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

(1) Jagadamma, L. K.; Al-Senani, M.; El-Labban, A.; Gereige, I.; Ngongang Ndjawa, G. O.; Faria, J. C. D.; Kim, T.; Zhao, K.; Cruciani, F.; Anjum, D. H.; McLachlan, M. A.; Beaujuge, P. M.; Amassian, A. Advanced Energy Materials 2015, 5, n/a.

(2) Pang, H.; Skabara, P. J.; Gordeyev, S.; McDouall, J. J. W.; Coles, S. J.; Hursthouse, M. B. Chemistry of Materials 2007, 19, 301.

(3) He, Z.; Xiao, B.; Liu, F.; Wu, H.; Yang, Y.; Xiao, S.; Wang, C.; Russell, T. P.; Cao, Y. Nat Photon 2015, 9, 174.

(4) Homyak, P. D.; Tinkham, J.; Lahti, P. M.; Coughlin, E. B. Macromolecules 2013, 46, 8873.

7

(5) Pang, H.; Skabara, P. J.; Crouch, D. J.; Duffy, W.; Heeney, M.; McCulloch, I.; Coles, S. J.; Horton, P. N.; Hursthouse, M. B. Macromolecules 2007, 40, 6585.

(6) Liang, Y.; Feng, D.; Wu, Y.; Tsai, S.-T.; Li, G.; Ray, C.; Yu, L. Journal of the American Chemical Society 2009, 131, 7792.

(7) Yu, J.; Zhao, B.; Nie, X.; Zhou, B.; Li, Y.; Hai, J.; Zhu, E.; Bian, L.; Wu, H.; Tang, W. New Journal of Chemistry 2015, 39, 2248.

(8) Greiner, R.; Blanc, R.; Petermayer, C.; Karaghiosoff, K.; Knochel, P. Synlett 2016, 27, 231.(9) Jiang, T.; Yang, J.; Tao, Y.; Fan, C.; Xue, L.; Zhang, Z.; Li, H.; Li, Y.; Huang, W. Polymer Chemistry 2016,

7, 926.

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