Influence of the donor size in panchromatic D-π-A-π-A dyes bearing 5-phenyl-5H-dibenzo-[b,f]azepine units for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2016.01.004

Two D-π-A-π-A organic dyes (YC-1 and YC-2) with 5-phenyl-5H-dibenzo[b,f]azepine derivatives as donor, thiophene as π bridge, and isoindigo and cyanoacrylic acid as acceptors were prepared. YC-1 and YC-2 show a panchromatic absorption between 300 nm and 800 nm both in solution and neat film. The photovoltaic performances of both dyes were evaluated in dye-sensitized solar cells based on iodide/triiodide electrolyte without any co-sensitizer. The YC-1 based device displays better device performance with open-circuit photocurrent density of 12.12 mA cm^-2, open-circuit voltage of 0.53 V, and fill factor of 68.9%, corresponding to overall conversion efficiency (η) of 4.38%. The inferior performance of device based on YC-2 (η = 1.46%) is ascribed to short electron lifetime as evidenced from electrochemical impedance spectroscopy measurement. This research provided a potential promising donor unit for organic dyes and revealed the influence of donor size in organic dyes for photovoltaic performances.

Full text of DOI:10.1016/j.dyepig.2016.01.004

Accepted Manuscript
Influence of the Donor Size in Panchromatic D-π-A-π-A Dyes Bearing 5-Phenyl-5H-
dibenzo-[b,f]azepine Units for Dye-Sensitized Solar Cells
Yafei Wang, Chuncheng Yang, Jue Chen, Hongrui Qi, Jianli Hua, Yu Liu, Etienne
Baranoff, Hua Tan, Jiang Fan, Weiguo Zhu
PII:
S0143-7208(16)00006-1
10.1016/j.dyepig.2016.01.004
DYPI 5056
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 18 August 2015
Revised Date: 3 November 2015
Accepted Date: 4 January 2016
Please cite this article as: Wang Y, Yang C, Chen J, Qi H, Hua J, Liu Y, Baranoff E, Tan H, Fan
J, Zhu W, Influence of the Donor Size in Panchromatic D-π-A-π-A Dyes Bearing 5-Phenyl-5H-
dibenzo-[b,f]azepine Units for Dye-Sensitized Solar Cells, Dyes and Pigments (2016), doi: 10.1016/
j.dyepig.2016.01.004.
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ACCEPTED MANUSCRIPT
Influence of the Donor Size in
Panchromatic D-π-A-π-A Dyes Bearing
5
-Phenyl-5H-dibenzo-[b,f]azepine Units for
Dye-Sensitized Solar Cells
*
Yafei Wang†ǁ , Chuncheng Yang†&, Jue Chen‡&, Hongrui Qi†, Jianli Hua‡, Yu Liu†,
∆
*
*
Etienne Baranoff , Hua Tan†, Jiang Fan†, Weiguo Zhu†
†
College of Chemistry, Key Lab of Environment-Friendly Chemistry and Application of the Ministry of Education,
Xiangtan University, Xiangtan 411105, China
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and
‡
Technology, 130 Meilong Road, Shanghai 200237, PR China
∆
School of Chemistry, University of Birmingham, B15 2TT UK
ǁInstitute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and
Devices, South China University of Technology, Guangzhou 510640, China
&
The authors contributed equally to this work.
*
To whom correspondence should be addressed
E. Baranoff) e.baranoff@bham.ac.uk
W. Zhu) zhuwg18@126.com
(
(
(Y. Wang) wangyf_miracle@hotmail.com
1

ACCEPTED MANUSCRIPT
Abstract
Two
D-π-A-π-A
organic
dyes
(YC-1
and
YC-2)
with
5
-phenyl-5H-dibenzo[b,f]azepine derivatives as donor, thiophene as π bridge, and
isoindigo and cyanoacrylic acid as acceptors were prepared. YC-1 and YC-2 show a
panchromatic absorption between 300 nm and 800 nm both in solution and neat film.
The photovoltaic performances of both dyes were evaluated in dye-sensitized solar
cells based on iodide/triiodide electrolyte without any co-sensitizer. The YC-1 based
device displays better device performance with open-circuit photocurrent density of
-2
1
2.12 mA cm , open-circuit voltage of 0.53 V, and fill factor of 68.9 %,
corresponding to overall conversion efficiency (ƞ) of 4.38 %. The inferior
performance of device based on YC-2 (ƞ = 1.46 %) is ascribed to short electron life
time as evidenced from electrochemical impedance spectroscopy measurement. This
research provided a potential promising donor unit for organic dyes and revealed the
influence of donor size in organic dyes for photovoltaic performances.
Keywords: 5-phenyl-5H-dibenzo[b,f]azepine; Isoindigo; D-π-A-π-A framework;
Photovoltaic performance; Dye-sensitizer solar cells;
2

ACCEPTED MANUSCRIPT
1
. Introduction
Dye-sensitized solar cells (DSSCs) have emerged as one promising candidate for
renewable and green energy owing to their low cost fabrication procedure and
attractive power conversion efficiency (ƞ) [1,2]. To date, a great deal of sensitizing
dyes, including metal complexes [3-8] and organic molecules [9-12], has been
exploited to achieve high ƞ. As such, several DSSCs show ƞ > 10% under AM1.5
–
2
simulated solar light (100 mW cm ) [13-17]. Compared to ruthenium-based
sensitizers, organic dyes are attractive because of their high molar extinction
coefficients, structural variety and possibly low cost due to the absence of platinoid
ion and exciting progresses have been made recently with these materials [18,19].
Generally, effective organic dyes for high performance of DSSCs are constructed
around a donor-π-bridge-acceptor (D-π-A) framework resulting in effective
intramolecular charge transfer [20-24]. Recently, the concept has been extended to
D-A-π-A [24-29] architecture, in which the additional acceptor unit is favorable to the
photophysical properties of the dyes, enhancing the photovoltaic performances and
the photostability of dyes. Among these DSSCs dyes, considerable endeavors have
been made to design new-type donor moieties, such as triphenylamine [10,19,25],
carbazole [23,24], phorphyrin [26], indoline [30] and phthalocyanine derivatives
[
31,32]. Triarylamine derivatives are the one of most effective dyes in DSSCs owing
to their good electron-donating character and generally reversible oxidation process
33]. Conversion efficiencies in the range of 9-10 % have been reported with
[
arylamine-based organic dyes [33]. For example, a D-A-π-A dye (WS-9) using
3

ACCEPTED MANUSCRIPT
indoline derivatives as the donor, cyanoacrylic acid as the acceptor, n-hexylthiophene
unit as the conjugated spacer and the benzothiadiazole unit as the additional acceptor
unit achieved ƞ = 9.04% with iodine electrolyte [34]. Hua and coworkers designed an
D-A-π-A motif based on arylamine organic dyes utilizing quinoxaline unit as the
3
+/2+
additional acceptor moiety (YA422), resulting in a ƞ = 10.65% with [Co(bpy)3]
electrolyte [35]. However, arylamine-based organic dyes remain some challenges
including lack of absorption in the near-infrared region and dye aggregation on the
TiO film. Therefore, further red-shifted absorption spectra can be obtained by
2
grafting additional conjugated spacer between the donor and acceptor units to form a
D-π-A-π-A framework owing to the expended conjugation. Lin and his coworkers
demonstrated the D-π-A-π-A motif possessed broad absorption spectra and efficient
light-to-electricity conversions [36]. Even so, the research of structure-property
relationship of this D-π-A-π-A motif is still overlooked.
Encouraged by the successful application of triphenylamine (TPA) to DSSCs,
herein we propose the use of 5-phenyl-5H-dibenzo[b,f]azepine, PDBAz, as a novel
donor group of organic dye for DSSC. Because of some structural similarity with TPA,
it was expected that PDBAz would still provide advantageous properties of TPA, such
as good hole transporting ability and sterically-hindered structure limiting dye
aggregations [37-39]. Furthermore, the additional conjugation in PDBAz would result
in red-shifted absorption, thus leading to an expected enhancement of Jsc.
We have prepared two dyes based on the D-π-A-π-A architecture with isoindigo
and cyanoacrylic acid as acceptor groups and thiophene as π-linker. 2-Ethylhexyl
4

ACCEPTED MANUSCRIPT
chains were grafted onto the isoindigo unit to ensure solubility and disrupt undesirable
dye aggregation. To study the effect of molecular structure on photovoltaic
performance, two donor groups were used: dye YC-1 has PDBAz while dye YC-2
uses bis-PDBA-aniline, a much bulkier donor moiety (Chart 1). Both dyes show
panchromatic absorption from 300 nm to 800 nm in solution and neat film. To study
the effect of the donor size on photovoltaic performance, YC-1 and YC-2 were used
as sensitizer for dye-sensitized solar cells with an iodide-based electrolyte. The device
using YC-1 as the sensitizer exhibited better photovoltaic performance (ƞ = 4.38%
–2
with J = 12.12 mA cm ) than the cell using YC-2 ( ƞ = 1.46% with J = 4.84 mA
sc
sc
-2
cm ) due to major difference in J . Compared to the analogous TPA-based dye (ID1:
SC
-2
3
.52 %, 9.89 mA cm ) [38], YC-1 shows enhanced performance in the same
condition. Density functional theory (DFT) calculations and electrochemical
impedance spectroscopy (EIS) have been used to understand the structure-property
relationship of both YC-1 and YC-2.
2
. Experimental section
2
.1. Materials
Transparent conducting oxide (TCO, 15 ꢀ /square, F-doped SnO from Geao Science
2
and Educational Co. Ltd.) was used as the substrate for the TiO thin-film electrode.
2
Methoxypropionitrile (MPN) was purchased from Aldrich. Tetra-n-butylammonium
hexafluorophosphate (TBAPF ) and lithium iodide were bought from Fluka. Iodine
6
(99.999%) was purchased from Alfa Aesar. Intermediate 7 has been reported in the
5

ACCEPTED MANUSCRIPT
previous literature [40]. The starting material of 10-methoxy-5H-dibenzo[b,f]azepine
is purchased from Energy Chemical. All other solvents and chemicals used in this
work were of reagent grade and used without further purification.
2
.2. Characterization
1
13
H NMR and C NMR spectra were obtained with a Brucker AM 400 spectrometer.
Mass spectra (MS) were recorded on a Bruker Autoflex MALDITOF instrument using
dithranol as a matrix. The UV-vis absorption and photoluminescence spectra were
measured with a Varian Cray 50. Cyclic voltammograms were performed with a
Versastat II electrochemical workstation (Princeton applied research) using a normal
three-electrode cell with a Pt working electrode, a Pt wire counter electrode, and
Ag/AgCl reference electrode. The photovoltaic characterization was performed on the
setup that constitutes a 450 W xenon lamp (Oriel), a Schott K113 Tempax sunlight
filter (PräzisionsGlas & Optik GmbH), and a source meter (Keithley 2400) which
applies potential bias and measures the photogenerated current. IPCE was obtained
using a SR830 lock-in amplifier, a 300 W xenon lamp (ILC Technology) and a
Gemini-180 double monochromator (Jobin-Yvon Ltd.). The electrochemical
impedance spectroscopy measurements of all the DSSCs were performed using a
Zahner IM6e Impedance Analyzer (ZAHNER-Elektrik GmbH & CoKG, Kronach,
Germany). The frequency range is 0.1 Hz-100 kHz. The applied voltage bias is from
-0.60 V to -0.85 V. The magnitude of the alternating signal is 5 mV. Intensity
modulated photovoltage spectroscopy was obtained by the Zahner IM6e Impedance
Analyzer (ZAHNER-Elektrik GmbH & CoKG, Kronach, Germany) and a
6

ACCEPTED MANUSCRIPT
light-emitting display array (λ = 457 nm, blue light). The frequency range is 0.1
Hz-100 kHz.
2
.3. Fabrication of DSSCs
Four layers of Dyesol 90-T TiO paste and a scattering layer were screen-printed onto
2
o
the FTO glass. The photoanodes was sintered gradually up to 500 C and kept at this
temperature before cooling. The photoanodes were immersed into 40 mM TiCl4
o
o
aqueous solution at 70 C for 30 min and sintered at 450 C for 30 min and cooled
o
-4
down to 80 C. Then the photoanodes were placed into 3×10 M dye bath in DCM
solution for 6 h. The dye-sensitized photoanodes were sealed with platinized counter
electrodes by a hot-melt film (25-ꢁm-thick Surlyn, Dupont). The electrolytes were
introduced to the cells via two pre-drilled holes in the counter electrodes. The
electrolyte consists of 0.05 M I , 0.05 M LiI, 0.5 M BMII, 0.1 M DMPII and 0.1 M
2
2
GuSCN in acetonitrile. The active area of all DSSCs is 0.12 cm .
2
5
.4. Synthesis
-(4-bromophenyl)-10-methoxy-5H-dibenzo[b,f]azepine (1)
To a solution of 10-methoxy-5H-dibenzo[b,f]azepine (223 mg, 1.0 mmol), sodium
tert-butoxide (t-BuONa) (288 mg, 3.0 mmol) and tris(dibenzylideneacetone)
-
dipalladium (Pd (dba) ) (11.6 mg, 0.01 mmol) in dry toluene (15 mL) was added
2 3
tri-tert-butylphosphine (P(t-Bu) ) (0.03 M in toluene, 1 mL, 0.03 mmol) and
3
1
8
-bromo-4-iodobenzene (390 mg, 1.3 mmol). The resulting mixture was refluxed for
h under nitrogen atmosphere. Then the reaction mixture was let to cool down to
room temperature and quenched with saturated aqueous NaHCO (20 mL). The
3
7

ACCEPTED MANUSCRIPT
organic layer was separated and the aqueous layer was extracted with
dichloromethane (DCM) (3 × 25 mL). The combined organic layers were washed with
water, dried over anhydrous MgSO , and then concentrated under vacuum to give the
4
crude product 1, which was purified by column chromatography on silica gel
1
(
PE/DCM, 5:1) to afford the pure compound 1 as a white solid (192 mg, 50%). H
NMR (400 MHz, CDCl , TMS), δ(ppm): 7.82 (d, J = 7.6 Hz, 1H), 7.53 (d, J = 7.2 Hz,
3
1
H), 7.49–7.34 (m, 5H), 7.28 (d, J = 12.5 Hz, 1H), 7.08 (d, J = 8.4 Hz, 2H), 6.25 (d, J
1
3
=
8.4 Hz, 2H), 6.04 (s, 1H), 3.80 (s, 3H). CNMR (100 MHz, CDCl ) δ: 156.15,
3
1
1
1
47.56, 142.80, 141.12, 135.90, 134.25, 131.26, 130.92, 130.17, 129.64, 129.61,
28.49, 127.70, 127.36, 127.18, 113.48, 109.77, 102.19, 55.42.
0-methoxy-5-(4-(thiophen-2-yl)phenyl)-5H-dibenzo[b,f]azepine (2)
To a mixture of 1 (500 mg, 1.33 mmol) and 2-(tributyltin)thiophene (740 mg, 2.0
mmol) in toluene (25 mL) was added Pd(PPh ) (57 mg, 0.05 mmol) and then
3
4
refluxed for 12 h under nitrogen atmosphere. After cooled down to room temperature
RT) and quenched with water, the mixture was extracted with DCM (3 × 30 mL).
(
The combine organic layers were washed with water and dried with anhydrous
MgSO . After filtration, the solvent was removed under vacuum and the residue was
4
purified by silica gel column chromatography with (PE/DCM, 6/1) as eluent to obtain
1
2
as a white solid (100 mg, 19%). H NMR (400 MHz, CDCl , TMS), δ(ppm): 7.82 (d,
3
J = 5.6 Hz, 1H), 7.66–7.19 (m, 9H), 7.18–6.91 (m, 3H), 6.41 (d, J = 5.9 Hz, 2H), 6.06
1
3
(
s, 1H), 3.80 (s, 3H). CNMR (100 MHz, CDCl ) δ: 156.30, 148.06, 145.18, 143.10,
3
1
36.10, 134.49, 130.90, 130.22, 129.78, 129.74, 128.53, 127.91, 127.69, 127.31,
8

ACCEPTED MANUSCRIPT
1
27.16, 126.62, 124.61, 123.00, 112.16, 102.41, 68.10, 55.43.
1
0-methoxy-5-(4-(5-(trimethylstannyl)thiophen-2-yl)phenyl)-5H-dibenzo[b,f]azep
ine (3)
2
0
.5 M n-BuLi (0.3 mL, 0.75 mmol) was added dropwise to a solution of 2 (98 mg,
.26 mmol) in dry THF (15 mL) at –78 °C under nitrogen. After stirring for 2 h,
trimethylchlorotin (0.5 mL, 1.9 mmol) was added. The reaction mixture was stirred
for additional 30 min and then gradually warmed to RT and stirred for 12 h. The
reaction was quenched with water (30 mL) and the resulting mixture was extracted
with DCM (3 × 25 mL). The organic layers were combined and washed with water
and dried over anhydrous MgSO . After filtration, the solvent was removed under
4
1
vacuum to give product 3 (60 mg, 32%). H NMR (400 MHz, CDCl , TMS), δ(ppm):
3
7
1
0
4
.81 (d, J = 7.8 Hz, 1H), 7.50 (dd, J = 21.9, 3H), 7.39 (d, J = 13.7 Hz, 5H), 7.17 (s,
H), 7.07 (t, J = 19.0 Hz, 2H), 6.40 (d, J = 8.6 Hz, 2H), 6.06 (s, 1H), 3.79 (s, 3H),
.46-0.20 (m, 9H).
-(thiophen-2-yl)aniline (4)
Pd(PPh ) (134 mg, 0.116 mmol) was added to a solution of 4-bromoanilines (2 g,
3
4
1
1.6 mmol) and 2-(tributyltin)thiophene (8.5 g, 22.7 mmol) in 50 mL toluene and the
mixture was heated at 110°C for 12 h under nitrogen atmosphere. After cooling to RT,
the mixture was poured into water (50 mL) and the organic layer separated. The
aqueous layer was extracted with DCM (3 × 25 mL). The combined organic layers
were washed with water and dried over anhydrous MgSO . The volatiles were
4
removed under vacuum to give crude product 4, which was purified by column
9

ACCEPTED MANUSCRIPT
chromatography on silica gel with (PE/DCM, 4/1) as eluent to obtain 4 as a light
1
yellow solid (1.8 g, 89%). H NMR (400 MHz, CDCl , TMS), δ(ppm): 7.42 (d, J =
3
6
.6 Hz, 2H), 7.15 (s, 2H), 7.02 (s, 1H), 6.69 (d, J = 6.7 Hz, 2H), 3.73 (s, 2H).
4
-(10-methoxy-5H-dibenzo[b,f]azepin-5-yl)-N-(4-(10-methoxy-5H-dibenzo[b,f]az
epin-5-yl)phenyl)-N-(4-(thiophen-2-yl)phenyl)aniline (5)
To a solution of compound 4 (0.35 g, 2 mmol), t-BuONa (1 g, 8.0 mmol) and
Pd (dba) (110 mg, 0.12 mmol) in dry toluene (50 mL) was added P(t-Bu) (1 M in
2
3
3
toluene, 0.3 mL) and 1 (2.26 g, 6.0 mmol). The resulting mixture was refluxed for 8 h
in nitrogen atmosphere. After cooling down to RT, the reaction mixture was quenched
with saturated aqueous NaHCO (20 mL). The organic layer was separated and the
3
aqueous layer was extracted with DCM (3 × 35 mL). The combine organic layers
were washed with water and dried over anhydrous MgSO . The volatiles were
4
removed under vacuum to give the crude product 5, which was purified by column
chromatography on silica gel with (PE/DCM, 5/1) as eluent to obtain 5 as a light
yellow solid (1.35 g, 87%). 1HNMR (400 MHz, CDCl , TMS), δ(ppm): 7.66 (d, J =
3
5
5
.5 Hz, 2H), 7.51 (s, 2H), 7.47 – 7.20 (m, 13H), 7.13 (s, 2H), 6.95 (s, 2H), 6.68 (d, J =
1
3
.5 Hz, 4H), 6.50 (d, J = 5.1 Hz, 2H), 6.27 – 5.96 (m, 6H), 3.69 (s, 6H). CNMR (100
MHz, DMSO) δ: 155.83, 148.70, 145.06, 144.39, 142.96, 141.24, 137.57, 136.07,
1
1
34.35, 131.43, 130.63, 130.09, 129.75, 128.66, 128.49, 128.08, 127.62, 127.42,
27.16, 126.55, 124.70, 124.13, 121.93, 118.08, 112.48, 102.96, 55.79. MALDI-MS
calcd for C H N O S [M]+, 769.281; found, 769.441.
5
3
40
2
2
1
0

ACCEPTED MANUSCRIPT
4
-(10-methoxy-5H-dibenzo[b,f]azepin-5-yl)-N-(4-(10-methoxy-5H-dibenzo[b,f]az
epin-5-yl)phenyl)-N-(4-(5-(trimethylstannyl)thiophen-2-yl)phenyl)aniline (6)
2
.5 M n-BuLi (0.3 mL, 0.75 mmol) was added dropwise to a solution of compound 5
o
(
551 mg, 1 mmol) in dry THF (15 mL) at –78 C under nitrogen. After stirring for 1 h
o
at –78 C, trimethyl tin (0.47 mL, 1.7 mmol) was added. The reaction mixture was
stirred at –78 °C for additional 30 min and then gradually warmed to room
temperature and further stirred for 12 h. Then the reaction was quenched by adding
water (30 mL) and the resulting mixture was extracted with DCM (3 × 25 mL). The
organic layers were combined, washed with water and dried over anhydrous MgSO4.
The volatiles were removed under vacuum to give compound 6 as a yellow oil (320
1
mg) in 45%. HNMR (400 MHz, CDCl , TMS), δ(ppm): 7.77 (s, 2H), 7.47 (br, 6H),
3
7
.34 (br, 8H), 7.08 (s, 2H), 6.77 (br, 6H), 6.29 (br, 4H), 6.06 (br, 2H), 3.78 (d, J = 25.4
Hz, 6H), 0.51-0.20 (m, 9H).
Synthesis of 8
A mixture of compound of 7 (1.28 g, 2 mmol), Pd (dba) (73 mg, 0.08 mmol),
2
3
P(o-tyl) (50 mg, 0.16 mmol), and Na CO (2.76 g, 20 mmol) in 60 mL of THF was
3
2
3
heated to 50 °C. After stirring for 30 min, a solution of 5-formylthiophen-2-yl boronic
acid (0.31 g, 2 mmol) in THF (10 mL) was injected to the mixture. The mixture was
then heated at 80 °C for 10 h. After cooling to RT and quenched with water, the
resulting solution was extracted with DCM (3 × 30 mL). The combined organic layers
were washed with water and dried over anhydrous MgSO . The volatiles were
4
removed under vacuum and the residue was purified by column chromatography
11

ACCEPTED MANUSCRIPT
silica gel with PE/DCM (V/V, 5/2) as eluent to obtain 8 as a purple solid (750 mg,
1
5
1
5%). HNMR (400 MHz, CDCl , TMS), δ(ppm): 9.96 (s, 1H), 9.23 (d, J = 8.3 Hz,
3
H), 9.08 (d, J = 8.6 Hz, 1H), 7.77 (s, 1H), 7.50 (s, 1H), 7.37 (d, J = 9.2 Hz, 1H), 7.16
(
d, J = 14.7 Hz, 1H), 7.04 (s, 1H), 6.92 (s, 1H), 3.69 (dd, J = 31.1, 9.4 Hz, 4H), 1.86
s, 2H), 1.33 (dd, J = 29.4, 18.9 Hz, 16H), 1.08-0.70 (m, 12H).
(
Synthesis of 9
Compound 3 (150 mg, 0.275 mmol), compound 8 (200 mg, 0.3 mmol), Pd(PPh ) (13
3
4
mg, 0.011 mmol) and 15 mL toluene were mixed together in a 25 mL flask and the
mixture was heated at 110 °C for 12 h. The mixture was then cooled down to room
temperature and extracted with DCM (3×10 mL). The combined organic layers were
washed with water and dried over anhydrous MgSO . The volatiles were removed
4
under vacuum and the residue was purified by column chromatography on silica gel
1
with PE/EA (V/V, 7/1) as eluent to obtain 9 as a black solid (120 mg, 44%). HNMR
(
400 MHz, CDCl , TMS), δ(ppm): 9.92 (s, 1H), 9.17 (dd, J = 23.9, 8.4 Hz, 2H), 7.84
3
(d, J = 7.9 Hz, 1H), 7.76 (d, J = 3.6 Hz, 1H), 7.61 – 7.30 (m, 15H), 7.10 (d, J = 3.3 Hz,
1
H), 7.03 (s, 1H), 6.95 (s, 1H), 6.44 (d, J = 8.6 Hz, 2H), 6.07 (s, 1H), 3.81 (s, 3H),
.71 (d, J = 15.6 Hz, 4H), 1.88 (s, 2H), 1.47-1.04 (m, 16H), 1.03-0.59 (m, 12H).
3
1
3
CNMR (100 MHz, CDCl ) δ: 182.38, 168.39, 155.94, 153.58, 148.44, 146.16,
3
1
1
1
1
45.71, 145.21, 141.29, 140.57, 138.36, 134.32, 133.34, 130.49, 130.13, 129.51,
28.46, 127.64, 126.37, 123.91, 122.93, 122.23, 122.21, 120.22, 119.50, 118.51,
18.07, 112.14, 105.30, 104.42, 102.14, 55.42, 44.13, 37.80, 30.86, 28.86, 23.09,
+
4.13, 14.09, 10.84. MALDI-MS calcd for C H N O S [M] , 976.426; found,
6
2
63
3
4 2
1
2

ACCEPTED MANUSCRIPT
9
76.549.
Synthesis of 10
Compound 6 (160 mg, 0.208 mmol), compound 8 (162 mg, 0.25 mmol), Pd(PPh3)4
12 mg, 0.01 mmol) and 15 mL toluene were mixed together in a 25 mL flask and the
(
mixture was heated at 110 °C for 12 h. The mixture was then cooled down to room
temperature and extracted with DCM (3×10 mL). The combined organic layers were
washed with water and dried over anhydrous MgSO . The volatiles were removed
4
under vacuum and the residue was purified by column chromatography on silica gel
1
with PE/EA (V/V, 7/1) as eluent to obtain 10 as a black solid (110 mg, 38%). H
NMR (400 MHz, CDCl ) δ(ppm): 9.91 (s, 1H), 9.20 (d, J = 8.4 Hz, 1H), 9.14 (d, J =
3
8
.2 Hz, 1H), 7.77 (br, 4H), 7.72 (s, 2H), 7.63–7.29 (m, 24H), 6.84 (dd, J = 120.7, 49.3
Hz, 9H), 6.32 (s, 5H), 6.07 (s, 2H), 3.82 (s, 6H), 3.73 (s, 4H), 1.35 (dd, J = 41.3, 20.1
13
Hz, 18H), 1.03–0.73 (m, 12H). CNMR (100 MHz, CDCl3) δ: 182.69, 168.63,
168.43, 156.23, 153.61, 149.35, 149.22, 149.12, 146.47, 146.05, 145.54, 145.17,
143.45, 142.93, 141.72, 140.96, 138.60, 137.66, 136.05, 135.62, 134.47, 133.38,
130.69, 130.48, 130.05, 129.79, 129.71, 128.32, 127.52, 127.00, 126.85, 126.58,
126.20, 124.80, 124.28, 122.98, 120.46, 118.46, 112.58, 104.44, 102.59, 55.43, 43.95,
37.81, 30.86, 28.88, 28.83, 24.31, 23.09, 14.15, 14.10, 10.86, 10.84.MALDI-MS
calcd for C H N O S [M]+, 1364.584; found, 1364.832.
8
9
83
5
5 2
Synthesis of YC-1
Compound 9 (200 mg, 0.19 mmol), 2-cyanoacetic acid (200 mg, 2.2 mmol),
ammonium acetate (320 mg) in acetic acid (30 mL) were heated to 120 °C under
1
3

ACCEPTED MANUSCRIPT
nitrogen atmosphere for 24 h. After cooling down to room temperature, the mixture
was poured into water. The precipitate was filtered off and purified by column
chromatography on silica gel using DCM/CH OH (15/1,V/V) as eluent to provide
3
1
YC-1 as a dark solid (80 mg, 38%). HNMR (400 MHz, DMSO-d6, TMS), δ(ppm):
9
2
6
1
.14–8.90 (m, 2H), 8.21 (s, 1H), 8.05 (s, 1H), 7.76 (d, J = 7.4 Hz, 2H), 7.62 (d, J =
4.3 Hz, 4H), 7.54–7.22 (m, 8H), 7.16 (d, J = 8.8 Hz, 2H), 7.05 (s, 1H), 6.93 (s, 1H),
.27 (d, J = 7.9 Hz, 1H), 6.19 (s, 1H), 3.75 (s, 3H), 3.62 (s, 4H), 1.76 (s, 2H),
.10–1.31 (m, 20H), 0.85 (d, J = 23.8 Hz, 8H). MALDI-MS calcd for C H N O S
5 2
6
5
64
6
+
[
M] , 1044.432; found, 1044.519.
Synthesis of YC-2
Compound 10 (150 mg, 0.11 mmol), 2-cyanoacetic acid (183 mg, 2.0 mmol),
ammonium acetate (290 mg) in acetic acid (25 mL) were heated to 120 °C under
nitrogen atmosphere for 24 h. After cooling to room temperature, the mixture was
poured into water. The precipitate was filtered off and purified by column
chromatography on silica gel using DCM/CH OH (15/1,V/V) as eluent to provide
3
1
YC-2 as a dark solid (63 mg, 40%). HNMR (400 MHz, DMSO-d6, TMS), δ (ppm):
8
7
6
1
.99 (s, 2H), 8.23 (s, 1H), 8.09 (s, 1H), 7.70 (dd, J = 22.6, 7.4 Hz, 2H), 7.53 (s, 1H),
.48 – 7.18 (m, 14H), 7.14 (s, 1H), 6.98 (s, 1H), 6.88 (s, 1H), 6.69 (d, J = 7.1 Hz, 4H),
.55 (d, J = 7.0 Hz, 1H), 6.27–6.09 (m, 5H), 3.72 (s, 6H), 3.60 (s, 4H), 1.75 (s, 2H),
.19 (d, J = 75.3 Hz, 20H), 0.96–0.63 (m, 8H). MALDI-MS calcd for C H N O S
9
2
84
6
6 2
+
[
M] , 1431.589; found, 1431.755.
1
4

ACCEPTED MANUSCRIPT
. Results and discussion
3
3
.1. Synthesis and characterized
The synthetic routes of the two dyes YC-1 and YC-2 are described in Scheme 1.
Intermediates 1 and 5 were prepared by the Buchwald-Hartwig amination via the
palladium-catalyzed cross-coupling of amines (10-Methoxy iminostilbene or 4) with
aryl halides (4-bromoiodobenzene or 1). Typical Stille coupling reactions between
aryl bromide (1 and 4-bromoaniline) and tributyl(thiophen-2-yl)stannane were carried
out to afford compound 2 in moderate yield (19%) and compound 4 in high yield
(
89%). Intermediates 2 and 5 were reacted with Sn(CH ) Cl to give compound 3 and
3 3
6
in yield of 32% and 45%, respectively. Compound 8 was obtained via Suzuki
coupling reaction between intermediate 7 and 5-formylthiophen-2-ylboronic acid in a
yield of 55%. The key aldehyde precursors of 9 and 10 were synthesized through
Stille coupling of 3 (or 6) and 8 in 44% and 38% yields, respectively. Finally,
Knoevenagel condensation of these aldehydes with cyanoacetic acid gave the dyes
1
YC-1 and YC-2. Both target dyes were characterised by H NMR and TOF-Mass.
3
.2. Photophysical properties
The UV-vis absorption spectra of YC-1 and YC-2 are shown in Figure 1 and the
corresponding data are listed in Table 1. Panchromatic absorption with two typical
bands is observed for YC-1 and YC-2 both in solution and on neat film. The
absorption band between 340 nm and 470 nm with the maximum extinction
4
–1
–1
coefficient (ε) of ≈ 3-5 × 10 M cm is attributed to the localized aromatic π–π*
4
–1
–1
transitions, while the band at 470–800 nm (ε ≈ 2-4 × 10 M cm ) is ascribed to the
1
5

ACCEPTED MANUSCRIPT
intramolecular charge transfer transition (ICT) [41]. YC-2 exhibits a 31 nm red shift
of the absorption band at long-wavelength both in solution and neat film compared to
YC-1, ascribed to the larger donor group in YC-2. Additionally, YC-2 displays a
lower ε at long-wavelength than YC-1 pointing to a lower oscillator strength for the
ICT transition. Compared to the absorption in solution, a similar absorption bands
with slight hypochromic shift (ca. 18 nm) are observed in neat film. This result can be
presumably explained by the formation of H-type aggregates [41].
When both the dyes were adsorbed onto 2.0 ꢁm thick TiO films, similar absorption
2
bands with remarkable blue shift are observed (23 nm for YC-1 and 40 nm for YC-2)
compared to that achieved in solution (Figure 2 and Table 1). According to previous
reports, this blue shift is caused by the deprotonation of the dyes on TiO film and
2
formation of H-aggregates [41,42].
3
.3. Electrochemical property
To investigate the possibility of electron transfer from the dyes to TiO , cyclic
2
voltammetry was carried out to estimate the redox potentials of YC-1 and YC-2 and
the data are summarized in Table 1. The oxidation potentials of YC-1 and YC-2 are
+
+
located at 0.81 V and 0.72 V (Fc/Fc is 0.46 V vs Ag/AgCl, Fc/Fc is 0.63 V vs NHE
[
43]) versus Ag/AgCl electrode, respectively (Figure 3). Both the values of oxidation
potential (YC-1: 0.98 V vs NHE, YC-2: 0.89 V vs NHE) are higher than that of
iodide/triiodide (0.35 V vs NHE) [18,42], indicating YC-1 and YC-2 can provide
ample driving force for the dye regeneration. Compared to YC-1, the additional
1
0-Methoxy iminostilbene and triphenylamine units in YC-2 decrease its oxidation
1
6

ACCEPTED MANUSCRIPT
potential by 90 mV owing to increased donor ability. On the other hand, the zero-zero
transition energies (E ) are estimated to be 1.67 V (YC-1) and 1.56 V (YC-2) based
0
-0
on the absorption thresholds. Estimated from oxidation potentials and E , the optical
0
-0
reduction potentials of both dyes are determined to be –0.69 V and –0.67 V (vs NHE)
for YC-1 and YC-2, respectively. Both dyes show almost the same reduction
potentials due to the same acceptor units. The optical reduction potentials lie above
the conduction band of TiO (–0.5 V vs NHE) [44], favoring the injection of electrons
2
from the dyes to the TiO2.
3
.4. Theoretical calculations
In order to investigate the relationship between electronic distribution and
molecular structure, density functional theory (DFT) calculations were performed to
optimize the geometry of both dyes based on Gaussian 09 at B3LYP/6-31G(d) level.
As shown in Figure 4, a similar pattern of highest occupied molecular orbitals
(HOMOs) are observed at thiophene and 5-phenyl-5H-dibenzo[b,f]azepine units for
YC-1 and YC-2. Conversely, the lowest unoccupied molecular orbitals (LUMOs)
display a different electron population. For YC-1, the LUMO is delocalized across the
anchoring group, isoindigo, thiophene and 5-phenyl-5H-dibenzo[b,f]azepine moieties,
while the LUMO of YC-2 is only located at anchoring group. This larger
HOMO-LUMO overlap in YC-1 points to a more effective intermolecular charge
transfer in YC-1 compared to YC-2.
3
.5. Photovoltaic performance
The performance of YC-1 and YC-2 as sensitizer for dye-sensitized solar cells
1
7

ACCEPTED MANUSCRIPT
–2
were evaluated at 100 mW cm under simulated AM1.5G solar light. The
current-density-voltage (J-V) curves of the devices are shown in Figure 5 and the
performance parameters are summarized in Table 2. The DSSCs based on YC-1
–
2
showed a short-circuit photocurrent density (J ) of 12.12 mA cm , an open-circuit
sc
voltage (V ) of 0.53 V, and a fill factor (FF) of 68.9%, corresponding to an overall ƞ
oc
–2
of 4.38%. YC-2-based DSSCs exhibited a J of 4.84 mA cm , a V of 0.48 V, and an
sc
oc
FF of 63.4%, achieving ƞ of 1.46%. All the photovoltaic parameters of the YC-1 are
higher than those of YC-2, in particular the difference in short-circuit current is
striking. The latter is mainly attributed to the large difference of IPCE between the
two dyes in the 450-650 nm interval, where the intensity of incident photon flux is the
highest.
To gain further insight into the influence of molecular structure on the current
density of the devices, the incident photo-to-current conversion efficiency (IPCE) of
the devices were measured (Figure 6). Both dyes show two well-separated peaks at
3
49 nm and 419 nm and one broad peak between 500 nm and 800 nm. The shape of
the spectra is reminiscent of the absorption spectra with a blue shift as observed in the
absorption spectra of the TiO films. Importantly the IPCE of YC-1 based device is
2
about 61 %, 48 % and 31 % at 349 nm, 419 nm 559 nm, respectively while YC-2
based device exhibits IPCE of only 51 %, 38 % and 15 % at the same wavelengths.
Therefore the IPCE of the YC-1 based device is about 10 % higher across the whole
visible wavelength range than that of YC-2 based device. This result is ascribed to the
better light harvesting properties of YC-1 (higher epsilon) than YC-2, which was
1
8

ACCEPTED MANUSCRIPT
expected from the absorption spectra (Figure 1). Consequently, the higher IPCE
values of YC-1 results in the much higher J compared to YC-2.
sc
3
.6. Electrochemical Impedance Spectroscopy (EIS)
Electrochemical impedance spectroscopy (EIS) was carried out to explore the
electron recombination dynamics in the devices. The Nyquist plots and bode plots of
DSSCs based on YC-1 and YC-2 were recorded in dark with applied bias of 0.55 V
and the results are shown in Figure 7 and Figure 8.
Estimated from the fitting of the EIS spectra with an electrochemical model [38,45],
the series resistances (R ), charge transfer resistances at the Pt/electrolyte interface
S
(
R ) and dye/TiO /electrolyte interface (R ) were calculated (Table 3). The R and
CE 2 rec S
RCE corresponding to the arc in high frequency region (Figure 4 inset) show almost
the same value in both DSSCs owing to the same electrode and electrolyte. Rrec
corresponds to the middle arc in low frequency region and indicates that the Rrec of
YC-1 based device is distinctly higher than that of YC-2 based device.
The intermediate frequency peak in the EIS Bode plots is assigned to the
recombination at the TiO /electrolyte interface. As seen from Figure 8, the
2
intermediate frequency peak of YC-1 based device shows a lower frequency than that
of YC-2 based device. Based on this result [46], the electron lifetimes (τ , 1/(2πf)) are
e
assessed to be 12.5 ms and 7.2 ms for YC-1 and YC-2, respectively. YC-1 possesses
a longer τ in titania films, which could be attributed to a lower rate of charge
e
recombination and higher V . Additionally, the radius of the intermediate arc in
oc
Figure 8 describes the charge transfer impedance at the TiO /electrolyte interface [42].
2
1
9

ACCEPTED MANUSCRIPT
YC-1 shows a larger arc than YC-2, implying that YC-1 has a more difficult charge
transfer than YC-2, which results in a higher electron density in the titania film
leading to superior performance for its corresponding device. Likewise, this result
could be derived from Nyquist plot (Figure 7).
4
. Conclusions
In conclusion, two D-π-A-π-A dyes based on 5-phenyl-5H-dibenzo[b,f]azepine
donor group were synthesized and characterized. Compared to the reported dyes ID1,
which uses a triphenylamine donor group, YC-1 and YC-2 showed red-shifted
absorption and enhancement of molar extinction coefficient due to extended
conjugation. YC-2 possesses stronger donor ability as shown by the decreased
oxidation potential compared to YC-1. The influence of the different donor groups on
the photovoltaic performance of sensitized cells was explored. YC-1 exhibited the
–
2
best power conversion efficiency of 4.38% (J = 12.12 mA cm , V = 0.53 V and FF
sc
oc
=
68.89) for the DSSCs without any co-sensitizer, which is almost three folds the
efficiency of YC-2. This result is attributed to the more effective ICT and longer
lifetime of electron injected in the titania of YC-1 as evidenced by DFT and EIS,
leading to an improvement of photocurrent and open circuit voltage, and, thus power
conversion efficiency. This research illustrated: (1) 5-phenyl-5H-dibenzo[b,f]azepine
could be a promising donor moiety in DSSCs dyes; (2) The balance between spatial
structure and intermolecular charge transfer plays a key role for the high efficiency in
sensitizer. Future work will be focus on optimized DSSCs with the co-sensitizer and
2
0

ACCEPTED MANUSCRIPT
cobalt electrolyte to further improve the performance.
Acknowledgements
Financial support from the National Natural Science Foundation of China (21202139,
5
1473140, 51273168 and 21172073), the Program for Innovative Research
Cultivation Team in University of Ministry of Education of China (1337304), the
Innovation Group in Hunan Natural Science Foundation (12JJ7002),. The Scientific
Research Fund of Hunan Provincial Education Department (12B123, 10C1294),
Natural Science Foundation of Hunan (12JJ4019, 11JJ3061). The Open Fund of the
State Key Laboratory of Luminescent Materials and Devices (South China University
of Technology) (2015-skllmd-08).
2
1

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ACCEPTED MANUSCRIPT
Tables Captions
Table 1 Photophysical data of YC-1 and YC-2
Table 2 Photovoltaic performance parameters of sensitized cells
Table 3 Parameters evaluated from fitting the EIS spectra of YC-1 and YC-2 based
DSSCs
2
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ACCEPTED MANUSCRIPT
Table 1
5
–1
–1
Absorption/nm(10 M cm )
a
opt
b
ox
c
red-opt
Dye
E
g
/eV
E /V
E
/V
Solution
Neat film
On TiO2
4
15 (0.45),
402,
562
414,
576
YC-1
410, 563
1.67
1.56
0.98
0.89
-0.69
-0.67
5
85 (0.45)
392 (0.48),
YC-2
391, 600
6
16 (0.35)
a
b
ox
estimated from the edge-absorption spectra; calculated from onset oxidation peaks. Edyes -EFC/FC++0.63
c
red-opt
g
ox
V; calculated from the formulation: E
= E -E
Table 2
-2
Dye
J
sc/mA cm
V /V
FF
PCE/%
4.38
oc
YC-1
YC-2
12.12
4.84
0.53
0.48
68.89
63.35
1.46
Table 3
Dye
R (ꢀ)
RCE (ꢀ)
56.47
Rrec (ꢀ)
96.55
τe (ms)
12.5
S
YC-1
YC-2
16.11
16.96
41.48
63.23
7.2
3
0