Synthesis of benzothiazole-benzoxazole dendrimers with triazole as bridging unit and their application in dye-sensitized solar cells

Article abstract of DOI:10.1039/c3nj00887h

Dendrimers with benzothiazole-benzoxazole surface groups and electron transporting triazole as bridging units and diphenyl sulfone as core unit were synthesized successfully by Cu(i)-catalysed Huisgen 'click reaction' using a convergent approach. All the dendrimers exhibit excellent optical and electrochemical properties. Higher-generation dendrimers exhibit better current generating capacity and show better power conversion efficiency than lower-generation dendrimers when used as additives in dye-sensitized solar cells.

Full text of DOI:10.1039/c3nj00887h

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Synthesis of benzothiazole–benzoxazole dendrimers
with triazole as bridging unit and their application in
dye-sensitized solar cells†
Cite this: NewJ.Chem., 2013,
37, 3692
Perumal Rajakumar,*a Venkatesan Kalpana,a Shanmugam Ganesanb and
Pichai Maruthamuthub
Dendrimers with benzothiazole–benzoxazole surface groups and electron transporting triazole as bridging
units and diphenyl sulfone as core unit were synthesized successfully by Cu(I)-catalysed Huisgen ‘click
reaction’ using a convergent approach. All the dendrimers exhibit excellent optical and electrochemical
properties. Higher-generation dendrimers exhibit better current generating capacity and show better
power conversion efficiency than lower-generation dendrimers when used as additives in dye-sensitized
solar cells.
Received (in Montpellier, France)
5th August 2013,
Accepted 12th August 2013
DOI: 10.1039/c3nj00887h
www.rsc.org/njc
Heterocycles12 such as thiazole,13 oxazole,14 imidazole15 systems
play a vital role in the conformational landscape because of charge–
Introduction
The world’s energy needs are growing constantly and all the
while more and more environmental issues are observed as a
consequence of increasing carbon dioxide emission. Hence
there is an increase in pressure to use renewable energy
sources, such as solar energy, but the production of solar cells
charge interaction and charge delocalization over two or more sites.
Such charges can rapidly and efficiently explore all sites of the
molecule with respect to their reactivity so affecting the photo-
physical properties. In this context benzothiazoles, benzoxazoles
and benzimidazoles have the capability of producing a range of
colours with good transport properties, so that they can be used as
both emissive and electron transport materials in OLEDs.
is still too costly to compete with available energy sources.
1
¨
Though in 1991, Michael Gratzel and Brian O’Regan invented
¨
the Gratzel cell in order to reduce the energy demand, still
Benzoheterazole-based dendrimer systems have been applied as
additives in DSSCs16 and light emitting materials. According to the
literature survey, only very few benzothiazole dendrimers have been
synthesized through click chemistry in order to study their applica-
tion in material chemistry. The architypal click reaction that has
gained tremendous attention since it was reported by Fokin and
Sharpless17 and which fulfills all the criteria for the construction of
dendritic structures is the Cu(I) salt catalyzed reaction of azides
with terminal alkynes. This reaction is highly efficient and occurs
under benign conditions in the presence of other reactive groups
with no byproducts. Due to its orthogonal, robust nature, click
chemistry has attracted significant attention of the material science
community, especially for the construction of complex polymeric
materials and dendrimers. Characteristics of click reactions include
high yields, short time duration, stereospecificity, mild reaction
conditions, inexpensive, environmentally as well as eco-friendly
reagents and with no protection and deprotection protocols, in
addition to air- and moisture-tolerant reaction conditions.
practical applications require further research to be carried out
in this area. Dye-sensitized solar cells (DSSCs) are low cost solar
cells characterized by thin films. DSSCs are electrochemical
devices comprising a light-absorbing molecule anchored on to
semiconducting titanium dioxide (TiO2) nanoparticles which
make use of sunlight to generate electricity.
Frechet and Vogtle made a revolutionary breakthrough in the
field of dendrimer chemistry through the synthesis of various types
of dendrimers with varied applications. Dendrimers have received
much attention in the areas of material science and biological
science such as in versatile biomimetic catalysts,4 drug delivery
systems,5 sensors,6 solar cell7 additives and efficient light harvesting
antenna,8 multivalent diagnostics for magnetic resonance imaging
(MRI),9 molecular encapsulation10 and bioconjugate11 chemistry.
2
3
¨
a Department of Organic Chemistry, University of Madras, Guindy Campus,
Chennai 600 025, Tamilnadu, India. E-mail: perumalrajakumar@gmail.com;
Tel: +91 44 2220 2810
b Department of Energy, University of Madras, Guindy Campus, Chennai 600 025,
Tamilnadu, India
Recently, water-soluble glycodendrimers,18 chalcone dendri-
mers,19 quinolone dendrimers,20 benzothiazole dendrimers16 and
diphenylamine dendrimers21 have been reported from our labora-
tory through click chemistry. We report herein the synthesis, photo-
physical properties as well as DSSC22 application of dendrimers 1–6
† Electronic supplementary information (ESI) available: General procedure for
fabrication of cell assemblies and NMR data for some compounds. See DOI:
10.1039/c3nj00887h
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Scheme 1 Molecular structures of dendrimers 1–6.
with benzothiazole–benzoxazole as surface groups and 1,2,3-triazole in Scheme 2. 2-(Prop-2-ynylthio)benzo[d]thiazole 723 and 2-(prop-
as bridging units and diphenyl sulfone as core unit through click 2-ynylthio)benzo[d]oxazole 823 were obtained in 95 and 94%
chemistry using a convergent approach (Scheme 1).
yield by the reaction of 2-mercaptobenzothiazole/2-mercapto-
benzoxazole with 1.1 equiv. of propargyl bromide, TBAB (5 mg)
and KOH (1.5 equiv.) in a mixture of toluene and water (1 : 1) at
room temperature for 2 h (Scheme 2). Reaction of 3,5-bis-
Results and discussion
The synthetic pathway leading to dendritic wedges 12 and 13 from (azidomethyl)phenol 921 with 2.1 equivalents of S-propargyl
2-mercaptobenzothiazole/2-mercaptobenzoxazole is outlined benzoheterazole 7 and 8 in the presence of CuSO4ꢀ5H2O
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Scheme 2 Reagents and conditions: (i) CuSO4ꢀ5H2O (5 mol%), NaAsc (10 mol%), THF–H2O (1 : 1), rt, 10 h, 10 (82%), 11 (80%), 14 (75%), 15 (74%); (ii) K2CO3, DMF,
60 1C, 48 h, 12 (85%), 13 (84%), 16 (79%), 17 (78%).
(5 mol%) and sodium ascorbate (10 mol%) in a mixture of
The propargyl dendritic wedges 12 and 13 were synthesized
water and THF (1 : 1) at room temperature afforded the pheno- from the phenolic dendrons 10 and 11. Reaction of 1.0 equiv. of
lic dendron G1-OH 10 and 11 in 82 and 80% yields, respectively. each of the phenolic dendrons 10 and 11 with 1.1 equiv. of the
In the 1H NMR spectrum, the phenolic dendritic wedge 10 propargyl bromide in the presence of K2CO3 in DMF at room
showed a set of four proton singlets at d 4.59 and 5.29 due to temperature for 24 h gave dendrons 12 and 13 in 85 and 84%
the S-methylene and N-methylene protons, respectively, and a yields, respectively. In the 1H NMR spectrum, compound 13
broad singlet at d 9.88 for the phenolic proton in addition to showed a triplet at d 2.49 for the acetylenic proton and a two
the signals for the aromatic protons. In the 13C NMR spectrum, proton singlet at d 4.53 for the O-methylene proton and a set of
the dendron 10 displayed S-methylene carbon at d 27.5 and four proton singlets at d 4.60 and 5.37 for S-methylene and N-
N-methylene carbon at d 53.8 in addition to the signals for the methylene protons, respectively, in addition to the signals for
aromatic carbons. Further, the structure of dendron 10 was the aromatic protons. In the 13C NMR spectrum, dendron 13
confirmed from the ESI-mass spectrum. Similarly the struc- displayed the S-methylene carbon at d 26.8 and O-methylene
ture of dendron 11 was also confirmed from the spectral and carbon at d 53.6 and N-methylene carbon at d 55.9 in addition
analytical data.
to the signals for the aliphatic and aromatic carbons. In the
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mass spectrum (ESI), dendron 13 displayed a peak at m/z 621 and N-methylene protons, respectively, in addition to the signals
corresponding to the molecular ion. The composition of den- for aromatic protons. In the 13C NMR spectrum, dendron 16
dron 13 was further confirmed from elemental analysis. Simi- displayed the S-methylene carbon at d 27.8 and O-methylene
larly, the structure of the first-generation propargyl dendron 12 carbon at d 53.5, O-methylene at d 53.7 and two N-methylene
was also confirmed from spectral and analytical data.
carbons at d 56.0 and 61.7 in addition to the aliphatic and aromatic
Dendrons 14 and 15 were synthesized in 75 and 74% yields, carbon signals. In the mass spectrum (ESI), dendron 16 showed a
respectively, by the reaction of 1 equiv. of 3,5-bis(azidomethyl)- peak at m/z 1548 corresponding to the molecular ion. The composi-
phenol 9 with 2.1 equiv. of propargyl dendrons 12 and 13 in the tion of dendron 16 was further confirmed from elemental analysis.
presence of a mixture of CuSO4ꢀ5H2O and sodium ascorbate in Similarly, the structure of the second-generation propargyl dendron
aqueous THF solution (1 : 1) (Scheme 2). In the 1H NMR spectrum, 17 was also confirmed from spectral and analytical data.
compound 14 showed two set of four and eight proton singlets at
The zeroth, first and second generation dendrimers 1 and 2
d 4.58, 4.92, 5.23 and 5.30 due to the S-methylene, O-methylene (G0), 3 and 4 (G1) and 5 and 6 (G2) could be achieved by ‘click’
and N-methylene protons, respectively, and one proton broad chemistry approach using 4,40-sulfonylbis(azidomethyl) benzene 18
singlet at d 9.97 for phenolic proton in addition to the signals as the core unit. Reaction of 1.0 equiv. of the bisazide 18 with
for aromatic protons. In the 13C NMR spectrum, dendron 14 2.1 equiv. of each of the alkyne dendrons 7 and 8 (G0) in 5 mol%
displayed the S-methylene carbon at d 27.7, O-methylene carbon CuSO4ꢀ5H2O with 10 mol% sodium ascorbate in a 1 : 1 solvent ratio
at d 53.5 and N-methylene carbons at d 53.7 and 61.7 in addition of THF and H2O for 12 h at room temperature afforded dendrimers
to the signals for aromatic carbons. Further the structure and 1 and 2 (G0) in 85 and 83% yields, respectively (Scheme 3). In the
composition of the dendron 14 was confirmed from its ESI-mass 1H NMR spectrum, compound 2 showed a set of four proton
spectrum and elemental analysis, respectively. Similarly the struc- singlets at d 4.55 and 5.51 for S-methylene and N-methylene
ture of dendron 13 was also confirmed based on spectral and protons, respectively, in addition to the signals for aromatic pro-
analytical data. The second-generation propargyl dendrons 16 tons. In the 13C NMR spectrum, dendrimer 2 displayed S-methylene
and 17 were synthesized in 79 and 78% yields, respectively, by carbon at d 26.7 and N-methylene carbon at d 53.2 in addition to
the treatment of 1.0 equiv. of phenolic dendrons 14 and 15 with the signals for aromatic carbon. In the mass spectrum (ESI)
1.1 equiv. of the propargyl bromide in the presence of K2CO3 in dendrimer 2 showed a peak at m/z 707 corresponding to the
DMF at room temperature for 24 h.
molecular ion. Similarly, the structure of the zeroth generation
In the H NMR spectrum, compound 16 showed a triplet at dendrimer 1 was also confirmed from spectral and analytical data.
d 2.48 for acetylenic proton, a two proton singlet at d 4.59 for Reaction of 1.0 equiv. of the bisazide 18 with 2.1 equiv. of each
1
O-methylene proton and a set of eight proton singlets at d 4.63 and of the alkyne dendrons 12 and 13 (G1) in 5 mol% CuSO4ꢀ5H2O
5.29 for S-methylene and N-methylene protons, respectively, and with 10 mol% sodium ascorbate in a 1 : 1 solvent ratio of THF and
a set of four proton singlets at d 4.98 and 5.44 for O-methylene H2O for 12 h at room temperature afforded dendrimers 3 and 4
Scheme 3 Reagents and conditions: (i) CuSO4ꢀ5H2O (5 mol%), NaAsc (10 mol%), THF–H2O (1 : 1), rt, 10 h, 1 (85%), 2 (83%), 3 (80%), 4 (81%), 5 (73%), 6 (72%).
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(G1) in 80 and 81% yields, respectively (Scheme 3). In the 1H NMR
spectrum, compound 3 showed a set of eight proton singlets at
d 4.56 and 5.23 for S-methylene and N-methylene protons and a
set of four proton singlets at d 4.91 and 5.48 for O-methylene and
N-methylene protons in addition to the signals for aromatic
protons. In the 13C NMR spectrum, dendrimer 3 displayed the
S-methylene carbon at d 27.7 and O-methylene at d 53.5 and two
N-methylene carbons at d 53.9 and 61.6 in addition to the
aromatic carbon signals. In the mass spectrum (ESI), dendrimer
3 showed a peak at m/z 1634 corresponding to the molecular ion.
The composition of the dendrimer 3 was further confirmed from
Table 1 Optical and electrochemical parameters measured for compounds 1–6
in CHCl3
Dendrimer
lmax/nm
lem a/nm
Epc a/V
Epa a/V
1
2
3
4
5
6
244, 278, 302
246, 279, 285
243, 278, 302
245, 278, 286
242, 279, 301
245, 279, 285
394
383
396
383
389
382
ꢂ0.52
ꢂ0.43
ꢂ0.55
ꢂ0.44
ꢂ0.60
ꢂ0.46
1.24
1.51
1.25
1.52
1.28
1.52
a
All spectra were recorded in CHCl3 at room temperature at 10ꢂ4 M.
elemental analysis. Similarly, the structure of the first-generation dendrimers 1, 3 and 5 are almost identical, exhibiting three
dendrimer 4 was also confirmed from spectral and analytical data. absorption bands between 242 and 302 nm.
Finally, the second generation dendrimers 5 and 6 were synthe-
This may be due to the electronic transition that occurs in
sized in 73 and 72% yields, respectively, by the reaction of 1.0 equiv. the benzothiazole unit. Similarly, dendrimers 2, 4 and 6 have
of bisazide 18 with 2.1 equiv. of each of the alkyne dendrons 16 and almost identical surface units, which showed three absorption
17 (G1) under click chemistry conditions for 12 h at room tem- bands, between 245 and 286 nm (Fig. 1), this may be due to
perature (Scheme 3). In the 1H NMR spectrum, compound 5 the electronic transition occurring in the benzoxazole unit. As
showed a four proton singlet at d 4.39 for O-methylene proton, a the number of triazole and benzothiazole–benzoxazole units
sixteen proton singlet at d 4.61 for S-methylene protons, an eight increases from zeroth generation to the second generation, the
proton singlet at d 4.97 for O-methylene protons, a sixteen proton absorption intensity also increases which indicates that the
singlet at d 4.99 for N-methylene protons, eight proton singlet at amount of light absorbed by the dendritic antenna increases
d 5.27 for N-methylene protons and a four proton singlet at d 5.40 from lower generation to higher generation which is otherwise
for N-methylene protons attached to the sulfonyl core, in addition known as valence effect in dendrimer chemistry.24
to the signals for the aromatic protons.
In the 13C NMR spectrum, dendrimer 5 displayed the
Emission studies
S-methylene carbon at d 27.8 and two O-methylene carbons at
d 53.2, 53.5 and three N-methylene carbons at d 53.6, 53.8 and
61.7 in addition to the aromatic carbon signals. The mass
spectrum (FAB-MS) of the dendrimer 5 showed a peak at m/z
3424 corresponding to the molecular ion. The composition of
the dendrimer 5 was further confirmed from elemental ana-
lysis. Similarly, the structure of the second-generation dendri-
mer 6 was also confirmed from spectral and analytical data.
The fluorescence properties of dendrimers 1–6 parallel the
absorption spectra. All the dendrimers showed only one intense
band between 382 and 396 nm and lem values of the fluores-
cence spectra are shown in Table 1.
Unlike in the absorption spectra, the emission spectra showed
fluorescence quenching25 with increasing number of triazole and
benzothiazole–benzoxazole units (Fig. 2). The fluorescence inten-
sity is found to decrease as the number of triazole and benzohe-
terazole units increases in the dendritic architectures.
Absorption studies
Electrochemical properties
The UV absorption emission spectra of compounds 1–6 in
CHCl3 are presented in Fig. 1. The lmax values of UV absorption The redox behaviour of dendrimers 1–6 were studied by cyclic
spectra are shown in Table 1. In their absorption spectra, voltammetry (CV) in CHCl3 (1 ꢁ 10ꢂ4 M) at room temperature.
Fig. 1 (A) UV absorption spectra of dendrimers 1, 3 and 5 in CHCl3 (1 ꢁ 10ꢂ5 M). (B) UV absorption spectra of dendrimers 2, 4 and 6 in CHCl3 (1 ꢁ 10ꢂ5 M).
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Table 2 Performance of cells fabricated with and without additives in the redox
couple
Electrolyte system
Jsc/mA cmꢂ2
Voc/mV
Fill factor
Z (%)
N3 dye TiO2/KI/I2/Pt
N3 dye TiO2/KI/I2/1/Pt
N3 dye TiO2/KI/I2/3/Pt
N3 dye TiO2/KI/I2/5/Pt
N3 dye TiO2/KI/I2/2/Pt
N3 dye TiO2/KI/I2/4/Pt
N3 dye TiO2/KI/I2/6/Pt
5.6
5.6
6.6
11.8
7.1
10.4
12.0
550
714
725
883
730
865
885
0.50
0.50
0.52
0.54
0.53
0.52
0.54
2.2
2.9
3.6
8.0
4.0
6.7
8.2
of triazole and benzoheterazole moiety in the dendritic
architectures.
Fig. 2 Fluorescence spectra of dendrimers 2,
4 and 6 in CHCl3 at room
temperature.
Dye sensitized solar cell studies
The nature of the electrolytic solution can improve the solar cell
performance in dye sensitized solar cells. Against the background
provided in the Introduction section, adding various nitrogen-
containing heterocycles such as pyrimidine and alkylpyridines to
the electrolyte solution increased the open circuit voltage (Voc)
and short circuit current density ( Jsc) values and improved the
efficiency (Z). Hence, nitrogen containing heterocyclic com-
pounds as additives might enhance Voc and Jsc due to the electron
donating ability of the nitrogen atom. In the current investiga-
tion, benzoheterazole dendrimers, whose heterocyclic ring con-
sisted of sulfur, nitrogen, oxygen and carbon were examined as
ꢂ
additives in DMF solution of Iꢂ/I3 redox electrolyte.
The solar cell performance of dendrimers 1–6 were compared to
that without an additive (Table 2). The photophysical and electro-
chemical properties of the dendrimers were also investigated. It
was found that on passing from zeroth to second generation
dendrimers, photophysical properties became altered, which helps
to elucidate the influence of the dendrimer additives on the dye-
sensitized solar cell performance.26
Fig. 3 Cyclic voltammogram of dendrimers 2, 4 and 6 in CHCl3 at room
temperature.
All the compounds exhibited an electrochemical response, and
the cyclic voltammogram of the dendrimers 2, 4 and 6 are
shown in Fig. 3, with redox potentials listed in Table 1. A glassy
carbon working electrode was used with Pt wire counter elec-
trode and Ag/AgCl as reference electrode. CV studies were carried
out from solutions of the substrates (1 mM) in CHCl3 using
tetrabutylammonium perchlorate (0.1 M) as supporting electrolyte.
The redox behaviour of zeroth-generation dendrimer 1 and second-
generation dendrimer 3 are similar, exhibiting one irreversible
cathodic peak which is shifted to slightly higher negative
potential in 3 and one anodic peak which is shifted towards
slightly higher positive potential in 3 due to the presence of
additional triazole and benzoheterazole units. The dendrimers
2, 4 and 6 also show a similar shift in redox potentials (Fig. 3).
Dendrimers 1, 3 and 5 showed a gradual decrease of Epc
from ꢂ0.52, ꢂ0.55 and ꢂ0.60 V and Epa increases slightly as
1.24, 1.25 and 1.28 V. Among the dendrimers 2, 4 and 6, Epc
varied as ꢂ0.43, ꢂ0.44 and ꢂ0.46 and Epa values as 1.51, 1.52
and 1.52 V. The change in Epc and Epa values for 1, 3 and 5 when
In our previous work,16 Voc, Jsc and Z values of 780 mV,
6.2 mA cmꢂ2 and 7.1%, respectively, were reported. In order to
achieve higher short circuit current density and efficiency, the
benzothiazole–benzoxazole dendrimers were synthesized up to
the second generation and used as additive in DSSCs. The
tested dendrimers significantly affected the performance of the
dye-sensitized solar cell as shown in Fig. 4. On passing from
zeroth-generation dendrimers to second-generation dendri-
mers, Voc, Jsc and current also increase due to the presence of
increasing number of triazole and benzoheterazole units. All of
the Voc values for solar cells containing dendrimers were greater
than that for the cell without an additive. Among the tested
dendrimers, second-generation dendrimers 5 and 6 displayed
the largest Voc (883, 885 mV) and Jsc (11.8, 12 mA cmꢂ2) values
and highest efficiency of 8.0 and 8.2%, respectively.
Conclusions
compared with 2, 4 and 6 could be due to change in the In conclusion, synthesis of dendritic structures with triazole
heterocyclic unit and there is only a gradual change on increas- as bridging unit and benzoheterazole as surface group has
ing the dendrimer generation. In conclusion, the electro- been achieved through a ‘click’ chemistry approach. UV studies
chemical behaviour is slightly altered on increasing the number indicate that as the number of triazole units increases, absorption
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Fig. 4 Current–voltage curves for triazole dendrimers 1–6 as additives.
also increases, and hence the light absorbed by the dendritic
Alkyne dendrons 12, 13, 16 and 17; general procedure.
antenna increases with increase in the dendrimer generation. The A mixture of the corresponding hydroxyl dendron (1.0 equiv.)
CV study reflects a mild change in the redox behaviour as a and propargyl bromide (1.1 equiv.) in the presence of anhydrous
function of the dendritic generation. Further as the number of potassium carbonate (5 equiv.) in dry DMF (10 mL) was stirred
triazole units increase in the dendrimers of higher generation, the for 12 h. The reaction mixture was filtered and evaporated to give
DSSC performance also increases. Dendrimers 5 and 6 exhibited the crude alkyne product which was purified by column chromato-
better power conversion efficiency (Z) values of 8.0 and 8.2%, graphy (SiO2).
respectively, when used as additives in DSSC than the control
Core 18. To a solution of 4,40-sulfonyl bis(bromomethyl)-
(N3 dye/KI/I2) as well as other benzothiazole/benzoxazole den- benzene (1.0 g, 2.48 mmol) in a mixture of acetone and water
drimers as additives. Synthesis of similar benzoheterazole (4 : 1, 30 mL), NaN3 (0.48 g, 7.43 mmol) was added and the
dendrimers and their DSSC studies with higher efficiency are mixture was refluxed for 3 h at 60 1C. When the reaction was
underway.
complete, the mixture was concentrated and the crude product was
dissolved with CHCl3 (2 ꢁ 100 mL), washed with water (200 mL)
and brine (50 mL), dried over Na2SO4 and concentrated to give a
residue, which was purified by column chromatography (SiO2).
Dendrimers 1–6; general procedure. A mixture of acetylenic
Experimental
General
All the reagents and solvents employed were of the best grade derivative (2.0 equiv.) and azide (1.0 equiv.) was dissolved in a
available and were used without further purification. Melting mixture of THF and H2O (1 : 1; 20 mL) and sodium ascorbate
points were determined by using a Toshniwal melting point (10 mol%) added followed by the addition of CuSO4ꢀ5H2O
apparatus by the open capillary tube method and are un- (5 mol%). The reaction mixture was stirred overnight at room
corrected. UV/Vis spectra were recorded on a Hitachi U-3210 temperature. The solvent was evaporated and the crude product
spectrophotometer. Fluorescence spectra were recorded on a was dissolved with CHCl3 (2 ꢁ 100 mL), washed with water
Hitachi F-3010 spectrophotometer. 1H and 13C NMR spectra (200 mL) and brine (50 mL), dried over Na2SO4 and concen-
were recorded on Bruker 300 MHz spectrometers. Elemental trated to give a residue, which was purified by column chromato-
analyses were performed on a Perkin-Elmer 240B instrument. graphy (SiO2).
Electrochemical studies were carried out on a CH Instrument
electrochemical analyser.
Phenolic dendron 10 (G1-OH). Yield 82%; mp 112–115 1C;
1H NMR (300 MHz, CDCl3): d 4.59 (s, 4H), 5.29 (s, 4H), 6.55
(s, 1H), 6.76 (s, 2H), 7.27 (t, J = 8.1 Hz, 2H), 7.39 (t, J = 7.8 Hz,
2H), 7.56 (s, 2H), 7.70 (d, J = 7.8 Hz, 2H), 7.81 (d, J = 8.1 Hz, 2H),
Syntheses
Phenolic dendrons 10, 11, 14 and 15; general procedure. 9.88 (br s, 1H) ppm; 13C NMR (75 MHz, CDCl3) d 27.5, 53.8,
A mixture of bisazide 9 (1.0 equiv.) and acetylenic derivative 115.5, 118.0, 121.1, 121.4, 123.3, 124.5, 126.2, 135.4, 136.7,
(2.0 equiv.) was dissolved in a mixture of THF and H2O (1 : 1; 144.2, 152.9, 158.6, 165.5 ppm; MS (ESI) m/z 615 (M+). Anal.
20 mL) and sodium ascorbate (10 mol%) added followed by calc. for C28H22N8OS4: C 54.70, H 3.61, N 18.23. Found: C 54.62,
the addition of CuSO4ꢀ5H2O (5 mol%). The reaction mixture H 3.72, N 18.31%.
was stirred overnight at room temperature. The solvent was
Phenolic dendron 11 (G1-OH). Yield 80%; mp 172–175 1C;
evaporated and the crude product was dissolved with CHCl3 1H NMR (300 MHz, CDCl3): d 4.56 (s, 4H), 5.33 (s, 4H), 6.61
(2 ꢁ 100 mL), washed with water (200 mL) and brine (50 mL), (s, 1H), 6.76 (s, 2H), 7.20–7.29 (m, 4H), 7.38–7.41 (m, 2H), 7.55
dried over Na2SO4 and concentrated to give a residue, which (d, J = 7.2 Hz, 2H), 7.64 (s, 2H), 9.66 (s, 1H) ppm; 13C NMR
was purified by column chromatography (SiO2).
(75 MHz, CDCl3): d 26.5, 53.9, 110.1, 115.5 118.2, 118.4, 123.2,
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124.2, 124.4, 136.7, 141.6, 143.9, 152.0, 158.6, 164.0 ppm; MS (m, 4H), 7.53 (s, 2H), 7.57 (s, 4H), 7.69 (d, J = 7.8 Hz, 4H), 7.83
(ESI) m/z 582 (M+). Anal. calc. for C28H22N8O3S2: C 57.72, H 3.81, (d, J = 8.1 Hz, 4H) ppm. 13C NMR (75 MHz, CDCl3): d 27.9, 53.6,
N 19.23. Found: C 57.62 H 3.72, N 19.31%.
53.7, 56.9, 61.4, 76.2, 78.4, 114.4, 114.9, 119.6, 120.5, 121.1,
1
Alkyne dendron 12 (G1). Yield 85%; mp 91–94 1C; H NMR 121.5, 123.2, 123.3, 124.5, 126.2, 135.4, 137.1, 137.2, 143.5,
(300 MHz, CDCl3): d 2.49 (t, J = 2.4 Hz, 1H), 4.49 (s, 2H), 4.63 144.3, 152.9, 158.4, 158.0, 165.9 ppm; MS (ESI) m/z 1483 (M+).
(s, 4H), 5.31 (s, 4H), 6.55 (s, 1H), 6.69 (s, 2H), 7.26 (t, J = 7.2 Hz, Anal. calc. for C73H58N22O7S4: C 59.10, H 3.94, N 20.77. Found:
2H), 7.38 (t, J = 7.5 Hz, 2H), 7.56 (s, 2H), 7.70 (d, J = 8.1 Hz, 2H), C 59.20, H 3.99, N 20.65%.
7.81 (d, J = 8.1 Hz, 2H) ppm; 13C NMR (75 MHz, CDCl3): d 27.7,
4,40-Sulfonylbis(azidomethyl)benzene 18. Yield 95%; mp
53.5, 55.8, 76.4, 76.8, 114.4, 120.0, 121.2, 121.4, 123.2, 124.5, 134–138 1C; 1H NMR (300 MHz, CDCl3): d 4.43 (s, 4H), 7.46
126.2, 135.4, 137.1, 144.3, 152.9, 158.3, 165.7 ppm; MS (ESI) m/z (d, J = 9.6 Hz, 4H), 7.95 (d, J = 9.0 Hz, 4H) ppm; 13C NMR
653 (M+). Anal. calc. for C31H24N8OS4: C 57.03, H 3.71, N 17.16. (75 MHz, CDCl3): d 53.9, 128.3, 128.7, 141.2, 141.4 ppm; MS
Found: C 57.12, H 3.79, N 17.21%.
Alkyne dendron 13 (G1). Yield 84%; mp 143–145 1C; 1H NMR N 25.59. Found: C 51.29, H 3.74, N 25.65%.
(300 MHz, CDCl3): d 2.49 (t, J = 2.4 Hz, 1H), 4.53 (s, 2H), 4.60
(ESI) m/z 328 (M+). Anal. calc. for C14H12N6O2S: C 51.21, H 3.68,
Dendrimer (G0) 1. Yield 85%; mp 78–80 1C; 1H NMR
(s, 4H), 5.37 (s, 4H), 6.71 (s, 1H), 6.73 (s, 2H), 7.21–7.3 (m, 4H), (300 MHz, CDCl3): d 4.66 (s, 4H), 5.50 (s, 4H), 7.01 (d, J = 8.4 Hz,
7.42 (d, J = 7.2 Hz, 2H), 7.57 (d, J = 6.9 Hz, 2H), 7.63 (s, 2H) ppm; 1H), 7.24–7.34 (m, 5H), 7.42 (d, J = 8.1 Hz, 2H), 7.59 (s, 2H), 7.66–
13C NMR (75 MHz, CDCl3): d 26.8, 53.6, 55.9, 76.4, 78.9, 110.0, 7.81 (m, 6H); 7.81 (s, 2H) ppm; 13C NMR (75 MHz, CDCl3): d 27.7,
114.6, 118.4, 120.1, 123.0, 124.1, 124.4 137.1, 141.7, 144.1, 53.2, 110.3, 121.2, 121.5, 123.2, 124.5, 124.7, 126.2, 128.5, 135.5,
152.0, 158.4, 164.2 ppm; MS (ESI) m/z 621 (M+). Anal. calc. for 140.3, 144.9, 152.9, 165.5 ppm; MS (EI) m/z 739 (M+). Anal. calc. for
C
31H24N8O3S2: C 59.99, H 3.90, N 18.05. Found: C 59.89, H 3.87, C34H26N8O2S5: C 55.26, H 3.55, N 15.16. Found: C 55.16, H 3.42,
N 18.15%. N 15.20%.
Phenolic dendron 14 (G2-OH). Yield 75%; mp 98–102 1C;
Dendrimer (G0) 2. Yield 83%; mp 135–136 1C; 1H NMR (300 MHz,
1H NMR (300 MHz, CDCl3): d 4.58 (s, 8H), 4.92 (s, 4H), 5.23 (s, CDCl3): d 4.55 (s, 4H), 5.51 (s, 4H), 7.07 (d, J = 8.1 Hz, 2H), 7.29
8H), 5.30 (s, 4H), 6.58 (s, 2H), 6.64 (s, 5H), 6.68 (s, 2H), 7.19– (s, 2H), 7.42 (d, J = 6.9 Hz, 2H), 7.53–7.55 (m, 2H), 7.66 (s, 2H),
7.27 (m, 4H), 7.34 (t, J = 7.8 Hz, 4H), 7.47 (s, 2H), 7.59 (s, 4H), 7.77–7.84 (m, 4H), 8.03 (s, 2H), 8.21 (s, 2H) ppm; 13C NMR
7.66 (d, J = 8.1 Hz, 4H), 7.78 (d, J = 8.1 Hz, 4H), 9.97 (br s, 1H); (75 MHz, CDCl3): d 26.7, 53.2, 110.1, 118.5, 123.3, 123.8, 124.3,
13C NMR (75 MHz, CDCl3): d 27.7, 53.5, 53.7, 61.7, 114.6, 115.5, 124.4, 124.5, 127.8, 128.6, 141.6, 148.8, 152.0, 163.6 ppm;
118.6, 119.8, 121.1, 121.4, 123.3, 123.5, 124.5, 126.2, 135.3, MS (EI) m/z 707 (M+). Anal. calc. for C34H26N8O4S3: C 57.78,
136.8, 137.1, 143.4, 144.1, 152.9, 158.6, 158.9, 165.7 ppm. MS H 3.71, N 15.85. Found: C 57.86, H 3.62, N, 15.90%.
(ESI): m/z 1510 (M+). Anal. calc. for C70H56N22O3S8: C 55.68, H
3.74, N 20.41. Found: C 55.59, H 3.79, N 20.31%.
Dendrimer (G1) 3. Yield 80%; mp 74–76 1C; 1H NMR
(300 MHz, CDCl3): d 4.56 (s, 8H), 4.91 (s, 4H), 5.23 (s, 8H),
Phenolic dendron 15 (G2-OH). Yield 74%; mp 120–123 1C; 5.48 (s, 4H), 6.55 (s, 2H), 6.62 (s, 4H), 7.17–7.21 (m, 2H), 7.27–
1H NMR (300 MHz, CDCl3): d 4.59 (s, 8H), 5.02 (s, 4H), 5.32 7.37 (m, 8H), 7.47 (s, 2H), 7.49 (s, 4H), 7.63 (d, J = 7.8 Hz, 4H),
(s, 8H), 5.56 (s, 4H), 6.67 (s, 2H), 6.72 (s, 5H), 7.21–7.24 (m, 6H), 7.74 (d, J = 8.1 Hz, 4H), 7.91 (d, J = 8.7 Hz, 6H) ppm; 13C NMR
7.35–7.41 (m, 4H), 7.54 (s, 2H), 7.57 (s, 4H), 7.64 (s, 4H), 7.91 (75 MHz, CDCl3): d 27.7, 53.5, 53.9, 61.6, 114.4, 119.7, 121.2,
(d, J = 8.1 Hz, 4H), 9.97 (br s, 1H) ppm; 13C NMR (75 MHz, 121.5, 123.3, 124.5, 126.2, 128.3, 128.5, 128.7, 128.8, 135.4,
CDCl3): d 26.8, 53.3, 53.5, 61.7, 110.0, 114.5, 118.4, 119.8, 123.2, 137.1, 140.1, 143.7, 144.4, 152.9, 158.9, 165.7 ppm; MS (ESI)
123.5, 124.1, 124.4, 128.6, 128.8, 137.1, 141.6, 141.7, 143.7, m/z 1634 (M+). Anal. calc. for C76H60N22O4S9: C 55.86, H 3.70,
144.0, 152.0, 158.9, 164.1 ppm; MS (ESI) m/z 1445 (M+). Anal. N 18.86. Found: C 55.75 H 3.62, N 18.90%.
calc. for C70H56N22O7S4: C 58.16, H 3.90, N 21.32. Found: C
Dendrimer (G1) 4. Yield 81%; mp 102–105 1C; 1H NMR
(300 MHz, CDCl3): d 4.59 (s, 8H), 5.02 (s, 4H), 5.32 (s, 8H), 5.56
58.24, H 3.85, N 21.21%.
1
Alkyne dendron 16 (G2). Yield 79%; mp 85–88 1C; H NMR (s, 4H), 6.67–6.73 (m, 6H), 7.19–7.29 (m, 10H), 7.38 (t, J = 9.0 Hz,
(300 MHz, CDCl3): d 2.48 (t, J = 2.4 Hz, 1H), 4.59 (s, 2H), 4.63 6H), 7.54–7.57 (m, 4H), 7.65 (s, 4H), 7.91 (d, J = 7.8 Hz, 6H) ppm;
(s, 8H), 4.98 (s, 4H), 5.29 (s, 8H), 5.44 (s, 4H), 6.61 (s, 2H), 6.70 13C NMR (75 MHz, CDCl3): d 26.8, 53.3, 53.5, 61.7, 110.0, 114.5,
(s, 4H), 6.81 (s, 3H), 7.22 (s, 1H), 7.23–7.28 (m, 3H), 7.34–7.40 118.4, 119.8, 123.2, 123.5, 124.1, 124.4, 128.6, 128.8, 137.1, 141.6,
(m, 4H), 7.53 (s, 2H), 7.57 (s, 4H), 7.69 (d, J = 7.8 Hz, 4H), 7.80 141.7, 143.7, 144.0, 152.0, 158.9, 164.1 ppm; MS (ESI) m/z 1569
(d, J = 8.1 Hz, 4H) ppm; 13C NMR (75 MHz, CDCl3): d 27.8, 53.5, (M+). Anal. calc. for C76H60N22O8S5: C 58.15, H 3.85, N 19.63.
53.7, 56.0, 61.7, 76.5, 77.7, 114.4, 114.9, 119.6, 120.5, 121.1, Found: C 58.25, H 3.79, N 19.70%.
121.5, 123.2, 123.3, 124.5, 126.2, 135.4, 137.1, 137.2, 143.5,
Dendrimer (G2) 5. Yield 73%; mp 136–140 1C; 1H NMR
144.3, 152.9, 158.4, 159.0, 165.7 ppm; MS (ESI) m/z 1548 (M+). (300 MHz, CDCl3): d 4.39 (s, 4H), 4.61 (s, 16H), 4.97 (s, 8H),
Anal. calc. for C73H58N22O3S8: C 58.64, H 3.78, N 19.91. Found: 4.99 (s, 16H), 5.27 (s, 8H), 5.40 (s, 4H), 6.59 (s, 4H), 6.67–6.79
C 58.22, H 3.85, N 19.81%.
(m, 14H), 7.22–7.27 (m, 4H), 7.31–7.44 (m, 12H), 7.54 (s, 4H),
Alkyne dendron (G2) 17. Yield 78%; mp 125–126 1C; 1H NMR 7.58 (s, 8H), 7.60 (s, 2H), 7.69 (d, J = 7.8 Hz, 8H), 7.80 (d, J = 8.1 Hz,
(300 MHz, CDCl3): d 2.49 (t, J = 2.4 Hz, 1H), 4.56 (s, 2H), 4.63 8H), 7.83–7.91 (m, 8H) ppm; 13C NMR (75 MHz, CDCl3): d 27.8,
(s, 8H), 4.98 (s, 4H), 5.29 (s, 8H), 5.46 (s, 4H), 6.61 (s, 2H), 6.70 53.2, 53.5, 53.6, 53.8, 61.7, 114.4, 114.6, 119.6, 120.2, 121.2, 121.5,
(s, 4H), 6.82 (s, 3H), 7.21 (s, 1H), 7.23–7.28 (m, 3H), 7.34–7.43 123.3, 123.5, 123.7, 124.5, 126.2, 128.3, 128.4, 128.7, 128.8, 135.4,
c
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NJC
Paper
9 E. C. Wiener, M. W. Brechbiel, H. Brothers, R. L. Magin,
D. A. Tomalia and P. C. Lauterbur, Magn. Reson. Med., 1994,
31, 1.
10 A. W. Kleiji, R. A. Gossage, R. J. M. K. Gebbink,
N. Brinkmann, E. Reijerse, F. Vogtle, V. Vicinelli, V. Ceroni,
M. Maestri and V. Balzani, Angew. Chem., Int. Ed., 2002,
41, 3595.
11 (a) Q. Wang, T. R. Chan, R. Hilgraf, V. V. Fokin, K. B.
Sharpless and M. G. Finn, J. Am. Chem. Soc., 2003,
125, 3192; (b) G. A. Burley, J. Gierlich, M. R. Mofid, H. Nir,
S. Tal, Y. Eichen and T. Carell, J. Am. Chem. Soc., 2006,
128, 1398.
137.2, 141.6, 143.5, 143.7, 144.3, 152.9, 158.9, 159.0, 165.7 ppm;
MS (FAB) m/z 3424 (M+). Anal. calc. for C160H128N50O8S17: C 56.12,
H 3.77, N 20.45. Found: C 56.20, H 3.79, N 20.50%.
Dendrimer (G2) 6. Yield 72%; mp 82–86 1C; 1H NMR
(300 MHz, CDCl3): d 4.40 (s, 4H), 4.71 (s, 16H), 4.97 (s, 8H),
4.99 (s, 16H), 5.37 (s, 8H), 5.40 (s, 4H), 6.59 (s, 4H), 6.67–6.80
(m, 14H), 7.22–7.28 (m, 4H), 7.31–7.44 (m, 12H), 7.54 (s, 4H),
7.58 (s, 8H), 7.60 (s, 2H), 7.69 (d, J = 7.8 Hz, 8H), 7.81 (d, J = 8.1 Hz,
8H), 7.83–7.95 (m, 8H) ppm; 13C NMR (75 MHz, CDCl3): d 26.7,
53.2, 53.5, 53.8, 56.0, 61.6, 110.0, 114.5, 115.1, 118.4, 119.7, 120.7,
121.4, 122.9, 123.3, 124.1, 124.4, 125.2, 127.7, 128.1, 128.4, 128.8,
137.2, 141.7, 143.9, 145.2, 151.9, 158.9, 159.3, 164.0 ppm; MS
(FAB) m/z 3295 (M+). Anal. calc. for C160H128N50O16S9: C 58.31,
H 3.91, N 21.25. Found: C 58.26, H 3.89, N 21.30%.
12 B. Jena, S. Sundar Manoharan and S. Prakash, J. Phys. Chem.
C, 2009, 113, 20942.
13 (a) R. Kannan, G. S. He, L. Yuan, F. Xu, P. Prasad and
A. G. Dombroskie, Chem. Mater., 2001, 13, 1896; (b) G. S.
Chiang, C. F. Shu and C. T. Chen, Org. Lett., 2005, 7, 3717.
14 C. W. Ko, Y. T. Tao, A. Danel, L. Krzeminska and
P. Tomasik, Chem. Mater., 2001, 13, 2441.
15 (a) F. C. Krebs and H. Spanggaard, J. Org. Chem., 2002,
67, 7185; (b) F. Liang, Q. Zhou, Y. Cheng, L. Wang, D. Ma,
X. Jing and F. Wang, Chem. Mater., 2003, 15, 1935;
(c) W. S. Huang, J. Limn, C. H. Chien, Y. T. Tao, S. Sun
and Y. S. Wen, Chem. Mater., 2004, 16, 280; (d) R. Y. Wang,
W. L. Jia, H. Aziz, G. Vamvounis, S. Wang, N. X. Hu,
Z. D. Popovic and J. A. Coggan, Adv. Funct. Mater., 2005,
15, 1483.
Acknowledgements
The authors thank the University Grants Commission (UGC) for
financial assistance and DIST-FIST New Delhi for NMR spectral
facility. V. K. thanks Council of Scientific and Industrial
Research (CSIR), New Delhi for a SRF fellowship.
References
1 B. O’Regan and M. Gratzel, Nature, 1991, 353, 737.
2 (a) C. J. Hawker and J. M. J. Frechet, Three-Dimensional
Dendritic Macromolecules: Design, Synthesis and Properties, in
New Methods of Polymer Synthesis, ed. J. R. Ebdon and 16 P. Rajakumar, V. Kalpana, S. Ganesan and P. Maruthamuthu,
G. C. Eastmond, Blackie Academic and Professional, Glasgow, Tetrahedron Lett., 2011, 52, 5812.
1995, vol. 2; (b) J. M. J. Frechet and D. A. Tomalia, Dendrimers 17 V. V. Rostovtsev, L. G. Green, V. V. Fokin and K. B.
and Other Dendritic Polymers, John Wiley & Sons, Ltd, Chichester, Sharpless, Angew. Chem., Int. Ed., 2002, 41, 2596.
2001; (c) C. J. Hawker and J. M. J. Frechet, J. Chem. Soc., Chem. 18 P. Rajakumar, R. Anandhan and V. Kalpana, Synlett, 2009,
Commun., 1990, 1010; (d) C. J. Hawker and J. M. J. Frechet, J. Am.
Chem. Soc., 1990, 112, 7638.
1417.
19 P. Rajakumar and S. Raja, Tetrahedron Lett., 2008, 49, 6539.
¨
3 E. Buhleier, W. Wehner and F. Vogtle, Synthesis, 1978, 155. 20 P. Rajakumar and R. Raja, Tetrahedron Lett., 2010, 51,
4 W. T. S. Huck, L. J. Prins, R. H. Fokkens, N. M. M. 4365.
Nibbering, F. C. J. M. van Veggel and D. N. Reinhoudt, 21 P. Rajakumar, C. Satheeshkumar and S. Raja, Tetrahedron
J. Am. Chem. Soc., 1998, 120, 6240. Lett., 2010, 51, 5167.
5 (a) S. H. Medina and M. E. H. El-Sayed, Chem. Rev., 2009, 22 (a) S. Raja, C. Satheeshkumar, P. Rajakumar, S. Ganesan
109, 3141; (b) N. A. Peppas, T. Nagai and M. Miyajima,
Pharm. Tech. Jpn., 1994, 10, 611; (c) S. Svenson and D. A.
Tomalia, Adv. Drug Delivery Rev., 2005, 57, 2106; (d) I. J.
Majoros, A. Myc, T. Thomas, C. B. Mehta and J. R. Baker, Jr.,
Biomacromolecules, 2006, 7, 572.
and P. Maruthamuthu, J. Mater. Chem., 2011, 21, 7700;
(b) P. Rajakumar, S. Raja, C. Satheeshkumar, S. Ganesan,
P. Maruthamuthu and S. A. Suthanthiraraj, New J. Chem.,
2010, 34, 2247; (c) N. Satoh, T. Nakashima and K. Yamamoto,
J. Am. Chem. Soc., 2005, 127, 13030; (d) K. Yamamoto and
K. Takanashi, Polymer, 2008, 49, 4033.
6 (a) V. Balzani, P. Ceroni, S. Gestermann, C. Kauffmann,
M. Gorka and F. Vogtle, Chem. Commun., 2000, 853; (b) T. D. 23 (a) K. Prabakaran and F. Nawaz Khan, Phosphorus, Sulfur
Jamnes, H. Shinmori, M. Takeuchi and S. Shinkai, Chem.
Commun., 1996, 705.
7 D. Astruc, E. Boisselier and C. T. Orneals, Chem. Rev., 2010,
110, 1857.
8 (a) G. M. Stewart and M. A. Fox, J. Am. Chem. Soc., 1996,
118, 4354; (b) T. H. Ghaddar, J. F. Wishart, D. W. Thompson,
Silicon Relat. Elem., 2010, 185, 825–831; (b) K. Rubina,
M. Fleisher, E. Abele, Yu. Popelis and E. Lukevits, Russ. J.
Org. Chem., 2003, 39, 963–967.
24 T. H. Kwon, M. K. Kim, J. Kwon, T. Y. Shin, S. J. Park,
C. L. Lee, J. J. Kim and J. I. Hong, Chem. Mater., 2007,
19, 3673.
J. K. Whitesell and M. A. Fox, J. Am. Chem. Soc., 2002, 25 P. Rajakumar, M. Dhanasekaran and S. Selvam, Synthesis,
124, 8285; (c) A. Adronov and J. M. J. Frechet, Chem. 2006, 1257.
Commun., 2000, 1701; (d) S. L. Gilat, A. Adronov and J. M. J. 26 P. M. Sirimanne, T. Shirata and T. Soga, J. Solid State Chem.,
Frechet, Angew. Chem., Int. Ed., 1999, 38, 1422. 2002, 166, 142–147.
c
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