High-performance dipolar organic dyes with an electron-deficient diphenylquinoxaline moiety in the π-conjugation framework for dye-sensitized solar cells

Article abstract of DOI:10.1002/chem.201201000

We report here the synthesis and electrochemical and photophysical properties of a series of easily prepared dipolar organic dyes and their application in dye-sensitized solar cells (DSSCs). For the six organic dyes, the molecular structures comprised a triphenylamine group as an electron donor, a cyanoacrylic acid as an electron acceptor, and an electron-deficient diphenylquinoxaline moiety integrated in the π-conjugated spacer between the electron donor and acceptor moieties. The incorporation of the electron-deficient diphenylquinoxaline moiety effectively reduces the energy gap of the dyes and broadly extends the spectral coverage. DSSCs based on dye 6 produced the best overall cell performance of 7.35 %, which translates to approximately 79 % of the intrinsic efficiency of the DSSCs based on the standard N719 dye under identical experimental conditions. The high performance of DSSCs based on dye 6 among the six dyes explored is attributed to the combined effects of high dye loading on a TiO₂ surface, rapid dye regeneration, and effective retardation of charge recombination. Copyright

Full text of DOI:10.1002/chem.201201000

FULL PAPER
DOI: 10.1002/chem.201201000
High-Performance Dipolar Organic Dyes with an Electron-Deficient
Diphenylquinoxaline Moiety in the p-Conjugation Framework for
Dye-Sensitized Solar Cells
Sie-Rong Li,[a] Chuan-Pei Lee,[b] Hui-Tung Kuo,[a] Kuo-Chuan Ho,*[b] and
Shih-Sheng Sun*[a]
Abstract: We report here the synthesis
and electrochemical and photophysical
properties of a series of easily prepared
dipolar organic dyes and their applica-
tion in dye-sensitized solar cells
(DSSCs). For the six organic dyes, the
molecular structures comprised a tri-
phenylamine group as an electron
donor, a cyanoacrylic acid as an elec-
tron acceptor, and an electron-deficient
diphenylquinoxaline moiety integrated
in the p-conjugated spacer between the
electron donor and acceptor moieties.
The incorporation of the electron-defi-
cient diphenylquinoxaline moiety effec-
tively reduces the energy gap of the
dyes and broadly extends the spectral
coverage. DSSCs based on dye 6 pro-
duced the best overall cell performance
of 7.35%, which translates to approxi-
mately 79% of the intrinsic efficiency
of the DSSCs based on the standard
N719 dye under identical experimental
conditions. The high performance of
DSSCs based on dye 6 among the six
dyes explored is attributed to the com-
bined effects of high dye loading on a
TiO2 surface, rapid dye regeneration,
and effective retardation of charge re-
combination.
Keywords: absorption
· diphenyl-
quinoxalines · dyes · green chemis-
try · solar cells
Introduction
ly catching up and recently the best photoconversion effi-
ciency of organic dye-sensitized solar cells up to 10% or
higher was observed.[18–20]
The development of efficient energy conversion materials
has been a popular research topic for searching for an
energy alternative for environmentally unfriendly fossil
fuels.[1–3] Dye-sensitized solar cells (DSSCs) represent a class
of low-cost photovoltaics relative to the traditional solid-
state semiconductor devices and have received considerable
attention recently.[4–6] Typically these devices are composed
of three functional regions: a light-harvesting dye adsorbed
on the nanocrystalline TiO2 film as the photosensitizer, a Pt-
coated counter electrode, and a liquid junction containing a
One of the crucial components in a DSSC is the photo-
sensitizer since it is responsible for light harvesting, funnel-
ling excitation energy to the semiconductor surface, and
eventually achieving charge separation. In recent years,
many studies have focused on screening various organic
dyes with a D–p–A system, which typically consists of an
electron donor (D), a p-conjugated spacer (p) between the
donor and acceptor, and an electron acceptor (A; most com-
monly a carboxylate group) for anchoring onto the TiO2 sur-
face. Photosensitizers possessing an extended p-conjugated
spacer would expect to narrow their HOMO–LUMO gaps
and increase the spectral coverage for the light harvest-
ing.[21–25] The structural design principle typically aims to fa-
cilitate efficient charge transfer upon light absorption, effec-
tively tuning the spectral coverage with enhanced light-har-
vesting efficiency, and increasing thermal and photochemical
stability.[26–28] Many organic dyes featuring substituted tri-
ꢀ
reusable redox couple (most commonly I3 /Iꢀ). Pioneered
by Grꢀtzel and co-workers,[7–10] DSSCs with RuII-based sen-
sitizers have shown the best photoconversion efficiency of
up to 11.5%.[11–17] However, the consumption of precious
metals for metal-complex dyes is still a concern in terms of
cost and long-term availability. On the other hand, the effi-
ciency of DSSCs based on metal-free organic dyes are rapid-
AHCTUNGTRENNUNG
arylamine,[29–34] coumarin,[35–37] indoline,[38] fluorene,[39] and
[a] Dr. S.-R. Li, Dr. H.-T. Kuo, Dr. S.-S. Sun
Institute of Chemistry
porphyrin[20,40–42] have been intensively employed as sensitiz-
ers for DSSCs.
Academia Sinica (Taiwan, ROC)
Fax : (+886)2-27831237
E-mail: sssun@chem.sinica.edu.tw
Alternatively, one way to extend the spectral coverage in
DSSCs is to integrate an electron-deficient fragment in the
p-conjugated spacer of organic dyes to form a low band gap
chromophore, which enhances the degree of charge separa-
tion and extends the absorption range of the sensitizers.
This strategy of reducing the HOMO–LUMO gap by intro-
ducing a linker to destabilize the HOMO and stabilize the
[b] C.-P. Lee, Prof. K.-C. Ho
Department of Chemical Engineering
National Taiwan University (Taiwan, ROC)
E-mail: kcho@ntu.edu.tw
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201201000.
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LUMO has been widely employed for organic bulk hetero-
junction photovoltaic materials,[43,44] but there are relatively
few reports for DSSC-exploited organic dyes incorporating
electron-deficient fragments. Notable examples for such
electron-deficient chromophores incorporated in sensitizers
for DSSCs include benzothiadiazole,[45,46] porphyrin-fused
quinoxaline,[47] boradiazaindacene,[48,49] quinoxaline,[50] and
others.[51] To explore the effect of an electron-deficient chro-
mophore and its relative linkage position in the p-conjugat-
ed framework on the performance of the resulting DSSCs,
we report herein the design and synthesis of a series of dipo-
lar organic sensitizers imparted with an electron-deficient
diphenylquinoxaline unit in the framework of the p-conju-
gated spacer to link the triphenylamine electron donor and
cyanoacrylic acid electron acceptor. An appreciable efficien-
cy of over 7.3% has been achieved with the dye 6 sensitized
the p-conjugated spacer are illustrated in Scheme 1. After
treating compound 7[52] with n-butyllithium and DMF at
ꢀ788C to form monoformylated diphenylquinoxaline pre-
cursor 8, the aldehydes 9 and 10 were synthesized by con-
ventional palladium-catalyzed Suzuki–Miyaura and Heck
coupling reactions, respectively. Dyes 1 and 2 were obtained
through a Knoevenagel condensation reaction by treating 9
and 10, respectively, with cyanoacetic acid in acetic acid at
reflux. Dyes 3, 4, and 5 were synthesized in a similar
manner by starting from 11,[53] 12,[53] and 13,[53] respectively,
by a series of boronation with n-butyllithium and excess
B(OMe)3 in THF, palladium-catalyzed Suzuki–Miyaura cou-
pling reaction, and a final condensation reaction. Dye 6,
which has a thiophene inserted between the diphenylqui-
noxaline and cyanoacrylic acid, was prepared from com-
pound 7 through two successive Suzuki coupling reactions
followed by Knoevenagel con-
densation with cyanoacetic
acid. All six dyes have been
fully characterized by NMR
spectroscopy, HRMS, and ele-
mental analysis. All six dipolar
dyes are air stable and possess
fairly good solubility in highly
polar solvents, especially in
DMF.
Photophysical and electrochem-
ical properties: The UV/Vis ab-
sorption spectra of the six dyes
measured in DMF are plotted
in Figure 1a and the related
photophysical data are summar-
ized in Table 1. The general fea-
tures of the absorption spectra
are composed of two broad
bands in the near UV and visi-
ble region, which are assigned
to a series of p–p* transitions
and a low-energy intramolecu-
lar charge-transfer (ICT) transi-
tion, respectively. The lowest
energy bands of the six dyes
range from 433 to 483 nm with
molar extinction coefficients
between 1.2ꢂ104 and 2.4ꢂ
DSSC. The high dye loading of dye 6 on the TiO2 surface
and effective retardation of charge recombination are attrib-
uted to the key factors for good DSSC performance, which
correlates well with the relative position of the electron-de-
ficient diphenylquinoxaline in the p-conjugated spacer.
104 mꢀ1 cmꢀ1. The absorption spectral coverage follows the
order of 5>3>4>2>6>1. As anticipated, dyes with a
more extended p-conjugation network display lower energy
absorption profiles and more extended spectral coverages.
The presence of a hexyl group in the thiophene adjacent to
the triphenylamine moiety in dye 4 results in a less planar
conformation due to the steric hindrance, which may ac-
count for the higher energy absorption profile in dye 4 rela-
tive to dye 3. Dyes 3 and 6 contain the same p-conjugated
fragments but differ in the relative position of thiophene
and diphenylquinoxaline. Direct connection of an electron-
Results and Discussion
Synthesis: The synthetic strategies for the preparation of the
six new organic dyes with a diphenylquinoxaline moiety in
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High-Performance Dipolar Organic Dyes
FULL PAPER
show fairly strong emission with
maxima
between
628
to
682 nm. The emission maximum
also follows the same trend as
the absorption spectra with
more p-conjugated molecules
showing red-shifted emission
profiles.
The absorption spectra of all
dyes adsorbed on a transparent
TiO2 film are depicted in Fig-
ure 1b. Relative to the spectra
measured in DMF, apparent
bathochromic shifts and broad-
ening of the absorption profiles
were observed for all six dyes
adsorbed on the TiO2 surface.
In particular, the spectral cover-
age of dye 5 is extended over
700 nm upon adsorption on the
TiO2 surface. Although the
exact nature of the dye aggre-
gate on the TiO2 surface is not
clear, the observed bathochro-
mic shift in the absorption spec-
tra is likely due to the coordina-
tion of the carboxylate group to
the surface Ti4+ that lowers the
p* level of the organic dyes.[54]
The amount of dye loadings on
the TiO2 surface estimated by
changes of absorbance of the
mother liquid of dyes before
and after adsorption are collect-
Scheme 1. Synthetic procedures for organic dyes 1–6: a) nBuLi, DMF, ꢀ78 to 08C, 2 h, 45%; b) 4-(diphenyl-
ACHTUNGTRENNUNGamino)phenylboronic acid, [PdCl2AHCTUNGTREN(NUNG PPh3)2], Na2CO3, H2O/THF (3:2), RT, 16 h; c) 2-cyanoacetic acid,
CH3COOH, NH4OAc, reflux, 12 h; d) N,N-diphenyl-4-vinylaniline, Pd
e) 1) nBuLi, THF, B 2A(PPh3)2], Na2CO3, 8, THF/H2O (3:2), RT, 16 h;
(OCH3)3, ꢀ788C to RT, 5 h; 2) [PdCl
f) 5-formylthiophen-2-ylboronic acid, [PdCl2A(PPh3)2], THF/H2O (3:2), Na2CO3, RT, 16 h.
ACHTUGNTREN(NUNG OAc)2, Na2CO3, DMF, 1208C, 16 h;
A
CHTUNGTRENNUNG
CTHUNGTRENNUNG
ed in Table 1. It generally cor-
relates well with the molecular
size of the dyes in which the
Table 1. Photophysical and electrochemical data and dye loading on a TiO2 surface.
smaller dyes displayed larger amounts of dye load-
ing on the TiO2 surface. Dye 6 shows the highest
dye loading on the TiO2 surface among the six dyes
even though its molecular size is larger than dyes 1
and 2. It is possible that the less space demanding
thiophene directly attached to cyanoacrylic acid
dramatically increases the number of dye molecules
that can be adsorbed on the TiO2 surface when
compared to other dyes with the more stericly de-
manding diphenylquinoxaline near the TiO2 surface
(see Table 1 and Figure 2). The orientation of the
adsorbed dyes on the TiO2 surface may also con-
tribute to the ultimate density of dye loading. The
dense packing of dye 6 on the TiO2 surface may
Dye lmax [nm]
(10ꢀ4 mꢀ1 cmꢀ1
lem
Dye loading
E (S+/S) E0–0
E (S+/S*) Egap
[a]
)
[nm][a] [10ꢀ7 molcm2] [V][b]
[eV][c] [V][d]
[V][e]
1
2
3
4
5
6
433 (1.2)
628
665
681
675
682
630
6.4
5.2
3.6
2.7
2.2
7.9
1.19
1.06
1.07
1.07
1.00
1.12
2.34
2.21
2.12
2.15
2.14
2.29
ꢀ1.15
ꢀ1.15
ꢀ1.05
ꢀ1.08
ꢀ1.14
ꢀ1.17
0.65
0.65
0.55
0.58
0.64
0.67
463 (2.4)
483 (1.9)
466 (1.8)
483 (2.0)
460 (2.2)
[a] Absorption and emission spectra were measured in DMF. [b] The oxidation poten-
tials (vs. the normal hydrogen electrode (NHE)) of dyes were measured in DMF with
tetrabutylammonium hexafluorophosphate (0.1m) as the supporting electrolyte.
[c] E0–0 values were estimated from the intersection of the normalized absorption and
emission spectra. [d] E (S+/S*) (vs. NHE) values were calculated from E (S+/S)ꢀE0–0
.
[e] The energy gap between the excited-state oxidation potential and the TiO2 conduc-
tion band edge.
ꢀ
also prevent the bulky I3 approaching closely to
rich thiophene to the triphenylamine donor site facilitates a
stronger ICT effect and more bathochromic shift of the ab-
sorption profile in dye 3 relative to the less effective ICT in
dye 6 in which an electron-deficient diphenylquinoxaline is
connected to the triphenylamine donor site. All six dyes
the surface and retard the charge recombination (vide
infra).
The redox potentials of dyes 1–6 were measured in DMF
by square-wave voltammetry and the results are also sum-
marized in Table 1. The first oxidation potential of the dyes,
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E (S+/S), which occurs at the triphenylamine site, falls in the
range between 1.00 to 1.19 V versus NHE and follows the
order of 1>6>2ꢁ3ꢁ4>5. The higher oxidation potentials
observed for dyes 1 and 6 relative to the others are attribut-
ed to the direct connection of an electron-deficient diphe-
nylquinoxaline moiety to the electrochemically active tri-
phenylamine site, which lowers the HOMO level. The 0–0
transition energy (E0–0) of the dyes, which was estimated at
the intersection of the normalized absorption and emission
spectra, follows the trend of p-conjugation length with
1>6>2>4>5>3. The E (S+/S*) values, corresponding to
the LUMO levels of the dyes, were calculated from
E (S+/S)ꢀE0–0 and fall in the range between ꢀ1.05 to
ꢀ1.17 V. As illustrated in Figure 3, the HOMO and LUMO
levels of all six dyes exhibit sufficient driving force for the
charge injection to the TiO2 conduction band (ꢀ0.5 V vs.
NHE), as well as the dye regeneration by iodide reduction
(0.4 V vs. NHE).
Molecular orbital calculations: Theoretical studies based on
DFT calculations by using the Gaussian 09 program package
at the B3LYP/6-31G* level of theory were carried out for
the six dyes to gain insight into the geometric electronic
strucutres.[55] Table 2 summarizes the frontier molecular or-
bitals for the six dyes. In Table 2 it can clearly be seen that
the electron density of the HOMOs is predominantly local-
ized on the triphenylamine donor moiety and diffusively dis-
tributed over the p-conjugation onto the diphenylquinoxa-
Figure 1. a) Absorption spectra measured in DMF and b) absorption
spectra measured with dyes adsorbed on a TiO2 surface.
Figure 2. A cartoon illustration of the steric demands of the dyes on adsorption on a TiO2 surface.
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High-Performance Dipolar Organic Dyes
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Table 2. Frontier molecular orbitals for dyes 1–6.
Dye HOMO
LUMO
1
2
3
Figure 3. Comparison of the ground- and excited-state oxidation poten-
tials of dyes 1–6 witt the conduction band of TiO2 and the oxidation po-
tential of iodide.
line moiety, which holds a small fraction of the overall elec-
tron density. In contrast, the electron density of the LUMOs
is extended over the diphenylquinoxaline and cyanoacrylic
acid acceptor moiety. It is notable that, by careful scrutiny
of the frontier MOs, dyes 1 and 6 exhibit more localized
electron density distribution than dyes 2–5, which implies a
more prominent charge separation in dyes 1 and 6 than dyes
2–5 upon photoexcitation. This subtle difference, in conjunc-
tion with other physical parameters, may contribute to the
different DSSC performance based on these six dyes.[56,57]
4
Photovoltaic performance: Figure 4 depicts the J–V curves
and action spectrum of incident photon-to-electron conver-
sion efficiency (IPCE) plots as a function of wavelength for
the DSSCs based on the six dyes studied in this work under
AM 1.5 illumination. The device performance and the rele-
vant data of the assembled DSSCs are summarized in
Table 3. The power conversion efficiencies are in the range
of 2.38–7.35% and in the order of 1<2<4<3<5<6 with
the best efficiency of dye 6 up to approximately 79% of in-
trinsic performance of the standard N719 dye under the
same experimental conditions. It is interesting to note that
5
6
Table 3. Photovoltaic performance parameters of the dyes.[a]
Dye
Voc
[V]
Jsc
[mAcm2]
ff
h
te
[ms][b]
G
[%]
replacing the diphenylquinoxaline unit in dye 6 with either a
phenyl or thiophene group resulted in DSSCs with lower
power conversion efficiencies than the device fabricated
based on dye 6.[58,59]
The spectral coverage of the IPCE plots becoming broad-
er toward the red region, consistent with the trend of the p-
conjugation of the organic dyes, generally matches well with
the absorption spectra of dyes adsorbed on the TiO2 surface.
In particular, the IPCE of dye 5 has extended into the near-
IR region. A similar trend of Jsc also follows the same order
with more extended p-conjugation network dyes generally
showing higher short-circuit current density. A notable ex-
1
2
3
4
5
6
0.57
0.53
0.56
0.57
5.97
6.99
9.90
8.84
0.70
0.70
0.71
0.71
2.38
2.61
3.94
3.58
0.95
0.77
0.87
1.02
1.65
2.29
3.56
0.59
12.34
0.69
5.01
0.70ꢂ0.004
15.40ꢂ0.26
0.68ꢂ0.02
7.35ꢂ0.10
N719
0.74ꢂ0.003
18.10ꢂ0.27
0.69ꢂ0.01
9.35ꢂ0.08
[a] Voc is the open-circuit voltage, Jsc is the short-circuit photocurrent den-
sity, ff is the fill factor, and h is the power conversion efficiency. Working
areas on an FTO substrate were 0.16 cm2. The cell parameters for dye 6
and N719 were averages with five measurements. [b] Electron lifetimes
(te) in the TiO2 film were obtained from the transient photovoltage
decay measurements.
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K.-C. Ho, S.-S. Sun et al.
ent chenodeoxycholic acid (CDCA) with dye 6 showed no
improvement of Voc but a reduced JSC, which is indicative of
dense packing of dyes on the TiO2 surface. The high dye
loading for dye 6 that results in a thick aggregate on the
TiO2 surface contributes not only to the improved light har-
vesting, but may also contribute to the effective retardation
of charge recombination and enhanced Voc. The dark current
measured with DSSCs based on dyes 1–6 also displayed a
reverse trend to the Voc values (see Figure S27 in the Sup-
porting Information). The incorporation of hydrophobic
hexyl groups in the p-conjugated framework of dyes 4 and 5
slightly increases the resulting Voc values relative to dyes 1–
3. On the contrary, dye 5 with two hydrophobic hexylthio-
phene units in the structure shows a lower Voc value and
shorter electron lifetime than dye 6 with a shorter conjugat-
ed spacer. A similar reduced Voc value has been reported in
organic dyes with the p-conjugated hexylthiophene unit in-
creasing from three to four and six.[63] Compared with the
tightly packed layer of dye 6, the more loosely packed layer
of dye 5 on the TiO2 surface may produce large interstices
ꢀ
that allows I3 to infuse close to the TiO2 surface and results
in faster charge recombination. The electron lifetimes (te),
obtained from the transient photovoltage decay measure-
ments, correlate well with the measured Voc value (see
Figure 5 and Table 3).[64] A longer electron lifetime indicates
Figure 4. a) The J–V characteristics of DSSCs based on the dye sensitiz-
ers. b) Incident photon-to-current efficiency (IPCE) spectra for the sensi-
tized DSSCs.
ception is dye 6, which exhibits the highest Jsc among all six
dyes. A DSSC based on dye 6 produced the maximum IPCE
of approximately 95% at 480 nm and maintained 80% or
higher over the spectral range from 400 to 570 nm. Those
based on other dyes produced maximum IPCE values below
60%. The outstanding IPCE of the DSSC based on dye 6 is
probably due to the combined effects of a particularly high
surface dye loading, a large driving force from the LUMO
level of the dye to the conduction band edge of TiO2, a high
molar extinction coefficient, and the efficient charge separa-
tion that led to a greater electron injection efficiency and
higher light-harvesting efficiency (LHE) than DSSCs based
on the other dyes studied here.[60,61]
Figure 5. Traces of the transient photovoltage decay of DSSCs based on
dyes 1–6 and N719.
a suppressed charge recombination arising from electrons in
ꢀ
the TiO2 film with the I3 ion in the electrolyte and, there-
fore, an enhanced open-circuit photovoltage was observed.
Electrochemical impedance spectroscopy (EIS) was fur-
ther employed to investigate the interfacial charge-transfer
dynamics in DSSCs.[65] In general, the EIS spectrum of an
FTO/TiO2/dye/electrolyte/Pt/FTO device (FTO=fluorine-
doped SnO2 conducting glass) shows three semicircles in the
frequency range of 10 mHz to 65 kHz. The ohmic serial re-
sistance (RS) corresponds to the overall series resistance
(see Figure S28 in the Supporting Information for the equiv-
alent circuit of an assembled DSSC). The first and second
semicircles correspond to the charge-transfer resistances at
Dye 6 exhibits a larger Voc than other dyes in the assem-
bled DSSCs. It is quite phenomenal that the Voc value from
a DSSC based on dye 6 is nearly comparable to that based
on N719.[4b,62] The bulky diphenylquinoxaline adjacent to
the triphenylamine donor in dye 6 forms a hydrophobic bar-
ꢀ
rier preventing the hydrophilic I3 ions approaching the
TiO2 surface, thereby retarding the charge recombination.[23]
Controlled experiments with DSSCs containing co-adsorb-
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the counter electrode (Rct1) and the TiO2/dye/electrolyte in-
terface (Rct2), respectively. The Warburg diffusion process of
Iꢀ/I3ꢀ in the electrolyte (Zw) is associated with the third sem-
icircle. However, in this work most of the conventional dif-
fusion resistance of the redox couple is apparently greatly
overlapped by Rct2 due to a short length for Iꢀ ion diffusion
available with the thin spacer used (25 mm thick), and owing
to the low viscosity of the solvents used in our electrolyte.
The Nyquist plots for DSSCs based on dyes 1–6 under AM
1.5 illumination are illustrated in Figure 6. The TiO2/dye/
electrolyte interface resistance decreased in the order of 1
which are attributed to the combined effects of high dye
loading on the TiO2 surface, efficient charge injection, and
dye regeneration. Further improvement of DSSCs with or-
ganic sensitizers based on the structure of dye 6 by molecu-
lar engineering aiming for better cell performance is in
progress.
Experimental Section
Materials and general procedures: The starting materials, N,N-diphenyl-
4-vinylaniline,[67] 5,8-dibromo-2,3-diphenylquinoxaline (7),[52] N,N-diphen-
yl-4-(thiophen-2-yl)benzenamine (11),[53] 4-(3-hexylthiophen-2-yl)-N,N-di-
phenylbenzenamine (12),[53] and 4-[3-hexyl-5-(3-hexylthiophen-2-yl)thio-
phen-2-yl]-N,N-diphenylbenzenamine (13),[53] were prepared according to
literature procedures. All other chemical reagents were commercially
available and were used without further purification unless otherwise
noted. Reactions were monitored by TLC using aluminum plates pre-
coated with a 0.25 mm layer of silica gel containing a fluorescent indica-
tor.
8-Bromo-2,3-diphenylquinoxaline-5-carbaldehyde (8): Under a nitrogen
atmosphere, a solution of 7 (1.0 g, 2.3 mmol) in THF (10 mL) was cooled
to ꢀ788C. After addition of 2.5m n-butyllithium in hexane (1.0 mL,
2.3 mmol) the reaction was stirred at a low temperature for 30 min and
slowly warmed to ꢀ108C. The reaction mixture was cooled to ꢀ788C
again and treated with DMF (0.2 mL, 2.6 mmol). The resulting mixture
was stirred for another 2 h followed by hydrolyzing with 1m HCl. The
mixture was extracted, dried over MgSO4, and purified by column chro-
matography (ethyl acetate/hexane 1:5). Compound 8 (0.4 g, 45%) was
obtained as
a
colorless solid. M.p. 186–1908C; 1H NMR (400 MHz,
CDCl3): d=7.34–7.44 (m, 6H), 7.61 (d, J=8.0 Hz, 2H), 7.65 (d, J=
7.6 Hz, 2H), 8.18 (s, 2H), 11.38 ppm (s, 1H); 13C NMR (100 MHz,
CDCl3): d=128.6, 129.4, 129.9, 130.0, 130.2, 130.4, 131.2, 132.0, 133.3,
138.2, 138.3, 138.6, 141.0, 154.4, 154.7, 191.0 ppm; HRMS (ESI): m/z
calcd for C21H13BrN2O: 389.0284 [M+H]+; found: 389.0283; elemental
analysis calcd (%) for C21H14BrN2O: C 64.80, H 3.37, N 7.20; found: C
64.81, H 3.35, N 7.24.
Figure 6. EIS Nyquist plots for DSSCs based on dyes 1–6 under AM 1.5
illumination.
(44.9)>2 (33.0)>4 (23.8)>3 (23.0)>5 (21.4)>6 (17.0 W),
which is in good agreement with the trend of the cell per-
formance. It is evident that an improved electron generation
and transport occurred in the cell due to the smaller TiO2/
dye/electrolyte interface resistance under illumination.[66]
General procedure for the syntheses of 9, 17, and 18: Under a nitrogen
atmosphere, a degassed mixture of THF (30 mL) and H2O (20 mL) was
added to a flask charged with the bromo-substituted compound (7, 8, or
17; 2.0 mmol), the aryl-substituted boronic acid (3.0 mmol), Na2CO3
(0.64 g, 6.0 mmol), and [PdCl2ACHTNUTRGNE(UGN PPh3)2] (84 mg, 0.12 mmol), and the result-
ing mixture was stirred for 16 h at room temperature. The solution was
extracted with ethyl acetate, dried over MgSO4, and purified by column
chromatography (ethyl acetate/n-hexane or dichloromethane/n-hexane)
to obtain the desired product.
Conclusion
We reported the synthesis of a series of dipolar organic dyes
with an electron deficient diphenylquinoxaline moiety incor-
porated within the p-conjugation framework and the per-
formance of standard DSSCs sensitized by these organic
dyes. DSSCs based on these dyes have shown photo-to-elec-
trical energy conversion efficiency up to 7.35% under simu-
lated AM 1.5 irradiation, which translates to approximately
79% of the intrinsic performance of a DSSC sensitized by
the N719 dye. The presence of the diphenylquinoxaline
moiety effectively lowers the energy gaps of organic dyes
and extends the absorption spectral coverage to the near-IR
8-[4-(Diphenylamino)phenyl]-2,3-diphenylquinoxaline-5-carbaldehyde
(9): The crude product was purified by column chromatography (ethyl
acetate/hexane 1:5) to yield pure compound 9 (1.07 g, 97%) as a red
solid. M.p. 238–2418C; 1H NMR (400 MHz, CDCl3): d=7.07 (t, J=
7.2 Hz, 2H), 7.18–7.20 (m, 6H), 7.27–7.31 (m, 6H), 7.33–7.41 (m, 4H),
7.56 (d, J=8.0 Hz, 2H), 7.65 (d, J=7.6 Hz, 2H), 7.77 (d, J=8.0 Hz, 2H),
7.94 (d, J=7.6 Hz, 1H), 8.38 (d, J=8.0 Hz, 1H), 11.44 ppm (s, 1H);
13C NMR (100 MHz, CDCl3): d=122.2, 123.7, 125.3, 128.5, 128.6, 129.1,
129.3, 129.4, 129.6, 129.7, 130.0, 130.2, 130.3, 130.8, 132.1, 138.1, 138.9,
141.2, 146.3, 147.6, 148.7, 152.6, 153.2, 191.5 ppm; HRMS (ESI): m/z
calcd for C39H28N3O: 554.2227 [M+H]+; found: 554.2238; elemental anal-
ysis calcd (%) for C39H27N3O: C 84.61, H 4.92, N 7.59; found: C 84.73, H
5.21, N 7.20.
region. The bulky and hydrophobic nature of diphenyl
oxaline could suppress the infusion of I3 ions toward the
TiO2 surface and retard the charge recombination. The per-
formance of the DSSC sensitized by dye 6 was the best
among the six dyes investigated. In particular, dye 6 shows
an outstanding light-harvesting ability and cell performance,
N
4-(8-Bromo-2,3-diphenylquinoxalin-5-yl)-N,N-diphenylaniline (17): The
crude product was purified by column chromatography (ethyl acetate/
hexane 1:7) to yield compound 17 (0.46 g, 38%) as a red solid. M.p. 256–
2578C; 1H NMR (400 MHz, CDCl3): d=7.05 (t, J=7.6 Hz, 2H), 7.17–
7.19 (m, 6H), 7.26–7.30 (m, 6H), 7.33–7.38 (m, 4H), 7.55 (d, J=7.6 Hz,
2H), 7.66–7.70 (m, 5H), 8.08 ppm (d, J=8.0 Hz, 1H); 13C NMR
(100 MHz, CDCl3): d=122.6, 122.8, 123.4, 125.0, 128.4, 128.5, 129.3,
ꢀ
Chem. Eur. J. 2012, 00, 0 – 0
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K.-C. Ho, S.-S. Sun et al.
129.5, 129.8, 130.3, 130.4, 131.3, 131.8, 133.2, 138.8, 138.9, 139.4, 139.9,
147.8, 147.9, 152.6, 153.0 ppm; elemental analysis calcd (%) for
C38H26BrN3: C 75.50, H 4.33, N 6.95; found: C 75.36, H 4.23, N 6.81.
8-{5-[4-(Diphenylamino)phenyl]-4-hexylthiophen-2-yl}-2,3-diphenyl
AHCTUNGERTGoNNUN xaline-5-carbaldehyde (15): Compound 15 was obtained as a red solid
ACHTUNGTRENNUNGquin-
(0.20 g, 48%) after purification by column chromatography (ethyl ace-
tate/hexane 1:4). M.p. 113–1148C; 1H NMR (400 MHz, CDCl3): d=0.88
(t, J=6.8 Hz, 3H), 1.27–1.39 (m, 6H), 1.68–1.74 (m, 2H), 2.73 (t, J=
7.6 Hz, 2H), 7.05 (t, J=7.6 Hz, 2H), 7.11 (d, J=8.4 Hz, 2H), 7.16 (d, J=
7.6 Hz, 4H), 7.28 (t, J=7.6 Hz, 4H), 7.33–7.42 (m, 8H), 7.65 (d, J=
8.0 Hz, 2H), 7.72 (d, J=8.0 Hz, 2H), 7.94 (s, 1H), 8.20 (d, J=8.0 Hz,
1H), 8.34 (d, J=8.4, 1H), 11.38 ppm (s, 1H); 13C NMR (100 MHz,
CDCl3): d=14.3, 22.8, 29.1, 29.5, 31.2, 31.9, 123.1, 123.5, 125.0, 125.6,
128.4, 128.5, 129.1, 129.3, 129.5, 129.6, 129.7, 129.9, 130.2, 130.7, 132.0,
135.3, 136.6, 138.6, 138.8, 138.9, 141.2, 144.1, 147.6, 147.7, 152.4, 153.5,
190.9 ppm; HRMS (MALDI): m/z calcd for C49H42N3OS [M+H]+:
720.3044; found: 720.3055; elemental analysis calcd (%) for C49H41N3OS:
C 81.75, H 5.74, N 5.84; found: C 81.45, H 5.82, N 5.77.
5-{8-[4-(Diphenylamino)phenyl]-2,3-diphenylquinoxalin-5-yl}thiophene-2-
carbaldehyde (18): Compound 18 was purified by column chromatogra-
phy (dichloromethane/hexane 1:1) to yield a red solid (1.04 g, 82%).
M.p. 251–2528C; 1H NMR (400 MHz, CDCl3): d=7.06 (t, J=7.2 Hz,
2H), 7.19–7.21 (m, 6H), 7.27–7.34 (m, 8H), 7.40 (br, 3H), 7.59 (d, J=
7.2 Hz, 2H), 7.72–7.73 (m, 2H), 7.75 (t, J=8.4 Hz, 2H), 7.81 (d, J=
3.2 Hz, 1H), 7.88 (d, J=7.6 Hz, 1H), 7.93 (d, J=3.6 Hz, 1H), 8.22 (d, J=
7.6 Hz, 1H), 9.97 ppm (s, 1H); 13C NMR (100 MHz, CDCl3): d=122.6,
123.5, 125.1, 127.2, 128.4, 128.5, 128.6, 129.3, 129.6, 129.5, 129.9, 130.3,
130.6, 131.3, 131.9, 135.9, 137.7, 138.6, 138.8, 138.9, 140.7, 145.1, 147.8,
148.1, 148.9, 152.2, 152.2, 183.6 ppm; HRMS (MALDI): m/z calcd for
C43H30N3OS: 636.2104 [M+H]+; found: 636.2164; elemental analysis
calcd (%) for C43H29N3OS·H2O: C 78.99, H 4.78, N 6.43; found: C 79.02,
H 4.71, N 6.43.
8-{5’-[4-(Diphenylamino)phenyl]-3,4’-di-n-hexylACTHNUTRGENUG[N 2,2’]bithiophen-5-yl}-2,3-
diphenylquinoxaline-5-carbaldehyde (16): Compound 16 was obtained as
a red solid (0.29 g, 63%) after purification by column chromatography
(ethyl acetate/hexane 1:4). M.p. 145–1468C; H NMR (400 MHz, CDCl3):
(E)-8-[4-(Diphenylamino)styryl]-2,3-diphenylquinoxaline-5-carbaldehyde
(10): Under a nitrogen atmosphere, degassed DMF (10 mL) was added
to a 50 mL flask containing 8 (0.20 g, 0.51 mmol), N,N-diphenyl-4-vinyl-
1
d=0.89 (t, J=6.8 Hz, 6H), 1.30–1.45 (m, 12H), 1.64–1.76 (m, 4H), 2.69
(t, J=7.6 Hz, 2H), 2.85 (t, J=7.6 Hz, 2H), 7.04 (t, J=7.6 Hz, 2H), 7.09–
7.15 (m, 7H), 7.27 (t, J=8.0 Hz, 4H), 7.33 (d, J=8.8 Hz, 2H), 7.36–7.42
(m, 6H), 7.66 (d, J=7.2 Hz, 2H), 7.75 (d, J=7.2 Hz, 2H), 7.85 (s, 1H),
8.17 (d, J=7.6 Hz, 1H), 8.30 (d, J=8.0 Hz, 1H), 11.35 ppm (s, 1H);
13C NMR (100 MHz, CDCl3): d=14.3, 14.4, 22.85, 22.86, 29.1, 29.5, 29.6,
29.7, 30.8, 31.2, 32.0, 123.2, 123.4, 124.9, 125.3, 128.2, 128.4, 128.6, 129.0,
129.3, 129.5, 129.6, 129.7, 130.0, 130.2, 130.8, 131.9, 133.9, 134.8, 136.5,
137.8, 138.5, 138.6, 138.7, 138.9, 139.1, 141.2, 147.4, 147.7, 152.4, 153.6,
190.9 ppm; HRMS (MALDI): m/z calcd for C59H56N3OS2: 886.3860
[M+H]+; found: 886.3884; elemental analysis calcd (%) for
C59H55N3OS2·0.5H2O: C 79.16, H 6.31, N 4.69; found: C 79.02, H 6.19, N
5.02.
ACHTUNGTRENNUNGaniline (0.17 g, 0.63 mmol), Na2CO3 (0.11 g, 1.02 mmol), and PdACHTUTGNREN(NUGN OAc)2
(5 mg, 0.02 mmol). The mixture was stirred at 1208C for 12 h. The mix-
ture was extracted with ethyl acetate, dried over MgSO4, and purified by
column chromatography (ethyl acetate/hexane 1:5). Compound 10
(0.28 g, 94%) was obtained as a red solid. M.p. 241–2428C; 1H NMR
(400 MHz, CDCl3): d=7.04–7.08 (m, 4H), 7.13 (d, J=7.6 Hz, 4H), 7.25–
7.29 (m, 4H), 7.33–7.40 (m, 6H), 7.52 (d, J=8.0 Hz, 2H), 7.55 (d, J=
16.0 Hz, 1H), 7.61–7.63 (m, 4H), 8.11 (d, J=7.6 Hz, 1H), 8.33 (d, J=
7.6 Hz, 1H), 8.36 (d, J=16.4 Hz, 1H), 11.38 ppm (s, 1H); 13C NMR
(100 MHz, CDCl3): d=120.9, 123.0, 123.7, 124.1, 125.2, 128.5, 128.6,
129.3, 129.4, 129.5, 129.6, 130.2, 131.0, 133.8, 138.0, 138.9, 139.1, 141.2,
142.8, 147.5, 148.7, 152.4, 153.8, 191.3 ppm; HRMS (MALDI): m/z calcd
for C41H30N3O: 580.2384 [M+H]+; found: 580.2395; elemental analysis
calcd (%) for C41H29N3O·1.5H2O: C 81.17, H 5.32, N 6.93; found: C
81.53, H 5.12, N 6.87.
General procedure for syntheses of dyes 1–6: Under a nitrogen atmos-
phere, the aldehyde (9, 10, 14, 15, 16, or 18; 0.3 mmol), cyanoacetic acid
(0.08 g, 0.9 mmol), ammonium acetate (9 mg), and glacial acetic acid
(5 mL) were mixed together and heated at reflux for 12 h. The red solu-
tion was then cooled to room temperature to yield dark-red precipitates,
which were collected by filtration and thoroughly washed with cold
acetic acid (2ꢂ10 mL), water (3ꢂ10 mL), and diethyl ether (2ꢂ10 mL).
The collected solids were further purified by washing with the dichloro-
methane/hexane mixture and subsequent recrystallization from dichloro-
methane/ethanol to yield the pure product.
General procedure for the syntheses of 14, 15, and 16: Under a nitrogen
atmosphere,
a solution of a thiophene derivative (11, 12, and 13;
2.5 mmol) in THF (12 mL) was cooled to ꢀ788C. After addition of 2.5m
n-butyllithium in hexane (1 mL, 2.5 mmol), the reaction was stirred for
1 h at ꢀ788C and gently warmed to ꢀ108C. The solution was cooled to
ꢀ788C again and treated with B
ACHTUNGERTN(NUNG OMe)3 (0.85 mL, 7.5 mmol). This was
then followed by gently warming to room temperature and the mixture
was stirred for another 5 h and then quenched with water. The mixture
was extracted with ethyl acetate, dried over MgSO4, and purified by flash
column chromatography (ethyl acetate/hexane 1:1). The boronic acid de-
rivatives thus obtained were not further purified and were instead used
(E)-2-Cyano-3-{8-[4-(diphenylamino)phenyl]-2,3-diphenylquinoxalin-5-
yl}acrylic acid (1): A dark-red solid (160 mg, 84%). M.p. 251–2528C;
1H NMR (400 MHz, CDCl3): d=7.08 (t, J=7.2 Hz, 2H), 7.19–7.21 (m,
6H), 7.28–7.32 (m, 7H), 7.35–7.42 (m, 3H), 7.56 (d, J=7.6 Hz, 2H), 7.66
(d, J=7.2 Hz, 2H), 7.80 (d, J=8.0 Hz, 2H), 7.97 (d, J=8.0 Hz, 1H), 8.99
(d, J=7.6 Hz, 1H), 9.93 ppm (s, 1H); 13C NMR (100 MHz, [D6]DMSO):
d=104.8, 116.3, 120.9, 123.8, 124.9, 127.4, 128.1, 128.3, 129.0, 129.1, 129.3
129.6, 129.7, 129.9, 132.0, 137.6, 138.3, 139.0, 143.5, 146.7, 147.8, 148.1,
152.2, 152.6, 163.2 ppm; HRMS (FAB): m/z calcd for C42H28N4O2:
620.2207; found: 620.2206; elemental analysis calcd (%) for
C42H28N4O2·0.5H2O: C 80.11, H 4.64, N 8.90; found: C 79.77, H 4.55, N
8.59.
directly for the subsequent reactions.
A degassed solution of THF
(12 mL) and H2O (8 mL) was added to a flask containing the boronic
acid derivative (0.48 mmol), compound 8 (0.16 g, 0.40 mmol), Na2CO3
(0.13 g, 1.20 mmol), and [PdCl2ACHTNUTRGNE(NUG PPh3)2] (14 mg, 0.02 mmol). The resulting
mixture was stirred for 16 h at room temperature under a nitrogen at-
mosphere. The mixture was extracted with dichloromethane, dried over
MgSO4, and purified by column chromatography to yield the desired al-
dehydes (14, 15, and 16).
(E)-2-Cyano-3-{8-[(E)-4-(diphenylamino)styryl]-2,3-diphenylquinoxalin-
5-yl}acrylic acid (2): A dark-red solid (160 mg, 82%). M.p. 236–2388C;
1H NMR (400 MHz, [D6]DMSO): d=7.00 (d, J=8.4 Hz, 2H), 7.09 (d, J=
8.0 Hz, 4H), 7.11 (t, J=7.2 Hz, 2H), 7.36 (t, J=8.0 Hz, 4H), 7.40–7.45
(m, 6H), 7.55–7.64 (m, 6H), 7.86 (d, J=16.4 Hz, 1H), 8.28 (d, J=
16.4 Hz, 1H), 8.46 (d, J=8.0 Hz, 1H), 8.80 (d, J=8.0 Hz, 1H), 9.51 ppm
(s, 1H); 13C NMR (125 MHz, [D6]DMSO): d=104.1, 116.5, 120.0, 122.0,
123.8, 124.7, 126.9, 128.2, 128.5, 129.1, 129.3, 129.7, 129.8, 129.9, 130.3,
134.1, 137.4, 138.4, 139.0, 140.4, 146.6, 147.9, 148.0, 152.5, 153.0,
163.3 ppm; HRMS (MALDI): m/z calcd for C44H31N4O2 [M+H]+:
647.2241; found: 647.2458; elemental analysis calcd (%) for C44H30N4O2:
C 81.71, H 4.68, N 8.66; found: C 81.52, H 4.70, N 8.64.
8-{5-[4-(Diphenylamino)phenyl]thiophen-2-yl}-2,3-diphenylquinoxaline-5-
carbaldehyde (14): Compound 14 was obtained as a red solid (0.19 g,
51%) after purification by column chromatography (ethyl acetate/hexane
1:4). M.p. 136–1378C; 1H NMR (400 MHz, CDCl3): d=7.03–7.10 (m,
4H), 7.12–7.14 (m, 4H), 7.26–7.30 (m, 4H), 7.31 (d, J=4.0 Hz, 1H),
7.35–7.42 (m, 6H), 7.55 (d, J=7.6 Hz, 2H), 7.65 (d, J=7.6 Hz, 2H), 7.73
(d, J=7.2 Hz, 2H), 7.99 (d, J=4.0 Hz, 1H), 8.23 (d, J=8.0 Hz, 1H), 8.34
(d, J=8.0 Hz, 1H), 11.38 ppm (s, 1H); 13C NMR (100 MHz, CDCl3): d=
122.6, 123.6, 125.0, 125.5, 126.8, 128.3, 128.5, 128.6, 129.2, 129.4, 129.6,
129.7, 130.0, 130.2, 130.7, 136.4, 136.5, 138.6, 138.8, 138.9, 141.3, 147.6,
148.1, 149.9, 152.5, 153.7, 191.0 ppm; HRMS (MALDI): m/z calcd for
C43H30N3OS: 636.2104 [M+H]+; found: 636.2117; elemental analysis
calcd (%) for C43H29N3OS: C 81.23, H 4.60, N 6.61; found: C 81.07, H
4.71, N 6.59.
(E)-2-Cyano-3-(8-{5-[4-(diphenylamino)phenyl]thiophen-2-yl}-2,3-di
ACHTUNGTRENNUNGphen-
AHCTUNGERTGyNNUN lquinoxalin-5-yl)acrylic acid (3): A dark-red solid (170 mg, 80%). M.p.
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High-Performance Dipolar Organic Dyes
FULL PAPER
264–2668C; 1H NMR (400 MHz, [D6]DMSO): d=7.03 (d, J=8.4 Hz,
2H), 7.09 (d, J=7.6 Hz, 4H), 7.11 (t, J=7.6 Hz, 2H), 7.35 (t, J=8.0 Hz,
4H), 7.42–7.48 (m, 6H), 7.56 (d, J=3.2 Hz, 1H), 7.59 (d, J=7.6 Hz, 2H),
7.64 (d, J=7.6 Hz, 2H), 7.68–7.69 (m, 2H), 8.21 (d, J=3.6 Hz, 1H), 8.62
(d, J=8.0 Hz, 1H), 8.80 (d, J=8.4 Hz, 1H), 9.49 ppm (s, 1H); 13C NMR
(125 MHz, [D6]DMSO): d=104.1, 116.6, 122.7, 123.6, 124.5, 125.6, 126.3,
126.5, 127.3, 128.2, 128.3, 129.4, 129.6, 129.8, 130.2, 130.4, 135.0, 135.6,
136.1, 137.7, 138.2, 139.0, 146.7, 147.2, 147.3, 149.1, 152.3, 152.8,
163.3 ppm; HRMS (MALDI): m/z calcd for C46H31N4O2S: 703.2162
[M+H]+; found: 703.2178.
spect to the TiO2) of PEG to the above solution to control the pore di-
ameters and to prevent the film from cracking during drying. For the
second type of TiO2 paste (scattering layer), the first type of TiO2 paste
was incorporated with 50 wt% of light scattering TiO2 particles (PT-
501A, 15 m2 gꢀ1, 100 nm, 99.74%, Ya Chung Industrial, Taiwan; with re-
spect to the 20 nm TiO2) for reducing the light loss by back scattering. A
fluorine-doped SnO2 conducting glass (FTO, 7 Wsqꢀ1, visible transmit-
tance ^80%, NSG America, USA) was first cleaned with a neutral
cleaner, and then washed with deionized water, acetone, and isopropyl al-
cohol (IPA), sequentially. The conducting surface of the FTO was treated
with a solution of TTIP (1 g) in 2-methoxyethanol (3 g, 99.5%, Sigma–
Aldrich) for obtaining a good mechanical contact between the conduct-
ing glass and TiO2 film, and to isolate the conducting glass surface from
the electrolyte. TiO2 pastes were coated onto the treated conducting glass
by using the doctor blade technique. For each coating TiO2 layer, the
dried TiO2 film was gradually heated to 5008C in an oxygen atmosphere,
and subsequently sintered at that temperature for 30 min. Before the
next coating, the sintered TiO2 film was treated with UV/Ozone for
15 min. The TiO2 photoanodes of the DSSCs employed in the experi-
ments were composed of a 12 mm thick transparent TiO2 layer and with a
scattering layer of 4 mm thickness. The prepared TiO2 photoanode was
treated with UV/Ozone for 30 min and heated to 808C, and then it was
immersed in a 5ꢂ10ꢀ4 m N719 dye solution at room temperature for 24 h.
N719 (Solaronix S.A., Switzerland) was dissolved in acetonitrile (ACN,
99.99%, Aldrich) and tert-butyl alcohol (tBA, 96%, Acros; volume ratio
of 1:1) as a standard dye solution. Various organic dye solutions were
prepared in dichloromethane (>99.8%, Sigma–Aldrich). The TiO2/dye
electrode thus prepared was placed on a platinum-sputtered conducting
glass electrode (FTO), keeping the two electrodes separated by a 25 mm-
thick surlyn (SX1170–25, Solaronix S.A., Switzerland). The two electro-
des were then sealed by heating. A mixture of 0.1m lithium iodide (LiI,
synthetic grade, Merck), 0.6m DMPII (Solaronix S.A., Switzerland),
0.05m iodine (I2, synthetic grade, Merck), and 0.5m 4-tert-butylpyridine
(TBP, 96%, Acros) in 3-methoxypropionitrile (MPN, Fluka)/ACN
(volume ratio of 1:1) was used as the electrolyte. The electrolyte was in-
jected into the gap between the electrodes by capillarity. The surface of
the DSSC was covered by a mask with a light-illuminated area of
0.16 cm2 and then illuminated by a class-A quality solar simulator (XES-
301S, AM 1.5G, SAN-EI ELECTRIC). Incident light intensity
(100 mWcmꢀ2) was calibrated with the reference solar cell and meter
(Oriel Instrument, model 91105).
(E)-2-Cyano-3-(8-{5-[4-(diphenylamino)phenyl]-4-hexylthiophen-2-yl}-
2,3-diphenylquinoxalin-5-yl)acrylic acid (4): A dark-red solid (210 mg,
1
89%). M.p. 258–2608C; H NMR (500 MHz, [D6]DMSO): d=0.84 (t, J=
6.5 Hz, 3H), 1.23–1.30 (m, 6H), 1.59–1.65 (m, 2H), 2.63 (t, J=7.5 Hz,
2H), 7.00 (d, J=8.5 Hz, 2H), 7.05 (d, J=8.5 Hz, 4H), 7.10 (t, J=7.0 Hz,
2H), 7.33 (t, J=8.5 Hz, 4H), 7.32–7.37 (m, 4H), 7.39–7.42 (m, 3H), 7.45
(t, J=7.0 Hz, 1H), 7.54 (d, J=8.0 Hz, 2H), 7.59 (d, J=8.0 Hz, 2H), 8.06
(s, 1H), 8.44 (d, J=8.5 Hz, 1H), 8.72 (d, J=8.5 Hz, 1H), 9.44 ppm (s,
1H); the 13C NMR spectrum was not acquired due to the low solubility
in CDCl3 and [D6]DMSO; HRMS (MALDI): m/z calcd for C52H43N4O2S
[M+H]+: 787.3101; found: 787.3111; elemental analysis calcd (%) for
C52H42N4O2S·0.5H2O: C 78.46, H 5.44, N 7.04; found: C 78.13, H 5.28, N
7.37.
(E)-2-Cyano-3-(8-{5’-[4-(diphenylamino)phenyl]-3,4’-di-n-hexyl[2,2’]bi-
ACHTUNGTRENNUNGthioACHTUNGTRENNUNGphen-5-yl}-2,3-diphenylquinoxalin-5-yl)acrylic acid (5): A dark-red
solid (240 mg, 82%). M.p. 246–2478C; 1H NMR (400 MHz, CDCl3): d=
0.89 (t, J=6.8 Hz, 6H), 1.30–1.45 (m, 12H), 1.60–1.77 (m, 4H), 2.69 (t,
J=7.6 Hz, 2H), 2.87 (t, J=7.6 Hz, 2H), 7.04 (t, J=7.2 Hz, 2H), 7.10 (d,
J=8.4 Hz, 2H), 7.14 (d, J=7.2 Hz, 4H), 7.28 (t, J=7.6 Hz, 4H), 7.33 (d,
J=8.8 Hz, 2H), 7.38–7.44 (m, 6H), 7.67 (d, J=7.2 Hz, 2H), 7.76 (d, J=
6.0 Hz, 2H), 7.89 (s, 1H), 8.24 (d, J=8.4 Hz, 1H), 8.97 (d, J=8.8 Hz,
2H), 9.86 ppm (s, 1H); the 13C NMR spectrum was not acquired due to
the low solubility in CDCl3 and [D6]DMSO; HRMS (FAB): m/z calcd for
C62H56N4O2S2: 952.3840 [M]+; found: 952.3844.
(E)-2-Cyano-3-(5-{8-[4-(diphenylamino)phenyl]-2,3-diphenylquinoxalin-
5-yl}thiophen-2-yl)acrylic acid (6): A dark-red solid (180 mg, 85%). M.p.
277–2848C; 1H NMR (500 MHz, [D6]DMSO): d=7.04 (d, J=8.5 Hz,
2H), 7.06–7.10 (m, 6H), 7.29–7.35 (m, 7H), 7.38–7.39 (m, 3H), 7.49 (d,
J=7.5 Hz, 2H), 7.72–7.74 (m, 5H; ArH), 7.84 (d, J=7.5 Hz, 1H), 7.97
(d, J=3.5 Hz, 1H), 8.20 (s, 1H), 8.35 ppm (d, J=8.0 Hz, 1H); 13C NMR
(125 MHz, [D6]DMSO): d=119.0, 127.1, 127.9, 128.1, 128.4, 129.0, 129.2,
129.4, 129.6, 129.7, 130.2, 130.8, 131.7, 134.4, 136.2, 137.5, 137.8, 138.5,
138.6, 139.8, 141.2, 143.1, 146.9, 147.1, 151.2, 151.6, 163.6 ppm; HRMS
(MALDI): m/z calcd for C46H31N4O2S: 703.2162 [M+H]+; found:
703.2181; elemental analysis calcd (%) for C46H30N4O2S: C 78.61, H 4.30,
N 7.97; found: C 78.46, H 4.27, N 7.89.
Photocurrent-density–voltage curves of the DSSCs were obtained with a
potentiostat/galvanostat (PGSTAT 30, Autolab, Eco-Chemie, the Nether-
lands). Electrochemical impedance spectra (EIS) were obtained by the
aforementioned potentiostat/galvanostat, equipped with an FRA2
module, under a constant light illumination of 100 mWcmꢀ2. The fre-
quency range explored was 10 mHz to 65 kHz. The applied bias voltage
was set at the open-circuit voltage of the DSSC, between the counter
electrode and the FTO-TiO2-dye working electrode by starting from the
short-circuit conditions; the corresponding ac amplitude was 10 mV. The
impedance spectra were analyzed by using an equivalent circuit model.
Incident photo-to-current conversion efficiency (IPCE) curves were ob-
tained at short-circuit conditions. The light source was a class A quality
solar simulator (PEC-L11, AM1.5G, Peccell Technologies, Inc.); light was
focused through a monochromator (Oriel Instrument, model 74100) onto
the photovoltaic cell. The monochromator was incremented through the
visible spectrum to generate the IPCE (l) as defined in Equation (1) in
Analytical instruments and measurements: Absorption and emission
spectra were recorded according to previously published procedures.[68]
Cyclic voltammetry experiments were carried out at room temperature
with a conventional three-electrode configuration consisting of a plati-
num working electrode, a platinum wire auxiliary electrode, and a non-
ACHTUNGTRENNUNG
aqueous Ag/AgNO3 reference electrode.[69] The potentials were quoted
against the ferrocene/ferrocenium internal standard and converted to
NHE by addition of 0.63 V. The solvent contained 1.0 mm of the dye and
0.1m tetrabutylammonium hexafluorophosphate as the supporting elec-
trolyte in all experiments. The geometry and electronic properties of the
dyes were calculated with the Gaussian program package.[55] All calcula-
tions were performed by using DFT with the B3LYP/6-31G* basis set.
which l is the wavelength, JSC is the short-circuit photocurrent (mAcmꢀ2
)
recorded with a potentiostat/galvanostat, and f is the incident radiative
flux (W mꢀ2) measured with an optical detector (Oriel Instrument, model
71580) and power meter (Oriel Instrument, model 70310).
For the TiO2 colloid solution, the TiO2 precursor was prepared by the
sol–gel method as in the following processes. TiIV tetraisopropoxide
(72 mL, TTIP, Aldrich, >98%) was added to a 0.1m nitric acid (430 mL,
70%, J. T. Baker) aqueous solution with constant stirring and heated to
858C simultaneously for 8 h. When the mixture was cooled to room tem-
perature, the resultant colloid was transferred to an autoclave (PARR
4540, USA), and then heated at 2408C for 12 h to allow the TiO2 parti-
cles to grow uniformly (ca. 20 nm). Consequently, the TiO2 colloid was
concentrated to 8 wt% (with respect to the TiO2). The first type of TiO2
paste (transparent layer) was prepared by the addition 25 wt% (with re-
IPCE ðlÞ ¼ 1240 ðJSC=lꢀÞ
ð1Þ
Photovoltage transients of the assembled devices were recorded with a
digital oscilloscope (model LT322, LeCroy, USA). Pulsed laser excitation
was applied by a frequency-doubled Q-switched Nd:YAG laser (model
Quanta-Ray GCR-3-10, Spectra-Physics laser) with a 2 Hz repetition rate
at 532 nm, and a 7 ns pulse width at half-height. The average electron
Chem. Eur. J. 2012, 00, 0 – 0
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K.-C. Ho, S.-S. Sun et al.
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Acknowledgements
We thank the National Science Council of Taiwan (grant no. 100–2113-
M-001-024-MY3), Academia Sinica, and the National Taiwan University
for support of this research. Dr. S.-R. Li thanks the postdoctoral fellow-
ship sponsored by the National Science Council of Taiwan (grant no. 099-
2811-M-001-121 and 100-2811-M-001-089) and Dr. H.-T. Kuo thanks the
postdoctoral fellowship sponsored by Academia Sinica. Mass spectrome-
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Received: March 23, 2012
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Chang, T. J. Chow, Tetrahedron 2009, 65, 9626–9632.
Revised: June 7, 2012
Published online: && &&, 0000
Chem. Eur. J. 2012, 00, 0 – 0
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K.-C. Ho, S.-S. Sun et al.
Solar Cells
S.-R. Li, C.-P. Lee, H.-T. Kuo,
K.-C. Ho,* S.-S. Sun* ........ &&&& — &&&&
High-Performance Dipolar Organic
Dyes with an Electron-Deficient
Diphenylquinoxaline Moiety in the p-
Conjugation Framework for Dye-
Sensitized Solar Cells
Donor–acceptor systems: Six organic
dyes comprised of a triphenylamine
group as an electron donor, a cyanoa-
crylic acid as an electron acceptor, and
an electron-deficient diphenylquinoxa-
line moiety integrated into the p-con-
jugated spacer between the electron
donor and acceptor were synthesized
(see figure). Dye-sensitized solar cells
(DSSCs) based on one of the dyes pro-
duced an overall cell performance of
7.35%, which translates to about 79%
of the intrinsic efficiency of the DSSC
based on standard N719.
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ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!