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323
for aliphatic and aromatic protons. The 13C NMR spectrum of 1
showed signal for N-methylene carbon at d 66.6 in addition to
the aliphatic and aromatic carbon signals. The 1H NMR spectrum
could not be unequivocally used to locate the trans olefinic protons
as it was mixed with the other aromatic protons.27 Moreover, in IR
spectrum of stilbenoid dendrimer 1 displayed the characteristic
absorption band of trans double bond at 958 cmÀ1 indicating that
HWE reaction has afforded the trans isomer. The mass spectrum
(ESI) of 1 showed a peak at m/z 721.4 (M+H)+ for molecular ion.
The structure of 1 was further confirmed from elemental analysis.
Similarly, the structure of dendrimers 2 and 3 was also confirmed
from spectral and analytical data.
Similarly, Michaelis–Arbuzov reaction of 1,3,5-tris(bromo-
methyl) benzene with triethyl phosphite at 160 °C afforded the
phosphonate esters 20 in 89% yield. HWE reaction of 1.0 equiv of
phosphonate diester 20 with 3.1 equiv of 10-n-heptylphenothi-
azine-3-carbaldehyde, 16/10-n-dodecylphenothiazine-3-carbalde-
hyde 17/10-n-hexadecylphenothiazine-3-carbaldehyde 18 using
NaH in THF afforded the phenothiazinostilbenoid dendrimers 4, 5,
and 6 in 77%, 66%, and 63% yields, respectively (Scheme 1). The
1H NMR spectra of 6 showed a triplet at d 3.72 (J = 6.9 Hz) for N-
methylene protons in addition to the signals for the aliphatic and
aromatic protons. The 13C NMR spectrum of 6 showed signal for
N-methylene carbon at d 47.4 in addition to the signals for aliphatic
and aromatic carbons. The IR spectra of stilbenoid dendrimer 6 dis-
played the absorption band of trans double bonds at 958 cmÀ1. The
mass spectrum (MALDI-TOF) of 6 showed the molecular ion peak at
m/z 1419.8. The structure of 6 was further confirmed from elemen-
tal analysis. Similarly, the structure of dendrimers 4 and 5 was also
confirmed from spectral and analytical data.
The attention was then focused on the synthesis of dendrons 13–
15 with phenothiazine units using HWE reaction. Michaelis–Arbu-
zov reaction of 1,3-bis(bromomethyl)-5-(prop-2-ynyloxy)benzene
with triethyl phosphite at 160 °C gave the phosphonate esters 21
in good yields. Further, HWE reaction of 1.0 equiv of phosphonate
diester 21 with 2.1 equiv of 10-n-heptylphenothiazine-3-carbalde-
hyde 16/10-n-dodecylphenothiazine-3-carbaldehyde 17/10-n-hex-
adecylphenothiazine-3-carbaldehyde 18 using NaH in THF gave
alkyne dendrons 13, 14, and 15 in 66%, 57%, and 50% yields, respec-
tively. The 1H NMR spectrum of propargyl dendron 14 displayed a
triplet at d 2.55 (J = 2.1 Hz) for acetylenic proton and doublet at d
4.75 (J = 2.1 Hz) for O-methylene protons, respectively, in addition
to the signals for aliphatic and aromatic protons. The 13C NMR spec-
trum of 14 showed O-methylene and acetylenic carbons at d 55.9,
75.7, and 78.6 respectively, in addition to the signals for aliphatic
and aromatic carbons. The IR spectra of stilbenoid dendrimer 14
displayed the absorption bands at 959 cmÀ1, 2118 cmÀ1, and
3268 cmÀ1 for trans double bond, –C„C– and „CAH units, respec-
tively. The mass spectrum (ESI) of 14 showed the molecular ion at
m/z 916.3 (M+H)+. Similarly, the structure of the dendron 13 and
15 was also confirmed from spectral and analytical data.
Further, in order to increase the number of stilbene and
phenothiazine units in the first-generation dendrimers 1.0 equiv
of 1,3,5-tris(azidomethyl)benzene 23 was reacted with 3.1 equiv
of propargyloxy conjugated dendrons 13, 14, and 15 under click
reaction conditions to give the first generation dendrimers 10,
11, and 12 in 82%, 81%, and 81% yields, respectively, (Scheme 1).
The 1H NMR spectrum of stilbenoid dendrimer 12 displayed the
N-methylene protons at d 5.14, the O-methylene protons at d
5.32, in addition to the other signals for the aliphatic and aromatic
protons. In the 13C NMR spectrum, dendrimer 12 showed
N-methylene and O-methylene carbons at d 53.3 and 62.1 in addi-
tion to the signals for the aliphatic and aromatic carbons. The mass
spectrum (MALDI-TOF) of 12 showed the molecular ion peak at m/z
3323.5. Similarly, the structure of dendrimers 10 and 11 was also
confirmed from spectral and analytical data.
The UV–visible absorption and fluorescence data of dendrimers
1–12 are summarized in Table 1. Two prominent absorption bands
were observed around 300 and 382 nm corresponding to the p–p*
transition of stilbene and phenothiazine units, respectively,
(Fig. 1a). Similar absorption spectral behavior is exhibited by all
the dendrimers, which shows the absence of aggregates and charge
transfer complexes even with increasing the number of stilbene
group. Further, the butterfly conformation of phenothiazine unit
also prevents the inter- and intra-molecular aggregations.28 No
shift in kmax or broadening of the absorption band was observed
in the UV–vis spectrum even with increasing the concentration
of dendrimer indicating that there is no aggregation of dendrimers
even at higher concentrations which may be due to butterfly con-
formation of phenothiazine that prevents the flat structure for the
dendrimers. Such nonaggregating nature of bulky molecules with
electron rich moieties is important for solar cell application. The
absorption maximum of dendrimers 1–3, 4–6, 7–9, and 10–12 does
not vary much with increasing alkyl chain length attached to the
phenothiazine unit which indicates that increasing the chain
length of the alkyl group on phenothiazine does not alter the
UV–visible absorption properties.
For dendrimers 1, 2, and 3 the shorter wavelength band and the
longer wavelength band centered around 291 and 377 nm where as
for dendrimers 4–12, the shorter wavelength band and the longer
wavelength band centered around 306 and 384 nm, respectively.
Table 1
Photophysical properties of the dendrimers 1–12
Dendrimers
1
kabs (nm)
e
105 MÀ1 cmÀ1
kem (nm)
Uf
sf (ns)
292
377
291
378
291
377
303
384
303
383
303
383
305
382
305
382
306
382
306
382
306
382
304
382
2.11
0.92
1.60
0.66
3.18
1.56
2.90
1.85
5.08
3.31
3.69
2.33
5.57
3.46
4.37
2.66
5.38
3.35
10.90
6.90
8.87
5.67
8.73
5.49
537
0.632
5.27
2
3
537
537
531
531
531
528
527
526
527
527
527
0.605
0.685
0.876
0.922
0.923
0.888
0.937
0.956
0.927
0.947
0.953
5.24
5.30
5.15
5.16
5.15
5.24
5.31
5.34
5.30
5.31
5.32
4
Having successfully synthesized the zero-generation dendri-
mers 1–6, we focused our attention on the convergent synthesis
of first-generation dendrimers 7–12 through click chemistry as
shown in Scheme 1. The reaction of 1.0 equiv of azide 22 with
2.1 equiv of propargyloxy conjugated dendrons 13, 14, and 15 in
the presence of Cu-(I) catalyzed click reaction conditions afforded
the first-generation dendrimers 7, 8, and 9 in 87%, 86%, and 83%
5
6
7
8
yields, respectively. The 1H NMR spectrum of dendrimer
7
9
displayed two singlets at d 5.16 and 5.21 corresponding to the
N-methylene and O-methylene protons, in addition to the signals
for aliphatic and aromatic protons. In the 13C NMR spectrum,
10
11
12
first-generation dendrimer
7 showed the N-methylene and
O-methylene carbons at d 49.4 and 61.9 along with signals for ali-
phatic and aromatic carbons. Similarly, structure of the dendrimers
8 and 9 was also confirmed from spectral and analytical data.