T-shaped (D)2–A–π–A type sensitizers incorporating indoloquinoxaline and triphenylamine for organic dye-sensitized solar cells

Article abstract of DOI:10.1016/j.electacta.2017.02.166

Four novel T-shaped metal-free organic sensitizers QX22–25 based on triphenylamine and indoloquinoxaline have been successfully designed and synthesized as a (D)₂–A–π–A type structure. These dye sensitizers have two triphenylamine donors attach

Full text of DOI:10.1016/j.electacta.2017.02.166

Accepted Manuscript
Title: T-shaped (D) –A–␲–A type sensitizers incorporating
2
indoloquinoxaline and triphenylamine for organic
dye-sensitized solar cells
Authors: Xing Qian, Xiaolin Lan, Rucai Yan, Yiming He,
Jiazheng Huang, Linxi Hou
PII:
S0013-4686(17)30446-2
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.electacta.2017.02.166
EA 29031
To appear in:
Electrochimica Acta
Received date:
Revised date:
Accepted date:
5-1-2017
26-2-2017
27-2-2017
Please cite this article as: Xing Qian, Xiaolin Lan, Rucai Yan, Yiming He,
Jiazheng Huang, Linxi Hou, T-shaped (D)2–A–␲–A type sensitizers incorporating
indoloquinoxaline and triphenylamine for organic dye-sensitized solar cells,
Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2017.02.166
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T-shaped (D) –A–π–A type sensitizers incorporating
2
indoloquinoxaline and triphenylamine for organic
dye-sensitized solar cells
Xing Qian, Xiaolin Lan, Rucai Yan, Yiming He, Jiazheng Huang, Linxi Hou*
College of Chemical Engineering, Fuzhou University, Xueyuan Road No. 2, Fuzhou
3
50116, China
*
Corresponding author. E-mail address: lxhou@fzu.edu.cn.
1

Graphical Abstract
Highlights:
1
2
3
4
. Four novel T-shaped (D) –A–π–A type organic dyes have been synthesized.
2
. Two triphenylamine donors and an indoloquinoxaline acceptor were combined.
. Different acceptors and π-bridges were used to tune the photoelectric properties.
. A highest PCE of 7.09% was achieved by the dye QX23.
Abstract: Four novel T-shaped metal-free organic sensitizers QX22–25 based on
triphenylamine and indoloquinoxaline have been successfully designed and
synthesized as a (D) –A–π–A type structure. These dye sensitizers have two
2
triphenylamine donors attaching on an indoloquinoxaline-based subordinate acceptor.
Different π-bridges (thiophene and furan) and acceptor/anchoring groups
(
cyanoacrylic acid and 2-(1,1-dicyanomethylene)rhodanine) were involved to tune the
2

photoelectrical properties. Their optical, electrochemical, and photovoltaic properties
as well as the DFT calculations have been systematically investigated, indicating
these four dyes are all capable as photosensitizers. A highest power conversion
−2
efficiency up to 7.09% with a Jsc of 12.9 mA cm and a Voc of 817 mV has been
achieved by the dye QX23 with a furan π-bridge and a cyanoacrylic acid
acceptor/anchoring group, indicating it is a promising strategy to construct efficient
(
D) –A–π–A type sensitizers by incorporating indoloquinoxaline and triphenylamine.
2
Keywords: Dye-sensitized solar cells; Organic sensitizers; Triphenylamine;
Indoloquinoxaline; (D) –A–π–A structure
2
1
. Introduction
Dye-sensitized solar cells (DSSCs) as promising photoelectric devices have shown
great potential due to their high power conversion efficiency (PCE) and low cost of
production [1,2]. A typical DSSC contains a semiconductor photoanode, an electrolyte,
a counter electrode, and a dye sensitizer [3–5]. In DSSC devices, dye sensitizers play
a key role on capturing sunlight, transferring charge, and injecting electron into
photoanode [6–10]. To date, a landmark PCE of 13% have been reported for zinc
porphyrin-based DSSC under AM 1.5G illumination [11]. Compared with the
well-known efficient porphyrin sensitizers, more and more attention has been devoted
in developing the metal-free organic sensitizers recently owing to their low cost, easy
3

synthetic progress, and high yields [12–16].
Donor–π–acceptor (D–π–A) type structure is the common conformation for most
organic sensitizers because of their promoted intramolecular charge transfer (ICT)
[17–21]. Recently, the D–A–π–A type structure with an additional acceptor involved
into the conventional D–π–A structure was intensively reported and received
increasing attention [22–26]. Through inserting a subordinate acceptor acting as the
electron trap to promote charge separation and facilitate electron transfer to the final
acceptor, the D–A–π–A type sensitizers exhibit broad responsive spectra and long
term photo-stability [27]. A series of electron-withdrawing groups such as quinoxaline
[28–30], benzothiadiazole [31–34], benzotriazole [35,36], benzoxadiazole [37], and
diketopyrrolopyrrole [38,39] etc. have been incorporated into the D–A–π–A type
sensitizers to conveniently tune the molecular energy level and modulate the
light-harvesting response range.
Herein, we report a novel T-shaped (D) –A–π–A type structure with double
2
triphenylamine donors and a indoloquinoxaline as the subordinate acceptor.
Triphenylamine containing an electron-rich nitrogen atom also has a nonplanar
propeller conformation in the ground state, which can suppress the intermolecular
aggregations. Thus, triphenylamine is widely used as the donor part for metal-free
organic sensitizers in DSSCs due to its unique electronic and optical properties
[40–43]. On the other hand, 6H-indolo[2,3-b]quinoxaline contains an
electron-deficient quinoxaline which is fused with an electron-rich indole unit and it
can be considered as a rigid planar built-in donor-acceptor chromophore [44]. This
4

unique dipolar structure endows indoloquinoxaline with an excellent natural potential
for ICT process [45]. On this research, our strategy is to introduce two triphenylamine
donors attached on an indoloquinoxaline acceptor to form a T-shaped (D)
and to combine the advantages of triphenylamine and indoloquinoxaline. This (D)
moiety can be used to connect with the π–A moiety to form a (D) –A–π–A system.
2
–A structure
2
–A
2
Based on their strong ICT characteristic and high synthetic flexibility of
triphenylamine and indoloquinoxaline, we have designed and synthesized four new
organic dyes QX22–25. Different acceptors (cyanoacetic acid with a -COOH
anchoring group and 2-(1,1-dicyanomethylene)rhodanine, DCRD, without a -COOH
anchoring group) and different π-bridges (thiophene and furan) have been investigated
to further tune the photoelectric properties of these dyes. The chemical structures of
the four dyes are shown in Fig. 1.
2
2
. Experimental section
.1. Materials and instruments
All reagents were obtained from Sigma-Aldrich, TCI and Alfa aesar. All solvents
were used after being purified through common standard purification methods. FTO
–
1
conductive glasses (a sheet resistance of 15 Ω sq , Nippon Sheet Glass) were used.
The H and 13C NMR measurements were performed on a Bruker 400 MHz
spectrometer. The UV-vis absorption spectra of all dyes were recorded on a UV-vis
spectrophotometer (Varian Cary 300 Conc). HR-MS data were recorded on a Varian
1
7
.0T FTMS system. Cyclic voltammetry (CV) and electrochemical impedance
5

spectroscopy (EIS) tests were performed using an electrochemical workstation
CHI660E, CH Instrument). CV measurements were conducted using a
(
three-electrode system with a glassy carbon electrode as the working electrode, an
+
Ag/Ag electrode as the reference electrode, and a platinum plate as the counter
electrode. The redox potentials were recorded in dichloromethane (DCM) solutions
using 0.1 M tetrabutylammonium hexafluorophosphate (n-Bu
4
NPF ) as the supporting
6
–
1
electrolyte with a scan rate of 100 mV S .
.2. Fabrication and Characterization of DSSCs
The TiO photoanodes were prepared according to our previous works [46]. A
transparent nanocrystalline TiO film (~12 µm) and a scattering layer (~4 µm) were
2
2
2
prepared over FTO glasses by screen-printing method. Afterwards, the electrodes
were gradually heated to 500 °C and sintered for 1 hour. Then, the prepared
electrodes were treated with 0.04 M TiCl aqueous solution at 70 °C for 1 hour and
4
sintered again at 500 °C for 1 hour. The Pt counter electrodes were produced by
thermal deposition of 20 mM chloroplatinic acid hexahydrate in isopropanol onto the
FTO glasses at 450 °C for 0.5 hour. The resulting TiO
the commercial N719 dye solution (0.3 mM in ethanol) for 12 hours. The sensitization
of the organic dyes on TiO photoanodes was performed in 0.3 mM dye solution for
iodine-based electrolyte composed of 0.3
,2-dimethyl-3-propylimidazolium iodide (DMPII), 0.1 M LiI, 0.05 M I , and 0.5 M
-tert-butylpyridine in acetonitrile (ACN) was used. The solar cells were fabricated
2
electrodes were immersed in
2
1
1
4
2
hours.
The
M
2
into a sandwich-type structure and illuminated by a standard solar simulator
6

(
XM-500W, Trusttech) under a standard 100 mW cm–2 irradiation, which was
calibrated by a standard silicon solar cell (91150V, Newport). The incident
photon-to-current conversion efficiency (IPCE) was tested by an IPCE system (QTest
2
000). The current-voltage (J-V) characteristic curves of the DSSC under simulated
irradiation were recorded by an electrochemical workstation (CHI660E, CH
Instrument).
2
2
.3. Synthesis
.3.1.
Synthesis
of
compound
1
(
4,4'-(benzo[c][1,2,5]thiadiazole-4,7-diyl)bis(N,N-diphenylaniline))
A mixture of 4,7-dibromo-2,1,3-benzothiadiazole (400 mg, 1.36 mmol),
-(diphenylamino)phenylboronicacid (983 mg, 3.40 mmol), Pd(dppf)Cl (55 mg, 0.07
CO (1.89 g, 13.6 mmol), toluene (24 mL) and DMF (8 mL) was refluxed
atmosphere for 20 hours. The mixture was poured into water and extracted
with DCM. After the organic phase was washed with water thoroughly and it was
dried over anhydrous Na SO . The solvent was removed under vacuum, and then the
4
2
mmol), K
2
3
under a N
2
2
4
crude product was purified by silica-gel column chromatography using DCM/PE (1:4)
as the eluent and the compound 1 as a red solid was obtained (yield: 95.3%). Mp
2
42–244 °C. IR (KBr) ν: 2955, 2923, 2853, 1588, 1480, 1400, 1384, 1314, 1274,
–
1 1
1
7
1
1
180, 1076, 1027, 884, 822, 752, 695, 620, 512 cm . H NMR (400 MHz, CDCl
3
) δ
.88 (d, J = 8.7 Hz, 4H), 7.74 (s, 2H), 7.30 (t, J = 7.8 Hz, 8H), 7.21 (t, J = 8.6 Hz,
1
3
2H), 7.07 (t, J = 7.3 Hz, 4H). C NMR (100 MHz, CDCl ) δ 154.17, 147.99, 147.51,
3
32.19, 131.02, 129.90, 129.36, 127.45, 124.90, 123.30, 122.95. HR-MS (ESI): m/z
7

+
[
M+H] calcd for C42
H
31
4
N S, 623.2264; found, 623.2250.
2
.3.2. Synthesis of 3
(
4,4'-(8-bromo-6H-indolo[2,3-b]quinoxaline-1,4-diyl)bis(N,N-diphenylaniline))
A mixture of 1 (400 mg, 0.64 mmol), zinc granule (2.00 g, 30.6 mmol),
hydrochloric acid (36%, 10 mL), CHCl
under a N atmosphere for 3 hours. The mixture was poured into water and extracted
with CH Cl . After being washed with water thoroughly, the organic phase was
separated and dried over anhydrous Na SO . After the solvent was removed, the
3
(20 mL) and EtOH (10 mL) was refluxed
2
2
2
2
4
compound 2 was obtained and used directly for the next reaction without further
purification because it was sensitive to air.
Then the product with 6-bromoisatin (145 mg, 0.64mmol), p-toluenesulfonic acid
(
200 mg) and toluene (40 mL) was refluxed at 140 °C for 4 hours. The mixture was
poured into water and extracted with CH Cl . The organic phase washed with water
and dried over anhydrous Na SO . After the solvent was removed under vacuum, the
2
2
2
4
product was mixed with potassium tert-butoxide (215 mg, 1.92 mmol), tetrabutyl
ammonium bromide (100 mg, 0.31 mmol) and THF (20 mL) and stirred at room
temperature for 1 hour before 2-ethylhexyl bromide (367 mg, 1.92 mmol) was added
to the reaction mixture. The mixture was refluxed for 2 hours at 80 °C before being
poured into water and extracted with CH
2
Cl
2
. After being washed with water
SO and the
thoroughly, the combined organic phase was dried over anhydrous Na
2
4
solvent was removed under vacuum. The crude product was purified by silica-gel
column chromatography using DCM/PE (1:2) as the eluent and the compound 3 as a
8

yellow solid was obtained (yield: 77.9%). Mp 203–205 °C. IR (KBr) ν: 2956, 2923,
852, 1594, 1511, 1493, 1404, 1314, 1277, 1192, 1178, 1063, 821, 752, 695, 510
2
–
1 1
cm . H NMR (400 MHz, CDCl
3
) δ 8.28 (d, J = 8.2 Hz, 1H), 7.91 (d, J = 7.5 Hz, 1H),
.88–7.72 (m, 5H), 7.65–7.60 (m, 1H), 7.48 (dd, J = 8.2, 1.4 Hz, 1H), 7.38–7.22 (m,
0H), 7.09 (q, J = 7.1 Hz, 4H), 4.37–4.17 (m, 2H), 1.48–1.26 (m, 7H), 1.25–1.19 (m,
7
2
2
1
1
1
3
1
3
H), 0.94–0.89 (m, 3H), 0.82 (t, J = 7.1 Hz, 3H). C NMR (100 MHz, CDCl ) δ
3
47.90, 147.85, 147.19, 146.91, 145.64, 144.96, 139.39, 138.74, 137.86, 137.80,
37.63, 133.53, 133.24, 131.95, 131.77, 129.30, 129.25, 129.03, 126.54, 124.73,
24.38, 124.11, 123.74, 123.18, 122.96, 122.89, 122.80, 118.79, 112.89, 45.88, 38.46,
+
0.66, 28.41, 23.94, 23.01, 13.98, 10.43. HR-MS (ESI): m/z [M+H] calcd for
C
58
H51BrN
5
, 896.3322; found, 896.3307.
2
.3.3.
Synthesis
of
compound
4
(
5-(1,4-bis(4-(diphenylamino)phenyl)-6-(2-ethylhexyl)-6H-indolo[2,3-b]quinoxali
n-8-yl)thiophene-2-carbaldehyde)
A mixture of 3 (310 mg, 0.35 mmol), 5-formyl-2-thiopheneboronicacid (135 mg,
0
.87 mmol), Pd(dppf)Cl
40 mL) and DMF (8 mL) was stirred under a N
Then the mixture was poured into water and extracted with CH
washed with water thoroughly, the organic phase was separated and dried over
anhydrous Na SO . The crude product was purified by silica-gel column
2
(30 mg, 0.04 mmol), K
2
CO
atmosphere for 4 hours at 80 °C.
Cl . After being
3
(500 mg, 1.73 mmol), toluene
(
2
2
2
2
4
chromatography using DCM/PE (1:1) as the eluent and the compound 4 as a red solid
was obtained (yield: 88.9%). Mp 118–120 °C. IR (KBr) ν: 2956, 2923, 2852, 1669,
9

1
593, 1510, 1492, 1466, 1409, 1314, 1275, 1217, 1178, 1019, 821, 753, 695, 669, 504
–
1 1
cm . H NMR (400 MHz, CDCl
3
) δ 9.94 (s, 1H), 8.43 (d, J = 8.1 Hz, 1H), 7.89 (d, J
=
7.5 Hz, 1H), 7.87–7.73 (m, 6H), 7.70 (s, 1H), 7.66 (s, 1H), 7.57 (d, J = 3.9 Hz, 1H),
7
1
.36–7.30 (m, 6H), 7.29–7.19 (m, 14H), 7.12–7.00 (m, 4H), 4.40–4.24 (m, 2H),
.43–1.34 (m, 5H), 1.28–1.23 (m, 6H), 0.93–0.88 (m, 3H), 0.79 (t, J = 7.2 Hz, 3H).
13C NMR (100 MHz, CDCl
) δ 182.76, 154.20, 147.90, 147.86, 147.21, 146.93,
3
1
1
1
2
9
2
45.49, 145.23, 143.07, 139.42, 138.86, 137.83, 137.76, 137.36, 134.99, 133.50,
33.25, 131.98, 131.78, 129.33, 129.28, 129.17, 126.55, 125.00, 124.75, 124.42,
23.71, 123.14, 123.00, 122.88, 122.84, 120.69, 118.90, 107.23, 45.75, 38.70, 29.74,
+
8.54, 24.06, 23.03, 14.01, 10.54.HR-MS (ESI): m/z [M+H] calcd for C63
H
54
N
5
OS,
28.4044; found, 928.4027.
.3.4.
Synthesis
of
5
(
5-(1,4-bis(4-(diphenylamino)phenyl)-6-(2-ethylhexyl)-6H-indolo[2,3-b]quinoxali
n-8-yl)furan-2-carbaldehyde)
The synthetic process of 5 resembles that of 4. The crude product was purified by
silica-gel column chromatography using DCM/PE (1:1) as the eluent and the
compound 5 as a red solid was obtained (yield: 89.6%). Mp 116–118 °C. IR (KBr) ν:
2
9
956, 2924, 2854, 1677, 1629, 1592, 1468, 1410, 1314, 1275, 1180, 1073, 1026, 972,
–
1 1
37, 821, 753, 695, 621, 512 cm . H NMR (400 MHz, CDCl ) δ 9.76 (s, 1H), 8.47
3
(d, J = 8.1 Hz, 1H), 7.91 (d, J = 7.5 Hz, 2H), 7.89–7.82 (m, 3H), 7.8–7.76 (m, 3H),
7
4
.42 (d, J = 3.7 Hz, 1H), 7.34 (d, J = 7.4 Hz, 6H), 7.31–7.23 (m, 14H), 7.12–7.05 (m,
H), 4.44–4.31 (m, 2H), 1.44–1.38 (m, 4H), 1.29 (s, 7H), 0.91 (s, 3H), 0.81 (t, J = 7.2
10

1
3
Hz, 3H). C NMR (100 MHz, CDCl
3
) δ 177.29, 159.31, 152.42, 147.91, 147.87,
1
1
1
2
47.19, 146.92, 145.47, 145.11, 139.40, 138.87, 137.81, 137.74, 133.52, 133.29,
31.99, 131.79, 130.61, 129.32, 129.28, 129.15, 126.50, 125.33, 124.74, 124.43,
23.47, 123.13, 122.99, 122.89, 122.84, 120.76, 117.78, 109.05, 105.92, 45.80, 38.59,
+
9.75, 28.52, 24.07, 23.02, 14.01, 10.56. HR-MS (ESI): m/z [M+H] calcd for
C
63
H
54
N
5
O
2
, 912.4272; found, 912.4259.
2
.3.5.
Synthesis
of
compound
QX22
(
(E)-3-(5-(1,4-bis(4-(diphenylamino)phenyl)-6-(2-ethylhexyl)-6H-indolo[2,3-b]qui
noxalin-8-yl)thiophen-2-yl)-2-cyanoacrylic acid)
A mixture of compound 4 (90 mg, 0.097 mmol), cyanoacetic acid (41 mg, 0.49
mmol), CH
hours under a N
with CH Cl . After washed with water thoroughly, the organic phase was separated
and dried over anhydrous Na SO . The crude product was purified by silica-gel
3
CN/CHCl
3
(10/20 mL) and piperidine (0.5 mL) was stirred at 85 °C for 5
2
atmosphere. Then the mixture was poured into water and extracted
2
2
2
4
column chromatography using DCM/MeOH (15:1) as the eluent and the compound
QX22 as a red solid was obtained (yield: 94.2%). Mp 270–272 °C. IR (KBr) ν:
2
8
7
1
1
955, 2924, 2854, 1629, 1593, 1509, 1493, 1458, 1399, 1384, 1276, 1180, 1081, 965,
–
1 1
20, 752, 696 cm . H NMR (400 MHz, DMSO-d ) δ 8.13 (d, J = 11.4 Hz, 2H),
6
.91–7.82 (m, 2H), 7.76 (d, J = 8.1 Hz, 2H), 7.74–7.61 (m, 5H), 7.57 (d, J = 8.3 Hz,
H), 7.43–7.26 (m, 8H), 7.19–6.93 (m, 16H), 4.27–4.13 (m, 2H), 1.25–1.17 (m, 6H),
1
3
.11–1.01 (m, 3H), 0.76 (t, J = 7.2 Hz, 3H), 0.65 (t, J = 7.1 Hz, 3H). C NMR (100
MHz, DMSO-d
6
) δ 152.40, 147.66, 146.65, 146.62, 145.23, 144.72, 142.98, 140.16,
11

1
1
1
2
39.17, 137.16, 135.86, 134.88, 134.49, 133.35, 133.18, 133.17, 132.15, 132.13,
32.06, 131.93, 131.91, 129.86, 129.75, 128.86, 124.79, 124.55, 124.35, 123.51,
23.38, 122.46, 122.41, 122.40, 122.34, 122.31, 122.28, 107.00, 106.98, 38.27, 30.46,
+
9.45, 28.21, 23.74, 22.89, 14.14, 10.59. HR-MS (ESI): m/z [M+H] calcd for
C
66
H
55
N
6
O
2
S, 995.4102; found, 995.4082.
Synthesis of
(E)-3-(5-(1,4-bis(4-(diphenylamino)phenyl)-6-(2-ethylhexyl)-6H-indolo[2,3-b]qui
2
.3.6.
compound
QX23
(
noxalin-8-yl)furan-2-yl)-2-cyanoacrylic acid)
The synthetic process of QX23 resembles that of QX22. The product was purified
by silica-gel column chromatography using DCM/MeOH (15:1) as the eluent and the
compound QX23 as a red solid was obtained (yield: 93.6%). Mp 240–242 °C. IR
(KBr) ν: 2956, 2924, 2854, 1594, 1551, 1511, 1493, 1467, 1395, 1314, 1276, 1187,
–
1 1
1
075, 1025, 968, 821, 793, 752, 696, 617, 510 cm . H NMR (400 MHz, DMSO-d )
6
δ 8.15 (d, J = 7.5 Hz, 1H), 8.00 (s, 1H), 7.91 (s, 1H), 7.78–7.62 (m, 7H), 7.43–7.29
m, 10H), 7.18–7.02 (m, 16H), 4.19–4.09 (m, 2H), 1.25 (d, J = 13.3 Hz, 6H), 1.03 (d,
(
1
3
J = 6.0 Hz, 3H), 0.75 (t, J = 7.1 Hz, 3H), 0.62 (t, J = 6.9 Hz, 3H). C NMR (100
MHz, DMSO-d ) δ 153.81, 153.79, 152.41, 151.38, 149.56, 143.45, 142.84, 141.83,
40.62, 139.79, 138.11, 137.93, 136.96, 136.92, 136.72, 136.22, 134.64, 134.53,
6
1
1
1
3
31.77, 131.07, 129.58, 129.53, 129.30, 128.28, 128.25, 128.16, 127.22, 127.19,
27.17, 127.12, 127.08, 124.02, 123.98, 119.22, 115.95, 111.08, 110.80, 42.60, 34.76,
+
4.22, 32.66, 28.21, 27.65, 18.86, 15.20. HR-MS (ESI): m/z [M+H] calcd for
C
66
H
55
N
6
3
O , 979.4330; found, 979.4314.
12

2
.3.7.
Synthesis
of
compound
QX24
(
(Z)-2-(5-((5-(1,4-bis(4-(diphenylamino)phenyl)-6-(2-ethylhexyl)-6H-indolo[2,3-b]
quinoxalin-8-yl)thiophen-2-yl)methylene)-4-oxothiazolidin-2-ylidene)malononitri
le)
A
mixture
-(4-oxothiazolidin-2-ylidene)malononitrile (30 mg, 0.18 mmol), ethanol/CHCl
0 mL) and 10 drops of NaOH (20%) was refluxed at 80 °C for 10 hours under a N
Cl . After
being washed with water thoroughly, the organic phase was separated and dried over
anhydrous Na SO The crude product was purified by silica-gel column
of
compound
5
(55
mg,
0.059
mmol),
2
3
3
(1:2,
2
atmosphere. Then the mixture was poured into water and extracted with CH
2
2
2
4
chromatography using DCM/MeOH (15:1) as the eluent and the compound QX24 as
a red solid was obtained (yield: 95.5%). Mp > 300 °C. IR (KBr) ν: 2956, 2924, 2853,
1
6
7
654, 1637, 1593, 1491, 1459, 1400, 1384, 1313, 1276, 1185, 1081, 1025, 821, 754,
–
1 1
98 cm . H NMR (400 MHz, DMSO-d
6
) δ 8.21 (d, J = 8.1 Hz, 1H), 8.00 (s, 1H),
.90 (d, J = 3.6 Hz, 1H), 7.85 (d, J = 7.5 Hz, 1H), 7.82–7.66 (m, 7H), 7.62 (d, J = 3.7
Hz, 1H), 7.44–7.30 (m, 8H), 7.23–7.00 (m, 16H), 4.38–4.24 (m, 2H), 1.30 (s, 5H),
.22–1.14 (m, 2H), 1.10 (d, J = 6.4 Hz, 2H), 0.83 (t, J = 7.2 Hz, 3H), 0.69 (t, J = 7.2
1
1
3
Hz, 3H). C NMR (100 MHz, DMSO-d
6
) δ 185.03, 183.25, 178.97, 152.40, 152.37,
1
1
1
1
51.64, 151.43, 150.37, 149.84, 145.05, 143.56, 142.94, 142.50, 141.92, 140.58,
39.00, 138.12, 138.02, 136.98, 136.80, 134.81, 134.69, 133.46, 131.83, 131.26,
29.57, 129.28, 128.45, 128.30, 127.36, 127.15, 125.94, 125.09, 123.55, 123.02,
21.65, 111.58, 99.99, 84.39, 53.70, 43.03, 35.32, 32.85, 27.62, 22.44, 18.95, 15.45.
13

–
HR-MS (ESI): m/z [M–H] calcd for C69
.3.8. Synthesis
(Z)-2-(5-((5-(1,4-bis(4-(diphenylamino)phenyl)-6-(2-ethylhexyl)-6H-indolo[2,3-b]
H
53
N
8
OS
2
, 1073.3789; found, 1073.3785.
2
of
compound QX25
(
quinoxalin-8-yl)furan-2-yl)methylene)-4-oxothiazolidin-2-ylidene)malononitrile)
The synthetic process of compound QX25 resembles that of compound QX24 and
the crude product was purified by silica-gel column chromatography using
DCM/MeOH (15:1) as the eluent and the compound QX25 as a deep red solid was
obtained (yield: 93.1%). Mp 226–228 °C. IR (KBr) ν: 2956, 2924, 2853, 1654, 1636,
1
595, 1491, 1468, 1400, 1384, 1314, 1277, 1189, 1075, 1028, 968, 821, 751, 696
–
1
1
cm . H NMR (400 MHz, DMSO-d
6
) δ 8.24 (d, J = 8.1 Hz, 1H), 7.84 (s, 1H),
.81–7.73 (m, 4H), 7.70 (dd, J = 8.1, 3.1 Hz, 3H), 7.42–7.31 (m, 11H), 7.17–7.06 (m,
6H), 4.22 (d, J = 7.1 Hz, 2H), 1.33–1.22 (m, 7H), 1.06–1.01 (m, 2H), 0.75 (t, J = 7.3
7
1
1
3
Hz, 3H), 0.63 (t, J = 7.3 Hz, 3H). C NMR (100 MHz, DMSO-d
6
) δ 191.48, 183.30,
1
1
1
1
3
1
60.60, 155.43, 152.39, 152.35, 151.54, 151.31, 149.91, 149.46, 143.52, 142.73,
42.17, 141.95, 141.72, 138.11, 137.71, 136.91, 136.68, 136.11, 134.75, 134.65,
31.28, 131.24, 129.63, 129.40, 129.09, 128.40, 128.30, 126.94, 124.00, 123.34,
22.41, 121.94, 121.22, 120.02, 116.35, 109.59, 84.40, 43.10, 36.35, 35.02, 34.22,
+
2.85, 27.63, 18.84, 15.35. HR-MS (ESI): m/z [M+H] calcd for C69
H
55
N
8
2
O S,
059.4163; found, 1059.4162.
3
3
. Results and discussion
.1. Dye synthesis
14

The syntheses of the four dyes QX22–25 were conducted by a concise synthetic
route which was displayed in Scheme 1. 2-(1,1-dicyanomethylene)rhodanine (DCRD)
was synthesized according to previous literatures [47,48]. Suzuki cross-coupling
reaction
of
,7-dibromo-2,1,3-benzothiadiazole produced the triphenylamine-modified compound
. After reduction of the compound 1 with zinc granule under an acidic condition,
4-(diphenylamino)phenylboronic
acid
with
4
1
ortho-diaminobenzene derivative 2 was obtained, which was used directly for next
procedure. Then, condensation reaction of 2 with 6-bromoindole-2,3-dione and
subsequent
alkylation
H-indolo[2,3-b]quinoxaline intermediate 3. the compound 1 or 3 with
-formylthiophene-2-boronic acid or 5-formylfuran-2-boronic acid gave the
reaction
produced
the
triphenylamine-modified
6
5
π-extended aldehydes 4 or 5. The aldehydes 4 and 5 finally reacted with cyanoacetic
acid or 2-(1,1-dicyanomethylene)rhodanine by Knoevenagel reaction to give these
four dyes QX22, QX23, QX24, and QX25, respectively. The H and 13C NMR
spectra of the new compounds are shown in the supporting information.
1
3
.2. Optical and electrochemical characterization
The UV-vis absorption spectra of QX22–25 in DCM solutions and on photoanode
TiO films are displayed in Fig. 2. The characteristic data are summarized in Table 1.
2
The four dyes all exhibited three strong distinct absorption bands in DCM solutions.
The two distinct absorption bands around 200–350 nm correspond to the localized
aromatic π–π* transitions or n–π* transitions of the conjugated backbone. The
15

absorption bands in the low-energy region around 400–600 nm can be assigned to the
intramolecular charge transfer (ICT) transition [49]. The maximum absorption peaks
of dyes QX22 and QX23 with cyanoacetic acid as the anchoring group were at 428
and
438
nm,
while
the
dyes
QX24
and
QX25
with
2
-(1,1-dicyanomethylene)rhodanine as the anchoring group were shown obviously red
shift by 34 nm and 38 nm compared to that of QX22 and QX23, respectively.
Meanwhile, the absorption peaks of QX23 and QX25 with a furan unit as the π-bridge
were shown a slight red shift than that of QX22 and QX24 with a thiophene unit as
the π-bridge. The molar extinction coefficients at λmax for QX22–25 were 2.47×104
4
4
4
–1
–1
(
428 nm), 2.74×10 (438 nm), 3.23×10 (462 nm), and 1.81×10 M cm (476
nm), respectively. As shown in Fig. 2b, when the four dyes were adsorbed on
photoanode TiO films, the absorption ranges of the four dyes showed much more
2
broader as compared to that of the corresponding absorption spectra in DCM solutions
which may be caused by the aggregation of dye molecules and the interactions
between the dye molecules and TiO semiconductor [50].
2
To investigate the electrochemical performance of the four dyes, cyclic
voltammetry (CV) measurements were tested in DCM solutions and calibrated against
–
1
ferrocene (0.63 V vs. NHE) [51] with a 100 mV s scan rate. The electrochemical
data of the four dyes were displayed in Table 1. The highest occupied molecular
orbitals (HOMOs) of QX22–25 corresponding to their first oxidation potentials (Eox
)
were 0.93, 0.96, 0.95, and 0.94 V (vs. NHE), respectively. The HOMO levels of these
dyes are more positive than the redox potential of the iodine/triiodide electrolyte (0.4
16

V vs. NHE), which ensures the regeneration of oxidized dyes. The band gap energies
0–0) of these four dyes were 2.21, 2.19, 2.17, and 2.16 eV, respectively, which were
(
E
calculated from the onsets of the absorption spectra of these dyes in DCM solutions.
The lowest unoccupied molecular orbitals (LUMOs) of QX22–25 calculated from the
equation Eox – E0–0 were –1.28, –1.23, –1.22, and –1.22 V (vs. NHE), respectively.
Obviously, the LUMO levels of the four dyes are also more negative than the
conduction band (CB) of the TiO
2
electrode (−0.5 V vs. NHE), which guarantees the
procedure of electron injection from the dyes to CB of TiO
2
.
3
.3. Theoretical calculations
In order to deep research the optimized configurations and frontier molecular
orbital electron distributions of the four dyes QX22–25, the density functional theory
DFT) calculations were performed by Gauss09w at B3LYP/6-31G* level. The
optimized configurations and electron distributions of the four dyes are shown in Fig.
. The optimized configurations show that each dye molecule present a T-shaped
(
4
structure with two propeller-shaped triphenylamine moieties attaching on planar rigid
indoloquinoxaline skeleton, which may be beneficial to suppress the intermolecular
aggregation and improve photovoltage. As a result, these dyes with an
indoloquinoxaline core combined with two triphenylamine moieties have a good
balanced structure, which can adequately involve the advantages of indoloquinoxaline
acceptor and triphenylamine donor. The dihedral angles between the π-bridge and
indoloquinoxaline were calculated to be 29.1, 1.9, 28.2, and 1.5° for QX22–25,
17

respectively. Obviously, the dyes QX22 and QX24 based on the furan π-bridge has a
much smaller dihedral angle than that of the dyes QX23 and QX25 based on the
thiophene π-bridge. The smaller dihedral angles between the π-bridge and
indoloquinoxaline acceptor can enhance the electron delocalization and transfer, and
expand the absorption spectra. As shown in Fig. 4, the four dyes have the similar
frontier molecular orbital distributions. The electron density of the HOMO and
HOMO-1 states for these dyes is uniformly distributed on the two triphenylamine
donors. On the other hand, the electron density of the LUMO states of the four dyes
mainly locates through the cyanoacrylic acid/DCRD acceptors and their nearby
π-bridge groups. While as for the LUMO+1 states of these dyes, the electron density
mainly locates on the subordinate acceptor indoloquinoxaline and slightly locates on
the π-bridges and cyanoacrylic acid/DCRD acceptors. This calculated result
demonstrates the T-shaped structure with an indoloquinoxaline core as the subordinate
acceptor and two triphenylamine moieties as the donors is beneficial to the electron
transfer from HOMO/HOMO-1 to LUMO/LUMO+1 by photoexcitation.
3
.4. Photovoltaic performance
The DSSC performances of the solar cells based on QX22–25 under AM 1.5G (100
–
2
mW cm ) irradiation were investigated using an iodine electrolyte composed of 0.3
M DMPII, 0.1 M LiI, 0.05 M I
2
, and 0.5 M 4-tert-butyl pyridine in ACN. The active
2
area of these assembled DSSCs was set as 0.16 cm using a metal mask. As is known
to all, the dye-bath solvents have significant influence on the photovoltaic
18

performances of organic dye-sensitized solar cells. The effects of different solvents
such as dichloromethane (DCM), toluene (PhMe), trichloromethane (TCM),
tetrahydrofuran (THF) and ethanol (EtOH) on the photovoltaic performance of DSSC
devices based on the dye QX22 were investigated and the results are shown in Table 2.
The corresponding photocurrent density-voltage (J-V) curves are displayed in Fig. 5.
Apparently, the cell using the photoanode sensitized in ethanol solution performed the
highest short-circuit current density (Jsc) and the highest open-circuit voltage (Voc
than that of those sensitized in other solutions, which resulted in the highest PCE,
indicating EtOH is the appropriate solvent for these (D) –A–π–A type dyes. The
)
2
photovoltaic performance data of DSSC devices based on other dyes QX23–25 using
different dye-bath solvents are also shown in the supporting information.
The J-V curves of the cells based on QX22–25 are shown in Fig. 6. The cell
performance parameters of QX22–25 using commercial N719 dye as a reference are
displayed in Table 3. The overall PCEs of the four dyes lay in the range of
4
.35–7.09%, with an order of QX24 < QX25 < QX22 < QX23. The dye QX23 with a
cyanoacrylic acid anchoring group and a furan π-bridge exhibited a highest Voc of 817
−2
mV, a highest Jsc of 12.9 mA cm , and a fill factor (FF) of 67.3%, generating a
highest PCE of 7.09%. Finally, the PCE of QX23-based cell reached about 88% of
−2
the conventional Ru(II) dye N719-based cell (Voc: 772 mV; Jsc: 15.8 mA cm ; PCE:
.10%) which was fabricated and measured under the same conditions. When the
π-bridge group was changed to thiophene, the dye QX22 presented a lower Jsc of 11.9
8
19

−2
mA cm , resulting in a lower PCE of 6.05% compared with QX23. As shown in Fig.
6
, QX24 and QX25 with a DCRD anchoring group exhibited a relatively poorer
−2
DSSC performances (QX24: a Voc of 707 mV, a Jsc of 9.03 mA cm , and a PCE of
−2
4
.35%; QX25: a Voc of 724 mV, a Jsc of 9.71 mA cm , and a PCE of 4.81%)
compared with QX22 and QX23 with a cyanoacrylic acid anchoring group.
Obviously, the anchoring groups have a great effect on the DSSC performances of
these dyes and the dye loading amount of QX22 and QX23 with a cyanoacrylic acid
−2
anchoring group was measured to be 121 and 130 nmol cm , respectively. However,
the adsorption amount of QX24 and QX25 with a DCRD anchoring group
−2
significantly decreased to 51.2 and 58.5 nmol cm , respectively, which may be one
of the reasons for the reduction of their Jsc values.
In order to investigate the reason of the different Jsc values of these four dyes, the
incident photon-to-current conversion efficiency (IPCE) measurements of the solar
cells sensitized by QX22–25 have been performed. The IPCE spectra for all these
cells exhibited a broad response within a wavelength range from 350 to 700 nm (Fig.
7
), indicating that all the dyes can efficiently convert the visible light into
photocurrent. The spectra onsets of QX22–25 were at 690, 700, 710, and 720 nm,
respectively. As shown in Fig 7, the cell sensitized by QX23 had the highest IPCE
−2
response corresponding to its highest Jsc value of 12.9 mA cm and gave over 60%
IPCE values from 400 to 600 nm with a maximum of 83% at 450 nm. The highest and
broadest IPCE response could explain its highest Jsc value in the J-V measurements.
20

3
.5. Electrochemical impedance spectroscopy
Electrochemical impedance spectroscopy (EIS) measurements for the four dyes
QX22–QX25 were performed to research the electron recombination in DSSCs
52,53]. The corresponding Nyquist plots and Bode phase plots are shown in Fig. 8.
[
These measurements were performed in a frequency range from 0.1 Hz to 100 kHz
under a bias of –0.80 V in complete darkness. As displayed in Fig. 8a, the larger
semicircle in the low-frequency region corresponds to the charge transfer resistance
(
R
ct) at the dyes/TiO
corresponds to the Rct value and the larger Rct value means the more efficient
hindrance of electron recombination at the dyes/TiO /electrolyte interface, the
2
/electrolyte interface [54,55]. The radius of this semicircle
2
decreased dark current and improved photovoltage [56,57]. The order of Rct values is
QX24 (31.5 Ω) < QX25 (38.4 Ω) < QX22 (54.9 Ω) < QX23 (80.0 Ω), which is
consistent well with the Voc values of QX24 (707 mV) < QX25 (724 mV) < QX22
(
797 mV) < QX23 (817 mV).
Besides, the peaks of the lower frequency region in the Bode plots also comply
with the order of Voc values. The electron lifetime (τ) as another important parameter
can be calculated from the equation τ = 1/(2πf), and the higher electron lifetime means
the more efficient hinder of charge recombination at the dyes/TiO /electrolyte
2
interface and higher photovoltage [59]. For the four dyes based devices, the calculated
electron lifetime values increased in the order of QX24 (10.8 ms) < QX25 (13.1 ms)
21

<
QX22 (19.3 ms) < QX23 (34.3 ms). On the basis of above studies, the best DSSC
performance of QX23 is mainly attributed to its significantly highest IPCE response
namely highest Jsc) and obviously smallest electron recombination rate (namely
highest Voc) compared with the other three dyes.
(
4
. Conclusions
A series of metal-free organic sensitizers (QX22–25) based on triphenylamine and
indoloquinoxaline have been successfully designed and synthesized as a T-shaped
D) –A–π–A type structure for DSSCs. Two triphenylamine donors were attached on
(
2
an indoloquinoxaline-based subordinate acceptor. Different π-bridges such as
thiophene and furan and acceptor/anchoring groups such as cyanoacrylic acid and
2
-(1,1-dicyanomethylene)rhodanine were used to tune their photoelectrical properties.
The corresponding optical, electrochemical, photovoltaic measurements as well as the
DFT calculations have been systematically investigated to certify these dyes are all
capable as photosensitizers. The dye QX23 with a furan π-bridge and a cyanoacrylic
acid acceptor/anchoring group have achieved a highest PCE of 7.09% with a Jsc of
1
2.9 mA cm−2 and a Voc of 817 mV, demonstrating it is a promising strategy to
construct efficient (D)
and triphenylamine.
2
–A–π–A type sensitizers by incorporating indoloquinoxaline
Acknowledgments
We are very grateful to the National Natural Science Foundation of China (No:
22

2
1376054) and Project of Education Department of Fujian Province (No: JAT160052)
for their generous financial support.
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