Pyrrolo[3,2,1-kl]phenothiazine-based D-π-A type organic dyes for efficient dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2016.12.025

Novel metal-free organic dyes based on polycyclic aromatic donor, pyrrolo[3,2,1-kl]phenothiazine, have been synthesized and applied in dye-sensitized solar cells. Combined with cyanoacrylic acid as the acceptor/anchoring group and simple π-spacers such as

Full text of DOI:10.1016/j.dyepig.2016.12.025

Dyes and Pigments 139 (2017) 292e299
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Pyrrolo[3,2,1-kl]phenothiazine-based D-p-A type organic dyes for
efficient dye-sensitized solar cells
Feng Li a , Yi-Zhou Zhu
,
b
a
,
**
, Shao-Chun Zhang , Huan-Huan Gao , Bin Pan ,
a
a
a
a, b, *
Jian-Yu Zheng
a
State Key Laboratory and Institute of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, China
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 September 2016
Received in revised form
Novel metal-free organic dyes based on polycyclic aromatic donor, pyrrolo[3,2,1-kl]phenothiazine, have
been synthesized and applied in dye-sensitized solar cells. Combined with cyanoacrylic acid as the
acceptor/anchoring group and simple
p-spacers such as thiophene (JY40), furan (JY41), and terthiophene
2
December 2016
(
JY42) as the conjugated linker, these dyes show good power conversion efficiency (PCE). The photo-
Accepted 11 December 2016
Available online 18 December 2016
physical, electrochemical and photovoltaic properties, as well as theoretical calculations of these dyes
were fully investigated. An attractive PCE up to 7.54%, with a short-circuit photocurrent density (Jsc) of
ꢀ
2
1
7.76 mA cm , an open-circuit photovoltage (Voc) of 726 mV, and a fill factor (FF) of 0.58 has been
Keywords:
Dye-sensitized solar cells
Organic dyes
Pyrrolo[3,2,1-kl]phenothiazine
Conjugated linker
achieved by the JY42-based cell under standard AM 1.5 G irradiation in conjunction with an iodine
electrolyte.
© 2016 Published by Elsevier Ltd.
II
III
1
. Introduction
using the Co /Co electrolyte under standard conditions [13].
Several high PCEs over 12% have also been achieved based on
metal-free organic dyes [14e17]. However, how to get high device
efficiency with a simple organic dye structure is still a challenge for
DSSCs and calls for more efforts on it.
As a promising photovoltaic technology, dye-sensitized solar
cells (DSSCs) have attracted considerable attention during the past
two decades because of their low cost, easy fabrication and
impressive device efficiency [1,2]. Owing to the decisive influence
of dyes on the device performance [3], various types of sensitizers
including metal complexes, porphyrins and metal-free organic dyes
have been successfully developed and applied in the highly effi-
cient DSSCs. Although ruthenium(II) complexes have been early
designed, and some well-known species such as N3, N719 and black
dye have achieved high conversion efficiency over 10% [4e6], high
cost and potential negative environmental impact unavoidably
limit their practical application in DSSCs. Research interests now
concern more about the porphyrin-based dye [7e9] and metal-free
organic dyes [10e12] due to the structural diversity, understand-
able design and facile modification. Recently, DSSC device sensi-
tized by zinc-porphyrin SM315 has reached an efficiency of ~13% by
Generally, organic dyes are designed with donor-
-A) configuration, for its intrinsic benefit in facilitating the intra-
molecular charge transfer (ICT) from the donor to the acceptor
upon photoirradiation and subsequent electron injection into TiO
semiconductor anode. Structural variation of the donor, -linker or
acceptor will evidently tune the absorption spectra, orbital energy
level and hence the photovoltaic performance of the resultant dye.
Efforts upon novel donor, p-bridge, acceptor and their combination
have been extensively made aiming at the improvement of DSSC
performance. Besides the classic arylamine donor system
[11,18e21], polycyclic aromatic ring has been reported to be
attractive as a donor component.
Some devices with promising photovoltaic performance have
been successfully fabricated using organic sensitizers containing
polycyclic aromatic ring such as carbazole [15,22e24], phenothia-
zine [25e27], phenoxazine [28,29], N-annulated perylene [16,17,30]
and indole derivatives [31,32] as the electron donors. To pursue
greater photovoltaic device performance, new donors derived from
abovementioned backbones with fusing heterocycles [33e36] or
benzene rings [37] have been further synthesized and given
p-acceptor (D-
p
2
p
*
Corresponding author. State Key Laboratory and Institute of Elemento-Organic
Chemistry, Nankai University, Tianjin 300071, China.
*
* Corresponding author.
E-mail addresses: zhuyizhou@nankai.edu.cn (Y.-Z. Zhu), jyzheng@nankai.edu.cn
(
J.-Y. Zheng).
http://dx.doi.org/10.1016/j.dyepig.2016.12.025
143-7208/© 2016 Published by Elsevier Ltd.
0

F. Li et al. / Dyes and Pigments 139 (2017) 292e299
293
enhanced PCE. Pyrrolo[3,2,1-kl]phenothiazine [38,39], a fused tet-
racyclic system of phenothiazine, can be regarded to have both
indole and phenothiazine components in core, which might show
strong electron delocalization and significant potential for intra-
molecular charge transfer (ICT). As far as we are concerned, the
photovoltaic application of pyrrolo[3,2,1-kl]phenothiazine de-
rivatives has not been reported yet. Herein, novel organic sensi-
tizers with pyrrolo[3,2,1-kl]phenothiazine as the electron donor
and cyanoacrylic acid as the electron acceptor/anchoring group
were designed and synthesized. For the purpose of gaining further
insight into such system, different conjugated linkers such as
thiophene (JY40), furan (JY41), and terthiophene (JY42) were
4-tert-butylpyridine in acetonitrile. The DSSCs were illuminated by
a solar simulator (CHF-XM-500W, Trusttech Co. Ltd.) under
ꢀ2
100 mW cm irradiation, which was calibrated by a standard sil-
icon solar cell (91150 V, Newport Corporation). The photocurrent
density-voltage (J-V) characteristic curves of the DSSC under
simulated sunlight were recorded using an electrochemical work-
station (Zahner Corporation). The incident photon-to-current
conversion efficiency (IPCE) was measured using a commercial
setup (QTest Station 2000 IPCE Measurement System, CROWN-
TECH, USA).
2.3. Synthesis and characterization
chosen to act as the
p-bridges. The chemical structures of these
dyes are displayed in Fig. 1.
2.3.1. 4-Bromo-2,6-diiodoaniline (2)
To a solution of 4-bromoaniline (6.88 g, 40 mmol) in anhydrous
ethanol (200 mL), I
2 2 4
(22.34 g, 88 mmol) and Ag SO (14.96 g,
2
. Experimental section
4
8 mmol) were added. After stirring for 24 h at room temperature,
the reaction mixture was filtered to remove AgI precipitate. The
filtrate was concentrated under reduced pressure and dissolved in
2.1. Materials and instruments
ethyl acetate. The organic layer was washed with aqueous Na
and brine, dried over Na SO , filtered and concentrated. The crude
product was purified by column chromatography on silica gel using
CH Cl /petroleum ether (1:6) as the eluent to afford compound 2 as
a white solid (8.46 g, yield: 50%). H NMR (400 MHz, CDCl
2 2 3
S O
All solvents were purified according to standard methods. NMR
2
4
solvents and other chemicals obtained from commercial sources
were used without further purification. 1H NMR and 13C NMR
2
2
spectra were recorded on Bruker 400 MHz spectrometer using TMS
as the internal standard. HR-MS data were obtained on a VG ZAB-
HS or Varian 7.0T FTMS. IR spectra were recorded with KBr pel-
lets on a Bio-rad FTS6000 spectrometer. UVevis spectra were
recorded on a Varian Cary 300 Conc UVevisible spectrophotometer.
Cyclic voltammetry and electrochemical impedance spectroscopy
1
3
)
d
7.73
145.62, 140.97,
n: 3399, 3300, 3195, 3061, 1731, 1607, 1548,
13
(
s, 2H), 4.63 (s, 2H). C NMR (101 MHz, CDCl
3
) d
1
09.87, 81.03. IR (KBr)
ꢀ
1
1439, 1341, 1280, 1232, 1107, 1053, 859, 701 cm . HRMS (ESI): m/z
þ
[MþH] calcd for C
6 5 2
H BrI N, 423.7695; found, 423.7675.
(
EIS) experiments were carried out on a Zennium electrochemical
2
.3.2. 4-Bromo-2-((4-(hexyloxy)phenyl)ethynyl)-6-iodoaniline (3)
A mixture of compound 2 (3.55 g, 8.38 mmol), 1-ethynyl-4-
workstation (Zahner Corporation).
(
0
hexyloxy)benzene (1.69 g, 8.35 mmol), Pd(PPh
.42 mmol) and CuI (161 mg, 0.84 mmol) in THF (20 mL) and
3
)
2
Cl
2
(295 mg,
2.2. Fabrication and characterization of DSSCs
triethylamine (20 mL) was stirred at room temperature for 18 h
under a nitrogen atmosphere. The reaction mixture was filtered,
and the filtrate was concentrated under reduced pressure. The
crude product was purified by column chromatography on silica gel
The photoanode used was the TiO
particles as the absorbing layer and ~4
the scattering layer) coated on fluorine-doped tin oxide (FTO,
Nippon Sheet Glass, 15 /sq) glass substrate by doctor-blade
electrodes were gradually heated to
2
thin film (~10
mm of 20 nm
mm of 200 nm particles as
U
using CH
pound 3 as a yellow solid (2.58 g, yield: 62%). H NMR (400 MHz,
CDCl
2 2
Cl /petroleum ether (1:3) as the eluent to afford com-
method. The resulting TiO
2
1
ꢁ
5
00 C and sintered for 60 min, followed by treatment with TiCl
4
3
)
d
7.69 (d, J ¼ 2.2 Hz, 1H), 7.49e7.37 (m, 3H), 6.94e6.80 (m,
ꢁ
(
0.04 M) aqueous solution at 70 C for 60 min and sintered again at
2
2
H), 4.73 (s, 2H), 3.97 (t, J ¼ 6.6 Hz, 2H), 1.78 (m, 2H), 1.52e1.40 (m,
ꢁ
5
00 C for 60 min. The counter electrode was obtained by ther-
PtCl (0.02 M in isopropanol) on the surface of
FTO glass at 400 C for 20 min. The adsorption of the dyes on TiO
was carried out in 0.3 mM dye solution in THF for 12 h. The elec-
13
H), 1.40e1.28 (m, 4H), 0.91 (t, J ¼ 7.0 Hz, 3H). C NMR (101 MHz,
mopyrolysis of H
2
ꢁ
6
3
CDCl ) d 159.84, 146.93, 140.51, 134.27, 133.21, 114.81, 114.20, 109.61,
2
108.92, 96.49, 83.07, 82.83, 68.29, 31.70, 29.27, 25.83, 22.74,14.18. IR
(
KBr)
n
: 3424, 3324, 2933, 2864, 2535, 2208, 1881, 1754, 1606, 1572,
trolyte was composed of 0.3 M DMPII, 0.1 M LiI, 0.05 M I
2
and 0.5 M
ꢀ1
1
510, 1448, 1291, 1254, 1176, 1057, 1025, 855, 828, 725 cm . HRMS
(
4
ESI): m/z [MþH]þ calcd for
20
C H22BrINO, 497.9929; found,
97.9918.
2.3.3. 4-Bromo-1-(4-(hexyloxy)phenyl)pyrrolo[3,2,1-kl]
phenothiazine (4)
In a 50 mL Schlenk tube, compound 3 (2.04 g, 4.1 mmol), CuI
(79 mg, 0.41 mmol), dimethylglycine (85 mg, 0.82 mmol) and
K
3
PO (2.18 g, 10.3 mmol) were added. The tube was evacuated and
4
backfilled with nitrogen (three times). Then a solution of 2-
bromobenzenethiol (1.18 g, 6.2 mmol) in DMSO (12.5 mL) was
ꢁ
added via syringe. The mixture was stirred at 90 C for about 18 h.
ꢁ
Then it was stirred at 130 C for 24 h. After being cooled to room
temperature, the mixture was poured into water and extracted
with ethyl acetate. The organic phase was washed with water for
three times and dried over anhydrous Na
solvent under vacuum, the crude product was purified by column
chromatography on silica gel using CH Cl /petroleum ether (1:6) as
the eluent to afford compound 4 as a white solid (870 mg, yield:
2 4
SO . After removal of the
2
2
1
Fig. 1. Chemical structures of JY40-42.
44%). H NMR (400 MHz, CDCl
3
)
d
7.45 (d, J ¼ 8.7 Hz, 2H), 7.31 (d,

294
F. Li et al. / Dyes and Pigments 139 (2017) 292e299
J ¼ 1.3 Hz, 1H), 7.13 (dd, J ¼ 7.7, 1.1 Hz, 1H), 6.96 (d, J ¼ 8.7 Hz, 2H),
2 2 2 2 3
Cl $CH Cl (15 mg, 0.018 mmol), K CO (150 mg, 1.08 mmol),
6
6
2
d
.93e6.87 (m, 2H), 6.86e6.78 (m, 1H), 6.72 (dd, J ¼ 8.3, 0.9 Hz, 1H),
toluene (10 mL) and methanol (2.5 mL) were added. The mixture
ꢁ
.46 (s, 1H), 4.01 (t, J ¼ 6.5 Hz, 2H), 1.96e1.71 (m, 2H), 1.55e1.44 (m,
was stirred at 90 C for 8 h under a nitrogen atmosphere. After
1
3
H), 1.37 (m, 4H), 0.93 (t, J ¼ 6.9 Hz, 3H). C NMR (101 MHz, CDCl
3
)
removal of the solvent, the crude product was purified by column
159.68, 141.35, 136.42, 135.98, 129.81, 128.61, 128.21, 127.25,
25.45, 124.67, 122.35, 120.02, 119.80, 119.51, 119.35, 116.72, 114.98,
07.74, 68.32, 31.75, 29.38, 25.90, 22.76, 14.19. IR (KBr) : 3063,
chromatography on silica gel using CH
2 2
Cl /petroleum ether (1:1) as
1
1
the eluent to afford compound 8 as a red liquid (140 mg, yield: 87%).
1
n
3
H NMR (400 MHz, CDCl ) d 9.81 (s, 1H), 7.57 (s, 1H), 7.47 (d,
2
1
932, 2861, 2741, 1891, 1605, 1562, 1492, 1434, 1328, 1248, 1133,
J ¼ 8.6 Hz, 2H), 7.40 (d, J ¼ 1.1 Hz, 1H), 7.24 (d, J ¼ 3.8 Hz, 1H), 7.16
(dd, J ¼ 7.6,1.2 Hz,1H), 7.11 (d, J ¼ 3.9 Hz, 2H), 7.06 (d, J ¼ 1.1 Hz,1H),
6.96 (d, J ¼ 8.7 Hz, 2H), 6.91 (td, J ¼ 7.6, 1.1 Hz, 1H), 6.85e6.79 (m,
047, 1001, 935, 832, 790, 720 cm . HRMS (ESI): m/z [MþH]þ calcd
ꢀ1
for C26H25BrNOS, 478.0840; found, 478.0832.
1
H), 6.76e6.71 (m, 1H), 6.53 (s, 1H), 4.01 (t, J ¼ 6.5 Hz, 2H),
2
.3.4. 5-(1-(4-(Hexyloxy)phenyl)pyrrolo[3,2,1-kl]phenothiazin-4-
yl)thiophene-2-carbaldehyde (6)
mixture of compound
formylthiophen-2-yl) boronic acid (162 mg, 1.04 mmol), Pd(dppf)
Cl $CH Cl (45 mg, 0.055 mmol), K CO (145 mg, 1.05 mmol) in
2.85e2.74 (m, 4H), 1.82 (m, 2H), 1.68 (dd, J ¼ 13.7, 6.5 Hz, 4H),
13
1.54e1.47 (m, 2H), 1.46e1.28 (m, 24H), 0.94e0.87 (m, 9H). C NMR
(101 MHz, CDCl 182.63, 159.54, 143.32, 141.41, 141.29, 141.20,
A
4
(250 mg, 0.52 mmol), (5-
3
) d
140.26, 140.11, 139.24, 138.82, 137.26, 136.10, 134.20, 130.34, 129.75,
128.61, 128.17, 127.95, 127.65, 127.09, 125.86, 125.78, 125.66, 124.54,
122.57, 119.25, 118.77, 115.14, 114.90, 114.65, 108.64, 68.28, 32.05,
32.01, 31.75, 30.73, 30.43, 29.91, 29.83, 29.67, 29.63, 29.57, 29.45,
2
2
2
2
3
toluene (10 mL) and methanol (2 mL) was stirred and heated at
reflux for 5 h under a nitrogen atmosphere. After removal of the
solvent, the crude product was purified by column chromatography
29.40, 29.38, 25.90, 22.83, 22.82, 22.76, 14.27, 14.20. IR (KBr) n:
on silica gel using CH
afford compound 6 as a yellow solid (234 mg, yield: 88%). H NMR
400 MHz, CDCl
9.86 (s, 1H), 7.71 (d, J ¼ 3.9 Hz, 1H), 7.49 (dd,
2
Cl
2
/petroleum ether (6:1) as the eluent to
3064, 2924, 2856, 2584, 1896, 1822, 1718, 1662, 1551, 1456, 1334,
1
ꢀ1
1286, 1247, 1172, 1155, 1020, 936, 834, 796, 749 cm . HRMS
þ
(
3
)
d
(MALDI-TOF): m/z [MþH] calcd for C55
2 4
H64NO S , 898.3820; found,
J ¼ 13.2, 5.0 Hz, 3H), 7.34 (d, J ¼ 3.9 Hz, 1H), 7.17 (dd, J ¼ 7.7, 1.3 Hz,
898.3818.
1
6
H), 7.10 (d, J ¼ 1.4 Hz, 1H), 7.01e6.94 (m, 2H), 6.94e6.89 (m, 1H),
.88e6.80 (m, 1H), 6.75 (d, J ¼ 7.6 Hz, 1H), 6.58 (s, 1H), 4.02 (t,
2.3.7. (E)-2-Cyano-3-(5-(1-(4-(hexyloxy)phenyl)pyrrolo[3,2,1-kl]
J ¼ 6.5 Hz, 2H), 1.89e1.75 (m, 2H), 1.49 (m, 2H), 1.38 (m, 4H), 0.93 (t,
phenothiazin-4-yl)thiophen-2-yl)acrylic acid (JY40)
13
J ¼ 7.0 Hz, 3H). C NMR (101 MHz, CDCl
41.77, 141.63, 138.11, 137.72, 135.86, 129.80, 129.48, 128.24, 127.60,
27.25, 125.42, 124.81, 123.60, 122.42, 119.37, 115.90, 115.48, 114.97,
08.64, 68.31, 31.75, 29.37, 25.90, 22.76, 14.21. IR (KBr) : 3109,
066, 2927, 2857, 2755, 1902, 1796, 1728, 1661, 1543, 1499, 1475,
3
) d 182.83, 159.68, 155.37,
A mixture of compound 6 (222 mg, 0.44 mmol), cyanoacetic acid
1
1
1
3
(186 mg, 2.19 mmol), ammonium acetate (255 mg, 3.3 mmol), and
acetic acid (10 mL) was heated to reflux for 4 h under a nitrogen
atmosphere. After cooling, it was poured into water and extracted
n
with CH
times and dried over anhydrous Na
under vacuum, and the crude product was purified by column
chromatography on silica gel using CH Cl /CH OH (8:1) as the
eluent to afford JY40 as an orange solid (226 mg, yield: 89%). H
NMR (400 MHz, DMSO-d
8.18 (s, 1H), 7.71 (d, J ¼ 3.8 Hz,1H), 7.56
2
Cl
2
. The organic phase was washed with water for three
ꢀ1
1435, 1329, 1289, 1245, 1113, 1036, 900, 867, 807, 756 cm . HRMS
2
SO . The solvent was removed
4
þ
(
5
MALDI-TOF): m/z [MþH] calcd for C31
2 2
H28NO S , 510.1561; found,
10.1559.
2
2
3
1
2.3.5. 5-(1-(4-(Hexyloxy)phenyl)pyrrolo[3,2,1-kl]phenothiazin-4-
6
) d
yl)furan-2-carbaldehyde (7)
(
s, 2H), 7.44 (d, J ¼ 7.6 Hz, 2H), 7.25 (d, J ¼ 4.4 Hz, 2H), 6.99 (t,
Compound 7 was synthesized by the same procedure described
J ¼ 8.4 Hz, 3H), 6.91 (t, J ¼ 7.7 Hz, 1H), 6.74e6.61 (m, 2H), 4.00 (t,
J ¼ 6.2 Hz, 2H), 1.73 (m, 2H), 1.44 (m, 2H), 1.32 (d, J ¼ 3.5 Hz, 4H),
1
for 6. (yellow oil, 205 mg, yield: 81%) H NMR (400 MHz, CD
2
Cl
2
)
d
1
9.58 (s, 1H), 7.69 (s, 1H), 7.47 (d, J ¼ 8.3 Hz, 2H), 7.32 (d, J ¼ 3.6 Hz,
1
3
0
1
1
1
3
1
.89 (t, J ¼ 6.8 Hz, 3H). C NMR (101 MHz, DMSO-d
6
) d 164.06,
H), 7.26 (s, 1H), 7.18 (d, J ¼ 7.7 Hz, 1H), 6.97 (d, J ¼ 8.3 Hz, 2H), 6.93
59.03, 149.83, 141.70, 140.87, 136.81, 135.15, 135.01, 129.38, 129.25,
28.16, 127.23, 124.88, 124.53, 123.93, 121.43, 118.61, 118.43, 115.12,
14.78, 108.60, 67.54, 31.02, 28.63, 25.20, 22.07, 13.87. IR (KBr)
034, 2931, 2862, 2215, 1892, 1602, 1501, 1467, 1393, 1289, 1247,
(
(
d, J ¼ 7.6 Hz,1H), 6.85 (t, J ¼ 7.8 Hz,1H), 6.80 (d, J ¼ 3.6 Hz,1H), 6.75
d, J ¼ 8.3 Hz, 1H), 6.61 (s, 1H), 4.02 (t, J ¼ 6.5 Hz, 2H), 1.86e1.75 (m,
n:
2
H), 1.54e1.44 (m, 2H), 1.40e1.33 (m, 4H), 0.92 (t, J ¼ 6.4 Hz, 3H).
1
3
C NMR (101 MHz, CDCl
38.15, 135.79, 129.77, 129.07, 128.19, 127.37, 127.16, 125.40, 125.25,
24.78, 122.49, 119.37, 119.19, 115.08, 114.93, 114.28, 108.73, 106.93,
3
) d 176.95, 160.27, 159.63, 151.73, 141.50,
ꢀ1
179, 1063, 1019, 939, 839, 805, 753 cm . HRMS (MALDI-TOF): m/z
1
þ
[MþH] calcd for C34
29 2 3 2
H N O S , 577.1620; found, 577.1620.
1
6
2
8.28, 31.73, 29.35, 25.88, 22.74, 14.19. IR (KBr)
n: 3111, 3061, 2927,
860, 2721,1895,1727,1669, 1609,1517,1455,1345,1285,1247,1176,
2.3.8. (E)-2-Cyano-3-(5-(1-(4-(hexyloxy)phenyl)pyrrolo[3,2,1-kl]
phenothiazin-4-yl)furan-2-yl)acrylic acid (JY41)
JY41 was synthesized by the same procedure described for JY40,
and the crude product was purified by column chromatography on
ꢀ
1
1
028, 969, 936, 838, 792, 756 cm . HRMS (MALDI-TOF): m/z
þ
[MþH] calcd for C31
3
H28NO S, 494.1790; found, 494.1789.
0
0
2
.3.6. 5 -(1-(4-(Hexyloxy)phenyl)pyrrolo[3,2,1-kl]phenothiazin-4-
silica gel using CH
an orange solid (122 mg, yield: 84%). H NMR (400 MHz, DMSO-d )
2 2 3
Cl /CH OH (12:1) as the eluent to afford JY41 as
00
0
00
1
yl)-3,3 -dioctyl-[2,2':5 ,2 -terthiophene]-5-carbaldehyde (8)
To a solution of compound 4 (215 mg, 0.45 mmol) in THF
6
d
7.82 (s, 1H), 7.79 (s,1H), 7.50 (d, J ¼ 8.6 Hz, 2H), 7.46 (s,1H), 7.32 (d,
(
10 mL), n-BuLi (0.37 mL, 0.59 mmol) was added drop-wise
J ¼ 6.4 Hz, 1H), 7.26 (s, 1H), 7.20 (d, J ¼ 3.5 Hz, 1H), 7.03 (m, 3H), 6.96
(t, J ¼ 7.8 Hz, 1H), 6.77 (s, 1H), 6.69 (d, J ¼ 8.0 Hz, 1H), 4.02 (t,
J ¼ 6.4 Hz, 2H), 1.78e1.68 (m, 2H), 1.49e1.40 (m, 2H), 1.37e1.27 (m,
ꢁ
at ꢀ78 C under argon. The solution was stirred at the same tem-
perature for 1 h, and B(OCH
stirring at the temperature for 4 h, the mixture was gradually
warmed to room temperature and poured into water. Then CH Cl
was added, the organic phase was washed with water for three
times and dried over anhydrous Na SO . After removal of the sol-
3 3
) (85 mg, 0.81 mmol) was added. After
1
3
4H), 0.89 (t, J ¼ 7.0 Hz, 3H). C NMR (101 MHz, DMSO-d
6
) d 163.24,
2
2
159.07, 141.01, 136.84, 134.99, 129.49, 128.32, 127.42, 127.07, 126.95,
125.49, 125.01, 124.92, 124.44, 121.41, 118.62, 118.22, 114.88, 113.94,
2
4
113.74, 108.66, 67.58, 31.00, 28.62, 25.19, 22.06, 13.88. IR (KBr)
n:
vent, the crude product was used for the next Suzuki reaction
without purification. To a 100 mL Schlenk tube, the previous ob-
tained crude product, compound 5 (105 mg, 0.18 mmol), Pd(dppf)
3054, 2933, 2863, 2216, 1611, 1509, 1459, 1391, 1353, 1286, 1247,
ꢀ1
1176, 1032, 936, 839, 796, 752, 723 cm . HRMS (MALDI-TOF): m/z
[MþH]þ calcd for C34
29 2 4
H N O S, 561.1848; found, 561.1848.

F. Li et al. / Dyes and Pigments 139 (2017) 292e299
295
0
0
2
.3.9. (E)-2-Cyano-3-(5 -(1-(4-(hexyloxy)phenyl)pyrrolo[3,2,1-kl]
first transformed into the corresponding boronic acid form, and
subsequent Suzuki coupling reaction was carried out with bromo-
substituted terthiophene [40]. Finally, the Knoevenagel condensa-
tions of compounds 6e8 with cyanoacetic acid were easily realized
in the presence of ammonium acetate and acetic acid to produce
the target dyes JY40-42 [41].
0
0
0
00
phenothiazin-4-yl)-3,3 -dioctyl-[2,2':5 ,2 -terthiophen]-5-yl)acrylic
acid (JY42)
JY42 was synthesized by the same procedure described for JY40,
and the crude product was purified by column chromatography on
Cl /CH OH (12:1) as the eluent to afford JY42 as
2 2 3
a deep red solid (95 mg, yield: 87%). H NMR (400 MHz, DMSO-d
silica gel using CH
1
6
)
d
2
8.28 (s, 1H), 7.76 (s, 1H), 7.44 (d, J ¼ 7.7 Hz, 3H), 7.33 (d, J ¼ 4.0 Hz,
3.2. Optical and electrochemical properties
H), 7.25 (d, J ¼ 6.9 Hz, 1H), 7.20e7.15 (m, 2H), 6.98 (m, 3H), 6.91 (t,
J ¼ 7.8 Hz,1H), 6.66 (d, J ¼ 8.2 Hz,1H), 6.63 (s,1H), 3.99 (t, J ¼ 6.4 Hz,
2 2
The UV/Vis absorption spectra of the new dyes in CH Cl solu-
tions were measured and shown in Fig. 2, and the relevant data are
summarized in Table 1. All dyes exhibit two distinct absorption
2
4
0
H), 2.75 (dd, J ¼ 16.2, 8.7 Hz, 4H), 1.79e1.71 (m, 2H), 1.68e1.55 (m,
H), 1.48e1.40 (m, 2H), 1.38e1.20 (m, 24H), 0.89 (t, J ¼ 6.9 Hz, 3H),
.82 (m, 6H). 13C NMR (101 MHz, THF-d
)
d
160.37, 141.84, 140.50,
bands in CH
correspond to the localized aromatic
the chromophores, and the other bands in the low-energy region
> 370 nm) can be ascribed to the intramolecular charge transfer
(ICT) transitions from the donor to the acceptor of the chromo-
phores [42,43]. The ICT absorption maximums ( max) for dyes JY40,
JY41 and JY42 are at 400, 418, and 461 nm, respectively. The
Cl
2
solutions, the high-energy bands (
l < 370 nm)
8
2
1
1
1
38.97, 137.79, 136.74, 134.87, 134.53, 131.13, 130.20, 129.30, 128.65,
28.47, 127.50, 126.51, 126.24, 125.08, 123.23, 119.74, 119.27, 115.35,
15.29, 114.93, 109.27, 68.48, 32.71, 32.40, 31.21, 30.93, 30.48, 30.32,
pep
* electronic transitions of
(l
3
2
0.24, 30.11, 30.03, 26.53, 23.39, 23.34,14.30,14.20. IR (KBr)
n: 3062,
926, 2856, 2215, 1673, 1608, 1559, 1502, 1464, 1399, 1260, 1213,
l
ꢀ1
1172, 1023, 936, 833, 789, 750 cm . HRMS (MALDI-TOF): m/z
þ
[MþH] calcd for C58
65 2 3 4
H N O S , 965.3878; found, 965.3879.
3
. Results and discussion
3.1. Synthesis and characterization
As shown in Scheme 1, para-bromoaniline (1) was first treated
with molecular I in the presence of Ag SO to give 4-bromo-2,6-
2 2 4
diiodoaniline (2). Selective palladium catalyzed Sonogashira
coupling was further achieved by controlling the molar ratio of
reactant 2 and 1-ethynyl-4-(hexyloxy)benzene to afford mono-
coupling product 3. A Cu(I)-catalyzed tandem CeS coupling/dou-
ble cyclization reaction was adopted to construct the tetracyclic
fused key intermediate 4 [39]. With the pyrrolo[3,2,1-kl]- pheno-
thiazine (4) in hand, a conventional Suzuki coupling with appro-
priate aldehyde substituted arylboronic acids was performed to
yield precursors 6 and 7. As for precursor 8, the intermediate 4 was
2 2
Fig. 2. UVevis spectra of JY40-42 in CH Cl solutions.
Scheme 1. Synthetic routes for JY40-42.

296
F. Li et al. / Dyes and Pigments 139 (2017) 292e299
Table 1
Photophysical and electrochemical data of JY40-42.
max/nma
3
/104
M
ꢀ1
cm
ꢀ1
E
ox/Vb
E
0e0/eVc
E
*
Dye
l
JY40
JY41
JY42
400
418
461
2.69
2.46
3.57
1.13
1.13
1.06
2.17
2.25
2.01
ꢀ1.04
ꢀ1.12
ꢀ0.95
a
solution (3 ꢂ 10ꢀ5 M) at room
2 2
The absorption spectra were measured in CH Cl
temperature.
b
First oxidation potentials (vs. NHE) were measured in benzonitrile with TBAPF
6
(
0.1 M) as the supporting electrolyte (working electrode: glassy carbon; counter
þ
electrode: Pt wire; reference electrode: Ag/Ag ).
c
E
0e0 values were estimated from the onset wavelength of the absorption spectra
in CH Cl
solutions using E0e0 ¼ 1240/
ox: The excited state oxidation potential, and Eox ¼ Eox e E0-0
2
*
2
onset
l .
d
*
E
.
significantly red-shifted and enhanced ICT absorption band of dye
ꢀ
1
ꢀ1
JY42 (461 nm, 35700 M
2
cm ) compared to JY40 (400 nm,
ꢀ
1
ꢀ1
ꢀ1
ꢀ1
Fig. 4. Schematic energy-level diagrams of JY40-42.
6900 M cm ) and JY41 (418 nm, 24600 M cm ) is due to the
better electron delocalization over the whole molecule with ter-
thiophene as -linker.
p
orbital configurations, density functional theory (DFT) calculations
were performed by using Gaussian 09 program package at the
B3LYP/6e31 G(d) level [46,47]. The optimized geometries and
electron distributions of the frontier molecular orbitals of JY40-42
are shown in Fig. 5. In the ground state, the geometry of pyrrolo
To evaluate the thermodynamic possibility of electron injection
and dye regeneration, the oxidation potentials of dyes were
measured in benzonitrile by using cyclic voltammetry (CV) with
tetra-n-butylammonium hexafluorophosphate (TBAPF
the supporting electrolyte. Ferrocene/ferrocenium couple (Fc/Fcþ,
.63 V vs. NHE) was employed as an external reference [44,45], and
6
, 0.1 M) as
[
3,2,1-kl]phenothiazine is not fully planar, and the angle between
0
ꢁ
the indole unit and the phenyl ring is 152 from the nitrogen side.
The nonplanar conformation of pyrrolo[3,2,1-kl]phenothiazine may
impede dye aggregation and the formation of intermolecular
excimer. The dihedral angles between the pyrrolo[3,2,1-kl]pheno-
the CV plots are displayed in Fig. 3. Corresponding to the first redox
potential (Eox), the highest occupied molecular orbital (HOMO)
energy levels of JY40-42 are 1.13, 1.13, and 1.06 V, respectively. The
band gap energies (E0ꢀ0) of JY40-42 are 2.17, 2.25 and 2.01 eV,
which are estimated from the onset wavelength of the absorption
thiazine donor and the
p
-linker of JY40-42 are estimated to be
ꢁ
ꢁ
ꢁ
27.1 , 0.9 , and 30.4 , respectively. For all dyes, the HOMO orbitals
2 2
spectra of the dyes in CH Cl solutions. The lowest unoccupied
are delocalized from the pyrrolo[3,2,1-kl]phenothiazine donor to
the cyanoacrylic acid acceptor, while the LUMO orbitals are mainly
molecular orbital (LUMO) energy levels of JY40-42 can be calcu-
lated by subtracting E0ꢀ0 from Eox, and the values are ꢀ1.04, ꢀ1.12,
and ꢀ0.95 V, respectively (Table 1). Based on the electrochemical
data, the energy level diagrams are depicted in Fig. 4. All the LUMO
localized at the cyanoacrylic acid and its adjacent
p-linkers. The
HOMO and LUMO of the dyes displayed partial overlap between the
donor group and the anchoring group, which would have a favor-
able effect on the charge separation within the dye and subsequent
electron injection from the excited dye into the conduction band of
levels of JY40-42 are more negative than the conduction band (Ecb
of TiO
2 are sufficiently more positive than the I /I
)
2
(ꢀ0.5 V vs. NHE). Meanwhile, all the HOMO levels of JY40-
ꢀ
ꢀ
3
redox potential
4
2
TiO upon photoexcitation.
(
0.4 V vs. NHE). These results ensure favorable electron injection
from excited dyes into the conduction band of TiO upon photo-
2
excitation and effective regeneration of the oxidized dye by the
electrolyte.
3.4. Photovoltaic performance of DSSCs
The photovoltaic properties of the DSSCs sensitized by JY40-42
ꢀ2
3.3. Theoretical calculations
under AM 1.5 G condition (100 mW cm ) were evaluated with an
iodine electrolyte (0.3 M DMPII, 0.1 M LiI, 0.05 M I and 0.5 M 4-tert-
2
To gain a deep insight into the molecular structure and frontier
butylpyridine in acetonitrile). Pertinent parameters are listed in
Table 2, and the photocurrent-voltage (J-V) curves are plotted in
Fig. 6a. The cell based on JY40 with a thiophene as the
p-linker gave
a PCE of 6.42%, with a short-circuit photocurrent density (Jsc) of
ꢀ2
1
3.38 mA cm , an open-circuit photovoltage (Voc) of 736 mV and a
fill factor (FF) of 0.65. When replacing the thiophene linker of JY40
by furan, the resultant JY41-based cell showed decreased Voc
ꢀ
2
(
705 mV) and Jsc (12.60 mA cm ), and hence showed a relatively
lower PCE of 5.60%. This is probably due to its stronger aggregation
on the TiO surface resulted from the relatively smaller dihedral
2
angle between the pyrrolo[3,2,1-kl]phenothiazine donor and the
furan linker. The increased Voc and Jsc of JY41 with co-adsorbent
chenodeoxycholic acid (CDCA) compared to that without CDCA in
the photovoltaic experiments (Supporting Information, Fig. S1 and
Table S1) also confirms the more serious aggregating tendency of
dye JY41. Notably, JY42 with terthiophene p-spacer gave a signifi-
ꢀ
2
cantly improved Jsc (17.76 mA cm ) due to its much broadened and
enhanced ICT absorption arised from its greatly extended conju-
gated structure. Although undesirable dye aggregation and charge
recombination aroused simultaneously by the expanded molecular
Fig. 3. Cyclic voltammograms of JY40-42 recorded in benzonitrile solutions.

F. Li et al. / Dyes and Pigments 139 (2017) 292e299
297
Fig. 5. The geometries and frontier orbitals of JY40-42 optimized by DFT calculations.
Table 2
Photovoltaic performance of JY40-42 with N719 as a reference.
a
DL/nmol cm 2
ꢀ
V
oc/mV
ꢀ2
Jsc/mA cm
FF
PCE/%
Dye
JY40
JY41
JY42
N719
158
175
136
e
736
705
726
762
13.38
12.60
17.76
17.40
0.65
0.63
0.58
0.60
6.42
5.60
7.54
7.95
a
2
Active area of the cells was 0.196 cm ; DL was the amount of dye loading; The
N719-sensitized cell was immersed in the commercial N719 dye solution (0.3 mM in
ethanol) for 12 h.
structure led to a relative low Voc (726 mV) compared to JY40
(
736 mV), JY42 finally achieved the highest PCE of 7.54%. This
reaches about 95% of the ruthenium dye N719-based standard cell
under the same conditions. As mentioned above, all these dyes
showed
2.6 mA cm
phenothiazine/phenoxazine-based dyes with similar structures
48,49], demonstrating pyrrolo[3,2,1-kl]phenothiazine is a prom-
a
high
V
oc over 700 mV and moderate
Jsc over
ꢀ
2
1
, which are comparable to those of the
[
ising electron donor to be used to construct highly efficient solar
cells.
To analyze the effect of
p-linker on the photocurrent features,
the incident photon-to-current conversion efficiency (IPCE) spectra
of JY40-42 were performed. As shown in Fig. 6b, the onsets of JY40
and JY41 were observed at 650 and 620 nm, respectively. In com-
parison to that of JY41, the red-shift of the IPCE onset for JY40 can
be attributed to the more electron-rich feature of thiophene rela-
tive to furan. Furthermore, the conjugation-extended terthiophene
linker causes a much better electron delocalization over the whole
molecule, thus the onset of JY42 was significantly red-shifted to
7
50 nm. Notably, over 60% IPCE value can be observed for JY42-
based cell in a broad region ranged from 380 to 620 nm, accom-
panying with a maximum IPCE value of 85% at 450 nm. The broader
IPCE response range and higher IPCE values of JY42 explains its
highest measured Jsc value.
Fig. 6. (a) The photocurrent density-voltage (J-V) curves and (b) the IPCE spectra of
JY40-42.
3
.5. Electrochemical impedance spectroscopy
the dark under a forward bias of ꢀ0.70 V with frequency range of
0
.1e100 kHz. For the Nyquist plots (Fig. 7a), the larger semicircle is
To further understand the photovoltaic behavior of the DSSCs,
2
related to the charge recombination resistance (Rrec) at the TiO /
electrochemical impedance spectroscopy (EIS) was carried out in

298
F. Li et al. / Dyes and Pigments 139 (2017) 292e299
based on JY42 exhibited an attractive PCE up to 7.54%, with a Jsc of
ꢀ2
1
7.76 mA cm , a Voc of 726 mV, and a FF of 0.58 under standard AM
1.5 G irradiation in conjunction with an iodine electrolyte. These
results suggest that pyrrolo[3,2,1-kl]phenothiazine is a promising
electron donor for high-performance DSSCs.
Acknowledgements
We are grateful to the 973 Program (2011CB932502), the Na-
tional Natural Science Foundation of China (Nos: 21572108 and
21272123)
and
Tianjin
Natural
Science
Foundation
(16JCYBJC16700) for their financial support.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2016.12.025.
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