A new class of organic dyes based on acenaphthopyrazine for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.jphotochem.2010.05.017

A new class of organic dyes based on acenaphthopyrazine derivatives, containing pyrazine group as the electron acceptor and o-dicarboxyl acids as the anchoring groups were designed and synthesized for application in dye-sensitized solar cells (DSCs). These dyes have short synthesis routes and are easily adsorbed on the surface of TiO₂. Under illumination of simulated AM1.5 solar light (100 mW cm^-2), a total solar energy conversion efficiency (η) of 4.04% was obtained for the 3-(diphenylamino) acenaphtho[1,2-b] pyrazine-8,9-dicarboxylic acid (AP-1) in the preliminary tests, in comparison with the conventional N719 dye (η = 7.05%) under the same conditions.

Full text of DOI:10.1016/j.jphotochem.2010.05.017

Journal of Photochemistry and Photobiology A: Chemistry 213 (2010) 152–157
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Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
A new class of organic dyes based on acenaphthopyrazine for dye-sensitized
solar cells
Zhixia Konga, Huizhi Zhoua, Jingnan Cuia,∗, Tingli Maa, Xichuan Yanga, Licheng Sunb,∗∗
a State Key Laboratory of Fine Chemicals, Dalian University of Technology (DUT), 158 Zhongshan Road, 116012 Dalian, China
b School of Chemical Science and Engineering, Center of Molecular Devices, Organic Chemistry, Royal Institute of Technology (KTH), Teknikringen 30,
10044 Stockholm, Sweden
a r t i c l e i n f o
a b s t r a c t
Article history:
A new class of organic dyes based on acenaphthopyrazine derivatives, containing pyrazine group as
the electron acceptor and o-dicarboxyl acids as the anchoring groups were designed and synthesized
for application in dye-sensitized solar cells (DSCs). These dyes have short synthesis routes and are easily
adsorbed on the surface of TiO2. Under illumination of simulated AM1.5 solar light (100 mW cm−2), a total
solar energy conversion efficiency (ꢀ) of 4.04% was obtained for the 3-(diphenylamino)acenaphtho[1,2-
b] pyrazine-8,9-dicarboxylic acid (AP-1) in the preliminary tests, in comparison with the conventional
N719 dye (ꢀ = 7.05%) under the same conditions.
Received 24 August 2009
Received in revised form 11 March 2010
Accepted 25 May 2010
Available online 4 June 2010
Keywords:
Dye-sensitized solar cells
Acenaphthopyrazine derivatives
o-Dicarboxyl groups
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
acetic acid unit served as electron acceptor and the anchoring
group. Our dyes are different from those dyes, and the anchoring
Dye-sensitized solar cells (DSCs) have attracted much attention
due to their capability to convert solar light into electricity with
low material and production costs since 1991 [1]. The pursuit of
highly efficient dyes has been one of the most active subjects in
the development of DSCs. In recent years, much attention has been
paid to the research of organic dyes as substitutes of noble metal
complexes due to many advantages, such as diversity of molecular
structures, simple synthesis as well as low cost and environmental
issues. For these reasons, different kinds of organic dyes have been
designed and synthesized, such as coumarin dyes [2–4], hemicya-
nine dyes [5–7], indoline dyes [8–10], triphenylamine dyes [11–13],
phenothiazine dyes [14], porphyrine dyes [15] and merocyanine
dyes [16]. Although some organic dyes are approaching the work
efficiency of N3/N719 dyes [17–20], the Ru dyes still represent the
most efficient sensitizers in DSCs so far. Searching for more efficient
organic dyes is still a challenging task. Therefore, it is necessary to
design and synthesize a new type of organic dyes for DSCs. Here,
we report a new class of organic dyes AP and APQ for DSCs based
on acenaphthopyrazine chromophore and using larger conjugate
system acenaphthene as a bridge group, pyrazine as electron-
withdrawing group, and o-carboxyl acids as the anchoring groups.
In most of reported organic dyes, cyanoacrylic acid or rhodanine
groups can easily binding to the semiconductor surface simultane-
ously with a consequent improvement in the electronic coupling
of the dye, because the o-dicarboxyl groups are placed in the same
benzene ring and ortho position. In order to investigate the effect of
space between anchoring and electron-withdrawing groups on the
efficiency of DSCs, the space difference between the carboxyl and
pyrazine groups by one benzene ring was designed in AP and APQ
dyes. According to the most effective DSCs’ dyes, a diphenylamine
or N-alkyl as the electron donor was also used in the AP and APQ
dyes. From the photophysical and photoelectrochemical measure-
ments, we found that this new class of acenaphthopyrazine dyes
showed moderate performance on DSCs’ applications.
2. Experimental
All chemicals were purchased commercially and used with-
out further purification except for 4,5-diaminophthalonitrile which
was synthesized according to Ref. [21] and toluene which was dis-
tilled from CaH2 and molecular sieves BaO.
2.1. Synthesis
There are four steps for the synthesis of acenaphthopyrazine
dyes AP and APQ in a moderate yield as shown in Scheme 1. First,
acenaphthylene-1,2-dione 1 was bromized, and the intermediates
3 were obtained via palladium-catalyzed aromatic C–N cou-
pling reactions of 2 with diphenylamine or N-alkylphenylamines
∗
Corresponding author. Tel.: +86 411 39893872; fax: +86 411 83673488.
Corresponding author. Tel.: +46 8 790 8127.
E-mail addresses: jncui@dlut.edu.cn (J. Cui), lichengs@kth.se (L. Sun).
∗∗
1010-6030/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2010.05.017

Z. Kong et al. / Journal of Photochemistry and Photobiology A: Chemistry 213 (2010) 152–157
153
Scheme 1. Synthesis of AP and APQ dyes.
[22–24], then followed by the condensation of the dione 3 with
2.1.3. 3-(Diphenylamino)acenaphtho[1,2-b]
2,3-diaminomaleonitrile and 4,5-diaminophthalonitrile to obtain
4 and 5, respectively. Finally, the acenaphthopyrazine dyes were
obtained after hydrolysis of 4 and 5 in alkaline aqueous solution.
pyrazine-8,9-dicarbonitrile (4-1) [25]
A
mixture of 2,3-diaminomaleonitrile (124 mg, 1.1 mmol)
and 3-1 (400 mg, 1.1 mmol) in dry 1-butanol (10 mL) was
refluxed for 4 h under N2. The mixture was extracted with
dichloromethane, the organic phase was washed with water
and dried over MgSO4. After removing the organic solvent, the
residue was purified by chromatography on silica gel column using
dichloromethane–petroleum ether (2/1, v/v) as an eluent to give
a dark-red solid (325 mg, 88.5% yield). 1H NMR (400 MHz, CDCl3):
ı 8.38 (d, J = 7.2 Hz, 1H), 8.30 (d, J = 8.0 Hz, 1H), 7.88 (d, J = 8.8 Hz,
1H), 7.58 (t, J = 7.8 Hz, 1H), 7.39 (d, J = 8.0 Hz, 1H), 7.34 (t, J = 7.8 Hz,
4H), 7.18 (m, 6H); TOF GCMS EI+ m/z, calcd for C28H15N5 421.1327,
found 421.1330.
2.1.1. 5-Bromoacenaphthylene-1,2-dione (2)
A mixture of acenaphthene quinine 1 (20.0 g, 109.8 mmol) and
liquid bromine (25.0 mL, 466.8 mmol) was stirred and refluxed
for 2 h. The redundant bromine was removed by adding sodium
bisulfite solution. Adding water into the reaction solution leads
to the formation of a precipitate which was filtered, and repeat-
edly washed by water until pH reached 7.0 in filtrate. The crude
product was purified by recrystallization in glacial acetic acid four
times, and brown-yellow needle crystals were obtained (25.8 g, 90%
yield). 1H NMR (400 MHz, DMSO-d6): ı 8.39 (d, J = 8.4 Hz, 1H), 8.21
(d, J = 7.6 Hz, 1H), 8.15 (d, J = 7.2 Hz, 1H), 8.04 (t, J = 7.6 Hz, 1H), 7.96
(d, J = 7.6 Hz, 1H); TOF HRMS EI+ m/z, calcd for C12H6O2 259.9473,
found 259.9475.
2.1.4. 3-(Diphenylamino)acenaphtho[1,2-b]
pyrazine-8,9-dicarboxylic acid (AP-1)
A mixture of compound 4-1 (325 mg, 0.77 mmol) in 4 M aque-
ous solution of sodium hydroxide (60 mL) was stirred at 90 ◦C for
48 h. After cooling, the mixture was diluted with water. The precip-
itate was filtrated, washed with ether, then suspended in diluted
hydrochloric acid solution for acidification, filtrated and red solid
(250 mg, 70.6% yield) was obtained by filtration. 1H NMR (400 MHz,
DMSO-d6): ı 8.37 (m, 2H), 7.81 (d, J = 8.4 Hz, 1H), 7.71 (t, J = 7.6 Hz,
1H), 7.36 (m, 5H), 7.13 (m, 6H); TOF HRMS ES− m/z, calcd for
C28H16N3O4 458.1141, found 458.1126.
2.1.2. 5-(Diphenylamino)acenaphthylene-1,2-dione (3-1)
This step was carried out under Ar with standard Schlenk tech-
niques. A mixture of 5-bromo-acenaphthene quinone 2 (1000 mg,
3.84 mmol), diphenylamine (650 mg, 3.84 mmol), Pd(OAc)2 (10 mg,
0.038 mmol), potassium tert-butanolate (520 mg, 4.60 mmol), and
tri-tert-butylphosphonium tetrafluoroborate (30 mg, 0.1 mmol) in
dry toluene (150 mL) was stirred at 80 ◦C under Ar till TLC monitor-
ing indicated complete consumption of the amine. After cooling,
the mixture was filtered. The filtrate was diluted with ether,
and the organic phase was washed with water and brine, and
dried over MgSO4. After removing the organic solvent, the residue
was purified by chromatography on a silica gel column using
dichloromethane–petroleum ether (2/1, v/v) as an eluent to give
a brown solid (353 mg, 26.4% yield). 1H NMR (400 MHz, CDCl3): ı
8.03 (d, J = 8.0 Hz, 1H), 7.97 (d, J = 6.8 Hz, 1H), 7.73 (d, J = 8.8 Hz, 1H),
7.49 (t, J = 7.8 Hz, 1H), 7.37 (d, J = 7.6 Hz, 1H), 7.32 (t, J = 7.6 Hz, 4H),
7.12 (m, 6H), MS APCI: [M+H]+ (350.1, m/z).
2.1.5. 3-(Butyl(phenyl)amino)acenaphtho[1,2-b]
pyrazine-8,9-dicarboxylic acid (AP-2)
The synthesis of AP-2 dye was similar to AP-1. 81.9% yield;
red solid; 1H NMR (400 MHz, DMSO-d6): ı 8.43 (d, J = 7.6 Hz, 1H),
8.34 (d, J = 6.8 Hz, 1H), 7.73 (d, J = 8.0 Hz, 1H), 7.70 (m, 2H), 7.26 (t,
J = 7.8 Hz, 2H), 6.97 (m, 3H), 4.02 (t, J = 7.4 Hz, 2H), 1.70 (m, 2H),
1.41 (m, 2H), 0.89 (t, J = 7.4 Hz, 3H); TOF HRMS ES− m/z, calcd for
C26H26N3O4 438.1454, found 438.1443.

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Z. Kong et al. / Journal of Photochemistry and Photobiology A: Chemistry 213 (2010) 152–157
2.1.6. 3-(Methyl(phenyl)amino)acenaphtho[1,2-b]
dimethyl-3-propylimidazolinium iodine (DMPII), 0.03 M I2, 0.5 M
pyrazine-8,9-dicarboxylic acid (AP-3)
4-tert-butylpyridine (TBP) and 0.10 M GuSCN in acetonitrile.
The synthesis of AP-3 dye was similar to AP-1. 65.6% yield;
orange-red solid; 1H NMR (400 MHz, DMSO-d6): ı 8.41 (d, J = 7.6 Hz,
1H), 8.34 (d, J = 6.8 Hz, 1H), 7.76 (d, J = 8.8 Hz, 1H), 7.67 (t, J = 7.6 Hz,
1H), 7.59 (d, J = 7.6 Hz, 1H), 7.30 (t, J = 7.6 Hz, 2H), 7.06 (d, J = 8.0 Hz,
2H), 7.01 (t, J = 7.4 Hz, 1H), 2.51 (s, 3H); TOF HRMS ES− m/z, calcd
for C23H14N3 O4 396.0984, found 396.0972.
2.3. Characterization of the dyes
1H NMR spectra were measured with VARIAN INOVA400 MHz
(USA) with the chemical shifts against TMS. Mass spectra were mea-
sured on an HP 1100 LC-MSD spectrometer, and high-resolution
mass spectra (HRMS) were obtained on HPLC-Q-Tof MS (Micro)
spectrometer. Absorption and emission spectra were recorded with
HP8453 (USA) and PTI700 (USA), respectively. Electrochemistry
was measured with BAS100W (USA). IR spectra were measured
with 20DXB. The geometrical and electronic properties of the dyes
were studied with DFT calculations using Gaussian 03 program
package.
2.1.7. 3-(Diphenylamino)acenaphtho[1,2-b]quinoxaline-9,10-
dicarboxylic acid (APQ-1)
The synthesis of APQ-1 dye was similar to AP-1. 32.6% yield;
red solid; 1H NMR (400 MHz, DMSO-d6): ı 8.40 (m, 3H), 7.80 (d,
J = 8.4 Hz, 1H), 7.73 (t, J = 7.4 Hz, 1H), 7.44 (d, J = 8.0 Hz, 1H), 7.35
(t, J = 7.8 Hz, 4H), 7.47 (m, 6H); TOF HRMS ES− m/z, calcd for
C32H18N3O4 508.1297, found 508.1286.
2.4. Photocurrent–voltage measurements
2.1.8. 3-(Butyl(phenyl)amino)acenaphtho[1,2-b]quinoxaline-
9,10-dicarboxylic acid (APQ-2)
The irradiation source for the photocurrent–voltage (J–V) mea-
surement was an AM 1.5 solar simulator (16S-002, Solar Light
Co. Ltd., USA). The incident light intensity was 100 mW cm−2 cali-
brated with a standard silicon solar cell. The current–voltage curves
were obtained by linear sweep voltammetry (LSV) method using an
electrochemical workstation (LK9805, Lanlike Co. Ltd., China). The
measurement of the incident photon-to-current conversion effi-
ciency (IPCE) was performed by a Hypermonolight (SM-25, Jasco
Co. Ltd., Japan).
The synthesis of APQ-2 dye was similar to AP-1. 32.0% yield;
orange-red solid; 1H NMR (400 MHz, DMSO-d6): ı 8.46 (m, 3H), 8.37
(d, J = 6.4 Hz, 1H), 7.73 (m, 3H), 7.25 (t, J = 8.0 Hz, 2H), 6.93 (m, 3H),
4.02 (t, J = 7.4 Hz, 2H), 1.70 (m, 2H), 1.41 (m, 2H), 0.90 (t, J = 7.4 Hz,
3H); TOF HRMS ES− m/z, calcd for C30H22N3O4 488.1610, found
488.1613.
2.1.9. 3-(Methyl(phenyl)amino)acenaphtho[1,2-b]quinoxaline-
9,10-dicarboxylic acid (APQ-3)
The synthesis of APQ-3 dye was similar to AP-1. 50.3% yield;
orange-red solid; 1H NMR (400 MHz, DMSO-d6): ı 8.95 (s, 1H),
8.94 (s, 1H), 8.46 (d, J = 7.2 Hz, 1H), 8.39 (d, J = 6.4 Hz, 1H), 7.78 (d,
J = 8.4 Hz, 1H), 7.11 (t, J = 7.6 Hz, 1H), 7.65 (d, J = 7.2 Hz, 1H), 7.27 (t,
J = 7.6 Hz, 2H), 7.11 (m, 3H), 3.58 (s, 3H); TOF HRMS ES− m/z, calcd
for C27H16N3O4 446.1141, found 446.1145.
3. Results and discussion
3.1. Photophysical and electrochemical properties
The absorption spectra of the dyes in the mixture of acetonitrile
(AN) and tert-butyl alcohol (v:v = 1:1, t-BuOH-AN) are displayed in
Fig. 1 and the data are listed in Table 1. The absorption maxima of
461, 457, and 444 nm were obtained for AP-1, -2 and -3 dyes, while
the APQ-1, -2 and -3 dyes showed 444, 435, 424 nm in the solu-
tion, respectively. Compared with the absorptions, ꢁmax of AP is
red-shifted than APQ dyes, indicating stronger electron push–pull
system in AP dyes than APQ dyes, although APQ dyes have bigger
conjugate system than AP dyes. The bigger conjugate system in APQ
dyes show higher extinction coefficients ε than AP dyes. When the
dyes were attached to TiO2, the absorption maxima of these dyes
were blue-shifted by 0, 6, 3, 8, 7, and 3 nm for AP-1, -2, -3, APQ-
1, -2 and -3, respectively. The deprotonation of carboxylic acid in
the dye molecule [19] and possible aggregation upon adsorption
on the TiO2 film, as discussed in the following IR analysis, may con-
tribute to this blue-shift. The aggregation of AP-1 dye in t-BuOH-AN
2.2. Fabrication of the nanocrystalline TiO2 solar cells
Titania paste from Solaronix-D (Solaronix, Switzerland) [17] was
deposited onto the conducting glass by doctor blading, with the
12 ␮m film thickness, 0.2 cm2 active area. The photoelectrode was
sintered at 500 ◦C for 30 min in air and cooled to room temper-
ature. For the dye uptake, the electrode was immersed into the
dye solutions [5 × 10−4 M in a mixture of acetonitrile and tert-butyl
alcohol (v/v, 1:1)], and kept at room temperature for 10 h, then
rinsed with EtOH and dried. A platinized counter electrode was
clipped onto the top of the TiO2 working electrode and electrolyte
solution was added between the two electrodes via capillary action.
The electrolyte was composed of 0.06 M lithium iodide, 0.60 M 1,2-
Fig. 1. Absorption spectra of acenaphthopyrazine dyes in t-BuOH-AN solution (left) and on TiO2 films (right).

Z. Kong et al. / Journal of Photochemistry and Photobiology A: Chemistry 213 (2010) 152–157
155
Table 1
Absorption, emission, electrochemical properties of the acenaphthopyrazine dyes.
Dye
Absorptiona
Emissiona
Oxidation potential
b
ꢁmax (nm)
ε at ꢁmax (M−1 cm−1
)
ꢁmax on TiO2 (nm)
ꢁmax (nm)
Eox (V) (versus NHE)c
E0–0 (V) (Abs/Em)d
Eox–0–0 (V) (versus NHE)
AP-1
AP-2
AP-3
APQ-1
APQ-2
APQ-3
461
457
444
444
435
424
0.87 × 104
0.63 × 104
0.31 × 104
1.22 × 104
1.04 × 104
1.10 × 104
461
451
441
436
428
421
603
599
585
590
608
607
1.33
1.28
1.25
1.31
1.26
1.21
2.17
2.31
2.32
2.35
2.39
2.45
−0.84
−1.03
−1.07
−1.04
−1.13
−1.24
a
Absorption and emission spectra were measured in t-BuOH-AN solution (2 × 10−5 M) at room temperature.
b
c
Absorption spectra on TiO2 were obtained through measuring the dye adsorbed on TiO2 film in t-BuOH-AN.
The electrochemical properties were measured in acetonitrile with 0.1 M tetra-butylammonium hexafluorophosphate (TBAPF6) as supporting electrolyte (working
electrode: glassy carbon; reference electrode: Ag/Ag+; calibrated with ferrocene/ferrocenium (Fe/Fe+) as an internal reference and converted to NHE by addition of 630 mV;
counter electrode: Pt).
d
The E0–0 was estimated from the intersection between the absorption and emission spectra.
Fig. 2. FT-IR spectra for dye AP-1 (a), sodium salt of AP-1 (b) and adsorbed on TiO2 film (c).
was not been observed by increasing concentration of the dye from
1.0 × 10−5 to 2.0 × 10−4 M.
absorption, indicating the two carboxyl groups bind to the TiO2 sur-
face simultaneously. The separation between the asymmetric and
symmetric peak for –COO− is calculated to be 253 cm−1, compara-
ble to that (232 cm−1) for the solid salt form, indicating a bidentate
coordination mode of the dye to the TiO2 surface [27].
Cyclic voltammetry (CV) was employed to measure the elec-
trochemical properties of these dyes and the results are shown in
Table 1. The first oxidation potential (Eox) corresponds to the HOMO
level of the dye. From the data in Table 1, it is known that the HOMO
levels of the dyes are sufficiently more positive than I−/I3 redox
−
3.3. Photovoltaic performance of DSCs
couple, indicating that the oxidized dyes could be reduced effec-
tively by electrolyte and then regenerated. The LUMO levels of these
dyes were calculated by Eox–E0–0, where E0–0 is the zeroth–zeroth
energy of the dyes estimated from the intersection between the
absorption and emission spectra, and the data are listed in Table 1.
To effectively inject the electron into the conducing band (CB) of
TiO2, the LUMO levels of the dyes must be sufficiently more nega-
tive than the conducing band energy (Ecb) of semiconductor, −0.5 V
(versus NHE) [26]. From the LUMO values, we can find that all
of acenaphthopyrazine dyes can complete the process of electron
injection into CB of TiO2 to form the oxidized dyes.
The data for photovoltaic performance are shown in Table 2 and
J–V curves of the dyes are shown in Fig. 3 (left). The highest value
of conversion efficiency (ꢀ) among these dyes was 4.04% for the
AP-1 in the preliminary tests, under the same conditions, the effi-
ciency of N719 was 7.05%. Similar to triphenylamine dyes [11–13],
the efficiency of AP-1 and APQ-1 dyes was higher than the other
dyes, which can be ascribed to the bigger conjugate system. The
incident photon-to-current conversion efficiencies (IPCEs) of these
dyes in DSCs are shown in Fig. 3 (right), which is in agreement
Table 2
Photovoltaic performance of acenaphthopyrazine dyes.a.
3.2. Binding mode of dye molecules to the TiO2 surface
Dyeb
JSC (mA cm−2
)
VOC (mV)
Fill factor (ff)
ꢀ (%)
In order to determine the detailed structure of acenaphthopy-
razine dyes adsorbed on the TiO2 surface, which is associated with
the interfacial electron injection, FT-IR absorption for AP-1 powder,
sodium salt of AP-1 and TiO2 films exposed to AP-1 solution was
measured, and the spectra are shown in Fig. 2. For the dye powder,
the 1725 cm−1 peak is assigned to carbonyl group in the carboxylic
acid. The peak at 1408 cm−1 results from the in-plane bending of
C–O–H. After the dye was adsorbed onto the TiO2 surface, the peak,
diagnostic of –COOH at 1725 and 1408 cm−1 disappeared, while
asymmetric and symmetric peaks for –COO− group were observed,
respectively, at 1621 and 1368 cm−1, owing to the deprotonation of
carboxylic acid upon dye adsorption and lead to the blue-shift of the
AP-1
AP-2
AP-3
APQ-1
APQ-2
APQ-3
N719
8.11
6.43
4.10
6.27
5.89
3.78
13.09
693
662
638
650
632
589
820
0.719
0.630
0.697
0.699
0.705
0.663
0.657
4.04
2.68
1.82
2.85
2.62
1.48
7.05
a
The photovoltaic performance of acenaphthopyrazine dyes was measured under
irradiation of AM 1.5 G simulated solar light (100 mW cm−2) at room temperature,
12 ␮m film thickness which adopted Solaronix-D titania paste, 0.2 cm2 working area.
b
The concentration of dyes is 5 × 10−4 M in a mixture of acetonitrile and tert-butyl
alcohol (v/v, 1:1), 0.6 M DMPII, 0.03 M I2, 0.1 M GuSCN, 0.5 M TBP, and 0.06 M LiI in
acetonitrile as electrolyte.

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Z. Kong et al. / Journal of Photochemistry and Photobiology A: Chemistry 213 (2010) 152–157
Fig. 3. J–V curves (left) and IPCE spectra (right) of DSCs based on acenaphthopyrazine dyes.
Fig. 4. The frontier orbitals of the AP-1 and APQ-1 dyes optimized at the B3LYP/6-31+G(d) level and B3LYP/3-21+G, respectively.
with the absorption spectra of the dyes. The highest value obtained
for AP-1 was 63% at 460 nm. It seems that introduction of long-
wavelength chromophore to AP dyes may broaden the spectrum
of absorption to get better IPCE. Compared to using APQ dyes, the
photovoltaic performance of DSCs using AP dyes is better despite
of APQ dyes have higher ε, indicating the space between anchor-
ing and electron-withdrawing groups is an important factor and the
small space is propitious to obtain the higher conversion efficiency.
4. Summary
We have designed and synthesized a new class of organic
dyes with acenaphthopyrazine chromophore containing larger
conjugate system acenaphthene as a bridge group, pyrazine
as electron-withdrawing group, and o-dicarboxyl acids as the
anchoring groups. DSCs based on these dyes were prepared with
nanostructured TiO2. The AP-1 dye has the highest efficiency
(ꢀ = 4.04%) among these dyes in the preliminary tests, in compar-
ison with N719 dye (ꢀ = 7.05%) under the same conditions. The
small space between anchoring and electron-withdrawing groups
could be propitious to obtain the better photovoltaic performance
of DSCs. By further structural modification of the acenaphthopy-
razine dyes and optimization of test conditions, we expect to get
even better DSC performance and related work is in progress.
3.4. Theoretical study of dye structure and electron distribution
In order to get a further insight into the difference in perfor-
mance of DSCs based on these dyes, density functional theory (DFT)
calculations [28] were performed for the geometry optimization.
Fig. 4 shows the frontier molecular orbitals of AP-1 and APQ-1. At
the HOMO of the dye, the electron density is mainly distributed in
the diphenylamine electron donor moiety, and with electronic exci-
tation at the LUMO level, intramolecular charge transfer induced
electron movement from the donor site to the acceptor moiety of
the dye. The better photovoltaic performance of AP-1 than APQ-1
indicates that the space between o-dicarboxyl groups and acenaph-
thopyrazine in AP-1 or APQ-1 is important and excited electrons
of AP-1 can easily inject into the conduction band of TiO2 via the
o-dicarboxyl groups.
Acknowledgments
We are grateful to the National Natural Science Foundation of
China (90713030; 20876025), the National Basic Research Program
of China (Grant No. 2009CB220009), the Program for Changjiang
Scholars and Innovative Research Team in University (IRT0711),
the Swedish Energy Agency, the Swedish Research Council, and the
K & A Wallenberg Foundation for financial support of this work.

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157
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