/DMF (5:1, v/v), 120 °C, 24 h, 51%.
1
Figure 1. The structure of PTB7-Th with donor-acceptor (D-A) units and its EDST-functionalized analogs EDST-PTB7-Th (6).
Identifying suitable reaction conditions for stannylating substrate 3 posed a significant synthetic
challenge in this study. In a classical approach, stannylation is done via lithiation followed by the addition of
the electrophile at -78 °C in dry solvent. Previously, monostannylated EDST was prepared by treating EDST
with LDA (pKa ~36) at -78 °C, followed by the addition of trimethyltin chloride in dry THF as the reaction
medium. However, when applied to substrate 3, this strategy was inefficient, leading to the formation of a
mixture of mono-, bis-stannylated products together with the nonreacted starting material, which was
difficult to separate by column chromatography. In our hands, changing from single to repetitive procedure
for adding the two reagents conferred no advantage. Moreover, we found that in the classical approach, it
was not possible to drive the reaction to completion by simply applying excess of reagents, which only
contributed to decomposition of the target product. The use of more powerful lithiation reagents such as n-
BuLi and tert-BuLi led to the formation of multiple byproducts, presumably due to instability of the 3,4-
ethylenediselena part under the reaction conditions. Therefore, these results caused us to pursue a non-
classical trapping approach, in which the lithiated species are quenched in-situ with the electrophile. Indeed,
by using LDA and tributyltin chloride, the reaction could be driven to completion indicating no sign of non-
reacted or mono-reacted product according to the TLC analysis. However, the purity of the stannylated
product required further improvement due to a small amount of a closely running byproduct, which was
difficult to eliminate by column chromatography. For this purpose, a less basic and more nucleophilic LDA
alternative, lithium tetramethylpiperidide8 (LiTMP, pKa ~37) was tested. Although no byproduct formation
was noticed under the same conditions with LiTMP in an analogous fashion, its lithiating power was not
sufficient to take the reaction to completion. Finally, we found that the formation of the byproduct could be
minimized by slowly adding 3.1 eq of LiTMP, followed by 2.4 equivalents of LDA in the presence of 6.1 eq
of tributyltin chloride in dry THF at -78 °C. With the desired bis-stannylated block 4 in hand, the target
polymer EDST-PTB7-Th was prepared in 51% yield by reacting the corresponding monomers 4 and 5 in the
presence of Pd(PPh3)4 as a catalyst with conventional heating.
The polymer EDST-PTB7-Th has been studied in terms of its physical, electrochemical, and thermal
properties in order to compare it to the parent polymer PTB7-Th, with the results summarized in Table 1.
Table 1. Physicochemical properties of EDST-PTB7-Th and PTB7-Th9
polymerMn/Mw
(kDa)PDI Tg [°C] Td [°C]
solution
λmax [nm]
film
λmax [nm]HOMO [eV]a LUMO [eV]a
EgEC
[eV]bEg
opt
[eV]c
EDST-PTB7-Th 36/122 4.38 ntf 310 539 565 -5.36 -3.54 1.82 1.75
PTB7-Th 45/94 2.09 ntf 383 692 (707) 780 -5.3 -3.71 1.59 1.59
aHOMO and LUMO levels were estimated from the onset of the oxidation and reduction peaks of the cyclic voltammogram and referenced to ferrocene, which has a HOMO of -4.8 eV. bElectrochemical bandgap was calculated from the cyclic voltammogram. cOptical bandgap was calculated from the onset of UV-vis spectra of neat film, Eg
opt = 1240/λonset. dIrreversible peak. eQuasi-reversible peak. fNo transition.
UV–vis absorption measurements were conducted for EDST-PTB7-Th as a solid thin film deposited
on a quartz glass cuvette and as a dichloromethane solution. For the thin film, the onset of the longest
2
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
-60
-40
-20
0
20
40
Cur
rent
/ A
Potential / V
film solution
(a) (b)
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
-80
-60
-40
-20
0
20
40
Cur
rent
/ A
Potential / V
film solution
wavelength absorption (λmax = 565 nm) band gives an optical band gap (Egopt
) of ≈1.75 eV, whereas the
estimated band gap from the onset of the absorption (λmax = 539 nm) in the solution is ≈1.84 eV.
400 600 8000.0
0.2
0.4
0.6
0.8
1.0
Nor
mal
ised
Abs
orba
nce
(a.u
)
Wavelength (nm)
film solution
Figure 1. UV−vis absorption spectra of EDST-PTB7-Th as a solid thin film deposited on a quartz glass cuvette and as a
dichloromethane solution.
Figure 2. Cyclic voltammetry of EDST-PTB7-Th in solution (dashed line) and thin-film (continuous line): (a) ferrocene as an internal reference, (b) ferrocene as an external reference. The experiments in solution were carried out in dichloromethane (0.19 mM based on the Mw of the repeating unit) using a glassy carbon electrode. A film was deposited from a solution of EDST-PTB7-EDST-Th in chloroform on a glassy carbon electrode and experiments were carried out in acetonitrile. In both cases, an Ag wire reference electrode and a Pt counter-electrode, in the presence of Bu4NPF6 (0.1 M), were used.
Cyclic voltammogram measurements of EDST-PTB7-Th in the form of as a solid thin film and in
dichloromethane solution were performed using ferrocene as both an external and internal standard. The
estimations were done using the empirical relations ELUMO = [(Ered – E1/2(ferrocene) + 4.8] eV or EHOMO = [(Eox –
E1/2(ferrocene) + 4.8] eV. Based on the obtained cyclic voltammetry results in solution, EDST-PTB7-Th shows
EHOMO = 5.74 eV, ELUMO = 3.56 eV and EgEC
= 2.18 eV. When measured as film, this polymer shows EHOMO =
3
5.36 eV, ELUMO = 3.54 eV and EgEC
= 1.82 eV. These energy gap values are larger than the energy gaps from
UV−vis absorption spectra (1.84 and 1.75 eV, respectively). A quasi-reversible reduction peak is observed
for the thin film in acetonitrile with Emax = 3.79 eV.
-1.82 -1.59
-1.96
-6.0
-5.5
-5.0
-4.5
-4.0
-3.5
B
Figure 3. Energy band diagram of EDST-PTB7-Th and PTB7-Th in addition to the work function of ITO and Al.
The band diagram (Figure 3) with HOMO/LUMO levels of EDST-PBT7-Th as compared to the
parent polymer, PTB7-Th, and PCBE in addition the ITO and Al work functions shows that the newly
designed material can be used as a potential active layer for organic solar cells.
100 200 300 400 500
60
80
100
Wei
ght (
%)
Temperature, oC
Figure 4. TGA plot of EDST-PTB7-Th polymer with a heating rate of 10 °C min-1 under nitrogen atmosphere
Thermogravimetric analysis (TGA) measurement was carried out to evaluate the thermal stability of
the polymer, and the TGA plot of the polymer is shown in Fig. 3. The TGA profile reveals that the
decomposition temperatures (Td) at 5 % weight loss is approximately 310 °C, which is lower than that of
PTB7-Th (383 °C), indicating that incorporation of EDST moiety decreases the thermal stability of the
polymer in comparison to the parent analog. Nevertheless, the thermal stability of EDST-PTB7-Th is still
acceptable for its application in PSCs.
4
ITO-4.7 eV
-3.54 eV-3.71 eV
-5.36 eV -5.3 eV
PCBE
-5.87 eV
-3.91 eVAl
-4.3 eV
As shown in Fig. 5, differential scanning calorimetry (DSC) of EDST-PTB7-Th showed no
evidence of a phase transition point, which is an analogous behavior to that reported for PTB7-Th.
50 100 150 200 250-1.4
-1.2
-1.0
-0.8
-0.6
Hea
t flo
w (m
V)
Temperature, oC
Figure 5. DSC trace of the neat polymer EDST-PTB7-Th.
Summary
We synthesized a unique polymer EDST-PTB7-Th featuring EDST structure introduced between
electron-rich (donor) and electron-deficient (acceptor) building blocks of a well-known OPV donor polymer,
PTB7-Th. A new bis-functionalization strategy for EDST core has been developed, which allows the design
of new materials with enhanced photovoltaic properties. The target polymer EDST-PTB7-Th was prepared
in 51% yield by Stille coupling of the bis-functional monomers. The target polymer displayed a wide
absorption range starting from 710 nm, the optical bandgap of 1.75 eV, and the LUMO level tuned to -5.36
eV.
5
Experimental section
2,6-Dibromo-4,8-bis[5-(2-ethylhexyl)-2-thienyl]-benzo[1,2-b:4,5-b']dithiophene (1). Compound 1
was synthesized using a procedure reported previously.4 NMR spectra were fully consistent with the data
already reported.
2-(Trimethylstannyl)-3, 4-ethylenediselenothiophene (2). Compound 2 was prepared according to
the synthetic routes reported previously.5 NMR spectra were in agreement with the previously published
data.
2,6-Bis(3,4-ethylenediselenathiophene)-4,8-bis[5-(2-ethylhexyl)-2-thienyl]-benzo[1,2-b:4,5-b']-
dithiophene, EDST-BDT-EDST (3). Compound 2 (440 mg, 1.02 mmol), compound 1 (365 mg, 0.51
mmol) and Pd(PPh3)4 (144 mg, 0.125 mmol) were weighted into a 5 mL clean and dry microwave vessel.
The vessel was subjected to three successive cycles of vacuum followed by refiling with nitrogen. Then
anhydrous toluene (3 mL) was added via a syringe. The reaction was carried out at 160 °C for 2 h under
nitrogen protection. After cooling the reaction mixture to room temperature, the reaction mixture was
concentrated in vacuo, and purified by column chromatography on silica gel using dichloromethane/hexane
(1/3, v/v) to yield 365 mg (66%) of the target product as yellow-orange solid; mp 140-142 °C. 1H NMR (400
MHz, CDCl3): δ 7.81 (s, J = 12.1 Hz, 1H), 7.36 (d, J = 3.5 Hz, 1H), 7.26 (d, J = 2.8 Hz, 1H), 6.91 (d, J = 3.5
Hz, 1H), 3.34 (s, 4H), 2.88 (d, J = 6.7 Hz, 2H), 1.74 – 1.64 (m, 1H), 1.57 – 1.29 (m, 8H), 0.96 (t, J = 7.5 Hz,
3H), 0.92 (t, J = 7.1 Hz, 3H). 13C NMR (101 MHz, CDCl3): δ 146.04 (Cq), 138.94 (Cq), 136.97 (Cq), 136.62
(Cq), 131.97 (Cq), 127.93 (CH), 125.67 (CH), 123.76 (Cq), 123.32 (Cq), 122.47 (CH), 122.29 (Cq), 121.93
(CH), 41.60 (CH), 34.41 (CH2), 32.64 (CH2), 29.08 (CH2), 25.87 (CH2), 24.36 (CH2), 23.17 (CH2), 22.82
(CH2), 14.35 (CH3), 11.12 (CH3). MALDI-TOF-MS: m/z = 1111.57 [M]+. HRMS, m/z: calcd for C46H50S6Se4
[M+H]+:1112.8906; found 1112.8953. Elemental analysis: found C, 50.92; H, 4.50%; calculated: C, 49.72;
H, 4.54%.
2,6-Bis[2-(tributylstannyl)-3,4-ethylenediselenothiophene]-4,8-bis[5-(2-ethylhexyl)-2-thienyl]-
benzo[1,2-b:4,5-b']-dithiophene (4). Under nitrogen atmosphere, EDST-BDT-EDST (3) (568 mg, 0.51
mmol) and tributyltin chloride 850 L (3.13 mmol) were added to anhydrous THF (9 mL) in a 25 mL clean
and dry flask at room temperature, and the mixture was cooled to -78 °C. A solution of freshly prepared
LiTMP8 (1.58 mmol, 2.5 mL, 0.63 M in THF) was added dropwise within 20 minutes, and the reaction was
stirred for 1 h at -78 °C and then for 1 h at room temperature. The reaction mixture was cooled down again
to -78 °C, and a solution of LDA (1.2 mL, 1.2 mmol, 1 M in THF) was added dropwise within 20 minutes.
The reaction mixture was allowed to stir at -78 °C for 1 h before it was warmed to room temperature
overnight. The mixture was quenched by addition of 20 mL of saturated ammonium chloride and extracted
by dichloromethane (50 mL) three times. The combined organic phase was washed with water, brine, dried
over anhydrous MgSO4, and concentrated under vacuum. The obtained residue was precipitated from ice-
cold methanol. The filtered crude product was purified by flash column chromatography on a silica gel
6
column, pretreated with 10% triethylamine in hexane and washed with hexane (2 × 250 mL), by eluting with
15% DCM in hexane. The obtained residue was sonicated in ethanol, cooled with ice-water, filtered, and
dried in vacuo to obtain the target compound 4 (632 mg, 73%) as a pale yellow-orange solid. 1H NMR (400
MHz, CD2Cl2): δ 7.83 (s, 2H), 7.39 (d, J = 3.5 Hz, 2H), 6.97 (d, J = 3.5 Hz, 2H), 3.38 – 3.27 (m, 8H), 2.91
(d, J = 6.7 Hz, 4H), 1.76 – 1.70 (m, 2H), 1.64 – 1.56 (m, 12H), 1.50 – 1.28 (m, 28H), 1.27 – 1.21 (m, 12H),
0.97 (t, J = 7.4 Hz, 6H), 0.95 – 0.88 (m, J = 7.4 Hz, 24H). 13C NMR (101 MHz, CD2Cl2): δ 146.75 (Cq),
139.21 (Cq), 137.63 (Cq), 137.55 (Cq), 137.50 (Cq), 137.41 (Cq), 133.53 (Cq), 128.44 (CH), 126.20 (CH),
125.22 (Cq), 124.01 (Cq), 121.85 (CH), 42.18 (CH), 34.81 (CH2), 33.10 (CH2), 29.61 (CH2), 29.52 (CH2),
27.82 (CH2), 26.36 (CH2), 25.72 (CH2), 24.91 (CH2), 23.65 (CH2), 14.57 (CH3), 14.01 (CH3), 11.94 (CH2),
11.36 (CH3).
4,6-Dibromo-3-fluorothieno[3,4-b]thiophene-2-ethylhexyl ester (5). Compound 5 was prepared
according to the synthetic routes reported previously.7 NMR spectra were in agreement with the previously
published data.
Polymer EDST-PTB7-Th (6). In a 25 mL 2-neck flask equipped with a water-cooled condenser, the
bis-stannylated compound 4 (329 mg, 0.20 mmol), ester 5 (92 mg, 0.20 mmol) and Pd(PPh3)4 (25 mg, 0.022
mmol) were dissolved in a toluene (5 mL) and DMF (1 ml) mixed solvent under nitrogen atmosphere. The
mixture was stirred at 120 °C for 24 hr. The polymerization proceeded for additional 12 h after adding 2-
bromothiophen (25 μL) and 2-tributyltin-thiophene (60 μL) as end-capping agents. After the resulting
solution was cooled down to room temperature, it was then poured into methanol (300 mL). The resulting
precipitate was collected by filtration, and the product was then further purified by Soxhlet extraction
consecutively with methanol (8 h), hexane (8 h), acetone (12h) and chloroform (24 h). The chloroform
solution of polymer 6 was filtered through Celite, concentrated by rotary evaporation to a volume of ca. 5
mL, and then precipitated from methanol (300 mL). The precipitated polymer was collected by filtration,
rinsed with methanol (50 mL), acetone (50 mL), hexane (50 mL), and then dried in high vacuum (4.8×10 -2
mbar) for 24 h at room temperature. The target polymer EDST-PTB7-Th (6) was obtained as a dark purple
solid; 142 mg, yield 51%. 1H NMR (400 MHz, CDCl3): δ 7.93 (br., 2H), 7.40 (br., 2H), 6.97 (br., 2H), 4.27
(br., 2H), 3.38 (br., 8H), 2.90 (br., 4H), 1.73 (br., 3H), 1.35 (br., 24H), 0.89 (br., 18H). 19F NMR (376 MHz,
CDCl3): δ -113.01, -113.11. GPC (vs. polystyrene standard) Mn 36000 g/mol, PDI 4.38.
Literature
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(2) Pang, H.; Skabara, P. J.; Gordeyev, S.; McDouall, J. J. W.; Coles, S. J.; Hursthouse, M. B. Chemistry of Materials 2007, 19, 301.
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8
1H NMR of 2,6-dibromo-4,8-bis[5-(2-ethylhexyl)-2-thienyl]-benzo[1,2-b:4,5-b']dithiophene (1) in CDCl3, Bruker-400
9
13C NMR of 2,6-dibromo-4,8-bis[5-(2-ethylhexyl)-2-thienyl]-benzo[1,2-b:4,5-b']dithiophene (1) in CDCl3, Bruker-400
10
13C DEPT NMR of 2,6-dibromo-4,8-bis[5-(2-ethylhexyl)-2-thienyl]-benzo[1,2-b:4,5-b']dithiophene (1) in CDCl3, Bruker-400
11
1H NMR of 2-(trimethylstannyl)-3,4-ethylenediselenothiophene (2) in CDCl3, Bruker-400
12
13C DEPT NMR of 2-(trimethylstannyl)-3,4-ethylenediselenothiophene (2) in CDCl3, Bruker-400
13
77Se NMR of 2-(trimethylstannyl)-3,4-ethylenediselenothiophene (2) in CDCl3, Bruker-400
14
1H NMR of EDST-PTB7-EDST (3) in CDCl3, Bruker-400
15
13C NMR of EDST-PTB7-EDST (3) in CDCl3, Bruker-400
16
13C DEPT NMR of EDST-PTB7-EDST (3) in CDCl3, Bruker-400
17
1H NMR of compound 4 in CDCl3, Bruker-400
18
13C NMR of compound 4 in CDCl3, Bruker-400
19
13C DEPT NMR of compound 4 in CDCl3, Bruker-400
20
1H NMR of 4,6-dibromo-3-fluorothieno[3,4-b]thiophene-2-ethylhexyl ester (5) in CDCl3, Bruker-500
21
13C NMR of 4,6-dibromo-3-fluorothieno[3,4-b]thiophene-2-ethylhexyl ester (5) in CDCl3, Bruker-500
22
13C DEPT NMR of 4,6-dibromo-3-fluorothieno[3,4-b]thiophene-2-ethylhexyl ester (5) in CDCl3, Bruker-500
23
19F NMR of 4,6-dibromo-3-fluorothieno[3,4-b]thiophene-2-ethylhexyl ester (5) in CDCl3, Bruker-500
24
res.
H2O
res.
CH
Cl3
1H NMR of polymer EDST-PTB7-Th (6) in CDCl3, Bruker-400
25
19F NMR of polymer EDST-PTB7-Th in CDCl3, Bruker-400
26