Phenothiazine dyes containing a 4-phenyl-2-(thiophen-2-yl) thiazole bridge for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.tet.2020.131102

Two novel phenothiazine dyes bearing a single or double cyanoacrylic acid acceptors, which share the same electron donor phenothiazine and the same 4-phenyl-2-(thiophen-2-yl)thiazole π-bridge, were synthesized. Thus, phenothiazine dyes with D-π-A and A-D-π-A framework were configured, and their photophysical properties and photovoltaic performance were investigated. The incorporation of another cyanoacrylic acid acceptor was found to benefit the loading amount on TiO₂, the light-harvesting ability, and the electron-injection efficiency. Dye with double cyanoacrylic acid acceptors showed a double short-circuit current compared with dye with a single acceptor. Therefore, dye with double cyanoacrylic acid acceptors achieved improved photoelectric conversion efficiency 4.35% (J_SC = 10.29 mA cm^?2, V_OC = 0.65 V, FF = 0.65) under standard global AM 1.5 G solar condition with dye N719 (7.19%) as a reference.

Full text of DOI:10.1016/j.tet.2020.131102

Tetrahedron xxx (xxxx) xxx
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Phenothiazine dyes containing a 4-phenyl-2-(thiophen-2-yl) thiazole
bridge for dye-sensitized solar cells
,
*
Liang Han a, Yaqian Chen a, Jin’ge Zhao a, Yanhong Cui a, Shaoliang Jiang b
a College of Chemical Engineering, Zhejiang University of Technology, Hangzhou, 310032, China
b College of Pharmaceutical Science, 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:
Two novel phenothiazine dyes bearing a single or double cyanoacrylic acid acceptors, which share the
same electron donor phenothiazine and the same 4-phenyl-2-(thiophen-2-yl)thiazole -bridge, were
synthesized. Thus, phenothiazine dyes with D- -A and A-D- -A framework were configured, and their
Received 29 December 2019
Received in revised form
18 February 2020
Accepted 2 March 2020
Available online xxx
p
p
p
photophysical properties and photovoltaic performance were investigated. The incorporation of another
cyanoacrylic acid acceptor was found to benefit the loading amount on TiO2, the light-harvesting ability,
and the electron-injection efficiency. Dye with double cyanoacrylic acid acceptors showed a double
short-circuit current compared with dye with a single acceptor. Therefore, dye with double cyanoacrylic
Keywords:
Phenothiazine
Double acceptors
4-Phenyl-2-(thiophen-2-yl)thiazole
Photovoltaic performance
Dye-sensitized solar cell
acid acceptors achieved improved photoelectric conversion efficiency 4.35% (JSC ¼ 10.29 mA cmꢀ2
,
VOC ¼ 0.65 V, FF ¼ 0.65) under standard global AM 1.5 G solar condition with dye N719 (7.19%) as a
reference.
© 2020 Elsevier Ltd. All rights reserved.
1. Introduction
photophysical properties [6].
Different chromophores are incorporated into molecules of full
Solar energy is the largest renewable carbon-free source and is
thus the superior candidate for replacing fossil energy. The appli-
cation of solar energy is extensively researched to alleviate the fast
depletion problems of fossil energy along with the environmental
problems related to its combustion [1]. Photovoltaic devices are
effective methods of converting solar energy into electricity power
through solar cells [2]. Among them, dye-sensitized solar cells
(DSSCs) have been widely investigated due to their low fabrication
cost, easy manufacturing process, flexibility in the scaling process,
and good photovoltaic performance [3].
In DSSCs, dyes absorb sunlight and inject electrons into TiO2,
generating electric current; the dyes play a key role in the photo-
electric conversion process [4]. From the earlier reported metal
complex sensitizing dyes to nowadays more attractive full organic
sensitizing dyes, most efforts are focused on the development of
novel dyes with improved light-harvesting ability and electron-
injection efficiency to enhance the efficiency of DSSCs [5e7]. Full
organic sensitizing dyes are greatly researched because of their
simple preparation procedures at low cost and easily tunable
organic sensitizing dyes, such as coumarin dyes [8e10], triaryl-
amine dyes [11e13], indoline dyes [14,15], and carbazole dyes
[16e18], with good photovoltaic performance. Various
p-bridges
with different combinations are incorporated into sensitizers to
enlarge the conjugate system and aid in intramolecular charge
transfer (ICT) from chromophores to electron acceptor [19,20].
Compared with donors and
p-bridges, electron acceptors have
fewer variations. In sensitizing dyes, the electron acceptor bearing
carboxylic acid group facilitates in anchoring dye molecules with
TiO2 and injecting the electron from the sensitizing dyes into the
conduction band of TiO2; the entire photoelectric conversion pro-
cess is then initiated. The commonly used acceptors are cyanoa-
crylic acid acceptor and rhodamine acetic acid acceptor [21,22]. The
former is preferred to the latter due to its better electron injection
ability. Some dyes bearing benzoic acid acceptor have shown
improved photoelectric conversion efficiency [23,24]. Most re-
ported full organic sensitizing dyes show one anchoring carbox-
ylate group, which is thought to be a reason for their inferior cell
efficiency compared with metal organic sensitizers with 1e4
anchoring groups [25]. Thus, some sensitizers have been developed
with more than one anchoring group, showing higher dye
adsorption, additional stability, and improved electron injection
efficiency than their counterpart with a single anchoring group
* Corresponding author.
E-mail address: shljiang@zjut.edu.cn (S. Jiang).
https://doi.org/10.1016/j.tet.2020.131102
0040-4020/© 2020 Elsevier Ltd. All rights reserved.
Please cite this article as: L. Han et al., Phenothiazine dyes containing a 4-phenyl-2-(thiophen-2-yl) thiazole bridge for dye-sensitized solar cells,
Tetrahedron, https://doi.org/10.1016/j.tet.2020.131102

2
L. Han et al. / Tetrahedron xxx (xxxx) xxx
[25e27]. However, additional sensitizers with more than one
anchoring group are still required to provide further information
about structure-property relationships.
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
deposited by screen-printing, resulting in the TiO2 electrodes. The
Phenothiazine chromophore is
a large p-conjugated rigid
molecule that exhibits a butterfly-shaped structure. Given their
excellent electron-donating ability, high molar extinction coeffi-
cient, and intense luminescence, phenothiazine derivatives are
extensively applied in light-emitting diodes, laser dyes, and
thickness of TiO2 film was 12 mm. The TiO2 electrodes were grad-
ually heated under an air flow at 325 ꢁC for 5 min, at 375 ꢁC for
5 min, and at 450 ꢁC for 15 min, and finally, at 500 ꢁC for 15 min.
After being cooled to 80 ꢁC, the TiO2 electrode was immersed into
the dye solution in DMF 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. Then the cells were fabricated with 0.3 M 1,2-dimethyl-3-
propylimidazolium iodide (DMPII), 0.03 M I2, 0.07 M LiI, 0.1 M
guanidine thiocyanate, and 0.4 M 4-tert-butylpyridine (TBP) in
CH3CN as the electrolyte. The active area of DSSCs was 0.36 cm2.
photovoltaic materials. Meanwhile, as a
p bridge, benzene favors
the enhancement of photovoltage, and thiophene is believed to be
beneficial to photocurrent improvement. 4-Phenyl-2-(thiophen-2-
yl)thiazole is an interesting
p-bridge that connects the benzene
ring with the thiophene ring through thiazole ring. Herein, two
novel phenothiazine sensitizing dyes bearing 4-phenyl-2-(thio-
phen-2-yl)thiazole
p-bridge were synthesized with single or dou-
ble cyanoacrylic acid acceptors (Scheme 1).
2. Experimental
2.1. Materials and characterization
2.3. Photovoltaic characterization
Nuclear magnetic resonance spectra were recorded on Bruker
Avance III 500 MHz and chemical shifts are expressed in ppm using
TMS as an internal standard. Mass spectra were measured using a
Waters Xevo Q-Tof Mass Spectrometer. Absorption spectra were
measured with SHIMADZU (model UV2550) UVevis spectropho-
tometer. Fluorescence spectra were obtained on a SHIMADZU RF-
5301PC spectrofluorometer.
The photovoltaic performance of the DSSCs was evaluated at AM
1.5 G illumination (100 mW/cm2; 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
(SM-25, Jasco, Japan). The cyclic voltammetry was recorded at a
constant scan rate of 100 mV sꢀ1 with a computer controlled
Scheme 1. Syntheses of phenothiazine dyes.
Please cite this article as: L. Han et al., Phenothiazine dyes containing a 4-phenyl-2-(thiophen-2-yl) thiazole bridge for dye-sensitized solar cells,
Tetrahedron, https://doi.org/10.1016/j.tet.2020.131102

L. Han et al. / Tetrahedron xxx (xxxx) xxx
3
electrochemical analyzer (IviumStat, Holland).
DMSO-d6)
d: 162.78, 161.99, 159.18, 142.88 (2C), 142.51, 137.74,
135.69, 131.94 (2C), 131.55, 131.05, 129.30, 129.03, 127.50 (3C),
127.39 (2C), 127.19 (3C), 126.19, 123.70 (3C), 122.63 (2C), 118.68;
2.4. General procedure for the synthesis of compounds
HRMS(ESI) m/z 534.0387[M
ꢀ
H]-, Calcd for C29H16N3O2S3:
2.4.1. Synthesis of 10-(4-(2-(thiophen-2-yl)thiazol-5-yl)phenyl)-
10H-phenothiazine I
534.0405.
Phenothiazine (1.00 g, 5.02 mmol), 4-(4-bromophenyl)-2-(thi-
ophen-2-yl)- thiazole (1.46 g, 4.54 mmol), CsCO3 (4.91 g,
15.06 mmol), Pd(OAc)2 (0.11 g, 0.50 mmol), P(t-Bu)3HBF4 (0.44 g,
1.50 mmol) were dissolved in xylene (50 mL). The reactant was
refluxed for 10 h. After being cooled to room temperature, the
mixture was filtrated and the solvent was evaporated to dryness.
The residue was dissolved with dichloromethane, washed with
saturated NaCl aqueous solution, and dried over anhydrous Na2SO4.
After removal of the solvent, the residue was purified by column
chromatography (VPE: VEtOAc ¼ 15:1) to give compound I as light
green solid (1.24 g, 62%). m.p. 212e213 ꢁC; 1H NMR (500 MHz,
2.4.4. Synthesis of 3-(4-(4-(3-(2-carboxy-2-cyanovinyl)-10H-
phenothiazin-10-yl)- phenyl)-2-(thiophen-2-yl)thiazol-5-yl)-2-
cyanoacrylic acid T-2
Compound IIb (0.25 g, 0.50 mmol), cyanoacetic acid (0.13 g,
1.50 mmol), piperidine (0.5 mL) were dissolved in CHCl3eCH3CN
cosolvent (15 mL, VCHCl3: VCH3CN ¼ 1:2). The reactant was refluxed
for 8 h. After being cooled to room temperature, the solution was
evaporated to dryness. The residue was purified by column chro-
matography (VCH2Cl2: VCH3OH: VAcOH ¼ 200:6:1) to give compound
T-1 as reddish orange solid (0.27 g, 87%). m.p. 241e242 ꢁC; 1H NMR
(500 MHz, DMSO-d6)
d
9.35 (bs, 2H), 8.22 (s, 1H), 7.95 (d, J ¼ 8.2 Hz,
CDCl3)
d
8.19 (d, J ¼ 8.5 Hz, 2H), 7.61 (dd, J ¼ 3.7, 1.1 Hz, 1H), 7.48 (d,
2H), 7.90 (d, J ¼ 4.4 Hz, 2H), 7.79 (s, 1H), 7.72 (s, 1H), 7.67 (d,
J ¼ 8.3 Hz, 2H), 7.49 (d, J ¼ 8.8 Hz, 1H), 7.27 (t, J ¼ 4.5 Hz,1H), 7.14 (d,
J ¼ 6.8 Hz, 1H), 7.01 (t, J ¼ 7.3 Hz, 1H), 6.94 (t, J ¼ 7.4 Hz, 1H), 6.30 (d,
J ¼ 8.7 Hz, 1H), 6.27 (d, J ¼ 8.2 Hz, 1H); 13C NMR (500 MHz, DMSO-
J ¼ 3.0 Hz, 2H), 7.47 (d, J ¼ 1.9 Hz, 1H), 7.45 (dd, J ¼ 5.1, 1.1 Hz, 1H),
7.14 (dd, J ¼ 5.1, 3.7 Hz, 1H), 7.06 (dd, J ¼ 7.4, 1.8 Hz, 2H), 6.90e6.87
(m, 2H), 6.86e6.83 (m, 2H), 6.33 (dd, J ¼ 8.1, 1.4 Hz, 2H). HRMS(ESI)
m/z 441.0545[MþH]þ, Calcd for C25H17N2S3:441.0554.
d6)
d 164.15, 163.05, 162.08, 158.83, 146.28, 145.17, 142.22, 140.69,
137.68, 135.64, 133.42, 132.42 (2C), 131.12, 130.43 (2C), 129.94,
129.37, 129.05, 127.73, 127.63, 126.95, 126.86, 126.60, 123.65, 119.95,
119.15, 119.06, 118.56, 116.73, 116.05, 113.18, 109.92; HRMS(ESI) m/z
631.0566 [MþH]þ, Calcd for C33H19N4O4S3: 631.0568.
2.4.2. Syntheses of 4-(4-(10H-phenothiazin-10-yl)phenyl)-2-
(thiophen-2-yl)thiazole-5- carbaldehyde IIa and 10-(4-(5-formyl-2-
(thiophen-2-yl)thiazol-4-yl)phenyl)-10H- phenothiazine-3-
carbaldehyde IIb
Dry DMF (0.73 g, 10.00 mmol) was mixed with POCl3 (1.53 g,
10.00 mmol) and stirred in ice bath for 0.5 h. Then the solution of
compound I (0.88 g, 2.00 mmol) in dichloroethane was added and
the reactant was stirred at 65 ꢁC for 10 h. After being cooled to room
temperature, the reactant was stirred for 0.5 h with H2O (50 mL).
The mixture was extracted with dichloromethane and then the
organic layer was washed with saturated NaCl aqueous solution,
dried over anhydrous Na2SO4. After removal of the solvent, the
2.5. X-ray crystal structure determination
Crystals of compound IIa and IIb suitable for X-ray data
collection were grown from petroleum-dichloromethane cosol-
vent. X-ray data of IIa and IIb were collected on a CCD diffrac-
tometer using Mo K
a
(
l
¼ 0.71073). The structures were solved by
direct methods using SHELXS-97 [28] and refined by full-matrix
least-squares refinement methods based on F2 using SHELXL-97
[29]. Crystallographic data (excluding structure factors) for the
structures in this paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication
1,866,985 and 1,866,990 CCDC.
residue was purified by column chromatography (VPE
:
VCH2Cl2 ¼ 1:1) to give monoaldehyde IIa as light yellow solid (0.42 g,
45%) and dialdehyde IIb as yellow solid (0.45 g, 46%). IIa: m.p.
207e208 ꢁC; 1H NMR (500 MHz, CDCl3)
d 10.09 (s, 1H), 7.94 (d,
J ¼ 8.5 Hz, 2H), 7.75 (dd, J ¼ 3.8, 1.1 Hz, 1H), 7.59 (dd, J ¼ 5.0, 1.1 Hz,
1H), 7.48 (d, J ¼ 8.5 Hz, 2H), 7.19 (d, J ¼ 1.7 Hz, 1H), 7.18 (dd, J ¼ 3.3,
1.6 Hz, 2H), 7.06e7.02 (m, 2H), 6.99e6.95 (m, 2H), 6.62 (dd, J ¼ 8.1,
1.2 Hz, 2H); HRMS(ESI) m/z 469.0506[MþH]þ, Calcd for
3. Results and discussion
3.1. Syntheses
C
26H17N2OS3:469.0503; IIb: m.p. 208e209 ꢁC; 1H NMR (500 MHz,
CDCl3)
d
10.14 (s, 1H), 9.75 (s, 1H), 8.09 (d, J ¼ 8.5 Hz, 2H), 7.78 (dd,
Phenothiazine chromophore was selected to construct novel
J ¼ 3.7, 0.9 Hz, 1H), 7.61 (dd, J ¼ 5.0, 0.9 Hz, 1H), 7.60 (d, J ¼ 1.8 Hz,
1H), 7.59 (d, J ¼ 1.8 Hz, 1H), 7.54 (d, J ¼ 1.9 Hz, 1H), 7.35 (dd, J ¼ 8.5,
1.9 Hz, 1H), 7.20 (dd, J ¼ 4.9, 3.9 Hz, 1H), 7.06e7.03 (m, 1H),
6.92e6.89 (m, 2H), 6.31 (d, J ¼ 8.5 Hz, 1H), 6.26e6.24 (m, 1H);
sensitizing dyes with 4-phenyl-2-(thiophen-2-yl)thiazole
and cyanoacrylic acid acceptor. Two linking modes occur between
phenothiazine donor and -bridge: phenothiazine links -bridge
p-bridge
p
p
through CeN or CeC bond forming a T- or rode-shaped molecule,
respectively. T-shaped phenothiazine dyes show preferable energy
level for electron injection into TiO2 and less electron recombina-
tion rate compared with rode-shaped dyes [30]. Therefore,
HRMS(ESI)
m/z
497.0452[MþH]þ,
Calcd
for
C
27H17N2O2S3:497.0452.
2.4.3. Synthesis of 3-(4-(4-(10H-phenothiazin-10-yl)phenyl)-2-
phenothiazine was coupled with p-bridge through CeN bond to
(thiophen-2-yl)thiazol- 5-yl)-2-cyanoacrylic acid T-1
prepare two T-shaped phenothiazine sensitizing dyes, T-1 and T-2.
4-(4-Bromophenyl)-2-(thiophen-2-yl)thiazole was synthesized
from 2-bromo-1-(4-bromophenyl)ethanone and thiophene-2-
carbothioamide as previously described [31]. Phenothiazine reac-
ted with 4-(4-bromophenyl)-2-(thiophen-2-yl)- thiazole through
Buchwald-Hartwig cross coupling to afford compound I. We then
attempted to introduce an aldehyde group onto thiophene ring to
obtain compound A through Vilsmeier reaction of compound I with
DMF and POCl3. Two products were obtained namely, mono-
aldehyde compound IIa and dialdehyde compound IIb, as proven
by 1H NMR and MS. To verify the position of another aldehyde
group in dialdehyde molecule, we cultivated the single crystals of
Compound IIa (0.23 g, 0.50 mmol), cyanoacetic acid (0.13 g,
1.50 mmol), piperidine (0.5 mL) were dissolved in CHCl3eCH3CN
cosolvent (15 mL, VCHCl3: VCH3CN ¼ 1:2). The reactant was refluxed
for 8 h. After being cooled to room temperature, the solution was
evaporated to dryness. The residue was purified by column chro-
matography (VCH2Cl2: VCH3OH: VAcOH ¼ 200:5:1) to give compound
T-1 as yellow solid (0.24 g, 90%). m.p. 248e249 ꢁC; 1H NMR
(500 MHz, DMSO-d6)
d
8.29 (s,1H), 8.01 (dd, J ¼ 3.8,1.1 Hz,1H), 7.97
(dd, J ¼ 5.0, 1.1 Hz, 1H), 7.85 (d, J ¼ 8.5 Hz, 2H), 7.49 (d, J ¼ 8.5 Hz,
2H), 7.31e7.28 (m, 2H), 7.27 (d, J ¼ 1.5 Hz, 1H), 7.17e7.13 (m, 2H),
7.06e7.03 (m, 2H), 6.71 (dd, J ¼ 8.1, 1.0 Hz, 2H); 13C NMR (125 MHz,
Please cite this article as: L. Han et al., Phenothiazine dyes containing a 4-phenyl-2-(thiophen-2-yl) thiazole bridge for dye-sensitized solar cells,
Tetrahedron, https://doi.org/10.1016/j.tet.2020.131102

4
L. Han et al. / Tetrahedron xxx (xxxx) xxx
monoaldehyde and dialdehyde compounds. Surprisingly, mono-
aldehyde IIa and dialdehyde IIb bore an aldehyde group on thiazole
ring instead of thiophene ring, whereas another aldehyde group
was present on phenothiazine ring in dialdehyde IIb molecule.
Thus, aldehyde intermediates IIa and IIb were prepared. Knoeve-
nagel condensation was proceeded between cyanoacetic acid and
IIa and IIb to synthesize compounds T-1 and T-2, respectively.
3.2. Crystal
The structures of monoaldehyde and dialdehyde compounds
were confirmed by single-crystal X-ray diffraction analysis (Figs. 1
and 2). The corresponding parameters are listed in Table 1. Com-
pound IIa crystallizes in the triclinic space group P-1 with
a ¼ 8.2537(13) Aꢁ, b ¼ 10.6284(16) Aꢁ, c ¼ 12.342(2) Aꢁ;
Fig. 1. The crystal structure of compound IIa.
a
¼ 83.455(4)ꢁ,
b
¼ 83.196 (3)ꢁ, and
g
¼ 85.362(3)ꢁ; Z ¼ 2. The
molecular structure of compound IIa shows an aldehyde group on
the thiazole ring. Compound IIb crystallizes in the monoclinic
space group P-1 with a ¼ 8.8080(5) Aꢁ, b ¼ 19.0291(13) Aꢁ,
c ¼ 13.5466(9) Aꢁ;
a
¼ 90ꢁ,
b
¼ 99.156 (3)ꢁ, and
g
¼ 90ꢁ; Z ¼ 4. Aside
from the aldehyde group on the thiazole ring, another aldehyde
group was observed on the phenothiazine ring in the case of
compound IIb. The dihedral angles between the thiazine ring in the
phenothiazine unit and benzene ring of
p
-bridge are 100.8ꢁ and
93.2ꢁ for dyes T-1 and T-2, respectively. The donor unit is almost
perpendicular to the benzene ring and with the introduction of
another aldehyde group, this dihedral angle decreased. In the case
of
p-bridge, five-membered thiophene is coplanar with five-
membered thiazole, and the corresponding dihedral angles are
10.2ꢁ and 5.5ꢁ for dyes T-1 and T-2, respectively. However, the
incorporation of bulky six-membered benzene produces a large
dihedral angle. The dihedral angle between the thiazole ring and
benzene ring increases to 53.6ꢁ and 51ꢁ for dyes T-1 and T-2,
respectively.
3.3. Photophysical properties
Fig. 2. The crystal structure of compound IIb.
The UVevis spectra of phenothiazine dyes were measured in
Table 1
Crystal structures and data refinement parameters.
Compound
IIa
IIb
Empirical Formula
Formula Weight
Temp (K)
C26H16
468.59
296(2)
c
C27H16N2O2S3
496.60
172.5
Crystal System/Space Group
a/Å
b/Å
Triclinic/P-1
8.2537(13)
10.6284(16)
12.342(2)
83.455(4)
83.196(3)
85.362(3)
2
Monoclinic/P21/n
8.8080(5)
19.0291(13)
13.5466(9)
90
99.156(3)
90
4
c/Å
ꢁ
ꢁ
ꢁ
/
a
b
g
/
/
Z
V/Å3
1065.5(3)
1.461
0.371
2241.6(2)
1.471
0.361
r
m
(g/cm3)
)
calc
(mmꢀ1
Crystal size (mm3)
F(000)
0.26 ꢂ 0.22 ꢂ 0.18
0.34 ꢂ 0.24 ꢂ 0.21
484.0
1024.0
2
Q
range for data collection (ꢁ)
3.342e61
6.348e50.742
Index ranges
ꢀ11 ꢃ h ꢃ 11, ꢀ14 ꢃ k ꢃ 13, ꢀ15 ꢃ l ꢃ 12
ꢀ9 ꢃ h ꢃ 10, ꢀ22 ꢃ k ꢃ 22, ꢀ16 ꢃ l ꢃ 16
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness of fit on F2
7472
12804
5143 [Rint ¼ 0.0223, Rsigma ¼ 0.0466]
4069 [Rint ¼ 0.0597, Rsigma ¼ 0.0608]
4069/0/307
1.008
5143/0/289
1.095
Final R indices [I > 2
s
(I)]
R1 ¼ 0.0550, wR2 ¼ 0.1652
R1 ¼ 0.0712, wR2 ¼ 0.2046
0.47/-0.51
R1 ¼ 0.0460, wR2 ¼ 0.0896
R1 ¼ 0.0841, wR2 ¼ 0.1024
0.30/-0.26
R indices (all data)
Largest difference peak/hole (e Åꢀ3
)
Please cite this article as: L. Han et al., Phenothiazine dyes containing a 4-phenyl-2-(thiophen-2-yl) thiazole bridge for dye-sensitized solar cells,
Tetrahedron, https://doi.org/10.1016/j.tet.2020.131102

L. Han et al. / Tetrahedron xxx (xxxx) xxx
5
its light-harvesting ability. In addition, the peak at 297 nm of T-2
shows slightly higher absorptivity than the peak at 400 nm. The
molar extinction coefficients of the former and latter are 27152 and
24777 molꢀ1 cmꢀ1, respectively.
In a DSSC, the dye is immobilized on TiO2 film and is responsible
for light absorption and electron injection. On TiO2 film, dyes
deprotonate to form carboxylate-TiO2 unit and anchor on TiO2
surface. Furthermore, dye molecules may form H-aggregates
through face-to-face alignment, which shifts the absorption peak
hypsochromically along with the deprotonation. Alternatively, J-
aggregates may be formed through edge-to-edge alignment and
shifts the absorption bathochromically. The occurrence of dye ag-
gregation and deprotonation may vary the absorption spectrum on
TiO2 film from that in solution. The absorption spectra of dye ag-
gregates on TiO2 film were measured, as listed in Fig. 3(b). The ICT
absorption of both dyes on TiO2 exhibits blue shift compared with
those in solution, indicating that H-aggregate is primarily formed.
Similar with that in solution, T-2 shows broader absorption spec-
trum and higher absorptivity than those of T-1 on TiO2. This feature
indicates its superior light-harvesting ability and benefits the
improvement of photocurrent. Furthermore, the broadening ab-
sorption spectrum of T-2 is shown in Fig. 3(b) related to that in
Fig. 3(a). The onset on TiO2 film shifts to 600 nm from 550 nm in
solution.
The different molecular structures of dyes determine the ag-
gregation form and the loading amount on TiO2. The loading
amount of phenothiazine dyes on TiO2 film was measured, as listed
in Table 2. The anchoring position on TiO2 surface of T-2 with two
cyanoacrylic acid acceptors may increase compared with that of T-1
with a single cyanoacrylic acid acceptor, which will increase the
loading amount. T-2 exhibits
a
loading amount of
4.2*10ꢀ8 mmol cmꢀ2, which is larger than the 3.7*10ꢀ8 mmol cmꢀ2
of T-1. A large loading amount on TiO2 surface of T-2 generates
increased light absorption and increasing anchoring position may
also improve electron-injection efficiency, which contribute to its
superior light-harvesting ability and photoelectric conversion
efficiency.
Fig. 3. UVevis spectra of phenothiazine dyes T-1 and T-2 (a) in (VCHCl3:V
¼ 10:1); (b) on TiO2 film.
CH3OH
CHCl3eCH3OH cosolvent and listed in Fig. 3. Two peaks were
observed in the absorption spectra of phenothiazine dyes: one peak
3.4. Computational analysis
at approximately 300 nm was caused by p/p* transition, whereas
Quantum chemical calculations using density functional theory
(DFT) provide the information of the optimized geometries, elec-
tronic distribution, and energy levels of dyes, which aids in un-
derstanding the relationship between molecular structure and
photophysical properties. On the basis of the crystal structures of
aldehyde intermediates, the optimized geometries and the electron
distribution of dyes T-1 and T-2 were calculated through DFT with
the Gaussian package using the hybrid B3LYP functional and the
standard 6-311G(d,p) basis set.
Fig. 4 lists the optimized geometries and the electron distribu-
tion of dyes T-1 and T-2. The corresponding dihedral data from
calculation are listed in Table 3 along with the aldehyde data from
the single-crystal X-ray diffraction analysis for comparison.
Phenothiazine units show a butterfly configuration, and large
dihedral angles are observed between benzene ring and pheno-
thiazine unit or thiazole ring. Bulky phenothiazine unit and ben-
zene ring are almost perpendicular to each another in the whole
molecules, which may be detrimental to the ICT but beneficial to
the inhibition of charge recombination. The thiazole rings show
the other peak at approximately 400 nm was ascribed to ICT
transition from the phenothiazine donor to the cyanoacrylic acid
acceptor. T-2 exhibits absorption peaks with similar wavelengths to
T-1 for
another acceptor minimally influences the energy level. T-1 ex-
hibits the similar molar extinction coefficients for * transition
p/p* and ICT transitions, indicating that the presence of
p/p
and ICT transition, which are 19873 molꢀ1 cmꢀ1 for the peak at
298 nm and 20016 molꢀ1 cmꢀ1 for the peak at 392 nm (Table 2).
With the incorporation of another cyanoacrylic acid acceptor, T-2
shows higher molar extinction coefficients for these two peaks and
achieves broader absorption range and higher absorption intensity
when compared with T-1. Therefore, the presence of another
acceptor in T-2 contributes to the transition probability, though
with minimal effect on the transition energy level, which benefits
Table 2
Optical properties of phenothiazine dyes T-1 and T-2.
εa /(molꢀ1$cmꢀ1
)
lmaxb /(nm)
LA (mmol.cmꢀ2
)
a
good coplanarity with thiophene. The planarity of
p-bridge is
Compd.
lmax /(nm)
impaired by the incorporation of six-membered benzene ring.
Approximately 40ꢁ of dihedral angles are observed between ben-
zene ring and thiazole ring (41ꢁ for T-1 and 39ꢁ for T-2). For both
dyes, the highest occupied molecular orbital (HOMO) is primarily
contributed by phenothiazine unit and the lowest unoccupied
T-1
T-2
298, 392
297, 400
19873, 20016
27152, 24777
382
375
3.7*10ꢀ8
4.2*10ꢀ8
a
Maximum absorption wavelength lmax and molar extinction coefficient at lmax
of dyes measured in CHCl3eCH3OH solvent (V:V ¼ 10:1).
b
Maximum absorption wavelength lmax of dyes on sensitized TiO2 electrodes.
Please cite this article as: L. Han et al., Phenothiazine dyes containing a 4-phenyl-2-(thiophen-2-yl) thiazole bridge for dye-sensitized solar cells,
Tetrahedron, https://doi.org/10.1016/j.tet.2020.131102

6
L. Han et al. / Tetrahedron xxx (xxxx) xxx
Fig. 4. Electronic distribution in the frontier molecular orbitals of phenothiazine dyes.
Table 4
Table 3
Energy levels of coumarin sensitizing dyes.
Dihedral parameters of phenothiazine compound.
Experimentala (ꢁ)
Compd
Calculatedb (ꢁ)
Compd
Experimentala (eV)
Calculatedd(eV)
Compd
HOMOa
E0-0
LUMOc
HOMO
E0-0
LUMO
b
:1c
:2d
:3e
:1c
:2d
:3e
T-1
T-2
ꢀ5.74
ꢀ5.76
2.89
2.85
ꢀ2.85
ꢀ2.91
ꢀ5.29
ꢀ5.67
2.00
2.25
ꢀ3.29
ꢀ3.42
IIa
IIb
100.8
93.2
53.6
51
10.2
5.5
T-1
T-2
84.0
81.7
40.7
39.1
2.4
2.5
a
a
The oxidation potential Eox in DMF was determined from cyclic voltammo-
The data was obtained from single crystal X-ray diffraction analysis.
Calculated in vacuo.
:1 is the dihedral angle between phenothiazine unit and benzene ring.
:2 is the dihedral angle between thiazole ring and benzene ring.
:3 is the dihedral angle between thiazole ring and thiophene ring.
b
c
grams and EHOMO ¼ e(EOXþ4.8 eV).
b
E0-0 was calculated from E0-0 ¼ 1240/lint and lint was the intersection of the
d
e
normalized absorption and emission spectra.
c
ELUMO was calculated from Eox-E0-0
Calculated in vacuo.
.
d
molecular orbital (LUMO) is located on the
p-bridge and carboxyl
comparison. The change trends of the experimental and calculated
values are similar.
group on thiazole ring. Therefore, HOMO and LUMO are well
separated, facilitating the exciton dissociation, which is beneficial
for the inhibition of the charge recombination [32].
DFT calculation generates eigenvalues of the Kohn-Sham, which
is considered as the HOMO energy level (EHOMO) of phenothiazine
dyes. The corresponding energy level data are listed in Table 4.
Compared with T-1, the incorporation of another carboxylic acid
group of T-2 shifts HOMO and LUMO down but has a very limited
effect on the energy gap of phenothiazine dyes. This result agrees
with their similar ICT absorption wavelength. We also listed the
experimental values obtained from cyclic voltammograms for
3.5. Photovoltaic performance
Phenothiazine dyes were loaded on TiO2 film to provide the
photoanode. The photoanode was fabricated with the Pd photo-
cathode by using the Iꢀ3 /Iꢀ electrolyte to provide the DSSC. The
monochromatic incident photon-to-electron conversion efficiency
(IPCE) spectra of these DSSCs were measured, as shown in Fig. 5(a).
T-2 shows an IPCE of over 40% from 360 nm to 575 nm with a
Please cite this article as: L. Han et al., Phenothiazine dyes containing a 4-phenyl-2-(thiophen-2-yl) thiazole bridge for dye-sensitized solar cells,
Tetrahedron, https://doi.org/10.1016/j.tet.2020.131102

L. Han et al. / Tetrahedron xxx (xxxx) xxx
7
Table 5
Parameters for DSSCs based on phenothiazine dyes.
Compd.
Voc(V)
Jsc(mA$cmꢀ2
)
ff
h(%)
T-1
T-2
N719
0.64
0.65
0.78
5.11
10.29
13.79
0.70
0.65
0.67
2.26
4.35
7.19
Double cyanoacrylic acid acceptors of T-2 improve light-harvesting
ability and electron-injection efficiency, which contribute to the
increase of the photocurrent density along with the photovoltage.
The presence of another cyanoacrylic acid acceptor brings about
higher Voc of T-2 (0.65 V) than that of T-1 with a single acceptor
(0.63 V). Therefore, superior JSC and improved VOC offers improved
photoelectric conversion efficiency of DSSC based on T-2 (4.35%),
which is almost doubled from DSSC based on T-1 (2.26%).
4. Conclusion
Two T-typed phenothiazine dyes were synthesized with 4-(4-
bromophenyl)-2-(thiophen-2-yl)thiazole as
p bridge and cyanoa-
crylic acid as the acceptor. The incorporation of aldehyde group
caused the production of monoaldehyde and dialdehyde com-
pounds. These compounds were characterized by single-crystal X-
ray diffraction analysis, and the aldehyde position was verified.
Knoevenagel condensation between aldehyde products and cya-
noacetic acid provided phenothiazine dyes with a single acceptor
and double acceptors. The similar structures of two phenothiazine
dyes generated similar UVevis absorption. However, the presence
of another acceptor contributes to improved light absorption in-
tensity, loading amount on TiO2, and charge injection efficiency.
Therefore, phenothiazine dye with double acceptors exhibits dou-
ble JSC and higher VOC, which determines its superior photoelectric
conversion efficiency, compared with its counterpart with a single
acceptor.
Declaration of competing interests
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
Fig. 5. IPCE (a) and J-V curve (b) for DSSCs based on phenothiazine sensitizing dyes.
Acknowledgement
maximum IPCE value of 52% at 492 nm. An IPCE of over 30% from
355 nm to 490 nm was observed in the case of T-1, which exhibits a
plateau of IPCE value 35% at 370e410 nm. In the IPCE evaluation,
TiO2 film in the photoanode is thicker than that used for absorption
measurement, which increased dye loading amount and, accord-
ingly, light harvesting. The light response region of both T-1 and T-2
is broadened, as shown in Fig. 5(a) related to the spectra in Fig. 3(b).
The IPCE onset of T-1 is shifted to 650 nm from 500 nm of its ab-
sorption spectrum on TiO2. Furthermore, the IPCE onset of T-2 is
observed at 730 nm, which is shifted bathochromically by 130 nm
compared with that on TiO2. Similar with the absorption spectra, T-
2 shows broader IPCE responsive region and stronger mono-
chromatic incident photon-to-electron conversion efficiency above
340 nm than T-1.
The authors gratefully acknowledge for the funding of Natural
Science Foundation of Zhejiang province (LY19B020009).
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tet.2020.131102.
References
[1] (a) N.S. Lewis, Science 351 (2016) 353;
(b) P. Nema, R.K. Nema, S. Rangnekar, Renew. Sustain. Energy Rev. 13 (2009)
2096.
[2] X.X. Liu, H.J. Chen, S.T. Tan, Renew. Sustain. Energy Rev. 52 (2015) 1527.
[3] (a) V. Rondan-Gomez, I. Montoya De Los Santos, D. Seuret-Jimenez, F. Ayala-
Mato, A. Zamudio-Lara, T. Robles-Bonilla, M. Courel, Appl. Phys. A-Mater. 125
(2019) 836;
Fig. 5(b) lists the J-V curve of DSSCs based on phenothiazine
dyes. The corresponding parameters short-circuit photocurrent
density (JSC), open-circuit photovoltage (VOC), fill factor (ff), and
(b) T.N. Murakami, N. Koumura, Adv. Energy Mater. 9 (2019), 1802967.
[4] J.M. Ji, H.R. Zhou, H.K. Kim, J. Mater. Chem. 6 (2018) 14518.
[5] A. Mishra, M.K.R. Fischer, P. Bauerle, Angew. Chem. Int. Ed. 48 (2009) 2474.
[6] C.P. Lee, R.Y.Y. Lin, L.Y. Lin, C.T. Li, T.C. Chu, S.S. Sun, J.T. Lin, K.C. Ho, RSC Adv. 5
(2015) 23810.
power conversion efficiency (h) are listed in Table 5, along with
those of the solar cell sensitized with dye N719 for comparison. A
broad and high IPCE responsive range of T-2 contributes to its
photocurrent density improvement. The photocurrent density of T-
2 is doubled, increasing to 10.29 mA cmꢀ2 from 5.11 mA cmꢀ2 of T-1.
[7] A. Carella, F. Brobone, R. Centore, Front. Chem. 6 (2018) 481.
[8] L. Han, H.B. Wu, Y.H. Cui, X.Y. Zu, Q. Ye, J.R. Gao, J. Photochem. Photobiol., A
Please cite this article as: L. Han et al., Phenothiazine dyes containing a 4-phenyl-2-(thiophen-2-yl) thiazole bridge for dye-sensitized solar cells,
Tetrahedron, https://doi.org/10.1016/j.tet.2020.131102

8
L. Han et al. / Tetrahedron xxx (xxxx) xxx
[21] B.Y. Zhu, M. Xu, C.X. Qian, L. Han, J.R. Gao, Q. Ye, Y.J. Li, Tetrahedron 44 (2018)
290 (2014) 54.
[9] L. Han, R. Kang, X.Y. Zu, Y.H. Cui, J.R. Gao, Photochem. Photobiol. Sci. 14 (2015)
2046.
[10] M.M. Jadhav, J.V. Vaghasiya, D.S. Patil, New J. Chem. 42 (2018) 5267.
[11] C.J. Zhong, J.R. Gao, Y.H. Cui, T. Li, L. Han, J. Power Sources 273 (2015) 831.
[12] Y. Adachi, T. Nabeya, T. Nomura, K. Kondo, K. Kawakami, Y. Ooyama,
J. Ohshita, Dalton Trans. 48 (2019) 16671.
3985.
[22] M.M. Jadhav, J.V. Vaghasiya, D.S. Patil, S.S. Soni, N. Sekar, New J. Chem. 42
(2018) 5267.
[23] Y.M. Ren, D.Y. Sun, Y.M. Cao, H.N. Tsao, Y. Yuan, S.M. Zakeeruddin, P. Wang,
M. Gratzel, J. Am. Chem. Soc. 140 (2018) 2405.
[24] Y.C. Cheng, G.S. Yang, H.R. Jiang, S.L. Zhao, Q.Y. Liu, Y.S. Xie, ACS Appl. Mater.
Interfaces 10 (2018) 38880.
[25] R. Sirohi, D.H. Kim, S.C. Yu, S.H. Lee, Dyes Pigments 92 (2012) 1132.
[26] D. Elsherbiny, E. Yildirim, F. El-Essawy, A. Abdel-Megied, A. El-Shafei, Dyes
Pigments 147 (2017) 491.
[27] Y. Farre, M. Raissi, A. Fihey, Y. Pellegrin, E. Blart, D. Jacquemin, F. Odobel, Dyes
Pigments 148 (2018) 154.
[28] G.M. Sheldrick, SHELXS-97: Program for Crystal Structure Determination,
University of Gottingen, Germany, 1997.
[29] G.M. Sheldrick, SHELXL-97: Program for the Refinement of Crystal Structure,
University of Gottingen, Germany, 1997.
[30] A. Baheti, K.R.J. Thomas, C.T. Li, C.P. Lee, K.C. Ho, ACS Appl. Mater. Interf. 7
(2015) 2249.
[13] A. Pathak, T. Thomer, K.R.J. Thomas, M.S. Fan, K.C. Ho, Electrochim. Acta 304
(2019) 1.
[14] L. Han, J.G. Zhao, B. Wang, S.L. Jiang, J. Photochem. Photobiol., A 326 (2016) 1.
[15] M. Matsui, N. Tanaka, K. Funabiki, Y. Haishima, K. Manseki, J. Jin, Y. Inoue,
S. Higashijima, Y. Kubota, Tetrahedron 74 (2018) 3498.
[16] T. Li, J.R. Gao, Y.H. Cui, C.J. Zhong, Q. Ye, L. Han, J. Photochem. Photobiol., A 303
(2015) 91.
[17] L. Han, X.Y. Zu, Y.H. Cui, H.B. Wu, Q. Ye, J.R. Gao, Org. Electron. 15 (2014) 1536.
[18] B. Hemavathi, V. Jayadev, S.C. Pardhan, G. Gokul, K. Jagadish,
G.K. Chandrashekara, P.C. Ramamurthy, R.K. Pai, K.N.N. Unni, T.N. Ahipa,
S. Soman, R.G. Balakrishna, Sol. Energy 174 (2018) 1085.
[19] Y.K. Eom, I.T. Choi, S.H. Kang, J. Lee, J. Kim, M.J. Ju, H.K. Kim, Adv. Energy Mater.
5 (2015) 1500300.
ꢀ
€
€
[31] S.P. Singh, S. Sehgal, Indian J. Chem. B 27B (1988) 941.
ꢀ ꢀ
[32] P. Matunova, V. Jirasek, B. Rezek, Phys. Chem. Chem. Phys. 21 (2019) 11033.
[20] Y.K. Eom, S.H. Kang, I.T. Choi, Y.J. Yoo, J.H. Kim, H.K. Kim, J. Mater. Chem. 5
(2017) 2297.
Please cite this article as: L. Han et al., Phenothiazine dyes containing a 4-phenyl-2-(thiophen-2-yl) thiazole bridge for dye-sensitized solar cells,
Tetrahedron, https://doi.org/10.1016/j.tet.2020.131102