Hyperbranched poly(aroxycarbonyltriazole)s: Metal-free click polymerization, light refraction, aggregation-induced emission, explosive detection, and fluorescent patterning

Article abstract of DOI:10.1021/ma400609m

The metal-free click polymerization (MFCP) of azide and alkyne has become a powerful tool for the synthesis of functional polytriazoles. Among which, the MFCP of propiolate and azide has been used to prepare functional linear poly(aroxycarbonyltriazole)s (PACTs). Their hyperbranched analogues, however, have been rarely prepared. In this paper, hyperbranched PACTs with satisfactory molecular weights and high regioregularities were synthesized in high yields by the MFCP of tripropiolates (1) and tetraphenylethene (TPE)-containing diazide (2) under the optimized reaction conditions without protection from oxygen and moisture. The resultant polymers are soluble in common organic solvents and thermally stable, with 5% loss of their weights at temperatures higher than 330°C. The polymers exhibit high refractive indices with low chromatic dispersion. Thanks to their contained TPE units, the polymers show the unique feature of aggregation-induced emission, and their aggregates can function as fluorescent sensors for the detection of explosives with the superamplification quenching effect. Furthermore, the polymers can be readily photo-cross-linked, yielding two-dimensional fluorescent patterns with high resolution.

Full text of DOI:10.1021/ma400609m

Article
pubs.acs.org/Macromolecules
Hyperbranched Poly(aroxycarbonyltriazole)s: Metal-Free Click
Polymerization, Light Refraction, Aggregation-Induced Emission,
Explosive Detection, and Fluorescent Patterning
Hongkun Li,† Haiqiang Wu,† Engui Zhao,‡ Jie Li,‡ Jing Zhi Sun,† Anjun Qin,*,†
and Ben Zhong Tang*,†,‡,§
†MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering,
Zhejiang University, Hangzhou 310027, China
‡Department of Chemistry, Institute for Advanced Study, and Institute of Molecular Functional Materials, The Hong Kong University
of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China
§Guangdong Innovative Research Team, State Key Laboratory of Luminescent Materials and Devices, South China University of
Technology, Guangzhou 510640, China
S
* Supporting Information
ABSTRACT: The metal-free click polymerization (MFCP) of azide and
alkyne has become a powerful tool for the synthesis of functional
polytriazoles. Among which, the MFCP of propiolate and azide has been
used to prepare functional linear poly(aroxycarbonyltriazole)s (PACTs).
Their hyperbranched analogues, however, have been rarely prepared. In
this paper, hyperbranched PACTs with satisfactory molecular weights and
high regioregularities were synthesized in high yields by the MFCP of
tripropiolates (1) and tetraphenylethene (TPE)-containing diazide (2)
under the optimized reaction conditions without protection from oxygen
and moisture. The resultant polymers are soluble in common organic
solvents and thermally stable, with 5% loss of their weights at temperatures
higher than 330 °C. The polymers exhibit high refractive indices with low
chromatic dispersion. Thanks to their contained TPE units, the polymers
show the unique feature of aggregation-induced emission, and their
aggregates can function as fluorescent sensors for the detection of explosives with the superamplification quenching effect.
Furthermore, the polymers can be readily photo-cross-linked, yielding two-dimensional fluorescent patterns with high resolution.
to be completely removed from the resultant polymers and can
cause cytotoxicity and deteriorate the photophysical properties
of polymeric materials.9 To circumvent this problem, the metal-
free click polymerizations (MFCPs) of activated alkynes
(aroylacetylenes/propiolates) and azide or activated azide and
alkynes have been successfully established, and functional linear
polytriazoles have been prepared under mild reaction
conditions.10
Among these methods, the MFCP of propiolates and azides
is promising to be utilized for the preparation of functional
hyperbranched polytriazoles but rarely reported.10c,d One of the
advantages of this polymerization is that the propiolate
monomers could be facilely prepared from commercially
available starting materials by one-step and one-pot reaction
under ambient conditions. Furthermore, it could be carried out
at a moderate temperature of 100 °C for 24 h in the mixed
solvent of DMF and toluene even without protection from
INTRODUCTION
■
Because of their unique architectures and properties, hyper-
branched polymers have gained increasing research interests
and are widely used in the areas of coatings, additives, drug
delivery carriers, supramolecular chemistry, etc.1,2 With
persistent efforts of polymer chemists, various polymerization
methods, such as polycondensation, polycyclotrimerization, and
polycouplings, have been developed, and functional hyper-
branched polymers have been prepared.3 Generally, these
polymerizations must be carried out under harsh reaction
conditions, which greatly limit their applications. Thus,
development of efficient polymerizations under mild reaction
conditions becomes the key issue to expand the application of
hyperbranched polymers.4
The azide−alkyne click polymerization, developed based on
the well-known azide−alkyne click reactions and sharing their
advantages of efficiency, mild reaction conditions, and function
tolerance, etc.,5 is an alternative efficient technique for the
preparation of functional hyperbranched polymers.6 Currently,
the click polymerizations have mainly been mediated by
metallic catalysts.7,8 However, the catalyst residue is difficult
Received: March 24, 2013
Revised: May 4, 2013
© XXXX American Chemical Society
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Monomer Preparation. The diazide of 1,2-bis[4-(azidomethyl)-
phenyl]-1,2-diphenylethene (2) was prepared according to the
experimental procedures described in our previous work.11 The
triynes of tripropiolates (1) were prepared by esterification of triols 4
with propiolic acid 3 in the presence of TsOH (Scheme 2). Detailed
experimental procedures for the synthesis of 1,1,1-trimethylolethane
tripropiolate (1a) are given below as an example.
oxygen and moisture, and soluble linear poly-
(aroxycarbonyltriazole)s (PACTs) with high molecular weights
(Mw up to 23 500) and regioregularities (fraction of 1,4-
disubstituted 1,2,3-triazole in PACTs up to ∼90%) in excellent
yields (up to ∼99%) could be produced (Scheme 1).
Scheme 1. Synthesis of Poly(aroxycarbonyltriazole) (PACT)
by Metal-Free Click Polymerization of Dipropiolate and
Diazide
Scheme 2. Synthetic Routes to Tripropiolates 1a and 1b
In a 250 mL round-bottom flask equipped with a Dean−Stark
apparatus were added 2.47 g (20 mmol) of 1,1,1-tris(hydroxymethyl)-
ethane (4a), 5.60 g (80 mmol) of propiolic acid (3), and 1.14 g (6
mmol) of TsOH in 100 mL of dry toluene. After refluxed for 48 h with
constant removal of yielded water, the reaction mixture was
concentrated, and the residues were dissolved in 100 mL of
dichloromethane (DCM). The organic phase was washed with 5%
aqueous NaHCO3 solution (30 mL × 3) and water (50 mL × 1) and
then dried over MgSO4 overnight. After filtration and solvent
evaporation, the crude product was purified by a silica gel column
using petroleum ether/ethyl acetate (6:1, v/v) as eluent. 1,1,1-
Trimethylolethane tripropiolate (1a) was obtained in 75.0% yield
(4.14 g) as a white solid. IR (thin film), ν (cm−1): 3274 (C−H
stretching), 2936, 2122 (CC stretching), 1722 (CO stretching),
Encouraged by these exciting results and to extend the
application of MFCP of propiolate and azide, in this paper, we
synthesized the first example of soluble multifunctional
hyperbranched PACTs (hb-PACTs) with high regioregularity
by this method based on optimal reaction conditions. The hb-
PACTs show high thermal stability and refractive indices and
feature the aggregation-induced emission characteristics, and
their aggregates could be used to detect explosive with
superamplification quenching effect. Moreover, two-dimen-
sional fluorescent patterns could readily be generated under UV
irradiation.
1
1594, 1472, 1382, 1228, 995, 752, 698. H NMR (400 MHz, DMSO-
d6), δ (TMS, ppm): 4.59 (s, 3H), 4.12 (s, 6H), 0.98 (s, 3H). 13C NMR
(100 MHz, DMSO-d6), δ (ppm): 152.3, 75.9, 74.1, 67.2, 38.4, 16.5.
Anal. Calcd for C14H12O6: C, 60.87; H, 4.38. Found: C, 60.72; H, 4.39.
Propane-1,2,3-triyl tripropiolate (1b) was prepared with the similar
procedures. Light-yellow viscous oil; yield 30.4%. IR (thin film), ν
(cm−1): 3280 (C−H stretching), 2971, 2123 (CC stretching),
1722 (CO stretching), 1453, 1376, 1215, 1093, 1013, 950, 754, 682,
EXPERIMENTAL SECTION
■
General Information. Tetrahydrofuran (THF) and toluene were
distilled from sodium benzophenone ketyl in an atmosphere of
nitrogen immediately prior to use. N,N-Dimethylformamide (DMF)
was predried over calcium hydride, distilled under reduced pressure,
and kept under nitrogen. Other solvents were purified by standard
methods. Propiolic acid and 1,1,1-tris(hydroxymethyl)ethane were
purchased from Alfa Aesar. Glycerin, p-toluenesulfonic acid, and picric
acid were obtained from Sinopharm Group Chemical Reagent Co.,
Ltd. (China). All other chemicals were purchased from Acros or
Aldrich and used as received without further purification.
1
600. H NMR (400 MHz, DMSO-d6), δ (TMS, ppm): 5.40 (m, 1H),
4.71 (s, 1H), 4.66 (s, 2H), 4.46−4.36 (m, 4H). 13C NMR (100 MHz,
DMSO-d6), δ (ppm): 151.7, 151.4, 76.8, 76.4, 73.6, 69.8, 62.9. HRMS
(MALDI-TOF), m/z calculated for C12H8O6Na [M + Na]+
=
271.0219; found = 271.0223.
Infrared (IR) spectra were taken on a Bruker Vector 22
Polymer Synthesis. All the polymerizations of tripropiolates and
diazide were carried out in air, unless otherwise stated. Typical
experimental procedures for the metal-free click polymerizations of 1
and 2 are shown below.
In a 15 mL Schlenk tube were placed 0.2 mmol of 1 and 0.3 mmol
of 2. Then, freshly distilled DMF (1.2 mL) was injected into the tube
to dissolve the monomers. After stirring at 60 °C for 5 h, the reaction
mixture was diluted with 10 mL of chloroform and added dropwise
into 300 mL of hexane through a cotton filter under stirring. The
precipitates were allowed to stand overnight and then collected by
filtration. The polymer was washed with hexane and dried to a
constant weight at ambient conditions.
1
spectrometer as thin films on KBr pellets. H and 13C NMR spectra
were recorded on Bruker ADVANCE2B 400 NMR or Bruker DMX
500 NMR spectrometers in CDCl3 or DMSO-d6 using tetramethylsi-
lane (TMS; δ = 0) as internal reference. UV spectra were measured on
a Varian VARY 100 Bio UV−vis spectrophotometer. Photo-
luminescence (PL) spectra were recorded on a RF-5301PC SERIES
spectrofluorometer. Fluorescence quantum yields (ΦF) were estimated
using quinine sulfate in 0.1 N H2SO4 (ΦF = 54.6%) as standard, and
the absorbance of the solutions was kept around 0.05 to avoid internal
filter effect. Element analysis was conducted on a Thermo Finnigan
Flash EA1112 system. MALDI−TOF mass spectra were recorded on a
GCT premier CAB048 mass spectrometer. Thermogravimetric
analysis (TGA) measurements were carried out on a PerkinElmer
TGA 7 under dry nitrogen at 20 °C/min. Refractive index (RI) values
were determined on a Metricon Models 2010 and 2010/M prism
coupler thin film thickness/refractive index measurement system.
Relative weight-average (Mw) and number-average (Mn) molecular
weights of the polymers and their polydispersity indices (PDI, Mw/
Mn) were estimated by a Waters 1515 gel permeation chromatography
(GPC) system equipped with an interferometric refractometer
detector, using a set of monodispersed linear polystyrenes (PS) as
calibration standards and THF as the eluents at a flow rate of 1.0 mL/
min.
Characterization Data of hb-PIa. The polymer was prepared by
polymerization of 1,1,1-trimethylolethane tripropiolate (1a) and 1,2-
bis[4-(azidomethyl)phenyl]-1,2-diphenylethene (2). White powder;
88.2% yield. Mw 7200; Mw/Mn 2.65 (GPC, determined in THF on the
basis of PS calibration). Fraction of 1,4-disubstituted 1,2,3-triazoles in
the polymer (F1,4): 89.3%. IR (thin film), ν (cm−1): 3268 (C−H
stretching), 2960, 2918, 2115 (N3 and CC stretching), 1727 (CO
1
stretching), 1545, 1467, 1389, 1213, 1046, 766, 699. H NMR (400
MHz, DMSO-d6), δ (TMS, ppm): 8.92, 8.64, 7.07, 6.93, 5.74, 5.55,
4.52, 4.31, 1.09. 13C NMR (125 MHz, CDCl3), δ (ppm): 160.3, 152.2,
144.4, 143.0, 140.6, 139.5, 133.6, 132.2, 131.3, 127.9, 126.6, 75.6, 74.1,
67.2, 54.1, 38.8, 16.7.
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Scheme 3. Synthesis of Hyperbranched Poly(aroxycarbonyltriazole)s hb-PIa and hb-PIb via Metal-Free Click Polymerizations of
Tripropiolates 1 and Diazide 2
hb-PIb. The polymer was synthesized by the polymerization of
propane-1,2,3-triyl tripropiolate (1b) and 1,2-bis[4-(azidomethyl)-
phenyl]-1,2-diphenylethene (2). White powder; 91.6% yield. Mw 4400;
Mw/Mn 2.21 (GPC, determined in THF on the basis of PS
calibration). F1,4: 82.6%. IR (thin film), ν (cm−1): 3268 (C−H
stretching), 3137, 2959, 2110 (N3 and CC stretching), 1735 (CO
systematically optimize polymerization conditions for the
preparation of soluble hb-PACTs.
We first investigated the effect of temperature on the
polymerization of 1a and 2 in DMF for 4 h with tripropiolate
concentration of 0.167 M (Table 1). Similar to our first try, the
1
stretching), 1541, 1446, 1359, 1203, 1051, 849, 761, 703. H NMR
a
Table 1. Effect of Temperature on the MFCP of 1a and 2
(400 MHz, DMSO-d6), δ (TMS, ppm): 8.83, 8.68, 7.09, 6.93, 5.73,
5.54, 4.61, 4.35. 13C NMR (125 MHz, CDCl3), δ (ppm): 160.1, 151.9,
144.4, 142.9, 140.9, 139.4, 132.1, 131.0, 127.9, 126.7, 73.9, 69.8, 62.9,
54.2.
Fabrication of Fluorescent Patterns. Photo-cross-linking
reactions of the polymer films were conducted in air at room
temperature using the 365 nm UV light with an intensity of 18.5 mW/
cm2 as light source. The polymer films were prepared by spin-coating
the polymer solution in 1,2-dichloroethane on silicon wafers and dried
at room temperature overnight under reduced pressure. The
photoresist patterns were generated using a copper photomask and
taken on an Olympus BX41 fluorescent optical microscope with a
330−385 nm wide-band UV excitation.
b
c
c
entry
T (°C)
S
Mw
PDI
yield (%)
1
2
3
4
5
100
90
gel
gel
√
√
√
80
70
60
52 700
20 600
5 200
7.08
6.06
2.07
85.9
81.7
66.2
a
b
Carried out in air for 4 h; [1a]/[2] = 2:3, [1a] = 0.167 M. Solubility
(S) tested in THF, DCM, chloroform, and DMF, etc.; √ = completely
c
soluble. Mw and PDI of polymer estimated by GPC in THF on the
basis of a linear polystyrene calibration.
RESULTS AND DISCUSSION
■
gels were formed at 100 and 90 °C. The polymerizations
carried out at 70 and 80 °C could produce soluble hb-PACTs
with high Mw in high yields, but with very broad polydispersity
indices (PDI > 6). Delightfully, soluble hb-PACT with the Mw
and PDI values of 5200 and 2.07, respectively, could be
obtained at lower reaction temperature of 60 °C. It is worth
noting that the Mw values of hyperbranched polymers are
normally underestimated as large as 7−30-fold by GPC using
linear polystyrene as calibration.12 Thus, 60 °C was selected as
our optimized polymerization temperature.
We then followed the time course of the polymerization of
1a and 2 in DMF at 60 °C with former concentration of 0.167
M (Table 2). The Mw and PDI values of the polymer products
generally increase with time except that for 8 h. The PDI values
became broad after 5 h, and the yields decrease slightly after 6
h. We thus chose 5 h as our optimal reaction time when taking
the Mw and PDI into account.
Monomer Synthesis. The tetraphenylethene (TPE)-
containing diazide monomer 2 was prepared according to our
reported procedures.11 The tripropiolates 1 were facilely
synthesized by a one-step and one-pot esterification of
commercially available triols (4) and propiolic acid (3) in the
presence of TsOH (Scheme 2). All the monomers were
carefully purified and characterized by spectroscopic methods,
from which satisfactory analysis data corresponding to their
molecular structures were obtained (see Experimental Section
for details).
Metal-Free Click Polymerization. In our previous work,
we have established an efficient metal-free click polymerization
(MFCP) of dipropiolate and diazide.10c,d These monomers
could readily react at a moderate temperature of 100 °C, and
functional linear PACTs with high molecular weights and
regioregularities in excellent yields could be produced in a short
reaction time. Encouraged by the high efficiency of this MFCP,
in this paper, we tried to use it to prepare hyperbranched
PACTs (hb-PACTs) (Scheme 3). Unfortunately, an insoluble
gel was formed when tripropiolate 1a and diazide 2 was reacted
in DMF at 100 °C for 3.5−4 h. Thus, it is necessary to
Finally, we studied the effect of the monomer concentration
on the polymerization of 1a with 2 in DMF at 60 °C for 5 h
(Table 3). With increasing monomer concentration from 0.100
to 0.200 M, the yields of the hb-PACTs increased but with
slight changes of their Mw and PDI. When considering the
C
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a
Table 2. Time Course on the MFCP of 1a and 2
b
b
entry
t (h)
Mw
PDI
yield (%)
1
2
3
4
5
4
5
6
7
8
5 200
7 800
2.07
2.72
5.95
6.34
5.48
66.2
68.5
78.5
76.0
17 300
18 700
12 400
75.7
a
b
Carried out in air at 60 °C; [1a]/[2] = 2:3, [1a] = 0.167 M. Mw and
PDI of polymer estimated by GPC in THF on the basis of a linear
polystyrene calibration.
Table 3. Effect of Monomer Concentration on the MFCP of
a
1a and 2
b
b
entry
[1a] (mol/L)
yield (%)
Mw
PDI
1
2
3
4
5
0.100
0.120
0.150
0.167
0.200
69.1
71.3
71.8
76.6
76.8
9200
8400
4.00
3.64
3.56
3.31
7800
Figure 1. TGA thermograms of hb-PIa and hb-PIb recorded under
nitrogen at a heating rate of 20 °C/min.
7200
11 400
4.44
a
b
Carried out in DMF at 60 °C in air for 5 h with [1a]/[2] = 2:3. Mw
obtained (see Experimental Section and Supporting Informa-
tion for details).
Examples of the IR spectra of hb-PIa and its monomers are
given in Figure 2. 1a shows absorption bands at 3274 and 2122
and PDI of polymer estimated by GPC in THF on the basis of a linear
polystyrene calibration.
yield, Mw, and PDI, we thus took 0.167 M as the optimized
monomer concentration of 1a.
Then, we used the optimized conditions to polymerize 1 and
2, which propagated smoothly, and hb-PIa and hb-PIb with
high molecular weights in high yields were obtained (Table 4).
a
Table 4. MFCPs of Tripropiolates 1 and Diazide 2
b
b
c
entry monomer polymer yield (%)
Mw
PDI
F1,4 (%)
1
2
3
1a + 2
1a + 2
1b + 2
hb-PIa
hb-PIa
hb-PIb
88.2
85.8
91.6
7200
6100
4400
2.65
2.44
2.21
89.3
88.5
90.9
d
a
Carried out at 60 °C in air for 5 h; [1]/[2] = 2:3; [1] = 0.167 M.
Mw and PDI of polymer estimated by a GPC system equipped with
b
an interferometric refractometer detector in THF on the basis of a
c
linear polystyrene calibration. Fraction of 1,4-disubstituted 1,2,3-
d
1
triazoles in the products calculated from H NMR spectra. Carried
out under nitrogen.
Figure 2. IR spectra of monomers 1a (A) and 2 (B) and their polymer
hb-PIa (C).
By comparing the polymerization results of 1a and 2 in air and
under nitrogen (cf. Table 4, entries 1 and 2), we can draw a
conclusion that oxygen and moisture exert little effect on the
polymerizations, which remarkably simplifies the reaction
procedures.
Processability and Stability. All the freshly prepared
hyperbranched polymers are completely soluble in commonly
used organic solvents, such as THF, DCM, chloroform, 1,2-
dichloroethane, and DMF. Furthermore, hb-PIa and hb-PIb can
be readily fabricated into high quality thin solid films by spin-
coating or solution casting process, which suggests that they are
processable. They are also thermally stable. As can be seen from
the TGA curves in Figure 1, the temperatures for 5% weight
loss are higher than 330 °C under nitrogen, revealing their
strong resistance to the thermolysis and oxidation at the
elevated temperatures.
cm−1 due to its C−H and CC stretching vibrations,
respectively. The strong absorption at 2101 cm−1 in the
spectrum of 2 is associated with its azido group. All these
absorption bands are still observed at similar wavenumbers but
become weaker in the spectrum of hb-PIa. This suggests that
most of the ethynyl and azido groups of the monomers have
been converted to the triazole rings of the polymer by the
MFCP. The spectral profile of hb-PIb is similar to that of hb-
PIa (Figure S1).
1
We have previously found that the H NMR spectra of
poly(aroyltriazole)s and PACTs measured in DMSO-d6 have
better resolution than in CDCl3.10f Thus, we performed the 1H
NMR analysis of hb-PI in DMSO-d6. The spectral profiles of
hb-PIa and hb-PIb are similar, implying that the alkyl chain
exerts little effect on the polymer structures (Figure 3 and
Figure S2). Figure 3 shows the 1H NMR spectra of polymer hb-
PIa and its monomers as an example. All resonance peaks of
polymer and monomers are assignable. The resonances of the
Structural Characterization. The polymers were charac-
terized by spectroscopic methods, and satisfactory results
corresponding to their expected molecular structures are
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hb-PIb, the important parameter of degree of branching is
hardly assessed.
Optical Transparency and Light Refractivity. Polymers
intended for photonic applications should have a high degree of
optical clarity.13 hb-PIa and hb-PIb absorb no light at
wavelengths longer than 390 nm in chloroform solutions
(Figure 4), indicating that they are highly transparent in the
Figure 4. Transmission spectra of chloroform solutions of hb-PIa and
hb-PIb. Polymer concentration: 10 μM.
1
Figure 3. H NMR spectra of monomer 1a (A) and 2 (B) and their
visible spectral region. This excellent optical transparency is due
to their contained alkyl and ester repeating units, which breaks
the electronic communications and decreases the effective
conjugation of the polymer chains. The high optical trans-
parency makes the hb-PACTs promising for photonic
applications.14
polymer hb-PIa (C) in DMSO-d6. The solvent and water peaks are
marked with asterisks.
ethynyl protons of 1a and the methylene protons adjacent to
the azido groups of 2 at δ 4.59 and 4.37, respectively, become
much weaker after polymerization. However, new peaks
resonated at δ 8.92, 8.64, 5.74, and 5.55 are observed in the
spectrum of hb-PIa. The strong peaks at δ 8.92 (c) and 5.55 (a)
are associated with the resonances of the protons in the triazole
ring and its neighboring methylene group in the 1,4-isomeric
units, while the weak signals at δ 8.64 (d) and 5.74 (b) stem
from those in the 1,5-isomeric units. The results indicate that
most of the ethynyl and azido groups of the monomers have
been transformed into the triazole rings of the polymer by the
MFCP, further confirming the conclusion drawn from the IR
spectra analysis. The resonance peaks at δ 8.92 and 8.64 are
well separated, which enables us to calculate the F1,4 value from
their integrals in the spectrum of hb-PIa, which is as high as
89.3%. Similarly, the F1,4 of hb-PIb can also be calculated by this
method, and an even higher value (90.9%) was deduced
(Figure S2 and Table 4).
Since the hb-PI contains polarized triazole rings and ester
groups, they are expected to show high refractive index (RI)
values (n).15 Wavelength-dependent refractivity measurements
confirm that hb-PIa and hb-PIb possess high RI values of
1.614−1.586 and 1.622−1.589 in the wavelength region of
600−1550 nm, respectively (Figure 5). It is worth noting that
these values are higher than those of the commercially
The 13C NMR spectrum of hb-PIa shows weak resonance
peaks of acetylenic carbon atoms at δ 75.6 and 74.1 compared
to those of its monomer. Furthermore, new peaks correspond-
ing to the absorptions of the triazole carbons are observed at δ
140.6 and 139.5. These results again indicate that the ethynyl
groups of 1a and azides of 2 have been converted into the
triazole rings in the polymer by the MFCP (Figure S3). Similar
phenomena were observed in the 13C NMR spectrum of hb-PIb
(Figure S4). Because of the poor resolution of related
Figure 5. Light refraction spectra of thin solid films of hb-PIa and hb-
PIb and variation of modified Abbe number (υ′D) with polymer
́
structures.
1
resonances in the H and 13C NMR spectra of hb-PIa and
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important optical plastics such as poly(methyl methacrylate) (n
= 1.489) and polycarbonate (n = 1.582). In addition, the
development of polymeric materials with low chromatic
dispersion is of importance for practical applications. The
Abbe number (υD) of a material is a measure of the variation or
́
dispersion in its RI values with wavelength. To evaluate the
application potential of an optical material, it is suggested to use
́
a modified Abbe number (υ′D) calculated by its RI values at the
nonabsorbing wavelengths of 1064, 1319, and 1550 nm.16 The
first two wavelengths are chosen in view of the practical interest
of commercial laser wavelengths, while the last one is the
wavelength for telecommunication. The modified Abbe
is defined as eq 1:
́
number
n1319 − 1
ν′D =
n1064 − n1550
(1)
where n1319, n1064, and n1550 are the RI values at 1319, 1064, and
1550 nm, respectively.
The υ′D values for hb-PIa and hb-PIb were calculated to be
101 and 75, which are much higher than those for the
conjugated polymers with absorptions in the visible spectral
region (υ′D ≈ 10−40). Since optical dispersion is inversely
́
proportional to the Abbe number, the hb-PIa and hb-PIb thus
have very low optical dispersion. Thus, this MFCP paves a
facile and effective way to prepare photonic hyperbranched
polytriazoles with high refractive index values and low
chromatic dispersion.
Aggregation-Induced Emission. As shown in Scheme 3,
hb-PIa and hb-PIb were synthesized by MFCP of triynes 1 and
TPE-containing diazide 2. Previously, we have found that TPE
is a typical luminogen exhibiting aggregation-induced emission
(AIE) characteristic, which refers to a unique phenomenon that
a series of propeller-shaped molecules are nonemissive when
molecularly dissolved but emit intensely when aggregated or
fabricated in solid films, and the restriction of intramolecular
rotation (RIR) has been rationalized as the mechanism.17,18
The AIE-active TPE-containing linear polytriazoles have been
prepared by the Cu(I)-catalyzed and metal-free click polymer-
izations and been well-investigated.19 The TPE-containing
hyperbranched polytriazoles showing AIE feature are, however,
rarely reported.7a,b
Figure 6. (A) PL spectra of hb-PIa in THF and THF/water mixtures.
Concentration: 10 μM; λex: 318 nm. (B) Variation in the quantum
yields (ΦF) of hb-PIa, hb-PIb and TPE in THF/water mixtures with
different water fractions. The ΦF values were estimated using quinine
sulfate in 0.1 M H2SO4 (ΦF = 54.6%) as standard. Inset in panel B:
solutions of hb-PIa in pure THF and a THF/water mixture with 90%
water; photographs taken under illumination of a hand-held UV lamp.
According to our previous work,7b the TPE-containing
hyperbranched polymers possessing flexible alkyl chains can be
endowed with the AIE feature. hb-PIa and hb-PIb also contain
such chains and are expected to preserve the AIE features. We
thus investigated their emission behaviors in the THF/water
mixtures with different water fraction (fw). Figure 6A shows the
PL spectra of hb-PIa as an example. When excited with the
maximum absorption at 318 nm, the PL curve of a diluted THF
solution of hb-PIa is almost a flat line parallel to the abscissa; in
other words, the polymer is virtually nonluminescent. On the
contrary, the fluorescence emission is progressively intensified
with its spectral profile remaining unchanged upon gradual
addition of water into its THF solution. The similar emission
behaviors of hb-PIb were also observed in its PL spectra
(Figure S5). Evidently, hb-PIa and hb-PIb are AIE-active.
The AIE characteristics of hb-PIa and hb-PIb were further
verified by the variations of their fluorescence quantum yields
(ΦF) in the solution and aggregate states. In pure THF, hb-PIa
and hb-PIb both show negligibly small ΦF values of 0.20 and
0.23%, respectively (Figure 6B). The ΦF values of them
increase progressively with gradual addition of water and reach
the highest values of 16.51 and 16.53% at the fw of 90%, which
are 82- and 72-fold higher than those in pure solvents,
respectively. Furthermore, these ΦF values of polymers are
close to that of TPE under the same experimental conditions at
the fw of 90%, suggesting that the emission behavior of TPE
was preserved in the polymers after MFCP.
The AIE features of hb-PIa and hb-PIb could be attributed to
their intrinsic architectures. According to the RIR mechanism,
even after the TPE units have been incorporated into the
polymer branches, their peripheral phenyl rings can still rotate
freely in the solution state, making the polymers nonemissive.
In the aggregate state, however, the intramolecular rotation is
restricted, which effectively blocks the nonradiative energy
dissipation channels of the polymers and turns on their light
emission.
Explosive Detection. Prompted by the AIE characteristics
of the hb-PIa and hb-PIb, we utilized them as fluorescent
sensors for the explosive detection, which is important in the
fields of homeland security and antiterrorism.20 Because 2,4,6-
trinitrophenol (picric acid, PA) is commercially available, we
then use it as a model explosive among the nitroaromatic
F
dx.doi.org/10.1021/ma400609m | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
compounds. The aggregates of polymers in THF/water
mixtures with fw of 90% were utilized as the fluorescent
sensors due to their highest emission intensities. For
comparison, the nanoaggregate of TPE was also used to detect
PA.
Figure 7A shows the PL behaviors of hb-PIa upon addition of
PA as an example. With gradual addition of PA, the PL
unreacted ethynyl and azide groups on their peripheries, which
can be cross-linked upon UV irradiation, the polymer films
could be employed to generate fluorescent patterns. When thin
solid films of hb-PIa and hb-PIb on silicon wafers were
irradiated under a UV light through copper photomasks, the
exposed region was cross-linked and bleached probably through
a nitrene-mediated photolysis mechanism,7c whereas the
unexposed area remains emissive. Two-dimensional fluorescent
patterns with sharp line edges were thus readily generated
without going through the development processes (Figure 8).
Figure 8. Two-dimensional fluorescent pattern of hb-PIa. The
photograph was taken under UV irradiation.
CONCLUSION
■
In summary, we have successfully prepared functional hb-
PACTs by the efficient metal-free click polymerization of
tripropiolates and TPE-containing diazide. Through inves-
tigation of the reaction conditions, we found that hb-PACTs
with satisfactory molecular weights and high regioregularities
(F1,4 up to 90.9%) could be produced in high yields (up to
91.6%) after the monomers reacted in DMF at 60 °C for 5 h
without protection from oxygen and moisture. The resultant
hb-PACTs are completely soluble in organic solvents and
thermally stable. Furthermore, they possess good film-forming
ability and display high refractive indices with low chromatic
dispersions. Thanks to the AIE feature of their contained TPE
units, the hb-PACTs are also AIE-active, and their aggregates
can work as fluorescent chemosensors for the detection of
explosives with superamplification quenching effect. The
multifunctional hb-PACTs make them promising candidates
for high-tech applications.
Figure 7. (A) PL spectra of the hb-PIa in THF/water mixtures (fw:
90%) containing different amounts of picric acid (PA). Concentration:
10 μM. λex: 318 nm. (B) Stern−Volmer plots of I0/I − 1 of hb-PIa, hb-
PIb, and TPE versus PA concentration, where I = peak intensity and I0
= peak intensity at [PA] = 0 μg/mL. Inset: the chemical structure of
PA.
ASSOCIATED CONTENT
■
S
* Supporting Information
1
IR and H NMR spectra of hb-PIb and their monomers; 13C
NMR spectra of hb-PIa and hb-PIb and their monomers; PL
spectra of hb-PIb in THF/water mixtures. This material is
available free of charge via the Internet at http://pubs.acs.org.
intensities of hb-PIa aggregates progressively decreased but
with no change in the spectral profiles. Similar PL behaviors are
observed for hb-PIb aggregates, and their detection limits for
PA could be as low as 1 μg/mL. The aggregates of TPE can
also be used to detect PA and the Stern−Volmer plot is almost
linear with PA concentration, giving a quenching constant of
13 400 M−1, whereas the plots of hb-PIa and hb-PIb show
linear correlation with quenching constants of 19 500 and 26
700 M−1, respectively, when the PA concentrations are lower
than 20 μg/mL (Figure 7B). Afterward, the curves bend
upward, which shows a unique superamplification quenching
effect, which are similar to our previous research.21
AUTHOR INFORMATION
Corresponding Author
*E-mail: qinaj@zju.edu.cn (A.J.Q.); tangbenz@ust.hk (B.Z.T).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was partially supported by the National Science
Foundation of China (21222402, 21174120 and 20974098);
the key project of the Ministry of Science and Technology of
Fluorescence Patterning. Thanks to the AIE feature, film-
forming ability of hb-PIa and hb-PIb, and the remaining
G
dx.doi.org/10.1021/ma400609m | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
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