Photophysical properties and dye-sensitized solar cell studies on thiadiazole-triazole-chalcone dendrimers

Article abstract of DOI:10.1016/j.tetlet.2011.12.098

Synthesis of charge-separable and hole-transporting dendrimers with chalcone at the periphery, thiadiazole as core and triazole as branching unit has been achieved through 'click' reaction. The dendrimers are used as a charge separator in dye-sensitized s

Full text of DOI:10.1016/j.tetlet.2011.12.098

Tetrahedron Letters 53 (2012) 1139–1143
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Photophysical properties and dye-sensitized solar cell studies on
thiadiazole–triazole–chalcone dendrimers
Perumal Rajakumar a, , Ayyavu Thirunarayanan a, Sebastian Raja a, Shanmugam Ganesan b,
⇑
Pichai Maruthamuthu b
a Department of Organic Chemistry, University of Madras, Maraimalai Campus, Chennai 600 025, India
b Department of Energy, University of Madras, Maraimalai Campus, Chennai 600 025, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Synthesis of charge-separable and hole-transporting dendrimers with chalcone at the periphery, thiadi-
azole as core and triazole as branching unit has been achieved through ‘click’ reaction. The dendrimers
are used as a charge separator in dye-sensitized solar cells (DSSCs) and exhibit higher open-circuit volt-
age than the bare film through the suppression of back electron transfer. As the dendrimer generations
increases the dimension of the dendrimer also increases, resulting in a stronger association with Iꢀ3 redox
couple and higher open-circuit.
Received 7 October 2011
Revised 21 December 2011
Accepted 23 December 2011
Available online 30 December 2011
Keywords:
Dendrimers
Chalcone
1,3,4-Thiadiazole
Alkyne
Ó 2012 Elsevier Ltd. All rights reserved.
Azide
Click
Photoisomerization
DSSC
Over the past two decades, considerable efforts have been fo-
cused on the conversion of solar energy into electricity through
dye-sensitized solar cells (DSSCs) due to their high power conver-
sion efficiency and their potential low cost.1–3 To improve the per-
formance of DSSCs, a great deal of research is being performed on
semiconductor nanocrystalline titanium oxide (TiO2) electrodes,4
molecular dyes,5 electrolytes,6 and counter electrodes.7 Electrolyte
is one of the critical components in DSSCs, which consist of iodine
(I2) and KI in DMF. The dye-sensitized solar cells consists of a
dye-adsorbed wide band gap semiconductor, typically TiO2 and
electrodes immersed in an electrolyte containing a redox couple
(Iꢀ/Iꢀ3 ). The presence of redox electrolyte will probably affect the va-
lance band edge and therefore the number of available donating
states in the semiconductor. Organic molecules play an indispens-
able role in photovoltaic and light harvesting systems due to their
excellent optical and electrical properties.8–10 Organic molecules
such as 4-tert-butylpyridine (TBP), pyrazole, imidazole, triazoles,
pyridine, pyrimidine, and pyrazine have been recently used as addi-
tives in the electrolytic solution of DSSC.11,12 Recent studies from
our laboratory13 have shown that dendrimers play an important
role as an additive to improve the efficiency of DSSCs. Application
of Diels–Alder reaction,14 Cu(I)-catalyzed click reaction15 and
thiol-yne reaction16 are the key issues in dendrimer synthesis. Click
chemistry refers to Cu(I) catalyzed Hüisgen 1,3-dipolar cycloaddi-
tion of azide to alkyne to give 1,4-disubstituted 1,2,3-triazole under
mild reaction conditions with excellent chemo selectivity and good
yield. 1,2,3-Triazole derivatives find a wide range of applications in
biological, medical, and industrial fields.17–19 Further, 1,3,4-thiadi-
azole skeleton acts as a strong electron acceptor unit, usually
employed to modulate the HOMO or LUMO energy levels for
low band gap features in optoelectronic and photophysical de-
vices.20–23 Furthermore, chalcone containing compounds exhibit a
wide variety of useful and emerging optical and electronic proper-
ties.24 Recently, we have reported novel dendrimers bearing
chalcone, carbohydrate and diphenylamine moieties at the
periphery by using ‘click’ approach.25,26 Inspired by the application
of dendrimers as an additive in solar cells, we report herein the
synthesis, photophysical and dye-sensitized solar cell studies of
dendrimers 1, 2, and 3 (Fig. 1) with chalcone as the surface group,
thiadiazole as the core, and triazole as branching units.
In order to synthesize the zeroth generation dendrimer 1, the
alkyne dendron 5 was synthesized in a 65% yield by the treatment
of 1.0 equiv of hydroxyl chalcone 4 with 1.2 equiv of propargyl bro-
mide in the presence of K2CO3 in DMF at room temperature for
24 h. The structure of the alkyne dendron 5 was confirmed from
spectral, analytical, and crystal data.27 In fact the structure of the
⇑
Corresponding author. Tel.: +91 44 22202810; fax: +91 44 22300488.
E-mail address: perumalrajakumar@gmail.com (P. Rajakumar).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.12.098

1140
P. Rajakumar et al. / Tetrahedron Letters 53 (2012) 1139–1143
N
N
S
N
N
N
N
O
O
N
N
MeO
MeO
O
O
OMe
OMe
1
OMe
OMe
MeO
OMe
OMe
OMe
MeO
MeO
N
N
S
N
N
O
O
N
N
O
O
O
N
N
N
N
N
N
N
N
N
N
N
N
N
N
O
O
2
O
MeO
MeO
O
O
OMe
OMe
OMe
MeO
MeO
OMe
OMe
OMe
MeO
MeO
N
N
N
N
N
N
N
N
O
O
N
O
O
O
O
O
O
O
N
N
MeO
MeO
N
O
N
N
S
N
N
N
N
OMe
OMe
OMe
OMe
OMe
N
N
N
N
N
N
N
N
N
N
N
N
N
N
MeO
MeO
O
O
OMe
3
MeO
MeO
N
N
N
N
N
N
N
N
N
O
O
N
N
O
O
O
O
O
O
O
O
MeO
MeO
N
OMe
OMe
OMe
MeO
Figure 1. Molecular structures of dendrimers 1, 2, and 3.
alkyne dendron 5 was confirmed from XRD data and crystal lattice
packing parameters were also determined. The first generation al-
kyne dendron 8 was synthesized from 5 via click chemistry. Reac-
tion of 1.0 equiv of bisazide 626 with 2.1 equiv of alkyne dendron 5
and 5 mol % CuSO4ꢁ5H2O, 10 mol % sodium ascorbate in a mixture
of water, t-BuOH (1:1) at room temperature for 12 h afforded the
hydroxyl chalcone 7 in an 84% yield, which was further treated
with 1.2 equiv of propargyl bromide in the presence of K2CO3 in
DMF at room temperature to give the first generation alkyne den-
dron 8 in an 81% yield. Similarly, the reaction of 1.0 equiv of bisaz-
ide 6 with 2.1 equiv of alkyne dendron 8 under click reaction
conditions afforded the phenolic dendron 9 in a 69% yield, which
was reacted with 1.5 equiv of propargyl bromide in the presence
of K2CO3 in DMF at room temperature for 24 h to give the second
generation alkyne dendron 10 in a 71% yield. The structures of
the alkyne dendrons 8 and 10 were confirmed from spectral and
analytical data (Scheme 1).
dendrimers 1, 2, and 3 were synthesized in 92%, 80%, 67% yields,
respectively, by the treatment of compound 12 with the corre-
sponding alkyne dendrons 5, 8, and 10 under click reaction condi-
tions (Scheme 2).
In 1H NMR spectrum, compound 1 showed two sharp singlets at
d 3.89 and d 3.92 for methoxy protons, two singlets at d 5.27 and
5.99 for N-methylene and O-methylene protons in addition to the
olefinic and aromatic protons. The 13C NMR spectrum of compound
1 displayed four peaks at d 55.9, d 56.2, d 48.0, and d 61.0 for two
different methoxy, N-methylene, and O-methylene carbons. The
carbonyl carbon appeared at d 188.6 in addition to the aromatic
carbons. The appearance of molecular ion peak EI-MS (70 ev) at
m/z = 901 also further confirmed the structure of the dendrimer
1. Similarly, structures of the dendrimers 2 and 3 were thoroughly
characterized and confirmed from spectral and analytical data.
The UV–vis absorption spectra of dendrimers 1, 2, and 3 in
CHCl3 revealed the absorption maxima as listed in Table 1. The
absorption spectra of the dendrimers 1, 2, and 3 showed two
absorption bands, a strong band at 282 nm and a weak band at
350 nm. As the dendrimer generation increases from the zero to
the second the absorption increases due to the increase in the
number of triazole unit and chalcone surface group. With more
To synthesize dendrimers 1, 2, and 3, 1,3,4-thiadiazole bisazide
12 was used as the core moiety, which was obtained in a 78% yield
from 1,3,4-thiadiazole bischloride 1128 and 2.1 equiv of sodium
azide in DMF. The structure of the bisazide 12 was confirmed from
spectral and analytical data. The zero, first, and second generation

P. Rajakumar et al. / Tetrahedron Letters 53 (2012) 1139–1143
1141
N3
HO
(6)
O
O
HO
OMe
OMe
O
OMe
OMe
N3
i
ii
OMe
OMe
5
4
OR
OR
N
N
N
N
N
N
N
N
N
N
N3
N
N
O
O
HO
(6)
O
O
N3
N
N
N
N
iv
N
N
N
N
N
N
N
N
O
O
MeO
MeO
OMe
OMe
O
O
O
O
OMe
OMe
7, R = H
iii
8, R = CH2CCH
O
O
O
O
OMe
OMe
MeO
MeO
OMe
MeO
OMe
MeO
OMe
OMe
MeO
OMe
9, R = H
10, R = CH2CCH
v
Scheme 1. Reagents and conditions: (i) 1.2 equiv propargylbromide, K2CO3, DMF, rt, 24 h. (ii) 1 equiv 6, CuSO4ꢁ5H2O (5 mol %), NaAsc (10 mol %), t-BuOH—H2O (1:1), rt, 12 h.
(iii) 1.2 equiv propargyl bromide, K2CO3, DMF, rt, 24 h. (iv) 1 equiv 6, CuSO4ꢁ5H2O (5 mol %), NaAsc (10 mol %), t-BuOH—H2O (1:1), rt, 12 h. (v) 1.2 equiv propargyl bromide,
K2CO3, DMF, rt, 24 h.
5
1
2
3
Table 1
Optical properties of dendrimers 1, 2, and 3
N
N
N N
ii
Dendrimer UV–visa
Absa
(a.u.)
Emissa
kmax (nm)
e
(mol/L)
Quantum
yield (UF)
i
8
S
S
kmax (nm)
Cl
Cl
N3
N3
1
2
3
289
341
0.22
0.53
0.86
410
435
468
410
435
468
409
434
465
1.54 ꢂ 105 0.052
10
11
1.44 ꢂ 105
12
1.53 ꢂ 105
283
343
4.26 ꢂ 104 0.061
4.90 ꢂ 104
Scheme 2. Reagents and conditions: (i) 2.1 equiv NaN3, DMF, rt, 48 h (ii)
CuSO4ꢁ5H2O (5 mol %), NaAsc (10 mol %), t-BuOH—H2O (1:1), rt, 12 h.
6.02 ꢂ 104
282
350
3.40 ꢂ 104 0.057
3.21 ꢂ 104
number of methoxy groups, stronger absorption was found for the
dendrimer 3 than the dendrimers 1 and 2. The absorbance inten-
sity increases with increase in the number of methoxy bearing
chalcone units in the dendritic architectures which indicates that
the amount of light absorbed by the dendritic antenna increases
from lower generation to higher generation.29
3.94 ꢂ 104
a
Measured in CHCl3 (1 ꢂ 10ꢀ4 M) solution.
fluorescence quantum efficiency. Hence, dendrimer 3 could be
expected to show the higher quantum efficiency among all the
dendrimers due to the valence effect. However, in dendrimer 3
extensive steric crowding suppresses the valency effect of the
chalcone surface units and hence the quantum yield decreases. In
dendrimer 3, steric factor and valency effect act in the opposite
direction and valency effect is suppressed by excess crowding at
the surface of the highly branched dendrimer 3. Moreover, a better
solute-solvent interaction for the compound with increasing charge
transfer may also decrease the quantum yield of compound 3.
Chalcone structural moeity is capable of undergoing photoin-
duced isomerization and as a consequence, the dendrimers with
Dendrimers 1, 2, and 3 exhibited fluorescence maxima at 410,
434, and 468 nm (kexc), respectively (Table 1). The fluorescence
enhancement was confirmed from the quantum yield values (UF
)
of the dendritic molecules. The fluorescence quantum yields of all
the dendrimers were recorded using Quinine sulfate (UF–0.546)
in 0.1 M H2SO4 as an internal standard. Quinine sulfate is mainly
used to standardize the absorbance in the fluorescence spectrum
and not the intensity and hence the concentration of the Quinine
sulfate is immaterial. The quantum yields of 1, 2, and 3 are 0.052,
0.061, and 0.057 (error 0.1%), respectively. In general, increase in
the number of chalcone units might lead to increase in the

1142
P. Rajakumar et al. / Tetrahedron Letters 53 (2012) 1139–1143
Table 2
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Photovoltaic properties of electrolytic system with undopped as well as dopped with
1 min.
2 min
3 min
4 min
5 min
6 min
7 min
8 min
9 min
10 min
11 min
12 min
dendrimers 1, 2, and 3 in KI and I2 redox couple under illumination of 40 mWcmꢀ2 at
AM 1.5
S. No.
Electrolyte
system
JSC
Voc
(mV)
Fill
factor
Efficiency
(%)
(mAcmꢀ2
)
g
1
2
3
4
5
KI/I2/S/Pt
KI/I2/1/Pt
KI/I2/2/Pt
KI/I2/3/Pt
KI/I2/3/Pt
2.5
5.4
6.3
6.9
6.9
420
620
685
760
760
0.49
0.52
0.51
0.54
0.54
1.4
4.3
5.5
7.1
7.1
current–volꢀtage (I–V) characteristics curve for the undopped elec-
trolyte (Iꢀ/I3 ) system and for the same electrolyte system dopped
with the dendrimers 1, 2, and 3 The photovoltaic properties via
short-circuit current (Jsc), the open-circuit voltage (Voc), the fill fac-
250
300
350
400
450
500
Wavelength (nm)
tor and electric energy efficiency (g) are summarized in Table 2.
The photoelectrochemical parameters clearly indicate that the
use of dendrimers 1, 2, and 3 as an additive to the redox electrolyte
improves the power conversion efficiency (g) of dye-sensitized so-
Figure 2. Photoisomerization of dendrimer 2 irradiated at 365 nm UV lamp in
(1 ꢂ 10ꢀ4 M) CHCl3 at room temperature.
lar cell (DSSC). From the present study it is clear that the dendri-
mer 3 with more number of electron donating atoms forms a
charge transfer complex with iodine (I2) in the redox couple, yield-
ing high performance efficiency in dye sensitized solar cell than the
dendrimers 1 and 2. It is well known that the improved hole collec-
tion by Iꢀ, would in turn increase the Voc of the solar cell.37 The de-
crease in the Iꢀ3 ion concentration at the electrodes may reduce the
transfer of the injected electron from the dye and Iꢀ3 ions, thereby
increasing the electron conduction near the TiO2 electrode, result-
ing in a significant effect on the performance of the cell, as reported
earlier.38
In conclusion, we have synthesized novel dendrimers 1, 2, and 3
with chalcone as the surface group, triazole as the branching and
thiadiazole as the core units, through click chemistry approach.
The optical properties indicated that as the generation increases
the intensity of the absorption also increases. PL studies also indi-
cated the same trend of increase in PL emission as the dendrimer
generation increases. Further DSSC studies revealed that the sec-
ond generation dendrimer namely dendrimer 3 shows the higher
3
7
2
6
1
5
4
3
S
2
1
0
0
100 200
300 400 500
600 700 800
Voltage (mV)
Figure 3. Current–voltage response of dendrimer 1, 2, 3 and undoped electrolyte S
at 40 mWcmꢀ2
.
power conversion efficiency (g) of 7.1% with Voc of 0.690 V than
the dendrimer 1 and 2 which show the efficiencies of 4.3% and
5.5% only with Voc of 0.540 V and 0.630 V, respectively.
such moeity can be model compounds for optical memory devices.
Such systems are capable of performing write—lock—read—unlock
erase cycles. Photoinduced trans–cis-isomerization behavior of
dendrimers 1, 2, and 3 were investigated by UV–vis spectropho-
tometry. The dendrimers 1, 2, and 3 (1 ꢂ 10ꢀ4 M in CHCl3) were
irradiated at 365 nm to induce trans–cis-isomerization. Figure 2
shows the photoisomerization of dendrimer 2 on UV irradiation.
Dendrimer 2 undergoes trans–cis-isomerization as revealed by
the decrease in the absorbance for the trans-isomer and increase
in the absorbance for the cis-isomer. The isosbestic point was ob-
served at 295 nm and the system needs 12 min to attain photosta-
tionary state. Similarly, dendritic architectures 1 and 3 also
underwent trans–cis-isomerization with isobestic point at 295 nm.
The organic nitrogenous compounds have influence on the effi-
ciency of DSSCs which can be explained mainly as due to the shift
of the conduction band potential (CBP).30 Significant improvement
of the solar cell performance can be obtained by the addition of
certain organic nitrogenous compounds as an additive to the redox
Acknowledgments
The authors thank UGC, New Delhi, for financial assistance and
DST-FIST for providing NMR facilities to the department. A.T.
thanks CSIR-UGC for JRF and National Centre for Ultra-Fast pro-
cesses, University of Madras for fluorescence studies.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.12.098.
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was a platinum coated fluorinated tin oxide (FTO) conducting glass. The
electrolyte solution was injected into the space between two electrodes. The
electrolyte solution was composed of KI is 2.2 ꢂ 10ꢀ5 M; I2 is 3 ꢂ 10ꢀ6 M, and
dendrimer 7.510ꢀ6
M
as additive in 10 ml DMF solvent. The
photoelectrochemical properties were measured under simulated solar light
at 40 mWcmꢀ2. The photo current–photovoltage (I–V curve) was measured
using a BAS 100A electrochemical analyser. The apparent cell area of TiO2
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