New hyperbranched polytriazoles containing isolation chromophore moieties derived from AB4 monomers through click chemistry under copper(I) catalysis: Improved optical transparency and enhanced NLO effects

Article abstract of DOI:10.1002/chem.201102872

By modifying a synthetic procedure, two new hyperbranched polytriazoles (HP1 and HP2) containing isolation chromophores were synthesized successfully through click chemistry reactions under copper(I) catalysis. For the first time, these two polymers were

Full text of DOI:10.1002/chem.201102872

DOI: 10.1002/chem.201102872
New Hyperbranched Polytriazoles Containing Isolation Chromophore
Moieties Derived from AB4 Monomers through Click Chemistry under
Copper(I) Catalysis: Improved Optical Transparency and Enhanced NLO
Effects
Wenbo Wu,[a] Cheng Ye,[b] Gui Yu,[b] Yunqi Liu,[b] Jingui Qin,[a] and Zhen Li*[a]
Abstract: By modifying a synthetic procedure, two new hyperbranched polytri-
ACHTUNGTRENNUNGazoles (HP1 and HP2) containing isolation chromophores were synthesized suc-
cessfully through click chemistry reactions under copper(I) catalysis. For the first
time, these two polymers were derived from an AB4-type monomer, although they
contain different end-capping chromophores. They are soluble in normal polar or-
ganic solvents and are well characterized. Thanks to the presence of the isolation
chromophore, the two polymers demonstrate good nonlinear optical (NLO) prop-
erties and optical transparency, making them promising candidates for practical
applications.
Keywords: chromophores
chemistry hyperbranched poly-
mers isolation chromophores
nonlinear optics · polymers
·
click
·
·
·
Introduction
ers, such as good solubility, low viscosity, and multifunction-
ality at the end groups. Therefore, hyperbranched polymers,
especially those derived from AB2 monomers with a con-
trolled structure more like that of dendrimers than that of
those prepared through the popular “A2 +B3” approach,
have been receiving more and more attention.[5]
Dendrimers and hyperbranched polymers, together classi-
fied as dendritic polymers, have been attracting more and
more interest in several areas of the physical, biological, and
engineering sciences due to their unique three-dimensional
architecture.[1–4] Dendrimers are defect-free and perfect
monodisperse macromolecules with a regular and highly
three-dimensional branched structure, and demonstrate
many advantages, such as their nanometer size, globular
shape, multivalent character, and the modularity of their as-
sembly. However, their preparation is generally tedious and
costly because of the repetitive protection, deprotection,
and purification steps involved, which directly limit their
large-scale application.[3] In contrast to dendrimers, hyper-
branched polymers are of special interest due to their easy
synthetic accessibility, typically by one-pot syntheses that
allow for their production in large quantities and their appli-
cation on an industrial scale.[4] Although the structures of
hyperbranched polymers are not imperfect and still contain
linear units, they inherit many of the properties of dendrim-
Interestingly, early in 2000, Ishida et al.[6] synthesized a
series of monomers (AB2, AB4, and AB8) and found that
the degree of branching (DB) of the corresponding hyper-
branched polymers increased with increasingly branched
monomers; thus, AB2 monomers gave a DB of only 32%,
AB4 monomers gave 72%, and AB8 gave 84%. This means,
with increasingly branched ABn-type monomers, the proper-
ties of the hyperbranched polymers they form would be
more similar to those of dendrimers. Thus, the attempt to
prepare hyperbranched polymers derived from ABn-type
(n>2) monomers (for example, AB4-type monomers)
became very important, although the syntheses should be
much more difficult as a result of too many reactive groups
being present for easy cross-linking.
Since 2001, the Huisgen 1,3-dipolar cycloaddition between
azides and alkynes to yield triazoles, which is often referred
to simply as the click reaction, has caused considerable in-
terest among researchers because of its remarkable features,
such as nearly quantitative yields, mild reaction conditions,
broad tolerance toward functional groups, low susceptibility
to side reactions, and simple product isolation.[7] As an effi-
cient covalent linking technique, this click chemistry has re-
cently been widely utilized to functionalize surfaces, poly-
mers, dendrimers, and, of course, hyperbranched polymers.
However, for hyperbranched polymers, the reported success-
ful examples are relatively scarce and most take an A2 +B3
approach.[8] For AB2 monomers, it was found to be difficult
to construct the corresponding soluble hyperbranched poly-
[a] W. Wu, Prof. J. Qin, Prof. Z. Li
Department of Chemistry, Hubei Key Laboratory on
Organic and Polymeric Opto-Electronic Materials
Wuhan University, Wuhan 430072 (P.R. China)
Fax : (+86)27-68756757
E-mail: lizhen@whu.edu.cn
[b] Prof. C. Ye, Prof. G. Yu, Prof. Y. Liu
Organic Solids Laboratories, Institute of Chemistry
The Chinese Academy of Sciences
Beijing 100080 (P.R. China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102872.
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FULL PAPER
mers through the CuI-catalyzed click polymerizations due to
the self-oligomerization side reaction.[9] Fortunately, by
modifying the synthetic procedure and using end-capping
groups, in 2009 we successfully prepared some soluble azo
chromophore containing AB2-type hyperbranched polytri-
mers could be boosted to several times higher by adding a
“suitable isolation group” to the NLO chromophore moiet-
ies.[17] However, nearly all of the previously used isolation
groups were nonpolar groups, utilized simply to decrease
the strong interactions between the chromophore moieties,
and did not directly contribute to the macroscopic NLO
effect, but decreased the effective concentration of the NLO
chromophore moieties. Very recently, we used a chromo-
phore with a lower mb value as the isolation group for an-
other chromophore with a high mb value to achieve high
poling efficiency as a result of the decreased strong electron-
ic interactions and the increased NLO effect. This result
showed that the introduction of isolation chromophore moi-
eties into NLO materials could improve their optical trans-
parency and poling efficiency. Thus, considering the interest-
ing results for the isolation chromophore moieties and the
aforementioned synthetic challenge, we attempted to design
and synthesize new NLO hyperbranched polymers
(Scheme 2) derived from AB4-type monomers, containing
two chromophores with regular structures. Fortunately, by
adjusting the amount of catalyst, the solvent, and the reac-
tion time, with the aid of the added end-capping groups, the
soluble hyperbranched polymers HP1 and HP2 were synthe-
sized for the first time. Thanks to the introduction of isola-
tion chromophores and their unique three-dimensional ar-
chitecture, HP1 and HP2 exhibit good NLO activities and
improved optical transparency. Herein, we present the syn-
ACHTUNGTRENNUNGazoles (Charts S1–S3 in the Supporting Information; the
synthetic route to a typical example, HP-N, is shown in
Scheme 1) by using different AB2-type monomers and end-
capping groups to those used in the new second-order non-
linear optical (NLO) polymeric materials.[5] These materials
are utilized in telecommunications, computing, embedded-
network sensing, THz wave generation and detection, and
many other applications. However, how about the construc-
tion of hyperbranched polymers derived from ABn-type
ACHTUNGTRENNUNG(n>2) monomers through the powerful click chemistry pro-
tocol? So far, there have been no reports concerning this
method, partially due to its inherent synthetic difficulty.
On the other hand, in 2006, by utilizing the site isolation
principle,[10] on the basis of the literature[11–16] and in an at-
tempt to partially resolve the challenge inherent in the
second-order NLO polymeric materials (to efficiently trans-
late the large b values of the organic chromophores into
high macroscopic NLO activities in polymers), we prepared
different kinds of NLO polymer, in which the size of the iso-
lation groups in the NLO chromophore moieties was
changed from small to large. The experimental results dem-
onstrated that the macroscopic nonlinearity of NLO poly-
Scheme 1. The structure and synthesis of HP-N derived from an AB2-type monomer.
Chem. Eur. J. 2012, 18, 4426 – 4434
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Z. Li et al.
Scheme 2. The synthesis of hyperbranched polymers HP1 and HP2.
theses, characterization, and properties of these new hyper-
branched polymers.
portant that the fluoroborate salt should be utilized as the
reagent and the reaction be conducted in a pure organic sol-
vent, since the generally used inorganic acids for the azo
coupling reaction might destroy the azido groups during the
long reaction time. The obtained pure AB4-type monomer,
similar to other AB2-type monomers, was very stable and
could be stored in a refrigerator for about half a year.
Due to self-oligomerization, it was very hard to obtain
soluble hyperbranched polymers by using the CuI-catalyzed
click polymerization of the AB2 monomers.[9] As to the
AB4-type monomer, the polymerization by click chemistry
reactions might be even more difficult due to the presence
of more azido groups. Since 2009, by adjusting the synthetic
procedure for the click reaction, we have successfully pre-
pared some soluble AB2-type hyperbranched polytriazoles
(Scheme 1 and Charts S1–S3 in the Supporting Information)
in two steps:[5] in the first stage of the polymerization pro-
cess, the hyperbranched polymeric intermediate HP-N3
(Chart S4 in the Supporting Information), which possesses a
large number of unreacted azido groups in the periphery,
was synthesized; then, in the second stage, the end-capping
group and another batch of catalysts were added into the re-
Results and Discussion
Synthesis: As reported in the literature and in our previous
examples, the azido and yne groups both possess high reac-
tivity,[5,7] and could therefore react with each other at tem-
peratures a little above room temperature. That is to say, if
the reaction temperature was a little higher, the monomer
containing both the azido and yne groups could not be ob-
tained. Fortunately, under the normal azo-coupling reaction
conditions, some AB2-type monomers (Scheme S1 in the
Supporting Information) were successfully prepared (as de-
scribed in our previous work)[5] because the reaction be-
tween the azido and yne groups could be avoided at the low
reaction temperature utilized (08C). Herein, we used a simi-
lar method to synthesize the monomer (Scheme 3). Unlike
the previous case, the reaction between S3 and S4 required
a much longer reaction time (about 3 days), possibly due to
the larger steric hindrance within S3. Thus, it was more im-
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Hyperbranched Polytriazoles
FULL PAPER
Scheme 3. The synthesis of the AB4-type monomers.
action mixture of HP-N3 to react with the azido groups on
the periphery of HP-N3. Herein, we attempt to prepare hy-
perbranched polymers from AB4 monomers by following
this method.
be long enough for the construction of the hyperbranched
structure of the hyperbranched polymers.
However, once the end-capping chromophore was added,
the viscosity of the reaction mixture increased, and only in-
soluble polymers were soon obtained. Analyzing the poly-
merization carefully, in the case of the AB4-type monomer,
unlike the polymerization of the AB2-type monomer, there
were more azido groups on the surface of the reactive inter-
mediate formed, and thus, considerably more end-capping
chromophores had to be used. Consequently, at the same
time as the addition of the end-capping chromophores, some
additional solvent was added, along with some additional
catalyst. As the end-capping chromophores were different in
HP1 and HP2, the amount of added solvent and the reac-
tion time were also different. The specific reaction time and
amounts of catalysts and solvent are presented in Table 1
and the Experimental Section. The obtained hyperbranched
polymers HP1 and HP2 were soluble in common polar or-
ganic solvents, such as CH2Cl2, CHCl3, THF, DMF, and
DMSO, and their solutions could be easily spin-coated into
thin solid films. Therefore, it was convenient to test their
NLO and other properties based on the solutions and thin
films.
It is clear that there are more azido groups and more
branched structures in AB4-type monomers that would
make the polymerization gelate more easily than the AB2
monomers. On the other hand, the larger steric effects of
AB4-type monomers would slow down the reaction. Thus, it
is necessary to optimize the polymerization time and the
concentration of the monomer and catalyst to avoid the pos-
sible gelation. First, the same concentrations of the mono-
mer and catalyst as those utilized in the polymerization of
the AB2-type monomers in our previous work were used in
the reaction. However, very quickly, many solids precipitat-
ed from the reaction solution, and the obtained polymer was
insoluble in all solvents. Consequently, the concentration of
the monomer was decreased by half. After about 30 h, we
introduced molecules containing only alkyne groups to react
with any unreacted azido groups and avoid the formation of
gels because if the reaction time was longer than 36 h, only
insoluble cross-linked polymers were produced. In compari-
son with the polymerization of the AB2-type monomers in
our previous work,[5] more end-capping groups needed to be
used. In the AB4-type monomer, there were four azido
groups present but only one alkyne group, and the one
alkyne group might react with one of the azido groups
during the polymerization process. Therefore, the number of
azido groups in the periphery might be three times the
number of monomers added, and thus, the number of end-
capping groups added should be at least three times the
number of monomers in order to react with all of the azido
groups at the periphery of the hyperbranched polymer. To
ensure the reaction is as complete as possible, we used a
little more of the end-capping groups than required, that is,
3.6 times the number of monomers. Although the number of
end-capping groups was high, it would not affect the con-
struction of the hyperbranched structure of the resulting
polymers because the end-capping groups were added to the
polymerization system after 30 h. This reaction time should
Table 1. Some data for the polymerizations.
[a]
[b]
[c]
[d]
[e]
[f]
[g]
c1
V1
V2
t1
t2
n1
n2
Yield[h]
[mmolmLꢀ1
]
[mL] [mL] [h]
[h] [%] [%] [%]
HP-N[i]
HP1
HP2
control 1 0.02
control 2 0.01
control 3 0.01
0.02
0.01
0.01
7.5
4.5
3
–
–
0
4.5
6
–
–
16
30
30
0.3
36
12 10
20 10
48 10
3.3 88.7
10
10
–
61.4
64.1
gel
–
–
1
–
–
–
–
gel
–
none 30
–
gel
[a] The concentration of the monomer. [b] The volume of DMF (solvent)
used in the polymerization stage. [c] The volume of extra DMF used
after the addition of the end-capping chromophores. [d] Polymerization
time before the addition of the end-capping chromophore. [e] Reaction
time after the addition of the end-capping chromophore. [f] Amount of
catalyst before the addition of the end-capping chromophore.
[g] Amount of catalyst after the addition of the end-capping chromo-
phore. [h] The yield for the polymerization reaction. [i] Prepared in our
previous work.
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Z. Li et al.
Thus, by modifying the synthetic procedure, soluble hy-
perbranched polymers, HP1 (with a sulfonyl-chromophore
as the end-capping group) and HP2 (with a nitro-chromo-
phore as the end-capping group), could be synthesized con-
veniently from the same AB4-type monomer through click
chemistry under copper(I) catalysis. Inspired by this prelimi-
nary result, it is believed that many other AB4-type, and per-
haps even ABn-type (n>4), hyperbranched polymers could
be obtained by carefully modifying the synthetic procedure.
the monomer, confirming that both the azido and yne moi-
eties were present in the monomer. After the polymeri-
zation and end-capping reaction, the absorption bands at
about 2098 and 3277 cmꢀ1 had disappeared, again confirm-
ing the success of the polymerization reaction.
The molecular weights of HP1 and HP2 were determined
by gel-permeation chromatography (GPC) with DMF as the
eluent and polystyrene standards as the calibration stand-
ards. As shown in Table 2 and the Experimental Section, the
Structural Characterization: The monomers and
Table 2. Characterization data for the hyperbranched polymers.
polymers were characterized by spectroscopic meth-
ods, and all gave satisfactory spectral data (Figur-
es S1–S7 in the Supporting Information). The AB4-
type monomer was a new compound, and its
1H NMR spectrum is shown in Figure S1 in the Sup-
porting Information. In comparison with the
1H NMR spectrum of compound S3, some new
peaks appeared, which could be assigned to the
[a]
[a]
[b]
[c]
[f]
[g]
Mw
[ꢁ105]
Mw/Mn
Tg
Td
Te[d]
ls[e]
d33
[pmVꢀ1
d33(1)
[pmVꢀ1
]
F[h]
G
[8C] [8C] [8C] [mm]
E
]
N
HP1 1.25
HP2 1.79
2.06
2.42
97
100
243
284
120
125
0.22
0.25
117.6
167.4
24.8
31.5
0.22
0.26
[a] Determined by GPC in DMF on the basis of a polystyrene calibration. [b] Glass
transition temperature (Tg) of the polymers detected by DSC analyses under argon at
a heating rate of 108Cminꢀ1. [c] Td is the temperature at which 5% of the polymer
weight has been lost, as detected by use of TGA analyses under nitrogen at a heating
rate of 108Cminꢀ1. [d] Te is the best poling temperature. [e] Film thickness. [f] Second-
harmonic-generation (SHG) coefficient. [g] The nonresonant d33 values calculated by
using the approximate two-level model. [h] Order parameter F=1ꢀA1/A0, in which
A0 and A1 are the absorbance of the polymer film before and after the corona poling,
ꢁ
protons of the -CH2- (1.99), -C CH (2.05), and
-SCH2- (3.27) groups, as well as some ArH protons
in the sulfonyl chromophore, indicating the success
of the azo-coupling reaction. It is a pity that both respectively.
the 1H NMR spectrum of HP1 and that of HP2
showed broad peaks in the range of 1.5–2.3 ppm,
which should be assigned to the protons of the -CH2- groups
(the peak in the spectrum of the monomer is at 1.99 ppm),
and so it was not easy to confirm whether there were still
some resonance peaks of the acetylene protons. Luckily,
apart from this, the chemical shifts were consistent with the
polymer structures given in Scheme 2, and no unexpected
resonance peaks were observed. However, the absence of
the alkyne group could be easily proven by use of the
13C NMR spectra. In comparison with the 13C NMR spec-
trum of the monomer, the peaks at 81.66 ppm, the typical
two hyperbranched polymers possessed high molecular
weights, indicating that the polymerization method reported
herein successfully constructed the hyperbranched polymer-
ic structure with high molecular weights and good solubility
from AB4 monomers through the click chemistry reaction.
The polymers were thermally stable (Figure S8 in the Sup-
porting Information), and the 5% weight-loss temperature
of the polymers is listed in Table 2. HP2 exhibited better
thermal stability than HP1, possibly due to its slightly
higher molecular weight. The glass-transition temperatures
(Tg) of HP1 and HP2 were investigated by using a differen-
tial scanning calorimeter (Table 2), and they possessed mod-
erate Tg values of 97 and 1008C, respectively. Their glass-
transition temperatures were almost the same, possibly due
to their similar chemical and topological structures.
ꢁ
signal of the -C CH group, had disappeared in the spectra
of both HP1 and HP2, confirming the success of the poly-
merization.
Figure 1 shows the IR spectra of the monomer and the hy-
perbranched polymers. The absorption bands associated
with the nitro and sulfonyl groups are at about 1515, 1338,
and 1140 cmꢀ1. It is easy to see that there are absorptions at
UV/Vis spectra and effects of the isolation chromophore:
The UV/Vis absorption spectra of the polymers in different
solvents are shown in Figures 2 and 3, with their maximum
absorption wavelengths summarized in Table 3. To investi-
gate the different effects of the two chromophores, the end-
capping chromophore for HP2 (S2), which has the same
chromophore and isolation groups as HP2, was used as the
reference spectrum.
2098 cmꢀ1 (-N3) and 3277 cmꢀ1 (C CH) in the spectrum of
ꢁ
For the solutions in THF, 1,4-dioxane, chloroform, and di-
chloromethane, the redshift in the absorption is clear in the
UV/Vis spectra of HP2, in comparison with the correspond-
ing spectra of chromophore S2. This phenomenon is abnor-
mal. As reported in the literature and our previous work,[5]
the chromophore moieties in hyperbranched polymers ex-
hibit blueshifted maximum absorption in comparison with
those of the free chromophore in the same solvent due to
Figure 1. The FTIR spectra of the hyperbranched polymers and their cor-
responding monomer.
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Hyperbranched Polytriazoles
FULL PAPER
phore moieties, all of the maximum absorptions for the hy-
perbranched polymers, HP1 and HP2, containing the isola-
tion chromophores, were blueshifted in comparison with
that only containing one type of chromophore (HP-N;
Table 3). The blueshifted maximum absorptions of dendrim-
ers would surely result in a wide optical transparency
window, and contribute to their practical application in the
photonics fields. Moreover, in comparison with HP2, HP1
exhibits better transparency as a result of the presence of
more sulfonyl chromophore moieties.
Figure 2. UV/Vis spectra of HP1 in different solvents (0.02 mgmLꢀ1).
NLO properties: To evaluate the NLO activity of the poly-
mers, their poled thin films were prepared. The most con-
venient technique to study the second-order NLO activity is
to investigate the second-harmonic-generation (SHG) pro-
cesses characterized by d33, an SHG coefficient. The method
for the calculation of the SHG coefficients (d33) for the
poled films has been reported in our previous papers.[17,18]
From the experimental data, the d33 values for HP1 and
HP2 were calculated to be 117.6 and 167.4 pmVꢀ1, respec-
tively, at the fundamental wavelength of 1064 nm (Table 2).
To check the reproducibility of this result, we repeated the
measurements at least three times.
Table 3. The maximum absorption of the hyperbranched polymers and
the end-capping chromophore (lmax [nm]).[a]
THF 1,4-dioxane CHCl3 CH2Cl2 DMF DMSO Film
HP1
HP2
HP-N 464
S2 440
434
442
433
441
455
436
433
437
453
436
430
439
459
433
443
454
472
456
451
462
478
465
458
466
480
–
[a] The maximum absorption wavelength of polymer solutions with the
concentrations fixed at 0.02 mgmLꢀ1
.
In our previous work[5] with compounds with similar
chemical and topological structures to HP1 and HP2, but
only containing one type of chromophore (the nitro-based
one), HP-N demonstrated good NLO properties, with a d33
value of 116.8 pmVꢀ1. Herein, compounds HP1 and HP2
demonstrate higher d33 values, especially HP2 (its d33 values
was more than 50 pmVꢀ1 higher than HP-N). This is proba-
bly due to the presence of the isolation chromophore (the
sulfonyl-based one). The mb value for the sulfonyl-based
chromophore is lower than that of the nitro-based one.
Thus, the sulfonyl-based moieties and the triazole groups act
as the isolation groups to facilitate the noncentrosymmetric
alignment of the nitro-based chromophore moieties in the
dendrimers in the electronic field, benefiting the macroscop-
ic NLO effect. However, unlike normal isolation groups,
that is, those used in our previous work and the literature,
the sulfonyl-based chromophore moieties themselves appear
to be sufficiently well isolated by the triazole moieties,
which should be big enough to shield the electronic interac-
tions between the chromophore moieties due to their rela-
tively low mb value, to contribute to the macroscopic NLO
effect.
On the other hand, the DB of HP1 and HP2, derived
from the AB4-type monomer, should be higher than that of
HP-N, derived from the AB2-type monomer, as mentioned
previously. Thus, the 3D structure of HP1 and HP2 might
be closer to ideal than that of HP-N, and thus, according to
the site-isolation principle, their d33 values should be higher
than that of HP-N. Interestingly, although HP1 and HP2
have similar chemical structures, there is still a large differ-
ence in their NLO effects. This may be attributed to the dif-
ference in the ratios of the number of isolation chromo-
phore moieties (sulfonyl chromophores) to the number of
main chromophore moieties (nitro chromophores). If the
the branched nature of the structure and the site-isolation
effects. This abnormal UV/Vis absorption behavior for HP2
suggests some kind of interaction between the two types of
chromophore moieties, that is, the nitro and sulfonyl groups.
Very recently, to investigate the effects of isolation chromo-
phores, a series of polymers P12–P15 were prepared through
Sonogashira coupling reactions (Scheme S2 in the Support-
ing Information). The maximum absorption wavelength of
the chromophore moieties of P15, constructed from two dif-
ferent chromophore moieties with the regular AB structure,
was clearly redshifted in comparison with that of the corre-
sponding chromophores, whereas there was no evidence of a
redshift in the UV/Vis absorption spectra of P12–P14 (Fig-
ure S9 in the Supporting Information). This abnormal phe-
nomenon reveals that there may be some kind of interaction
between the two types of chromophore in the polymers with
a regular AB structure, and these interactions would cause
higher NLO effects. However, to gather more information
about these interactions, further studies are still needed.
In HP2 and S2, the ratio of the number of chromophores
to the number of isolation chromophores and to the number
of normal isolation groups was the same, that is, two chro-
mophores (the nitro-based ones) to one isolation chromo-
phore (the sulfonyl-based one) to six normal isolation
groups (four benzene rings and two triazole groups). How-
ever, hyperbranched polymer HP2 possessed a better 3D
topological structure than S2. As shown in Figure 3 and
Table 3, the difference between the maximum absorption
wavelengths in HP2 in different solvents (chloroform and
DMSO) was less than that for the corresponding chromo-
phore S2 (25 vs. 32 nm). This is probably due to the effective
site-isolation effect derived from a better 3D topological
structure.[5] In addition, for the presence of sulfonyl chromo-
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Z. Li et al.
concentration of the isolation chromophore moieties was
too high, the concentration of the main chromophore moiet-
ies becomes too low, leading to the comparably decreased
NLO effect. However, if there are too few isolation chromo-
phore moieties, the isolation effect becomes weak. Thus, the
difference in the ratio of the two chromophores would lead
to different d33 values for the corresponding polymers, and
there might be an optimum ratio between the isolation and
main chromophores to achieve the highest d33 value. The re-
sults for HP1 and HP2 could partially confirm this point,
but to gather more information, further studies are still
needed.
Because there might be some resonant enhancement due
to the absorption of the chromophore moieties at 532 nm,
the NLO properties of dendrimers should be smaller, as
shown by the nonresonant d33(1) values given in Table 2.
Due to their high-quality wide-optical transparency, as well
as the large d33 values, the d33(1) values for HP1 and HP2
were still high (24.8 and 31.5 pmVꢀ1, respectively), making
them promising candidates for potential applications in the
field of optics. This indicates that the introduction of the iso-
lation chromophore moieties into polymers is a new way to
further improve macroscopic NLO effects. To explore the
alignment of the chromophore moieties in the polymers, we
measured their order parameter, F. Figures S10 and S11 in
the Supporting Information contain the UV/Vis spectra of a
film of HP1 and HP2 before and after poling. It is easily
seen that, after poling, the dipole moments of the chromo-
phore moieties in the polymers were aligned and the absorp-
tion curves decrease because of birefringence.[19] From the
absorption change, the F values for the polymers could be
calculated by utilizing the equation presented in Table 2,
footnote h, and are found to be relatively high, showing the
good alignment of the chromophore moieties in HP1 and
HP2.
Figure 4. Decay curves of the SHG coefficients of the hyperbranched
polymers HP1 and HP2 as a function of temperature.
bility of the polymers is relatively good. This, coupled with
its large NLO effects and improved optical transparency
means that HP2 may be a promising candidate for practical
NLO applications.
Conclusion
By modifying a synthetic procedure, we have successfully
synthesized two new hyperbranched polytriazoles, contain-
ing isolation chromophores derived from AB4-type mono-
mers through click chemistry under copper(I) catalysis. The
two polymers are soluble in organic solvents and demon-
strate higher macroscopic NLO effects than a similar hyper-
branched polymer that just contains one kind of chromo-
phore (the nitro-based chromophore), synthesized from an
AB2-type monomer. This, coupled with their good thermal
stability suggests that these compounds are promising candi-
dates for practical photonic applications. We believe that
many other AB4-type, and even ABn-type (n>4), hyper-
branched polymers could also be obtained by carefully mod-
ifying the synthetic procedure.
The dynamic thermal stabilities of the NLO activities of
the polymers were investigated by depoling experiments, in
which the real-time decays of their SHG signals are moni-
tored as the poled films are heated from 35 to 1208C in air
at a rate of 48Cminꢀ1. Figure 4 shows the decay in SHG co-
efficient for HP1 and HP2 as a function of temperature; the
onset temperatures for decay were found to be around
808C. These results indicate that the long-term temporal sta-
Experimental Section
Materials and instruments: Tetrahydrofuran (THF) was dried over and
distilled from K–Na alloy under an atmosphere of dry nitrogen. Dime-
thylformamide (DMF) was dried over CaH2 and distilled under reduced
pressure before use. Chromophores S1, S3, S5, diazonium salt S4 and S6
were synthesized in our previous work.[5a, 17g,20] The synthetic route for S2,
the end-capping chromophore in HP2, is shown in Scheme 4 and its spe-
cific synthetic process is listed in the Supporting Information. All other
reagents were used as received.
1H NMR spectra were measured on a Varian Mercury300 spectrometer
by using tetramethylsilane (TMS; d=0 ppm) as the internal standard.
The Fourier-transform infrared (FTIR) spectra were recorded on a Perki-
nElmer-2 spectrometer in the region of 3000–400 cmꢀ1. UV/Vis spectra
were obtained by using a Shimadzu UV-2550 spectrometer. Elemental
analyses (EA) were performed by a CARLOERBA-1106 microelemental
analyzer. Matrix-assisted laser desorption ionization time-of-flight mass
spectra were measured on a Voyager-DE-STR MALDI-TOF mass spec-
Figure 3. UV/Vis spectra of HP2 in different solvents (0.02 mgmLꢀ1).
4432
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 4426 – 4434

Hyperbranched Polytriazoles
FULL PAPER
tion into methanol, from its solution in
THF, to afford HP1 as a red powder
(90.1 mg, 61.4%). Mw =1.25ꢁ105; Mw/
Mn =2.06 (GPC, polystyrene calibra-
tion); IR (KBr): u˜ =1716 (C=O), 1513,
1339 (-NO2), 1131 cmꢀ1 (-SO2-);
1H NMR (300 MHz, CDCl3, 298 K,
TMS): d=1.8–2.3 (brs;-CH2-), 2.3–2.5
(brs; -CH2-), 2.6–2.9 (brs; -CH2-), 2.9–
3.1 (brs; -SCH2-), 3.5–4.2 (brs;
-NCH2-), 4.2–4.7 (brs; -OCH2-), 6.4–
6.7 (brs; ArH), 6.7–7.0 (brs; ArH),
7.2–8.0 ppm (brs; ArH); 13C NMR
(75 MHz, CDCl3, 298 K, TMS): d=
22.54, 47.18, 49.79, 55.14, 61.71, 111.89,
122.84, 125.91, 128.42, 128.94, 129.53,
133.20, 138.45, 144.16, 146.19, 150.91,
156.11, 166.37 ppm; UV/Vis (THF,
0.02 mgmLꢀ1): lmax =434 nm.
Synthesis of HP2: The monomer
(41.7 mg, 0.03 mmol) was dissolved in
Scheme 4. The synthetic route to the end-capping chromophore S2.
DMF (3 mL) under a nitrogen atmos-
phere, and then an aqueous solution
of CuSO4 (75 mL, 0.04m) and NaAsc
trometer (MALDI-TOF MS; ABI, American) equipped with a 337 nm
nitrogen laser and a 1.2 m linear flight path, in positive ion mode. Gel-
permeation chromatography (GPC) was used to determine the molecular
weights of the polymers. GPC analyses were performed on a Waters
HPLC system equipped with a 2690D separation module and a 2410 re-
fractive-index detector. Polystyrene standards were used as the calibra-
tion standards for GPC, DMF was used as the eluent, and the flow rate
was 1.0 mLminꢀ1. Thermal analyses were performed on a NETZSCH
STA449C thermal analyzer at a heating rate of 108Cminꢀ1 under a nitro-
gen atmosphere at a flow rate of 50 cm3 minꢀ1 for thermogravimetric
analysis (TGA) and the thermal transitions of the polymers. The thick-
ness of the films was measured with an Ambios Technology XP-2 profil-
ometer.
(75 mL, 0.08m) was added. After the reaction was stirred for 30 h at room
temperature, the end-capping chromophore S2 (170.7 mg, 0.10 mmol)
and DMF (6 mL) were added. At the same time, another batch of CuSO4
(150 mL, 0.04m) and NaAsc (150 mL, 0.08m) were added. The reaction
mixture was stirred for a further 48 h. A large amount of water was
poured into the mixture, which was then filtered and washed with metha-
nol and acetone. The obtained solid was further purified by reprecipita-
tion into methanol from its solution in THF, to afford HP2 as a red
powder (125 mg, 64.1%). Mw =1.79ꢁ105; Mw/Mn =2.42 (GPC, polystyr-
ene calibration); IR (KBr): u˜ =1716 (C=O), 1514, 1338 (-NO2),
1139 cmꢀ1 (-SO2-); 1H NMR (300 MHz, CDCl3, 298 K, TMS): d=1.7–2.0
(brs; -CH2-), 2.0–2.1 (brs; -CH2-), 2.1–2.4 (brs; -CH2-), 2.7–3.0 (brs;
-CH2-), 3.0–3.1 (brs; -SCH2-), 3.5–4.2 (brs; -NCH2-), 4.2–4.7 (brs;
-OCH2-), 6.4–6.8 (brs; ArH), 6.8–7.0 (brs; ArH), 7.2–8.0ppm (brs;
ArH); 13C NMR (75 MHz, CDCl3, 298 K, TMS): d=21.79, 28.42, 47.20,
49.85, 51.09, 61.71, 68.65, 109.33, 111.92, 116.44, 122.93, 126.12, 128.43,
129.53, 133.23, 144.89, 146.89, 148.17, 149.29, 150.84, 155.06, 166.37 ppm;
UV/Vis (THF, 0.02 mgmLꢀ1): lmax =442 nm.
Synthesis of the monomer: Chromophore S3 (231.2 mg, 0.2 mmol) and
diazonium salt S4 (96.6 mg, 0.3 mmol) were dissolved in DMF (4 mL) at
08C. The reaction mixture was stirred for 3 d at 08C, and then treated
with H2O, extracted with CH2Cl2, and washed with brine. The organic
layers were combined and dried over anhydrous sodium sulfate. The
crude product was purified by column chromatography on silica gel by
using ethyl acetate/chloroform (2:1 v/v) as the eluent to afford a deep-
ꢁ
red solid (230 mg, 82.7%). IR (KBr): u˜ =3277 (C CH), 2098 (-N3), 1514,
1338 (-NO2), 1141 cmꢀ1 (-SO2-); 1H NMR (300 MHz, CDCl3, 298 K,
Acknowledgements
ꢁ
TMS): d=1.99 (brs, 2H; -CH2-), 2.05 (s, 1H; C CH), 2.24 (brs, 4H;
-CH2-), 2.33 (brs, 2H; -CH2-), 2.95 (brs, 4H; -CH2C-), 3.27 (brs, 2H;
-SCH2-), 3.58 (brs, 8H; -NCH2-), 3.69 (brs, 12H; -NCH2-, -CH2N3-), 4.15
(brs, 4H; -NCH2-), 4.38 (brs, 4H; -OCH2-), 6.57 (brs, 2H; ArH), 6.79
(brs, 5H; ArH), 7.23 (m, 4H; C=CH, ArH), 7.62 (brs, 2H; ArH), 7.80–
7.92 ppm (m, 11H; ArH); 13C NMR (75 MHz, CDCl3, 298 K, TMS): d=
13.52, 17.06, 18.97, 21.55, 21.66, 28.29, 30.35, 47.03, 48.56, 50.47, 50.98,
54.85, 65.37, 68.45, 70.15, 77.21, 81.66, 109.06, 111.53, 111.74, 116.26,
117.22, 122.19, 122.81, 125.77, 125.98, 128.63, 144.38, 144.85, 146.54,
147.00, 148.13, 149.27, 149.75, 154.99, 155.64 ppm; MALDI-TOF MS: m/
z calcd for C63H67N29O8S: 1390.46 [M+]; found: 1390.44; elemental analy-
sis calcd (%) for C63H67N29O8S: C 54.42, H 4.86, N 29.21; found: C 54.44,
H 5.29, N 28.45.
We are grateful to the National Science Foundation of China (No.
21034006) for financial support.
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Published online: February 23, 2012
4434
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Chem. Eur. J. 2012, 18, 4426 – 4434