Dipolar organic pyridyl dyes for dye-sensitized solar cell applications

Article abstract of DOI:10.1016/j.tet.2011.10.038

New dipolar dyes containing arylamine as the electron donor, 2-cyanoacrylic acid as the acceptor, and a conjugated spacer with incorporation of 2,5-pyridyl entity have been synthesized. Photophysical and electrochemical measurements, and theoretical compu

Full text of DOI:10.1016/j.tet.2011.10.038

Tetrahedron 68 (2012) 767e773
Contents lists available at SciVerse ScienceDirect
Tetrahedron
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Dipolar organic pyridyl dyes for dye-sensitized solar cell applications
,
b,
*
Hsien-Hsin Chou a, Chih-Yu Hsu a, Ying-Chang Hsu a, Yu-Sheng Lin b, Jiann T. Lin a , Chiitang Tsai
*
a Institute of Chemistry, Academia Sinica, No. 128, Sec. 2, Academia Road, Taipei 115, Taiwan
b Department of Chemistry, Chinese Culture University, Taipei 320, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 June 2011
Received in revised form 22 September 2011
Accepted 11 October 2011
Available online 13 November 2011
New dipolar dyes containing arylamine as the electron donor, 2-cyanoacrylic acid as the acceptor, and
a conjugated spacer with incorporation of 2,5-pyridyl entity have been synthesized. Photophysical and
electrochemical measurements, and theoretical computation were carried on these dyes. The solar cell
devices using these dyes as the sensitizers exhibited light-to-electricity efficiencies in the range of
4.28e5.27%, which reaches 60e72% of N719-based device fabricated and measured under similar con-
ditions. Better DSSC performance can be achieved with the dye where pyridine group is attached to
thienyl or fluorenyl group because of favorable resonance energy and/or coplanarity for more effective
charge transfer.
Keywords:
Dye-sensitized solar cells
Pyridine
Dipolar compounds
Metal-free dyes
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
performances. Dyes having 4,40-bis(hexyloxylphenyl)amino-
benzene as the donor, 2-cyanoacrylic acid as the acceptor and
Since the beginning of the 21st century, depletion of fossil fuel
reserve and global warming have become important worldwide
issues, which boost up tremendous growth of investigations on
environmental friendly new energy sources as well as more effi-
cient energy conversion and storage. Solar energy is very attractive
because of its environmental benign and endless supply. Un-
fortunately, up to date the crystalline and thin-film silicon-based
photovoltaics still suffer from major drawback, that is, request of
high purity materials and therefore accompanying with high cost.
oliogothiophenes as the conjugated spacer has the conversion ef-
ficiency at w10%, which is the best performance ever reported.16
We have been interested in pushepull type17 sensitizers using
arylamine as the electron donor18 and 2-cyanoacrylic acid as the
electron acceptor. In one of our reports, dipolar organic dyes with
a fluorene and a heteroaromatic ring in the conjugated spacer
exhibited high cell efficiencies reaching w95% of N719-based
(N719¼bis(tetrabutyl-ammonium)-cis-di(thiocyanato)-N,N0-bis(4-
carboxylato-40-carboxylic acid-2,20-bipyridine)ruthenium(II)) stan-
dard cell.19 However, these sensitizers have relatively short absorp-
tion wavelengths (w420 nm). In comparison, sensitizers with
implanted benzothiadiazole or benzoselenadiazole in the spacer
were found to have much longer absorption wavelengths.8a,20aec
Nevertheless, the dyes developed by us have the cell efficiency
reaching only ꢀ71% of N3-based device.8a Sensitizers with electron-
withdrawing cyanovinyl unit in the conjugated spacer also have
long absorption wavelengths (up to 578 nm), but relatively lower
efficiencies reaching only ꢀ69% of N719-based standard cell.20d
Density functional calculations indicated that charge trapping oc-
curred at the cyanovinyl group upon vertical excitation, which
hampered efficient electron injection from the sensitizer to the TiO2
nanoparticles. Wethereforesearchedforotherelectron-withdrawing
entityasthebuildingblockofthespacerinthehopeofredshiftingthe
absorption spectra without charge trapping. We then turned our at-
tention to pyridine, which is more electron deficient than thiophene
and benzene, and has resonance energy (32 kcal molꢁ1) lying be-
tween the two (thiophene: 29; benzene: 36 kcal molꢁ1).21 To our
knowledge, there was only two reports on metal-free pyridine-based
Dye-sensitized solar cells (DSSCs) are considered to be attractive
1
€
alternative of low cost solar cells since Gratzel’s pioneer work. An
ideal molecule to be used as the sensitizer for DSSCs requires
a larger electronic absorption coefficient, broad spectral coverage
even to near infrared region and good stability.2 At present, the
solar cell device fabricated with ruthenium-based polypyridine
complexes records the highest conversion efficiency above 11%.3
Nevertheless, the rather expensive cost of transition metal cou-
pled with lower absorption coefficient due to MLCT nature push
researchers searching for metal-free dyes as alternatives. Couma-
rin-,4 indoline-,5 cyanine-,6 hemicyanine-,7 triphenylamine-,8 tet-
rahydroquinoline-,9 truxene-,10 polyene-,11 thienothiophene-,12
oligothiophene-,13 furan-,14 and pyrrole-based15 metal-free dyes
have been reported for various purposes to enlarge cell
* Corresponding authors. Tel.: þ886 2 27898522; fax: þ886 2 27831237; e-mail
addresses: hhchou@chem.sinica.edu.tw (H.-H. Chou), jtlin@chem.sinica.edu.tw (J.T.
Lin).
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.10.038

768
H.-H. Chou et al. / Tetrahedron 68 (2012) 767e773
sensitizers.22 In this paper, we report new series of dipolar metal-free
compounds and DSSCs based on these sensitizers.
2. Results and discussions
2.1. Synthesis of the materials
New dipolar dyes shown in Fig. 1 were prepared in moderate to
good yields. Scheme 1 illustrates the synthetic protocol of the new
compounds. Suzuki coupling23 of the arylamino-substituted aryl-
boronic acids and 2-bromo-5-pyridylaldehyde to give the dipolar
aldehyde compounds 1ae4a in moderate yields. Similar coupling of
the arylamino-substituted arylboronic acid and 2-bromothiazole
followed by formylation of thiazole derivative gave aldehyde de-
rivative 5a. Finally, condensation of cyanoacetic acid and 1ae5a
with cyanoacetic acid yielded the desired products Y1eY5. The
dyes are colored in intense red and stable both in solid form and in
solution.
NC
NC
CO2H
CO2H
N
N
N
N
N
N
Y1
Y2
NC
NC
CO2H
CO2H
CO2H
S
N
N
Y3
Y4
NC
NC
CO2H
S
S
Fig. 2. Absorption (a) and emission (b) spectra of the dyes recorded in THF.
N
N
N
N
Y5
1P-PPS
attributable to localized
from 382 to 436 nm is attributed to the superposition of more delo-
calized * and charge-transfer transitions. The lmax value of dye Y3
pep* transition; another band at the region
pep
S
(416 nm) is greater than that of Y2 (382 nm), indicating that the large
twist angle between the two internal phenyl groups in Y2 deteriorates
S
CO2H
NC
effective
p-conjugation. Y1 has a similar absorption wavelength
DPAPTT
(415 nm) as Y3 despite of its shorterconjugation spacer. Clearly, Y1 has
more charge-transfer character than Y3 (see computation results).
Fig. 1. The structures of the dyes.
NC
(b)
CO2H
(a)
O
Ar
N
=
Ar
B(OH)2
Ar
1a-4a
N
N
N
N
H
Y1
Y2
Y3
Y4
Ar
S
NC
CO2H
(c) then (d)
O
H
(b)
S
S
N
N
B(OH)2
N
N
N
Y5
5a
Scheme 1. (a) 2-Bromo-5-pyridylaldehyde, cat. Pd(PPh3)4, Na2CO3, EtOH, toluene, 85 ꢂC (b) NCCH2CO2H, cat. piperidine, CH3CN, 80 ꢂC (c) 2-bromothiazole, cat. Pd(PPh3)4, Na2CO3,
toluene, EtOH, 85 ꢂC (d) i. n-BuLi, THF, ꢁ78 ꢂC; ii. DMF.
2.2. Photophysical properties
Furtherredshiftoftheabsorptionspectracanbeachievedbyreplacing
the internal phenyl unit with a thienyl group, as in Y4 (436 nm). Such
an outcome may be attributed to the smaller resonance energy of
thiophene than benzene ring, which facilitates the charge transfer
from the donor to the acceptor. It is interesting to note that re-
placement of the pyridine ring in Y2 with the more electron-deficient
Representative absorption and emission spectra of the dyes in THF
are displayed in Fig. 2 andthedataarecompiled inTable 1. As shown in
Fig. 2a, the dyes exhibit two intense absorption bands in UVevis re-
gion uponphotoexcitation: the absorption band lies at 300e312 nm is

H.-H. Chou et al. / Tetrahedron 68 (2012) 767e773
769
Table 1
Electrooptical parameters of the dyesa and performance parameters of DSSCs fabricated with these dyese
labs (10ꢁ4
3 ) [nm]
lem
nst
[cmꢁ1
E
(
D
Ep)b [mV]
HOMO/LUMO
[eV]
E0e0
[eV]
E0e0
[V]
*
VOC
[V]
JSC
h
[%]
FF
Rct
Dye loading
c
d
f
ox
1=2
[nm]
]
[mAcm2]
[U
]
[10ꢁ7 mol cmꢁ2
]
Y1
Y2
Y3
Y4
300 (2.27), 415 (2.39)
308 (2.64), 382 (2.67)
312 (2.58), 416 (2.89)
304 (1.47), 345 (1.27),
436 (2.51)
519
539
541
553
4829
7625
5554
4853
611 (233)
544 (205)
505 (113)
518 (126)
5.81/3.38
5.74/3.08
5.71/3.24
5.72/3.42
2.43
2.66
2.47
2.30
ꢁ1.12
ꢁ1.42
ꢁ1.26
ꢁ1.08
0.67
0.65
0.66
0.63
10.20
9.72
10.71
12.33
4.67
4.28
4.87
5.27
0.68
0.68
0.69
0.68
15.9
17.3
16.5
9.8
3.18
4.90
7.59
5.29
Y5
N719
308 (1.82), 413 (2.50)
542
5763
575 (160)
5.78/3.31
2.47
ꢁ1.19
0.68
0.70
9.90
15.08
4.61
7.18
0.68
0.68
11.6
d
3.78
d
a
All experiments were conducted in THF. labs: absorption wavelength;
3
: extinction coefficient; lem: emission wavelength; nst: Stokes shift.
b
Scan rate, 100 mV/s; electrolyte, (n-Bu)4NPF6;
DEp is the separation between the anodic and cathodic peaks. Potentials are quoted with reference to the internal ferrocene
ox
standard (E ¼þ212 mV vs Ag/AgNO3). The HOMO and LUMO energies are calculated using formula HOMO¼5.2þ(E1/2ꢁEFc) and LUMO¼HOMOꢁEgap, where 5.2 refers to
1=2
energy level of ferrocene in vacuo.
c
The optical bandgap (E0e0) defines the onset value of lowest-energy absorption peak.
d
The bandgap E0e0* defines the reduction potential corresponding to NHE using the formula E0e0*¼LUMOꢁ4.5 V.
e
Experiments were conducted using TiO2 photoelectrodes with approximately 18 m U/sq.) substrates.
m thickness and 0.25 cm2 working area on the FTO (15
f
Charge transfer resistances (Rct) were obtained from the radius of the intermediate frequency semicircle in Fig. 6.
thiazole ring, Y5, results in red shift of lmax, albeit with smaller ex-
ground-state geometry. The dye Y1 emits at the shortest wave-
length (519 nm) because of its shortest spacer. In comparison,
compound Y4 has longer emission wavelength than Y1eY3, in
conformity with its longest effective conjugation length.
tinction coefficient: Y2 (lmax¼382 nm;
3
¼2.67ꢃ10ꢁ4 Mꢁ1 cmꢁ1) ver-
sus Y5 (lmax¼413 nm;
3
¼2.50ꢃ10ꢁ4 Mꢁ1 cmꢁ1). In comparison,
replacement of the pyridine ring in Y2 with the more electron-rich
thiophene ring, 1P-PPS (Fig. 1),24 comparable lmax and larger extinc-
tion coefficient was observed: Y2 (lmax¼382 nm;
3
¼2.67ꢃ10ꢁ4
2.3. Electrochemical properties
M
ꢁ1 cmꢁ1) versus 1P-PPS (lmax¼380 nm,
3
¼3.02ꢃ10ꢁ4 Mꢁ1 cmꢁ1). In
contrast, substitution of the pyridine ring in Y4 (lmax¼436 nm,
The electrochemical data of the dyes obtained from cyclic vol-
tammetry (CV) are listed in Table 1 and the differential pulse vol-
tammograms (DPV) are displayed in Fig. 4. All of these dyes exhibit
a quasi-reversible one-electron redox process at ca. 500e600 mV,
which corresponds to the oxidation of the amino group. The oxi-
dation potentials increase in the order of Y3 (505 mV)<Y4
(518 mV)<Y2(544 mV)<Y5 (575 mV)<Y1 (611 mV). Y1 has the
largest Eox value because its shortest conjugation spacer allows
stronger electronic communication between the arylamine and
electron-withdrawing 2-cyanoacrylic acid and pyridine. As the
spacer becomes longer, there is diminished influence of the ac-
ceptor. Consequently, the aromatic segment nearby the arylamine
will have more significant influence, and compounds with more
electron-donating fluorenyl (Y3) or thienyl (Y4) groups appear to
have smaller Eox than other compounds. As expected, Y5, which
contains an electron-deficient thiazolyl entity is more difficult to be
oxidized than Y2.
3
¼2.51ꢃ10ꢁ4 Mꢁ1 cmꢁ1) bya thiophene ring,1P-PSS (Fig.1),24 leadsto
red shift of absorption as well as larger extinction coefficient
(lmax¼461 nm,
3
¼2.71ꢃ10ꢁ4 Mꢁ1 cmꢁ1). These observations pre-
sumably stem from higher resonance energy (32 kcal molꢁ1) and/or
less coplanarity of the pyridine ring with the neighboring aromatic
ring in the spacer, which hamper the charge transfer capability.
The absorption spectra of these compounds exhibit a negative
solvatochromism, and representative spectra are displayed in Fig. 3
for Y4. The lmax values for Y4 are in the order of dioxane (456 nm)>
THF (433 nm)>MeOH (429 nm)>acetone (426 nm)>DMF
(418 nm)>MeCN (412 nm). This can be explained by the stronger
interaction between the OeH bond of the carboxylic acid with polar
solvent molecules, which weakens the OeH bond and decreases
the electron-withdrawing nature of the carboxylic group. Similar
behavior was reported for metal-free dipolar compounds by us13b
and others.25 The compounds emit weakly in the range of
519e553 nm. There is large Stokes shift of the emission ranging
from 4829 to 5763 cmꢁ1 (Fig. 2b), suggesting that there exists
prominent geometrical change between the emission and the
ground states in these compounds. Compound Y2 has the largest
Stokes shift (7625 cmꢁ1) among all due to relatively twisted
1.2
dioxane
acetone
1.0
MeCN
DMF
0.8
MeOH
THF
0.6
0.4
0.2
0.0
Fig. 4. Differential pulse voltammograms of the compounds recorded in THF.
2.4. Photocurrentevoltage characteristics
400
500
600
The solar cells, with an effective area of 0.25 cm2, were fabricated
with nanocrystalline anatase TiO2 and sensitized using the organic
pyridyl dyes. The electrolyte is composed of 0.05 M I2/0.5 M LiI/0.5 M
Wavelength (nm)
Fig. 3. Absorption spectra of the dye Y4 recorded in different solvents.

770
H.-H. Chou et al. / Tetrahedron 68 (2012) 767e773
4-tert-butylpyridine in acetonitrile solution. The device perfor-
mance statistics under AM 1.5 illumination are collected in Table 1.
Fig. 5 shows both the photocurrentevoltage (JeV) curves and the
incident photon-to-current conversion efficiencies (IPCE) of the
cells, respectively. All devices fabricated with these dyes exhibited
higher quantum efficiencies (65e76%) than N719 in the wavelength
range of 400e500 nm. The action spectra (IPCE) of the device based
on Y4 extend even to the near infrared region at 700 nm. As a result,
the cell of Y4 has the highest current density and the overall con-
40
30
20
10
0
Y1
Y2
Y3
Y4
Y5
version efficiency (h), reaching 72% of N719-based standard device
despite of its somewhat lower VOC. The lower photocurrent of Y2-
and Y5-based device can be rationalized by the significant deviation
of the biphenyl spacer from planarity, which decreases the light-
harvesting efficiency (shorter absorption wavelength as well as
lower absorption intensity) and consequently hampers the cell ef-
ficiency. On the other hand, the device Y3 has current density and
cell performance better than the device Y1, probably because of
better light-harvesting of the fluorene-containing dye.
0
10
20
30
40
50
60
70
80
90
Z' (Ohm)
Fig. 6. Electrochemical impedance spectra (Nyquist plots) for the DSSCs measured
under illumination (AM 1.5).
These results may be rationalized by different dye densities on TiO2
surface. The dye loading amount on TiO2 surface is listed in Table 1.
An outstanding dye loading amount was found for Y3, and com-
paratively lower loading was found for Y5. Moreover, the high
photocurrent performance of Y4 is resulted from the low charge
transfer resistance and high dye loading intensity.
Performance of DSSCs in this study was compared with that
based on similar dyes where the pyridyl ring was replaced by a 2,5-
thienyl ring in order to evaluate the effect of pyridyl entity. The
efficiency of the device from Y2 is lower than that of 1P-PPS by 7%
(comparing to the standard device based on N719). Similarly, the
device of Y4 has an efficiency 9% lower that of 1P-PSS.24 It is be-
lieved that unfavorable twist angle between the pyridyl ring and
the neighboring aromatic ring, and a much higher resonance en-
ergy of pyridine than thiophene play important roles for de-
terioration of the cell efficiency. Adopting of 2,5-pyrimidyl ring in
the spacer of the sensitizer was reported to have high cell effi-
ciency,26 indicating that alleviating of the steric congestion in the
spacer as well as decreasing the resonance energy of the hetero-
aromatic ring will be beneficial to the cell performance.
2.5. Theoretical approach
Calculations applying density functional theory at the level of
B3LYP/6-31G* were conducted in order to gain further insight into
the correlation between the structure and the physical properties
as well as the cell performance. Time-dependent DFT results for
computation are included in Table S1 (Supplementary data). Fig. 7
Fig. 5. (a) JeV curves of DSSCs based on the dyes. (b) IPCE plots for the DSSCs.
In order to realize the interfacial charge transfer resistance from
the dyes injected intoTiO2, EIStechniques wereapplied on the DSSCs
as shown in Fig. 6 and some of the data are compiled in Table 1. The
radius of the intermediate frequency semicircle (1e20 Hz) corre-
sponds to the charge transfer resistance at the TiO2/dye/electrolyte
interface. The lower resistance implies higher electron injection rate
from the excited dyes to the TiO2 and thus resulting in higher pho-
tocurrent. The charge transfer resistance decreases in the orderof Y2
(17.3
U)>Y3 (16.5 U)>Y1 (15.9 U)>Y5 (11.6 U)>Y4 (9.8 U), which
conforms to the order of JSC values except Y3 and Y5. Relatively high
photocurrent was observed for the cell of Y3 in spite of its higher
interfacial resistance. In comparison, Y5 exhibits opposite behavior.
Fig. 7. Dihedral angles between the neighboring units of the dyes.

H.-H. Chou et al. / Tetrahedron 68 (2012) 767e773
771
displays the dihedral angles between neighboring groups in the
4. Experimental section
4.1. General information
conjugated spacer. The dihedral angles between the two phenyl
rings of the biphenyl spacer appear to be large: 29.2ꢂ and 34.0ꢂ for
Y2 and Y4, respectively. The dihedral angle of the pyridyl ring/
phenyl ring is significantly larger than those of the pyridyl ring/
thienyl ring and the phenyl ring/thiazolyl ring, and the latter two
have negligible values. Fig. 8 displays the HOMO and LUMO orbitals
of the compounds with their corresponding energies, and other
frontier orbitals are displayed in Fig. S1. S0/S1 transition mainly
comes from HOMO to LUMO and the charge-transfer character of
the transition is prominent. The calculated HOMO energies are in
the order of Y3 (ꢁ4.99 eV)>Y2 (ꢁ5.07 eV)wY4 (ꢁ5.07 eV)>Y5
(ꢁ5.13 eV)>Y1 (ꢁ5.23 eV), which are in the same trend as the re-
sults from electrochemical data. For dyes Y1 and Y4 the oscillator
strength (0.70 and 0.68, respectively) is relatively higher than that
in Y2 and Y5 (0.44 and 0.39, respectively). This is evidenced in
aforementioned calculated coplanarity and perfectly meets the
trend found in photophysical data.
Unless otherwise specified, all the reactions were performed
under nitrogen atmosphere using standard Schlenk techniques. All
solvents used were purified by standard procedures, or purged with
nitrogen before use. 1H NMR and 13C spectra were recorded on
a Bruker 400 MHz spectrometer operating at 400.135 MHz and
100.613 MHz, respectively. Absorption spectra were recorded on
a Cary 50 probe UVevis spectrophotometer. All chromatographic
separations were carried out on silica gel (60 M, 230e400 mesh).
Mass spectra (FAB) were recorded on a VG70e250S mass spec-
trometer. Elementary analyses were performed on a PerkineElmer
2400 CHN analyzer. The TiO2 nanoparticles and N719 were pur-
chased from Solaronix S. A., Switzerland.
4.2. Synthetic details and characterization
4.2.1. 6-(4-(Diphenylamino)phenyl)pyridine-3-carbaldehyde (1a). A
round-bottom flask weighted with 4-(diphenylamino)phenyl-
boronic acid (1.16 g, 4.01 mmol), 2-bromopyridine-5-carbaldehyde
(0.50 g, 2.69 mmol), and tetrakis(triphenylphosphine) palladium
(0.38 g, 0.32 mmol) was added 15 mL of toluene, 5 mL of ethanol,
and 7 mL of aq Na2CO3 (2 M). The resulting solution was heated at
85 ꢂC for 1 day. After the volatiles were removed under vacuum, the
resulting residue was dissolved in CH2Cl2, washed with brine and
dried over MgSO4. The crude product was further purified by col-
umn chromatography (silica gel) using CH2Cl2 as the eluent to give
a yellow powder (0.76 g, Yield 51%). Mass (FAB, m/z): 350.1 (Mþ). 1H
NMR (400 MHz, CDCl3):
d
10.07 (s, 1H), 9.04 (d, JHH¼1.6 Hz,1H), 8.16
(dd, JHH¼2.0 and 8.4 Hz, 1H), 7.95 (d, JHH¼8.8 Hz, 1H), 7.80 (d,
JHH¼8.4 Hz, 1H), 7.29 (t, 4H), 7.14 (d, JHH¼7.6 Hz, 4H), 7.10e7.08 (d,
JHH¼8.8 Hz, 1H), 7.08 (t, 2H). 13C NMR (100 MHz, CDCl3):
d 190.36,
161.57, 152.60, 150.12, 147.03, 136.28, 130.76, 129.54, 129.21, 128.57,
125.38, 124.0, 122.04, 119.50.
Compounds 2ae4a were synthesized by the same procedures as
described above for 1a using appropriate boronic acids. Compound
2a: Yield 51%. Mass (FAB, m/z): 426.2 (Mþ). 1H NMR (400 MHz,
CDCl3):
d
10.07 (s, 1H), 9.04 (d, JHH¼1.6 Hz, 1H), 8.16 (dd, JHH¼2.0
and 8.4 Hz, 1H), 7.95 (d, JHH¼8.8 Hz, 1H), 7.80 (d, JHH¼8.4 Hz, 1H),
7.29 (t, 4H), 7.14 (d, JHH¼7.6 Hz, 4H), 7.10e7.08 (d, JHH¼8.8 Hz, 1H),
7.08 (t, 2H). 13C NMR (100 MHz, CDCl3):
d 190.61, 162.00, 152.71,
148.05, 147.72, 142.82, 136.67, 136.39, 133.84, 129.96, 129.55, 128.20,
127.96, 127.22, 124.86, 123.75, 123.42, 120.50. Compound 3a: Yield
58%. Mass (FAB, m/z): 494.3 (Mþ). 1H NMR (400 MHz, CDCl3):
d
10.10 (s, 1H), 9.10 (s, 1H), 8.19 (d, JHH¼8.0 Hz, 1H), 8.04 (m, 2H),
7.93 (d, JHH¼8.0 Hz, 1H), 7.70 (d, JHH¼8.0 Hz, 1H), 7.59 (d,
JHH¼8.0 Hz, 1H), 7.24 (t, 4H), 7.11 (d, JHH¼8.4 Hz, 4H), 7.01 (m, 3H).
13C NMR (100 MHz, CDCl3):
d 190.62, 162.76, 152.73, 152.37, 150.95,
Fig. 8. Selected frontier orbitals and corresponding energy levels of the dyes.
148.26, 148.08, 144.17, 136.50, 136.00, 135.65, 129.72, 129.45, 127.04,
124.32, 123.54, 123.00, 122.00, 121.21, 120.59, 119.62, 119.10, 56.54,
32.83, 8.82. Compound 4a: Yield 73%. Mass (FAB, m/z): 432.2 (Mþ).
3. Conclusions
1H NMR (400 MHz, CDCl3):
d 10.01 (s, 1H), 8.93 (s, 1H), 8.10 (dd,
JHH¼1.6 and 8.0 Hz, 1H), 7.72 (d, JHH¼8.4 Hz, 1H), 7.64 (d,
JHH¼3.6 Hz, 1H), 7.51 (d, JHH¼8.4 Hz, 2H), 7.26 (t, 4H), 7.25e7.24 (d,
JHH¼3.2 Hz, 1H), 7.11 (d, JHH¼7.6 Hz, 4H), 7.04 (t, 2H), 7.05 (d,
In summary, a series of dipolar dyes implanted with pyridine-
containing spacer were successfully synthesized and solar cell de-
vices were fabricated and tested. The photo-to-current conversion
efficiencies of the cells are in the range of 4.28e5.27%. These values
are above 60% of the N719 standard device, with the best efficiency
reaching 72% of the standard cell based on N719. The device per-
formance of pyridine dyes falls behind that of thiophene dyes. The
somewhat large dihedral angle between the pyridine and the
neighboring phenyl group together with the large resonance en-
ergy of pyridine hamper effective charge transfer from the donor to
the acceptor.
JHH¼8.4 Hz, 2H). 13C NMR (100 MHz, CDCl3):
d 190.04, 157.28,
153.06, 149.57, 148.40, 147.44, 141.43, 136.22, 129.61, 129.49, 128.49,
127.53, 126.88, 125.08, 123.70, 123.20, 118.55.
4.2.2. 2-(40-(Diphenylamino)biphenyl-4-yl)thiazole-5-carbaldehyde
(5a). At nitrogen atmosphere 40-(diphenylamino)biphenyl-4-
ylboronic acid (1.06 g, 2.90 mmol), 2-bromothiazole (0.40 mL,
4.44 mmol), and Pd(PPh3)4 (0.080 g, 0.070 mmol) were weighted
into a round-botton flask. An aqueous solution of Na2CO3 (2 M,

772
H.-H. Chou et al. / Tetrahedron 68 (2012) 767e773
7 mL), 15 mL of toluene and 5 mL of ethanol were then added and
the resulting solution was heated at 85 ꢂC for 1 day. The solvent
was removed under vacuum and the residue was dissolved in
CH2Cl2, washed with brine and dried over MgSO4. The crude
product was purified by column chromatography using CH2Cl2 as
the eluent to give a yellow intermediate (0.83 g, 71%). 1H NMR
1H), 7.12 (d, JHH¼8.0 Hz, 4H), 7.06 (t, 3H), 2.17e1.96 (m, 4H), 0.38 (t,
6H). 13C NMR (100 MHz, THF-d8):
163.820, 161.414, 154.842,
d
153.055, 151.459, 149.186, 144.937, 138.418, 137.261, 136.792,
130.942, 129.360, 128.462, 126.876, 125.923, 125.245, 124.576,
124.337, 123.618, 123.357, 122.994, 122.846, 121.512, 121.073,
119.885, 116.452, 105.891. Compound Y4: Yield 81%. Anal. Calcd for
C31H21N3O2S: C, 74.53; H, 4.24; N, 8.41; Found: C, 74.58; H, 4.18; N,
8.44. HRMS m/z [MþHþ] calcd for C31H22N3O2S: 500.1432 Found:
(400 MHz, CDCl3):
d
8.00 (d, JHH¼8.4 Hz, 2H), 7.86 (d, JHH¼3.2 Hz,
1H), 7.63 (dd, JHH¼8.4 Hz, 2H), 7.50 (d, JHH¼8.4 Hz, 2H), 7.31 (d,
JHH¼3.6 Hz, 1H), 7.26 (t, 4H), 7.14 (m, 6H), 7.03 (t, 2H). This yellow
intermediate (0.70 g, 1.73 mmol) was further dissolved in 20 mL
of THF under nitrogen and the solution was cooled using acetone
bath to ꢁ78 ꢂC, n-butyl lithium (1.6 M in hexane, 1.18 mL,
1.90 mmol) was added to the cold solution via syringe in 5 min.
After the solution was stirred for 35 min, 0.26 mL of DMF
(0.87 mmol) was added all at once. The resulting mixture was
stirred for 90 min at ꢁ35 ꢂC and was allowed to warm to room
temperature and stirred for 30 min. The solution was then
quenched by aqueous NH4Cl, diluted with Et2O, and washed with
brine for several times. The organic extract was dried over MgSO4
and purified by column chromatography using hexanes/ethyl
acetate (2:3, v/v) as the eluent. The product was obtained as
a yellow powder (0.50 g, 58%). Mass (FAB, m/z): 432.1 (Mþ). 1H
500.1444. 1H NMR (400 MHz, acetone-d6):
d 9.04 (s, 1H), 8.61 (d,
JHH¼6.8 Hz, 1H), 8.35 (s, 1H), 8.08 (d, JHH¼8.8 Hz, 1H), 7.92 (d,
JHH¼3.6 Hz, 1H), 7.68 (d, JHH¼8.4 Hz, 2H), 7.49 (d, JHH¼4.0 Hz, 1H),
7.35 (t, 3H), 7.14e7.05 (m, 8H). 13C NMR (75 MHz, DMSO-d6):
d
163.540, 155.081, 153.405, 151.273, 148.262, 148.022, 147.139,
141.859, 137.079, 130.153, 129.788, 127.356, 127.167, 126.014,
125.066, 124.185, 122.924,118.910, 116.629, 104.366. Compound Y5:
Yield 90%. Anal. Calcd for C31H21N3O2S: C, 74.53; H, 4.24; N, 8.41;
Found: C, 74.59; H, 4.15; N, 8.46. HRMS m/z [MþHþ] calcd for
C31H22N3O2S: 500.1432; Found: 500.1441. 1H NMR (400 MHz, ace-
tone-d6):
JHH¼8.4 Hz, 2H), 7.71 (d, JHH¼8.4 Hz, 2H), 7.35 (t, 3H), 7.15e7.09 (m,
8H). 13C NMR (75 MHz, DMSO-d6):
173.714, 163.449, 155.898,
d
8.63 (s, 1H), 8.60 (s, 1H), 8.20 (d, JHH¼8.4 Hz, 2H), 7.87 (d,
d
148.108, 147.272, 145.131, 143.340, 132.475, 131.533, 130.799,
130.139, 128.229, 128.135, 127.378, 125.022, 124.092, 123.080,
116.531, 102.550.
NMR (400 MHz, CDCl3):
d 10.02 (s, 1H), 8.41 (s, 1H), 8.05 (d,
JHH¼8.0 Hz, 2H), 7.67 (d, JHH¼8.4 Hz, 2H), 7.50 (d, JHH¼8.4 Hz, 2H),
7.27 (t, 4H), 7.12 (d, JHH¼8.0 Hz, 6H), 7.05 (t, 2H). 13C NMR
(100 MHz, CDCl3):
d
182.20, 175.59, 152.65, 148.38, 147.61, 144.28,
4.3. Assembly and characterization of DSSCs
138.86, 133.17, 131.09, 129.59, 127.97, 127.93, 127.28, 125.00,
123.58, 123.49.
The photoanode used was the TiO2 thin film (12
mm of 20 nm
particles as the absorbing layer and 6 m of 400 nm particles as
m
4.2.3. (E)-3-(6-(4-(Diphenylamino)phenyl)pyridin-3-yl)-2-
cyanoacrylic acid (Y1). A round-bottom flask equipped with 6-(4-
(diphenylamino)phenyl)nicotinaldehyde (0.50 g, 1.43 mmol) and 2-
cyanoacetic acid (0.24 g, 2.82 mmol) was charged with 15 mL of
MeCN under nitrogen. After adding a few drops of piperidine, the
resulting solution was heated at 90 ꢂC for 1 day. Then volatiles of
the solution were removed under vacuum. The resulting residue
was extracted with dichloromethane and water, dried over MgSO4,
followed by column chromatography using in ethylacetate (with 5%
of acetic acid) as the eluent. Further recrystallization of the product
from acetone gives a red powder (0.22 g, 36%). Anal. Calcd for
C27H19N3O2: C, 77.68; H, 4.59; N, 10.07; Found: C, 77.65; H, 4.67; N,
9.56. HRMS m/z [MþHþ] calcd for C27H20N3O2: 418.1555; Found:
the scattering layer) coated on FTO glass substrate27 with a di-
mension of 0.5ꢃ0.5 cm2, and the film thickness measured by
a profilometer (Dektak3, Veeco/Sloan Instruments Inc., USA). A
platinized FTO produced by thermopyrolysis of H2PtCl6 was used
as a counter electrode. The TiO2 thin film was dipped into the THF
or MeCN solution containing 3ꢃ10ꢁ4 M dye sensitizers for at least
12 h. After rinsing with THF or MeCN, the photoanode adhered
with a polyester tape of 60
mm in thickness and with a square
aperture of 0.36 cm2 was placed on top of the counter electrode
and tightly clipping them together to form a cell. Electrolyte was
then injected into the space and then sealing the cell with the Torr
SealÒ cement (Varian, MA, USA). The electrolyte was composed of
0.5 M lithium iodide (LiI), 0.05 M iodine (I2), and 0.5 M 4-tert-
butylpyridine that was dissolved in acetonitrile. The standard cell
based on N719 was fabricated and measured under similar con-
ditions. The electrochemical impedance spectra (EIS) of the cells
were recorded using a potentiostat/galvanostat (CHI, model 440)
under 100 mW cmꢁ2 light intensity. The frequency range explored
in impedance measurements was 10 mHze100 kHz with sinu-
418.1566. 1H NMR (400 MHz, CDCl3):
d 8.90 (s, 1H), 8.68 (d,
JHH¼9.2 Hz, 1H), 8.28 (s, 1H), 7.95 (d, JHH¼8.4 Hz, 2H), 7.81 (d,
JHH¼8.4 Hz, 1H), 7.29 (t, 3H), 7.16e7.08 (m, 8H). 13C NMR (75 MHz,
THF-d8):
d 163.681, 160.621, 154.265, 151.482, 151.148, 148.345,
137.479, 132.066, 130.430, 129.277, 126.693, 126.321, 124.841,
122.859, 119.877, 116.480, 105.093.
The syntheses of compounds Y2eY5 follow the same procedure
as that of compound Y1 using appropriate aldehydes. Compound
Y2: Yield 46%. Anal. Calcd for C33H23N3O2: C, 80.31; H, 4.70; N, 8.51;
Found: C, 77.98; H, 4.57; N, 7.93. HRMS m/z [MþHþ] calcd for
C33H24N3O2: 494.1868; Found: 494.1878. 1H NMR (400 MHz,
soidal signal (single sine) and an ac amplitude (DEac) of 10 mV. The
applied bias voltage was set at the open-circuit voltage of the
DSSC, between the Pt counter electrode and the TiO2-dye working
electrode.
CDCl3):
d
8.99 (s, 1H), 8.72 (d, JHH¼6.4 Hz, 1H), 8.32 (s, 1H), 8.15 (d,
4.4. Quantum chemistry computation
JHH¼6.0 Hz, 2H), 7.94 (d, JHH¼7.2 Hz, 1H), 7.72 (d, JHH¼6.0 Hz, 2H),
7.53 (d, JHH¼6.4 Hz, 2H), 7.25 (d, JHH¼10.0 Hz, 4H), 7.13 (d,
JHH¼6.8 Hz, 6H), 7.05 (d, JHH¼6.0 Hz, 2H). 13C NMR (100 MHz,
The computation was performed with Q-Chem 3.0 software.28
Geometry optimization of the molecules were performed using
hybrid B3LYP functional and 6-31G* basis set. For each molecule,
a number of possible conformations were examined and the one
with the lowest energy was used. The same functional was also
applied for the calculation of excited states using time-dependent
density functional theory (TDeDFT). There exist a number of pre-
vious works that employed TDeDFT to characterize excited states
with charge-transfer character.29 In some cases underestimation of
the excitation energies was seen.29,30 Therefore, in the present
work, we use TDeDFT to visualize the extent of transition moments
DMSO-d6):
d 172.835, 163.364, 158.497, 154.671, 152.615, 149.326,
147.402, 143.145, 141.611, 137.290, 136.285, 133.252, 130.105,
128.036, 126.958, 124.842, 124.068, 123.912, 123.413, 123.134,
120.510, 117.420. Compound Y3: Yield: 33%. Anal. Calcd for
C38H31N3O2: C, 81.26; H, 5.56; N, 7.48; Found: C, 81.36; H, 5.75; N,
7.39. HRMS m/z [MþHþ] calcd for C38H32N3O2: 562.2494; Found:
562.2508. 1H NMR (400 MHz, acetone-d6):
d 9.20 (s, 1H), 8.65 (d,
JHH¼7.6 Hz, 1H), 8.42 (s, 1H), 8.32 (s, 1H), 8.26 (d, JHH¼5.6 Hz, 2H),
7.86 (d, JHH¼7.6 Hz, 1H), 7.79 (d, JHH¼8.0 Hz, 1H), 7.32 (t, 3H), 7.18 (s,

H.-H. Chou et al. / Tetrahedron 68 (2012) 767e773
773
€
Zakeeruddin, S. M.; Gratzel, M.; Wang, P. Energy Environ. Sci. 2009, 2, 92; (d) Li,
G.; Zhou, Y.-F.; Cao, X. B.; Bao, P.; Jiang, K.-J.; Lin, Y.; Yang, L.-M. Chem. Commun.
2009, 2201.
as well as their charge-transfer characters, and avoid drawing
conclusions from the excitation energy.
13. (a) Justin Thomas, K. R.; Lin, J. T.; Hsu, Y. C.; Ho, K. C. Chem. Commun. 2005,
4098; (b) Justin Thomas, K. R.; Hsu, Y.-C.; Lin, J. T.; Lee, K.-M.; Ho, K.-C.; Lai, C.-
H.; Cheng, Y.-M.; Chou, P.-T. Chem. Mater. 2008, 20, 1830; (c) Choi, H.; Baik, C.;
Acknowledgements
€
Kang, S. O.; Ko, J.; Kang, M.-S.; Nazeeruddin, M. K.; Gratzel, M. Angew. Chem., Int.
Ed. 2008, 47, 327.
We thank the Institute of Chemistry, Academia Sinica, and Na-
14. (a) Lin, J. T.; Chen, P. C.; Yen, Y. S.; Hsu, Y. C.; Chou, H. H.; Yeh, M. C. P. Org. Lett.
2009, 11, 97; (b) Li, R.; Lv, X.; Shi, D.; Zhou, D.; Cheng, Y.; Zhang, G.; Wang, P. J.
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tional Science Council, Taiwan, for financial support.
Supplementary data
The details of quantum chemistry computation, cyclic voltam-
mograms and the 1H and 13C NMR spectra are found. Supplemen-
tary data associated with this article can be found, in the online
version, at doi:10.1016/j.tet.2011.10.038.
16. Zhang, G.; Bala, H.; Cheng, Y.; Shi, D.; Lv, X.; Yu, Q.; Wang, P. Chem. Commun.
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