Organic dyes containing coplanar diphenyl-substituted dithienosilole core for efficient dye-sensitized solar cells

Article abstract of DOI:10.1021/jo100762t

(Figure Presented) Two new organic dyes adopting coplanar diphenyl-substituted dithienosilole as the central linkage have been synthesized, characterized, and used as the sensitizers for dye-sensitized solar cells (DSSCs). The best DSSC exhibited a high power conversion efficiency up to 7.6% (TP6CADTS) under AM 1.5G irradiation, reaching ～96% of the ruthenium dye N719-based reference cell under the same conditions.

Full text of DOI:10.1021/jo100762t

pubs.acs.org/joc
Organic Dyes Containing Coplanar Diphenyl-Substituted Dithienosilole
Core for Efficient Dye-Sensitized Solar Cells
Li-Yen Lin,† Chih-Hung Tsai,‡ Ken-Tsung Wong,*,† Tsung-Wei Huang,‡ Lun Hsieh,‡
Su-Hao Liu,‡ Hao-Wu Lin,‡ Chung-Chih Wu,*,‡ Shu-Hua Chou,† Shinn-Horng Chen,§
and An-I Tsai§
†Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan, ‡Department of Electrical
Engineering, Graduate Institute of Photonics and Optoelectronics, and Graduate Institute of Electronics
Engineering, National Taiwan University, Taipei 10617, Taiwan, and Energy Material Research Group,
§
Eternal Chemical Co., Ltd., Kaohsiung 821, Taiwan
kenwong@ntu.edu.tw; chungwu@cc.ee.ntu.edu.tw
Received April 19, 2010
Two new organic dyes adopting coplanar diphenyl-substituted dithienosilole as the central linkage
have been synthesized, characterized, and used as the sensitizers for dye-sensitized solar cells (DSSCs).
The best DSSC exhibiteda high power conversion efficiencyup to7.6% (TP6CADTS) under AM 1.5G
irradiation, reaching ∼96% of the ruthenium dye N719-based reference cell under the same conditions.
Introduction
dye,5 have achieved remarkable power conversion efficiency
of 10-11% under standard global air mass 1.5 (AM 1.5G)
illumination. However, the rarity and high cost of the ruthe-
nium metal may limit their development for large-scale appli-
cations. Consequently, many researchers have focused on
developing metal-free organic sensitizers,7-18 and some of
these endeavors have achieved decent efficiencies in the past
Increasing energy demands and concerns about global
warming have encouraged scientists to develop cheap and
easily accessible renewable energy sources in recent years. In
comparison with the traditional silicon-based solar cells,1
dye-sensitized solar cells (DSSCs) are one of the most pro-
mising next-generation photovoltaic cells due to their versa-
tile, energy-saving, and environmentally friendly nature.
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Yao, B.; Xie, Z.; Wang, L. Angew. Chem., Int. Ed. 2007, 46, 1149.
Therefore, DSSCs have attracted much attention since the
€
2
breakthrough made by Gratzel et al. in 1991. Several
ruthenium-based sensitizers, such as N3,3 N719,4 and black
€
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4778 J. Org. Chem. 2010, 75, 4778–4785
Published on Web 06/17/2010
DOI: 10.1021/jo100762t
r
2010 American Chemical Society

Lin et al.
JOCArticle
few years.10b,c,13c,14b Among them, the donor-(π-spacer)-
acceptor (D-π-A) system is the basic structure for design-
ing the organic sensitizers due to their effective photoinduced
intramolecular charge-transfer characteristics. Generally,
arylamine-based moieties, which have also been widely used
in materials of organic light-emitting diodes (OLEDs), are
employed as the electron donor due to the strong electron-
donating nature.6 On the other hand, carboxylic acid, cyano-
acrylic acid, or rhodanine-3-acetic acid were introduced into
the (D-π-A) system as the electron acceptor as well as the
anchoring group to the TiO2 surface.7 Various chromophores
such as oligoene,8 coumarin,9 indoline,10 oligothiophene,11
dithienothiophene,12 thienothiophene,13 oligophenyleneviny-
lene,14 fluorene,15 spirobifluorene,16 phenoxazine,17 and tri-
phenylamine18 have been incorporated as the conjugated
spacer between the electron donor and electron acceptor for
tuning the wavelength ranges, absorption capability, and
other characteristics required for DSSCs.
Among various organic conjugated materials, dithienosilole
(DTS) derivatives and dithienosilole-based polymers are promis-
ing functional organic materials that have been widely used in
organic optoelectronic devices such as light-emitting diodes,19
field effect transistors,20 and polymer solar cells.21 However,
there are few reports of utilizing the DTS moiety as the con-
jugated spacer for the organic sensitizers of DSSCs. During the
preparation of this manuscript, Wang et al. reported a sensitizer
named C219 incorporating dihexyl-substituted DTS and 3,4-
ethylenedioxythiophene unit as a binary π-conjugated spacer
between a lipophilic alkoxy-substituted triphenylamine electron
donor and a hydrophilic cyanoacrylic acid electron acceptor,
leading to a high efficiency of 10.0-10.3% under AM 1.5 G
irradiation.22 In the meantime, Ko et al. also reported a new
class of organic dyes featuring DTS units equipped with
various substituents at the 3-position of DTS as D-A spacers
and also yielding good efficiencies from 6.73% to 7.50%.23
In this paper, we report our design, synthesis, and charac-
terization of two new organic dyes that contain a simple
triphenylamine or dihexyloxy-substituted triphenylamine moi-
ety as the electron donor and cyanoacrylic acid as the electron
acceptor. These two building blocks are bridged by diphenyl-
substituted DTS, which serves as a coplanar π-conjugated
spacer to give two organic D-π-A dyes TPCADTS and
TP6CADTS (Scheme 1). We adopted the simple D-π-A
structure with a shorter spacer for facilitating the electronic
coupling between the electron donor and electron acceptor,
resulting in a red-shifted absorption. Hence, efficient harvest-
ing of sunlight can be reasonably anticipated. In addition, the
introduction of the diphenyl-substituted DTS as the central
linkage for the organic sensitizer possesses two advantages: (1)
The HOMO-LUMO gap of DTS is smaller than that of 2,20-
bithiophene and carbon-bridged bithiophene due to the lower
lying LUMO,19b which in turn originates from the interaction
of silicon σ*-orbital and bithiophene π*-orbital, namely,
σ*-π* conjugation.24 The smaller band gap of DTS is highly
beneficial for harvesting of sunlight. (2) The tetrahedral silicon
center of diphenyl-substituted DTS does not distort the copla-
narity of π-conjugated system.19b Instead, the diphenyl sub-
stitution on silicon atom may reduce the π-π stacking
interactions of dyes and may be beneficial to high electron
injection yield and the power conversion efficiency.25
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SCHEME 1. Synthesis of Dyes TPCADTS and TP6CADTS
Results and Discussion
The sensitizers TPCADTS and TP6CADTS were ob-
tained in several steps by the synthetic pathways illustrated
in Scheme 1. Different from the two previously reported
DTS-containing dyes,22,23 we adopted 5,50-bis(trimethyl-
silyl)-3,30-diphenylsilylene-2,20-bithiophene (1) as the start-
ing material, which was synthesized according to the pub-
lished procedures.19b Selective monobromination of 1 gave
the key intermediate 5-bromo-50-trimethylsilyl-3,30-diphe-
nylsilylene-2,20-bithiophene (2) in 66% yield. Compound 2
was then desilylated with TFA to produce 5-bromo-3,30-
diphenylsilylene-2,20-bithiophene (3) with an isolated yield
of 98%. The Suzuki cross-coupling reaction of 3 and boronic
acids 4a26 afforded 5a in 83% yield, whereas 5b was synthe-
sized in 61% yield by Stille coupling reaction of 3 with
tributyltin-substituted compound 4b.27 The free C2-position
of DTS core in compound 5 was subsequently converted to
their corresponding carbaldehydes 6 by performing the
Vilsmeier-Haack reaction, affording 6a and 6b in 64%
FIGURE 1. Absorption (solid lines) and emission (dashed lines)
spectra of TPCADTS (blue lines) and TP6CADTS (red lines)
measured in 2-methyl-2-propanol-acetonitrile (1:1) solution
(10-5 M).
and 78% yield, respectively. Finally, the aldehydes were con-
densed with cyanoacetic acid to give the target compounds
TPCADTS and TP6CADTS in 85% and 86% yield, respec-
€
tively, via Knoevenagel reaction in the presence of ammo-
nium acetate.
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TABLE 1. Photophysical and Electrochemical Parameters of the Dyes
dye
λabsa/nm
εa/M-1 cm-1
λema/nm
λ
abs on TiO2b/nm
E0-0c/eV
Eoxd/V
Erede/V
Eox*f/V
TPCADTS
TP6CADTS
495
511
44500
36900
579
605
458
470
1.954
1.865
1.065
0.875
-0.963
-0.987
-0.889
-0.99
aAbsorption and emission spectra were measured in 2-methyl-2-propanol-acetonitrile (1:1) solution (10-5 M). bAbsorption spectra on TiO2 were
obtained through measuring the dye adsorbed on porous TiO2 nanoparticle film (thickness: 7 μm) in 2-methyl-2-propanol-acetonitrile (1:1) solution.
cE0-0 was estimated from the onset point of the absorption spectra. dThe oxidation potentials of the dyes were measured in DMF with 0.1 M
tetrabutylammonium hexafluorophosphate (TBAPF6) as electrolyte. eThe reduction potentials of the dyes were measured in THF with 0.1 M
tetrabutylammonium perchlorate (TBAP) as electrolyte. For footnotes d and e: working electrode: glassy carbon; reference electrode: Ag/Agþ; counter
electrode: Pt; scan rate: 100 mV/s; calibrated with ferrocene/ferrocenium (Fc/Fcþ) as an internal reference and converted to NHE by addition of 630
28 f
mV.
E * = Eox - E0-0.
ox
FIGURE 2. Absorption spectra of the dyes anchoring on the 7 μm
porous TiO2 nanoparticle film.
The UV-vis absorption and normalized emission spectra of
dyes TPCADTS (blue lines) and TP6CADTS (red lines) in
2-methyl-2-propanol-acetonitrile (1:1) solution are depicted in
Figure 1, and the corresponding data are summarized in Table 1.
The absorption spectrum of TPCADTS shows an absorption
maximum (λmax) centered at 495 nm (ε = 44500 M-1 cm-1),
which is assigned tothe π-π* transition. TP6CADTS exhibits
an absorption λmax at 511 nm (ε = 36900 M-1 cm-1) that is
red-shifted by 16 nm as compared to that of TPCADTS due to
the stronger electron-donating nature of hexyloxy substitu-
ents. TPCADTS and TP6CADTS exhibit emission bands
centered at 579 and 605 nm, respectively, as excited on their
absorption maxima. Upon anchoring on the 7 μm porous
TiO2 nanoparticle film, the absorption maxima of these two
dyes showblue-shiftedbehavior (Figure2) in comparisonwith
those in 2-methyl-2-propanol-acetonitrile (1:1) solution. The
blue shift is attributed to the deprotonation of carboxylic acid
upon adsorption onto the TiO2 surface, and the resulting
carboxylate-TiO2 unit is a weaker electron acceptor com-
pared to the carboxylic acid.29
To gain more insight into the electronic structures of
TPCADTS and TP6CADTS, density function theory (DFT)
calculations were performed at a B3LYP/6-31G(d) level for
the geometryoptimization. AsshowninFigure 3, the HOMOs
of both sensitizers are mainly populated over the triphenyla-
mine and dithienosilole blocks with considerable contribution
from the former, whereas LUMOs are delocalized through
the dithienosilole and cyanoacrylic acid fragments with
sizable contribution from the latter. Examination of the
frontier orbitals of both sensitizers suggests that the HOMO-
LUMO excitation would shift the electron density distri-
bution from the triphenylamine unit to the cyanoacrylic acid
FIGURE 3. Frontier molecular orbitals (HOMO and LUMO) of
TPCADTS (a) and TP6CADTS (b) calculated with DFT on the
B3LYP/6-31G(d) level.
moiety, facilitating efficient photoinduced/interfacial elec-
tron injection from excited dyes to the TiO2 electrode.
Quasi-reversible oxidation and reduction waves (Eox and
Ered in Table 1) were observed for the dyes TPCADTS and
TP6CADTS in the cyclic voltammetry (CV) measurements
(Figure 4). The oxidation can be attributed to the tripheny-
lamine moieties, while the reduction can be assigned to the
cyanoacrylic acid. Both oxidation and reduction potentials
of TP6CADTS are more negative than those of TPCADTS,
yet the difference in reduction potentials of the two dyes is
smaller than that in oxidation potentials. It suggests that the
introduction of a hexyloxy substituent on the triphenylamine
moiety has a significant influence on the ground-state oxida-
tion potential but a more limited effect on the ground-state
reduction potential of TP6CADTS. The different influence
on the ground-state oxidation and reduction potential by
incorporating the stronger donor leads to a narrow energy
gap of TP6CADTS. The zero-zero excitation energy (E0-0
)
estimated from the absorption onset and the ground-state
oxidation potential (Eox) were used to calculate the excited-
state oxidation potential (Eox*). The deduced Eox* values of
these two dyes are more negative than the conduction band
edge of the TiO2 (-0.5 V vs NHE),30 indicating that the
electron injection process shall be energetically favorable. On
the other hand, the ground-state oxidation potentials (Eox in
Table 1) of these two dyes are more positive than the redox
potential of the iodide/triiodide couple (0.4 V vs NHE). It
suggests that the oxidized dyes shall be able to accept
€
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FIGURE 4. Cyclic voltammograms of TPCADTS (blue lines) and
TP6CADTS (red lines); 0.1 M TBAP (reduction) in THF and 0.1 M
TBAPF6 (oxidation) in DMF were used as supporting electrolytes.
A glassy carbon electrode was used as the working electrode; scan
rate: 100 mV/s.
FIGURE 5. IPCE spectra of DSSCs based on TPCADTS and
TP6CADTS.
electrons from I- thermodynamically for effective dye re-
generation and to avoid the charge recombination process
between oxidized dyes and photoinjected electrons in the
TiO2 film.
The photovoltaic characteristics of TPCADTS and
TP6CADTS as the sensitizers in DSSCs were evaluated with
a sandwich DSSC cell comprising 0.6 M 1-butyl-3-methyli-
midazolium iodide (BMII), 0.05 M LiI, 0.03 M I2, 0.5 M
4-tert-butylpyridine, and 0.1 M guanidinium thiocyanate in
a mixture of acetonitrile-valeronitrile (85: 15, v/v) as the
redox electrolyte. Details of the device preparation and
characterization are described in the Experimental Section.
It was reported that the incorporation of 4-tert-butylpyri-
dine can decrease the dark current and shift the conduction
band edge of the TiO2 to a more negative potential. As a
result, the open-circuit voltage and fill factor can be im-
proved, leading to an improvement on overall conversion
efficiencies.31 The incident monochromatic photon-to-cur-
rent conversion efficiency (IPCE) spectra of the DSSCs are
shown in Figure 5. The IPCE spectrum of TP6CADTS is
broader than that of TPCADTS, as can be expected from the
absorption spectrum of TP6CADTS anchoring on the 7 μm
porous TiO2 film or its absorption in solutions. The IPCE of
TP6CADTS exceeds 70% from 350 to 560 nm and reaches a
maximum of 84%at450 nm. Amongthesetwo sensitizer dyes,
TP6CADTS shows higher peak IPCE, although its maximal
absorption coefficient may not necessarily be higher than that
of TPCADTS. The higher peak IPCE of TP6CADTS than
that of TPCADTS may be ascribed to the reduced charge
recombination at the TiO2/dye/electrolyte interface by tailor-
ing with two longer aliphatic hexyloxy chains.13a
FIGURE 6. Photocurrent density vs voltage for DSSCs based on
TPCADTS and TP6CADTS under AM 1.5 G simulated solar light
(100 mW cm-2).
η value of 6.65% (see Table 2). For a fair comparison, the
N719-sensitized DSSC was also fabricated under the same
conditions. The efficiency of the TP6CADTS-based cell reaches
∼96% of the N719 cell efficiency (7.93%). The higher Jsc value
of the TP6CADTS-based cell than that of the TPCADTS-
based cell is consistent with the values for the IPCE spectra,
reflecting the better sunlight-harvesting ability of TP6CADTS.
The two hexyloxy chains of TP6CADTS may also contribute to
inhibition of dye aggregation and thus to enhancement of Jsc.13a
In addition, with the existence of two aliphatic hexyloxy chains,
one also notes that the TP6CADTS-based cell gives a 50 mV
increase in Voc in comparison with the TPCADTS-based cell.
The effects of the two longer hexyloxy chains on photo-
voltaic characteristics were further elucidated by electroche-
mical impedance spectroscopy (EIS). The EIS is a useful tool
for characterizing important interfacial charge-transfer pro-
cesses in DSSCs, such as the charge recombination at the
TiO2/dye/electrolyte interface, electron transport in the
TiO2 electrode, electron transfer at the counter electrode,
and I3- transport in the electrolyte etc.32-34 In this study, the
Figure 6 shows the current-voltage (J-V) curves of the
DSSCs under standard global AM 1.5G solar irradiation.
The short-circuit photocurrent density (Jsc), open-circuit
voltage (Voc), and fill factor (ff) of the solar cell based on
TP6CADTS are 13.39 mA cm-2, 0.81 V, and 0.70, respec-
tively, yielding an overall conversion efficiency (η) of 7.60%.
Under the same conditions, the device based on TPCADTS
shows relatively lower Jsc and Voc, leading to an inferior
(32) Andrade, L.; Zakeeruddin, S. M.; Nazeeruddin, M. K.; Ribeiro,
€
H. A.; Mendes, A.; Gratzel, M. ChemPhysChem 2009, 10, 1117.
(33) Zhang, J.; Yang, G.; Sun, Q.; Zheng, J.; Wang, P.; Zhu, Y.; Zhao, X.
J. Renewable Sustainable Energy 2 2010, 013104.
(31) (a) Hara, K.; Dan-oh, Y.; Kasada, C.; Ohga, Y.; Shinpo, A.; Suga, S.;
Sayama, K.; Arakawa, H. Langmuir 2004, 20, 4205. (b) Boschloo, G.;
€
Haggman, L.; Hagfeldt, A. J. Phys. Chem. B 2006, 110, 13144.
(34) Fabregat-Santiago, F.; Bisquert, J.; Palomares, E.; Otero, L.;
Kuang, D.; Zakeeruddin, S. M.; Gra1tzel, M. J. Phys. Chem. C 2007, 111,
6550.
4782 J. Org. Chem. Vol. 75, No. 14, 2010

Lin et al.
JOCArticle
TABLE 2. DSSC Performance Parameters of the Dyesa
dye
Jsc/mA cm-2
Voc/V
ff
η (%)
TPCADTS
TP6CADTS
N719
12.70
13.39
14.65
0.76
0.81
0.77
0.69
0.70
0.70
6.65
7.60
7.93
aThe concentration of the dyes was maintained at 0.5 mM in
2-methyl-2-propanol-acetonitrile (1:1, v/v) solution, with 0.5 mM
deoxycholic acid (DCA) as a co-adsorbent. Performances of DSSCs
were measured with a 0.125 cm2 working area. Irradiating light: AM
1.5 G (100 mW cm-2).
FIGURE 8. EIS Bode plots (i.e., the phase of the impedance vs the
frequency) for DSSCs based on TPCADTS and TP6CADTS dyes.
related to the charge recombination rate, and its reciprocal is
associated with the electron lifetime.35,36 As can be seen in
Figure 8, the lower frequency peak of the TP6CADTS cell
shifts to a lower frequency (compared to the TPCADTS cell),
indicating that the electron lifetime was effectively prolonged
by the two longer hexyloxy chains near the TiO2/dye/electro-
lyte interface. EIS results of Figures 7 and 8 clearly suggest
that the two longer hexyloxy chains of TP6CADTS act as
effective blocking units between TiO2 surface and electrolyte,
preventing hydrophilic I3- ions approaching the TiO2 surface
and thereby leading to suppressed charge recombination/dark
current and lengthened electron lifetimes.11c,d,37 These factors
may in turn lead to the higher Voc (50 mV higher) of the
TP6CADTS cell than that of the TPCADTS cell.
FIGURE 7. EIS Nyquist plots (i.e., minus imaginary part of the
impedance -Z00 vs the real part of the impedance Z0 when sweeping
the frequency) for DSSCs based on TPCADTS and TP6CADTS dyes.
impedance spectroscopy was carried out by subjecting the
cell to the constant AM 1.5G 100 mW/cm2 illumination and
to the bias at the open-circuit voltage Voc of the cell (namely,
under the conditions of no dc electric current),34 for better
manifesting the process at the TiO2/electrolyte interface of
a cell under operation. Figure 7 shows the EIS Nyquist plots
(i.e., minus imaginary part of the impedance -Z00 vs the real
part of the impedance Z0 when sweeping the frequency) for
DSSCs based on TPCADTS and TP6CADTS dyes. For the
frequency range investigated (20 Hz to 1 MHz), in both cells,
a larger semicircle occurs in the lower frequency range (∼20 Hz
to 1 kHz) and a smaller semicircle occurs in the higher fre-
quency range. With the bias illumination and voltage applied,
the larger semicircle at lower frequencies corresponds to the
charge-transfer processes at the TiO2/dye/electrolyte inter-
face, while the smaller semicircle at higher frequencies corres-
ponds to the charge-transfer processes at the Pt/electrolyte
interface. The two cells hardly show a difference in smaller
semicircles at higher frequencies, but the difference in larger
semicircles at lower frequencies is significant. The larger width
of the lower frequency semicircle of the TP6CADTS cell
indicates a larger charge-transfer resistance at the TiO2/dye/
electrolyte interface.
Figure 8 shows the EIS Bode plots (i.e., the phase of the
impedance vs the frequency) of the DSSCs based on TPCADTS
and TP6CADTS dyes. For the frequency range investigated,
both EIS Bode plots also exhibit two peak features; the one at
higher frequencies corresponds to the charge transfer at the
Pt/electrolyte interface and the one at lower frequencies corres-
ponds to the charge transfer at the TiO2/dye/electrolyte inter-
face. Again, the difference in bode plots of both cells mainly
occurs in the lower frequency range. The characteristic
frequency of the lower frequency peak in the Bode plot is
Conclusions
In summary, we had successfully designed and synthesized
two new organic sensitizers (TPCADTS and TP6CADTS)
containing coplanar diphenyl-substituted dithienosilole as the
centrallinkagefor high-performance dye-sensitizedsolarcells.
By incorporating the diphenyl-substituted DTS core, these
two dyes exhibited the enhanced light-capturing abilities and
suppressed dye aggregation. These two dyes had been em-
ployed as the organic sensitizers in DSSCs. A solar-cell device
based on the sensitizer TP6CADTS yielded a higher overall
conversionefficiency up to 7.6%, reaching∼96%of theruthe-
nium dye N719-based reference cell under the same condi-
tions. The introduction of aliphatic hexyloxy chains on the
donor (e.g., TP6CADTS) appears to be beneficial to suppres-
sion of electronic coupling and charge recombination, etc. at
the TiO2/dye/electrolyte interface, which could contribute to
higher Voc and IPCE in DSSCs. The results disclosed in this
paper are believed to be useful for further development of
other low-cost metal-free organic dyes by different molecular
and energy-level engineering processes.
Experimental Section
Synthesis of the Dyes. General Methods. All chemicals and
reagents were used as received from commercial sources without
(35) Adachi, M.; Sakamoto, M.; Jiu, J.; Ogata, Y.; Isoda, S. J. Phys.
Chem. B 2006, 110, 13872.
€
(36) Wang, Q.; Moser, J.; Gratzel, M. J. Phys. Chem. B 2005, 109, 14945.
(37) Kim, S.; Kim, D.; Choi, H.; Kang, M. S.; Song, K.; Kang, S. O.; Ko,
J. Chem. Commun. 2008, 4951.
J. Org. Chem. Vol. 75, No. 14, 2010 4783

Lin et al.
JOCArticle
purification. Solvents for chemical synthesis were purified by
distillation. All chemical reactions were carried out under an
argon or nitrogen atmosphere.
6.99 (d, J = 8.8 Hz, 4H), 6.87 (d, J = 8.8 Hz, 4H), 6.75 (d, J =
8.4 Hz, 2H), 3.89 (t, J = 6.2 Hz, 4H), 1.67 (m, 4H), 1.38 (m, 4H),
1.29 (m, 8H), 0.86 (t, J= 6.2 Hz, 6H); 13CNMR(CD2Cl2, 100 MHz)
δ 156.1, 151.0, 148.7, 148.2, 146.8, 141.6, 140.6, 139.3, 135.7, 132.2,
130.7, 130.0, 128.6, 127.2, 126.6, 126.4, 126.3, 124.4, 120.4, 115.7,
68.8, 32.2, 29.9, 26.4, 23.3, 14.5; HRMS (m/z, FABþ) calcd for
C50H51NO2S2Si 789.3130, found 789.3133.
5-Bromo-50-trimethylsilyl-3,30-diphenylsilylene-2,20-bithiophene
(2). To a stirring solution of 1 (13.55 g, 27.60 mmol) in N,N-
dimethylformamide (275 mL) was dropwise added N-bromosucci-
nimide (5.86 g, 33.12 mmol) in N,N-dimethylformamide (275 mL)
at 0 °C under exclusion of light, and the reaction mixture was then
warmed to room temperature. After being stirred for 12 h, the re-
action mixture was poured into water and extracted with diethyl
ether, and the combined extracts were washed with brine, dried
over anhydrous magnesium sulfate, and filtered. The solvent was
removed by rotary evaporation. The crude product was dissolved
in hexane, and the undissolved solid was filtered. The filtrate was
evaporated to dryness, and the resulting solid was washed with
ethanol to afford 2 as a white solid (9.04 g, 66% yield): mp
5-[N,N-Bis(phenylamino)phenyl]-50-formyl-3,30-diphenylsilylene-
2,20-bithiophene (6a). To a stirring solution of 5a (826 mg, 1.4
mmol) in dry N,N-dimethylformamide (30 mL) was dropwise
added phosphorus oxychloride (0.22 mL, 2.38 mmol) at -40 °C
under argon. The reaction mixture was then heated to 60 °C and
stirred for 4 h. After being cooled to room temperature, the
reaction mixture was quenched with 10% sodium acetate aqueous
solution (210 mL) and extracted with ethyl acetate, and the
combined extracts were washed with brine, dried over anhydrous
magnesium sulfate, and filtered. The solvent was removed by
rotary evaporation, and the crude product was purified by column
chromatography on silica gel with ethyl acetate/hexane (v/v, 1:10)
as eluent to afford 6a as a red solid (553 mg, 64% yield): mp
235-237 °C; IR (KBr) ν 3031, 2809, 1657, 1589, 1487, 1115, 825
cm-1; 1H NMR (CDCl3, 400 MHz) δ 9.87 (s, 1H), 7.86 (s, 1H),
7.65 (dd, J = 8.0, 1.2 Hz, 4H), 7.49-7.45 (m, 4H), 7.41-7.37 (m,
5H), 7.30-7.26 (m, 4H), 7.12 (dd, J = 8.4, 0.8 Hz, 4H), 7.08-7.04
(m, 4H);13C NMR (CDCl3, 100 MHz) δ 182.2, 159.7, 149.9, 147.8,
147.1, 146.7, 145.8, 144.7, 139.9, 139.4, 135.2, 130.7, 130.2, 129.2,
128.3, 127.3, 126.7, 124.9, 124.6, 123.3, 123.1; HRMS (m/z, FABþ)
calcd for C39H27NOS2Si 617.1303, found 617.1304.
5-[N,N-Bis(4-hexyloxyphenylamino)phenyl]-50-formyl-3,30-di-
phenylsilylene-2,20-bithiophene (6b). To a stirring solution of 5b
(2 g, 2.53 mmol) in dry N,N-dimethylformamide (54 mL) was
dropwise added phosphorus oxychloride (0.4 mL, 4.3 mmol) at
-40 °C under argon. The reaction mixture was then heated to
60 °C and stirred for 3 h. After being cooled to room temperature,
the reaction mixture was quenched with 10% sodium acetate
aqueous solution (380 mL) and extracted with ethyl acetate, and
the combined extracts were washed with brine, dried over anhy-
drous magnesium sulfate, and filtered. The solvent was removed
by rotary evaporation, and the crude product was purified by
column chromatography on silica gel with ethyl acetate/hexane (v/
v, 1:10) as eluent to afford 6b as a red solid (1.62 g, 78% yield): mp
65-67 °C; IR (KBr) ν 3061, 2953, 2856, 2706, 1664, 1599, 1506,
1237, 1139, 825 cm-1; 1H NMR (DMSO-d6, 400 MHz) δ 9.87 (s,
1H), 8.40 (s, 1H), 7.86 (s, 1H), 7.70 (d, J = 7.2 Hz, 4H), 7.55-7.41
(m,8H), 7.03(d, J= 8.8 Hz, 4H), 6.90 (d, J= 8.8Hz, 4H), 6.74 (d,
J = 8.4 Hz, 2H), 3.92 (t, J = 6.8 Hz, 4H), 1.69 (m, 4H), 1.39 (m,
4H), 1.30 (m, 8H), 0.87 (t, J = 6.8 Hz, 6H); 13C NMR (CD2Cl2,
100 MHz) δ 182.6, 159.9, 156.4, 150.9, 149.4, 146.5, 146.4, 145.2,
140.3, 140.2, 139.8, 135.7, 131.1, 130.9, 128.7, 127.5, 126.9, 125.5,
124.7, 119.9, 115.7, 68.8, 32.2, 29.9, 26.3, 23.3, 14.5; HRMS (m/z,
FABþ) calcd for C51H51NO3S2Si 817.3080, found 817.3077.
3-[5-[N,N-Bis(phenylamino)phenyl]-3,30-diphenylsilylene-2,20-
bithiophene-50-yl]-2-cyanoacrylic Acid (TPCADTS). A mixture
of 6a (370 mg, 0.6 mmol), cyanoacetic acid (153 mg, 1.8 mmol),
and ammonium acetate (39 mg) in glacial acetic acid (13 mL)
was stirred and heated at 120 °C for 12 h. After the mixture was
cooled to room temperature, the resulting precipitate was collec-
ted by filtration and thoroughly washed with water, methanol,
and diethyl ether to afford TPCADTS as a black solid (350 mg,
85% yield): mp 294-296 °C; IR (KBr) ν 3056, 2217, 1718, 1682,
1592, 1429, 824 cm-1; 1H NMR (DMSO-d6, 400 MHz) δ 8.46 (s,
1H), 8.32 (s, 1H), 7.96 (s, 1H), 7.69-7.64 (m, 6H), 7.52-7.48 (m,
2H), 7.46-7.42 (m, 4H), 7.35-7.31 (m, 4H), 7.11-7.05 (m, 6H),
6.97 (d, J = 8.8 Hz, 2H); 13C NMR (DMSO-d6, 100 MHz) δ
163.8, 158.8, 149.8, 147.5, 146.6, 146.4, 145.5, 143.4, 139.8,
137.3, 135.0, 130.9, 129.9, 129.7, 128.5, 126.9, 126.6, 126.3,
124.6, 123.7, 122.5, 116.9; HRMS (m/z, FABþ) calcd for
C42H28N2O2S2Si 684.1361, found 684.1357.
140-142 °C; IR (KBr) ν 3065, 2950, 1587, 1427, 1245, 995 cm-1
;
1H NMR (CDCl3, 400 MHz) δ 7.63 (dt, J = 6.4, 1.6 Hz, 4H),
7.45-7.36 (m, 6H), 7.31 (s, 1H), 7.18 (s, 1H), 0.36 (s, 9H); 13C
NMR (CDCl3, 100 MHz) δ 154.8, 150.6, 142.5, 140.3, 136.1,
135.2, 132.1, 131.1, 130.3, 128.1, 111.9, 0.3; HRMS (m/z, FABþ)
calcd for C23H21BrS2Si2 495.9807, found 495.9803.
5-Bromo-3,30-diphenylsilylene-2,20-bithiophene (3). To a stir-
ring solution of 2 (3.30 g, 6.63 mmol) in chloroform (165 mL)
was dropwise added trifluoroacetic acid (0.98 mL, 13.26 mmol)
at room temperature under exclusion of light. After being stirred
for 12 h, the reaction mixture was poured into water and extrac-
ted with chloroform, and the combined extracts were washed
with brine, dried over anhydrous magnesium sulfate, and fil-
tered. The solvent was removed by rotary evaporation to afford
3 as a light yellow solid (2.75 g, 98% yield): mp 173-175 °C; IR
(KBr) ν 3090, 3018, 1586, 1427, 1250, 943 cm-1 1H NMR
;
(CDCl3, 400 MHz) δ 7.64 (d, J = 6.8 Hz, 4H), 7.47-7.37 (m,
6H), 7.30 (d, J = 4.8 Hz, 1H), 7.24 (d, J = 4.8 Hz, 1H), 7.20 (s,
1H); 13C NMR (CDCl3, 100 MHz) δ 150.5, 149.6, 139.9, 138.7,
135.2, 132.0, 130.9, 130.4, 129.5, 128.1, 126.2, 111.8; HRMS (m/z,
FABþ) calcd for C20H13BrS2Si 423.9411, found 423.9416.
5-[N,N-Bis(phenylamino)phenyl]-3,30-diphenylsilylene-2,20-
bithiophene (5a). A mixture of 3 (2.33 g, 5.48 mmol), 4a (1.90 g,
6.58 mmol), tetrakis(triphenylphosphine)palladium(0) (316 mg,
0.27 mmol), and potassium carbonate (2 M aqueous solution,
13.7 mL, 27.40 mmol) in toluene (270 mL) was stirred and heated
to reflux under argon for 2.5 h. The reaction mixture was poured
into water and extracted with dichloromethane, and the com-
bined extracts were washed with brine, dried over anhydrous
magnesium sulfate, and filtered. The solvent was removed by
rotary evaporation, and the crude product was purified by
column chromatography on silica gel with dichloromethane/
hexane (v/v, 1:5) as eluent to afford 5a as a yellow solid (2.68 g,
83% yield): mp 230-232 °C; IR (KBr) ν 3059, 3023, 1590, 1494,
1273, 826 cm-1; 1H NMR (CDCl3, 400 MHz) δ 7.67 (dt, J = 6.4,
1.6 Hz, 4H), 7.48-7.22 (m, 15 H), 7.12 (dd, J = 8.4, 1.2 Hz, 4H),
7.07-7.02 (m, 4H); 13C NMR (CDCl3, 100 MHz) δ 150.5, 148.4,
147.3, 146.9, 145.7, 141.1, 139.0, 135.3, 131.6, 130.2, 129.5, 129.2,
128.4, 128.1, 126.3, 125.8, 124.6, 124.3, 123.6, 122.9; HRMS (m/z,
FABþ) calcd for C38H27NS2Si 589.1354, found 589.1349.
5-[N,N-Bis(4-hexyloxyphenylamino)phenyl]-3,30-diphenylsily-
lene-2,20-bithiophene (5b). A mixture of 3 (2.94 g, 6.91 mmol), 4b
(7.61 g, 10.37 mmol), tetrakis(triphenylphosphine)palladium(0)
(404 mg, 0.35 mmol) in toluene (35 mL) was stirred and heated at
100 °C under argon for 24 h. After the mixture was cooled to
room temperature, the solvent was removed by rotary evapora-
tion, and the crude product was purified by column chromato-
graphy on silica gel with dichloromethane/hexane (v/v, 1:3) as
eluent to afford 5b as a yellow solid (3.32 g, 61% yield): mp
55-57 °C; IR (KBr) ν 3063, 2955, 2853, 1601, 1505, 1475, 1011,
824 cm-1; 1H NMR (DMSO-d6, 400 MHz) δ 7.71 (s, 1H), 7.66
(d, J = 6.8 Hz, 4H), 7.56 (d, J = 4.4 Hz, 1H), 7.50-7.37 (m, 9H),
4784 J. Org. Chem. Vol. 75, No. 14, 2010

Lin et al.
3-[5-[N,N-Bis(4-hexyloxyphenylamino)phenyl]-3,30-diphenylsily-
JOCArticle
dye-adsorbed TiO2 working electrode and a counter electrode
were then assembled into a sealed DSSC cell with a sealant spacer
between the two electrode plates. A drop of electrolyte solution
[0.6 M 1-butyl-3-methylimidazolium iodide (BMII), 0.05 M LiI,
0.03 M I2, 0.5 M 4-tert-butylpyridine, and 0.1 M guanidinium
thiocyanate in a mixture of acetonitrile-valeronitrile (85:15, v/v)]
was injected into the cell through a drilled hole. Finally, the hole
was sealed using the sealant and a cover glass. A mask with an
aperture area of 0.125 cm2 was covered on a testing cell during
photocurrent-voltage and incident photon-to-current conver-
sion efficiency measurements.
Photocurrent-Voltage Measurement. The photocurrent-
voltage characteristics of the DSSCs were measured under
illumination of AM1.5G solar light from a 300-W xenon lamp
solar simulator. The incident light intensity was calibrated as
100 mW/cm2. Photocurrent-voltage curves were obtained by
applying an external bias voltage to the cell and measuring the
generated photocurrent.
lene-2,20-bithiophene-50-yl]-2-cyanoacrylic Acid (TP6CADTS). A
mixture of 6b (327 mg, 0.4 mmol), cyanoacetic acid (102 mg, 1.2
mmol), and ammonium acetate (26 mg) in glacial acetic acid (9 mL)
was stirred and heated at 120 °C for 12 h. After being cooled to
room temperature, the resulting precipitate was collected by filtra-
tion and thoroughly washed with water, methanol, and hexane to
afford TP6CADTS as a black solid (305 mg, 86% yield): mp
244-246 °C; IR (KBr) ν 3062, 2930, 2854, 2215, 1719, 1682, 1565,
1373, 822 cm-1; 1H NMR (DMSO-d6, 400 MHz) δ 8.46 (s, 1H),
8.31 (s, 1H), 7.87 (s, 1H), 7.67 (d, J = 7.2 Hz, 4H), 7.56-7.42 (m,
8H), 7.03(d, J= 8.8 Hz, 4H), 6.90 (d, J= 8.8 Hz, 4H), 6.75 (d, J=
8.4 Hz, 2H), 3.92 (t, J = 6.8 Hz, 4H), 1.69 (m, 4H), 1.40 (m,
4H), 1.30 (m, 8H), 0.88 (t, J = 6.8 Hz, 6H); 13C NMR (DMSO-d6,
100 MHz) δ 164.5, 156.3, 151.2, 149.4, 147.2, 145.5, 140.3, 139.9,
137.8, 135.7, 131.6, 130.7, 129.2, 127.8, 127.4, 126.2, 124.9, 119.3,
116.2, 68.3, 31.7, 29.4, 25.9, 22.7, 14.6; HRMS (m/z, FABþ) calcd
for C54H52N2O4S2Si 884.3138, found 884.3146.
Theoretical Calculation. Density functional theory (DFT)
calculations were conducted by using the B3LYP hybrid func-
tional for the geometry optimizations. The molecular orbital
levels of HOMO and LUMO were achieved with the 6-31G(d)
basis set implemented in the Gaussian 03 package.
Measurement of Absorption Spectra of Dye-Loaded Nanoporous
TiO2 Films. A 7 μm thick transparent porous TiO2 nanoparticle
layer (adopting 20 nm anatase TiO2 nanoparticles) was coated on
a glass plate by the doctor-blade method. After being sintered at
500 °C for 30 min, the TiO2 film was immersed into the dye
solutions of 0.5 mM dye in acetonitrile and 2-methyl-2-propanol
(volume ratio 1:1) at room temperature for 24 h. The UV-vis
absorption spectrum of the dye-loaded TiO2 film was then
recorded on a spectrophotometer.
Fabrication of Dye-Sensitized Solar Cells. To prepare the
DSSC working electrodes, the FTO glass plates were first
cleaned in a detergent solution using an ultrasonic bath for 15
min and then rinsed with water and ethanol. A layer of 20 nm
anatase TiO2 nanoparticles for the transparent nanocrystalline
layer was first coated on the FTO glass plates by the doctor-
blade method. After the film was dried at 120 °C, another layer
of 400 nm anatase TiO2 nanoparticles was then deposited as the
light-scattering layer of the DSSC. The resulting working elec-
trode was composed of a 7 μm thick transparent TiO2 nano-
particle layer (particle size: 20 nm) and a 5 μm thick TiO2
scattering layer (particle size: 400 nm). The nanoporous TiO2
electrodes were then sequentially heated at 150 °C for 10 min,
at 300 °C for 10 min, at 400 °C for 10 min, and finally, at 500 °C
for 30 min. After cooling, the nanoporous TiO2 electrodes were
immersed into the dye solutions of 0.5 mM dye and 0.5 mM
deoxycholic acid (DCA) in acetonitrile and 2-methyl-2-propanol
(volume ratio 1:1) at room temperature for 24 h. Counterelec-
trodes of the DSSC were prepared by depositing 40 nm thick
Pt films on the FTO glass plates by e-beam evaporation. The
Incident Monochromatic Photon-to-Current Conversion Effi-
ciency Measurement. The incident monochromatic photon-to-
current conversion efficiency (IPCE) spectra were measured by
using a 75-W xenon lamp as the light source coupled to a
monochromator.38 The IPCE data were taken by illuminating
monochromatic light on the solar cells (with a wavelength
sampling interval of 10 nm from 350 to 700 nm) and measuring
the short-circuit current of the solar cells. The IPCE measure-
ment was performed with a lock-in amplifier, a low-peed
chopper, and a bias light source under full computer control.
Electrochemical Impedance Spectroscopy Measurement. The
electrochemical impedance spectroscopy (EIS) of the cells was
measured by using an impedance analyzer with a frequency
range of 20 Hz to 1 MHz. In this study, during the impedance
measurement, the cell was under the constant AM 1.5G 100 mW/
cm2 illumination. The impedance of the cell (throughout the
frequency range of 20 Hz to 1 MHz) was then measured by
applying a bias at the open-circuit voltage Voc of the cell (namely,
under the condition of no dc electric current) and by using an ac
amplitude of 10 mV.
Acknowledgment. This work was financially supported by
the National Science Council of Taiwan.
Supporting Information Available: 1H and 13C NMR spec-
tra for all new compounds. This material is available free of
charge via the Internet at http://pubs.acs.org.
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