Asymmetric double donor-π-acceptor dyes based on phenothiazine and carbazole donors for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.tet.2017.09.018

Four organic dyes with asymmetric double donor-π-acceptor system were designed, synthesized and utilized in dye-sensitized solar cells (DSSCs), where phenothiazine and carbazole were introduced as double donors, butyl or octyl chain as the linker and cyanoacetic acid or rhodanine acetic acid as the electron acceptor. The design of double donor-π-acceptor system contributes to a distinctive increase of the molar extinction coefficient and benefits light-harvesting capability of these dyes. Due to superior electron injection efficiency, CPC-series dyes with cyanoacetic acid acceptor exhibit broader IPCE spectra and high IPCE responsive value compared with rhodanine-3-acetic acid acceptor, leading to higher J_SC. Long alkyl chain favors antiaggregation effect and low charge recombination rate is observed for dyes with octyl chain which display higher V_OC than those with butyl chain. Among four dyes, dye with octyl chain and cyanoacetic acid acceptor exhibits the highest short-circuit photocurrent density (9.64 mA cm^?2) and open-circuit photovoltage (720 mV), leading to the best overall power conversion efficiency of 4.76%.

Full text of DOI:10.1016/j.tet.2017.09.018

Tetrahedron xxx (2017) 1e9
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Asymmetric double donor-p-acceptor dyes based on phenothiazine
and carbazole donors for dye-sensitized solar cells
*
Bo-ye Zhu, Liang Wu, Qin Ye, Jian-rong Gao, Liang Han
College of Chemical Engineering, Zhejiang University of Technology, Hangzhou, 310032, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Four organic dyes with asymmetric double donor-p-acceptor system were designed, synthesized and
utilized in dye-sensitized solar cells (DSSCs), where phenothiazine and carbazole were introduced as
double donors, butyl or octyl chain as the linker and cyanoacetic acid or rhodanine acetic acid as the
Received 4 August 2017
Accepted 12 September 2017
Available online xxx
electron acceptor. The design of double donor-p-acceptor system contributes to a distinctive increase of
the molar extinction coefficient and benefits light-harvesting capability of these dyes. Due to superior
electron injection efficiency, CPC-series dyes with cyanoacetic acid acceptor exhibit broader IPCE spectra
Keywords:
DSSC
Asymmetric organic dyes
Phenothiazine
Carbazole
and high IPCE responsive value compared with rhodanine-3-acetic acid acceptor, leading to higher JSC
.
Long alkyl chain favors antiaggregation effect and low charge recombination rate is observed for dyes
with octyl chain which display higher VOC than those with butyl chain. Among four dyes, dye with octyl
chain and cyanoacetic acid acceptor exhibits the highest short-circuit photocurrent density
Double donor-p-acceptor
ꢀ
2
(
9.64 mA cm ) and open-circuit photovoltage (720 mV), leading to the best overall power conversion
efficiency of 4.76%.
©
2017 Elsevier Ltd. All rights reserved.
1. Introduction
photosensitizers and hence power conversion efficiency.6,7 A lot of
molecular modification approaches, such as introducing double
Dye-sensitized solar cells (DSSCs), a promising approach of solar
donor/acceptor and extending
ieve high light absorption efficiency.
strategy, which combines dyes with complementary absorption
p-bridge, have been taken to ach-
8
e10
energy utilization, have attracted worldwide scientific research
attention because of their high efficiency, low cost, and facile
Cosensitization is another
1
11,12
fabrication, ever since Gr a€ tzel and O'Regan reported the pioneer
properties to obtain a broader absorption spectrum.
However,
work in 1991.2 The sensitizers, which harvest the sunlight and
inject the excited electrons into a photoanode, play a critical role in
the performance and operation range of the solar cell. To date, some
cells with ruthenium complexes as sensitizers have achieved high
competitive adsorption of these dyes on TiO
sometimes de-
2
13
teriorates the photovoltaic performance.
Recently, organic dyes containing double D-
p
-A chains with a
non-conjugated linkage, coded DB dyes, have been reported to
improve the light-harvesting units on TiO and hence the photo-
Compared with single D- -A dye, organic
dyes with two symmetric D- -A chains exhibit higher but similar
absorption spectrum range. Then dyes with two asymmetric D-
A chains were designed to broaden the absorption spectra and
3
power conversion efficiency (PCE) of over 11e12%. As a promising
2
14,15
category of sensitizers, the metal-free organic dyes possess several
advantages such as facile fabrication, less environmental issues,
larger molar extinction coefficient and variable structure adjust-
voltaic performance.
p
p
h
1
6
p
-
4
ability in comparison with the metal based organic dyes. Metal-
1
4,17
free organic dyes with D-
p-A structural motif have been widely
improve light-harvesting capability.
Different D-p-A chains,
18
17
investigated, many of which exhibited an energy-to-electricity
based on phenothiazine donor, diphenylamine donor and indole
donor etc, were incorporated into DB dyes. Optimal photoelectric
5
19
conversion efficiency close to that of a benchmark dye N719.
Through molecular engineering, much progress has been wit-
nessed in improving the light-harvesting ability of organic
conversion efficiency 6.1% was achieved with double phenothiazine
donors.
18,20
On the other hand, carbazole donor was also introduced into DB
dyes and yet exhibited unsatisfactory photovoltaic performance
21
with symmetric carbazole DB dyes or asymmetric diphenylam-
*
Corresponding author.
E-mail address: hanliang814@163.com (L. Han).
17
ine-carbazole DB dyes.
Because of their short maximum
https://doi.org/10.1016/j.tet.2017.09.018
040-4020/© 2017 Elsevier Ltd. All rights reserved.
0
Please cite this article in press as: Zhu B-y, et al., Asymmetric double donor-
p-acceptor dyes based on phenothiazine and carbazole donors for
dye-sensitized solar cells, Tetrahedron (2017), https://doi.org/10.1016/j.tet.2017.09.018

2
B.-y. Zhu et al. / Tetrahedron xxx (2017) 1e9
absorption wavelength (ꢁ400 nm), low photoelectric conversion
efficiency of DB dyes based on carbazole donor is mainly ascribed to
the weak light-harvesting capability in the NIR-IR region. Hence,
we introduced phenothiazine donor with good light-harvesting
property in the visible region into DB dyes based on carbazole
donor to enhance light collection. Butyl chain and octyl chain were
utilized to connect phenothiazine and carbazole donors and the
influence of alkyl chain length on the charge recombination was
investigated. The acceptors based on cyanoacetic acid or rhodanine
UVeVis absorption spectra were measured on SHIMADZU (model
UV-2550) UVevis spectrophotometer.
2.2. Fabrication of DSSCs
The FTO glass was first cleaned in a detergent solution using an
ultrasonic bath for 15 min, and then rinsed with water and ethanol.
For the fabrication of photoelectrodes, titanium nanoxide paste was
2
deposited by screen-printing, resulting in the TiO electrodes. The
2
acetic acid were used to anchor the dye with TiO and help the
thickness of TiO
2
film was 12
m
m. The TiO
2
electrodes were grad-
photoinduced electrons inject into the photoanode. Therefore, four
phenothiazine-carbazole DB dyes were synthesized (Scheme 1) and
the influence of alkyl bridge and electron acceptor on the physical
properties and photovoltaic performance were investigated.
ꢃ
ꢃ
ually heated under an air flow at 325 C for 5 min, at 375 C for
ꢃ
ꢃ
5
min, and at 450 C for 15 min, and finally, at 500 C for 15 min.
ꢃ
After being cooled to 80 C, the TiO
2
electrode was immersed into
the dye solution in a mixture of MeOH and CHCl
3
(VMeOH:
V
CHCl3 ¼ 1:10) and kept at room temperature for 24 h to assure
complete dye uptake, which were then rinsed with MeOH to
remove excess dye. Meanwhile, the counter electrodes were pre-
pared by screen-printing a 50 nm Pt layer on the cleaned FTO
plates. Open cells were fabricated in air by clamping the different
photoelectrodes with platinized counter electrodes. The electrolyte
2
. Experimental
2.1. Materials and characterization
All of the chemicals used in the current study were purchased
used here is composed of the CH
3
CN solution of 0.3 M 1,2-dimethyl-
, 0.07 M LiI, 0.1 M
from commercial vendors and used as received without further
purification, unless otherwise noted. All solvents were purified and
dried using standard methods prior to use. Nuclear magnetic
3-propylimidazolium iodide (DMPII), 0.03 M I
2
guandine thiocyanate, and 0.4 M 4-tert-butylpytiding (TBP). The
active area of DSSCs was 0.36 cm .
2
1
13
resonance ( H and C) spectra were recorded on a Bruker AM 500
spectrometer (Bruker) with chemical shifts reported as ppm at 500
and 125 MHz, respectively, (in CDCl
3
or DMSO-d
6
as internal
2.3. Photovoltaic characterization
standard). Fluorescence spectra were obtained with an F-7000
Fluorescence Spectrophotometer in a solution of 5 ꢂ 10ꢀ mol L
5
ꢀ1
.
The photovoltaic performance of the DSSCs was evaluated at AM
Scheme 1. Syntheses of phenothiazine-carbazole DB dyes.
Please cite this article in press as: Zhu B-y, et al., Asymmetric double donor-
p-acceptor dyes based on phenothiazine and carbazole donors for
dye-sensitized solar cells, Tetrahedron (2017), https://doi.org/10.1016/j.tet.2017.09.018

B.-y. Zhu et al. / Tetrahedron xxx (2017) 1e9
3
2
1
.5 G illumination (100 mW/cm ; Sol3A, Newport, USA) using a
Keithley digital source meter (Keithley 2420, USA). The action
spectra of monochromatic incident photon-to-current conversion
efficiency (IPCE) for solar cell were performed by a Hypermonolight
product under reduced pressure. The crude product was purified by
silica gel chromatography eluting (silica gel, PE: DCM ¼ 5:1) leading
1
to white oil (6.9 g, 72%). H NMR (500 MHz, CDCl
3
)
d: 8.42 (d,
J ¼ 7.7 Hz, 2H), 7.77 (t, J ¼ 7.6 Hz, 2H), 7.64 (d, J ¼ 8.2 Hz, 2H), 7.56 (t,
J ¼ 7.4 Hz, 2H), 7.45 (d, J ¼ 7.6 Hz, 2H), 7.41e7.38 (m, 2H), 7.17 (d,
J ¼ 7.3 Hz, 2H), 7.06 (d, J ¼ 5.5 Hz, 2H), 4.40 (t, J ¼ 7.0 Hz, 2H), 3.96 (t,
J ¼ 6.7 Hz, 2H), 2.00e1.94 (m, 4H), 1.59e1.43 (m, 8H).
(
(
SM-25, Jasco, Japan). Electrochemical impedance spectroscopy
EIS) experiments were conducted using a computer-controlled
potentiostat (ZeniumZahner, Germany). The measured frequency
ranged from 100 mHz to 1 MHz while the AC amplitude was set to
1
0 mV. The bias of all EIS measurements was set to the VOC of the
2.4.5. Synthesis of 10-(4-(3-formyl-9H-carbazol-9-yl)butyl)-10H-
phenothiazine-3- carbaldehyde 3a
corresponding dyes.
Under nitrogen atmosphere, a stirred solution of 2a (4.2 g,
2
2
3
.4. General procedure for the synthesis of compounds
10 mmol) in anhydrous DMF (20 mL) and DCE (100 mL) was stirred
ꢃ
at 0 C, POCl
3
(6.1 g, 40 mmol) was slowly syringed. After stirred at
ꢃ
ꢃ
.4.1. Synthesis of 9-(4-bromobutyl)-9H-carbazole 1a
To a stirred mixture of carbazole (10 g, 60 mmol) and TBAB (1 g,
mmol) in methylbenzene (250 mL), NaOH (36 g, 900 mmol) in
0 C for 30 min, the reaction mixture was heated up to 90 C. Then,
after stirred at 90 C for 4 h, the reaction mixture was poured into
ꢃ
NaOH (1 M, 300 mL), extracted with ethyl acetate (3 ꢂ 100 mL) and
washed by NaCl saturated solution (3 ꢂ 100 mL), concentrated to
give crude product under reduced pressure. The crude product was
purified by silica gel chromatography eluting (silica gel, PE:
DCM ¼ 2:1) leading to luminous yellow solid (2.4 g, 50%). Mp:
water (20 mL) was added. After stirred at room temperature for
0 min, 1,4-dibromobutane (25.6 g, 120 mmol) was added. After
stirred overnight, the reaction mixture was poured into water
3
(
1000 mL), extracted with DCM (3 ꢂ 300 mL) and washed by NaCl
ꢃ
1
saturated solution (3 ꢂ 300 mL), concentrated to give crude
product under reduced pressure. The crude product was purified by
silica gel chromatography eluting (silica gel, PE: DCM ¼ 8:1) leading
165e166 C. H NMR (500 MHz, CDCl ) d: 10.10 (s, 1H), 9.78 (s, 1H),
3
8.55 (s, 1H), 8.10 (d, J ¼ 7.7 Hz, 1H), 7.96 (d, J ¼ 8.5 Hz, 1H), 7.51 (dd,
J ¼ 9.3 Hz, 2H), 7.48 (d, J ¼ 6.5 Hz, 1H), 7.35 (t, J ¼ 8.5 Hz, 2H), 7.31 (t,
J ¼ 7.3 Hz, 1H), 7.10 (t, J ¼ 6.5 Hz, 2H), 6.97 (t, J ¼ 7.9 Hz, 1H), 6.76 (d,
J ¼ 7.9 Hz, 1H), 6.71 (d, J ¼ 8.3 Hz,1H), 4.33 (t, J ¼ 6.9 Hz, 2H), 3.90 (t,
ꢃ
1
to white solid (11.2 g, 62%). Mp: 104e105 C. H NMR (500 MHz,
CDCl
3
)
d: 8.14 (d, J ¼ 7.7 Hz, 2H), 7.51 (d, J ¼ 7.6 Hz, 2H), 7.43 (d,
J ¼ 8.2 Hz, 2H), 7.28 (t, J ¼ 7.0 Hz, 2H), 4.37 (t, J ¼ 7.0 Hz, 2H), 3.40 (t,
J ¼ 6.3 Hz, 2H), 2.11e2.05 (m, 2H), 1.89e1.84 (m, 2H); HRESIMS m/z
þ
J ¼ 6.5 Hz, 2H), 2.13e2.09 (m, 2H), 1.97e1.91 (m, 2H).
477.1638 [MþH] , cacld C30
25 2 2
H N O S for: 477.1631.
2
.4.2. Synthesis of 9-(8-bromooctyl)-9H-carbazole 1b
To a stirred mixture of carbazole (10 g, 60 mmol) and TBAB (1 g,
mmol) in methylbenzene (250 mL), NaOH (36 g, 900 mmol) in
2.4.6. Synthesis of 10-(8-(3-formyl-9H-carbazol-9-yl)octyl)-10H-
phenothiazine-3- carbaldehyde 3b
3
Under nitrogen atmosphere, a stirred solution of 2b (4.8 g,
water (20 mL) was added. After stirred at room temperature for
0 min, 1,8-dibromooctane (32.3 g, 120 mmol) was added. After
stirred overnight, the reaction mixture was poured into water
10 mmol) in anhydrous DMF (20 mL) and DCE (100 mL) was stirred
ꢃ
3
at 0 C, POCl
3
(6.1 g, 40 mmol) was slowly syringed. After stirred at
ꢃ
ꢃ
0 C for 30 min, the reaction mixture was heated up to 90 C. Then,
after stirred at 90 C for 4 h, the reaction mixture was poured into
ꢃ
(
1000 mL), extracted with DCM (3 ꢂ 300 mL) and washed by NaCl
saturated solution (3 ꢂ 300 mL), concentrated to give crude
NaOH (1 M, 300 mL), extracted with ethyl acetate (3 ꢂ 100 mL) and
washed by NaCl saturated solution (3 ꢂ 100 mL), concentrated to
give crude product under reduced pressure. The crude product was
purified by silica gel chromatography eluting (silica gel, PE:
product under reduced pressure. The crude product was purified by
silica gel chromatography eluting (silica gel, PE: DCM ¼ 8:1) leading
1
to white oil (12.2 g, 57%). H NMR (500 MHz, CDCl
3
)
d: 8.41 (d,
1
J ¼ 7.7 Hz, 2H), 7.76 (t, J ¼ 7.3 Hz, 2H), 7.61 (d, J ¼ 8.2 Hz, 2H), 7.55 (t,
DCM ¼ 2:1) leading to an luminous yellow oil (2.5 g, 47%). H NMR
J ¼ 7.4 Hz, 2H), 4.38 (t, J ¼ 7.1 Hz, 2H), 3.55 (t, J ¼ 6.8 Hz, 2H),
3
(500 MHz, CDCl ) d: 10.08 (s, 1H), 9.75 (s, 1H), 8.57 (s, 1H), 8.13 (d,
2
.03e1.94 (m, 4H), 1.56e1.46 (m, 4H), 1.45e1.34 (m, 4H).
J ¼ 7.7 Hz, 1H), 7.99 (d, J ¼ 8.5 Hz, 1H), 7.61e7.57 (m, 1H), 7.56e7.50
(
m, 2H), 7.42 (d, J ¼ 8.6 Hz, 2H), 7.32 (t, J ¼ 7.5 Hz, 1H), 7.14 (t,
2.4.3. Synthesis of 10-(4-(9H-carbazol-9-yl)butyl)-10H-
J ¼ 7.0 Hz, 1H), 7.09 (dd, J ¼ 7.6, 1.5 Hz, 1H), 6.97e6.93 (m, 1H), 6.83
(t, J ¼ 8.0 Hz, 2H), 4.26 (t, J ¼ 7.2 Hz, 2H), 3.82 (t, J ¼ 7.0 Hz, 2H),
phenothiazine 2a
A stirred solution of 1a (6 g, 20 mmol), phenothiazine (5 g,
1.86e1.80 (m, 2H), 1.77e1.71 (m, 2H), 1.42e1.32 (m, 8H); HRESIMS
þ
2
5 mmol) and NaOH (2 g, 50 mmol) in DMF (250 mL) was stirred at
m/z 533.2258 [MþH] , cacld C34
33 2 2
H N O S for: 533.2263.
room temperature for 6 h. The reaction mixture was poured into
water (1000 mL), extracted with DCM (3 ꢂ 300 mL) and washed by
NaCl saturated solution (3 ꢂ 300 mL), concentrated to give crude
product under reduced pressure. The crude product was purified by
silica gel chromatography eluting (silica gel, PE: DCM ¼ 5:1) leading
2.4.7. Synthesis of (E)-3-(9-(4-(3-((E)-2-carboxy-2-cyanovinyl)-
10H-phenothiazin-10- yl)butyl)-9H-carbazol-3-yl)-2-cyanoacrylic
acid CPC-1
A solution of 3a (1.5 g, 3 mmol), cyanoacetic acid (1 g, 12 mmol)
and piperidine (0.5 g, 6 mmol) in chloroform (50 mL) was stirred
under nitrogen atmosphere and refluxing for 6 h. The reaction
mixture was concentrated to give crude product under reduced
pressure. The crude product was purified by silica gel chromatog-
raphy eluting (silica gel, DCM: methanol: acetic acid ¼ 100:5:1)
ꢃ
1
to white solid (6.4 g, 76%). Mp: 171e172 C. H NMR (500 MHz,
CDCl
3
)
d
: 8.10 (d, J ¼ 7.7 Hz, 2H), 7.43 (t, J ¼ 7.6 Hz, 2H), 7.31 (d,
J ¼ 8.2 Hz, 2H), 7.23 (t, J ¼ 7.4 Hz, 2H), 7.17e7.11 (m, 4H), 6.93 (t,
J ¼ 7.5 Hz, 2H), 6.78 (d, J ¼ 8.1 Hz, 2H), 4.30 (t, J ¼ 7.1 Hz, 2H), 3.86 (t,
J ¼ 6.5 Hz, 2H), 2.07e2.02 (m, 2H), 1.90e1.85 (m, 2H).
ꢃ
1
leading to dark red solid (1.3 g, 72%). Mp: 206e207 C. H NMR
2
.4.4. Synthesis of 10-(8-(9H-carbazol-9-yl)octyl)-10H-
(500 MHz, DMSO-d
6
)
d
: 8.65 (s, 1H), 8.35 (s, 1H), 8.12 (d, J ¼ 7.2 Hz,
phenothiazine 2b
1H), 8.02 (d, J ¼ 7.7 Hz, 1H), 7.99 (s, 1H), 7.65 (d, J ¼ 8.6 Hz, 1H), 7.60
(s, 1H), 7.51 (d, J ¼ 8.8 Hz, 1H), 7.47 (d, J ¼ 8.2 Hz, 1H), 7.42 (t,
J ¼ 7.5 Hz, 1H), 7.23 (t, J ¼ 7.3 Hz, 1H), 7.08 (t, J ¼ 7.6 Hz, 1H), 7.02 (d,
J ¼ 7.3 Hz,1H), 6.91 (t, J ¼ 7.5 Hz,1H), 6.87 (d, J ¼ 8.2 Hz,1H), 6.84 (d,
J ¼ 8.7 Hz, 1H), 4.35 (t, J ¼ 6.6 Hz, 2H), 3.87 (t, J ¼ 5.9 Hz, 2H),
A stirred solution of 1b (7.2 g, 20 mmol), phenothiazine (5 g,
5 mmol), NaOH (2 g, 50 mmol) in DMF (250 mL) was stirred at
2
room temperature for 6 h. The reaction mixture was poured into
water (1000 mL), extracted with DCM (3 ꢂ 300 mL) and washed by
NaCl saturated solution (3 ꢂ 300 mL), concentrated to give crude
1
3
1.97e1.88 (m, 2H), 1.81e1.73 (m, 2H);
C NMR (500 MHz,
Please cite this article in press as: Zhu B-y, et al., Asymmetric double donor-
p-acceptor dyes based on phenothiazine and carbazole donors for
dye-sensitized solar cells, Tetrahedron (2017), https://doi.org/10.1016/j.tet.2017.09.018

4
B.-y. Zhu et al. / Tetrahedron xxx (2017) 1e9
ꢃ
DMSOþCDCl
3
)
d
: 164.20, 163.69, 154.81, 152.01, 148.32, 142.73,
acid ¼ 100:5:1) leading to black solid (1.1 g, 40%). Mp: 165e166 C.
1
1
42.56, 140.49, 131.01, 128.87, 127.87, 127.64, 127.00, 126.55, 125.55,
H NMR (500 MHz, DMSO-d
6
)
d
: 8.48 (s, 1H), 8.28 (d, J ¼ 7.7 Hz, 1H),
124.79, 124.03, 123.45, 122.82, 122.55, 122.31, 121.95, 120.31, 120.23,
8.06 (s, 1H), 7.79e7.74 (m, 2H), 7.73e7.71 (m, 1H), 7.65 (d, J ¼ 8.4 Hz,
1H), 7.52 (d, J ¼ 7.6 Hz, 1H), 7.42 (dd, J ¼ 9.0, 1.5 Hz, 1H), 7.35 (d,
J ¼ 2.1 Hz, 1H), 7.30 (t, J ¼ 7.2 Hz, 1H), 7.19 (t, J ¼ 7.8 Hz, 1H), 7.13 (t,
J ¼ 7.5 Hz, 1H), 7.08 (d, J ¼ 8.8 Hz, 1H), 7.05e6.95 (m, 2H), 4.78 (s,
117.06, 116.46, 116.17, 115.38, 109.69, 109.66, 99.92, 98.49, 46.22,
þ
4
2.15, 25.00, 23.32; HRESIMS m/z 610.1765 [M] , cacld C36
26 4 4
H N O S
for: 610.1675.
2
H), 4.74 (s, 2H), 4.43 (t, J ¼ 6.5 Hz, 2H), 3.87 (t, J ¼ 6.6 Hz, 2H),
13
2
1
.4.8. Synthesis of (E)-3-(9-(8-(3-((E)-2-carboxy-2-cyanovinyl)-
0H-phenothiazin-10- yl)octyl)-9H-carbazol-3-yl)-2-cyanoacrylic
acid CPC-2
1.81e1.74 (m, 2H), 1.68e1.61 (m, 2H), 1.48e1.30 (m, 8H). C NMR
(500 MHz, DMSO) : 195.90, 195.38, 174.83, 170.18, 169.25, 169.07,
d
149.79, 145.64, 144.24,143.48, 138.53, 135.74, 133.56, 132.08, 131.34,
130.69, 129.99, 129.61, 127.20, 126.61, 126.36, 126.23, 125.81, 124.96,
124.68, 123.62, 122.84, 121.31, 119.99, 119.05, 118.99, 118.65, 113.18,
A solution of 3b (1.7 g, 3 mmol), cyanoacetic acid (1 g, 12 mmol)
and piperidine (0.5 g, 6 mmol) in chloroform (50 mL) was stirred
under nitrogen atmosphere and refluxing for 6 h. The reaction
mixture was concentrated to give crude product under reduced
pressure. The crude product was purified by silica gel chromatog-
raphy eluting (silica gel, DCM: methanol: acetic acid ¼ 100:5:1)
112.72, 58.88, 49.57, 47.88, 45.25, 31.16, 28.77, 28.65, 28.48, 23.85,
-
21.35; HRESIMS m/z 877.1362 [M-H] , cacld C44
H
37
N
4
O
6
S
5
for:
877.1317.
ꢃ
1
leading to dark red solid (1.5 g, 73%). Mp: 152e153 C. H NMR
500 MHz, DMSO-d
: 8.68 (s, 1H), 8.33 (s, 1H), 8.13 (d, J ¼ 8.5 Hz,
H), 8.04 (d, J ¼ 7.4 Hz, 1H), 7.97 (s, 1H), 7.71 (d, J ¼ 8.0 Hz, 1H), 7.63
3. Results and discussion
(
1
6
) d
3.1. Synthesis
(
s, 1H), 7.50 (d, J ¼ 8.6 Hz, 1H), 7.45 (s, 1H), 7.22 (s, 1H), 7.09 (t,
J ¼ 7.4 Hz, 1H), 7.00 (d, J ¼ 6.6 Hz, 1H), 6.92e6.79 (m, 4H), 4.26 (t,
Carbazole was alkylated at N atom with dibromoalkane under
the catalysis of TBAB to synthesize N-bromoalkyl carbazole, then
phenothiazine was introduced through nucleophilic substitution
reaction to give double donor intermediates 2. Intermediates 2
were treated with Vilsmeier reagent to form dialdehyde com-
pounds 3. Knoevenagel condensation between dialdehyde com-
pounds and cyanoacetic acid or rhodanine acetic acid afforded four
target compounds CPC-1, CPC-2, CPR-1 and CPR-2.
J ¼ 6.9 Hz, 2H), 3.76 (t, J ¼ 7.9 Hz, 2H), 1.76e1.74 (m, 2H), 1.67e1.58
13
(
1
1
1
m, 2H), 1.35e1.27 (m, 8H); C NMR (500 MHz, DMSOþCDCl
3
) d:
55.02, 152.23, 148.73 (2C), 142.53 (2C), 140.54, 131.37, 128.58,
28.11, 127.45 (2C), 126.93 (2C), 126.55, 123.67, 123.23 (2C), 122.60,
22.43, 122.25, 121.97, 120.24, 120.11, 117.06, 116.46, 115.86 (2C),
114.94 (2C), 109.49, 109.45, 54.04, 46.97, 28.52, 28.26, 26.23, 25.86,
þ
2
8.84, 25.77; HRESIMS m/z 667.2370 [MþH] , cacld C40
35 4 4
H N O S
for: 667.2379.
3.2. Optical characterization
2.4.9. Synthesis of (E)-5-((9-(4-(3-((E)-(3-carboxy-4-oxo-2-
thioxothiazolidin-5-ylidene)- methyl)-10H-phenothiazin-10-yl)
3 3
The UVevis spectra of the dyes in CHCl -CH OH solution and on
butyl)-9H-carbazol-3-yl)methylene)-4-oxo-2-thioxothiazolidine-3-
2
TiO film are displayed in Fig. 1(a) and (b), respectively, and the
carboxylic acid CPR-1
corresponding data are summarized in Table 1. The CPC-series dyes,
in solution, exhibit two absorption peaks: one peak at about
320 nm is ascribed to p/p* transition and the other peak is found
A solution of 3a (1.5 g, 3 mmol), rhodanine acetic acid (2.3 g,
2 mmol) and NH OAc (0.46 g, 6 mmol) in acetic acid (50 mL) was
1
4
stirred under nitrogen atmosphere and refluxing for 6 h. The re-
action mixture was concentrated to give crude product under
reduced pressure. The crude product was purified by silica gel
chromatography eluting (silica gel, DCM: methanol: acetic
at 395 nm caused by intramolecular charge transfer (ICT) from the
donor to acceptor. The CPR-series dyes display absorption peaks of
p/p* transition at 344 nm and ICT absorption peaks at 445 nm.
Distinctly broad absorption spectra are observed with the intro-
duction of rhodanine acetic acid acceptor and ICT absorption peaks
of CPR-series dyes are shift bathochromically by 50 nm when
compared with those of CPC-series dyes. Moreover, the molar
extinction coefficients of CPR-series dyes for ICT absorption peaks
are almost two times higher than those of CPC-series dyes as shown
in Table 1. Therefore, replacement of cyanoacetic acid acceptor with
rhodanine acetic acid acceptor facilitates the efficient ICT and ex-
pands the light absorption region, improving the light-harvesting
capability of CPR-series dyes.
ꢃ
acid ¼ 100:5:1) leading to black solid (1.2 g, 47%). Mp: 231e232 C.
1
H NMR (500 MHz, DMSO-d
.03 (s, 1H), 7.70 (d, J ¼ 7.1 Hz, 2H), 7.67 (d, J ¼ 9.0 Hz, 1H), 7.62 (d,
6
)
d
: 8.38 (s, 1H), 8.23 (d, J ¼ 7.7 Hz, 1H),
8
J ¼ 8.3 Hz, 1H), 7.50 (t, J ¼ 7.7 Hz, 1H), 7.29 (t, J ¼ 7.3 Hz, 1H), 7.25 (d,
J ¼ 8.5 Hz, 2H), 7.16 (t, J ¼ 7.8 Hz, 1H), 7.13 (d, J ¼ 7.6 Hz, 1H), 7.01 (d,
J ¼ 8.0 Hz,1H), 6.98 (d, J ¼ 7.4 Hz,1H), 6.95 (d, J ¼ 8.6 Hz,1H), 4.74 (s,
2
2
H), 4.71 (s, 2H), 4.47 (t, J ¼ 6.9 Hz, 2H), 3.93 (t, J ¼ 6.6 Hz, 2H),
13
.01e1.92 (m, 2H), 1.80e1.74 (m, 2H); C NMR (500 MHz, DMSO) d:
1
1
1
1
4
C
93.09, 192.58, 171.98, 167.27, 166.44, 166.29, 146.66, 143.09, 141.33,
40.66, 135.75, 132.99, 130.45, 129.30, 128.58, 127.85, 127.17, 126.83,
26.74, 124.35, 124.19, 123.55, 123.44, 123.01, 122.60, 121.84, 120.75,
18.54, 117.15, 116.40, 116.16, 113.92, 110.42, 109.99, 46.22, 45.09,
As listed in Table 1, the molar extinction coefficients of CPC-1
ꢀ
1
ꢀ1
ꢀ1
ꢀ1
and CPC-2 reach 43200 M cm and 41500 M cm , respec-
tively, which are two times higher than those of single D- -A dyes
p
-
21
ꢀ1
ꢀ1
16
2.10, 40.03, 25.11, 23.37; HRESIMS m/z 821.0710 [M-H] , cacld
for: 821.0691.
single-CBZ
(ε ¼ 10090 M
carbazole D- -A dye and single phenothiazine D-
(ε
¼
21406
M
cm
cm ). Therefore, the incorporation of single
-A dye into these
)
and 1(PTZCA)
ꢀ
1
ꢀ1
40 30 4 6 5
H N O S
p
p
2
.4.10. Synthesis of (E)-5-((9-(8-(3-((E)-(3-carboxy-4-oxo-2-
DB dyes leads to a mutual enhancement of light absorptive in-
tensity and contributes to distinctively stronger light absorption
response. In addition, CPC-series dyes also show higher molar
extinction coefficients than the corresponding double symmetric
thioxothiazolidin-5-ylidene)- methyl)-10H-phenothiazin-10-yl)
octyl)-9H-carbazol-3-yl)methylene)-4-oxo-2-thioxothiazolidine-3-
carboxylic acid CPR-2
21
ꢀ1
ꢀ1
A solution of 3b (1.7 g, 3 mmol), rhodamine-3-acetic acid (2.3 g,
carbazole D-
p
-A dye double-CBZ (ε ¼ 38344 M cm ) and
15
12 mmol) and NH
4
OAc (0.46 g, 6 mmol) in acetic acid (50 mL) was
double symmetric phenothiazine D-p-A dye 2(PTZCA)
ꢀ
1
ꢀ1
stirred under nitrogen atmosphere and refluxing for 6 h. The re-
action mixture was concentrated to give crude product under
reduced pressure. The crude product was purified by silica gel
chromatography eluting (silica gel, DCM: methanol: acetic
(ε ¼ 32000 M cm ).
The absorption spectra of phenothiazine-carbazole DB dyes on
TiO film are shown in Fig. 1(b). These dyes on TiO film exhibit
broader absorption spectra by 50 nm than in solution due to the
2
2
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B.-y. Zhu et al. / Tetrahedron xxx (2017) 1e9
5
Fig. 1. UVevis absorption spectra of phenothiazine-carbazole DB dyes (a) dyes in CHCl
3
-CH
3
OH solution (VCHCl3:VCH3OH ¼ 10:1); (b) dyes on TiO
2
film.
Table 1
Optical properties of dyes.
maxa/(nm)
εa/(Mꢀ1$cmꢀ1
)
l
maxb/(nm)
l
emc/(nm)
ꢀ8
ꢀ2
Adsorption capacity/(10 mol cm )
Compd.
l
CPC-1
CPC-2
CPR-1
CPR-2
322, 395
319, 396
344, 446
344, 445
46800, 43200
51300, 41500
36600, 80080
36200, 78200
392
392
462
464
573
574
638
636
12.3
12.8
8.9
9.6
a
Maximum absorption wavelength
Maximum absorption wavelength
Maximum emission wavelength measured in CHCl
l
max and molar extinction coefficient at
max of dyes on sensitized TiO electrodes.
-CH OH solution.
3 3
lmax of dyes measured in CHCl -CH OH solution.
b
c
l
2
3
3
molecular aggregation. On TiO
the dye molecule deprotonates to anchor with TiO
meantime dyes may form H-aggregates through parallel arrange-
ment, leading to a hypochromatic shift of max. Another head to tail
2
film, the carboxylic acid group in
According to the side view, the dihedral angles between the
carbazole unit and the phenothiazine unit are all close to 0 , and
long octyl chain increases the bulkiness of the molecule compared
with short butyl chain which may depress the formation of ag-
ꢃ
2
and in the
l
arrangement called J-aggregates usually induce the absorption
peak shift bathochromically. Generally, compounds with better
coplanarity are easier to aggregate. As can be seen, CPC-series dyes
2
gregates on TiO .
Fig. 3 and Table 2 show that the energy gaps of CPC-series dyes
are located at 2.95e2.97 eV while those of CPR-series dyes are
found at 2.76~-2.80 eV. In CPR-series dyes, the incorporation of
rhodanine acetic acid acceptor shifts the LUMO energy level
downwards by 0.15e0.17 eV when compared with CPC-series dyes,
which accounts for narrowing their energy gap and leads to a
bathochromical shift of
Fig. 4 presents the electron distribution in the frontier molecular
orbital HOMO and LUMO. Similar to the reference,
show similar maximum absorption wavelength on TiO
solution. A red shift of the maximum absorption wavelength is
observed when CPR-series dyes are adsorbed on TiO indicating the
2
with in
2
presence of J-aggregates, which may be ascribed to better copla-
narity of CPR-series dyes derived from larger conjugation system.
Noteworthily, CPC-series dyes achieve the same level of maximum
max
l .
1
7,18
absorption intensity with CPR-series dyes on TiO
former exhibits half of the molar extinction coefficients of the latter,
which benefits the light-harvesting capability of TiO electrode
sensitized with CPC-series dyes. The raising of the absorption in-
tensity for CPC-series dyes on TiO may thank to their higher
adsorption capacity than CPR-series dyes. The loading amount of
2
though the
the HOMO of
these dyes are mainly populated on phenothiazine donor which is
the electron-richest part of the whole molecule and spread up to
2
the
move from the HOMO to the LUMO through an intramolecular
charge transfer, and are finally located in the -bridge and the
acceptor of phenothiazine D- -A chain. The good overlap of the
HOMO and LUMO ensures an efficient photoinduced electron
p-bridge. Upon the light illumination, electrons are excited and
2
p
ꢀ
8
ꢀ2
CPC-series dyes on TiO
2
is about 12e13 ꢂ 10 mol cm while no
p
ꢀ8
ꢀ2
more than 10 ꢂ 10 mol cm loading amount is observed for CPR-
series dyes.
2
transfer from the dye to the TiO electrode.
3.3. Computational analysis
3.4. Photovoltaic performance
To gain insight of the relationship between the structure and the
The action spectra of the incident photo-to-current conversion
efficiency (IPCE) for phenothiazine-carbazole DB dyes sensitized
DSSCs are shown in Fig. 5(a). The photovoltaic performances of
these dyes sensitized DSSCs under standard conditions (AM 1.5G,
100 mW cm ) were measured using an iodine electrolyte and the
J-V curves are shown in Fig. 5(b). The detailed photovoltaic pa-
rameters of short-circuit photocurrent (JSC), open-circuit photo-
photophysical properties, the ground state geometries of CPC-
series dyes and CPR-series dyes have been optimized by the
B3LYP hybrid density functional method in conjunction with the d-
polarized 6-31G (d) basis set implemented in the Gaussian 03
program.
The optimized structures of these dyes are presented in Fig. 2. As
shown in the front view, both the CPC-series dyes and CPR-series
dyes display a butterfly-shaped structure and good planarity.
ꢀ2
voltage (VOC), fill factor (FF), power conversion efficiency (h) are
summarized in Table 3.
Please cite this article in press as: Zhu B-y, et al., Asymmetric double donor-p-acceptor dyes based on phenothiazine and carbazole donors for
dye-sensitized solar cells, Tetrahedron (2017), https://doi.org/10.1016/j.tet.2017.09.018

6
B.-y. Zhu et al. / Tetrahedron xxx (2017) 1e9
Fig. 2. Optimized geometries of phenothiazine-carbazole DB dyes at the B3LYP/6-31G(d) level.
Fig. 3. The energy level of the frontier molecular orbitals of phenothiazine-carbazole DB dyes at B3LYP/6-31G(d).
Similar with the absorption spectra on TiO
2
, these dyes show
photo-to-current conversion efficiency. Moreover, the anchoring
carboxyl group with TiO in CPR-series dyes is disjointed with
rhodanine cycle and hence the conjugation system of the whole
molecule due to the presence of the methylene group. Accordingly,
cyanoacetic acid acceptor in CPC-series dyes, forming a good
conjugation with the whole molecule, plays a better role in electron
broad IPCE spectra among 300e650 nm. However, CPC-series dyes
show distinctively superior IPCE spectra to CPR-series dyes and
exhibit broader IPCE responsive region with higher IPCE value.
CPR-1 and CPR-2 reach the maximum IPCE value 22% and 30% at
50 nm respectively while the IPCE value of CPC-1 exceeds 30% at
40e520 nm. CPC-2 shows the best IPCE action spectrum among
2
4
3
2
injection process into TiO than rhodanine acetic acid acceptor in
four dyes which keeps a plateau from 410 nm to 490 nm with 50%
IPCE values. As mentioned above, CPC-series dyes have higher
CPR-series dyes. Therefore, besides higher loading amount, CPC-
series dyes with cyanoacetic acid acceptor achieve better overlap
of the electron density of the dyes with the semiconductor and
higher electron injection efficiency. Improved light-harvesting
ability and better electron injection efficiency contribute to supe-
rior IPCE action spectra of CPC-series dyes.
loading amount on TiO
enhanced absorption intensity compared with in solution. With
further increase of TiO film thickness as IPCE is measured, the
2
than CPR-series dyes and therefore show
2
loading amount of CPC-series dyes may continually raise, which
contributes to increased light capture and benefits better incident
On the other hand, the linker between carbazole donor and
phenothiazine donor plays its role. Compared with short butyl
linker in CPC-1 and CPR-1, long octyl linker brings about broader
and higher IPCE spectra to CPC-2 and CPR-2, indicating that the
Table 2
Energy level data of phenothiazine-carbazole DB dyes at the B3LYP/6-31G(d).
linker of longer alkyl chain could restrict the
p-p stacking of dye
Compd
HOMO
E
L-H
LUMO
more efficiently and further promote the electron injection
ability.
Generally, better IPCE action spectra correspond to higher JSC. JSC
for four dyes ranks by the order of CPC-2 > CPC-1 > CPR-2 > CPR-1.
CPC-2 and CPR-2 with octyl linker show higher JSC than their
19
CPC-1
CPC-2
CPR-1
CPR-2
ꢀ5.51
ꢀ5.40
ꢀ5.49
ꢀ5.38
2.97
2.95
2.80
2.76
ꢀ2.54
ꢀ2.45
ꢀ2.69
ꢀ2.62
Please cite this article in press as: Zhu B-y, et al., Asymmetric double donor-
p-acceptor dyes based on phenothiazine and carbazole donors for
dye-sensitized solar cells, Tetrahedron (2017), https://doi.org/10.1016/j.tet.2017.09.018

B.-y. Zhu et al. / Tetrahedron xxx (2017) 1e9
7
Fig. 4. Frontier molecular orbitals (HOMO, LUMO) of phenothiazine-carbazole DB dyes.
Fig. 5. Incident photo-to-current conversion efficiency (IPCE) curve (a) and current density-potential (IeV) curve (b) of phenothiazine-carbazole DB dyes.
counterpart with butyl linker. CPC-2 achieves the highest JSC among
four dyes thanks for its superior IPCE spectrum. In addition,
consistent with their higher IPCE responsive value, CPC-series dyes
have higher JSC than CPR-series dyes. JSC mainly depends on the
light-harvesting ability and the electron injection efficiency. Infe-
rior electron injection efficiency of CPR-series dyes counteracts
3.5. Electrochemical properties
Electrochemical impedance spectroscopy (EIS) is used to
investigate charge recombination of DSSCs based on phenothiazine
-carbazole DB dyes and conducted in the dark under the bias of VOC
The corresponding Nyquist and Bode spectra are presented in Fig. 6
along with the equivalent circuit. The corresponding resistance
parameters were obtained through software ZSimpWin and listed
in Table 4.
In the Nyquist spectrum, CPC-series dyes show two semicircles
while one semicircle is observed for CPR-series dyes, which is
similar with our previous work. Generally, the semicircle at
0.05e1 Hz relates to the diffusion of I
perhaps due to the low viscosity of the present electrolyte. The
additional semicircle of CPC-series dyes at 1k-100k Hz is assigned
to the redox reaction at the platinum counter electrode which may
.
their better light-harvesting ability and leads to low JSC
.
VOC of CPR-series dyes is also brought down by unsatisfactory
electron injection efficiency and less than 0.6 eV, while CPC-series
dyes exhibit VOC higher than 0.7 eV. On the other hand, anti-
aggregation effect of octyl chain favors the improvement of VOC
.
2
2
CPC-2 and CPR-2 exhibit higher VOC than CPC-1 and CPR-1,
respectively. Therefore, among four dyes, CPC-2 with cyanoacetic
acid acceptor and octyl chain gives the optimal photovoltaic per-
ꢀ
3
, which is often not observed
2
3
ꢀ
2
formance of 4.79% thanks to its superior JSC of 9.64 mA cm and
OC of 0.72 V.
V
2
4
be too quick in the case of CPR-series dyes to be observed. The
semicircle at 1-1k Hz in the Nyquist diagram is attributed to the
electron transfer at the photoanode/electrolyte interface and cor-
responds to Rct in Table 4. CPC-series dyes with cyanoacetic acid
acceptor exhibit larger semicircles and higher Rct than their coun-
terpart with rhodanine acetic acid acceptor. Larger semicircle cor-
responds to higher Rct, indicating slow charge recombination rate,
less dark current and high VOC. With larger conjugation system and
better coplanarity, CPR-series dyes may have an increasing
Table 3
Parameters for DSSCs based on phenothiazine-carbazole DB dyes.
Compd
V
OC (V)
J
SC (mA cmꢀ2)
FF
h %
CPC-1
CPC-2
CPR-1
CPR-2
0.70
0.72
0.52
0.56
7.06
9.64
3.69
4.29
0.85
0.69
0.65
0.68
4.23
4.79
1.25
1.63
Please cite this article in press as: Zhu B-y, et al., Asymmetric double donor-
p-acceptor dyes based on phenothiazine and carbazole donors for
dye-sensitized solar cells, Tetrahedron (2017), https://doi.org/10.1016/j.tet.2017.09.018

8
B.-y. Zhu et al. / Tetrahedron xxx (2017) 1e9
Fig. 6. Electrochemical impedance spectra for DSSCs based on phenothiazine-carbazole DB dyes (a) Nyquist plot; (b) Bode plot; (c) equivalent circuit.
tendency to aggregate and be easier to go through charge recom-
bination. In addition, long octyl chain benefits the antiaggregation
effect and contributes to the inhibition of charge recombination.
Both CPC-2 and CPR-2 show larger semicircles and higher Rct than
CPC-1 and CPR-1, respectively. These charge recombination results
roughly agree with VOC order. Though it shows similar charge
recombination rate with CPC-1, CPR-2 has a lower VOC by 0.14 V
than CPC-1 due to its inferior electron injection since VOC depends
on both the electron injection efficiency and the charge recombi-
nation rate.
long octyl chain favors antiaggregation effect and brings about
broader IPCE spectra and lower charge recombination rate. There-
fore, dye with cyanoacetic acid acceptor and octyl chain achieves
the optimal photovoltaic performance owing to its superior elec-
tron injection efficiency.
Acknowledgement
We gratefully acknowledge the financial supported by the
funding of National Natural Science Foundation of China
(21406202).
4
. Conclusion
Appendix A. Supplementary data
Four asymmetric double D-
p
-A dyes were synthesized with
-A dye through the
connection of butyl chain or octyl chain. Compared with the re-
ported D- -A counterparts, these dyes exhibit significant increase
of molar extinction coefficients which were at the range of
phenothiazine D-
p-A dye and carbazole D-
p
Supplementary data related to this article can be found at
https://doi.org/10.1016/j.tet.2017.09.018.
p
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4
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dye-sensitized solar cells, Tetrahedron (2017), https://doi.org/10.1016/j.tet.2017.09.018