Synthesis of triazole dendrimers with a dimethyl isophthalate surface group and their application to dye-sensitized solar cells

Article abstract of DOI:10.1039/c0nj00044b

Dendritic architectures with 1,2,3-triazole as the building unit and dimethyl isophthalate as the surface group are synthesized through a convergent approach employing click chemistry and tested for their application in dye-sensitized solar cells (DSSCs).

Full text of DOI:10.1039/c0nj00044b

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Synthesis of triazole dendrimers with a dimethyl isophthalate surface
group and their application to dye-sensitized solar cells
Perumal Rajakumar,*a Sebastian Raja,a Chinnadurai Satheeshkumar,a
Shanmugam Ganesan,b Pichai Maruthamuthub and Samuel Austin Suthanthirarajb
Received (in Montpellier, France) 20th January 2010, Accepted 12th May 2010
DOI: 10.1039/c0nj00044b
Dendritic architectures with 1,2,3-triazole as the building unit and dimethyl isophthalate
as the surface group are synthesized through a convergent approach employing click
chemistry and tested for their application in dye-sensitized solar cells (DSSCs).
The studies revealed that the presence of triazole and isophthalate groups in dendritic
structures significantly altered the absorption, electrochemical and DSSC behaviors.
In DSSCs, the dendritic structures exhibited higher Voc values and higher power
À
conversion efficiency (Z) in the IÀ/I3 redox couple under simulated conditions
at 40 mW cmÀ2
.
applications as fluorophores, chemosensors and charge transfer
agents.8
Introduction
Dye-sensitized solar cells (DSSCs) have received much
attention in recent years due to rapidly increasing energy
demands worldwide, their low-cost and high conversion
performance.1 DSSCs with 11% power conversion efficiency
were well developed by Gratzel et al. during 1991.2
Recently, we have reported novel dendrimers bearing
chalcone and carbohydrate moieties at the periphery through
the click chemistry approach.9 Inspired by the wide application
of triazoles, we report herein 1,2,3-tirazole and methyl
carboxylate based dendrimers 1a–c, 2a–c, 3a,b and 4a,b
(Fig. 1) through a click chemistry approach. In addition
the optical and electrochemical properties of the resulting
dendrimers and their performance in DSSCs have also
been systematically investigated. All the dendrimers reported
herein have both triazole and dimethyl isophthalate units
and hence might show a conjugative effect of both function-
alities with respect to photophysical and electrochemical
properties.
DSSCs are fabricated from nanocrystalline TiO2 film
absorbed with [cis-di-thiocyanato-N,N-bis(2,2-bipyridyl-4,4-
dicarboxylic acid)Àruthenium(II)] (N3 dye), a redox electrolyte
such as the IÀ/I3 redox couple and platinum as a counter
electrode. Recently, organic materials with excellent optical
and electrochemical properties have been developed and used
in photovoltaic devices and light harvesting systems.3
Among several methodologies for improved functional
organic materials, a building block approach based on the
dendrimer structure has been suggested to be an efficient
design criterion for new artificial light harvesting and solar
energy conversion devices.4 Dendrimers provide perfect
monodisperse macromolecules with a regular and highly
branched three-dimensional architecture for a wide variety
of applications in optoelectronic and electronic devices.5
Recently, Yamamato et al.6 reported novel dendrimers and
their application in DSSCs.
Methodology has played a crucial role in the synthesis of
dendrimers in recent years. In particular, click chemistry
happens to be an important tool for the synthesis of
dendrimers in modern chemistry. Click chemistry refers to
Cu(I) catalyzed 1,3-dipolar cycloaddition of azide to alkyne to
give 1,2,3-triazole. 1,2,3-Triazole is an important heterocycle
with a wide range of applications in the field of biology and
pharmacology.7 Moreover, their derivatives find industrial
a Department of Organic Chemistry, University of Madras,
Guindy Campus, Chennai 600 025, India.
E-mail: perumalrajakumar@gmail.com; Fax: +91 044 22300488;
Tel: +91 044 22351269 x213
b Department of Energy, University of Madras, Guindy Campus,
Chennai 600 025, India
Fig. 1 Molecular structure of dendrimers 1–4.
ꢀc
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Results and discussion
Synthesis
The propargyl dendron 6 was synthesized in 91% yield by
treatment of 1.0 equiv. of methyl-5-hydroxyisophthalate (5)
with 1.25 equiv. of propargyl bromide in the presence of
K2CO3 in DMF at 60 1C for 24 h (Scheme 1). Zero generation
dendrimers 1a–c could be achieved by click chemistry
methodology. Reaction of 1.0 equiv. of the bis-azides 7a–c10
with 2.1 equiv. of alkyne 6 in 5 mol% CuSO4Á5H2O and
10 mol% sodium ascorbate in THF–H2O (1 : 1) for 12 h at
room temperature, gave the dendrimers 1a–c in 83%, 86% and
90% yields, respectively (Scheme 1). The structure of the
dendrimers 1a, 1b and 1c was confirmed from spectral and
analytical data.
Scheme 2 Reagents and conditions: (i) 2.1 equiv. NaN3, DMF, rt,
48 h; (ii) 2.0 equiv. KOH, ethanol, reflux, 3 h; (iii) 2.1 equiv. 6, CuSO4Á
5H2O (5 mol%), NaAsc, (10 mol%), THF–H2O (1 : 1), rt, 12 h;
(iv) K2CO3, DMF, 60 1C, 48 h.
Dibromide 811 was converted into bis-azide 9 in 75% yield
using 2.1 equiv. of sodium azide in DMF at room temperature
for 48 h, followed by deacylation using ethanolic KOH at
reflux for 3 h. The structure of the bis-azide 9 has been
confirmed from spectral and analytical data. Treatment of
1.0 equiv. of bis-azide 9 with 2.1 equiv. of alkyne 6 in 5 mol%
CuSO4Á5H2O with 10 mol% sodium ascorbate in THF–H2O
(1 : 1) for 12 h at room temperature afforded the hydroxyl
dendron 10 in 64% (Scheme 2).
Scheme 3 Reagents and conditions: (i) 3.2 equiv. 6, CuSO4Á5H2O
(5 mol%), NaAsc, (10 mol%), THF–H2O (1 : 1), rt, 12 h.
The synthesis of the first generation dendrimers 2a–c could
be achieved by O-alkylation methodology using o-, m- and
p-xylenyl dibromides 11a–c as core units and a phenolic
dendritic arm 10 as the surface and branching unit. Reaction
of 2.1 equiv. of dendron 10 with 1.0 equiv. of corresponding
o-, m- and p-xylenyl dibromides 11a–c in the presence of
K2CO3 in DMF at 60 oC for 48 h afforded the first generation
dendrimers 2a–c in 67%, 75% and 79% yields, respectively
(Scheme 2). The structure of dendrimers 2a, 2b and 2c were
confirmed from spectral analytical data.
Scheme 4 Reagents and conditions: (i) 3.2 equiv. 10, K2CO3, DMF,
60 1C, 48 h.
Zero generation dendrimers 3a and 3b with tris branching
were obtained in 89% and 87% yields, respectively by the
treatment of 1.0 equiv. of triazides 12a and 12b12 with
3.2 equiv. of alkyne 6 under the click reaction conditions as
mentioned earlier (Scheme 3). Spectroscopic and analytical
methods were used to confirm the structure of the dendrimers
3a and 3b.
The reaction of 1.0 equiv. of tribromides 13a and 13b13 with
3.2 equiv. of hydroxyl dendron 10 in the presence of K2CO3
in DMF at 60 1C for 48 h afforded the first generation
dendrimers 4a and 4b in 85% and 83% yields, respectively
(Scheme 4).
The 1H NMR spectrum of 4a showed a singlet for thirty-six
protons at d 3.82 for the methyl protons of the carboxylate
ester, a six-proton singlet at d 4.92 for O-methylene protons
attached to the benzene ring, a set of twelve proton-singlets
at d 5.12 and d 5.41 for N-methylene and O-methylene
protons in addition to thirty-six aromatic protons. In the
13C NMR spectrum, dendrimer 4a showed four peaks at
d 51.4, d 52.7, d 61.3 and d 68.6 for methyl, N-methylene
and two sets of O-methylene carbons in addition to twelve
aromatic carbons and the carbonyl carbon at d 164.9. The
mass spectrum (MALDI-TOF) of 4a showed a peak at m/z 2216
corresponding to the molecular ion. The structure of 4a
was further confirmed from elemental analysis. Similarly, the
structure of dendrimer 4b was also confirmed from spectral
and analytical data.
UV-vis absorption studies
The UV–vis absorption data of compounds 1a–c, 2a–c, 3a,b
and 4a,b in DMSO are presented in Fig. 2. The data of lmax
peaks of UV–vis absorption spectra in DMSO are listed in
Table 1.
Scheme
1 Reagents and conditions: (i) 1.25 equiv. propargyl
The absorption spectra of all the zero generation dendrimers
1a–c and 3a,b are almost identical, and exhibited only one
bromide, K2CO3, DMF, 60 1C, 24 h; (ii) CuSO4Á5H2O (5 mol%),
NaAsc, (10 mol%), THF–H2O (1 : 1), rt, 12 h.
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Electrochemical studies
The redox behavior of dendrimers 1a–c, 2a–c, 3a,b and
4a,b were studied by cyclic voltammetry (CV) in DMSO
(1 Â 10À5 M) at room temperature. All the compounds
have an electrochemical response, and the cyclic voltammo-
gram of all the dendrimers are shown in Fig. 3, with redox
potentials listed in Table 1. The redox behaviour of zero
generation dendrimers 1a–c, 3a,b are almost similar, exhibiting
only one irreversible cathodic peak corresponding to a
triazole unit. Similarly, the reduction potential of first genera-
tion dendrimers 2a–c and 4a,b are almost similar however a
multi-reduction peak was observed in the cyclic voltammo-
gram (Fig. 3). All the first generation dendrimers showed
more than one irreversible reduction peak from cyclic
voltammetry measurements. For example, dendrimer 4b
has several triazole units and exhibits three irreversible reduc-
tion waves at À0.96 V, À1.07 V and À1.22 V respectively
(Fig. 3C).
The CV profile for different dendrimers show that they
exhibit generation dependant reduction peaks. Furthermore,
as the higher generation dendrimers contain a higher number
of triazole units, multi reduction is observed. The presence of
the triazole unit is responsible for the CV behaviour of the
dendrimers. In conclusion, the electrochemical behavior is
altered on increasing the number of triazole-carboxylate
moieties in the dendritic architectures.
Fig. 2 (a) UV-vis absorption spectra of zero generation dendrimers
1a–c, 3a and 3b in DMSO at room temperature. (b) UV-vis absorption
spectra of first generation dendrimers 2a–c, 4a and 4b in DMSO at
room temperature.
Dye-sensitized solar cell studies
Organic dye-sensitized solar cells have generated a lot of
interest during recent years15 due to the emerging importance
of energy need worldwide. During attempts to improve
the performance of solar cells, studies were carried out to
examine the role of the electrolytic solution in the cell.16
In general, the amine derivatives drastically increase the
open-circuit voltage (Voc) due to the electron donating
properties of the nitrogen lone pair.17 Various nitrogen based
organic molecules like aminopyridine, pyrimidine, pyrrolidino-
pyridine, pyrrolidinoethylpyridine have been used as
additives in DSSC studies.18 In the present investigation,
1,2,3-triazole based dendrimers with dimethyl isophthalate
groups could be tested for better performance in DSSCs
studies. Dendrimers 1a–c, 2a–c, 3a,b and 4a,b form
charge transfer complexes with iodine in the redox couple
(IÀ/I3À) in photovoltaic devices. From the present investigation,
Table 1 Optical and electrochemical parameters were measured for
the compounds 1a–c, 2a–c, 3a,b and 4a,b in DMSO
a
UV-Vis /nm
a
Epc1 (v)
a
Epc2 (v)
a
Epc3 (v)
Compound
1a
1b
1c
2a
2b
2c
3a
3b
4a
4b
308
310
308
287, 308
288, 308
286, 308
310
308
285, 308
285, 308
À0.64
À0.96
À0.99
À1.04
À0.84
À0.64
À0.70
À0.65
À0.64
À0.90
—
—
—
—
À0.91
À0.76
—
À0.75
À0.75
À0.97
—
—
—
—
—
—
À2.50
À2.55
—
À1.06
a
All spectra were recorded in DMSO at room temperature at
c = 1 Â 10À5M.
À
it is clear that the dendrimer additive in IÀ/I3 electro-
lyte solution influences Ru DSSC performance and the
Voc values are drastically altered. Typically, cells with
dendrimers display higher Voc values when compared to a
cell without a dendrimer additive. The chemical reaction of
charge-transfer complexation in the electrolyte solution is
given below.
absorption band in between 307–310 nm (Fig. 2A). Similarly,
the first generation dendrimers 2a–c and 4a,b are also nearly
identical, showing two absorption bands – a weak band at
285–287 nm and strong band at 307–308 nm, respectively for
all the dendrimers (Fig. 2B).
À
Dendrimer + I3 - Dendrimer I2 + IÀ
As the number of triazole and carboxylate units increase
from the zero generation to the first generation the absorption
also increases which indicates that the amount of light
absorbed by the dendritic antenna increases from one generation
to the next.14
Dendrimers bearing triazole and ester groups effectively form
charge transfer complexes with I2 in the redox complexation
reaction which results in a decrease in the sublimation
of iodine and also reflects the high performance of DSSC.
ꢀc
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Fig. 4 (a) Current–voltage response of zero generation dendrimer
1a–c, 3a and 3b at 40 mW cmÀ2. (b) Current–voltage response of first
generation dendrimer 2a–c, 4a and 4b at 40 mW cmÀ2
.
structures, which is usually referred to as the multivalent
effect19 in dendrimer chemistry.
The additives 4a and 4b drastically enhance the open-circuit
voltage (Voc) and the solar energy conversion efficiency (Z)
and hence show a better performance in DSSC studies due to
the higher number of electron donating triazole units. The
photovoltaic performance data for the dendrimers 1a–c, 2a–c,
3a,b and 4a,b are summarized in Table 2.
Fig. 3 (a) Cyclic voltammogram of dendrimers 1a–c in DMSO at
room temperature. (b) Cyclic voltammogram of dendrimers 2a–c in
DMSO at room temperature. (c) Cyclic voltammogram of dendrimers
3a,b and 4a,b in DMSO at room temperature.
Conclusions
In conclusion, the synthesis of dendritic structures with
triazole as the building block and dimethyl isophthalate
as the surface group has been achieved through a click
chemistry approach. The optical properties indicate that as
the number of triazole units increases absorption also
increases and hence the light absorbed by the dendritic
antenna increases as the generation increases. The CV studies
clearly reflect multireduction peaks as the dendritic generation
increases from lower to higher. Further as the number of
triazole units increases in the dendrimers of higher generation,
DSSC performance also increases. Among all the dendritic
structures, dendrimers 4a and 4b exhibited higher Voc of
0.610V and 0.620V and higher Z values of 5.5% and 5.4%
respectively than the other dendrimers synthesized in the
present investigation.
The photovoltaic performance of the dendrimers at 40 mW
cmÀ2 are shown in Fig. 4. Fig. 4A shows the current–voltage
response of the zero generation dendrimers 1a–c, 3a and 3b.
The performance of zero generation dendrimers are almost
similar and the presence of two or three triazole moieties do
not affect the current–voltage curve to a greater extent.
Fig. 4B displays the photovoltaic response of first generation
dendrimers at 40 mW cmÀ2. It is found that the photo-
electrochemical properties increase with increasing core length
of the dendrimers leading to better performance. Moreover,
the efficiency of the DSSCs increase with the increase in the
number of triazole units in the first generation dendritic
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Table 2 The photovoltaic properties of the dendrimers with KI and I2 redox couple under illumination of 40 mW cmÀ2
S. No
Electrolyte system
Jsc/mAcmÀ2
Voc/mV
fill factor
efficiency% (Z)
1
2
3
4
5
6
7
8
KI/I2
1.8
3.7
4.2
4.6
5.0
5.2
5.9
4.3
4.6
6.3
6.7
420
395
410
450
470
480
509
440
410
610
620
0.44
0.43
0.45
0.48
0.50
0.55
0.56
0.42
0.50
0.58
0.60
0.8
1.5
1.9
2.5
2.9
3.4
4.2
2.0
2.4
5.5
5.4
KI/I2/1a
KI/I2/1b
KI/I2/1c
KI/I2/2a
KI/I2/2b
KI/I2/2c
KI/I2/3a
KI/I2/3b
KI/I2/4a
KI/I2/4b
9
10
11
as counter and reference electrodes, respectively. Prior to each
electrochemical experiment, this GCE was mechanically
polished with 0.05 micron alumina powder and cleaned in a
1 : 1 acetone/ethanol mixture in an ultrasonic bath to remove
impurities, rinsed with water and then dried in air. Then the
electrode was cleaned by cycling between the potentials of
À1.6 to +0.6 V versus AgCl in 1 Â 10À5 M of tetrabutyl-
ammoniumhexafluorophosphate (TBAPF6) in DMSO at a
scan rate of 50 mV sÀ1 for approximately 30 min until
reproducible scans were recorded. All the electrochemical
experiments were performed in a quiescent solution at room
temperature (25 Æ 1 1C).
Experimental
Materials and methods
Chemicals and solvents (AR quality) were used as received
without any further purification. All melting points are
1
uncorrected. H NMR and 13C NMR spectra were recorded
1
on BRUKER 300 MHz instruments. H NMR spectra were
recorded using deuterated chloroform (CDCl3) as solvent.
Tetramethylsilane (TMS) was used as the internal standard.
Mass spectra were recorded by JEOL-DX303 HF mass
spectrometer. Elemental analyses were carried out by Perkin-
Elmer CHNS 2400 instrument. Column chromatography was
performed on silica gel (ACME, 100–200 mesh). Routine
monitoring of the reaction was made using thin layer
chromatography developed on glass plates coated with silica
gel-G (ACME) of 25 mm thickness and visualized with iodine.
The UV-vis spectra were recorded on a Shimadzu 260
spectrophotometer.
General procedure (C): Synthesis of compounds 1a–c and 3a,b
A mixture of azide, alkyne and CuSO4/sodium ascorbate
mixture in an aqueous THF solution (1 : 1) was stirred for
12 h at room temperature. The residue obtained after
evaporation of the solvent was washed thoroughly with water
and dissolved in CHCl3 (150 mL). The organic layer was
separated, washed with brine (1 Â 150 mL), dried (anhydrous
Na2SO4) and evaporated to give the crude triazole, which was
purified by column chromatography (SiO2) using the mixture
of CHCl3 with MeOH (99 : 1) as the eluent.
General procedure (A) for dye-sensitized solar cell studies
The TiO2 photoelectrode was prepared as described earlier.20
The N3 dye was adsorbed on to the TiO2 surface by soaking
the TiO2 photoelectrode in an ethanol solution of the N3 dye
(5 Â 10À5 M concentration) for 24 h at room temperature. The
photoelectrode was washed, dried and immediately used for
the measurement of solar cell performance. A sandwich-type
photoelectrochemical cell was composed of a dye-coated TiO2
photoanode, and a platinum coated Fluorinated Tin Oxide
(FTO) conducting glass to act as the counter electrode. The
electrolyte solution was injected into the space between
two electrodes. The electrolyte solution was composed of
General procedure (D): Synthesis of compounds 2a–c and 4a,b
A mixture of bromide (1.0 equiv.) and phenol (2.1 equiv./
3.2 equiv.) in dry DMF was stirred at 60 1C for 48 h. The reaction
mixture was then allowed to cool to room temperature and
poured into ice water. The resulting precipitate was filtered,
washed thoroughly with water and dissolved in CHCl3
(150 mL). The organic layer was separated, washed with brine
(1 Â 150 mL), dried (anhydrous Na2SO4) and evaporated to
give the crude dendrimer, which was purified by column
chromatography (SiO2) using CHCl3–MeOH (99 : 1) as the
eluent.
3.2 Â 10À5 M of KI, 4.1 Â 10À6 M of I2, and 6.4 Â 10À6
M
of the dendrimer additives in 10 ml DMF solvent. The solar to
electric energy conversion efficiency was measured under
simulated solar light at 40 mWcmÀ2. The photocurrent-photo-
voltage was measured using a BAS 100A Electrochemical
analyzer. The apparent cell area of TiO2 photoelectrode was
1 cm2 (1 cm  1 cm).
Compound 1a. Following the general procedure C,
compound 1a was obtained as a colorless solid in 83% yield.
1
Mp = 136–139 1C; EI-MS (70 eV): m/z 684 (M+); H NMR
General procedure (B) for electrochemical studies
(300 MHz, CDCl3): 3.92 (s, 12H); 5.24 (s, 4H); 5.67 (s, 4H)
7.27–7.31 (m, 2H); 7.40–7.43 (m, 2H); 7.57 (s, 2H); 7.79 (m,
4H); 8.28 (s, 2H). 13C NMR (75 MHz, CDCl3): 51.3, 52.4,
62.4, 120.1, 123.1, 123.6, 129.9, 130.6, 131.9, 133.1, 143.9,
158.2, 165.9. Anal. calc. for C34H32N6O10: C, 59.65; H, 4.71;
N, 12.27. Found: C, 59.73; H, 4.84; N, 12.36.
Cyclic voltammetric measurements were performed in a
conventional three electrode system on CHI model 1100A
series electrochemical analyzer (CH Instrument, USA). Glassy
carbon electrode (GCE) was used as working electrode with Pt
foil (large surface area) and a silver-silver chloride (Ag/AgCl)
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Compound 1b. Following the general procedure C,
compound 1b was obtained as a colorless solid in 86% yield.
Compound 3b. Following the general procedure C,
compound 3b was obtained as a colorless solid in 87% yield.
Mp = 106 1C; QTOF-MS: m/z 1030 (M+); 1H NMR
(300 MHz, CDCl3): 2.37 (s, 9H); 3.83 (s, 18H); 5.12 (s, 6H);
5.62 (s, 6H); 7.41 (s, 3H); 7.66 (s, 6H); 8.14 (s, 3H). 13C NMR
(75 MHz, CDCl3): 16.7, 49.1, 52.5, 62.3, 119.9, 122.4, 123.5,
130.6, 131.8, 139.9, 143.4, 158.1, 165.9. Anal. calc. for
C51H51N9O15: C, 59.47; H, 4.99; N, 12.24. Found: C, 59.39;
H, 4.90; N, 12.17.
1
Mp = 67–70 1C; EI-MS (70 eV): m/z 684 (M+); H NMR
(300 MHz, CDCl3): 3.83 (s, 12H); 5.15 (s, 4H); 5.46 (s, 4H);
7.15–7.21 (m, 3H); 7.27–7.32 (m, 1H); 7.57 (s, 2H); 7.69 (s,
4H); 8.17 (s, 2H). 13C NMR (75 MHz, CDCl3): 51.5, 52.8,
61.4, 118.9, 122.0, 122.5, 126.5, 127.3, 129.0, 130.8, 134.6,
142.8, 157.2, 164.9. Anal. calc. for C34H32N6O10: C, 59.65; H,
4.71; N, 12.27. Found: C, 59.53; H, 4.81; N, 12.36.
Compound 4a. Following the general procedure D,
compound 4a was obtained as a colorless solid in 85% yield.
Compound 1c. Following the general procedure C,
compound 1c was obtained as a colorless solid in 90% yield.
1
Mp = 103À108 1C; MALDI-TOF MS: m/z 2216 (M+); H
1
Mp = 137–140 1C; EI-MS (70 eV): m/z 684 (M+); H NMR
NMR (300 MHz, CDCl3): 3.82 (s, 36H); 4.92 (s, 6H); 5.12 (s,
12H); 5.41 (s, 12H); 6.75 (s, 9H); 7.20 (s, 3H); 7.59 (s, 6H); 7.67
(s, 12H); 8.15 (s, 6H). 13C NMR (75 MHz, CDCl3): 51.4, 52.7,
61.3, 68.6, 113.7, 118.8, 118.9, 122.1, 122.4, 124.9, 130.8,
136.2, 136.3, 142.8, 157.2, 158.5, 164.9. Anal. calc. for
(300 MHz, CDCl3): 3.93 (s, 12H); 5.26 (s, 4H); 5.56 (s, 4H);
7.29 (s, 4H); 7.59 (s, 2H); 7.81 (s, 4H); 8.29 (s, 2H). 13C NMR
(75 MHz, CDCl3): 52.5, 53.7, 62.5, 120.1, 122.8, 123.6, 128.8,
131.9, 135.2, 143.9, 158.2, 165.9. Anal. calc. for C34H32N6O10:
C, 59.65; H, 4.71; N, 12.27. Found: C, 59.55; H, 4.83; N, 12.18.
C
111H102N18O33: C, 60.16; H, 4.64; N, 11.38. Found: C,
Compound 2a. Following the general procedure D,
compound 2a was obtained as a colorless solid in 67% yield.
Mp = 96–100 1C; QTOF-MS: m/z 1503 (M+); 1H NMR
(300 MHz, CDCl3): 3.82 (s, 24H); 4.96 (s, 4H); 5.13 (s, 8H);
5.42 (s, 8H); 6.73 (s, 6H); 7.30 (m, 4H); 7.64 (s, 4H); 7.68
(s, 8H); 8.15 (s, 4H). 13C NMR (75 MHz, CDCl3): 52.4, 53.7,
62.3, 68.2, 114.6, 119.8, 120.0, 123.3, 123.5, 128.8, 129.2, 131.9,
134.3, 137.2, 143.7, 158.2, 159.5, 165.9. Anal. calc. for
C76H70N12O22: C, 60.72; H, 4.69; N, 11.18. Found: C, 60.83;
H, 4.59; N, 11.29.
60.05; H, 4.56; N, 11.45.
Compound 4b. Following the general procedure D,
compound 4b was obtained as a colorless solid in 83% yield.
Mp = 116–120 1C; MALDI-TOF MS: m/z 2258 (M+);
1H NMR (300 MHz, CDCl3): 2.25 (s, 9H); 3.82 (s, 36H);
4.94 (s, 6H); 5.16 (s, 12H); 5.44 (s, 12H); 6.74 (s, 3H); 6.74
(s, 6H); 7.61 (s, 6H); 7.69 (s, 12H); 8.16 (s, 6H). 13C NMR
(75 MHz, CDCl3): 15.1, 51.5, 52.8, 61.4, 64.3, 113.5, 119.0,
122.1, 122.2, 122.5, 130.3, 130.9, 136.3, 138.5, 142.9, 157.2,
159.1, 164.9. Anal. calc. for C114H108N18O33: C, 60.63; H, 4.82;
N, 11.16. Found: C, 60.75; H, 4.93; N, 11.27
Compound 2b. Following the general procedure D,
compound 2b was obtained as a colorless solid in 75% yield.
Mp = 98–102 1C; QTOF-MS: m/z 1503 (M+); 1H NMR
(300 MHz, CDCl3): 3.89 (s, 24H); 4.98 (s, 4H); 5.22 (s, 8H);
5.47 (s, 8H); 6.79 (s, 2H); 6.79 (s, 4H); 7.35 (s, 4H); 7.61 (s,
4H); 7.75 (s, 8H); 8.25 (s, 4H). 13C NMR (75 MHz, CDCl3):
52.6, 53.8, 62.4, 69.9, 114.8, 119.9, 120.1, 120.3, 123.1, 123.6,
127.3, 131.9, 136.7, 137.2, 143.9, 158.3, 159.7, 166.0. Anal.
calc. for C76H70N12O22: C, 60.72; H, 4.69; N, 11.18. Found: C,
60.63; H, 4. 80; N, 11.31.
Acknowledgements
The authors acknowledge UGC, New Delhi, India for
financial support. We thank DST, New Delhi for NMR
facilities under the DST-FIST programme to the Department
of Organic Chemistry, University of Madras, Chennai, India.
SR thanks UGC, New Delhi for Fellowship.
References
Compound 2c. Following the general procedure D,
compound 2c was obtained as a colorless solid in 79% yield.
Mp = 92 1C; QTOF-MS: m/z 1503 (M+); 1H NMR
(300 MHz, CDCl3): 3.83 (s, 24H); 4.89 (s, 4H); 5.17 (s, 8H);
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