Synthesis and DSSC application of novel dendrimers with benzothiazole and triazole units

Article abstract of DOI:10.1016/j.tetlet.2011.08.126

Synthesis of novel dendrimers with benzothiazole as surface group and triazole as branching unit is achieved through click chemistry. The presence of more number of benzothiazole and triazole units increases the molar absorption coefficient and alters the

Full text of DOI:10.1016/j.tetlet.2011.08.126

Tetrahedron Letters 52 (2011) 5812–5816
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis and DSSC application of novel dendrimers with benzothiazole
and triazole units
Perumal Rajakumar a, , Venkatesan Kalpana a, Shanmugam Ganesan b, Pichai Maruthamuthu b
⇑
a Department of Organic Chemistry, University of Madras, Guindy Campus, Chennai 600 025, India
b Department of Energy, University of Madras, Guindy 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 novel dendrimers with benzothiazole as surface group and triazole as branching unit is
achieved through click chemistry. The presence of more number of benzothiazole and triazole units
increases the molar absorption coefficient and alters the fluorescence as well as electrochemical behav-
iors in the dendrimers. Dye-sensitized solar cell (DSSC) studies reveal that dendrimers with more number
of benzothiazole and triazole groups exhibit better current generating capacity than the dendrimers with
lesser number of benzothiazole and triazole groups. Dendrimer 2b shows the maximum current conver-
Received 9 June 2011
Revised 20 August 2011
Accepted 23 August 2011
Available online 28 August 2011
Keywords:
sion efficiency (g) of 7.1%.
Dendrimer
Benzothiazole
DSSC
Ó 2011 Elsevier Ltd. All rights reserved.
Click chemistry
Dye-sensitized solar cells (DSSCs) have been widely used as the
next-generation solar cells because of their simple structure, light
weight, and high performance in energy conversion.1 Gratzel and
co-worker2 first proposed DSSCs in 1991 and ever since intensive
studies were carried out to improve the efficiency of energy
conversion as well as to reduce the cost of DSSCs.3 DSSCs usually
have a mesoporous TiO2 working electrode, a redox couple such as
IÀ/I3,À and a counter electrode. Under illumination, excited electrons
from the dye are injected into the conduction band of TiO2 and the
electrons travel to a conductive band. The resulting oxidized dye is
reduced by a suitable redox couple. The solar cell performance can
be improved by the addition of certain organic nitrogenous
compounds as additive to the electrolyte. The organic nitrogenous
compound significantly improves the short-circuit current density
(Jsc) and open-circuit voltage (Voc)4 and hence the synthesis of such
organic nitrogenous compounds with excellent optical and electro-
chemical properties has been the focal point of investigation for the
synthetic chemist. Among the new active organic materials, dendri-
mers5 have been the focal point of investigation for DSSC application
because of their well-defined molecular structure with precise
molecular weight, relatively simple, reproducible synthesis, easy
purification, and monodisperse nature with highly branched globu-
lar three-dimensional architecture. The minimal structural modifi-
cation in dendrimers has resulted in functionalized dendrimers
which could act as better light-harvesting antennae,6 sensors,7 solar
cells8 Organic Light Emitting Devices (OLEDS) than the unfunction-
alized dendrimers.
Nowadays various methodologies are used for the synthesis of
dendrimers and recently Cu(I)-catalyzed 1,3-dipolar cycloaddition
of an azide to a terminal alkyne to give triazole, usually referred as
click chemistry9,10 happens to be an important tool for the synthe-
sis of dendrimers. Benzothiazole11 derivatives are known to exhibit
excellent thermal and electrochemical properties and are also used
as organic dyes.12 The use of 2-arylbenzothiazoles as imaging
agents for b-amyloid, antitumor agents, antituberculotics, antipar-
asitics, chemiluminescent agents and as photosensitizers13–18 has
been reported in the literature. Recently, synthesis of dendrimers
with chalcone,19 glucose20 and methyl carboxylate at the periphery
has been reported from our laboratory. We wish to report herein
the synthesis and DSSC application of dendrimers 1, 1a, 1b, 2, 2a,
and 2b as they are the only examples of dendrimers with benzothi-
azole surface group.
The synthetic pathway leading to 2-(4-(azidomethyl)phenyl)
benzothiazole 4 is outlined from 2-(4-(bromomethyl)phenyl)ben-
zothiazole 3 in Scheme 1. Bromide 3 was obtained in 90% yield from
2-(4-(methyl)phenyl)benzothiazole21 and N-bromosuccinimide in
CCl4 in the presence of BPO under refluxing conditions. The reaction
of bromide 3 with NaN3 in acetone at 60 °C afforded azide 4 in 91%
yield.
In order to synthesize the dendritic chloride 7, methyl-3,5-
dihydroxybenzoate was treated with propargyl bromide in the
presence of K2CO3 in DMF to give methyl-3,5-bis(propargyl-
oxy)benzoate 5 which was then reduced with LiAlH4 to give the
⇑
Corresponding author. Tel.: +91 044 22351269x213; fax: +91 44 22300488.
E-mail address: perumalrajakumar@gmail.com (P. Rajakumar).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.08.126

P. Rajakumar et al. / Tetrahedron Letters 52 (2011) 5812–5816
5813
S
N
O
5, X = COOMe
6, X = CH2Cl
N
N
(ii)
X
N
N
O
O
N
O
Cl
E
S
(iii)
3, E = CH2Br
N
N
(i)
N
S
4, E = CH2N3
7
Scheme 1. Reagent and conditions: (i) 1.5 equiv NaN3, acetone–H2O (4:1), 60 °C, 1–3 h; (ii) LAH, THF, rt then followed by SOCl2/Py; (iii) CuSO4 (5 mol %), NaAsc (10 mol %),
tBuOH–H2O (1:1), rt, 10 h.
corresponding alcohol that on further treatment with SOCl2 in
DCM in the presence of pyridine gave 3,5-bis(propargyloxy)ben-
zylchloride 6. Reaction of 1.0 equiv of chloride 6 with 2.1 equiv
of azide 4 under the Cu(I)-catalyzed Huisgen ‘click reaction’ condi-
tions gave the dendritic chloride 7 in 90% yield (Scheme 1).
Dendrimers 1 and 1b were obtained by the O-alkylation of
resorcinol 8 with bromide 3 and chloride 7. However, dendrimer
1a was obtained by the click reaction of azide 4 with bispropargyl-
oxy resorcinol 10. O-Alkylation of resorcinol 8 with 2.1 equiv of
bromide 3 and chloride 7 in the presence of K2CO3 in DMF at 60 °C
for 48 h afforded 1 and 1b in 87% and 73% yields, respectively. Reac-
tion of resorcinol 8 with two equivalents of propargyl bromide in the
presence of K2CO3 in DMF afforded the bispropargylated derivative
10 which under click reaction conditions with azide 4 in the pres-
ence of CuSO4.5H2O (5 mol %) and sodium ascorbate (10 mol %) in
a mixture of water and tBuOH (1:1) at room temperature afforded
dendrimer 1a in 82% yield (Scheme 2). The structure of the dendri-
mers 1, 1a, and 1b was confirmed from spectral and analytical data.
A similar sequence was employed for the synthesis of dendri-
mers 2, 2a, and 2c. Reaction of one equivalent of phloroglucinol
R
HO
OH
8, R = H
3
7
9, R = OH
R
(i)
(i)
(i)
R
Br
O
O
R
O
N
O
N
O
N
O
O
O
O
O
N
N
N
N
N
S
S
N
N
N
N
10, R = H
11, R =
N
N
O
(ii)
4
1, R = H
2, R =
S
N
N
S
R
N
S
S
N
S
N
O
O
N
O
N
S
N
1b, R = H
2b, R =
N
N
N
N
N N
N
O
O
O
N
N
S
N
N
S
N
S
N
1a, R = H
2a, R =
O
N
N
N
S
N
Scheme 2. Synthesis and molecular structures of dendrimers 1, 1a, 1b, 2, 2a and 2b. Reagents and conditions: (i) K2CO3, DMF, 60 °C, 48 h; (ii) CuSO4 (5 mol %), NaAsc
(10 mol %), tBuOH–H2O (1:1), rt, 10 h.

5814
P. Rajakumar et al. / Tetrahedron Letters 52 (2011) 5812–5816
Table 1
and benzothiazole units due to self-quenching and the enhance-
ment of non-radiative transitions. Figure 1 shows the fluorescence
Photophysical properties of dendrimers 1, 1a, 1b, 2, 2a, and 2b in DMF
spectra of dendrimers
1, 1a, 1b, 2, 2a, and 2b in DMF
Compound
kabs max (nm)
e
(mol/L)
kem max (nm)
(1 Â 10À4 mol/L).
1
304
304
304
304
304
304
0.74 Â 10À4
1.44 Â 10À4
1.55 Â 10À4
0.89 Â 10À4
1.12 Â 10À4
1.68 Â 10À4
376
376
374
375
376
376
Cyclic voltammetric (CV) studies were carried out to understand
the electrochemical behavior of the dendrimers in DMF medium in
1a
1b
2
2a
2b
0.04
1
1a
1b
(A)
0.03
9 with 3.1 equiv of bromide 3 and chloride 7 in the presence of
K2CO3 in DMF at 60 °C for 48 h afforded 2 and 2b in 77% and 70%
yields, respectively. Further phloroglucinol 9 was alkylated with
three equivalents of propargyl bromide in the presence of K2CO3
in DMF to give the tris-propargylated compound 11 which on fur-
ther reaction with 3.1 equiv of azide 4 under click reaction condi-
tions in the presence of CuSO4Á5H2O (5 mol %) and sodium
ascorbate (10 mol %) in a mixture of water and tBuOH (1:1) at room
temperature afforded dendrimer 2a in 81% yield (Scheme 2). The
structure of the dendrimers 2, 2a, and 2b was thoroughly charac-
terized from spectral and analytical data.
0.02
0.01
0.00
-0.01
-0.02
-0.03
The UV absorption spectra of compounds1, 1a, 1b, 2, 2a, and 2b
were recorded in DMF (1 Â 10À4 mol/L) and the kmax values are
presented in Table 1. The absorption spectra of the compounds 1,
1a, 1b, 2, 2a, and 2b are almost identical and exhibit an intense
absorption band at 304 nm. Molar absorption coefficient increases
significantly as the generation of dendrimer increases. There is no
spectral broadening or spectral shift with increase in generation
which reflects the symmetrical nature of the dendrimers. Com-
pounds 1b and 2b showed higher absorbance intensity among all
the dendrimers synthesized. The absorbance intensity is found to
increase with the increase in the number of triazole and benzothi-
azole units which indicates that the amount of light absorbed by
the dendritic antenna increases from one generation to the next.22
Upon excitation at 320 nm, the dendrimers 1, 1a, 1b, 2, 2a, and
2b displayed emission maxima at 374–376 nm which is the char-
acteristic emission of the benzothiazole unit. In general, as the gen-
eration of dendrimer increases, the intensity of the emission peak
decreases significantly. The decrease in the intensity of emission
peak in dendrimer 1b could be due to the presence of more num-
ber of triazole groups when compared to 1a and in fact in dendri-
mer 1, the triazole group is absent and hence increases in the
intensity of emission peak. Similar trend is observed with respect
to dendrimers 2, 2a, and 2b also. Moreover the earlier reports from
our laboratory23 have also showed that the presence of the triazole
group influences the emission spectrum in the same manner.
Unlike in the absorbance spectra, the emission spectra showed a
fluorescence quenching with increasing the number of triazole
1.0
0.5
0.0
-0.5
-1.0
-1.5
Potential (V) vs Ag/AgNO3
0.02
0.01
( B )
2
2a
2b
(B)
0.00
-0.01
-0.02
1.0
0.5
0.0
-0.5
-1.0
-1.5
Potential (V) vs Ag/AgNO3
Figure 2. (A) Cyclic voltammogram of dendrimers 1, 1a, and 1b in DMF. (B) Cyclic
voltammogram of dendrimers 2, 2a, and 2b in DMF.
35
35
(B)
2
2a
2b
(A)
1
1a
1b
30
25
20
15
10
5
30
25
20
15
10
5
0
0
350
400
450
500
550
350
400
450
500
550
Wavelength (nm)
Wavelength (nm)
Figure 1. (A). Fluorescence spectra of the dendrimers 1, 1a, and 1b; (B). Fluorescence spectra of dendrimers 2, 2a, and 2b.

P. Rajakumar et al. / Tetrahedron Letters 52 (2011) 5812–5816
5815
Table 2
Electrochemical parameters of dendrimers 1, 1a, 1b, 2, 2a, and 2b in DMF
-
N
+e-
Compound
D
Ep (V)
Eo
Ecp (V)
N
S
Eap1 (V)
Eap3 (V)
.
R
R
1
À1.35
À1.39
À1.42
À1.37
À1.41
À1.38
À1.17
À1.20
À1.25
À1.25
À1.12
À1.24
0.18
0.19
0.17
0.12
0.29
0.14
À1.260
À1.295
À1.335
À1.310
À1.265
À1.311
0.53
0.56
0.52
0.61
0.56
0.62
S
1a
1b
2
2a
2b
In order to investigate photovoltaic properties of the dendri-
mers, the DSSC26 studies were carried out with all the dendrimers
using IÀ/I3À as the electrolyte. DSSC studies revealed that the stabil-
ity and lifetime of the redox couple IÀ/I3À have been very low due to
the sublimation of iodine. The organic materials with excellent
electronic properties are known to exhibit potential applications
in photovoltaic devices.27 During recent times organic nitrogenous
compounds, such as pyrimidine28 alkylpyridine29 are used as addi-
tives to increase the performance of DSSC by increasing the Voc va-
All potentials were measured w.r.t ferrocene as standard.
the presence of TBAP (0.1 M) as the supporting electrolyte with
glassy carbon rod as the working electrode, Ag/AgNO3 (0.1 M) as
the reference electrode, and Pt as the counter electrode. The redox
behavior of all the dendrimers are shown in Figure 2. All the dendri-
mers are comprised of four different types of aromatic moieties viz.,
resorcinol, phloroglucinol, triazole, and benzothiazole. Among
these, benzothiazole moiety preferably undergoes redox reaction
as reported in the literature.24 Compounds 1 and 1a have almost
similar branching arrangements and hence the cyclic voltammo-
gram of the dendrimers shows similar redox reaction. However,
the structural arrangement of the dendrimer 1b is different from
that of 1 and 1a and hence exhibits CV with slight variation.
In the three systems1, 1a, and 1b the cathodic scan direction and
one quasireversible redox reaction were observed whereas during
the reverse scan in the anodic direction, an irreversible peak was ob-
served. The resulting peak potential, peak separation (Ep) and formal
reduction potential (Eo) values are given in Table 2. From these stud-
ies it is clear that benzothiazole molecule undergoes quasireversible
single electron transfer process in cathodic scan direction.
The redox process is almost identical when compared with pre-
vious reported results.25 For example the Eo value at À1.29 V ver-
sus Ag/AgNO3 is due to electrochemical reversible reduction that
occurs at the benzothiazole ring (–C@N bond). Due to the single
electron reduction, the benzothiazole ring forms a stable radical
anion as shown below. During the reverse scan, an irreversible oxi-
dation peak was observed with a peak potential of ꢀ+0.53 V versus
Ag/AgNO3 in the anodic side which may due to the formation of the
radical cation. Dendrimers2, 2a, and 2b show a well resolved cyclic
voltammogram response for the phloroglucinol core. In the dendri-
mers 2, 2a, and 2b each benzothiazole ring is separated spatially
and hence all the peak potential and peak current are well resolved
for the system. From these investigations, the redox property of the
dendrimers provides a strong evidence for the utility of the
molecules in optoelectronic application.
lue and improving the efficiency (g
).30 The nitrogen containing
heterocyclic additives enhance Voc due to electron donating ability
of lone pairs of electrons on the nitrogen atom. The symmetric and
spherical structures of the dendrimers and their ability to form
charge transfer complexes result in decreasing the sublimation of
iodine and enhancing the power conversion efficiency. Although
there are many reports illustrating the role of dendrimers31 on
dye sensitized solar cells, dendrimer additives with benzothiazole
and triazole units have not been reported in DSSC to enhance the
power conversion efficiency. From the I–V curve (Fig. 3) and Table
3, it is seen that the efficiency of DSSC with dendrimers was high
when compared with the blank experiments. Moreover the effi-
ciency of the DSSC performance increases with increase in the num-
ber of benzothiazole and triazole units in the dendrimers. Dendron
7 and all the dendrimers 1, 1a, 1b, 2, 2a, and 2b have symmetric
structures with more number of electron donating atoms such as
oxygen, nitrogen, and sulfur which are useful for the charge separa-
tion and hence assist the fabrication of DSSC.
The synthesized dendrimers can easily form charge transfer com-
plex with iodine in the redox couple (IÀ/I3À) because of the presence
of lone pair of electrons on the oxygen, nitrogen, and sulfur atoms.
The complex formation decreases the sublimation of iodine and en-
hances the efficiency of DSSCs. While comparing the efficiency of
fabricated DSSC using dendrimers 1, 1a, and 1b the efficiency and
Voc increases relatively to a small extent as passing from dendrimer
1 to dendrimer 1a due to slight increase in the number of heterocy-
clic rings such as triazole. However, on passing from dendrimer 1a to
dendrimer 1b, the efficiency and Voc increase enormously due to the
presence of more number of triazole and benzothiazole units. The
dendrimer with more number of heterocyclic rings, such as triazole
and benzothiazole shows higher Voc and Jsc values.
0.0055
0.0065
(1b)
(B)
0.0050
0.0045
0.0040
0.0035
0.0030
0.0025
0.0020
0.0015
0.0010
0.0005
0.0000
(A)
0.0060
(2b)
0.0055
0.0050
0.0045
0.0040
(1a)
0.0035
0.0030
0.0025
0.0020
0.0015
0.0010
0.0005
0.0000
(2a)
(1)
(2)
0
100
200
300
400
500
600
700
0
100
200
300
400
500
600
700
800
Voltage (mV)
Voltage (mV)
Figure 3. (A) Current–voltage(I–V) response of resorcinol core dendrimers 1, 1a, and 1b at 40 mW cmÀ2. (B) Current–voltage response of phloroglucinol core dendrimers 2,
2a, and 2b at 40 mW cmÀ2
.

5816
P. Rajakumar et al. / Tetrahedron Letters 52 (2011) 5812–5816
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The photovoltaic properties of dendrimers with KI and I2 redox couple under
illumination of 40 mW cmÀ2
S. no
Electrolyte system
Jsc/mA cmÀ2
Voc/mV
FF
Efficiency
1
2
3
4
5
6
7
KI/I2
2.1
2.6
3.5
5.1
2.8
4.0
6.2
440
460
520
660
548
570
780
0.51
0.50
0.52
0.56
0.49
0.54
0.59
1.2
1.5
2.4
4.7
1.9
3.0
7.1
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KI/I2/1
KI/I2/1a
KI/I2/1b
KI/I2/2
KI/I2/2a
KI/I2/2b
A similar trend in the power conversion performance is ob-
served among the dendrimers 2, 2a, and 2b. Dendrimers 1b and
2b exhibit better power conversion efficiency and higher Voc values
than all the dendrimers. The DSSC performance characteristics are
summarized in Table 3. To conclude, the presence of electron
donating units and heterocyclic units, such as triazole and benzo-
thiazole significantly alter the Voc and Jsc values of dye sensitized
solar cells and thereby increase the efficiency of DSSC.
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In conclusion, dendrimers with benzothiazole as the surface unit
were synthesized by convergent synthetic strategy in good yields.
All the dendrimers exhibit excellent optical and electrochemical
properties. The cyclic voltammetric studies reveal that the benzothi-
azole molecule undergoes a quasireversible single electron transfer
process in cathodic scan direction and irreversible oxidation was ob-
served at anodic direction for all the dendritic architectures. DSSC
studies clearly show that as the number of benzothiazole and tria-
zole units increase, DSSC performance also increases. Among all
the dendritic structures, dendrimer 2b exhibits better power con-
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26. Fabrication of cell assembly
version efficiency (g) of 7.1% than all other dendrimers.
Acknowledgments
The DSSC, whose photo active area being 1 cm2 (1 cm x 1 cm), was fabricated
on a conducting glass covered with Fluorinated Tin Oxide (F:SnO2) (FTO) and
nano-crystalline TiO2 (degussa) cast by the procedure reported earlier.32 The
electrodes were immersed in a 5X10À5 M solution of the sensitizer namely, cis-
dithiocyanoto bis(2, 2’-bipyridyl-4,4’-dicarboxylate)-ruthenium(II) (N3 dye) in
ethanol for 20 h at room temperature before being washed with ethanol and
dried in air. The dendrimer based electrolyte solution was injected into the
space between two electrodes. The electrolyte solution was composed of
2.2 Â 10À5 M of KI, 3 X 10À6 M of I2, 8.5 X 10À6 M of the dendrimer additives in
DMF (10 mL) solvent. Subsequently, the platinum counter electrode was
pressed on top of the polymer film without any special sealing. With the aid of
a BAS 100A Electrochemical Analyzer, photovoltaic tests of the fabricated
DSSCs were carried out by measuring the current–voltage (I–V) characteristics
under illumination of 40 mW cmÀ2 at AM 1.5 condition using 150 W Xe lamp
as light source.
The authors thank the University Grant Commission (UGC),
New Delhi for financial assistance and the DST-FIST, New Delhi
for NMR spectral facility. V.K thanks Dr. K. Pandian, Department
of Inorganic chemistry, the University of Madras for CV studies.
Supplementary data
Supplementary data (experimental procedure, synthetic and
spectroscopic data for various dendrimers) associated with this
article can be found, in the online version, at doi:10.1016/
j.tetlet.2011.08.126.
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