Effect of structural modification on the performances of phenothiazine-dye sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2015.05.012

Three novel dyes DX1, DX2 and DX3 containing phenothiazine are designed and synthesized for dye-sensitized solar cells (DSSCs). Photophysical, electrochemical and photovoltaic properties of the three dyes have been systematically investigated. The results

Full text of DOI:10.1016/j.dyepig.2015.05.012

Accepted Manuscript
Effect of structural modification on the performances of phenothiazine-dye sensitized
solar cells
Xuxu Liu, Jun Long, Guo Wang, Yong Pei, Bin Zhao, Songting Tan
PII:
S0143-7208(15)00183-7
10.1016/j.dyepig.2015.05.012
DYPI 4775
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 8 January 2015
Revised Date: 31 March 2015
Accepted Date: 11 May 2015
Please cite this article as: Liu X, Long J, Wang G, Pei Y, Zhao B, Tan S, Effect of structural modification
on the performances of phenothiazine-dye sensitized solar cells, Dyes and Pigments (2015), doi:
10.1016/j.dyepig.2015.05.012.
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Graphical Abstract
Effect of structural modification on the performances of
phenothiazine-dye sensitized solar cells
Xuxu Liu, Jun Long, Guo Wang, Yong Pei, Bin Zhao, Songting Tan
N719
DX1+CDCA
14
12
10
8
DX2+CDCA
DX3+CDCA
DX1
DX2
DX3
6
4
2
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Voltage (V)

ACCEPTED MANUSCRIPT
Effect of structural modification on the performances of phenothiazine-dye
sensitized solar cells
Xuxu Liua, Jun Longa, Guo Wanga, Yong Peia, Bin Zhaoa,b, Songting Tana,b,
*
a College of Chemistry, and Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of
Education, Xiangtan University, Xiangtan 411105, PR China
b Key Laboratory of Advanced Functional Polymeric Materials of College of Hunan Province, Key Laboratory of
Polymeric Materials & Application Technology of Hunan Province, Xiangtan University, Xiangtan 411105, PR
China
* Corresponding author. E-mail address: tanst2008@163.com (S. Tan).
ABSTRACT: Three novel dyes DX1, DX2 and DX3 containing phenothiazine are designed and synthesized for
dye-sensitized solar cells (DSSCs). Photophysical, electrochemical and photovoltaic properties of the three dyes have
been systematically investigated. The results show that the DX1-based DSSC with 0.5 mM chenodeoxycholic acid
(CDCA) obtains the power conversion efficiency (PCE) of 5.69%. When an additional electron-deficient
benzothiadiazole (BT) unit is introduced into the molecular structures of the dyes DX2 and DX3, the absorption
spectra are broadened. But the short-circuit photocurrent density (Jsc) of the devices are decreased due to the
blocked electron transfer, so the DSSC device based on DX2 only obtains the PCE of 3.43%. Furthermore, a
triphenylamine (TPA) unit with high electron-donating ability is joined onto the nitrogen atom of phenothiazine
donor in DX3, which enhances the electron injection efficiency and reduces the dye aggregation. Thus, the Jsc is
improved, resulting in a higher PCE of 4.41% in the DX3-based dye than the DX2-based one.
Key words: dye-sensitized solar cells, phenothiazine, benzothiadiazole, photovoltaic performances, synthesis
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1. Introduction
As a promising alternative to conventional inorganic photovoltaic devices, dye-sensitized solar cells (DSSCs)
have drawn much attention since their introduction in 1991 [1], due to their potentially low-cost fabrication,
possibility of transparency and color selectivity, which can be integrated into building and automobile applications
[2-5]. As is well-known, the sensitizer is always a crucial element in DSSCs, exerting a significant influence on the
power conversion efficiency (PCE) as well as the device stability. To date, DSSCs incorporating ruthenium based
dyes and zinc–porphyrin based co-dyes have reached high efficiency over 11% [6,7], and 12% [8,9], respectively.
Thereinto, a new dye based on zinc–porphyrin, SM315, which was reported by Grätzel et al., showed a record PCE
of 13.0% [10]. However, metal-free organic dyes, commonly constructed with donor-π bridge-acceptor (D-π-A)
configuration, have become increasingly attractive for the merits of high molar extinction coefficients, low cost,
environment-friendly property and high flexibility of molecular design [6,11]. Thus, some dyes such as
triphenylamine- (TPA-), carbazole-, phenothiazine-, and indoline-based ones have achieved relatively high PCEs by
using iodide/triiodide-based electrolytes [12-21]. However, most of the D-π-A dyes tend to form intermolecular
aggregation on the TiO2 surface, which affects the light absorption and loss in the photo-generated electrons.
Therefore, further studies are needed to develop new dyes to maximize the electron accumulation in the TiO2
conduction band, reduce the charge recombination and absorb light intensely in the red to near-infrared (NIR)
region, which accounts for about 50% of solar energy [22-23].
Phenothiazine-based dyes have been intensively explored in recent years. In phenothiazine system, two phenyl
groups are arranged with a small torsion angle related to the nitrogen and sulfur atoms, ensuring that the
π-delocalization can be extended over the entire chromophore [24-29]. Meanwhile, attaching a bulky or branched
alkyl chain to the nitrogen atom of phenothiazine unit lead to the non-planar butterfly conformation of phenothiazine,
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which can sufficiently inhibit molecular aggregation and further enhance the charge separation on the TiO2 interface
[30,31]. Furthermore, a new type of dyes with an additional electron-deficient unit introduced between the donor and
π-bridge have been designed and characterized as a D-A-π-A architecture [32]. In the novel D-A-π-A configuration
dyes, the additional acceptor chromophore, which extends the range of π-electron delocalization, can facilitate
intramolecular charge transfer and adjust the band gap for harvesting more near-infrared (NIR) light [33]. Among all
the additional electron-deficient units, benzothiadiazole (BT) has been one of the most widely utilized in DSSCs
[34-38]. Here, we designed and synthesized three novel dyes (DX1, DX2 and DX3, Scheme 1), which contained
phenothiazine donors and cyanoacrylic acid acceptors bridged by a 3-hexylthiophene unit as a π-spacer. Additionally,
a BT unit was applied as an additional acceptor into the dyes DX2 and DX3 to realize a broadened absorption
spectrum. Notably, the 3D nonplanar TPA [39,40] was introduced to the 10-position nitrogen atom of phenothiazine
donor in DX3 to obtain a double-donor structure, which resulted in enhanced electron-donating ability and reduced
dye aggregation. Photophysical, electrochemical and photovoltaic properties of the three dyes were systematically
investigated.
2. Experimental section
2.1. Materials
All the chemicals were purchased from Alfa Aesar and Chem Greatwall Chemical Company (Wuhan, China)
and used without further purification. Toluene and tetrahydrofuran (THF) were dried and distilled over sodium and
benzophenone. N,N-Dimethylformamide (DMF) was dried over and distilled from CaH2 under an atmosphere of
dry nitrogen. Phosphorus oxychloride and 1,2-dichloroethane were atmospheric distillation. All other commercially
available
materials
were
used
as
received
unless
noted
otherwise.
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4-Bromo-7-(4-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazole
(5)
[41],
3-bromo-10-(2-ethylhexyl)-10H-phenothiazine (7) [42] were prepared according to literature procedures.
2.2. Synthesis
The synthetic routes and molecular structures of the dyes are shown in Scheme 1. The detailed synthetic processes
are as follows.
2.2.1. 2-Bromo-4-hexylthiophene (1)
A solution of 3-hexylthiophene (8.00 g, 47.53 mmol) in anhydrous THF (100 mL) was cooled down to –78 oC
under argon atmosphere, and n-BuLi (2.5 M in hexane, 22.82 mL, 57.04 mmol) was added dropwise. After stirring
for 3 h, carbon tetrabromide (18.92 g, 57.04 mmol) in anhydrous THF (30 mL) was added dropwise and the reaction
medium was slowly warmed to the room temperature. Water was added for quenching the reaction, and the solution
was extracted with CH2Cl2. The organic layer was washed with H2O and dried over MgSO4. Solvent was removed by
rotary evaporation. The crude product was purified on silica gel column chromatography with petroleum ether to get
a yellow liquid (11.50 g, 97.9%). 1H NMR (CDCl3, 400MHz, δ/ppm): 6.88 (s, 1H), 6.80 (s, 1H), 2.56–2.53 (t, 2H, J =
7.5 Hz), 1.29–1.26 (m, 8H), 0.89–0.87 (t, 3H, J = 6.2 Hz). GC-MS (C10H15BrS) m/z: calcd for 247.2; found 248.0.
2.2.2. 5-Bromo-3-hexylthiophene-2-carbaldehyde (2)
The compound 2 was synthesized by Vilsmeier reaction. Under nitrogen atmosphere, to a solution of compound
1 (11.50 g, 46.52 mmol) in dichloroethane (100 mL), anhydrous DMF (4.32 mL, 55.83 mmol) and POCl3 (5.20 mL,
55.83 mmol) was added slowly at 0 oC. The mixture was stirred at 0 oC for 20 min and then heated to 85 oC for 24 h.
After cooled to room temperature, sodium acetate solution (50 mL) was added to hydrolyse the reaction for 4 hours.
Then the mixture was extracted with CH2Cl2 and washed with water. The organic phase was dried over anhydrous
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MgSO4 and the solvent was removed by rotary evaporation. The crude product was purified on silica gel
chromatography using a petroleum ether/ dichloromethane mixture (1/1 by volume) as eluent to afford a yellow
liquid (5.21 g, 40.7%). 1H NMR (CDCl3, 400MHz, δ/ppm): 9.92 (s, 1H), 6.85 (s, 1H), 2.90–2.86 (t, 2H, J = 7.6 Hz),
1.66–1.25 (m, 8H), 0.89–0.87 (t, 3H, J = 6.3 Hz). GC-MS (C11H15BrOS) m/z: calcd for 275.2; found 276.0.
2.2.3. 4-Hexyl-[2,2'-bithiophene]-5-carbaldehyde (3)
The compound 3 was synthesized by Stille coupling reaction. Compound 2 (1.50 g, 5.45 mmol) and
tributyl(thiophen-2-yl)stannane (3.05 g, 8.18 mmol) were dissolved in anhydrous toluene (40 mL) and deoxygenated
with nitrogen for 15 min. Pd(PPh3)4 (190 mg, 0.17 mmol) was then added under nitrogen and the mixture was again
o
flushed with nitrogen for 30 min. Then the reaction mixture was stirred at 110 C for 2 days under nitrogen
atmosphere. After cooled to room temperature, the mixture was poured into water (50 mL) and extracted with
dichloromethane. The organic phase was washed with water and dried over anhydrous MgSO4. After concentration,
the crude product was purified on silica gel chromatography using a petroleum ether/dichloromethane mixture (2/1
by volume) as eluent to afford an orange liquid (1.51 g, 98.6%). 1H NMR (CDCl3, 400MHz, δ/ppm): 9.99 (s, 1H),
7.35–7.34 (d, 2H, J = 3.4 Hz), 7.07–7.05 (d, 2H, J = 4.6 Hz), 2.95–2.91 (t, 2H, J = 7.7 Hz), 1.74–1.26 (m, 8H), 0.90
(s, 3H). GC-MS (C15H18OS2) m/z: calcd for 278.4; found 278.1.
2.2.4. 5'-Bromo-4-hexyl-[2,2'-bithiophene]-5-carbaldehyde (4)
Compound 3 (1.31 g, 4.70 mmol) was dissolved in DMF (20 mL). The mixture of N-bromosuccinimide (NBS)
(1.00 g, 5.65 mmol) in DMF (15 mL) was added dropwise to the solution and the solution was stirred for 24 hours in
the dark. Then, the mixture was poured into water and extracted with diethyl ether, then washed by water. The organic
phase was dried over anhydrous MgSO4, and then the solvent was removed by rotary evaporation. The crude product
was purified on silica gel chromatography using a petroleum ether/dichloromethane mixture (4/1 by volume) as
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1
eluent to afford an orange liquid (1.18 g, 70.1%). H NMR (CDCl3, 400MHz, δ/ppm): 9.99 (s, 1H), 7.08 (s, 1H),
7.03–7.02 (d, 1H, J = 3.4 Hz), 6.99 (s, 1H), 2.94–2.90 (t, 2H, J = 7.5 Hz), 1.70–1.26 (m, 8H), 0.90 (s, 3H).
MALDI-TOF MS (C15H17BrOS2) m/z: calcd for 357.329; found 357.078.
2.2.5. 5-(7-Bromobenzo[c][1,2,5]thiadiazol-4-yl)-3-hexylthiophene-2-carbaldehyde (6)
The synthetic procedure for compound 6 was similar to that for compound 2, except that compound 5 (3.00 g,
7.87 mmol) was used instead of compound 1. The crude product was purified on silica gel chromatography using a
1
petroleum ether/dichloromethane mixture (1/1 by volume) as eluent to afford a yellow solid (2.50 g, 78.1%). H
NMR (CDCl3, 400MHz, δ/ppm): 10.14 (s, 1H), 8.07 (s, 1H), 7.95–7.85 (d, 2H, J = 7.6 Hz), 3.09–3.06 (t, 2H, J = 7.5
Hz), 1.82–1.77 (m, 2H), 1.47 (s, 2H), 1.38 (s, 2H), 1.30 (s, 2H), 0.94 (s, 3H). MALDI-TOF MS (C17H17BrN2OS2)
m/z: calcd for 409.364; found 409.077.
2.2.6. 10-(2-Ethylhexyl)-3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolane-2-yl)-10H-phenothiazine (8)
o
A solution of the compound 7 (7.99 g, 20.48 mmol) in anhydrous THF (40 mL) was cooled down to –78 C
under argon atmosphere, and n-BuLi (2.5 M in hexane, 8.19 mL, 20.48 mmol) was added dropwise. After stirring for
3 h, 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (4.18 mL, 20.48 mmol) was added dropwise and the
reaction medium was slowly warmed to the room temperature. Water was added for quenching the reaction, and the
aqueous layer was then extracted with CH2Cl2. The organic layer was washed with H2O and dried over MgSO4.
Solvent was removed by rotary evaporation. The crude product was purified on silica gel column chromatography
with a petroleum ether/ethyl acetate mixture (30:1 by volume) to afford a yellow-green oil (4.75 g, 53.0%). 1H NMR
(CDCl3, 400MHz, δ/ppm): 7.58 (s, 2H), 7.13–7.12 (d, 2H, J = 6.8 Hz), 6.92–6.91 (d, 1H, J = 6.9 Hz), 6.86–6.85 (m,
2H), 3.74–3.73 (d, 2H, J = 5.3 Hz), 1.93–1.90 (m, 1H), 1.44–1.32 (m, 8H), 1.26 (s, 12H), 0.87–0.83 (t, 6H, J = 7.1
Hz). MALDI-TOF MS (C26H36BNO2S) m/z: calcd for 437.446; found 437.328.
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2.2.7. 5'-(10-(2-Ethylhexyl)-10H-phenothiazin-3-yl)-4-hexyl-[2,2'-bithiophene]-5-carbaldehyde (9)
The compound 9 was synthesized by Suzuki reaction. Compound 8 (0.78 g, 1.79 mmol), compound 4 (0.53 g,
1.49 mmol), potassium carbonate (1.03 g, 7.45 mmol) dissolved in water (3 mL) and thimbleful tetrabutylammonium
bromide (TBAB) were dissolved in anhydrous DMF (30 mL) and deoxygenated with nitrogen for 15 min. Pd(PPh3)4
(52 mg, 0.045 mmol) was then added under nitrogen and the mixture was again flushed with nitrogen for 30 min.
o
Then the reaction mixture was stirred at 100 C for two days under nitrogen atmosphere. After cooled to room
temperature, the mixture was poured into water (50 mL) and extracted with dichloromethane. The organic phase was
washed with water and dried over anhydrous MgSO4. After concentration, the crude product was purified on silica
gel chromatography using a petroleum ether/dichloromethane mixture (2/1 by volume) as eluent to afford an
1
orange-red oil (0.86 g, 97.2%). H NMR (CDCl3, 400MHz, δ/ppm): 9.98 (s, 1H), 7.39–7.37 (d, 2H, J = 6.3 Hz),
7.19–7.14 (m, 3H), 7.05 (s, 1H), 6.96–6.86 (m, 4H), 3.76–3.74 (d, 2H, J = 6.9 Hz), 2.95–2.91 (t, 2H, J = 7.6 Hz),
1.94 (m, 1H), 1.72–1.69 (m, 3H), 1.52–1.26 (m, 12H), 0.90–0.84 (m, 9H). 13C NMR (100 MHz, CDCl3, δ/ppm):
181.27, 153.85, 146.12, 145.83, 145.23, 145.22, 135.59, 134.52, 127.99, 127.66, 127.36, 126.99, 126.57, 126.12,
125.11, 124.77, 124.56, 123.21, 122.75, 116.10, 116.05, 51.21, 36.07, 31.60, 31.31, 30.81, 29.04, 28.65, 28.61, 24.14,
23.09, 22.59, 14.09, 14.03, 10.58. MALDI-TOF MS (C35H41NOS3) m/z: calcd for 587.901; found 587.309.
2.2.8.
5-(7-(10-(2-Ethylhexyl)-10H-phenothiazin-3-yl)benzo[c][1,2,5]thiadiazol-4-yl)-3-hexylthiophene-2-carbaldehyde
(10)
The synthetic procedure for compound 10 was similar to that for compound 9, except that compound 6 (0.80 g,
1.95 mmol) was used instead of compound 4. The crude product was purified on silica gel chromatography using a
petroleum ether/dichloromethane mixture (2/1 by volume) as eluent to afford a black red solid (1.09 g, 85.2%). 1H
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NMR (CDCl3, 400MHz, δ/ppm): 10.10 (s, 1H), 8.06 (s, 1H), 8.01–7.99 (d, 1H, J = 7.4 Hz), 7.85–7.82 (m, 2H),
7.70–7.69 (d, 1H, J = 7.4 Hz), 7.20–7.16 (m, 2H), 7.03–7.01 (d, 1H, J = 8.3 Hz), 6.97–6.91 (m, 2H), 3.81–3.80 (d,
2H, J = 6.8 Hz), 3.06–3.02 (t, 2H, J = 7.5 Hz), 2.02–1.99 (m, 1H), 1.79–1.74 (m, 2H), 1.54–1.30 (m, 14H),
0.92–0.89 (t, 9H, J = 7.0 Hz). 13C NMR (100 MHz, CDCl3, δ/ppm): 182.03, 153.71, 153.27, 152.69, 147.60, 146.53,
145.19, 137.46, 133.59, 130.93, 130.53, 128.29, 128.02, 127.71, 127.38, 127.26, 126.55, 126.08, 125.44, 124.48,
122.75, 116.12, 115.76, 51.24, 36.09, 31.61, 31.45, 30.86, 29.07, 28.75, 28.67, 24.17, 23.09, 22.58, 14.04, 14.00,
10.56. MALDI-TOF MS (C37H41N3OS3) m/z: calcd for 639.936; found 639.298.
2.2.9. 4-Iodo-N,N-di-p-tolylaniline (11)
A suspended solution of 4-methyl-N-phenyl-N-(p-tolyl)aniline (10.00 g, 36.59 mmol), iodine (3.99 g, 15.70
mmol), and periodic acid (1.49 g, 6.53 mmol) in 95% ethanol (10 mL) was refluxed under nitrogen for 4 h. The
reaction mixture was cooled to room temperature and then saturated sodium subsulfite aqueous solution (50 mL) was
added. The crude product was extracted into dichloromethane, washed with water, and dried over anhydrous sodium
sulfate. After removing solvent under reduced pressure, the residue was purified on silica gel chromatography using
petroleum ether as eluent to yield a white solid (12.66 g, 86.6%). 1H NMR (CDCl3, 400MHz, δ/ppm): 7.46–7.44 (d,
2H, J = 8.1 Hz), 7.08–7.06 (d, 4H, J = 7.7 Hz), 6.99–6.97 (d, 4H, J = 7.7 Hz), 6.78–6.76 (d, 2H, J = 8.1 Hz), 2.32 (s,
6H). MALDI-TOF MS (C20H18IN) m/z: calcd for 399.268; found 399.130.
2.2.10. N-(4-(10H-phenothiazin-10-yl)phenyl)-4-methyl-N-(p-tolyl)aniline (12)
Compound 11 (10.00 g, 25.05 mmol), 10H-phenothiazine (4.16 g, 20.87 mmol), copper powder (0.80 g, 12.52
mmol) and potassium carbonate (4.32 g, 31.31 mmol) were mixed and stirred for 8 h at 180 oC, then after the mixture
was cooled to 130 oC, ethyl acetate (30 mL) was added by means of a syringe, and the solution was stirred for 1 h.
After the solution was cooled to room temperature, the solvent was removed by rotary evaporation. The crude
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product was purified on silica gel chromatography using a petroleum ether/dichloromethane mixture (4/1 by volume)
1
as eluent to afford a white powder (1.79 g, 18.2%). H NMR (CDCl3, 400MHz, δ/ppm): 7.16–7.12 (m, 12H),
7.00–6.98 (d, 2H, J = 7.1 Hz), 6.90–6.87 (m, 2H), 6.81–6.78 (m, 2H), 6.32–6.30 (d, 2H, J = 7.7 Hz), 2.34 (s, 6H).
MALDI-TOF MS (C32H26N2S) m/z: calcd for 470.627; found 470.172.
2.2.11. 4-(3-Bromo-10H-phenothiazin-10-yl)-N,N-di-p-tolylaniline (13)
The synthetic procedure for compound 13 was similar to that for compound 4, except that compound 12 (1.79 g,
3.80 mmol) was used instead of compound 3. The crude product was purified on silica gel chromatography using
petroleum ether as eluent to afford a light green solid (1.49 g, 71.4%). 1H NMR (CDCl3, 400MHz, δ/ppm): 7.14–7.08
(m, 13H), 6.97–6.87 (m, 3H), 6.82–6.80 (m,1H), 6.30–6.29 (d, 1H, J = 7.4 Hz), 6.16–6.14 (d, 1H, J = 8.5 Hz), 2.34 (s,
6H). MALDI-TOF MS (C32H25BrN2S) m/z: calcd for 549.523; found 550.154.
2.2.12.
4-Methyl-N-(p-tolyl)-N-(4-(3-(4,4,5-trimethyl-1,3,2-dioxaborolan-2-yl)-10H-phenothiazin-10-yl)phenyl)aniline (14)
The synthetic procedure for compound 14 was similar to that for compound 8, except that compound 13 (0.71 g,
1.30 mmol) was used instead of compound 7. The crude product was purified on silica gel column chromatography
with a petroleum ether/ethyl acetate mixture (30:1 by volume) to obtain a yellow-green oil (0.57 g, 73.2%). 1H NMR
(CDCl3, 400MHz, δ/ppm): 7.39 (s, 1H), 7.30–7.28 (d, 1H, J = 8.3 Hz), 7.14–7.11 (m, 12H), 6.96–6.94 (d, 1H, J = 7.3
Hz), 6.87–6.83 (m, 1H), 6.80–6.76 (m, 1H), 6.27–6.23 (t, 2H, J = 9.0 Hz), 2.34 (s, 6H), 1.30 (s, 12H). MALDI-TOF
MS (C38H37BN2O2S) m/z: calcd for 596.589; found 596.309.
2.2.13.
5-(7-(10-(4-(Di-p-tolylamino)phenyl)-10H-phenothiazin-3-yl)benzo[c][1,2,5]thiadiazol-4-yl)-3-hexylthiophene-2-c
arbaldehyde (15)
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The synthetic procedure for 15 was similar to that for 10, except that compound 14 (0.56 g, 0.94 mmol) was used
instead of compound 8. The crude product was purified on silica gel chromatography using a petroleum
ether/dichloromethane mixture (2/1 by volume) as eluent to afford a black red solid (0.52 g, 82.7%). 1H NMR (CDCl3,
400MHz, δ/ppm): 10.09 (s, 1H), 8.05 (s, 1H), 8.01–7.99 (d, 1H, J = 7.4 Hz), 7.68–7.66 (d, 1H, J = 7.5 Hz), 7.63 (s,
1H), 7.60–7.57 (d, 1H, J = 8.7 Hz), 7.19–7.10 (m, 12H), 7.02–7.00 (d, 1H, J = 7.0 Hz), 6.92–6.89 (m, 1H), 6.84–6.81
(m, 1H), 6.45–6.43 (d, 1H, J = 8.6 Hz), 6.34–6.32 (d, 1H, J = 8.0 Hz), 3.06–3.02 (t, 2H, J = 7.5 Hz), 2.35 (s, 6H),
1.79–1.74 (m, 2H), 1.52–1.12 (m, 6H), 0.98–0.84 (m, 3H). 13C NMR (100 MHz, CDCl3, δ/ppm): 182.01, 153.68,
148.42, 147.64, 145.05, 144.87, 144.00, 137.43, 133.52, 133.40, 131.42, 130.68, 130.48, 130.19, 128.02, 127.40,
126.99, 126.95, 126.65, 126.17, 125.42, 124.40, 122.81, 122.62, 120.13, 115.99, 115.71, 31.62, 31.45, 29.68, 29.08,
28.75, 22.59, 20.89, 14.06. MALDI-TOF MS (C49H42N4OS3) m/z: calcd for 799.079; found 798.289.
2.2.14. (E)-2-cyano-3-(5'-(10-(2-ethylhexyl)-10H-phenothiazin-3-yl)-4-hexyl-[2,2'-bithiophen]-5-yl)acrylic acid
(DX1)
Compound 9 (0.23 g, 0.39 mmol), cyanoacetic acid (0.33 g, 3.90 mmol), and piperidine (0.19 mL) were
dissolved in CHCl3 (20 mL), and the solution was refluxed at 80 °C for 12 h under nitrogen atmosphere. Then the
solution was poured into a diluted aqueous HCl (10 mL) solution and extracted with dichloromethane. The organic
phase was dried over anhydrous MgSO4, and the solvent was removed by rotary evaporation. The crude product was
purified using silica gel column chromatography (eluent: CH2Cl2/CH3OH = 50:1) to give a prunosus solid (0.18 g,
71.8%). 1H NMR (DMSO, 400 MHz, δ/ppm): 8.27 (s, 1H), 7.57–7.56 (d, 1H, J = 3.2 Hz), 7.49 (s, 4H), 7.21–7.12 (m,
2H), 7.06–7.04 (d, 2H, J = 7.9 Hz), 6.97–6.96 (d, 1H, J = 7.1 Hz), 3.78 (s, 2H), 2.76 (s, 2H), 1.80 (s, 1H), 1.58 (s, 2H),
1.36–1.20 (m, 13H), 0.84–0.79 (m, 9H). 13C NMR (100 MHz, CDCl3, δ/ppm): 168.64, 157.45, 147.55, 146.29,
145.99, 145.13, 144.80, 133.93, 128.86, 128.05, 127.75, 127.65, 127.32, 126.54, 125.57, 125.03, 124.78, 124.54,
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123.46, 122,77, 116.09, 116.01, 51.19, 36.03, 31.53, 31.09, 30.78, 29.20, 29.02, 28.62, 24.09, 23.04, 22.54, 14.03,
13.97, 10.51. MALDI-TOF MS (C38H42N2O2S3) m/z: calcd for 654.947; found 654.247.
2.2.15.
(E)-2-cyano-3-(5-(7-(10-(2-ethylhexyl)-10H-phenothiazin-3-yl)benzo[c][1,2,5]thiadiazol-4-yl)-3-hexylthiophen-2-y
l)acrylic acid (DX2)
The synthetic procedure for DX2 was similar to that for DX1, except that compound 10 (0.20 g, 0.31 mmol) was
used instead of compound 9. The crude product was purified on silica gel chromatography (eluent: CH2Cl2/CH3OH =
50:1) to give a black red solid (0.20 g, 93.4%). 1H NMR (DMSO, 400 MHz, δ/ppm): 8.34 (s, 1H), 8.25–8.24 (d, 1H,
J = 7.3 Hz), 8.18 (s, 1H), 7.93–7.89 (m, 3H), 7.23–7.17 (m, 3H), 7.10–7.08 (d, 1H, J = 7.8 Hz), 7.00–6.96 (m, 1H),
3.84 (s, 2H), 2.83 (s, 2H), 1.86 (s, 1H), 1.64 (s, 2H), 1.39–1.21 (m, 14H), 0.85–0.81 (m, 9H). 13C NMR (100 MHz,
CDCl3, δ/ppm): 145.16, 130.74, 130.19, 128.25, 127.98, 127.86, 127.71, 127.23, 126.34, 125.97, 125.41, 123.79,
122.74, 116.11, 115.69, 51.21, 36.07, 31.61, 31.26, 30.84, 29.30, 29.08, 28.65, 24.12, 23.08, 22.58, 14.05, 13.99,
10.52. MALDI-TOF MS (C40H40N4O2S3) m/z: calcd for 706.982; found 706.315.
2.2.16.
(E)-2-cyano-3-(5-(7-(10-(4-(di-p-tolylamino)phenyl)-10H-phenothiazin-3-yl)benzo[c][1,2,5]thiadiazol-4-yl)-3-hexy
lthiophen-2-yl)acrylic acid (DX3)
The synthetic procedure for DX3 was similar to that for DX1, except that compound 15 (0.20 g, 0.25 mmol) was
used instead of compound 9. The crude product was purified on silica gel chromatography (eluent: CH2Cl2/CH3OH =
50:1) to give a black solid (0.20 g, 91.0%). 1H NMR (DMSO, 400MHz, δ/ppm): 8.29 (s, 1H), 8.17–8.15 (d, 1H, J =
7.4 Hz), 8.13 (s, 1H), 7.82–7.80 (d, 1H, J = 7.4 Hz), 7.73 (s, 1H), 7.61–7.59 (d, 1H, J = 8.5 Hz), 7.22–7.13 (m, 7H),
7.07–7.06 (d, 6H, J = 7.2 Hz), 6.97–6.94 (m, 1H), 6.86–6.83 (m, 1H), 6.32–6.30 (d, 1H, J = 8.6 Hz), 6.26–6.24 (d,
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1H, J = 8.0 Hz), 2.81–2.78 (m, 2H), 2.28 (s, 6H), 1.62 (s, 2H), 1.29–1.21 (m, 6H), 0.85 (s, 3H). 13C NMR (100 MHz,
CDCl3, δ/ppm): 148.32, 144.89, 143.90, 133.43, 132.80, 131.41, 130.41, 130.14, 127.91, 126.87, 126.60, 125.36,
122.88, 122.52, 119.44, 115.90, 115.61, 31.69, 31.14, 29.69, 29.22, 29.12, 22.59, 20.86, 14.08. MALDI-TOF MS
(C52H43N5O2S3) m/z: calcd for 866.125; found 865.307.
2.3. Instruments and characterizations
1H and 13C NMR spectra were measured on a Bruker Avance 400 instrumentusing, using tetramethylsilane
(TMS; δ = 0 ppm) as internal standard. GC-MS spectrometric measurements were obtained on Agilent 7890-5975.
MALDI-TOF mass spectrometric measurements were performed on Bruker Autoflex ꢀ. UV-Visible spectra of the
dyes were measured on a Perkin-Elmer Lamada 25 spectrometer. The photoluminescence (PL) spectra were
obtained using Perkin-Elmer LS-50 luminescence spectrometer. Electrochemical redox potentials were obtained by
cyclic voltammetry (CV) using a three-electrode configuration and an electrochemistry workstation (ZAHNER
ZENNIUM) at a scan rate of 50 mV s–1. The working electrode was a glassy carbon electrode; the counter electrode
was a Pt electrode, and saturated calomel electrode (SCE) was used as reference electrode. Tetrabutylammonium
perchlorate (TBAP) 0.1 M was used as supporting electrolyte in nitrogen-purged anhydrous CHCl3.
Ferrocene/ferrocenium redox couple was used for potential calibration [5].
2.4. Fabrication and characterization of DSSCs
Fluorine-doped SnO2 conducting glass (FTO) were cleaned and immersed in aqueous 40 mM TiCl4 solution at
o
70 C for 30 min, then washed with water and ethanol, sintered at 450oC for 30 min. The TiO2 suspension was
prepared from P25 (Degussa AG, Germany) [43] and 1 wt% magnesium acetate solution [44] on following a
literature procedure. Then the paste was deposited onto the FTO glass by blade coating. Subsequently, a 3 µm thick
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200 nm particle sized TiO2 scattering layer was deposited again by blade coating. The TiO2-coated FTO glass was
sintered at 450 oC for 30 min, then treated with TiCl4 solution at 70 oC for 30 min and calcined at 450 oC for 30 min
again. After the film was cooled to room temperature, it was immersed into 0.5 mM dye solution with or without
chenodeoxycholic acid (CDCA) (0.5mM) in the dark overnight. The sensitized electrode was then rinsed with
ethanol and dried. A drop of electrolyte was deposited onto the surface of the electrode and a Pt foil counter electrode
was clipped onto the top of the TiO2 electrode to assemble a DSSC for photovoltaic performance measurements. The
electrolyte consisted of 0.5 M LiI, 0.05 M I2, and 0.5 M 4-tert-butylpyridine (TBP) in 3-methoxypropionitrile and the
efficient irradiated area of the cell was 0.2 cm2. The current density-voltage (J–V) curves were measured by a
Keithley 2602 Source Meter under 100 mW cm–2 standard AM 1.5G spectrum using a Sol 3A Oriel solar simulator.
The incident light intensity was calibrated using a standard Si solar cell. The power conversion efficiency (η) of the
DSSC is calculated from short-circuit photocurrent (Jsc), the open-circuit photovoltage (Voc), the fill factor (FF) and
the intensity of the incident light (Pin) according to the following equation:
Jsc (mA cm-2) ¡ÁVoc (V) ¡ÁFF
¦Ç=
Pin (mW cm-2)
The measurement of monochromatic incident photo-to-current conversion efficiency (IPCE) for the solar cell was
also detected with a Zolix Solar Cell Scan 100 QE/IPCE measurement system.
3. Result and discussion
3.1. Synthesis
The detailed synthetic routes of the three dyes DX1, DX2 and DX3 are shown in Scheme 1. The compound 2
and 6 were synthesized via Vilsmeier reaction. The compound 3 was synthesized by Stille coupling reaction. The
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compound 1 and 13 were gained by two different bromination reactions, while the compound 11 was synthesized
via an iodination reaction. The compound 12 was obtained through Ullmann coupling reaction. The compound 7
and 13 were transformed into the corresponding diboronic ester 8 and 14 via lithiation and subsequent quenching
with 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, respectively. Then the compound 9, 10 and 15 were
synthesized by Suzuki reaction. Finally, the target dyes DX1, DX2 and DX3 were obtained via Knoevenagel
condensation. The structures of the dyes DX1, DX2 and DX3 were verified by 1H NMR, 13C NMR, GC-MS and
MALDI-TOF.
3.2. Photophysical properties
The UV-Vis absorption spectra of the dyes DX1, DX2 and DX3 in diluted (10–5 M) chloroform solutions are
depicted in Fig. 1(a), and the corresponding data are listed in Table 1. The shorter-wavelength absorption peaks at
ca. 300 nm and longer-wavelength absorption bands at ca. 450–600 nm could be assigned to the localized aromatic
π-π* transitions of the conjugated aromatic moieties and intramolecular charge transfer (ICT) between the
phenothiazine donor and the cyanoacrylic acid acceptor, respectively [42]. While the extra wavelength absorption
bands (350–450 nm) in DX2 and DX3 appeared due to the introduction of BT moiety as a spacer [45]. The
maximum absorption wavelengths (λmax) of DX1, DX2 and DX3 appeared at 494, 507 and 524 nm, and the
corresponding molar extinction coefficients (ε) were 2.62×104, 2.21×104 and 1.70×104 M–1cm–1, respectively. In
comparison with DX1, the ICT absorption peaks of DX2 and DX3 displayed red-shifts of 13 and 30 nm,
respectively, which could be attributed to a better delocalization of electrons over the π-conjugated molecules,
when the BT unit was used as an additional acceptor unit [46]. Moreover, a bathochromic shift of 17 nm of DX3
compared to DX2 in ICT absorption peak was observed, which might be interpreted on the basis of the increase in
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the conjugation dimension for DX3 with the participation of TPA unit, resulting in the enhanced π-electron density
to the acceptor [47]. Simultaneously, the absorption onset was extended by 100 nm from 600 nm (DX1) to 700 nm
(DX2 and DX3) in a dilute solution. However, the ε values were decreased in DX2 and DX3, which may lead to a
drop in short-circuit photocurrent density (Jsc) and photovoltaic performance.
In addition, as shown in Fig. 1(b), the maximum absorption peaks of three dyes on TiO2 films were observed
to severely blue-shift by 44, 87 and 25 nm with respect to those in solution for DX1, DX2 and DX3, respectively,
which could be mainly attributed to the H-type of aggregation and the partial deprotonation of carboxylic acid unit
caused by the interaction between the dye and TiO2 [48,49]. Apparently, DX2 revealed a sharper hypsochromic
shift than DX1 and DX3 in the absorption spectrum, indicating that DX2 had a higher degree of H-type aggregation
on the TiO2 film [50]. Remarkably, a distinct red-shift of absorption onset from 700 (DX1) to 800 nm (DX2 and
DX3) could be seen in Fig. 1(b). Accordingly, the absorption spectra of DX2 and DX3 in solutions and on TiO2
films showed a markedly broad profile, which was beneficial to light utilization [51].
3.3. Electronchemical properties
Cyclic voltammetry (CV) was carried out to estimate the possibility of electron injection and dye regeneration.
As shown in Fig. 2 and Table 2, the first oxidation potentials (Eox), which corresponded to the highest occupied
molecular orbital (HOMO) energy levels of DX1, DX2 and DX3, were 0.96, 0.98 and 0.90 V, respectively, vs a
normal hydrogen electrode (NHE), which were calibrated by addition of 0.65 V to the potential (vs. SCE) vs. Fc/Fc+
by CV. All of them were more positive than the redox couple (I−/ I3− , 0.42 V), suggesting sufficient driving force for
dye regeneration [52]. The similarity of HOMO energy levels of DX1 and DX2 was mainly attributed to their same
donor unit, as the HOMO levels of compounds were mainly depended on the electron-donating moiety [53].
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Additionally, the HOMO energy level of DX3 was higher than those of DX1 and DX2, which verified that the
introduction of TPA unit led to stronger electron-donating property of DX3. Estimated from intersection of the
normalized absorption and the emission spectrum (λint) in Fig. 3, the zero–zero excitation energy (E0-0) values were
2.24, 2.27 and 2.25 V for DX1, DX2 and DX3, respectively. The reduction potentials (Ered vs. NHE), corresponding
to the lowest unoccupied molecular orbital (LUMO) energy level, were –1.28, –1.29 and –1.35 V for DX1, DX2 and
DX3, respectively, calculated by Eox – E0-0. Apparently, the LUMO levels of the three dyes were more negative than
the conduction band (Ecb) of TiO2 (about –0.5 V vs. NHE), indicative of thermodynamic feasibility for electron
injection from the excited dyes into the TiO2 electrode [54]. Furthermore, the LUMO level of DX3 was higher than
the other dyes, which was beneficial for higher electron injection efficiency, namely, more sufficient electron flow
from the excited dyes to the TiO2 surface [55].
3.4. Density functional theory (DFT) calculations
To explain the structural properties of the dyes and understand the possibility of charge transfer from donor to
acceptor on electronic excitation, the optimized structures of three dyes were calculated geometrically using the
density functional theory (DFT) at the B3LYP/ 6-31G* level. The optimized structures with torsion angles and
electron densities of HOMOs and LUMOs of the dyes are shown in Table 3. In the optimized structures, the torsion
angles between the hexylthiophene moiety and its adjoining aromatic units in three dyes were similar, but the
changing tendency of torsion angles between the phenothiazine moiety and the adjacent units was in the order of
DX1 < DX2 < DX3, indicating that the structure of DX1 possesses relatively better planarity. In dye system, the
planar structure can enhance the aromatic character of the heterocyclic atom, increasing the degree of electronic
resonance between donor and acceptor moieties in the dye molecules and facilitating the electron transfer from
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donor to cyanoacrylic acceptor. However, it increases the stacking of the dye molecules, resulting in more dye
aggregation and electron recombination [56]. Thus, the electron transfer could be more effective in DX1, which
corresponded to the emission spectra in Fig. 3. On the other hand, the electron densities of HOMOs in DX1 and
DX2 were mainly localized over the phenothiazine moiety, whereas that in DX3 was mostly localized over the TPA
unit, which may due to the relatively strong electron-donating property of TPA unit. Additionally, the electron
density of LUMO in DX1 is mainly located on the cyanoacrylic segment and its adjoining hexylthiophene unit,
suggesting the well-inductive electron tendency from phenothiazine donor to the cyanoacrylic acceptor. However,
the electron densities of LUMOs in DX2 and DX3 were predominantly located on cyanoacrylic acceptor and
extended to the hexylthiophene and BT moieties, facilitating the ICT between phenothiazine donor and the BT unit,
and this in turn decreased the ICT between phenothiazine donor and the cyanoacrylic acceptor, and then reduced
the utilization to long wave. From those observations, it is clearly evident that, in DX1, the HOMO-LUMO
excitation induced by light irradiation could move the electron distribution from the phenothiazine moiety to the
cyanoacrylic moiety more effectively, and the photo-induced electrons can be more efficiently transferred from the
dye to the TiO2 surface by the electric charge separation.
3.5. Photovoltaic properties of DSSCs
The photocurrent density-voltage (J-V) curves and the incident photon to current conversion efficiency (IPCE)
spectra of DSSCs based on DX1, DX2 and DX3 with or without CDCA are shown in Fig. 4 and Fig. 5, with the
detailed photovoltaic parameters summarized in Table 4. The IPCE value of DX1 was over 60% in relatively
narrow wavelength ranges from 400 to 550 nm, with a maximum IPCE value of 72% at 465 nm and an onset at 700
nm, showing a higher IPCE value than the other two dyes, which was in good accordance with the Jsc variation
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obtained in J-V measurements. On the other hand, DX2 and DX3 got maximum IPCE values of 38% at 470 nm and
47% at 465 nm, respectively, with the photocurrent signal up to ca. 800 nm. In accord with the UV-Vis absorption
spectra in Fig. 1, DX2 and DX3 showed lower IPCE values compared to DX1.
As we all know, CDCA can work as an anti-aggregation compound to improve the performance in the DSSC
devices [57,58]. Thus, the photovoltaic performances with CDCA as the co-adsorbent were studied in the DSSCs.
Upon co-adsorption with 0.5 mM CDCA, The IPCE responses of DSSCs were remarkably increased compared to
the dyes only. The plateaus of IPCE curves were lifted to 79%, 49% and 57%, and the Jsc increased to 11.46, 8.01
and 10.28 mA cm– 2 for DX1, DX2 and DX3, respectively (Fig. 4). The remarkable enhancement of IPCE was
attributed to the break-up of dye aggregation upon co-adsorption. The increased Jsc was due to the enhancive
injection efficiency resulting from relatively independent dye molecules arraying on TiO2 surface [51], while the
Voc values were slightly increased, may due to the retardation of charge recombination arising from the prevention
of π-π stacking [59].
Consequently, the photovoltaic performances of DSSCs with CDCA were better than those without CDCA. As
is well-known, a high Jsc and Voc can lead to a high PCE. Despite the panchromatic responses of DX2 and DX3, they
showed lower Jsc values because of the lower IPCE values resulting from the lower ε values with respect to DX1. As
a result, the DX1-based DSSC with 0.5 mM CDCA as the co-adsorbent exhibited the best PCE of 5.69%, and the
DSSCs based on DX2 and DX3 with CDCA obtained PCEs of 3.43% and 4.41%, respectively, while the reference
ruthenium dye of N719 gives the PCE of 6.56% at the same conditions.
3.6. Electrochemical impedance spectroscopy studies
To further understand the relationship between the charge transfer process and photovoltaic properties of the
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DSSCs, electrochemical impedance spectroscopy (EIS) was employed under a forward bias of –0.65 V in the dark.
The Nyquist plots for the DSSCs based on DX1, DX2 and DX3 are displayed in Fig. 6 (a). The first semicircle (Rce)
is attributed to charge transfer at the counter electrode/electrolyte interface, while the second semicircle (Rrec) is
accorded to charge transfer at the TiO2/dye/electrolyte interface [60]. The similar Rce values (for DX1, DX2 and
DX3) were probably due to the same counter electrode and electrolyte. A large Rrec means a small dark current and
a low charge recombination rate [61]. The Rrec values for DX1, DX2 and DX3 were estimated to be 90, 65 and 49
ꢀ, respectively, as shown in Table 4. Obviously, the Rrec value of DX1 was much larger than the other two dyes,
which could restrain the charge recombination between injected electron and electron acceptor (I–3 ) in the
electrolyte, thereby contributing to the Voc [53].
On the other hand, the Bode plots for the DSSCs based on DX1, DX2 and DX3 are shown in Fig. 6 (b). The
peaks located at the high-frequency (right) and middle-frequency (left) respectively correspond to the small
semicircle (left) and large semicircle (right) in the Nyquist plots (Fig. 6 (a)). The reciprocal of the peak frequency
for the middle-frequency peak is regarded as the electron lifetime since it represents the charge transfer process at
the TiO2/dye/electrolyte interface. So the trend of electron lifetimes was in the order of DX1 > DX2 > DX3, which
was well in accordance with the changing tendencies of Voc values obtained in J–V curves. Evidently, the longer
electron lifetime of DX1 compared to the other two dyes could suppress the generation of dark current and reduce
electron recombination more effectively, resulting in a higher Voc in DX1-based device [50].
4. Conclusions
In summary, we reported the design and synthesis of three novel phenothiazine-based dyes DX1, DX2 and
DX3. By replacing the thiophene unit with the strong electron-withdrawing BT moiety, DX2 and DX3 displayed
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remarkable responses in the NIR region as well as a panchromatic response in DSSCs. However, the lower Jsc
resulted from the lower IPCE values led to lower photovoltaic performances in DX2- and DX3-based devices
compared to the DX1-based one. Additionally, with the introduction of strong electron-donating TPA unit, the dye
aggregation was reduced and electron injection efficiency was increased, thus the Jsc of the DX3-based device was
increased, resulting in a higher PCE in the DX3-based device with respect to the DX2-based one. Upon
co-adsorption with 0.5 mM CDCA, The photovoltaic performances of DSSCs were remarkably increased compared
to the dyes only. Consequently, the device based on DX1 with 0.5 mM CDCA obtained the highest PCE of 5.69%,
and the cells based on DX2 and DX3 with 0.5 mM CDCA got PCEs of 3.43% and 4.41%, respectively, while the
reference ruthenium dye of N719 gives the PCE of 6.56% at the same conditions. The researches on structure
tuning of phenothiazine-based dyes enrich their application in DSSCs. And we believe that with further
optimization to the molecular structure and the device, the photovoltaic performances of phenothiazine-based dyes
can be improved tremendously.
Acknowledgments
This work was supported by the National Nature Science Foundation of China (51173154), and Hunan Provincial
Natural Science Foundation of China (13JJ2025).
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Captions for Tables, Scheme and Figures
Table 1 Maximum absorption and emission data of the dyes DX1, DX2 and DX3.
Table 2 Electrochemical data of the dyes DX1, DX2 and DX3.
Table 3 Optimized structures, torsion angles and electronic distributions in HOMO and LUMO levels of the dyes
DX1, DX2 and DX3.
Table 4 Photovoltaic performance parameters of DSSCs based on the dyes DX1, DX2 and DX3.
Scheme 1. Synthetic routes and molecular structures of the three dyes DX1, DX2 and DX3.
Fig. 1. Absorption spectra of DX1, DX2 and DX3 in diluted CHCl3 solutions (a) and on TiO2 thin-films (b).
Fig. 2. Cyclic voltammogram (a) and energy level diagram (b) of the dyes DX1, DX2 and DX3.
Fig. 3. Emission spectra of the dyes DX1, DX2 and DX3 in diluted CHCl3 solutions.
Fig. 4. IPCE action spectra for the DSSCs based on the DX1, DX2 and DX3 with or without CDCA on
dye-sensitized transparent TiO2 films.
Fig. 5. J–V curves for DSSCs with the dyes DX1, DX2 and DX3 with or without CDCA under AM 1.5 simulated
sunlight (100 mW cm–2) illumination.
Fig. 6. EIS Nyquist plots (a) and Bode plots (b) for DSSCs based on DX1, DX2 and DX3 measured under –0.65 V
bias in the dark.
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Table 1
Maximum absorption and emission data of the dyes DX1, DX2 and DX3
Dye
λabs/nm (ε(×104 M-1cm-1) a
λmax/nm (on TiO2 ) b
λem /(nm) c
641
DX1
DX2
DX3
494(2.62)
450
333
397
420(1.89)
422(1.80)
507(2.21)
524(1.70)
582
572
a Maximum absorption in CHCl3 solution (10–5 M) at 25 oC.
b Maximum absorption on TiO2 film.
c Maximum emission of the dyes in CHCl3 solution (10–5 M) at 25 oC.
Table 2
Electrochemical data of the dyes DX1, DX2 and DX3.
Dye
λint/nm
554
E0-0/eV
2.24
Eox/V vs NHE
0.96
Ered/V/ vs NHE
–1.28
Egap/V
0.78
DX1
DX2
DX3
545
2.27
0.98
–1.29
0.79
552
2.25
0.90
–1.35
0.85
E0-0 values were calculated from intersection of the normalized absorption and the emission spectrum (λint): E0-0 = 1240/λint. The first
oxidation potential (vs NHE), Eox was measured in CHCl3 and calibrated by addition of 0.65 V to the potential vs Fc/Fc+. The reduction
potential, Ered, was calculated from Eox – E0-0. Egap is the energy gap between the Ered of dye and the conductive band level of TiO2 (-0.5
V vs NHE).
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