Efficient Organic Sensitizers with Pyridine-N-oxide as an Anchor Group for Dye-Sensitized Solar Cells

Article abstract of DOI:10.1002/cssc.201402208

Five organic dyes with pyridine-N-oxide as the anchor group and electron acceptor have been synthesized and applied in dye-sensitized solar cells (DSSCs). Benzothiadiazole was introduced in the conjugation system to increase the electron withdrawing properties, FTIR spectra showed that the coordination was between the pyridine-N-oxide and the Br?nsted acid site on the TiO₂surface. The relationship between different dye structures and the performance of the DSSCs was investigated systematically. The location of the thiophene unit was studied, and the direct linkage of benzothiadiazole with pyridine-N-oxide was beneficial to broaden the absorption. The donor–acceptor–acceptor-configured dye WL307, which has 2-ethylhexyloxy chains in the donor part, showed the best efficiency of 6.08 % under 100 mW cm^?2light illumination. The dye series showed a fairly good stability during the one month test period.

Full text of DOI:10.1002/cssc.201402208

Supporting Information
ꢀ Copyright Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, 2014
Efficient Organic Sensitizers with Pyridine-N-oxide as an
Anchor Group for Dye-Sensitized Solar Cells
Lei Wang,[a] Xichuan Yang,*[a] Jianghua Zhao,[a] Fuguo Zhang,[a] Xiuna Wang,[a] and
Licheng Sun[a, b]
cssc_201402208_sm_miscellaneous_information.pdf

General synthetic procedure
1
MS data were obtained with GC-Tof MS (UK), HP1100 LC/MSD (USA), and LC/Q-TOF MS (UK). H NMR spectra were
taken with VARIAN INOVA 400MHz (USA) using TMS as standard. Commercial reagents and solvents were used as
received without further purification. 4-(5-(4, 4, 5, 5-tetramethyl-1, 3, 2-dioxaborolan-2-yl)thiophen-2-yl)pyridine, N,
N-diphenyl-4-(4, 4, 5, 5 -tetramethyl-1, 3, 2-dioxaborolan-2-yl)aniline (3), N, N-diphenyl-4-(5-(4, 4, 5, 5-tetramethyl-1, 3, 2-
dioxaborolan-2-yl)thiophen-2-yl)aniline (4) were synthesized according to the previous publication.
The synthesis route is summarized in Scheme 2.
4,7-dibromobenzo(1,2,5)thiadiazole (1). Diazosulfide (13.6 g, 10 mmol) and Br2(35.16 g, 2.2 equiv) were dissolved in the
HBr(100 ml), and then were refluxed overnight, extracted with CH2Cl2. The organic layer was combined and removed by
rotary evaporation. The residue was purified by column chromatography on silica (CH2Cl2/petroleum ether = 5/1, v/v) to
give white needle-like solid (27 g, 92 mmol). Yeild: 91.8%. Complex (1) 1H NMR (400 MHz, Acetone) δ 7.93 (s, 2H).
TOF-HRMS calcd. For C6H2N2SBr [M+] 291.8314, found 291.8305.
4-(octan-3-yloxy)-N-(4-(octan-3-yloxy)phenyl)-N-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)aniline(3’).
To a solution of Compound(2’) (7.00 g, 12.1mmol) in THF (70 ml) nBuLi (11.3 ml, 2.5 M in n-hexane) was added dropwisely
at –78 oC under N2. The reaction was kept at –78 oC for 1 h. Then 2–isopropoxy–4, 4, 5, 5–tetramethyl–1, 3,
2–dioxaborolane (5.20 g, 15.2 mmol) was added. The mixture was allowed to warm to room temperature overnight. The
reaction was quenched by water and then was extracted with ether. The combined organic layers were dried over
anhydrous Na2SO4 and evaporated to dryness. The residue was purified by chromatography on silica gel eluting with
1
hexane/ethyl acetate (4/1) to afford 3’ as white oil (6.00 g, 9.61 mmol). Yield: 79%. Compound (3’) H NMR (400 MHz,
Acetone) δ 7.53 (d, J = 8.6 Hz, 2H), 7.11 – 7.02 (m, 4H), 6.96 – 6.89 (m, 4H), 6.79 (t, J = 9.6 Hz, 2H), 1.78 – 1.63 (m, 4H),
1.58 – 1.41 (m, 2H), 1.39 – 1.32 (m, 16H), 1.30 (d, J = 5.1 Hz, 12H), 0.92 (dt, J = 10.8, 7.3 Hz, 12H).
4-(octan-3-yloxy)-N-(4-(octan-3-yloxy)phenyl)-N-(4-(5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)thiophen-2-yl)ph
enyl)aniline(4’). Compound (2’) reacted with thiophen-2-ylboronic acid to get 4-(octan-3-yloxy)-N-(4-
(octan-3-yloxy)phenyl)-N- (4-(thiophen-2-yl)phenyl)aniline, and then reacted as step (b). Compound (4’) 1H NMR (400 MHz,
Acetone) δ 7.93 – 7.88 (m, 1H), 7.81 – 7.76 (m, 2H), 7.59 (t, J = 7.0 Hz, 1H), 7.10 – 7.03 (m, 4H), 6.94 – 6.84 (m, 6H), 1.77
– 1.63 (m, 4H), 1.58 – 1.40 (m, 2H), 1.39 – 1.32 (m, 16H), 1.30 (d, J = 5.1 Hz, 12H), 0.92 (dt, J = 10.8, 7.3 Hz, 12H).
4-(7-bromobenzo[c][1,2,5]thiadiazol-4-yl)-N,N-diphenylaniline (5). Complex (1) (415 mg, 1.4 mmol) was dissolved with
complex (3) (430.77 mg, 1.5 mmol), Pd(PPh3)4 (70 mg, 0.07 mmol), and K2CO3 (1.38 g, 10.0 mmol) in 50 ml of THF and 5
ml of H2O were heated to 70 °C under a nitrogen atmosphere for 8 h. After cooling to room temperature, the mixture was
extracted with CH2Cl2. The organic portion was combined and removed by rotary evaporation. The residue was purified by
column chromatography on silica (CH2Cl2/petroleum ether = 1/5, v/v) to give an orange solid (339.3 mg, 0.74 mmol). Yeild:
49.3%. Complex (5) 1H NMR (400 MHz, Acetone) δ 8.06 (d, J = 7.7 Hz, 1H), 7.99 – 7.94 (m, 2H), 7.76 (d, J = 7.7 Hz, 1H),
7.40 – 7.32 (m, 4H), 7.21 – 7.14 (m, 6H), 7.14 – 7.09 (m, 2H). TOF-HRMS calcd. For C24H16N3SBr [M+] 227.0248, found
227.0298.
4-(7-bromobenzo[c][1,2,5]thiadiazol-4-yl)-N,N-bis(4-(octan-3-yloxy)phenyl)aniline (5’). Complex (1) and complex (3’)
were reacted to get Complex (5’). Complex (5’) 1H NMR (400 MHz, Acetone) δ 8.02 (d, J = 7.7 Hz, 1H), 7.87 (d, J = 8.8 Hz,
2H), 7.70 (d, J = 7.7 Hz, 1H), 7.26 – 7.07 (m, 4H), 6.97 (d, J = 8.9 Hz, 6H), 3.99 – 3.82 (m, 4H), 1.73 (dq, J = 12.1, 6.0 Hz,
2H), 1.61 – 1.42 (m, 7H), 1.43 – 1.29 (m, 9H), 0.99 – 0.83 (m, 12H). TOF-HRMS calcd. For C40H48BrN3O2S [M+] 713.2651,
found 713.2673.
4-(5-(7-bromobenzo[c][1,2,5]thiadiazol-4-yl)thiophen-2-yl)-N,N-diphenylaniline (6). Complex (1) and complex (4) were
reacted as step d to get Complex (6). Complex (6) 1H NMR (400 MHz, Acetone) δ 8.22 (d, J = 4.0 Hz, 1H), 8.03 (d, J = 7.8
Hz, 1H), 7.96 (d, J = 7.8 Hz, 1H), 7.72 – 7.66 (m, 2H), 7.52 (d, J = 4.0 Hz, 1H), 7.38 – 7.31 (m, 4H), 7.17 – 7.11 (m, 5H),
7.10 – 7.07 (m, 3H). TOF-HRMS calcd. For C28H18N3S2Br [M+] 539.0137, found 539.0126.
4-(5-(7-bromobenzo[c][1,2,5]thiadiazol-4-yl)thiophen-2-yl)-N,N-bis(4-(octan-3-yloxy)phenyl)aniline (6’). Complex (1)
and complex (4) were reacted as step d to get Complex (6’). Complex (6’) 1H NMR (400 MHz, Acetone) δ 8.18 (d, J = 3.9
Hz, 1H), 8.00 (d, J = 7.8 Hz, 1H), 7.91 (d, J = 7.8 Hz, 1H), 7.57 (d, J = 8.7 Hz, 2H), 7.44 (dd, J = 14.8, 6.2 Hz, 1H), 7.09 (t, J
= 6.1 Hz, 4H), 6.96 (t, J = 6.1 Hz, 4H), 6.89 (d, J = 8.7 Hz, 2H), 3.96 – 3.87 (m, 4H), 1.73 (dq, J = 12.1, 6.0 Hz, 2H), 1.48

(tdd, J = 23.3, 11.3, 5.2 Hz, 7H), 1.41 – 1.31 (m, 9H), 0.96 – 0.87 (m, 12H). TOF-HRMS calcd. For C44H50N3O2S2Br [M+]
795.2524, found 795.2528.
N,N-diphenyl-4-(7-(5-(pyridin-4-yl)thiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)aniline (WL302Q). Complex (5) was
reacted with 4-(5-(4, 4, 5, 5-tetramethyl-1, 3, 2- dioxaborolan-2-yl)thiophen-2-yl)pyridine to get WL302Q as a red solid.
WL302Q 1H NMR (400 MHz, CDCl3) δ 8.64 (s, 2H), 8.13 (d, J = 3.9 Hz, 1H), 7.99 (d, J = 7.5 Hz, 1H), 7.89 (d, J = 8.6 Hz,
2H), 7.73 (s, 1H), 7.63 (d, J = 4.0 Hz, 1H), 7.60 (d, J = 5.2 Hz, 2H), 7.31 (t, J = 7.8 Hz, 4H), 7.21 (t, J = 7.5 Hz, 6H), 7.08 (t,
J = 7.3 Hz, 2H). TOF-HRMS calcd. For C33H22N4S2 [M+] 538.1293, found 538.1286.
4-(5-(7-(4-(diphenylamino)phenyl)benzo[c][1,2,5]thiadiazol-4-yl)thiophen-2-yl)pyridine-1-oxide(WL302).
3-chloroperoxybenzoic acid (190 mg, 1.10 mmol ) in 15 ml CH2Cl2 were added in dropwise to WL302Q (410.9 mg, 0.90
mmol) in 50 ml of CH2Cl2, then stirred at RT overnight. The mixture was dried by rotary evaporation. The residue was
purified by column chromatography on silica (CH2Cl2/CH3OH = 100/1, v/v) to give a red solid (226.9 mg, 0.48 mmol). Yield:
54.7%. WL302 1H NMR (400 MHz, CDCl3) δ 8.24 (d, J = 6.6 Hz, 2H), 8.11 – 8.08 (m, 1H), 7.97 (d, J = 7.5 Hz, 1H), 7.89 (dd,
J = 8.7, 3.3 Hz, 2H), 7.73 (d, J = 7.6 Hz, 1H), 7.58 (d, J = 6.6 Hz, 2H), 7.51 (d, J = 3.9 Hz, 1H), 7.33 – 7.27 (m, 4H), 7.24 –
7.15 (m, 6H), 7.13 – 7.03 (m, 2H). TOF-HRMS calcd. For C33H22N4OS2 [M+] 554.1245, found 554.1235.
N,N-diphenyl-4-(7-(pyridin-4-yl)benzo[c][1,2,5]thiadiazol-4-yl)aniline (WL301Q). Complex (5) was reacted with
pyridin-4-ylboronic acid to get WL301Q as a red solid. WL301Q 1H NMR (400 MHz, CDCl3) δ 8.63 (d, J = 6.0 Hz, 2H), 8.14
(d, J = 4.0 Hz, 1H), 8.01 (d, J = 7.5 Hz, 1H), 7.92 – 7.85 (m, 2H), 7.75 (d, J = 7.6 Hz, 1H), 7.66 (t, J = 5.3 Hz, 3H), 7.32 –
7.27 (m, 3H), 7.23 – 7.17 (m, 5H), 7.08 (dd, J = 14.3, 7.0 Hz, 2H). TOF-HRMS calcd. For C29H20N4S [M+] 456.1414, found
456.1409.
N,N-diphenyl-4-(5-(7-(pyridin-4-yl)benzo[c][1,2,5]thiadiazol-4-yl)thiophen-2-yl)aniline(WL303Q). WL303Q 1H NMR
(400 MHz, CDCl3) δ 8.77 (d, J = 5.3 Hz, 2H), 8.18 (d, J = 3.9 Hz, 1H), 7.96 (dd, J = 6.9, 3.1 Hz, 3H), 7.85 (d, J = 7.5 Hz, 1H),
7.62 – 7.51 (m, 2H), 7.34 (d, J = 3.9 Hz, 1H), 7.32 – 7.23 (m, 5H), 7.14 (d, J = 7.6 Hz, 4H), 7.13 – 7.04 (m, 4H). TOF-HRMS
calcd. For C33H22N4S2 [M+] 538.1290, found 538.1286.
4-((2-ethylhexyl)oxy)-N-(4-((2-ethylpentyl)oxy)phenyl)-N-(4-(7-(pyridin-4-yl)benzo[c][1,2,5]thiadiazol-4-yl)phenyl)ani
line(WL306Q). WL306Q 1H NMR (400 MHz, CDCl3) δ 8.78 (d, J = 4.5 Hz, 2H), 8.05 (d, J = 5.2 Hz, 2H), 7.91 (d, J = 7.4 Hz,
1H), 7.84 (d, J = 8.6 Hz, 2H), 7.77 (d, J = 7.5 Hz, 1H), 7.14 (d, J = 8.7 Hz, 4H), 7.06 (d, J = 8.6 Hz, 2H), 6.87 (d, J = 8.7 Hz,
4H), 3.79 (t, J = 30.7 Hz, 4H), 1.54 – 1.38 (m, 2H), 1.36 – 1.30 (m, 16H), 0.98 – 0.88 (m, 12H). TOF-HRMS calcd. For
C45H52N4O2S [M+] 712.3807, found 712.3811.
4-((2-ethylhexyl)oxy)-N-(4-((2-ethylhexyl)oxy)phenyl)-N-(4-(5-(7-(pyridin-4-yl)benzo[c][1,2,5]thiadiazol-4-yl)thiophe
n-2-yl)phenyl)aniline(WL307Q). WL307Q 1H NMR (400 MHz, CDCl3) δ 8.72 (t, J = 25.8 Hz, 2H), 8.17 (d, J = 3.9 Hz, 1H),
7.97 (d, J = 5.8 Hz, 2H), 7.93 (d, J = 7.6 Hz, 1H), 7.84 (d, J = 7.6 Hz, 1H), 7.50 (d, J = 8.6 Hz, 2H), 7.29 (d, J = 3.9 Hz, 1H),
7.09 (d, J = 8.8 Hz, 4H), 6.95 (d, J = 6.3 Hz, 2H), 6.85 (d, J = 8.8 Hz, 4H), 3.84 (t, J = 7.7 Hz, 4H), 1.72 (dd, J = 12.1, 6.0 Hz,
2H), 1.55 – 1.40 (m, 7H), 1.40 – 1.27 (m, 9H), 1.01 – 0.85 (m, 12H). TOF-HRMS calcd. For C49H54N4O2S2 [M+] 794.3721,
found 794.3688.
4-(7-(4-(diphenylamino)phenyl)benzo[c][1,2,5]thiadiazol-4-yl)pyridine 1-oxide (WL301). WL301Q was oxidized as
step g to get WL301 as a red solid. WL301 1H NMR (400 MHz, Acetone) δ 8.37 (d, J = 6.9 Hz, 2H), 8.30 (d, J = 7.0 Hz, 2H),
8.18 (d, J = 7.6 Hz, 1H), 8.05 (d, J = 8.6 Hz, 2H), 7.99 (d, J = 7.5 Hz, 1H), 7.38 (t, J = 7.8 Hz, 4H), 7.18 (dd, J = 8.4, 2.7 Hz,
5H), 7.14 (t, J = 7.7 Hz, 2H). TOF-HRMS calcd. For C29H20N4So [M+H+] 473.1425, found 473.1436.
4-(7-(5-(4-(diphenylamino)phenyl)thiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)pyridine 1-oxide(WL303). WL303 1H
NMR (400 MHz, CDCl3) δ 8.39 (d, J = 6.7 Hz, 2H), 8.20 (d, J = 3.9 Hz, 1H), 8.15 (d, J = 6.7 Hz, 2H), 7.95 (d, J = 7.6 Hz, 1H),
7.85 (d, J = 7.7 Hz, 1H), 7.61 – 7.54 (m, 2H), 7.35 (d, J = 3.9 Hz, 1H), 7.33 – 7.27 (m, 4H), 7.18 – 7.12 (m, 4H), 7.12 – 7.05
(m, 4H). TOF-HRMS calcd. For C33H22N4OS2 [M+] 554.1214, found 554.1235.
4-(7-(4-((4-((2-ethylhexyl)oxy)phenyl)(4-((2-ethylpentyl)oxy)phenyl)amino)phenyl)benzo[c][1,2,5]thiadiazol-4-yl)pyri
dine 1-oxide(WL306). WL306 1H NMR (400 MHz, CDCl3) δ 8.39 (s, 2H), 8.16 (d, J = 5.4 Hz, 2H), 7.86 (dd, J = 16.8, 7.7
Hz, 3H), 7.76 (d, J = 7.2 Hz, 1H), 7.14 (d, J = 8.0 Hz, 4H), 7.05 (d, J = 7.9 Hz, 2H), 6.87 (d, J = 8.6 Hz, 4H), 3.84 (d, J = 4.9
Hz, 4H), 1.57 – 1.34 (m, 2H), 1.33 (dd, J = 8.8, 5.3 Hz, 16H), 0.97 – 0.88 (m, 12H). TOF-HRMS calcd. For C45H52N4O3S
[M+] 728.3807, found 728.3760.
4-(7-(5-(4-(bis(4-((2-ethylhexyl)oxy)phenyl)amino)phenyl)thiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)pyridine
1-oxide(WL307). WL307 1H NMR (400 MHz, CDCl3) δ 8.39 (d, J = 6.8 Hz, 2H), 8.18 (d, J = 3.9 Hz, 1H), 8.15 (d, J = 6.8 Hz,
2H), 7.92 (d, J = 7.6 Hz, 1H), 7.84 (d, J = 7.6 Hz, 1H), 7.49 (d, J = 8.8 Hz, 2H), 7.30 (d, J = 3.9 Hz, 1H), 7.08 (m, 6H), 6.93
(s, 4H), 3.85 (d, J = 8.1 Hz, 4H), 1.72 (dq, J = 12.2, 6.1 Hz, 2H), 1.55 – 1.37 (m, 7H), 1.33 (dd, J = 8.9, 5.2 Hz, 9H), 0.93 (dt,
J = 9.4, 7.2 Hz, 12H). TOF-HRMS calcd. For C49H54N4O3S2 [M+] 810.3674, found 810.3637.

Table S1 Photophysical and electrochemical properties of WL30X dyes.
Dye
Optimized Structure
HOMO
LUMO
WL301
WL302
WL303
WL306
WL307
Figure S1 DPV curves of WL30X dyes.

Figure S2 The IPCE spectra of DSSCs based on WL30X dyes.
Figure S3 FTIR spectra of WL30X dye powder and dye on the TiO2.