Y-shaped dyes based on triphenylamine for efficient dye-sensitized solar cells

Article abstract of DOI:10.1016/j.tet.2012.02.077

Three new triphenylamine-based dyes with Y-shaped conformation bearing triphenylamino-vinyl, 10-octyl-10H-phenothiazine-vinyl and 9-octyl-9H-carbazole- vinyl as arms (TT, TP, and TC) have been synthesized. From electrochemical investigations it is found that they can be employed in DSSCs due to the balanced HOMO and LUMO energy levels. Notably, the photo-to-electrical conversion efficiency of the DSSCs sensitized with branched TT, TP, and TC reach 5.12%, 4.84%, and 3.63%, which are higher than that sensitized with T (2.79%), and the DSSC sensitized with TT shows higher IPCE response and better photovoltaic performances (J_sc=12.37 mA/cm², V _oc=0.72 V and ff=0.58) than others. These results reveal that the introduction of branched Y-shaped extended π-conjugated donors to D-π-A dyes cannot only enlarge the spectral response range, but also suppress the molecular aggregation on TiO₂ films to a certain extent, which would enhance the performance of DSSCs.

Full text of DOI:10.1016/j.tet.2012.02.077

Tetrahedron 68 (2012) 3626e3632
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Y-shaped dyes based on triphenylamine for efficient dye-sensitized solar cells
,
b
a
a,
Junhui Jia a, Kaiyu Cao a, Pengchong Xue a , Yuan Zhang , Huipeng Zhou , Ran Lu
*
*
a State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, PR China
b College of Chemistry, Beijing Normal University, Beijing 100875, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Three new triphenylamine-based dyes with Y-shaped conformation bearing triphenylamino-vinyl,
10-octyl-10H-phenothiazine-vinyl and 9-octyl-9H-carbazole-vinyl as arms (TT, TP, and TC) have
been synthesized. From electrochemical investigations it is found that they can be employed in
DSSCs due to the balanced HOMO and LUMO energy levels. Notably, the photo-to-electrical con-
version efficiency of the DSSCs sensitized with branched TT, TP, and TC reach 5.12%, 4.84%, and 3.63%,
which are higher than that sensitized with T (2.79%), and the DSSC sensitized with TT shows higher
IPCE response and better photovoltaic performances (Jsc¼12.37 mA/cm2, Voc¼0.72 V and ff¼0.58)
Received 7 November 2011
Received in revised form 15 February 2012
Accepted 28 February 2012
Available online 13 March 2012
Keywords:
Dye-sensitized solar cells (DSSCs)
Triphenylamine
Y-shaped conformation
Carbazole
than others. These results reveal that the introduction of branched Y-shaped extended
p-conjugated
donors to D- -A dyes cannot only enlarge the spectral response range, but also suppress the mo-
p
lecular aggregation on TiO2 films to a certain extent, which would enhance the performance of
DSSCs.
Phenothiazine
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
compared to the ruthenium sensitizers.5 Therefore, on the one
hand, many attempts have been made to design organic dye
Sustained increase in energy demands and concerns about
global warming have led to a greater focus on renewable energy
sources. It is well-known that solar energy is the largest re-
newable carbon-free resource. Among the devices that can con-
vert solar energy into electricity, dye-sensitized solar cells
(DSSCs) have gained widespread interest due to their high-en-
ergy conversion efficiency and low production cost.1 Although
the high power conversion efficiencies above 11% under standard
solar illumination have been reported using Ru(II) polypyridyl
complexes as the sensitizers in DSSCs,2 the limited resources and
elaborated purification of Ru complexes make the wide applica-
tion become difficult. In recent years, metal-free organic sensi-
tizers have emerged as competitive alternatives because of
relatively low cost, ease of structural tuning, high molar extinc-
tion coefficients, and more environmentally friendly properties.3
Although the energy conversion efficiencies of the DSSCs sensi-
tized with organic dyes have been improved greatly,4 the con-
version efficiencies still lag behind those of the Ru complexes. It
is suggested that the narrow absorption bands in the visible re-
gion and the formation of aggregates on the semiconductor
surfaces are responsible for the low efficiencies of organic dyes
molecules with higher molar extinction coefficients and broader
spectral responses, and it is found that most of the efficient or-
ganic dyes synchronously contain donor and acceptor bridged by
a
p-conjugated linker (D-p-A) because their spectral responses
can be tuned by varying donor, spacer, and acceptor moieties. For
example, the derivatives of porphyrin,4i thiophene,4m,n fluo-
rene,4k phenothiazine,4p,q carbazole,4f,r coumarin,4s indoline,4g
cyanine,4t hemicyanine,4u merocyanine,4v perylene,4w and tri-
arylamine,4a,j,6e8 used as the sensitizers have yielded good con-
version efficiencies. On the other hand, in order to avoid
molecular aggregation of the organic dyes starburst units were
employed in the conjugated molecules.4h,n,9 With these in mind,
we designed new branched triphenylamine-based conjugated
molecules in Y-shaped conformation, in which two arms of
triphenylamino-vinyl, 10-octyl-10H-phenothiazine-vinyl or 9-
octyl-9H-carbazole-vinyl were anchored by a triphenylamine
moiety connected with the acceptor of cyanoacrylic acid (TT, TP,
and TC, Scheme 1). Such extended
p-conjugated systems with
branched arms are expected to exhibit strong light-harvesting
abilities and weak aggregation tendency. It has been found that
the total solar-to-energy conversion efficiencies of the devices
sensitized with Y-shaped TT, TP, and TC are 5.12%, 4.84%, and
3.63%, respectively, which are higher than that of the device
sensitized with T (2.79%). It suggests that the introduction of the
branched Y-shaped donor with extended
dyes would enhance the performance of the DSSCs.
p-conjugation to D-p-A
* Corresponding authors. Tel.: þ86 431 88499179; e-mail addresses:
xuepengchong@mail.jlu.edu.cn (P. Xue), luran@mail.jlu.edu.cn (R. Lu).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2012.02.077

J. Jia et al. / Tetrahedron 68 (2012) 3626e3632
3627
intermediates and the target products were purified by column
chromatography, and the new compounds were characterized with
FTIR, 1H NMR, 13C NMR, elemental analysis, and MALDI/TOF mass
spectroscopy. The compounds TT, TP, and TC exhibited vibration
absorption bands around 962 cmꢁ1 in the IR spectrum, suggesting
the presence of the trans-double bond (C]C).6,11 In addition, the 1H
NMR spectra of TT, TP, and TC further confirmed that all the ethenyl
groups adopted the trans-conformation on account of the absence
of the signal at w6.5 ppm assigned to the protons in cis-double
bonds (CH]CH).6,11,13 The obtained dyes TT, TP, and TC showed
good solubility in most organic solvents, including toluene, ben-
zene, o-dichlorobenzene, CH2Cl2, THF, CHCl3, DMF, DMSO, etc.
2.2. Photophysical properties
As shown in Fig. 1a, it was found that the
pep transition ab-
*
Scheme 1. Molecular structures of triphenylamine-based dyes T, TT, TP, and TC.
sorption band of T in THF (2.0ꢂ10ꢁ5 M) appeared at 300 nm, and its
intermolecular charge transfer (ICT) transition band emerged at
370 nm. As to TT, TP, and TC, the absorption bands originated from
ICT transition red-shifted to 407, 409, and 395 nm (Table 1), re-
spectively, compared with that of T. Such spectral changes may be
2. Results and discussion
2.1. Syntheses of TT, TP, and TC
due to the extended
p-conjugation of the branched Y-shaped tri-
The synthetic routes for triphenylamine-based dyes TT, TP, and
TC are sketched in Scheme 2. Compounds 1e3 were synthesized
according to the reported procedures.10,11 The Heck reaction be-
tween compounds 1 and 2 catalyzed by Pd(OAc)2 at 110 ꢀC for 24 h
afforded compound 5 in a yield of 78%.10 Then, compound 5 could
be easily transformed into compound TT via Knoevenagel reaction
in a yield of 74%.6 Similarly, compound TP was prepared from
compound 6, which was obtained from compounds 1 and 3 under
Heck reaction condition. Moreover, Wittig reaction between
methyltriphenylphosphonium iodine and 9-octyl-9H-carbazole-3-
carbaldehyde12 gave compound 4, which reacted with compound
1 via Heck reaction to yield compound 7. Thus, compound TC was
synthesized by Knoevenagel reaction between cyanoacetic acid
and compound 7 in a yield of 68%. In addition, we synthesized 3-(4-
diphenylaminophenyl)-2-cyanoacrylic acid (T) as the reference
compound according to the reported procedures.7 All the
phenylamines. In addition, the molar extinction coefficients of TT
(6.67ꢂ104 Mꢁ1 cmꢁ1 at 407 nm), TP (5.40ꢂ104 Mꢁ1 cmꢁ1 at
409 nm) and TC (4.74ꢂ104 Mꢁ1 cmꢁ1 at 395 nm) were higher than
that of T (0.94ꢂ104 Mꢁ1 cmꢁ1 at 370 nm, Table 1) meaning that the
stronger light-harvesting abilities of the triphenylamine-based
dyes than that of T could be expected. Moreover, the UVevis ab-
sorption spectra of triphenylamine-based dyes deposited on
transparent TiO2 films (2 mm in thickness) were shown in Fig. 1b.
The maximal absorption peaks for T, TT, TP and TC on TiO2 films
appeared at 387, 408, 444 and 428 nm, respectively, exhibiting red-
shift compared with those in solutions, which might originate from
the aggregation or electronic coupling of dyes on the TiO2 sur-
faces.14 It should be noted that there is a red-shift of 1 nm for TT on
TiO2 film compared with that in solution was detected, meaning
that the branched conformation of TT could suppress the formation
of the aggregates effectively on TiO2 surface. Furthermore, the
Scheme 2. Syntheses of triphenylamine-based dyes TT, TP, and TC.

3628
J. Jia et al. / Tetrahedron 68 (2012) 3626e3632
Fig. 1. Normalized UVevis absorption spectra of triphenylamine-based dyes T, TT, TP, and TC in THF (a, 2.0ꢂ10ꢁ5 M) and deposited on TiO2 films (b).
Table 1
Photophysical and electrochemical data of dyes T, TT, TP, and TC
a
Dye
labs (nm) (
3
ꢂ10ꢁ4 Mꢁ1 cmꢁ1
)
Eox (V) versus NHEb
E0e0 (eV)c
2.49
ELUMO (V) versus NHEd
ꢁ1.51
Egape (V)
HOMOf (eV)
LUMOf (eV)
T
300 (0.15)
370 (0.94)
300 (3.26)
407 (6.67)
281 (3.45)
409 (5.40)
300 (2.82)
395 (4.74)
0.98
1.01
ꢁ5.45
ꢁ2.25
TT
TP
TC
0.47
0.57
0.64
2.29
2.34
2.28
ꢁ1.82
ꢁ1.77
ꢁ1.64
1.32
1.27
1.14
ꢁ4.80
ꢁ4.88
ꢁ4.85
ꢁ2.33
ꢁ2.36
ꢁ2.28
a
Measured in THF (2ꢂ10ꢁ6 M),
3
(ꢂ10ꢁ4 Mꢁ1 cmꢁ1).
b
c
d
e
f
The ground station potential (first oxidation peak) of the dyes was measured in CH2Cl2 with ferrocene/ferrocenium (Fc/Fcþ) as an internal reference.
E0e0 was determined from the edge of the absorption spectrum without adding TBAOH.
ELUMO¼EoxꢁE0e0
.
Egap is the energy gap between the ELUMO of the dye and the conduction band level of TiO2 (ꢁ0.5 eV vs NHE).
Energy levels of HOMO and LUMO versus vacuum were given after geometrical optimization.
absorption band of TT on TiO2 film became very broader compared
with those of T, TP and TC since the former one might originate
from the overlap of the electronic transition bands of TT from
a large amount of monomeric species and parts of aggregated
species, and the later ones mainly from the aggregated species of T,
TP and TC. Therefore, the strong light-harvesting ability of TT with
less aggregation on TiO2 film would be favoring the performance of
DSSC.4p In addition, the amount of dyes adsorbed on TiO2 films was
estimated by desorbing the dye with NaOH solution (Table 2).6,7
redox potential value (0.4 V vs NHE, Fig. 3), so the oxidized dyes
formed after electron injection into the conduction band of TiO2
would accept electrons from Iꢁ thermodynamically.
In addition, the excited-state oxidation potential of the dye plays
an important role in the electron-injection process from the excited-
state dye toTiO2 electrode, and it can be evaluated from the equation:
ELUMO¼EoxꢁE0e0, in which Eox is the first oxidation potentials and
E0e0 is the 0e0 excitation energy. We gained the E0e0 values of T, TT,
TP, and TC from the edge of absorption spectra of the dyes in
dichloromethane, which were 2.49 V, 2.29 V, 2.34 V, and 2.28 V, re-
spectively. As a result, ELUMO values for T, TT, TP, and TC were ꢁ1.51 V,
ꢁ1.82 V, ꢁ1.77 V, and ꢁ1.64 V versus NHE, respectively. The excited-
state oxidation potentials of the three Y-shaped triphenylamine-
based dyes are more negative than the equivalent potential of the
TiO2 conduction band edge (ꢁ0.5 V vs NHE),1 providing thermody-
namic driving force (1.14e1.32 V, Table 1) for electron injection.
Therefore, the obtained sensitizers might be employed in DSSCs as
sensitizers based on the assumption that energy gap of 0.2 eV is
necessary for efficient electron injection.7,15e17
Table 2
Photovoltaic parameters of the DSSCs sensitized with T, TT, TP, and TC in full sunlight
(AM 1.5 G, 100 mW/cm2)
Dye
Dye loada (10ꢁ7 mol/cm2)
Voc (V)
Jsc (mA/cm2)
ff
h (%)
T
1.68
2.01
2.08
1.80
0.64
0.72
0.74
0.73
6.88
12.37
11.11
9.42
0.63
0.58
0.59
0.53
2.79
5.12
4.84
3.63
TT
TP
TC
a
Amount of the dyes absorbed on TiO2 film.
2.3. Electrochemical properties
2.4. Molecular orbital calculations
To estimate the HOMO levels of the dyes, cyclic voltammetry
was employed using a three-electrode cell and an electrochemistry
workstation (CHI 604). The working electrode was a glass carbon
disc, the auxiliary electrode was a Pt wire, and Ag/Agþ was used as
reference electrode. Tetrabutylammonium tetrafluoroborate
(TBABF4, 0.1 M) was used as the supporting electrolyte in dry
CH2Cl2 and the ferrocenium/ferrocene (Fc/Fcþ) redox couple was
used as an internal potential reference.15 It was found that the first
oxidation potentials (Eox), corresponding to the HOMO levels, for
TT, TP, and TC were 0.47 V, 0.57 V, and 0.64 V (vs NHE, Table 1),
respectively, which were more positive than the iodine/iodide
To gain an insight into the geometrical and electronic structures
of the dyes, density functional theory (DFT) calculations for the four
triphenylamine-based sensitizers were performed. Fig. 2 showed
the molecular frontier orbitals of T, TT, TP, and TC. We could find
that the HOMOs of TT, TP, and TC were mainly localized on the
donors of triphenylamino-vinyl, 10-octyl-10H-phenothiazine-vinyl,
and 9-octyl-9H-carbazole-vinyl arms, while the HOMO of T was
a
p-orbital delocalized throughout the entire molecule. On the
other hand, the LUMOs of TT and TP were chiefly localized at cya-
noacrylic acid groups. It suggested that the modifying of the donor
moieties in triphenylamine functionalized cyanoacrylic acid with

J. Jia et al. / Tetrahedron 68 (2012) 3626e3632
3629
assembled into
a
sealed sandwich type cell. The photo-
currentevoltage curves of the devices were shown in Fig. 4. The
photovoltaic parameters, such as conversion efficiency ( ), short-
h
circuit photocurrent density (Jsc), the open-circuit photovoltage
(Voc), and the fill factor (ff) of the devices under full sunlight (AM 1.5
G, 100 mW/cm2) were summarized in Table 2. Under standard
global AM 1.5 solar conditions, the power conversion efficiencies
(
h
) of the cells sensitized with TC, TP, and TT were 3.63%, 4.84%, and
5.12%, respectively, which were higher than that sensitized with
(2.79%), illustrating that the introduction of branched
T
conjugated arms would improve the conversion efficiencies of the
DSSCs. As reported by other groups, the DSSCs sensitized with
triphenylamine-based dyes have shown the power conversion ef-
ficiencies in the range of 3.5e5.8%,4o,7,8 so the synthesis of the
branched triphenylamine-based dyes would be helpful for design
of the sensitizers employed in DSSCs. In addition, the Jsc and Voc
values for the devices based on TT, TP, and TC were also increased
compared with that based on T. The increased Jsc value suggested
high extinction coefficient of the Y-shaped dyes,9 and the increased
Voc values of TT (0.72 V), TP (0.74 V), and TC (0.73 V) compared with
T (0.64 V) could be attributed to the effective retardation of charge
recombination between the electrons at the TiO2 and the oxidized
branched dyes.18,19 To reveal the origin of the increased Voc value for
the cells based on TT, TP, and TC, the dark IeV curves were given in
Fig. 3. The onsets of the dark current of the DSSCs based on TT, TP,
and TC were 769 mV, 770 mV, and 796 mV, respectively, were
higher than T-based cell (660 mV), indicating that the increasing of
Voc was associated with the decreasing of the electron re-
combination in TiO2 with the Iꢁ/I3ꢁ redox couple in the electrolyte.20
Notably, we could find that the TT-sensitized cell exhibited better
photovoltaic performances (Jsc¼12.37 mA/cm2, Voc¼0.72 V, ff¼0.58,
Fig. 2. The frontier orbital plots of the HOMO and LUMO of T, TT, TP and TC.
and
h
¼5.12%) than other dyes, which might be due to the weakest
aggregation of TT on TiO2 film according to UVevis absorption re-
sults. Moreover, the best IPCE (monochromatic incident photon-to-
current conversion efficiency) response of TT-based DSSC among
the obtained DSSCs was also observed as shown in Fig. 5. It was
found that the characteristics of the spectral response of the pho-
tocurrent for the cells based on triphenylamine-based dyes were
similar to their absorption spectra, but the spectral response of the
device sensitized by TT, TP, and TC was broader than that of T due to
their large conjugation. It should be noted that the IPCE curve of the
device sensitized with TT showed a high plateau from 420 to
520 nm, with maximum of 73% at 423 nm, which was higher than
the DSSCs sensitized with other dyes. We deduced that the high
IPCE response of the DSSC based on TT is probably due to its high
Fig. 3. Cyclic voltammograms of T, TT, TP, and TC.
triphenylamine or 10-octyl-10H-phenothiazine could lead to
a better electron separation between the HOMO and LUMO, which
would suppress the electron recombination from the conduction of
TiO2 to the oxidized dyes. However, the LUMO for TC was localized
not only on cyanoacrylic acid group but also on the carbazole-based
arms, which would be unfavorable for the conversion efficiency of
the cell, which was confirmed by the following results.
2.5. Photovoltaic properties of DSSCs
The electrochemical cell used for photovoltaic measurements
consisted of a dye-adsorbed TiO2 electrode (the thickness of TiO2
Fig. 4. Current densityevoltage curves under full sunlight (real) (AM 1.5 G, 100 mW/
cm2) and in the dark (dashed) of devices sensitized with T, TT, TP, and TC using
a volatile electrolyte.
film is 7
mm), a counter electrode, and an organic electrolyte. The
dye-adsorbed TiO2 electrode and Pt-counter electrode were

3630
J. Jia et al. / Tetrahedron 68 (2012) 3626e3632
carried out on a Shimadzu RF-5301 luminescence spectrometer. IR
spectra were measured using a Germany Bruker Vertex 80v FT-IR
spectrometer by incorporating samples in KBr disks. Mass spectra
were performed on Agilent 1100 MS series and AXIMA CFR MALDI/
TOF (matrix assisted laser desorption ionization/time-of-flight) MS
(COMPACT). C, H, and N analyses were taken on a Vario EL cube el-
emental analyzer. Cyclic voltammetry was carried out with a CHI
604B electrochemical working station at room temperature at a scan
rate of 50 mV/s. Tetrahydrofuran (THF) was distilled over sodium
and benzophenone. CH3CN and DMF were distilled from phospho-
rous pentoxide. CH2Cl2 was distilled by hydrogenated calcium and
other chemicals were used as received without further purification.
FTO glasses (2.2 mm thickness, 15
pasted with nanocrystalline-TiO2 in thickness of 7
U
/cm2, Nippon Sheet Glass)
m and Pt elec-
m
trodes were bought from Dalian HeptaChroma SolarTech Co., Ltd.
4.2. Synthetic methods and characterizations
Fig. 5. Incident photo-to-current conversion efficiency of devices sensitized with T, TT,
TP, and TC.
4.2.1. Compounds 1e3. Compounds 1 (4-(bis(4-iodophenyl)amino)
benzaldehyde),
2 (N,N-diphenyl-4-vinylaniline), 3 (10-octyl-3-
molar extinction coefficient as well as large energy gap between the
LUMO level of the dye and the conduction band edge of TiO2, which
leads to increased electron injection efficiency relative to those of T,
TP, and TC.7,15 Moreover, the branched features (Fig. S2) of the
synthesized triphenylamine derivatives would passivate the dyes
by blocking the acceptor species from being attracted to the dye
molecules.21 Long-term stability measurements of the devices with
quasi-solid-state electrolyte were performed over 720 h at full
sunlight and at 50 ꢀC. The variations of the conversion efficiency of
solar cells sensitized with T, TT, TP, and TC were shown in Fig. S1. As
for the full sunlight soaking test, all the four cells show an increase
vinyl-10H-phenothiazine) and T (2-cyano-3-(4-(diphenylamino)
phenyl)acrylic acid) were synthesized according to the reported
procedures.7,10,11
4.2.2. 9-Octyl-3-vinyl-9H-carbazole (4). Methyltriphenylphospho-
nium iodine (7.90 g, 19.5 mmol), compound 1 (4.00 g, 13 mmol),
and t-BuOK (2.30 g, 20 mmol) were dissolved in 25 ml dry THF at
0 ꢀC, and the solution was stirred at room temperature for 3 h. After
filtrated to remove the solid, the solvent was removed under re-
duced pressure. The residue was purified using column chroma-
tography (petroleum ether/CH2Cl2¼4:1) to give green-yellow oil
(3.2 g, 80% in yield). 1H NMR (500 MHz, CDCl3)
d¼8.05 (t, J¼8.5 Hz,
h
during the initial 60 h, which may be caused by filling of elec-
trolyte into the inner mesopores of TiO2 electrode. On continuous
720 h of sunlight soaking test, the solar cell performances of the
DSSCs based on the new Y-shaped triphenylamine-based dyes (TT,
TP, and TC) show little degradation (3%, 4%, and 7%, respectively),
while T decreased sharply (18%). Therefore, the stability of TT and
TP under sunlight and air was good.14 As a result, Y-shaped
triphenylamine-based dyes can become competitive candidate in
sensitizers employed in DSSCs with high performance on account
2H), 7.51 (d, J¼8.5 Hz, 1H), 7.39 (t, J¼7.5 Hz, 1H), 7.31 (d, J¼8.0 Hz,
1H), 7.25 (d, J¼8.5 Hz, 1H), 7.17 (t, J¼7.5 Hz, 1H), 6.86 (m, 1H), 5.73
(d, J¼17.5 Hz, 1H), 5.14 (t, J¼14 Hz, 1H), 4.15 (t, J¼7.5 Hz, 2H),
1.18e1.26 (m, 12H), 0.84 (t, J¼7.0 Hz, 3H) (Fig. S3). Anal. Calcd (%) for
C22H27N: C, 86.51; H, 8.91; N, 4.59. Found: C, 86.39; H, 8.83; N, 4.74.
4.2.3. 4-(Bis-(4-(4-diphenylaminostyryl)phenyl)-amino)benzalde-
hyde (5). To a solution of compound 1 (5.25 g, 10 mmol), com-
pound 2 (7.0 g, 26 mmol) and Bu4NBr (9.85 g, 30 mmol) in dry DMF
(50 mL), anhydrous K2CO3 (4.2 g, 30 mmol), and Pd(OAc)2 (5.0 mg,
0.02 mmol) were added. The mixture was stirred under a N2
atmosphere at 110 ꢀC for 24 h. After cooling to room temperature,
the mixture was poured into water (500 mL) and extracted with
CH2Cl2. The combined organic phases were washed with brine, and
dried with anhydrous Na2SO4. After the solvent was removed, the
residue was purified by column chromatography (silica gel, petro-
leum ether/CH2Cl2¼2:1) to give 6.3 g yellow solid (78% in yield). Mp
of their strong light-harvesting ability (extended
p-conjugation),
weak aggregation (branched structure) on TiO2 films, and balanced
oxidationereduction potential.
3. Conclusions
In summary, we have synthesized three new Y-shaped
triphenylamine-based dyes (TT, TP, and TC), which can be used as
sensitizers in DSSCs. It was found that the photo-to-electrical con-
version efficiency of the DSSCs sensitized with branched TT, TP, and
TC were 5.12%, 4.84%, and 3.63%, which were higher than that sen-
sitized with T (2.79%). Notably, the DSSC sensitized with TT gave
higher IPCE response and better photovoltaic performances
(Jsc¼12.37 mA/cm2, Voc¼0.72 V, and ff¼0.58) than others. Therefore,
the branched Y-shaped triphenylamine-based dyes with extended
160.0e162.0 ꢀC. 1H NMR (500 MHz, CDCl3)
d¼9.83 (s, 1H), 7.71 (d,
J¼8.5 Hz, 2H), 7.45 (d, J¼8.5 Hz, 4H), 7.38 (d, J¼8.5 Hz, 4H), 7.27 (m,
8H), 7.13 (m, 12H), 7.03 (m, 12H), 6.96 (d, J¼16.5 Hz, 2H) (Fig. S4).
MS (MALDI-TOF): m/z 811.9 [Mþ] (Fig. S5, calcd for C59H45N3O
811.4). Anal. Calcd (%) for C59H45N3O: C, 87.27; H, 5.59; N, 5.17; O,
1.97. Found: C, 87.15; H, 5.79; N, 5.37.
p-conjugation could afford high performance of DSSCs on account of
their strong light-harvesting ability, weak aggregation onTiO2 films,
and balanced oxidationereduction potential. This strategy would be
helpful for the design of competitive dyes employed in DSSCs.
4.2.4. 4-(Bis-4-(E-2-(10-octyl-10H-phenothiazin-2-yl)vinyl)phenyl)
aminobenzaldehyde (6). The synthetic method for compound 6
was similar to that of compound 5. It was purified by column
chromatography (silica gel, petroleum ether/CH2Cl2¼4/1) to give
yellow solid (60% in yield). Mp 65.0e66.0 ꢀC. 1H NMR (500 MHz,
4. Experimental section
CDCl3)
d
¼9.83 (s, 1H), 7.70 (d, J¼8.5 Hz, 2H), 7.42 (d, J¼8.0 Hz, 4H),
4.1. Materials and measurements
7.27 (d, J¼17.0 Hz, 4H), 7.13 (d, J¼8.5 Hz, 8H), 7.09 (d, J¼8.5 Hz, 2H),
6.90 (d, J¼33.5 Hz, 10H), 3.75 (t, J¼6.5 Hz, 4H), 1.25e1.44 (m, 24H),
0.86 (t, J¼6.5 Hz, 6H) (Fig. S6). MS (MALDI-TOF): m/z 944.4 [MþHþ]
(Fig. S7, calcd for C63H65N3OS2 943.5). Anal. Calcd (%) for
UVevis absorption spectra were determined on a Shimadzu UV-
1601PC spectrophotometer. Photoluminescence (PL) spectra were

J. Jia et al. / Tetrahedron 68 (2012) 3626e3632
3631
C63H65N3OS2: C, 80.13; H, 6.94; N, 4.45; O, 1.69; S, 6.79. Found: C,
80.30; H, 6.81; N, 4.71.
24H), 0.81 (t, J¼7.0 Hz, 6H) (Fig. S16). 13C NMR (125 MHz, DMSO-d6)
d
¼165.1, 153.3, 152.2, 146.1, 144.9, 141.3, 140.7, 135.7, 133.5, 130.9,
130.3, 130.0, 129.0, 128.3, 127.1, 126.9, 126.4, 125.5, 123.3, 123.0,
121.2,119.8,119.4,118.2,110.3,100.4, 43.2, 32.0, 29.9, 29.6, 29.4, 27.3,
22.9, 14.8 (Fig. S17). MS (MALDI-TOF): m/z 947.2 [MþHþ] (Fig. S18,
calcd for C66H66N4O2: 946.5). Anal. Calcd (%) for C66H66N4O2: C,
83.68; H, 7.02; N, 5.91; O, 3.38. Found: C, 83.59; H, 6.88; N, 6.07.
4.2.5. 4-(Bis-4-((E)-2-(9-octyl-9H-carbazol-2-yl)vinyl)phenyl)ami-
nobenzaldehyde (7). The synthetic method for compound 7 was
similar to that of compound 5. It was purified by column chroma-
tography (silica gel, petroleum ether/CH2Cl2¼1:1) to give yellow
solid (65% in yield). Mp 76.0e78.0 ꢀC. 1H NMR (500 MHz, CDCl3)
d
¼9.84 (s, 1H), 8.22 (d, J¼5 Hz, 2H), 8.12 (d, J¼7.5 Hz, 2H), 7.65e7.73
4.3. Theoretical calculation methods
(m, 4H), 7.52 (t, J¼9.0 Hz, 4H), 7.47 (t, J¼7.5 Hz, 4H), 7.39e7.45 (m,
4H), 7.27e7.36 (m, 2H), 7.21e7.34 (m, 4H), 7.10e7.21(m, 4H) 4.29 (t,
J¼7.0 Hz, 4H), 1.19e1.54 (m, 24H), 0.86 (t, J¼7.0 Hz, 6H) (Fig. S8). MS
(MALDI-TOF): m/z 880.3 [MþHþ] (Fig. S9, calcd for C63H65N3O
879.5). Anal. Calcd (%) for C63H65N3O: C, 85.97; H, 7.44; N, 4.77; O,
1.82. Found: C, 85.85; H, 7.66; N, 4.65.
The geometrical structures of the synthesized triphenylamine-
based dyes were optimized by employing the density functional
theory at the B3LYP/6-31 level with the Gaussian 03W program
package.22 Molecular orbitals were visualizable using Gaussview.
4.4. Preparation of dye-absorbed TiO2 films
4.2.6. (E)-3-(4-(Bis-(4-(4-diphenylaminostyryl))-phenyl)amino)phe-
nyl-2-cyanoacrylic acid (TT). A mixture of compound 5 (0.10 g,
12.32 mmol), cyanoacetic acid (20.95 mg, 25 mmol), and ammo-
nium acetate (20 mg) in glacial acetic acid (10 mL) was stirred at
120 ꢀC for 12 h. After it was cooled to room temperature, the
resulting precipitate was collected by filtration and washed with
water. The crude product was purified by column chromatography
(silica gel) using methanol/dichloromethane (v/v¼1:10) as the el-
uent. 80 mg of red solid was obtained (74% in yield). Mp
174.0e176.0 ꢀC. IR (KBr, cmꢁ1): 961, 1070, 1134, 1437, 1612, 2026,
The dye-anchored TiO2 film as a working electrode was pre-
pared by following procedure. The area of the nanocrystalline-TiO2
film coated on FTO glasses (2.2 mm thickness, 15 U
/cm2, Nippon
Sheet Glass) was 0.24 cm2 (0.55 cm of diameter). The film was dried
in air at 100 ꢀC for 15 min followed by another 15 min at 150 ꢀC.
Then, the film was calcined at 350 ꢀC for 10 min. After that, the film
was treated at 450 ꢀC for 30 min under oxygen atmosphere. When
the temperature decreased to 100 ꢀC, the electrodes were im-
mersed in a dry CH2Cl2 solution of the dye (0.5 mM) and kept at
room temperature for 12 h.
2218, 2370, 2447, 2927, 3124, 3377 and 3441 cmꢁ1
.
1H NMR
(500 MHz, DMSO-d6)
d
¼8.12 (s, 1H), 7.93 (d, J¼9.0 Hz, 2H), 7.60 (d,
A sandwich cell was prepared by using the dye-anchored TiO2
film as a working electrode and Pt electrode as a counter one, which
was assembled with a hot-melt-ionomer film of Surlyn polymer
gasket. The electrodes were tightly held and heated at 120 ꢀC for 25 s
in seal machine (DHS-ES2) to seal the two electrodes. The aperture
of the Surlyn frame was 2 mm larger than that of the area of TiO2 film
and its width was 1 mm. The hole in the counter electrode was
sealed bya film of Surlyn. Adrop of an electrolytewas put on the hole
in the back of the counter electrode. It was introduced into the cell
via vacuum backfilling. Finally, the hole was sealed using Surlyn film
and a cover glass (0.13e0.17 mm thickness). The edge of the FTO
outside of the cell was roughened with sandpaper. An electrolyte
solution used was 0.6 M 1,2-dimethyl-3-methylimidazolium iodide,
0.1 M LiI, 0.05 M I2, 0.5 M TBP, 4-tert-butylpyridine, and 0.05 mM
GUSCN (guanidine thiocyanate) in dry acetonitrile.
J¼8.5 Hz, 4H), 7.51 (d, J¼8.5 Hz, 4H), 7.32 (t, J¼8.0 Hz, 8H), 7.16 (m,
8H), 7.05 (m, 12H), 7.00 (d, J¼9.0 Hz, 2H), 6.96 (d, J¼8.5 Hz, 4H)
(Fig. S10). 13C NMR (125 MHz, DMSO-d6)
d
¼164.9, 154.1, 152.5,
147.6, 145.5, 145.0, 135.2, 133.8, 132.2, 130.9, 130.4, 128.5, 127.3,
127.0, 126.6, 125.0, 124.1, 123.9, 119.7, 119.0, 117.8, 99.0 (Fig. S11). MS
(MALDI-TOF): m/z 878.7 [Mþ] (Fig. S12, calcd for C62H46N4O2:
878.4). Anal. Calcd (%) for C62H46N4O2: C, 84.71; H, 5.27; N, 6.37; O,
3.64. Found: C, 84.50; H, 5.16; N, 6.67.
4.2.7. (E)-3-(4-(Bis-(4-(E-2-(10-octyl-10H-phenothiazin-2-yl))vinyl)
phenyl)amino)phenyl-2-cyanoacrylic acid (TP). The synthetic pro-
cedure for TP was similar to that of TT. The crude product was pu-
rified by column chromatography (silica gel) using methanol/
dichloromethane (v/v¼1:10) as the eluent to give dark-red solid
(64% in yield). Mp 164.0e166.0 ꢀC. IR (KBr, cmꢁ1): 962, 1040, 1130,
1180, 1330, 1460, 1500, 1581, 2213, 2360, 2850, 2923, 3398,
4.5. Photovoltaic characterizations
3430 cmꢁ1
;
1H NMR (500 MHz, DMSO-d6)
d¼8.04 (s, 1H), 7.88 (d,
J¼8.50 Hz, 2H), 7.56 (d, J¼8.0 Hz, 4H), 7.38 (d, J¼7.5 Hz, 4H), 7.19 (t,
J¼7.5 Hz, 2H), 7.04e7.15 (m, 10H), 6.95 (m, 8H), 3.86 (m, 4H),
1.19e1.37 (m, 24H), 0.81 (t, J¼7.0 Hz, 6H) (Fig. S13). 13C NMR
Photoelectrochemical data were measured using a 150 W xenon
light source that was focused to give 100 mW/cm2 at the surface of
the test cell. The spectral output of the lamp was matched with the
aid of a Mega-9 AM 1.5 sunlight filter so as to reduce the mismatch
between the simulated and the true solar spectrum. The applied
potential and current were measured using a CHI 604B electro-
chemical working station. A similar data acquisition system was
used to control the incident photon-to-current conversion effi-
ciency (IPCE) measurement. Under full computer control, light from
a 250 W halogen lamp was focused through a high throughput
(125 MHz, DMSO-d6)
d¼164.9, 152.1, 151.4, 145.2, 144.9, 135.0, 133.1,
132.5,128.5,128.0,127.8,127.0,126.9,126.6,125.4,124.7,124.0,123.3,
120.9,118.7,116.7, 99.7, 67.8, 47.4, 32.0, 29.5, 29.4, 27.0, 26.9, 22.9,14.8
(Fig. S14). MS (MALDI-TOF): m/z 1011.3 [MþHþ] (Fig. S15, Calcd for
C66H66N4O2S2: 1010.5). Anal. Calcd (%) for C66H66N4O2S2: C, 78.38; H,
6.58; N, 5.54; O, 3.16; S, 6.34. Found: C, 78.29; H, 6.57; N, 5.43.
4.2.8. (E)-3-(4-(Bis(4-((E)-2-(9-octyl-9H-carbazol-2-yl)vinyl)phenyl)
amino)phenyl)-2-cyanoacrylic acid (TC). The synthetic procedure
for TC was similar to that of TT. The crude product was purified by
column chromatography (silica gel) using methanol/dichloro-
methane (v/v¼1:9) as the eluent to give a red solid (68% in yield).
Mp 126.0e128.0 ꢀC. IR (KBr, cmꢁ1): 962, 1070, 1133, 1268, 1382,
1440, 1612, 1710, 2030, 2220, 2440, 2925, 3135, 3378, 3441 cmꢁ1. 1H
monochromator (Omni-l150) onto the photovoltaic cell. The IPCE
values were determined at 2 nm intervals.7
The solar energy-to-electricity conversion efficiency (h) of the
DSSCs is calculated from the short-circuit photocurrent density
(Jsc), the open-circuit photovoltage (Voc), the fill factor of the cell
(ff), and the intensity of the incident light (Pin).7
NMR (500 MHz, DMSO-d6)
d
¼8.38 (s, 1H), 8.18 (d, J¼8.0 Hz, 2H),
Acknowledgements
7.94 (m, 4H), 7.73 (m, 2H), 7.65 (t, J¼8.0 Hz, 4H), 7.59 (d, J¼8.0 Hz,
4H), 7.41e7.47 (m, 4H), 7.38 (t, J¼8.0 Hz, 2H), 7.26 (m, 4H), 7.17e7.21
(m, 2H), 6.94 (d, J¼9.0 Hz, 2H), 4.38 (t, J¼7.0 Hz, 4H), 1.04e1.27 (m,
This work was supported by the Scientific Research Foundation
for the Returned Overseas Chinese Scholars, State Education

3632
J. Jia et al. / Tetrahedron 68 (2012) 3626e3632
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MOL files and InChIKeys of the most important compounds
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