Synthesis and performance of new quinoxaline-based dyes for dye sensitized solar cell

Article abstract of DOI:10.1016/j.dyepig.2015.05.019

New quinoxaline-based dyes were synthesized and applied as a sensitizer for dye-sensitized solar cell. A quinoxaline moiety was used as an electron withdrawing group and triphenylamine and phenothiazine derivatives were introduced as electron donating gro

Full text of DOI:10.1016/j.dyepig.2015.05.019

Dyes and Pigments 121 (2015) 204e210
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis and performance of new quinoxaline-based dyes for dye
sensitized solar cell
Chang Young Jung, Cheol Jun Song, Wang Yao, Jong Min Park, In Ho Hyun,
Dong Hoon Seong, Jae Yun Jaung*
Department of Organic and Nano Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul 133-791, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
New quinoxaline-based dyes were synthesized and applied as a sensitizer for dye-sensitized solar cell. A
quinoxaline moiety was used as an electron withdrawing group and triphenylamine and phenothiazine
derivatives were introduced as electron donating groups. The dyes synthesized with the phenothiazine
moiety showed greater conversion efficiency than dyes with the triphenylamine moiety leading to the
best overall power conversion efficiency of 4.36%.
Received 24 March 2015
Received in revised form
13 May 2015
Accepted 17 May 2015
Available online
© 2015 Elsevier Ltd. All rights reserved.
Keywords:
Quinoxaline
Triphenylamine
Phenothiazine
Dye-sensitized solar cells
Sensitizers
IPCE
1. Introduction
Generally, metal-free dyes for DSSCs feature a push-pull struc-
ture, in which the electron transfer occurs efficiently. Once the
With increasing interest in alternative energy, researchers have
focused on solar energy due to its sustainability and ease of access.
Dye-sensitized solar cells (DSSC) convert light to electric energy [1].
DSSCs have attracted research because of their low cost and high
power-conversion efficiency [2]. Dye molecules play a key role as
sensitizers and have been studied to improve the efficiency of the
electrons in the dye are excited due to the absorption of light, an
electron donor pushes and an electron acceptor pulls the electrons
from the donor. The electrons transported to the acceptor can be
injected into the conduction band of a semiconductor through
anchoring groups linked to the acceptor.
Over the last several decades, various functional groups and
their derivatives have been introduced as electron donors or ac-
ceptors. Among these, triphenylamine derivatives are excellent
electron donors due to their electron-donating property and non-
planar molecular configuration, which prevents aggregation
[21,22].
Phenothiazine derivatives have shown better light conversion
efficiency than similarly-shaped triphenylamine groups because a
phenothiazine moiety with electron-rich heteroatoms, nitrogen
and sulfur, has a strong electron donating ability. In addition, a non-
planar structure like butterfly suppresses aggregation via inter-
molecular interaction [23].
€
solar energy to electricity conversion in DSSC [3]. Grazzel reached
an efficiency of over 11% using N719 dye, a Ru-metal complex [4];
however, this dye is inefficient for large scale applications because
the ruthenium atom is rare and expensive. For those reasons,
metal-free dyes have gained interest due to easily-tunable ab-
sorption regions, high molecular extinction coefficients and lower
cost when compared to metal complexes. Carbazole [5], coumarine
[6], indoline [7,8], porphyrin [9,10], perylene [11,12], phthalocya-
nine [13,14], phenothiazine [15e18], squaraine [19] have been
introduced in the literature as sensitizers for improving efficiency.
€
Recently, the Grazzel group achieved an efficiency of 13% using
porphyrin dye [20].
Quinoxaline moieties have recently been investigated as elec-
tron acceptors in DSSCs, utilizing their strong electron-
withdrawing property that stems from a high electron affinity
derived from two symmetric unsaturated nitrogen atoms in the
heterocycle [24]. Dong Wook Chang et al. [25] modified horizon-
tally- or vertically-structured quinoxaline derivatives, and Takahiro
* Corresponding author. Tel.: þ82 2 2220 0492; fax: þ82 2 2220 4092.
E-mail address: jjy1004@hanyang.ac.kr (J.Y. Jaung).
http://dx.doi.org/10.1016/j.dyepig.2015.05.019
0143-7208/© 2015 Elsevier Ltd. All rights reserved.

C.Y. Jung et al. / Dyes and Pigments 121 (2015) 204e210
205
Kono et al. [26] and Jie Shi et al. [27] studied quinoxaline-based
DSSCs using the triphenylamine group as the electron donor,
though neither study has introduced more powerful electron do-
nors such as triphenylamine derivatives with alkoxy moieties. To
the best of our knowledge, there has not yet been any attempt to
utilize the phenothiazine and quinoxaline groups as electron do-
nors and acceptors, respectively, for DSSCs.
In this study, triphenylamine and phenothiazine derivatives
with an alkoxy group were introduced to 2, 3-positions of qui-
noxaline as an electron donor for the Y-shape, and a carboxylic
group was introduced to the 6-positon as an anchoring group.
Introduction of the alkoxy groups on the electron donor was ex-
pected to improve the electron donating ability. The synthesized
structures, NQX1-4, exhibited photovoltaic properties.
holes and the holes were sealed with a cover glass using the
additional surlyn film. The active area of the dye-coated TiO2 film
was 0.25 cm2.
2.4. Measurements
The photocurrent density-voltage (J-V) curves were collected
using a Keithley Model 2400 source meter and a solar simulator
with a 300W Xenon arc-lamp Newport under AM 1.5 illumination
(100 mWcmꢁ2). A photomask on the residual area of the dye
fabricated cell was used to reduce scattered light (the active area of
the DSSC was 0.25 cm2). The incident photon-current conversion
efficiency (IPCE) was recorded in the wavelength range of
300e800 nm using a model QEX7 solar cell spectral response
measurement system (PV Measurements, Inc).
2. Experimental
2.5. Synthesis
2.1. Materials
2.5.1. 1-iodo-4-(isopentyloxy)benzene (1b)
All reagents and solvents were purchased commercially and
used without further purification. (isopentyloxy)benzene, 4-(bis(4-
methoxyphenyl)amino)benzaldehyde (3a), 10-(4-methoxyphenyl)-
10H-phenothiazine-3-carbaldehyde (5a) and 4-acetamidobenzoic
acid (6) were prepared following methods reported in literature
[23,28e30].
Sulfuric acid (24.2 ml) was carefully added to methanol (400 ml)
in a 3-neck flask under stirring and then cooled to 0 ꢀC. Potassium
iodide (50.7 g, 1.82 mol) and (isopentyloxy)benzene (49.2 g,
0.3 mol) was slowly added sequentially to the solution. After
warming to room temperature, 30% hydrogen peroxide (63.6 ml)
was slowly added drop wise and then stirred at 60 ꢀC for 14 h. The
resulting mixture was cooled to room temperature, poured into
water and extracted with chloroform. The organic phase was
washed with a sodium metabisulfate aqueous solution, dried over
anhydrous sodium sulfate, and concentrated to produce a yellow oil
2.2. Characterization
1H and 13C NMR spectra were recorded on a VARIAN UnityInova
300 MHz FT-NMR spectrometer. MALDI-TOF MS analysis was con-
ducted on a Waters Limited MALDI-TOF spectrometer using a
dithranol matrix. The UVevis absorption spectra were measured on
a Jasco V-670 spectrophotometer. Cyclic voltammetry (CV) was
performed on a Bio-Logic (SP-200) with a three-electrode cell in
DMF containing 0.1 M tetrabutylammonium hexafluorophosphate
at a scan rate of 100 mVs-1. A carbon electrode, Ag/AgCl electrode,
and a platinum wire were used as working, reference, and counter
electrodes, respectively.
(87 g, 0.3 mol). 1H NMR (300 MHz, (CD3)2SO):
d
(ppm) ¼ 7.58-7.55
(d, J ¼ 9.0 Hz, 2H, ArH), 6.79-6.76 (d, J ¼ 9.0 Hz, 2H, ArH), 3.97-3.93
(t, J ¼ 6.6 Hz, 2H, -CH2-),1.80-1.70 (m, J ¼ 6.6 Hz,1H, -CH-),1.61-1.55
(q, J ¼ 6.6 Hz, 2H, -CH2-), 0.92-0.89 (d, 6H, -CH3). 13C NMR
(150 MHz, (CD3)2SO):
24.5, 22.4.
d
(ppm) ¼ 158.5, 137.9, 117.2, 82.8, 66.0, 37.3,
2.5.2. 4-(isopentyloxy)-N-(4-(isopentyloxy)phenyl)-N-
phenylaniline (2b)
Copper powder (26.8 g, 0.42 mol) and copper iodide (0.187 g,
0.98 mmol) were added to a solution of 1b (87 g, 0.3 mol) and
aniline (13.1 g, .0.14 mol) in o-dichlorobenzene (100 ml), and po-
tassium carbonate (77.6 g, 0.56 mol) under a N2 condition. The
resulting mixture was vigorously refluxed for 24 h, cooled to room
temperature and filtered with benzene. The filtrate was concen-
trated and purified by column chromatography on silica gel using
dichloromethane/hexane (1:2) as an eluent. The yield of the yellow
viscous liquid was 35 g (60%). 1H NMR (300 MHz, CDCl3):
2.3. DSSC fabrication
A 0.1M titanium (Ⅳ) bis(ethyl acetoacetate)diisopropoxide so-
lution in butanol was coated on the FTO glass plate (Pilkington-
TEC8) by a spin coater (at 1000 rpm for 10s and then, at 2000 rpm
for 40s) and the substrate was heated to 450 ꢀC for 30 min in a
furnace to form a 20-nm thickness. A 20-nm TiO2 paste (PST-18 NR,
CCIC) was coated by a doctor blading method and annealed at
450 ꢀC for 30 min (~11
doctor blading method using a 400-nm TiO2 particle (PST-400C,
CCIC) and treated under 450 ꢀC for 30 min (~4
m) to improve light
m
m). The scattering layer was developed by a
d
(ppm) ¼ 7.22-7.17 (t, 2H, ArH), 7.09-7.06 (d, J ¼ 9.0 Hz, 4H, ArH),
6.99-6.97 (d, J ¼ 7.5 Hz, 4H, ArH), 6.94-6.89 (t, J ¼ 7.5 Hz, 1H, ArH),
6.87-6.84 (d, J ¼ 9.0 Hz, 2H, ArH), 4.01-3.97 (t, J ¼ 6.6 Hz, 4H, -CH2-),
1.93-1.84 (m, J ¼ 6.6 Hz, 2H, -CH-), 1.75-1.68 (q, J ¼ 6.6 Hz, 4H, -CH2-
), 1.02-1.00 (d, 12H, -CH3). 13C NMR (150 MHz, CDCl3):
m
absorption. The treated substrate was further exposed to an
aqueous 50 mM TiCl4 solution at 70 ꢀC for 20 min and sintered at
450 ꢀC for 30 min. After sintering, the substrate was immersed in a
5ⅹ10ꢁ4 M dye solution of chloroform for 2 h at room temperature
for dye adsorption (in the case of N719 dye, the substrate was
immersed for 24 h), rinsed with chloroform and dried. To prepare
the counter electrode, a 0.01M H2PtCl6 solution in isopropyl alcohol
was coated on the cleaned FTO plate using a spin coater (at
1000 rpm for 10s and at 2000 rpm for 40s, subsequently) and
heated at 450 ꢀC for 30 min. The Pt-coated electrode was then
drilled to make two holes, washed with ionized water and dried.
The prepared electrodes were assembled into a sandwich-type
d
(ppm) ¼ 155.2, 148.8, 140.9, 128.8, 126.3, 120.7, 115.3, 115.1, 66.5,
38.1, 25.0, 22.6.
2.5.3. 4-(bis(4-(isopentyloxy)phenyl)amino)benzaldehyde (3b)
The solution of 2b (30 g, 0.072 mol) in DMF (320 ml) was stirred
at 0 ꢀC. Phosphorus oxychloride (17.7 g, 0.115 mol) was added drop
wise, and stirred at room temperature for an additional 30 min. The
resulting mixture was heated to 80 ꢀC, stirred for 12 h, cooled to
room temperature, and poured into water. After stirring for 30 min,
the emulsion was extracted with ether. The organic solvent was
washed with brine, dried over anhydrous sodium sulfate and
concentrated. The residue was purified by column chromatography
on silica gel using chloroform as an eluent to produce a yellow
structure using surlyn film (25 m
m) with a hot press at 90 ꢀC. Af-
ter assembly, an electrolyte solution (0.6 M DMPII, 0.1M LiI, 0.1M I2,
0.5M TBP in acetonitrile) was injected into the cell through the

206
C.Y. Jung et al. / Dyes and Pigments 121 (2015) 204e210
viscous liquid product (18.9 g, 59%). 1H NMR (300 MHz, CDCl3):
2.5.7. 2,3-bis((E)-4-(bis(4-methoxyphenyl)amino)styryl)
d
(ppm) ¼ 9.75 (s, 1H, -CHO), 7.64-7.61 (d, J ¼ 8.7 Hz, 2H, ArH), 7.13-
quinoxaline-6-carboxylic acid (NQX1)
7.10 (d, J ¼ 9.0 Hz, 4H, ArH), 6.90-6.83 (m, 6H, ArH), 4.00-3.96 (t,
J ¼ 6.6 Hz, 4H, -CH2-), 1.90-1.81 (m, J ¼ 6.6 Hz, 2H, -CH-), 1.72-1.65
(q, J ¼ 6.6 Hz, 4H, -CH2-), 0.99-0.96 (d, 12H, CH3). 13C NMR
A mixture of 7 (0.26g, 1.0 mmol), 3a (0.9g, 2.1 mmol) and a small
portion of piperidine in anhydrous toluene (10 ml) was refluxed
under a N2 atmosphere. The reaction was confirmed using thin
layer chromatography. After cooling to room temperature, the
remaining liquid was concentrated and purified by column chro-
matography on silica gel using chloroform/methanol (20:1) as an
eluent to produce a red-brown solid product (0.9 g, 84%). m.p.
(150 MHz, CDCl3):
d
(ppm) ¼ 190.2, 156.8, 154.0, 138.5, 131.3, 128.0,
127.6, 116.6, 115.5, 66.5, 37.9, 24.9, 22.5.
2.5.4. 10-(4-(isopentyloxy)phenyl)-10H-phenothiazine (4b)
165 ꢀC. 1H NMR (300 MHz, (CD3)2SO):
d
(ppm) ¼ 13.29 (br, 1H,
1b (87 g, 0.3 mol), phenothiazine (40.8 g, 0.2 mol), copper
powder (25.4 g, 0.4 mol), potassium carbonate (111 g, 0.8 mol) and
18-crown-6 (5.2 g, 0.02 mol) were suspended in o-dichlorobenzene
(230 ml) under a N2 condition. The suspension was refluxed for 24 h
and then cooled to room temperature and filtered with chloroform.
The filtrate was evaporated in vacuo, and then purified by recrys-
tallization with methanol to produce an ivory solid (59 g, 80%). m.p.
-COOH), 8.44 (s, 1H, ArH), 8.11-8.07 (d, J ¼ 8.7 Hz, 1H, ArH), 7.97-
7.88 (m, 3H, ArH, -CH-), 7.72-7.66 (m, 6H, ArH, -CH-), 7.10-7.07 (d,
J ¼ 9.0 Hz, 8H, ArH), 6.95-6.92 (d, J ¼ 9.0 Hz, 8H, ArH), 6.74-6.70 (m,
4H, ArH), 3.75 (s, 12H, -CH3). 13C NMR (150 MHz, (CD3)2SO):
d
(ppm) ¼ 166.7, 156.3, 150.5, 150.0, 149.5, 149.4, 142.7, 139.9, 139.2,
138.3, 137.8, 130.7, 129.4, 129.3, 128.3, 127.4, 127.2, 118.4, 117.8, 117.7,
115.0, 55.4. MS (MALDI-TOF) m/z 832.32, Calcd: 832.94. Anal. Calcd
for C53H44N4O6: C, 76.42; H, 5.32, N, 6.73. Found: C, 76.07; H, 5.68,
N, 6.51.
90 ꢀC. 1H NMR (300 MHz, (CD3)2SO):
d
(ppm) ¼ 7.33-7.30 (d,
J ¼ 9.0 Hz, 2H, ArH), 7.21-7.18 (d, J ¼ 9.0 Hz, 2H, ArH), 7.05-7.03 (d,
J ¼ 7.5 Hz, 2H, ArH), 6.94-6.80 (m, 4H, ArH), 6.16-6.13 (d, J ¼ 8.1 Hz,
2H, ArH), 4.10-4.06 (t, J ¼ 6.6 Hz, 2H, -CH2-),1.85-1.78 (m, J ¼ 6.6 Hz,
1H, -CH-), 1.70-1.63 (q, J ¼ 6.6 Hz, 2H, -CH2-), 0.97-0.95 (d, 6H,
2.5.8. 2,3-bis((E)-4-(bis(4-(isopentyloxy)phenyl)amino)styryl)
quinoxaline-6-carboxylic acid (NQX2)
-CH3). 13C NMR (150 MHz, (CD3)2SO):
d
(ppm) ¼ 158.4, 144.0, 132.2,
NQX2 was synthesized with a method similar to NQX1. A
mixture of 7 (0.44 g, 2.18 mmol), 3b (2.1 g, 4.71 mmol) and a small
portion of piperidine in anhydrous toluene (20 ml) was refluxed
under a N2 condition. The reaction was confirmed using thin layer
chromatography. After cooling to room temperature, the remaining
liquid was concentrated and purified by column chromatography
on silica gel using chloroform/methanol (20:1) as an eluent to
produce a red-brown solid product (1.1 g, 48%). m.p. 135 ꢀC. 1H NMR
131.9, 127.2, 126.5, 122.4, 118.7, 116.5, 115.5, 66.2, 37.4, 24.6, 22.4.
2.5.5. 10-(4-(isopentyloxy)phenyl)-10H-phenothiazine-3-
carbaldehyde (5b)
The solution of 4b (30 g, 83.0 mmol) and DMF (67.8 ml,
829.9 mmol) in 1,1,2-trichloroethane (200 ml) was stirred at 0 ꢀC.
Phosphorus oxychloride (31 ml, 332.0 mmol) was slowly added to
the solution, stirred at 0 ꢀC for 30 min, and heated to 95 ꢀC. After
stirring for an additional 4 h, the mixture was cooled to room
temperature, poured into ice water and stirred for 30 min. The
emulsion was extracted with chloroform. The organic phase was
dried over anhydrous sodium sulfate, concentrated with an in
vacuo rotary evaporator and purified by column chromatography
on silica gel using benzene as an eluent to produce a yellow solid
product (25.5 g, 79%). m.p. 105 ꢀC. 1H NMR (300 MHz, CDCl3):
(300 MHz, (CD3)2SO):
d
(ppm) ¼ 13.30 (br, 1H, -COOH), 8.45 (s, 1H,
ArH), 8.12-8.08 (d, J ¼ 8.7 Hz, 1H, ArH), 7.99-7.89 (m, 3H, ArH, -CH-),
7.74-7.66 (m, 6H, ArH, -CH-), 7.07-7.04 (d, J ¼ 8.4 Hz, 8H, ArH), 6.93-
6.90 (d, J ¼ 8.4 Hz, 8H, ArH), 6.73-6.67 (m, 4H, ArH), 3.98-3.93 (t,
J ¼ 6.6 Hz, 8H, -CH2-), 1.82-1.73 (m, J ¼ 6.6 Hz, 4H, -CH-), 1.63-1.56
(q, J ¼ 6.6 Hz, 8H, -CH2-), 0.93-0.91 (d, 24H, -CH3). 13C NMR
(150 MHz, (CD3)2SO):
d
(ppm) ¼ 166.7, 155.8, 155.7, 150.5, 150.0,
149.6, 149.4, 142.7, 139.9, 139.1, 139.0, 129.4, 129.3, 127.4, 127.3,
127.2, 127.1, 118.3, 117.7, 117.6, 115.5, 66.0, 37.5, 24.6, 22.4. MS
(MALDI-TOF) m/z 1057.57, Calcd: 1057.36. Anal. Calcd for
d
(ppm) ¼ 9.68 (s, 1H, -CHO), 7.44 (s, 1H, ArH), 7.29-7.24 (m, 3H,
ArH), 7.13-7.10 (d, J ¼ 8.7 Hz, 2H, ArH), 6.96-6.93 (m, 1H, ArH), 6.84-
6.81 (m, 2H, ArH), 6.21-6.18 (d, J ¼ 8.7 Hz,1H, ArH), 6.18-6.14 (m,1H,
ArH), 4.09-4.05 (t, J ¼ 6.6 Hz, 2H, -CH2-),1.94-1.85 (m, J ¼ 6.6 Hz,1H,
C69H76N4O6: C, 78.38; H, 7.24, N, 5.30. Found: C, 78.07; H, 7.56, N,
5.21.
-CH-), 1.78-1.71 (q, J ¼ 6.6 Hz, 2H, CH2-), 1.02-1.00 (d, 6H, -CH3). 13
C
NMR (150 MHz, CDCl3):
d
(ppm) ¼ 189.7, 159.2, 149.5, 142.8, 132.0,
2.5.9. 2,3-bis((E)-2-(10-(4-methoxyphenyl)-10H-phenothiazin-3-
yl)vinyl)quinoxaline-6-carboxylic acid (NQX3)
131.6, 130.8, 129.9,127.4, 127.0, 126.5,123.5, 119.9, 118.9, 116.6, 116.4,
114.9, 66.7, 37.9, 25.0, 22.6.
NQX3 was synthesized with a method similar to NQX1. A
mixture of 7 (0.75 g, 3.71 mmol), 5a (2.72 g, 8.16 mmol) and a small
portion of piperidine in anhydrous toluene (20 ml) was refluxed
under a N2 condition. The reaction was confirmed using thin layer
chromatography. After cooling to room temperature, the remaining
liquid was concentrated and purified by column chromatography
on silica gel using chloroform/methanol (20:1) as an eluent to
produce a red-brown solid product (1.0 g, 32%). m.p. 230 ꢀC. 1H
2.5.6. 2,3-dimethylquinoxaline-6-carboxylic acid (7)
6
(5 g, 27.5 mmol) and tin chloride dehydrate (31.0 g,
137.3 mmol) were suspended in ethanol (30 ml) under N2 condi-
tions. The suspension was refluxed for 5 h, and then cooled to room
temperature. 2,3-butandione (2.37 g, 27.5 mmol) was added to the
suspension and the suspension was refluxed for an additional hour.
The resulting mixture was cooled to room temperature, alkalized to
pH 11 by adding a 2M NaOH aqueous solution. The alkalized sus-
pension was filtered using celite, and the filtrate was evaporated to
remove the organic solvent. The residue was acidificated to pH 2
with a 2M HCl aqueous solution, and then the precipitate was
filtered, and purified by recrystallization with methanol. The
product was obtained as a gray solid to produce 5.1 g (92%). m.p.
NMR (300 MHz, (CD3)2SO):
d
(ppm) ¼ 13.30 (br,1H, -COOH), 8.46 (s,
1H, ArH), 8.14-8.11 (d, J ¼ 8.7 Hz, 1H, ArH), 8.01-7.98 (d, J ¼ 8.7 Hz,
1H, ArH), 7.86-7.84 (m, 4H, ArH,-CH-), 7.75-7.73 (s, 2H, ArH), 7.41-
7.36 (m, 6H, ArH, -CH-), 7.25-7.22 (d, J ¼ 9.0 Hz, 4H, ArH), 7.11-7.08
(d, J ¼ 7.2 Hz, 2H, ArH), 6.97-6.84 (m, 4H, ArH), 6.14-6.10 (d,
J ¼ 8.4 Hz, 4H, ArH), 3.88 (s, 6H, -CH3). 13C NMR (150 MHz,
(CD3)2SO):
d
(ppm) ¼ 166.7, 159.1, 150.2, 149.7, 144.6, 144.5, 143.1,
230 ꢀC. 1H NMR (300 MHz, (CD3)2SO):
d
(ppm) ¼ 13.25 (br, 1H,
142.7, 140.0,137.1, 136.6,132.1,131.8,130.5,130.4,130.3,128.6,128.5,
128.4, 127.3, 126.6, 125.3, 122.8, 119.8, 119.0, 118.2, 116.3, 115.7, 115.3,
55.4. MS (MALDI-TOF) m/z 831.64, Calcd: 832.99. Anal. Calcd for
-COOH), 8.43 (s, 1H, ArH), 8.18e8.15 (d, J ¼ 8.4 Hz, 1H, ArH), 8.00-
7.97 (d, J ¼ 8.4 Hz, 1H, ArH), 2.71 (s, 6H, -CH3). 13C NMR (150 MHz,
(CD3)2SO):
d
(ppm) ¼ 166.8, 156.1, 155.2, 142.4, 139.5, 130.6, 130.0,
C51H36N4O4S2: C, 73.54; H, 4.36, N, 6.73; S, 7.70. Found: C, 73.26; H,
128.4, 128.0, 23.0, 22.8.
4.68, N, 6.47; S, 7.48.

C.Y. Jung et al. / Dyes and Pigments 121 (2015) 204e210
207
Scheme 1. Synthesis of NQX1-4. (a) aniline, K2CO3, Cu, CuI, o-dichlorobenzene, reflux, 24 h; (b) POCl3, DMF, 0 ꢀCe80 ꢀC, 12 h; (c) phenothiazine, K2CO3, Cu, CuI, o-dichlorobenzene,
reflux, 24 h; (d) POCl3, DMF, 0 ꢀCe80 ꢀC, 12 h; (e) SnCl2, EtOH, reflux, 5h, 2,3-butandione, reflux, 1 h; (f) 3 or 5, piperidine, toluene, reflux.
2.5.10. 2,3-bis((E)-2-(10-(4-methoxyphenyl)-10H-phenothiazin-3-
yl)vinyl)quinoxaline-6-carboxylic acid (NQX4)
Starting from the corresponding aldehyde derivatives 3 or 5 and
7 in the presence of piperidine and toluene as solvent, the target
compounds were prepared through a knoevenagel condensation.
On the other hand, the 1H NMR spectrum of NQX1-4 indicated that
the ethylene protons appeared as doublets at 7.90e7.95 ppm
(J ¼ 15.3 Hz), and 7.67e7.72 ppm (J ¼ 15.3 Hz). According to the
coupling constant, NQX1-4 should exist in the trans-configuration
(Fig. 1).
NQX4 was synthesized with a method similar to NQX1. A
mixture of 7 (0.75 g, 3.71 mmol), 5b (3.19 g, 8.16 mmol) and a small
portion of piperidine in anhydrous toluene (20 ml) was refluxed
under a N2 condition. The reaction was confirmed using thin layer
chromatography. After cooling to room temperature, the remaining
liquid was concentrated and purified by column chromatography
on silica gel using chloroform/methanol (20:1) as an eluent to
produce a red-brown solid product (0.9 g, 26%). m.p. 275 ꢀC. 1H
3.2. UVevis absorption property
NMR (300 MHz, (CD3)2SO):
d
(ppm) ¼ 13.42 (br,1H, -COOH), 8.46 (s,
1H, ArH), 8.16-8.13 (d, J ¼ 8.7 Hz, 1H, ArH), 7.99-7.96 (d, J ¼ 8.7 Hz,
1H, ArH), 7.86-7.84 (m, 4H, ArH, -CH-), 7.75-7.72 (m, 2H, ArH), 7.38-
7.35 (m, 6H, ArH, -CH-), 7.24-7.21 (d, J ¼ 9.0 Hz, 4H, ArH), 7.11-7.08
(d, J ¼ 7.2 Hz, 2H, ArH), 6.96-6.84 (m, 4H, ArH), 6.15-6.12 (d,
J ¼ 8.7 Hz, 4H, ArH), 4.13-4.09 (t, J ¼ 6.6 Hz, 4H, -CH2-),1.89-1.80 (m,
J ¼ 6.6 Hz, 2H, -CH-), 1.72-1.65 (q, J ¼ 6.6 Hz, 4H, -CH2-), 0.99-0.97
The results of computational calculations (Fig. 3) revealed that
the basic chromophores of the target compounds have strong
intramolecular charge transfer (ICT) system in which the alkox-
yaminophenyl unit acts as a donor and the quinoxaline unit as an
acceptor moiety. Fig. 2 shows the UVevis spectra of NQX1-4 in
chloroform solution. The synthesized sensitizers showed a broad
absorption band up to approximately 600 nm. NQX1 and NQX2
have three absorption peaks. The two peaks located near 300 and
400 nm resulted from two triphenylamine groups as light
(d, 12H, -CH3). 13C NMR (150 MHz, (CD3)2SO):
d
(ppm) ¼ 166.7,
158.5,150.2,149.7, 144.6, 144.5, 143.2, 143.1, 142.7, 140.0, 137.5, 137.1,
131.9, 131.7, 130.5, 130.4, 130.2, 128.6, 128.4, 128.3, 127.3, 126.5,
125.4, 122.8, 119.0, 118.2, 116.7, 115.7, 115.4, 66.3, 37.5, 24.6, 22.5. MS
(MALDI-TOF) m/z 944.55, Calcd: 945.20. Anal. Calcd for
absorbing antennae and an efficient p-p* transition [31]. The peak
near 500 nm caused intramolecular charge transfer (ICT) between
C
59H52N4O4S2: C, 74.97; H, 5.55, N, 5.93; S, 6.78. Found: C, 74.75; H,
5.83, N, 5.65; S, 6.47.
3. Result and discussion
3.1. Synthesis
The synthetic routes followed to prepare compounds are pre-
sented in Scheme 1. The triphenylamine and phenothiazine alde-
hydes were prepared according to reported procedures and the
analytical data are presented in the supporting information. The
starting compound 2, 3-dimethylquinoxaline-6-carboxylic acid, 7
was prepared by the reported method [30]. The 1H NMR spectra
showed that the methyl proton of compound 7 were observed at
2.71 ppm. Due to the strong electron withdrawing effect of the
carboxy group and the quinoxaline ring, the methyl group of 7 was
able to undergo a condensation reaction with arylaldehydes.
Fig. 1. Molecular structures of quinoxaline-based dyes.

208
C.Y. Jung et al. / Dyes and Pigments 121 (2015) 204e210
NQX3 and NQX4. Their molar coefficient (ε) were observed in the
range of 29,700e46,300 M-1 cmꢁ1
.
3.3. Electrochemical property
Cyclovoltametry was used to measure the electrochemical
property of the synthesized sensitizers in DMF with a three elec-
trode configuration consisting of a carbon disk working electrode, a
Ag/AgCl reference electrode and a Pt wire counter electrode. 0.1 M
tetra-n-butylammonium tetrafluroroborate (TBABF4) in a DMF so-
lution was used as the supporting electrolyte. The highest occupied
molecular orbital (HOMO) level was calculated using HOMO
(eV) ¼ ꢁ4.8 - (Eonset - E1/2(Ferrocene)). The Eonset is the oxidation
potential (V) and E1/2(Ferrocene) is the half-wave potential in so-
lution. The band gap (Eg) was derived from the wavelength at a 10%
maximum absorption intensity. The lowest unoccupied molecular
orbital (LUMO) level was estimated using LUMO ¼ HOMO þ Eg.
The electrochemical properties of the dyes are shown in Table 1.
The dyes showed similar energy levels. The HOMO levels of the
dyes were less than the redox level of Iꢁ/Iꢁ3 (ꢁ4.6 eV), indicating
that the HOMO levels were adequate for efficient dye regeneration.
The LUMO levels were greater than the conduction band of the
metal oxide semiconductor (ꢁ3.8 eV, TiO2), implying that excited
electrons can be efficiently injected into the semiconductor.
Therefore, the electrochemical properties of the dyes were appro-
priate for DSSC.
Fig. 2. Absorption spectra of dyes in a chloroform solution (2.0 ꢂ 10ꢁ5 M).
the donor and acceptor. Similarly, NQX3 and NQX4 showed strong
absorption peaks near 350 and 500 nm. The lmax values of NQX1
and NQX2 substituted with tetra-alkoxy groups on the triphenyl-
amine exhibited a slight red-shift absorption compared to those of
Fig. 3. Optimized structures and electronic distributions in the HOMO, HOMO-1 and LUMO levels of the dyes.

C.Y. Jung et al. / Dyes and Pigments 121 (2015) 204e210
209
Table 1
Table 2
Photophysical and electrochemical properties of NQX1-4.
Photovoltaic performance of the dyes.
lmax (nm) εa (M-1cmꢁ1
)
Fmaxb (nm) HOMOc Eg
LUMOe
Dye
Voc (V)
Jsc (mA/cm2)
FF
h (%)
a
d
Dyes
NQX1 482
NQX2 484
NQX3 477
NQX4 478
37,700
46,300
33,900
29,700
684
685
698
699
ꢁ4.98
ꢁ4.99
ꢁ4.92
ꢁ4.92
2.20 ꢁ2.78
2.19 ꢁ2.80
2.18 ꢁ2.75
2.17 ꢁ2.75
N719
NQX1
NQX2
NQX3
NQX4
0.69
0.65
0.70
0.64
0.61
12.77
8.72
7.60
9.77
9.99
0.75
0.73
0.75
0.67
0.71
6.57
4.10
3.98
4.18
4.36
a
Measured in a chloroform solution (2.0 ꢂ 10ꢁ5 M).
b
c
Excited at their lmax values.
HOMO ¼ ꢁ4.8 e (Eonset e E1/2(Ferrocene)), Eonset was determined from the
oxidation onset, scan rate: 100 mVs-1
.
over-measured photocurrent due to light scattering near the edge
of the active area [32].
Under AM 1.5 illumination, the NQX4-based cells showed the
best photovoltaic performance among NQX1-4. The cell with NQX4
exhibited a short circuit current density (Jsc) of 9.99 mA/cm2, an
open circuit voltage (Voc) of 0.61 V, and a fill factor (FF) of 0.71,
d
Measured from the 10% intensity of lmax
LUMO ¼ HOMO þ Eg.
.
e
3.4. Theoretical approach
resulting in an overall power conversion efficiency
h of 4.36%.
Geometric optimization of the designed structure using
computational calculations was applied to explain the minimal
effect of the substituent. Accelrys Materials Studio 4.3 was used for
molecular design and the calculation employed the Forcite tool
(molecular mechanics) and VAMP (semi-empirical method) with
PM3 Hamiltonian methods.
Fig. 3 shows the electron distribution of the HOMO and LUMO of
dyes and the dihedral angles between the quinoxaline ring and
adjacent ethylene moieties. The HOMO-1 and HOMO levels repre-
sent the highest electron density near triphenylamine or pheno-
thiazine moieties as the electron donor, while LUMO is located near
the quinoxaline unit as the electron acceptor suggesting that the
photo-induced electron transfer occurs more efficiently.
The donor groups of the optimized structures are located as far
from each other as possible to avoid overlap. The dihedral angles of
NQX1 (17.42ꢀ and 18.70ꢀ) between the electron donor and qui-
noxaline ring are larger than those of NQX2 (25.56ꢀ and 25.00ꢀ)
due to steric strain, which results from the increased presence of
bulkier isopentyloxy moieties over methoxy moieties. The opti-
mized structures of NQX3-4 show that only alkoxy phenyl rings are
distorted significantly, with the exception of other rings linked by
sulfur atoms. The dihedral angles for NQX3 are slightly smaller than
those for NQX4 due to the varied bulkiness of the alkoxy moieties.
The cell fabricated with NQX1 has higher efficiency than that
with NQX2, possibly caused by a large difference in distortions
between triphenylamine and quinoxaline groups leading to a lower
Jsc [27]. The efficiency of NQX4 is slightly higher than that of NQX3.
It might be ascribed to rise in photo-to-current conversion effi-
ciency is influenced by the varied electron donating ability of
alkoxy chains.
According to the IPCE spectra, the synthesized dye based cells
had high maximum IPCE values near the wavelength of 500 nm,
which were greater than 70%. The NQX4-based cell had an IPCE
value of 81% at 490 nm, which was much greater than the
maximum IPCE value of the N719-based cell. The fabricated cells
had onsets of their IPCE spectra below 700 nm. Though the cell
fabricated with NQX4 had a lower overall efficiency than N719 due
to narrow light absorption, further modifications of the structure
would lead to red-shift of the absorption band and improvements
in the conversion efficiency of the cell.
4. Conclusion
We successfully designed and synthesized quinoxaline-based
dyes (NQX1-4) containing triphenylamine and phenothiazine de-
rivatives with alkoxy moieties as an electron donor. The triphe-
nylamine and phenothiazine derivatives with alkoxy chains and the
quinoxaline group were shown as an efficient electron donor and
acceptor for DSSC, respectively. The cells fabricated with NQX3 and
NQX4 using phenothiazine moieties had greater overall efficiency
than the cells fabricated with NQX1 and NQX2 using triphenyl-
amine moieties. The solar cell based on NQX4 showed the best
power conversion efficiency of 4.36% and a high IPCE value near
500 nm. These results indicate that further study of metal-free
3.5. Solar cell performance
The effects of the electron donors with an alkoxy chain on the
efficiency of the cells were evaluated by measuring their cell per-
formance relative to N719 as the reference dye. The photocurrent
density-voltage (J-V) curves and the IPCE spectra of DSSCs based on
the synthesized dyes are shown in Fig. 4, and the numerical results
are summarized in Table 2. A photomask was used to prevent the
Fig. 4. (a) J-V curves and (b) IPCE spectra of the DSSCs based on N719, NQX1, NQX2, NQX3, and NQX4.

210
C.Y. Jung et al. / Dyes and Pigments 121 (2015) 204e210
sensitized solar cells by 3D molecular structuralization.
2010;132:4054e5.
J Am Chem Soc
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Acknowledgment
This work was supported by the Technology Innovation Program
(No. 10047756, Development of tetra-pyrrole type for Color, light-
emitting, detecting Devices) funded by the Ministry of Trade, In-
dustry & Energy (MI, Korea).
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phenothiazine unit for dye-sensitized solar cells. Mol Cryst Liq Cryst
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Appendix A. Supplementary data
[18] Hung WI, Liao YY, Hsu CY, Chou HH, Lee TH, Kao WS, et al. High-performance
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