Synthesis and photovoltaic properties of new [1,2,5]thiadiazolo[3,4-c] pyridine-based organic Broadly absorbing sensitizers for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.tet.2014.04.039

In this paper, by introducing [1,2,5]thiadiazolo[3,4-c]pyridine (PT) as an auxiliary acceptor into the molecular design of organic sensitizers, we have synthesized four new dyes (PT1-PT4) for dye-sensitized solar cells (DSSCs) with triphenylamine or N,N-d

Full text of DOI:10.1016/j.tet.2014.04.039

Tetrahedron xxx (2014) 1e8
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis and photovoltaic properties of new [1,2,5]thiadiazolo[3,4-
c]pyridine-based organic Broadly absorbing sensitizers for dye-
sensitized solar cells
,
,
,
Weijiang Ying a y, Xiaoyu Zhang a y, Xin Li b, Wenjun Wu a, Fuling Guo a, Jing Li a
,
*
b
a,
*
ꢀ
Hans Agren , Jianli Hua
a Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, Shanghai 200237, China
b Department of Theoretical Chemistry and Biology, School of Biotechnology, KTH Royal Institute of Technology, Stockholm 10691, Sweden
a r t i c l e i n f o
a b s t r a c t
Article history:
In this paper, by introducing [1,2,5]thiadiazolo[3,4-c]pyridine (PT) as an auxiliary acceptor into the
molecular design of organic sensitizers, we have synthesized four new dyes (PT1ePT4) for dye-
sensitized solar cells (DSSCs) with triphenylamine or N,N-diphenylthiophen-2-amine as the donor
Received 28 January 2014
Received in revised form 9 April 2014
Accepted 10 April 2014
Available online xxx
units and thiophene or benzene as the
p-bridges, respectively. All the structures, optical and electro-
chemical properties were fully characterized. Nanocrystalline TiO2 dye-sensitized solar cells were also
fabricated using these dyes. Among them, PT2-based DSSCs showed the highest overall conversion ef-
ficiency of 6.11% with Voc¼668 mV, Jsc¼12.61 mA cmꢀ2 and a fill factor (FF)¼0.74 after a chenodeox-
ycholic acid (CDCA) treatment under standard illumination condition (100 mW cmꢀ2 simulated AM 1.5
solar light).
Keywords:
[1,2,5]Thiadiazolo[3,4-c]pyridine
N,N-Diphenylthiophen-2-amine
Sensitizer
Ó 2014 Published by Elsevier Ltd.
Dye-sensitized solar cells
1. Introduction
withdrawing unit between the donor and the
p-bridge as an in-
ternal acceptor, which is known as the DeAe
p
eA configuration.7 It
Dye-sensitized solar cells (DSSCs) have drawn extensive atten-
tion as a promising alternative to traditional silicon-based solar
cells during the past two decades due to their low-cost fabrication
and relatively high performance.1 The sensitizers, which have
a crucial effect on the performance of DSSCs, are basically divided
into two big groups, that is, metal complexes and organic dyes.2,3
Ruthenium (Ru) complexes, as one of the most successful metal-
complex sensitizers, have roughly reached the photoelectric con-
version efficiency of 10e12% under the standard measurement
conditions.4 To date, a record efficiency of 12.3% has been kept by
using zinceporphyrin-based sensitizer.5 On the other hand, metal-
free organic sensitizers have also attracted increasing attention due
to their high molar extinction coefficients, tunable absorption
properties, relatively high efficiency and low cost. Owing to the
sensitizers’ intramolecular charge-transfer (ICT) property, the
has been reported that sensitizers with those auxiliary acceptor
units, such as benzothiadiazole,8 benzotriazole,9 quinoxaline,10
diketopyrrolopyrrole,11 pyrido[3,4-b]pyrazine,12 and thieno[3,4-c]
pyrrole-4,6-dione,13 have displayed several advantages involving
tuning of the molecular energy levels, red-shifting of the charge-
transfer absorption band, and improvement of photovoltaic per-
formance and stability.
[1,2,5]Thiadiazolo[3,4-c]pyridine (PT), with the only molecular
difference of nitrogen atom on the pyridine ring from benzothia-
diazole (BT), has a higher electron affinity than BT. PT-based poly-
mers have been widely used in the fields of field-effect transistors
(FET) and bulk heterojunction (BHJ) solar cells.14 It is notable that,
in 2011, Heeger and his co-workers reported the highest conversion
efficiency of 6.7% for solution-processed small molecular solar cells
of that time by incorporating PT as the acceptor unit inside the
small molecule, indicating that PT is a promising electron acceptor
unit.15,16
donorep bridgeeacceptor (DepeA) configuration is often used in
the design of organic sensitizers.6 Recently, a renewed configura-
tion has been put forward by introducing an additional electron-
Triphenylamine, however, as a good hole-transport group, is
limited by its relatively narrow absorption range. It is reported
recently that replacing one benzene ring of triphenylamine with
a thiophene ring can improve its electron-donating ability and
broaden the absorption spectrum, by which the overall conversion
efficiency of 7.64% was reached.17 This report inspired us to do some
* Corresponding authors. Tel.: þ86 21 64250940; fax: þ86 21 64252758 (J.H.),
Tel./fax: þ86 21 64252758(J.L.); e-mail addresses: lijhy@ecust.edu.cn (J. Li), jlhua@
ecust.edu.cn (J. Hua).
y
These authors contributed equally to this work.
0040-4020/$ e see front matter Ó 2014 Published by Elsevier Ltd.
http://dx.doi.org/10.1016/j.tet.2014.04.039
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2
W. Ying et al. / Tetrahedron xxx (2014) 1e8
further investigation into the application of N,N-diphenylthiophen-
2-amine as the donor moiety to dye-sensitized solar cells. So in this
paper, we have designed and synthesized four DeAepeA featured
organic sensitizers PT1ePT4 using PT as the auxiliary acceptor unit,
triphenylamine or N,N-diphenylthiophen-2-amine between the
donor unit and thiophene or benzene as the p-bridge (see Scheme
1). The photovoltaic performance was also measured and analyzed.
According to our expectation, the absorption spectra of the dyes
would be broadened and even extended into NIR region.
Scheme 1. Molecular structures of the dyes PT1ePT4.
2. Results and discussion
2.1. Synthetic procedures
Scheme 2. The synthetic procedures of the dyes PT1ePT4.
Scheme 2 depicts the synthesis of PT dyes. 2,5-Dibromopyr-
idine-3,4-diamine (1) was synthesized by refluxing the commer-
cially available 3,4-diaminopyridine in a mixture of hydrobromic
acid and bromine. The crucial intermediate (2) was obtained by the
subsequent cyclization reaction according to published pro-
cedures.18 Stille coupling reaction of PT with N,N-diphenyl-4-
(tributylstannyl)aniline and N,N-diphenyl-5-(tributylstannyl)thio-
phen-2-amine resulted in the compounds 3 and 6, respectively. In
the next step, Suzuki coupling reaction of 3 and 6 with 5-
formylthiophen-2-ylboronic acid and 4-formylphenyl-boronic
acid afforded the corresponding aldehyde precursors (4, 5, 7, 8),
respectively. Finally, the target products PT1ePT4 were synthe-
sized via Knoevenagel condensation of respective aldehydes with
cyanoacetic acid in the presence of ammonium acetate. In the Stille
coupling reaction, preferential oxidative coupling occurred at the
more electron deficient 4-carbon position of pyridine derivates
leading to the pyridal N-atoms proximal to the donor unit.19 It is
important to highlight the regiochemistry of 3 and 6, where the N-
atoms of pyridine rings of each PT acceptor unit are orientated
towards the donor moiety (proximal configuration). This confor-
Fig. 1. Absorption spectra of PT1ePT4 dyes in CH2Cl2.
mation is observed from the corresponding 1H NMR, as the
d data of
transitions caused by the conjugation of PT and donor moiety. The
middle absorption bands of PT1 and PT3 peak at 415 nm and
430 nm while that of PT2 and PT4 peak at 360 nm and 373 nm,
respectively, suggesting these bands can be ascribed to localized
the benzene and thiophene attached to the PT-core shifted to the
downfield (8.40e8.50 ppm), which was affected by the N-atoms of
pyridine rings (see Supplementary data). Such a phenomenon has
also been observed in other small molecules containing pyridine
rings.12,15 Also, it can be confirmed by the molecular weight of
compounds 3 and 6. The four organic PT-based sensitizers were
fully characterized with elemental analysis, 1H NMR, 13C NMR, and
HRMS (see Supplementary data).
aromatic
p
e
p
*
transitions caused by the conjugation of PT and
thiophene or benzene moiety of the
p
-bridge.20 The latter ab-
sorption band can be assigned to the intramolecular charge-
transfer (ICT) between the donor and acceptor moiety.12,21 The
maximum absorption wavelengths of PT1ePT4 were 535, 500,
620 and 575 nm, respectively. With the same donor moieties, the
35 nm red-shift of PT1 compared with PT2 and 45 nm red-shift of
PT3 compared with PT4 in the absorption spectra can be contrib-
2.2. Optical and electrochemical properties
The absorption spectra of the four dyes in the CH2Cl2 solvent are
shown in Fig. 1. From the absorption spectra, we can see that all the
dyes exhibit three major absorption bands. The former absorption
bands of PT1ePT4 are typical triarylamine absorptions, peaking at
uted to the different p-bridges. Fig. 1 is also an evidence of that the
changing of donor unit can cast a notable influence on the ab-
sorption spectrum. As a matter of fact, replacing triphenylamine
unit with N,N-diphenylthiophen-2-amine unit resulted in
75e85 nm bathochromic shift among the four dyes, which
the range of 305e320 nm, which can be attributed to
pep
*
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W. Ying et al. / Tetrahedron xxx (2014) 1e8
3
PT1ePT4 on the TiO2 films were 503, 476, 554 and 524 nm, which
were blue-shift 32, 24, 66 and 51 nm compared with that in CH2Cl2,
respectively. To our knowledge, this may be caused by two reasons:
(1) the deprotonation of carboxylic acid when it absorbs on the
surface of TiO2, since the formed carboxylateeTiO2 unit is a weaker
electron acceptor than carboxylic acid; (2) the H-aggregation of
dyes. We noted that dyes with triphenylamine donor units were
less hypochromatic-shifted than those with N,N-diphenylth-
iophen-2-amine donor units. This may be the result of the non-
planar molecular configuration of triphenylamine, which reduced
the H-aggregation. However, the absorption spectra were broaden
when the dyes were absorbed on the TiO2 films, which was bene-
ficial to the light harvesting. All the absorption data are listed in
Table 1.
Table 1
Fig. 2. Absorption spectra of PT1ePT4 dyes on 2 mm transparent TiO2 films.
Optical properties and electrochemical properties of the dyes PT1ePT4
Dye lmaxa/nm (
3
ꢁ10ꢀ4 Mꢀ1 cmꢀ1
)
lmax
HOMOc/V E0e0d/eV LUMOe/V
b
TiO2/nm (vs NHE)
(vs NHE)
expanded the absorption wavelength range efficiently. According to
the optimized structures from density functional theory (DFT)
calculations, the pyridine segment together with the thiadiazole
unit is prone to form H-bonding with the proximal thiophene or
phenyl rings, leading to nearly full planarity. The only fragment that
would cause twisting is the distal phenyl ring. These features are
clearly represented in Fig. 3, Fig. S2 and Table S2. It appears that the
stronger electron-donating ability of N,N-diphenylthiophen-2-
amine than triphenylamine is mainly responsible for the bath-
ochromic shifts. Unfortunately, this was on the expense of the
molar extinction coefficient, which may affect the light harvesting
efficiency. The cut-off wavelength of PT3 reached 740 nm, in-
dicating that narrow-energy-gap groups and strong electron-
donating moieties can broaden the absorption spectrum, even to
the near-infrared region.
PT1 308(0.68), 415(0.35), 535(0.73) 503
PT2 313(1.35), 360(0.67), 500(0.94) 476
PT3 315(0.35), 430(0.33), 620(0.61) 554
PT4 317(0.36), 373(0.27), 575(0.55) 524
1.13
1.13
0.97
0.97
1.97
2.07
1.78
1.90
ꢀ0.84
ꢀ0.94
ꢀ0.81
ꢀ0.93
a
Absorption maximum in CH2Cl2 solution.
Absorption maximum on TiO2 film(without CDCA).
HOMO energy levels were measured in CH2Cl2 with 0.1
b
c
M
tetra-n-buty-
lammoniumhexafluorophosphate (TBAPF6) as electrolyte (working electrode: Pt;
reference electrode: SCE; calibrated with ferrocene/ferrocenium (Fc/Fcþ)).
d
E0e0 was estimated from the intercept of the normalized absorption and
emission spectra of the dyes, E0e0¼1240/
l.
e
LUMO energy levels were estimated by subtracting E0e0 from the HOMO energy
level.
One of the conditions that a good sensitizer has to meet is its
lowest unoccupied molecular orbital (LUMO) energy level have to
be higher than the conduction band energy level (Ecb) of TiO2 so
that the excited electrons can be injected into the conduction band
efficiently, and its highest occupied molecular orbital (HOMO) must
be lower than the redox potential of the electrolyte so that the
oxidized dyes can be reduced by accepting the electrons from the
redox species in the electrolyte. To estimate the feasibility of the
electron injection into the conduction band of TiO2 and dye re-
generation, cyclic voltammetry (CV) plots were recorded in CH2Cl2
solution with 0.1 M tetra-n-butylammoniumhexafluorophosphate
(TBAPF6) as electrolyte with a three-electrode system (working
electrode: Pt; counter electrode: Pt wire; reference electrode: sat-
urated calomel electrode; calibrated with ferrocene/ferrocenium
(Fc/Fcþ) as an internal reference, the results were converted to
those for the normal hydrogen electrode (NHE)). The cyclic vol-
tammograms of PT1ePT4 are shown in Fig. S1. The redox potentials
(Eox) of PT1ePT4, which were corresponding to the HOMO energy
levels, were located at 1.13, 1.13, 0.97, and 0.97 V versus NHE, re-
spectively. The same HOMO energy levels of PT1 and PT2 along
with PT3 and PT4 indicated that the HOMO energy level mainly
depended on the donor moiety. The reason for higher HOMO en-
ergy levels of PT3 and PT4 compared to that of PT1 and PT2 can be
attributed to the stronger electron-donating ability of N,N-diphe-
nylthiophen-2-amine unit than that of triphenylamine unit. The
stronger electron-donating ability the donor unit has, the higher
HOMO energy level the sensitizer has. The parameters of electro-
chemical properties of PT1ePT4 can be found in Table 1. The zero-
zero transition energies (E0e0) of the four dyes were estimated from
the intercept of the normalized absorption and emission spectra,
which were 1.97, 2.07, 1.78, and 1.90 eV, respectively. The LUMO
energy levels, calculated from EHOMOꢀE0e0, were ꢀ0.84, ꢀ0.94,
ꢀ0.81, and ꢀ0.93 V versus NHE, respectively. The LUMO values are
much more negative than the Fermi level of TiO2 (ꢀ0.5 V vs NHE),
Fig. 3. The electron distribution in HOMO and LUMO levels of PT1ePT4 dyes.
Absorption spectra of PT1ePT4 dyes on transparent TiO2 films
are shown in Fig. 2. The maximum absorption wavelengths of
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4
W. Ying et al. / Tetrahedron xxx (2014) 1e8
ensuring an efficient electron injection process from the excited
state of the dyes into the TiO2 electrode. And the HOMO values are
much more positive than the redox potential of Iꢀ=I3ꢀ (0.4 V vs
NHE), which assure an efficient dye regeneration process. Cyclic
voltammetry proves that PT1ePT4 can be applied to DSSCs in
theory.
2.3. Molecular orbital calculations
To further study the processes of intermolecular electron
transfer and the properties of absorption spectrum caused by exi-
ted states, DFT calculations were carried out to optimize the
structures of compounds PT1ePT4 at the B3LYP/6-31G level of
*
theory using the Gaussian 09 program packages.22,23 With solvent
effects of CH2Cl2 taken into account by the polarizable continuum
model (PCM),24 the lowest singlet excited states (S1) of the sensi-
tizer dyes were simulated by time-dependent (TD) DFT calculations
employing the range-separated CAM-B3LYP functional.25
Fig. 4. The IPCE spectra for DSSCs sensitized by the dyes PT1ePT4 and 20 mM CDCA-
treated PT2.
The calculated parameters of the lowest excitation processes are
shown in Table S1, including the energies of excited states, oscil-
lator strengths and the contribution components of molecular or-
bitals. The obtained lowest excitation process of each compound
corresponds to charge-transfer (CT) from HOMO to LUMO, that is,
electronic transition from the donor moiety to the cyanoacrylic acid
acceptor moiety, which ensures effective injection of electrons into
TiO2. According to Table S1, PT1 and PT2 had two components
(HOMO/LUMO and HOMOꢀ1/LUMO) for S0/S1 transition
while PT3 and PT4 had only one component (HOMO/LUMO),
owing to different nature of the donor units. The maximum ab-
sorption peaks caused by the electron transition from HOMO to
LUMO were 524, 476, 577 and 528 nm, consistent with the exper-
imental data.
factor (FF) and the overall conversion efficiency (h) are tabulated in
Table 2. And the IeV curves are illustrated in Fig. 5. There are three
possible reasons of why the performance of triphenylamine-based
DSSCs is better than that of N,N-diphenylthiophen-2-amine-based
DSSCs: (1) The molar extinction coefficients of PT1 and PT2 are
higher, which enable them to harvest the sunlight more efficiently.
(2) According to the DFT calculations, the HOMOs of PT3 and PT4
extend from N,N-diphenylthiophen-2-amine to PT and
p-bridge,
leaving low electron density on the donor moiety, which goes
against the electron transfer from the donor unit to the acceptor
unit. This can also be reflected by the IPCE spectra; (3) The distinct
Voc decreases of DSSCs based on PT3 and PT4 might be the main
reason for the low conversion efficiencies. Since the planarity of the
compound became better by replacing the benzene ring by a thio-
phene ring, the dye aggregation and the electron recombination
The electron distributions in HOMO and LUMO levels of
PT1ePT4 dyes are displayed in Fig. 3, from which we can see that
the HOMOs of PT1 and PT2 are largely localized on the triphenyl-
might not be restricted effectively, resulting in decreasing Voc
.
To optimize the performance of PT2-based DSSCs, we carried
out CDCA treatments with different concentrations.26 All the data
are tabulated in Table 2. CDCA acts as a co-adsorbent with spatial
configuration to reduce the dye aggregation effectively, thus can
enhance the electron injection efficiency of the dye and Jsc. IeV
curves of cells based on PT2 co-sensitized with CDCA of different
concentrations are presented in Fig. 6. When the concentration of
CDCA increased from 0 to 20 mM, the Jsc increased while Voc de-
creased slightly, and the overall conversion efficiency was en-
hanced. Especially when the concentration of CDCA increased from
0 to 10 mM, the Jsc rose from 7.72 to 11.44 mA cmꢀ2, which was
a strong evidence that CDCA can reduce the dye aggregation and
increase the electron injection efficiency. However, when the con-
centration increased to 30 mM, the Jsc dropped and Voc barely
changed, resulting in a decrease of the overall conversion efficiency.
This might be the result of the excess CDCA, which reduced the
adsorption of the dye. Eventually, with the 20 mM CDCA treatment,
PT2-based DSSCs obtained the highest conversion efficiency of
6.11%.
amine donor units, and a small part of them are on the
through the PT moiety. Whilst, the HOMOs of PT3 and PT4 show
extension to the N,N-diphenylthiophen-2-amine, PT and -bridge,
leaving low electron density on the donor moiety, which goes
against the electron transfer from the donor unit to the acceptor
unit. Meanwhile, the LUMOs of four dyes are concentrated on the
p-bridge
p
PT,
p-bridge and cyanoacrylic acid acceptor moiety. This indicates
that PT plays a role of an electron trap. When the dyes adsorb on the
surface of TiO2, the photoinduced electrons on the donor part can
flow to the PT heterocycle smoothly, then to the cyanoacrylic acid
moiety, and finally into the conduction band of TiO2. The Cartesian
coordinates of optimized geometries are provided in
Supplementary data.
2.4. Photovoltaic performance of DSSCs
Fig. 4 shows the incident photon-to-electron conversion effi-
ciency (IPCE) as a function of incident wavelength for DSSCs based
on these dyes. The maximum IPCE values of DSSCs based on PT1
and PT2 are about 60%, while PT3 and PT4 are about 45%. Even
though the IPCE spectra of PT3 and PT4 have red-shifted compared
with that of PT1 and PT2, the corresponding IPCE values in the
range of 470e610 nm are much smaller, which lead to the lower
short circuit current densities. After the treatment with 20 mM
chenodeoxycholic acid (CDCA), the IPCE values of PT2 in the region
of 400e600 nm are increased by about 20%. Moreover, the IPCE
spectrum is also broadened a little. Therefore, the relatively high Jsc
2.5. Electrochemical impedance spectroscopy
The electrochemical impedance spectroscopy (EIS) was engaged
in further study upon the effect of different donors and
p-bridges
on Voc in DSSCs. The impedance spectra of DSSCs were obtained at
ꢀ0.70 V bias in the dark within the frequency range from 0.1 Hz to
100 Hz.
and
h
have been obtained.
Rs, Rres, and RCE represent the series impedance, the charge-
transfer impedances at the interface of TiO2/dye/electrolyte and the
interface of Pt/electrolyte, respectively (see Table S3). In Fig. 7a, the
biggest semicircle corresponds to the Rres. It is known that generally
The photovoltaic performance of DSSCs is characterized under
the AM1.5 simulated sunlight (100 mW cmꢀ2). All the values of the
short circuit current density (Jsc), the open circuit voltage (Voc), fill
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W. Ying et al. / Tetrahedron xxx (2014) 1e8
5
Table 2
Photocurrent-voltage characteristics of untreated DSSCs based on PT1ePT4 and
CDCA with different concentrations of 10, 20 and 30 mM co-adsorbed DSSCs based
on PT2
Dye
CDCA (mM)
Jsc (mA cmꢀ2
)
Voc (mV)
FF
h (%)
PT1
PT2
0
0
10
20
30
0
7.58
7.72
11.44
12.61
10.99
5.70
484
727
656
668
680
324
385
0.65
0.75
0.73
0.73
0.74
0.50
0.55
2.40
4.23
5.45
6.11
5.51
0.94
1.23
PT3
PT4
0
5.76
Illumination condition: AM 1.5 G simulated sunlight (100 mW cmꢀ2).
Electrolyte: 0.05 M I2, 0.5 M LiI in acetonitrile and 3-methoxypropionitrole (7:3, v/v).
Fig. 5. The IeV performance of solar cells sensitized by PT1ePT4 dyes.
Fig. 7. The impedance spectra (a) Nyquist plots and (b) Bode phase plots and (c)
equivalent circuits for DSSCs based on PT1ePT4 dyes measured at ꢀ0.70 V bias in the
dark. The lines of (a) and (b) show theoretical fits using the equivalent circuits (c).
and PT2 are 324, 385, 484 and 727 mV, respectively, which are
completely consistent with the sequence of the electron lifetime.
3. Conclusions
In summary, a series of new sensitizers (PT1ePT4) containing
[1,2,5]thiadiazolo[3,4-c]pyridine moiety as the auxiliary acceptor
were designed and synthesized. By replacing the triphenylamine unit
with N,N-diphenylthiophen-2-amine, the absorption range was fur-
ther broadened. Their absorption spectra, electrochemical and pho-
tovoltaic properties and photovoltaic performance were fully
characterized. Electrochemical measurement data indicate that the
tuning the LUMO and HOMO energy levels can be conveniently ac-
Fig. 6. IeV curves of cells based on PT2 co-sensitized with different concentrations of
CDCA.
complished by tuning the donor and
p-bridge moiety. All of these
the bigger this semicircle is, the higher Rres is, which will restrain
the recombination of electrons. According to Table S3, the Rs and
RCE of the four dyes are similar to each other. However, when it
comes to Rres, we can find the values showing the order of
PT3<PT4<PT1<PT2, which indicates that their electron transfer
resistances at the interface of TiO2/dye/electrolyte are in the same
order. From the Bode phase plots (Fig. 7b), the calculated electron
lifetimes of PT3, PT4, PT1 and PT2 are 0.03, 0.05, 0.06 and 3.38 ms,
respectively. It’s interesting that PT2 has a much longer electron
lifetime than the others. This suggests that PT2 can inhibit the dark
current and reduce the electron recombination, thus we can obtain
a relatively high open circuit voltage. The Voc data of PT3, PT4, PT1
dyes were employed as sensitizers for DSSC photovoltaic perfor-
mance test, PT2-based DSSC showed the best conversion efficiency of
6.11% (Jsc¼12.61 mA cmꢀ2, Voc¼668 mV, FF¼0.74) after treatment
with 20 mM CDCAunder standard global AM 1.5 solar light condition.
4. Experimental
4.1. Instruments and characterization
A Brucker AM 400 spectrometer was employed to obtain 1H
€
NMR and 13C NMR spectra. Mass spectra were measured using an
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6
W. Ying et al. / Tetrahedron xxx (2014) 1e8
ESI mass spectrometer. A Varian Cary 500 spectrophotometer was
utilized to measure the UVevis spectra. The cyclic voltammograms
of dyes were obtained with a Versastat II electrochemical work-
station (Princeton applied research) using a normal three-electrode
system (a Pt working electrode, a Pt wire counter electrode, and
a regular calomel reference electrode in saturated KCl solution).
The photovoltaic characterization was performed on the setup
that constitutes a 450 W Xenon lamp (Oriel), a Schott K113 Tempax
4.5. 7-Bromo-4-chloro-[1,2,5]thiadiazolo[3,4-c]pyridine (2)
The mixture of 2,5-dibromopyridine-3,4-diamine (2 g,
7.5 mmol) with 40 mL thionyl chloride in a 100 mL flask was heated
to reflux for 16 h then cooled to the room temperature. The excess
of thionyl chloride was evaporated and the crude product was
dissolved with 15 mL CH2Cl2. The CH2Cl2 layer was washed using
a saturated solution of NaHCO3 (50 mL) and water (50 mL). The
organic layer was dried with MgSO4 and the solvent was removed
under reduce pressure. The residue was purified by flash chroma-
tography using PE (petroleum ether)/DCM (dichloromethane)¼2:1
(v/v) to obtain 1.57 g of pale yellow needles. (Yield: 84%). 1H NMR
€
sunlight filter (Prazisions Glas & Optik GmbH), and a source meter
(Keithley 2400), which applies potential bias and measures the
photogenerated current. IPCE was measured using an SR830 lockin
amplifier, a 300 W xenon lamp (ILC Technology) and a Gemini-180
double monochromator (Jobin-Yvon Ltd.). The EIS measurements
were carried out using a Zahner IM6e Impedance Analyzer (ZAH-
NER-Elektrik GmbH & CoKG, Kronach, Germany) under the applied
voltage bias of ꢀ0.70 V. The frequency range is 0.1 Hze100 kHz and
the magnitude of the alternating signal is 10 mV.
(400 MHz, CDCl3, ppm):
d
¼8.56 (s, 1H); 13C NMR (100 MHz, CDCl3,
ppm):
d
¼156.10, 148.48, 145.19, 144.62, 110.88; HRMS (ESI, m/z):
[MþH]þ calcd for (C5H2N3SClBr), 249.8841; found, 249.8855.
4.6. 4-(7-Bromo-[1,2,5]thiadiazolo[3,4-c]pyridin-4-yl)-N,N-di-
phenylaniline (3)
4.2. Fabrication of dye-sensitized solar cells
A mixture of compound 2 (498 mg, 2 mmol), N,N-diphenyl-4-
(tributylstannyl)aniline (1.1 g, 2.1 mmol), 20 mg bis(-
triphenylphosphine)palladium(II) dichloride (PdCl2(PPh3)2) in
20 mL of argon-saturated dimethylformamide (DMF) was heated to
95 ꢂC under Ar for 12 h. After cooling to room temperature, the
mixture was poured into 100 mL of water and extracted with
CH2Cl2 (3ꢁ20 mL). The combined organic layers were washed with
water and brine, dried with anhydrous MgSO4 and the solvent was
removed under reduce pressure. The crude product was purified by
flash chromatography with (PE/DCM¼2:1 to 1:1, v/v) to yield
687 mg red solid. Yield: 75%. 1H NMR (400 MHz, CDCl3, ppm):
Two layers of Dyesol 90-T TiO2 paste and a scattering layer were
screen-printed onto the FTO glass and sintered up to 500 ꢂC. After
the treatment of 40 mM TiCl4 in deionized water at room tem-
perature for 12 h, the photoanode was calcinated at 450 ꢂC for
30 min. The photoanodes were cooled down to 80 ꢂC and immersed
in a solution of CDCA with different concentrations in ethanol for
6 h before in a 3ꢁ10ꢀ4 M dye bath in CH2Cl2 solution for 12 h at
room temperature. After rinsed with ethanol, the dye-sensitized
photoanodes were sealed with platinized counter electrode. Sur-
lyn (Dupont, 25-mm-thick) was used as a binder and a spacer. The
d
¼8.74 (s, 1H), 8.48 (d, J¼8 Hz, 2H), 7.43e7.30 (m, 4H), 7.21e7.16 (m,
electrolytes were introduced to the cells via pre-drilled holes in the
counter electrodes. The compositions of the electrolyte were as
follows: 0.05 M iodine, and 0.5 M lithium iodide in acetonitrile/3-
methoxypropionitrole (7:3, v/v). The active area of DSSCs was
0.25 cm2.
6H), 7.14e7.10 (m, 2H); 13C NMR (100 MHz, CDCl3, ppm):
d¼156.65,
152.38, 149.41, 146.79, 130.94, 129.16, 128.14, 125.28, 123.67, 122.24,
121.19, 113.69, 107.92; HRMS (ESI, m/z): [MþH]þ calcd for
(C23H16N4SBr), 459.0279; found, 459.0275.
4.7. 5-(4-(4-(Diphenylamino)phenyl)-[1,2,5]thiadiazolo[3,4-c]
pyridin-7-yl)thiophene-2-carbaldehyde (4)
4.3. Materials and reagents
Fluorine-doped SnO2 conducting glass (FTO glass, transparency
5-Formylthiophen-2-ylboronic acid (156 mg, 1.0 mmol),
Pd(PPh3)4 (20 mg) and 2 M K2CO3 (aq, 5 mL) were added into the
solution of compound 3 (229 mg, 0.5 mmol) in 15 mL THF. The
mixture was heated at 80 ꢂC under Ar for 12 h. The mixture was
cooled to room temperature and diluted with 100 mL water and
100 mL of CH2Cl2. The organic layer was separated and washed
with brine. After drying with anhydrous MgSO4, the solvent was
evaporated. The residue was purified by flash chromatography with
PE/DCM¼2:1 to 1:1 (v/v) to give a red solid (100 mg). Yield: 41%. 1H
>90%, sheet resistance 15
U/sq) was obtained from the Geao Sci-
ence and Educational Co. Ltd. of China. Acetonitrile, tetra-n-butyl
ammonium hexafluorophosphate (TBAPF6), 4-tert-butylpyridine
(4-TBP), and lithium iodide were bought from Fluka and iodine
(99.999%) was purchased from Alfa Aesar. The starting materials
N,N-diphenyl-4-(tributylstannyl)aniline and N,N-diphenyl-5-
(tributylstannyl)thiophen-2-amine were prepared according to
published procedures.27 All other solvents and chemicals used in
this work were of reagent grade and used without further
purification.
NMR (400 MHz, CDCl3, ppm):
d
¼9.99 (s, 1H), 9.03 (s, 1H), 8.60 (d,
J¼8 Hz, 2H), 8.20 (d, J¼4 Hz, 1H), 7.87 (d, J¼4 Hz, 1H), 7.35e7.31 (m,
4H), 7.22e7.18 (m, 6H), 7.15e7.12 (m, 2H); 13C NMR (100 MHz,
4.4. 2,5-Dibromopyridine-3,4-diamine (1)
CDCl3, ppm):
d
¼182.74, 165.14, 155.16, 152.58, 146.68, 144.24,
142.36, 136.43, 130.87, 129.27, 127.63, 125.58, 124.21, 121.01; HRMS
(ESI, m/z): [MþH]þ calcd for (C28H19N4OS2), 491.1000; found,
491.0992.
The mixture of 3,4-diaminopyridine (5.45 g, 50 mmol) with 48%
hydrobromic acid (24 mL) in a 50 mL flask was heated to reflux
while stirring. After adding bromine (8 mL, 155.7 mmol) dropwise,
the reaction mixture was heated for 16 h at 80 ꢂC then cooled down
to the room temperature. The mixture was filtered and washed
with a saturated solution of Na2S2O3 (150 mL), NaHCO3 (150 mL)
and finally water (150 mL). The residue was first purified by flash
chromatography using PE (petroleum ether)/EA (ethyl acetate)¼1:2
(v/v) as eluent and recrystallized in ethanol to give 4 g orange solid.
4.8. 4-(4-(4-(Diphenylamino)phenyl)-[1,2,5]thiadiazolo[3,4-c]
pyridin-7-yl)benzaldehyde (5)
Compound 5 was synthesized in a similar way to 4. The crude
product was purified by flash chromatography with PE/DCM¼1:1
to 1:2 (v/v) to give a red solid. Yield: 78%. 1H NMR (400 MHz, CDCl3,
(Yield: 30%). 1H NMR (400 MHz, DMSO-d6, ppm):
d¼7.53 (s, 1H),
ppm):
J¼8 Hz, 2H), 8.07 (d, J¼4 Hz, 2H), 7.35e7.31 (m, 4H), 7.23e7.20 (m,
6H), 7.14e7.11 (m, 2H); 13C NMR (100 MHz, CDCl3, ppm):
¼191.62,
156.47, 150.34, 149.43, 146.79, 143.45, 140.71, 135.88, 131.06, 130.01,
d¼10.12 (s, 1H), 8.84 (s, 1H), 8.58 (d, J¼8 Hz, 2H), 8.21 (d,
5.99 (s, 2H), 5.05 (s, 2H); 13C NMR (100 MHz, DMSO-d6, ppm):
d
¼140.66, 140.45, 129.81, 128.30, 106.24.; HRMS (ESI, m/z): [MþH]þ
d
calcd for (C5H5N3Br2), 264.8850; found, 264.8844.
Please cite this article in press as: Ying, W.; et al., Tetrahedron (2014), http://dx.doi.org/10.1016/j.tet.2014.04.039

W. Ying et al. / Tetrahedron xxx (2014) 1e8
7
129.63, 129.19, 125.75, 124.32, 121.20; HRMS (ESI, m/z): [MþH]þ
DCM/ethanol¼10;1 (v/v) to yield 87 mg red solid. Yield: 76%. 1H
calcd for (C30H21N4OS), 485.1436; found, 485.1435.
NMR (400 MHz, THF-d8, ppm):
d
¼9.02 (s, 1H), 8.66 (d, J¼8 Hz, 2H),
8.32 (s, 1H), 8.20 (d, J¼4 Hz, 1H), 7.90 (d, J¼4 Hz, 1H), 7.24e7.20 (m,
4.9. 5-(7-Bromo-[1,2,5]thiadiazolo[3,4-c]pyridin-4-yl)-N,N-di-
4H), 7.10e7.08 (m, 4H), 7.04e6.99 (m, 4H); 13C NMR (100 MHz, THF-
phenylthiophen-2-amine (6)
d8, ppm):
d
¼162.79, 154.41, 151.20, 150.61, 148.72, 144.96, 141.61,
137.23, 130.61, 127.08, 125.12, 123.57, 120.26, 118.07; HRMS (ESI, m/
z): [MþH]þ calcd for(C31H20N5O2S2), 558.1058; found, 558.1058.
Anal. Calcd for C31H19N5O2S2: C 66.77, H 3.43, N 12.56. Found C
66.58, H 3.52, N 12.39.
A mixture of compound 2 (498 mg, 2 mmol), N,N-diphenyl-5-
(tributylstannyl)thiophen-2-amine (1.08 g, 2.0 mmol), PdCl2(PPh3)2
(20 mg) in 20 mL of argon-saturated DMF was heated at 95 ꢂC
under Ar for 12 h. After cooling to room temperature, the mixture
was poured into 100 mL of water and extracted with CH2Cl2
(3ꢁ20 mL). The combined organic layers were washed with water
and brine, dried with anhydrous MgSO4 and the solvent was re-
moved under reduce pressure. The crude product was purified by
flash chromatography with PE/DCM¼2:1 to 1:1 (v/v) to yield
687 mg purple solid. Yield: 85%. 1H NMR (400 MHz, CDCl3, ppm):
4.13. 2-Cyano-3-(4-(4-(4-(diphenylamino)phenyl)-[1,2,5]thia-
diazolo-[3,4-c]pyridin-7-yl)phenyl)acrylic acid (PT2)
A procedure similar to that for the dye PT1 but with compound
5 (96 mg, 0.2 mmol) instead of compound 4 giving the dye PT2 as
a red solid (100 mg, yield: 91%). 1H NMR (400 MHz, THF-d8, ppm):
d
¼8.47 (d, J¼8 Hz,1H), 8.45 (s, 1H), 7.35e7.33 (m, 4H), 7.31e7.29 (m,
d
¼8.54 (s, 1H), 8.47e8.43 (m, 2H), 8.42 (s, 1H), 8.00e7.95 (m, 4H),
7.20e7.16 (m, 4H), 7.09e7.05 (m, 4H), 7.03e6.99 (m, 4H); 13C NMR
(100 MHz, THF-d8, ppm):
4H), 7.19e7.16 (m, 2H), 6.58 (d, J¼4 Hz, 1H); 13C NMR (100 MHz,
CDCl3, ppm):
d
¼156.25, 147.83, 146.45, 144.61, 133.38, 129.61,
d¼168.31, 156.43, 153.16, 146.88, 143.39,
129.26, 125.35, 124.96, 123.20, 115.84; HRMS (ESI, m/z): [MþH]þ
136.38, 133.02, 129.15, 128.71, 125.67, 124.24, 120.97, 118.20; HRMS
(m/z): [MþH]þ calcd for (C33H22N5O2S), 552.1494; found, 552.1491.
Anal. Calcd for C33H21N5O2S: C 71.85, H 3.84, N 12.70. Found C 71.67,
H 3.99, N 12.61.
calcd for (C21H14N4S2Br), 464.9843; found, 464.9846.
4.10. 5-(4-(5-(Diphenylamino)thiophen-2-yl)-[1,2,5]thiadia-
zolo[3,4-c]pyridin-7-yl)thiophene-2-carbaldehyde (7)
4.14. 2-Cyano-3-(5-(4-(5-(diphenylamino)thiophen-2-yl)-
[1,2,5]-thiadiazolo[3,4-c]pyridin-7-yl)thiophen-2-yl)acrylic
acid (PT3)
5-Formylthiophen-2-yl boronic acid (156 mg, 1.0 mmol),
Pd(PPh3)4 (20 mg) and 2 M K2CO3 (aq, 5 mL) were added into the
solution of compound 6 (232 mg, 0.5 mmol) in 15 mL THF. The
mixture was heated at 80 ꢂC under Ar for 12 h. The solution was
cooled to room temperature, and extracted with 100 mL water and
100 mL of CH2Cl2. The organic layer was separated and washed
with brine. After drying with anhydrous MgSO4, the solvent was
evaporated. The residue was purified by flash chromatography with
PE/DCM¼2:1 to 1:1 (v/v) to give a purple solid (136 mg). Yield: 55%.
A procedure similar to that for the dye PT1 but with compound
7 (100 mg, 0.2 mmol) instead of compound 4 giving the dye PT3 as
a purple solid (93 mg, yield: 82%). 1H NMR (400 MHz, THF-d8,
ppm):
d
¼8.74 (s, 1H), 8.45 (d, J¼4 Hz, 1H), 8.30 (s, 1H), 8.11 (d,
J¼4 Hz, 1H), 7.88 (d, J¼4 Hz, 1H), 7.29e7.25 (m, 4H), 7.23e7.21 (m,
4H), 7.10e7.07 (m, 2H), 6.48 (d, J¼4 Hz, 1H); 13C NMR (100 MHz,
1H NMR (400 MHz, CDCl3, ppm):
d
¼9.96 (s, 1H), 8.76 (s, 1H), 8.56 (d,
THF-d8, ppm):
d¼167.65, 156.82, 146.95, 145.25, 144.76, 142.29,
J¼4 Hz, 1H), 8.12 (d, J¼4 Hz, 1H), 7.84 (d, J¼4 Hz, 1H), 7.39e7.36 (m,
137.35, 133.65, 129.21, 124.76, 120.30, 115.16; HRMS (m/z): [MþH]þ
calcd for (C29H18N5O2S3), 564.0623; found, 564.0623. Anal. Calcd
for C29H17N5O2S3: C 61.79, H 3.04, N 12.42. Found C 61.63, H 3.16, N
12.30.
4H), 7.34e7.32 (m, 4H), 7.22e7.19 (m, 2H), 6.60 (d, J¼4 Hz, 1H); 13
C
NMR(100 MHz, CDCl3, ppm):
d
¼182.67, 160.74, 154.88, 148.34,
148.01, 146.46, 146.00, 142.69, 134.66, 129.61, 125.40, 125.09, 116.70,
115.54; HRMS (ESI, m/z): [MþH]þ calcd for (C26H17N4OS3),
497.0565; found, 497.0532.
4.15. 2-Cyano-3-(4-(4-(5-(diphenylamino)thiophen-2-yl)-
[1,2,5]-thiadiazolo[3,4-c]pyridin-7-yl)phenyl)acrylic acid (PT4)
4.11. 4-(4-(5-(Diphenylamino)thiophen-2-yl)-[1,2,5]thiadia-
zolo[3,4-c]pyridin-7-yl)benzaldehyde (8)
A procedure similar to that for the dye PT1 but with compound
8 (100 mg, 0.2 mmol) instead of compound 4 giving the dye PT4 as
a purple solid (94 mg, yield: 84%). 1H NMR (400 MHz, THF-d8,
Compound 8 was synthesized in similar way to 7. The crude
product was purified by flash chromatography with PE/DCM¼1:1
to 1:2 (v/v) to give a purple solid. Yield: 77%. 1H NMR (400 MHz,
ppm):
d
¼8.58 (s, 1H), 8.45 (d, J¼4 Hz, 1H), 8.21 (d, J¼8 Hz, 2H), 8.11
(d, J¼8 Hz, 1H), 7.28e7.24 (m, 4H), 7.22e7.20 (m, 4H), 7.09e7.05 (m,
CDCl3, ppm):
d
¼10.09 (s, 1H), 8.58 (s, 1H), 8.56 (d, J¼4 Hz, 1H), 8.15
2H), 6.50 (d, J¼4 Hz, 1H); 13C NMR (100 MHz, THF-d8, ppm):
(d, J¼8 Hz, 2H), 8.03 (d, J¼8 Hz, 2H), 7.39e7.36 (m, 4H), 7.35e7.32
d
¼160.33, 157.29, 154.18, 143.35, 139.33, 133.80, 132.03, 129.15,
(m, 4H), 7.21e7.17 (m, 2H), 6.62 (d, J¼4 Hz, 1H); 13C NMR (100 MHz,
127.37, 124.76, 122.00; HRMS (m/z): [MþH]þ calcd for
CDCl3, ppm):
d¼195.82, 159.82, 156.04, 148.74, 146.63, 143.70,
(C31H20N5O2S2), 558.1056; found, 558.1058. Anal. Calcd for
140.90,134.09, 129.92, 129.03, 124.92,122.23,115.85; HRMS (ESI, m/
C31H19N5O2S2: C 66.77, H 3.43, N 12.56. Found C 66.64, H 3.54, N
z): [MþH]þ calcd for (C28H19N4OS2), 491.1000; found, 491.0977.
12.42.
4.12. 2-Cyano-3-(5-(4-(4-(diphenylamino)phenyl)-[1,2,5]thia-
diazolo-[3,4-c]pyridin-7-yl)thiophen-2-yl)acrylic acid (PT1)
Acknowledgements
This work was supported by NSFC/China (2116110444,
21172073, 91233207 and 21372082) and National Basic Research
973 Program (2013CB733700).
Cyanoacetic acid (85 mg, 1 mmol) and ammonium acetate
(120 mg) were added into the solution of compound 4 (98 mg,
0.2 mmol) in acetic acid (10 mL). The reaction mixture was heated
to reflux under Ar for 12 h. The solution was cooled to room tem-
perature and diluted with 100 mL of water and 100 mL of CH2Cl2
was added. The organic layer was separated and washed with brine
and water. After drying with anhydrous MgSO4, the solvent was
evaporated. The residue was purified by flash chromatography with
Supplementary data
Supplementary data associated with this article can be found in
the online version, at http://dx.doi.org/10.1016/j.tet.2014.04.039.
Please cite this article in press as: Ying, W.; et al., Tetrahedron (2014), http://dx.doi.org/10.1016/j.tet.2014.04.039

8
W. Ying et al. / Tetrahedron xxx (2014) 1e8
12. Ying, W. J.; Yang, J. B.; Wielopolski, M.; Moehl, T.; Moser, J. E.; Comte, P.; Hua, J.
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Please cite this article in press as: Ying, W.; et al., Tetrahedron (2014), http://dx.doi.org/10.1016/j.tet.2014.04.039