Copper-catalyzed polycoupling of diynes, primary amines, and aldehydes: A new one-pot multicomponent polymerization tool to functional polymers

Article abstract of DOI:10.1021/ma501477w

Development of new methodology for the preparation of functional polymers with practical applications is a fundamental important research area in macromolecular science. In this paper, a new polymerization route for the synthesis of functional polymers is developed. The three-component polycoupling reactions of diynes [HC ≡ C-R-C ≡ CH, R = (C₆H ₅)₂C = C(C₆H₅)₂, (C ₆H₅)₄SiC₄(C₄H ₉)₂, (C₆H₅)₄SiC ₄(C₆H₅)₂, C₁₂H ₈(OCH₂)₂, (CH₂)₄], primary amines, and aldehydes are catalyzed by copper(I) chloride in toluene at 100 °C for 2 h, affording soluble and regular poly(dipropargyl amine)s (PDAs) with high molecular weights (M_w up to 43 800) in high yields (up to 89%). The polymerization reaction is insensitive to moisture. All the PDAs are thermally stable and film forming. Their thin films show good optical transparency and high refractive indices (RI = 1.8322-1.7458) with low optical dispersions (D down to 0.0117). The tetraphenylethene or silole-containing PDAs exhibit a phenomenon of aggregation-induced emission. The PDAs are photosensitive and cross-link upon UV irradiation, generating negative photoresist fluorescent patterns. The polymer aggregates can function as sensitive fluorescent chemosensors for detecting explosives, such as picric acid, 2,4-dinitrotoluene, and 4-nitrobenzoyl chloride, with large quenching constants of up to 2.7 × 10⁵ L/mol.

Full text of DOI:10.1021/ma501477w

Article
pubs.acs.org/Macromolecules
Copper-Catalyzed Polycoupling of Diynes, Primary Amines, and
Aldehydes: A New One-Pot Multicomponent Polymerization Tool to
Functional Polymers
Yajing Liu,†,‡ Meng Gao,†,‡ Jacky W. Y. Lam,*,†,‡ Rongrong Hu,†,‡ and Ben Zhong Tang*,†,‡,§
†HKUST-Shenzhen Research Institute, No. 9 Yuexing first RD, South Area, Hi-tech Park, Nanshan Shenzhen 518057, China
‡Department of Chemistry, Division of Life Science, State Key Laboratory of Molecular Neuroscience, Institute for Advanced Study,
Institute of Molecular Functional Materials, Division of Biomedical Engineering, The Hong Kong University of Science and
Technology, Clear Water Bay, Kowloon Hong Kong, China
§Guangdong Innovative Research Team, SCUT-HKUST Joint Research Laboratory, State Key Laboratory of Luminescent Materials
and Devices, South China University of Technology, Guangzhou 510640, China
ABSTRACT: Development of new methodology for the preparation of
functional polymers with practical applications is a fundamental
important research area in macromolecular science. In this paper, a
new polymerization route for the synthesis of functional polymers is
developed. The three-component polycoupling reactions of diynes
[ H C  C − R − C  C H ,
R
=
( C 6 H 5 ) 2 C  C ( C 6 H 5 ) 2 ,
(C6H5)4SiC4(C4H9)2, (C6H5)4SiC4(C6H5)2, C12H8(OCH2)2, (CH2)4],
primary amines, and aldehydes are catalyzed by copper(I) chloride in
toluene at 100 °C for 2 h, affording soluble and regular poly(dipropargyl
amine)s (PDAs) with high molecular weights (Mw up to 43 800) in high
yields (up to 89%). The polymerization reaction is insensitive to
moisture. All the PDAs are thermally stable and film forming. Their thin
films show good optical transparency and high refractive indices (RI =
1.8322−1.7458) with low optical dispersions (D down to 0.0117). The
tetraphenylethene or silole-containing PDAs exhibit a phenomenon of aggregation-induced emission. The PDAs are
photosensitive and cross-link upon UV irradiation, generating negative photoresist fluorescent patterns. The polymer aggregates
can function as sensitive fluorescent chemosensors for detecting explosives, such as picric acid, 2,4-dinitrotoluene, and 4-
nitrobenzoyl chloride, with large quenching constants of up to 2.7 × 105 L/mol.
Konakahara,4 Wang,5 Damavandi,6 and Cai7 augmented the
INTRODUCTION
■
reaction scope and enriched the catalyst system during the past
decade. In 2012, Li reported the copper-catalyzed A3-coupling
reaction of alkyne, amino acid, and aldehyde under ambient
aqueous conditions, generating dipropargylated products in
high yields.8
For many years, our group has been working on the
construction of functional macromolecules from monoynes,
diynes, and triynes by means of one or two-component
metathesis polymerization, polycoupling, polycyclotrimeriza-
tion, and “click” reaction.9 Recently, we succeeded in the
generation of soluble polymers with well-defined structures by
indium-catalyzed three-component polycoupling of diyne,
dialdehyde, and secondary amine (Scheme 1).10 The polymers
have been found to exhibit a variety of unique properties such
as strong emission in the aggregated state, high refractive
indices, and precursors to nanostructured magnetic ceramics.11
The development of a novel polymerization route to functional
polymer with unique properties is a fundamental important
area in macromolecule science. Compared with numerous
existing polymers with single-bond structures, those con-
structed from acetylenic monomers tend to be more electroni-
cally active. The conjugated olefin scaffolds endow the
polymers with unique electronic, mesomorphic, photonic, and
biological properties. However, most of these polymers are
obtained from single monomer or polymerization between two
components. Multicomponent polymerization tools, however,
are scarcely reported in the past decade.1
Multicomponent reactions (MCRs) have attracted much
attention from organic chemists because of their various
advantages.2 MCRs proceed in chemo- and regioselective
fashion with high atom economy and diminution of waste
production due to the reduced synthetic and isolation steps,
thus allowing the preparation of complex compounds from
simple precursors. In 2002, Li and co-workers discovered the
A3-coupling reactions of alkynes, aldehydes, and amines, and
since then they have made remarkable progress in this area.3
Received: July 18, 2014
Published: July 23, 2014
© 2014 American Chemical Society
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Scheme 1. Indium-Catalyzed Polycoupling of Diyne, Amine, and Dialdehyde
Scheme 2. Copper-Catalyzed Polycoupling of Diynes, Amines, and Aldehydes
The indium-catalyzed three-component polymerization,
however, has some drawbacks: the catalyst (InCl3) is expensive,
and the polymerization only proceeds smoothly at a high
temperature of 140 °C for a long reaction time of 20 h.
Therefore, it would be nice if a similar polymerization pathway
can be developed, which can perform under milder reaction
conditions without the requirement of costly catalyst or
additive.
In this paper, we show that the polycoupling reactions of
diynes, primary amines, and aldehydes can be carried out at a
lower temperature of 100 °C for only 2 h in the presence of a
less expensive catalyst of copper(I) chloride, furnishing soluble
poly(dipropargylamine)s (PDAs) with high molecular weights
in high yields (Scheme 2). The polymerization reaction is
insensitive to moisture. The obtained PDAs are thermally
stable. Their thin films show high refractive indices and cross-
link upon UV irradiation, generating negative photoresist
patterns. The PDAs carrying tetraphenylethene (TPE) and
silole chromophores exhibit a phenomenon of aggregation-
induced emission (AIE), and their aggregates can function as
sensitive fluorescent chemosensors for explosive detection. All
these attributes make them to find potential high technological
applications.
EXPERIMENTAL SECTION
■
Materials and Instrumentation. Tetrahydrofuran (THF) was
distilled from sodium benzophenone ketyl under nitrogen immediately
before use. All the primary amines, aldehydes, and other reagents were
purchased from Aldrich and used as received without further
purification. Monomers, named 1,2-bis(4-ethynylphenyl)-1,2-dipheny-
lethene (1a),12a 2,5-bis(4-ethynylphenyl)-3,4-diphenyl-1,1-dibutylsi-
lole (1c),12c 2,5-bis(4-ethynylphenyl)-1,1,3,4-tetraphenylsilole
(1d),12c and 1-(4-ethynylphenyl)-1,2,2-triphenylethene (9),12d were
prepared according to the literature methods. 4,4′-Bis(2-ethynyloxy)-
biphenyl (1e) was prepared by nucleophilic substitution of 4,4′-
biphenol with propargyl bromide.12b Weight-average molecular
weights (Mw) and polydispersities (Mw/Mn) of the polymers were
estimated on a Waters gel permeation chromatography (GPC) system
using THF as eluent. Details about the experimental setup can be
found in our previous publication.10
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Scheme 3. Synthetic Route to Tetraphenylethene-Containing Diyne
IR spectra were recorded on a PerkinElmer 16 PC FTIR
refluxed for 4 h. After cooling to room temperature, the organic layer
was washed with 10% aqueous NaHCO3 solution (3 × 30 mL) and
dried over anhydrous magnesium sulfate. After filtration and solvent
evaporation, the crude product was purified on a silica gel column
chromatography using hexane/ethyl acetate (10:1 v/v) as eluent.
1
spectrophotometer. H and 13C NMR spectra were measured on a
Bruker AV 400 spectrometer in deuterated chloroform using
tetramethylsilane (TMS; δ = 0) as internal reference. UV spectra
were measured on a Milton Ray Spectronic 3000 Array spectropho-
tometer. Photoluminescence (PL) spectra were recorded on a
PerkinElmer LS 55 spectrophotometer. Thermogravimetric analysis
(TGA) was carried on a TA TGA Q5000 under nitrogen at a heating
rate of 10 °C/min. Particle sizes of the polymer aggregates were
measured on a Brookhaven Zeta potential analyzer. Refractive indices
were determined on a J.A. Woollam variable angle ellipsometry system
with a wavelength tunability from 300 to 1000 nm.
1
Yellow solid; yield 71.5%. H NMR (400 MHz, CDCl3), δ (TMS,
ppm): 7.20 (m, 4H), 7.09 (m, 8H), 6.97 (m, 6H), 0.22 (s, 16H).
1,1-Bis(4-ethynylphenyl)-2,2-diphenylethene (1b). Into a 250
mL two-necked round-bottom flask was added 8 (2.1 g, 12.8 mmol),
excess potassium carbonate, tetrahydrofuran (50 mL), and methanol
(50 mL). After stirring for 3 h, the reaction was quenched with the
addition of 10% aqueous solution of hydrochloric acid. The organic
layer was extracted with dichloromethane (3 × 50 mL), and the
combined organic layers were dried over anhydrous magnesium
sulfate. After filtration and evaporation, the crude product was purified
on a silica gel column chromatography using hexane as eluent. White
Monomer Synthesis. Monomer 1b was synthesized according to
the synthetic route shown in Scheme 3. Detailed procedures are shown
below.
Synthesis of 4,4′-Bis(2-trimethylsilylethynyl)benzophenone
(6). Into a 500 mL two-necked round-bottom flask were added 4,4′-
dibromobenzophenone (5, 10 g, 29.4 mmol), Pd(PPh3)2Cl2 (619 mg,
0.88 mmol), CuI (223 mg, 1.17 mmol), PPh3 (154 mg, 0.59 mmol),
THF (150 mL), and triethylamine (50 mL) under a nitrogen
atmosphere. After all the solids were completely dissolved,
(trimethylsilyl)acetylene (16.6 mL, 117.6 mmol) was injected into
the flask. After stirring at 70 °C overnight, the reaction was quenched
with the addition of an aqueous solution of ammonium chloride. The
reaction mixture was extracted with dichloromethane (3 × 50 mL).
The organic layers were combined and dried over anhydrous
magnesium sulfate. The solvent was evaporated, and the resulting
crude product was purified on a silica gel column chromatography
using hexane as eluent. White solid; yield 81.6%. 1H NMR (400 MHz,
CDCl3), δ (TMS, ppm): 7.72 (d, 4H), 7.57 (d, 4H), 0.27 (18H).
1,1-Bis[4-(2-trimethylsilylethynyl)phenyl]-2,2-diphenyle-
thene (8). To a solution of diphenylmethane (7, 3.23 g, 19.2 mmol)
in THF (20 mL) was added dropwise 2 M solution of n-butyllithium
in hexane (7.68 mL, 15.36 mmol) at 0 °C under nitrogen. After
stirring for 30 min, a solution of 6 (2.1 g, 12.8 mmol) in THF was
added into the solution. After warming to room temperature and
stirring overnight, the reaction was quenched with the addition of an
aqueous solution of ammonium chloride. The organic layer was
extracted with dichloromethane (3 × 50 mL) and saturated aqueous
solution of sodium chloride. The combined organic layers were dried
over anhydrous magnesium sulfate. After filtration, the solvent was
evaporated and the resulting crude alcohol (containing excess
diphenylmethane) was dissolved in 80 mL of toluene in a 250 mL
two-necked round-bottom flask equipped with a condenser. A catalytic
amount of p-toluenesulfonic acid was added, and the mixture was
1
solid; yield 70.8%. H NMR (400 MHz, CDCl3), δ (TMS, ppm):
7.24−7.22 (m, 4H), 7.12−7.11 (m, 6H), 7.01−6.95 (m, 8H), 3.04 (s,
2H). 13C NMR (100 MHz, CDCl3), δ (TMS, ppm): 143.02, 142.59,
139.23, 131.28, 127.85, 126.89, 120.07, 83.67. HRMS (MALDI-TOF):
m/z 380.1525 [M+, calcd 380.1565].
Polymer Synthesis. All the polymerization reactions were carried
out under nitrogen using the standard Schlenk technique. A typical
experimental procedure for the polymerization of 1a with 2a and 3a is
given below as an example.
Into a 10 mL Schlenk tube equipped with a magnetic stirrer were
placed 1a (0.1 mmol), 2a (0.1 mmol), and 3a (0.3 mmol, 38 wt %
formaldehyde solution), CuCl (0.5 equiv to 1a), and NaHCO3 (0.1
mmol) in 0.1 mL of distilled toluene. The reaction mixture was stirred
under nitrogen in an oil bath at 100 °C for 2 h. Afterward, the
polymerization reaction was stopped by pouring the reaction mixture
into 150 mL of hexane via a cotton filter to remove any, if possible,
insoluble substances and the Cu catalyst. The precipitates were washed
with hexane and dried in vacuum overnight at 40 °C to a constant
weight.
Characterization Data for P1a/2a/3a. Yellow solid; yield 82.0%
(Table 6, entry 1). Mw 29 200; Mw/Mn 4.6 (GPC, polystyrene
calibration). IR (KBr), υ (cm−1): 3031, 2951, 1731, 1602, 1498, 1442.
1H NMR (400 MHz, CDCl3), δ (TMS, ppm): 7.24, 7.19, 7.17, 7.10,
6.99, 6.94, 3.88, 3.48, 3.10. 13C NMR (100 MHz, CDCl3), δ (TMS,
ppm): 172.16, 144.54, 143.39, 142.89, 141.21, 140.98, 140.75, 140.54,
137.46, 131.73, 131.17, 129.09, 128.27, 127.79, 126.72, 126.46, 120.99,
119.72, 85.01, 81.84, 65.76, 51.45, 40.97, 36.39, 36.26, 31.34.
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P1b/2a/3a. Greenish-yellow solid; yield 59.2% (Table 6, entry 2).
Mw 16 700; Mw/Mn 2.0 (GPC, polystyrene calibration). IR (film), υ
Model Reaction. Model compound 4 was prepared by coupling
reaction of 9, L-phenylalanine methyl ester hydrochloride (2a), and
formaldehyde (3a). The experimental procedure was similar to that for
the preparation of P1a/2a/3a but in the absence of organic solvent.
Light yellow solid; yield 27.3%. IR (film), υ (cm−1): 3066, 2954, 2862,
2355, 2249, 1953, 1887, 1809, 1731, 1596, 1496, 1445. 1H NMR (400
MHz, CDCl3), δ (TMS, ppm): 7.16, 7.14, 7.12, 7.10, 7.09, 7.02, 7.01,
7.00, 3.88, 3.87, 3.49, 3.11. 13C NMR (100 MHz, CDCl3), δ (TMS,
ppm): 172.23, 143.85, 143.41, 140.24, 137.55, 131.23, 129.21, 128.79,
127.78, 126.63, 126.57, 120.78, 85.29, 84.86, 51.57, 41.11, 36.37.
HRMS (MALDI-TOF): m/z 915.3992 (M+, calcd 915.4076).
Photopatterning. The photo-cross-linking reaction of the
polymer films was conducted in air at room temperature using 365
nm light obtained from a Spectroline ENF-280C/F UV lamp. The
procedures were similar to those described in our previous
publication.12e
1
(cm−1): 2926, 2857, 1735, 1663, 1600, 1496, 1451. H NMR (400
MHz, CDCl3), δ (TMS, ppm): 7.24, 7.21, 7.18, 7.17, 7.15, 7.10, 7.00,
6.99, 6.94, 6.92, 3.88, 3.48, 3.10. 13C NMR (100 MHz, CDCl3), δ
(TMS, ppm): 143.40, 143.18, 137.55, 131.24, 131.14, 129.19, 129.02,
128.37, 128.21, 127.80, 126.79, 126.55, 125.28, 121.00, 85.13, 51.56,
41.06, 30.91.
P1c/2a/3a. Orange solid; yield 50.7% (Table 6, entry 3). Mw 15
700; Mw/Mn 2.2 (GPC, polystyrene calibration). IR (KBr), υ (cm−1):
3025, 2956, 2921, 2855, 1734, 1656, 1597, 1437. 1H NMR (400 MHz,
CDCl3), δ (TMS, ppm): 7.19, 7.00, 6.82, 6.77, 3.89, 3.49, 3.12, 2.36,
1.32, 0.99, 0.81. 13C NMR (100 MHz, CDCl3), δ (TMS, ppm):
155.79, 142.94, 142.43, 140.47, 138.54, 131.35, 129.22, 129.00, 128.70,
128.34, 127.47, 126.35, 125.27, 119.82, 26.04, 25.64, 21.43, 11.62.
P1d/2a/3a. Yellow solid; yield 52.7% (Table 6, entry 4). Mw 2700;
Mw/Mn 2.0 (GPC, polystyrene calibration). IR (film), υ (cm−1): 2958,
1
RESULTS AND DISCUSSION
2918, 2850, 1735, 1666, 1597, 1431. H NMR (400 MHz, CDCl3), δ
■
(TMS, ppm): 7.64, 7.38, 7.10, 7.04, 6.98, 6.86, 6.84, 6.56, 3.52, 301,
2.81. The 13C NMR data were not obtained due to the low solubility
of the obtained polymer.
Monomer Preparation. In order to develop the one-pot
A3-coupling reaction of alkyne, primary amine, and aldehyde
P1e/2a/3a. Yellow solid; yield 14.6% (Table 6, entry 5). Mw 2100;
Mw/Mn 1.8 (GPC, polystyrene calibration). IR (film), υ (cm−1): 2924,
1723, 1604, 1495, 1446. 1H NMR (400 MHz, CDCl3), δ (TMS, ppm):
7.46, 7.44, 7.43, 7.31, 7.19, 7.10, 7.02, 7.00, 6.99, 4.75, 4.73, 4.72, 3.67,
3.50, 2.98, 2.55. 13C NMR (100 MHz, CDCl3), δ (TMS, ppm):
171.03, 129.05, 128.28, 127.67, 115.08, 60.28, 55.78, 30.80, 29.60,
20.92, 14.08.
P1f/2a/3a. Red oil; yield 12.7% (Table 6, entry 6). Mw 1500; Mw/
Mn 1.6 (GPC, polystyrene calibration). 1H NMR (400 MHz, CDCl3),
δ (TMS, ppm): 7.20, 7.17, 3.75−3.72, 3.53, 3.02, 2.23, 1.62. The 13C
NMR data were not obtained due to the low solubility of the obtained
polymer.
Table 1. Solvent Effect on the Polymerization of 1a, 2a, and
3a
a
b
b
entry
solvent
yield (%)
Mw
Mw/Mn
1
2
3
4
5
6
toluene
DMF
73.9
62.0
24800
9400
2200
1200
1200
gel
2.2
1.7
1.3
1.5
1.6
dioxane
methanol
ethanol
THF
67.2
12.7
50.2
∼100
a
Polymerization at 100 °C under nitrogen for 5 h in the presence of
P1a/2b/3a. Brown solid; yield 56.7% (Table 6, entry 7). Mw 11
CuCl. [1a] = [2a] = 1 M, [3a] = 3 M, [Cu] = 0.5 [1a]. Abbreviation:
DMF = dimethylformamide, THF = tetrahydrofuran. Determined by
800; Mw/Mn 2.2 (GPC, polystyrene calibration). IR (KBr), υ (cm−1):
b
1
3031, 2951, 1731, 1602, 1498, 1442. H NMR (400 MHz, CDCl3), δ
GPC in THF on the basis of a polystyrene calibration.
(TMS, ppm): 7.24, 7.19, 7.17, 7.10, 6.99, 6.94, 3.88, 3.48, 3.10. 13C
NMR (100 MHz, CDCl3), δ (TMS, ppm): 172.16, 144.54, 143.39,
142.89, 141.21, 140.98, 140.75, 140.54, 137.46, 131.73, 131.17, 129.09,
128.27, 127.79, 126.72, 126.46, 120.99, 119.72, 85.01, 81.84, 65.76,
51.45, 40.97, 36.39, 36.26, 31.34.
Table 2. Temperature Effect on the Polymerization of 1a, 2a,
a
and 3a
b
b
entry
1
temp (°C)
yield (%)
Mw
Mw/Mn
P1a/2c/3a. Brown solid; yield 57.1% (Table 6, entry 8). IR (KBr),
υ (cm−1): 2923, 1735, 1656, 1600, 1493, 1438, 1211, 1166, 1105,
1016. The GPC and NMR data were not obtained as the obtained
polymer was insoluble.
P1a/2d/3a. Orange solid; yield 7.2% (Table 6, entry 9). Mw 4100;
Mw/Mn 1.7 (GPC, polystyrene calibration). IR (film), υ (cm−1): 2957,
1715, 1604, 1512, 1437. 1H NMR (400 MHz, CDCl3), δ (TMS, ppm):
7.95, 7.15, 7.07, 6.99, 6.94, 6.89, 6.87, 4.38, 3.88, 3.85. The 13C NMR
data were not obtained due to the low solubility of the obtained
polymer.
P1a/2e/3a. Yellow solid; yield <10% (Table 6, entry 10). Mw 4400;
Mw/Mn 1.7 (GPC, polystyrene calibration). IR (film), υ (cm−1): 2926,
2857, 1730, 1598, 1495, 1449. 1H NMR (400 MHz, CDCl3), δ (TMS,
ppm): 7.11, 6.98, 3.75, 3.69, 3.68, 1.55, 1.25, 0.88, 0.71. The 13C NMR
data were not obtained due to the low solubility of the obtained
polymer.
80
100
120
67.6
73.9
24.2
19900
24800
3800
2.0
2.2
2.4
c
2
3
a
Polymerization in toluene under nitrogen for 5 h in the presence of
b
CuCl. [1a] = [2a] = 1 M, [3a] = 3 M, [Cu] = 0.5 [1a]. Determined
by GPC in THF on the basis of a polystyrene calibration. Data taken
c
from Table 1, entry 3.
Table 3. Concentration Effect on the Polymerization of 1a,
a
2a, and 3a
b
b
entry
[1a] (M)
yield (%)
Mw
Mw/Mn
1
2
3
0.5
1.0
1.5
40.9
73.9
89.0
9600
24800
43800
2.2
2.2
3.0
P1a/2f/3a. Red oil; yield <10% (Table 6, entry 11). Mw 4500; Mw/
a
Mn 1.5 (GPC, polystyrene calibration). IR (film), υ (cm−1): 2925,
Polymerization at 100 °C in toluene under nitrogen for 5 h in the
b
1
presence of CuCl. [1a]:[2a]:[3a]:[Cu] = 1:1:3:0.5. Determined by
GPC in THF on the basis of a polystyrene calibration. Data taken
2857, 1736, 1674, 1599, 1497, 1457. H NMR (400 MHz, CDCl3), δ
c
(TMS, ppm): 7.11, 6.98, 4.05, 4.02, 3.81, 3.72, 3.61, 3.60, 1.56, 1.25,
0.85. The 13C NMR data were not obtained due to the low solubility
of the obtained polymer.
from Table 1, entry 3.
P1a/2a/3b. Yellow solid; yield 68.9% (Table 6, entry 12). Mw 5300;
Mw/Mn 1.9 (GPC, polystyrene calibration). IR (film), υ (cm−1): 2954,
2390, 1731, 1603, 1504, 1441. 1H NMR (400 MHz, CDCl3), δ (TMS,
ppm): 7.12, 6.98, 5.01, 3.79, 3.52, 3.49, 3.03, 2.93. 13C NMR (100
MHz, CDCl3), δ (TMS, ppm): 178.36, 142.68, 135.78, 131.46, 128.25,
125.52, 91.92, 88.45, 83.47, 67.97, 53.70, 49.01, 34.22.
into a versatile methodology for the construction of functional
polymers, we designed and prepared a group of diyne
monomers with different molecular structures (Scheme 2).
While monomers 1a, 1c, 1d, and 1e were synthesized according
to the literature methods,12 compound 1b was prepared by the
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Table 4. Effects of Different Catalysts and Their Loading on
a
the Polymerization of 1a, 2a, and 3a
b
b
entry
catalyst
CuCl
loading (%)
yield (%)
Mw
Mw/Mn
1
2
3
4
50
50
50
30
73.9
10.0
29.7
52.5
24800
2800
3000
4000
2.2
1.2
1.3
1.2
CuI
Cu(OAc)2
CuCl
a
Polymerization at 100 °C in toluene under nitrogen for 5 h. [1a] =
b
[2a] = 1 M, [3a] = 3 M. Determined by GPC in THF on the basis of
a polystyrene calibration. Data taken from Table 1, entry 3.
c
Table 5. Time Course on the Polymerization of 1a, 2a, and
a
3a
b
b
entry
time (h)
Mw
Mw/Mn
1
2
3
1
2
5
26800
26900
24800
1.7
1.7
2.2
a
Polymerization at 100 °C in toluene under nitrogen in the presence
of CuCl. [1a] = [2a] = 1 M, [3a] = 3 M, [Cu] = 0.5 [1a].
b
Determined by GPC in THF on the basis of a polystyrene calibration.
a
Table 6. Polymerization Results of Different Monomers
b
b
c
d
entry
monomer
yield (%)
Mw
Mw/Mn
DP
S
Figure 1. IR spectra of (A) 1a, (B) 2a, (C) model compound 4, and
(D) polymer P1a/2a/3a.
1
1a/2a/3a
1b/2a/3a
1c/2a/3a
1d/2a/3a
1e/2a/3a
1f/2a/3a
1a/2b/3a
1a/2c/3a
1a/2d/3a
1a/2e/3a
1a/2f/3a
1a/2a/3b
82.0
59.2
50.7
52.7
14.6
12.7
56.7
57.1
7.2
29200
16700
15700
2700
4.6
2.0
2.1
2.0
1.8
1.6
2.2
10.9
14.3
10.0
1.7
√
√
√
Δ
√
Δ
√
×
√
√
√
√
2
3
shown in Table 1, the solvents exerts a strong influence on the
polymerization. Among all the tested organic solvents, toluene
is proved to be the most suitable one, giving a polymer with the
highest molecular weight (Mw = 24 800) in the highest yield of
73.9% (Table 1, entry 3). It is noteworthy that 3a is utilized in
aqueous solution, revealing that the polymerization is some-
what moisture-insensitive. Satisfactory results are also obtained
in dimethylformamide, but PDAs with lower molecular weights
are generated in dioxane, methanol, and ethanol despite that
the yield, in some occasions, exceeds 50%. Gelation was
observed when the polymerization was carried out in THF,
probably due to the formation of polymers with extremely high
molecular weights.
4
5
2100
2.5
6
1500
3.0
7
11800
9.2
8
9
4100
4400
4500
5300
1.7
1.7
1.5
1.9
4.4
4.7
5.3
3.8
10
11
12
<10
<10
68.9
a
Polymerization at 100 °C in toluene for 2 h under nitrogen in the
presence of CuCl. [1a] = [2a] = 1 M, [3a] = 3 M, [Cu] = 0.5 [1a].
Determined by GPC in THF on the basis of a polystyrene calibration.
Degree of polymerization. Solubility (S) tested in common organic
b
c
d
We then investigated the temperature effect on the
polymerization. At 80 °C, a polymer with a high molecular
weight (Mw = 19 900) was generated in ∼68% yield (Table 2,
entry 1). The yield and molecular weight were further
enhanced when the temperature was raised to 100 °C. Further
increment of the polymerization temperature to 120 °C led to a
poorer result, probably because of the undesired side-reactions
triggered by the high temperature.
Table 3 shows the effect of diyne concentration on the
polymerization. Generally, polymerization at a higher monomer
concentration will give a better result due to the more efficient
intermolecular collision between molecules. Indeed, when the
diyne concentration was gradually increased from 0.5 to 1.0 M
solvents, such as dichloromethane, chloroform, and tetrahydrofuran:
√ = completely soluble, Δ = partially soluble, × = insoluble.
synthetic route shown in Scheme 3. Monomers 1f, 2a−f, and
3a−b were commercially available and used without
purification. All the synthesized monomers were characterized
by standard spectroscopic techniques with satisfactory analysis
data (see data given in the Experimental Section).
Polymerization Reaction. With the aim of optimizing the
reaction conditions, we first studied the effect of different
solvents on the polymerization using 1a, 2a, and 3a as model
monomers. The polymerization was carried out at 100 °C
under nitrogen for 5 h in the presence of copper(I) chloride. As
Scheme 4. Synthesis of Model Compound 4
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Figure 2. 1H NMR spectra of (A) 1a, (B) 2a, (C) model compound 4,
and (D) polymer P1a/2a/3a in (A, C, and D) CDCl3 and (B) D2O.
The solvent peaks are marked with asterisks.
Figure 3. 13C NMR spectra of (A) 1a, (B) 2a, (C) 4, and (D) P1a/
2a/3a in (A, C, and D) CDCl3 and (B) D2O. The solvent peaks are
marked with asterisks.
and finally 1.5 M, the yield and molecular weight of the
obtained polymer were enhanced by more than 2- and 4-fold,
respectively. Since most of the monomers were not
commercially available, a diyne concentration of 1 M was
utilized for the subsequent investigation.
An array of copper salts is then examined for their efficiency
to catalyze the A3-coupling polymerization. It turns out that
copper(I) chloride is the most efficient catalyst for the
polymerization (Table 4, entry 1). On the contrary, neither
copper(I) iodide nor copper acetate can initiate the
polycoupling reaction. Previous study shows that the copper(I)
cation takes an important role in the mechanism of the A3-
coupling reaction.13 The higher ionic character of CuCl than
CuI and Cu(OAc)2 should be, in most cases, responsible for its
higher catalytic efficiency. Both the molecular weight and yield
of the resultant polymer were decreased significantly when a
reduced catalyst loading was used for the polymerization (Table
4, entry 4).
We then followed the time course on the polymerization by
GPC analysis of a small amount of reaction mixture extracted at
a particular time using a pipet. As shown in Table 5, stirring a
toluene solution of 1a, 2a, and 3a for 1 h already gives a
polymer with a reasonably high molecular weight. Prolonging
the reaction time to 2 h affects little the molecular weight but
improves the yield to a large extent. No further enhancement in
the polymerization result was observed when the reaction time
was lengthened to 5 h.
Figure 4. TGA thermograms of P1a−e/2a/3a recorded under
nitrogen at a heating rate of 10 °C/min.
Table 6 shows the polymerization results of various
combinations of diynes, primary amines, and aldehydes. Most
of the polymerizations proceed smoothly, generating soluble
PDAs with moderate to high molecular weights (Mw = 9600−
43 800) in moderate to high yields (50.7−89.0%). PDAs
generated from sterically more crowded diynes (1b and 1d)
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Figure 5. (A) UV and (B) PL spectra of P1a−c/2/3 in THF solutions. Solution concentration: 10 μM; excitation wavelength (nm): 371 (P1a/2a/
3a), 350 (P1b/2a/3a), 395 (P1c/2a/3a).
in Scheme 4. The coupling reaction of TPE-containing
monoyne 9, 2a, and 3a was carried out under a semineat
condition without organic solvent at 100 °C for 2 h. After
purification by column chromatography, compound 4 was
characterized by standard spectroscopic techniques.
Table 7. Optical Properties of the PADs
a
a
b
polymer
λabs (nm)
λem (nm)
ΦF,S (%)
P1a/2a/3a
P1b/2a/3a
P1c/2a/3a
371
350
395
505
491
527
1.8
2.6
1.9
Structural Characterization. All the polymers were
characterized by IR and NMR spectroscopies. An example of
the IR spectra of 1a, 2a, 4, and P1a/2a/3a is given in Figure 1.
While the absorption bands at 2106 and 3024 cm−1 in the
spectrum of 1a are stemmed from its C−H and CC
stretching vibrations, that at 3276 cm−1 in 2a is associated with
the absorption of its amino functionality. Both these bands,
however are not observed in the spectrum of P1a/2a/3a. On
the other hand, the spectrum of the polymer shows CO and
CC stretching absorption bands of 4 at 1746 and 2247 cm−1,
revealing the occurrence of the polymerization.
a
In dilute THF solution (10 μM). Abbreviation: λabs = absorption
b
maximum, λem = emission maximum. ΦF,S = fluorescence quantum
yields of the polymer thin films measured by an integrated sphere.
normally possess lower molecular weights than those prepared
from the less bulky ones (Table 6, entries 1−4). On the other
hand, diynes (1e and 1f) seem to show a lower polymerizability
than their counterparts with aromatic rings (1a−1c). Regarding
the primary amines, those with carboxylate group next to the
amino functionality and phenyl ring (1a−1c) tend to be good
monomers for the polymerization, probably due to the involved
electronic effect.
Synthesis of Model Compound. To confirm the
occurrence of the A3-polycoupling reaction and to assist
structural characterization of the obtained polymers, a model
reaction was carried out according to the synthetic route shown
1
Figure 2 depicts the H NMR spectra of P1a/2a/3a, its
monomers (1a and 2a), and model compound 4. The acetylene
proton of 1a resonates at δ 3.04, which disappears in the
spectrum of P1a/2a/3a. This suggests the complete con-
sumption of the triple bonds of 1a by the polymerization.
Instead, a new peak corresponding to the resonance of the
Figure 6. (A) PL spectra of 4 in THF and THF/H2O mixtures with different water fractions (fw). Concentration: 10 μM; excitation wavelength: 326
nm. Inset in (B): fluorescent photos of THF/water mixtures of 4 with different fw taken under 365 nm UV irradiation. (B) Plot of relative PL
intensity (I/I0) versus the composition of the THF/H2O mixture of 4.
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Figure 7. (A) Fluorescent photos of THF/H2O mixtures of P1a/2a/3a with different water fraction (fw) taken under 365 nm UV irradiation. PL
spectra of (B) P1a/2a/3a and (D) P1c/2a/3a in THF and THF/H2O mixtures with different (fw). Concentration: 10 μM; excitation wavelength
(nm): 371 (P1a/2a/3a) and 395 (P1c/2a/3a). Plot of relative PL intensity (I/I0) versus the composition of the THF/H2O mixtures of (C) P1a/
2a/3a and (E) P1c/2a/3a.
methylene protons next to the triple bond functionality is
emerged at δ 3.88 in P1a/2a/3a. Other absorption peaks can
be readily assigned by comparing with the spectrum of 4. This
suggests that the polymeric product is indeed P1a/2a/3a with a
molecular structure as shown in Scheme 2.
P1b/2a/3a and P1c/2a/3a retain 60−70% of their weight after
being heated to 800 °C. Thus, they can be applied as heat-
resistance materials in many different fields.
Optical Properties. Figure 5A shows the UV spectra of 4
and PDAs in dilute THF solutions. They vary in shape and
intensity due to their different structures and are peaked at
329−393 nm (Table 7). Among them, those containing silole
moieties (P1c/2a−b/3a) display redder absorption maximum.
From the UV analysis, it seems that P1a/2a/3a possesses a
higher conjugation than P1b/2a/3a as the orbitals are
distributed in a more linear and extended manner than in the
former. When their dilute THF solutions are photoexcited, the
PDAs emit green light at 505−541 nm (Figure 5B).
Similar information is provided by the 13C NMR analysis.
The internal acetylene carbon atom of 1a absorbs at δ 83.65,
which shifts to δ 84.96 after the polymerization (Figure 3).
Meanwhile, a new peak associated with the resonance of the
methylene carbons adjacent to the internal CC bond is
emerged at δ 41.03 in the spectrum of P1a/2a/3a. These
1
results are in well consistence with the results from IR and H
NMR measurements, further confirming the polymer structure.
Thermal Stability. As shown in Figure 4, all the polymers
are thermally stable, losing merely 5% of their weight at high
temperatures (Td) ranging from 250 to 291 °C under nitrogen.
Emission quenching in the solid or aggregated state is often
an obstacle to the practical applications of chromophoric
polymers. The strong interactions between polymer chains in
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Figure 10. Negative photopatterns generated by photolithography of a
film of P1a/2a/3a through a copper mask taken under (A) normal
light illumination and (B) UV irradiation. Excitation wavelength: 330−
385 nm.
induced to emit intensely upon aggregate formation. We
proposed the restriction of intramolecular motions as the main
cause of the AIE effect. In the solution state, the periphery
phenyl rings undergo free motions, which consumes the energy
of the excited state through nonradiative relaxation channel. In
solid or aggregated state, these motions are restricted, thus
enabling the luminogens to relax radiatively. As a result, the
polymers constructed from TPE- or silole-containing mono-
mers are anticipated to show the AIE phenomenon as well.
As depicted in Figure 6A, the dilute THF solution (10 μM)
of the model compound 4 emits almost no light upon
photoexcitation. The PL intensity, however, rises gradually
when an increasing amount of water is added to the THF
solution. At 99% water content, the PL intensity is 160-fold
higher than that in pure THF solution. Since 4 is insoluble in
water, its molecules must form aggregates in aqueous solutions
with high water contents. Clearly, 4 is AIE-active. Polymer
P1a/2a/3a is also proved to be AIE-active, as suggested by the
fluorescent photos shown in Figure 7A. The emission from the
THF solution is still weak when up to 50% of water is added to
the solution. Afterward, the PL becomes stronger progressively.
Similar result was also obtained from the PL analysis (Figure
7B). The slight drop in the fluorescence intensity at 99% water
fraction (Figure 7C) is probably due to the formation of
extensive aggregates, which has lowered the “effective” solute
concentration and hence the light emission. P1c/2a/3a shows
similar emission behaviors, but it emits at longer wavelength
region due to its higher conjugation (Figure 7D,E).
Figure 8. Particle size distributions of aggregates of P1c/2a/3a
suspended in THF/water mixtures (10 μM) with water fractions (fw)
of (A) 50, (B) 70, and (C) 90 vol %. Abbreviation: de = effective
diameter, dm = mean diameter, PDI = polydispersity.
Figure 9. Wavelength dependence of refraction index of thin films of
P1a−d/2a/3a.
It is noteworthy that all the aqueous mixtures of P1a/2a/3a
and P1c/2a/3a are visually transparent, suggesting that the
polymer aggregates are of nanodimension. Indeed, the particle
size analysis shows that the existence of particles in THF/H2O
mixtures of P1c/2a/3a with effective diameters ranging from
481 to 243 nm (Figure 8). When the water fraction was
progressively increased from 50% to 70% and then to 90%, the
particle size was decreased accordingly. This tendency is
reasonable: at low water fraction, the polymer chains aggregate
partially, and the dissolved chains slowly cluster to form “larger”
particles. On the other hand, a large proportion of the polymer
chains may aggregate quickly in the presence of a large amount
of water, which finally gives nanoparticles with “smaller”
effective diameters. The fluorescence quantum yields of thin
films of P1a−c/2a/3a measured by an integrated sphere are
within 1.8−2.6% (Table 7). Although these values are much
lower than that (33.7%) of model compound 4, probably due
to the partial emission quenching by the defects in the
polymers, the PDAs can be fabricated into thin solid films by a
simple spin-coating process, which makes them suitable for the
manufacture of large-area flat-panel devices.
Table 8. Refractive Indices and Chromatic Dispersions of
P1/2a/3a
a
polymer
n632.8
νD
D
P1a/2a/3a
P1b/2a/3a
P1c/2a/3a
P1d/2a/3a
1.6498
1.6555
1.7389
1.5623
46.7626
12.4045
31.4025
85.1818
0.0214
0.0806
0.0318
0.0117
a
Abbreviation: n = refractive index, νD = Abbe
́
number = (nD − 1)/(nF
− nC), where nD, nF, and nC are the RI values at wavelengths of
Fraunhofer D, F, and C spectral lines of 589.2, 486.1, and 656.3 nm,
respectively; D = 1/νD.
the aggregated state usually lead to the formation of detrimental
species such as excimers and exciplexes, which generally decay
via nonradiative pathways. This effect is well-known as
aggregation-caused quenching (ACQ).15 Recently our group
has observed a phenomenon the aggregation-induced emission
(AIE) that is the exact opposite of the ACQ effect. We
discovered that some propeller-shaped molecules such as TPE
and hexaphenylsilole are nonemissive in solutions but are
Light Refraction. Advanced photonic devices are often
composed of working units with high contrast of refractive
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Figure 11. PL spectra of P1b/2a/3a in THF/H2O mixture (1:9 v/v) containing different amounts of (A) picric acid (PA), (B) 2,4-dinitrotoluene
(DNT), and (C) 4-nitrobenzoyl chloride (NBC). (D) Stern−Volmer plots of relative intensity (I0/I − 1) versus the explosive concentration. I0 = PL
intensity in the absence of explosives.
commercial polymers,17 such as polystyrene (n = 1.602−1.589)
and polycarbonate (n = 1.593−1.576). Apparently, the high
refractivity of the polymers bestows them with potential
applications as photonic devices.
Chromatic Dispersion. The chromatic dispersion (D) is
concerned with the wavelength dependence of the refractive
index of a material. Large D value leads to undesired effect such
as degradation of images resolution and limit. Thus, polymers
with low chromatic dispersions are promising for applications
́
as optical materials. The Abbe number (νD) is adopted to
measure the variation or dispersion of a material and is defined
as (nD − 1)/(nF − nC), where nD, nF, and nC are the RI values at
wavelengths of Fraunhofer D, F, and C spectral lines of 589.2,
486.1, and 656.3 nm, respectively.18 The D value is the
reciprocal of νD. As shown in Table 8, the νD and the
corresponding D values of the polymers fall in the scale of
12.4−85.2 and 0.0117−0.0806. Evidently, most of our polymers
Figure 12. Normalized absorption spectra of DNT, NBC, and PA in
THF and PL spectrum of P1b/2a/3a in THF/H2O mixture (1:9 v/v).
́
have high Abbe numbers and hence low optical dispersions.
This enables them to find an array of photonic applications
such as coating materials and high-performance CMOS image
sensors.16
Photopatterning. UV irradiation is a convenient and
flexible source for the fabrication of luminescent patterns.
Taking its advantages of good film-forming ability and strong
emission in the solid state, UV irradiation of a spin-coated film
of P1a/2a/3a on a silicon wafer through a copper mask can
generate a well-resolved fluorescent pattern (Figure 10). The
exposed parts (lines) of the films are cross-linked and are
index (RI). Polymers with high RI values can serve as
promising candidates for a wide range of photonic applications,
such as organic light-emitting diodes, lenses, waveguides, and
image sensors.16 Theoretically, polymers with highly polarized
aromatic rings, heteroatoms, and acetylene units are liable to
exhibit high RI values. As illustrated in Figure 9, P1a−d/2a/3a
show RI of 1.8322−1.7458, 1.7787−1.6297, 1.7247−1.6645,
and 1.6026−1.5719, respectively, in a wide wavelength region
of 400−1000 nm, whose values are higher than those of
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photobleached. Thus, they appear dark under UV illumination.
The unexposed squares, on the other hand, emit brightly in the
presence of UV light. The distinct pattern with sharp edges has
proved the high efficiency of the technique for the construction
of photonic and electronic devices and biological sensing and
probing systems such as LCD, OLED, and medicinal diagnostic
biochip.19
dispersions. Some PDAs are AIE-active and exhibit strong
emissions in the solid or aggregated state. They are photo-
sensitive and can generate negative photoresist patterns upon
UV irradiation. They can function as fluorescent chemosensors
for sensitive detection of explosives. Since the present polymers
contain abundant triple bonds in their backbones, they are
promising candidates for organometallic materials and magnetic
ceramics. Additionally, the propargylamine substructure of the
polymers enables them to be further functionalized by reactions
such as “click” chemistry. As the amino acid segments are chiral
and capable of forming hydrogen bond, they may impart the
polymers with novel chiroptical and self-assembly properties.
All these possibilities are currently under investigation in our
laboratory.
Explosive Detection. The AIE property of P1b/2a/3a
inspires us to explore its potential applications as a chemo-
sensor for explosive because of the involved antiterrorism
implication. 2,4,6-Trinitrophenol (picric acid, PA), 2,4-
dinitrotoluene (DNT), and 4-nitrobenzoyl chloride (NBC)
are used as model compounds for their commercial availability
and distinction in high nitro group content. The nano-
aggregates in 90% aqueous mixture with a concentration of
10 μM are utilized as probes. As shown in Figure 11, the PL of
the nanoaggregates is weakened gradually when an increasing
amount of explosives is added to the aqueous solution. At PA,
DNT, and NBC concentration of 150 μM, 0.9 mM, and 1.3
mM, respectively, the emission is completed quenched. The
detection limit is found to be 1 μM. The Stern−Volmer plots of
relative PL intensity (I0/I − 1) versus the explosive
concentration all give curves bending upward, indicating a
more sensitive and efficient quenching process with an
increasing quencher concentration (Figure 11D).
There is a noticeable disparity in the quenching efficiency
toward different explosives. As deduced in Figure 11D, the
quenching constants of PA, DNT, and NBC determined from
the linear region of the Stern−Volmer plots are 268 500, 100
800, and 9300 L/mol, respectively. To explain such selectivity,
the absorption of PA, DNT, and NBC was measured and
compared with the PL spectrum of P1b/2a/3a (Figure 12). In
the wavelength region of 415−500 nm, the UV spectrum of PA
and the PL spectrum of the polymer are overlapped. This
should lead to energy transfer from the excited state of P1b/
2a/3a to the ground state of PA and hence nonradiative
relaxation.20 On the contrary, DNT and NBC do not absorb at
wavelengths where the polymer emits, thus accounting for their
weak quenching effect. On the other hand, the Lewis acid−base
interaction between the electron-efficient polymer chains and
the electron-deficient quencher molecules may also account for
the quenching phenomenon. Considering its highest nitro
group content, PA is supposed to exert the strongest electron-
withdrawing effect among the three explosives and hence shows
higher quenching efficiency. Generally, aggregation of fluo-
rescent conjugated polymers causes the ACQ problem and thus
reduces their sensing performance. However, aggregation is
beneficial to the PL of P1b/2a/3a. Moreover, its aggregates
have many cavities to bind with quencher molecules and
provide additional interchain diffusion pathways for excitons to
migrate. These collective effects thus make the PL annihilation
a highly efficient process.
AUTHOR INFORMATION
Corresponding Authors
*E-mail chjacky@ust.hk; Ph +852-2358-7375(7242); Fax
+852-2358-1594 (J.W.Y.L.).
■
*E-mail tangbenz@ust.hk; Ph +852-2358-7375(7242); Fax
+852-2358-1594 (B.Z.T.).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was partially supported by the National Basic
Research Program of China (973 Program; 2013CB834701),
the Research Grants Council of Hong Kong (604913, 604711,
6602212, and N_HKUST620/11), the Nissan Chemical
Industries, Ltd., the Innovation and Technology Commission
(ITCPD117-9), and the University Grants Committee of Hong
Kong (AoE/P-03/08). B.Z.T. thanks the support of the
Guangdong Innovative Research Team Program
(201101C0105067115).
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■
In this work, we succeeded in developing a new synthetic route
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and aldehydes. This one-pot A3-coupling polymerization is
catalyzed by a cheap catalyst of copper(I) chloride and is
tolerant to moisture. The polymerization reaction can be
completed in a short period of time, generating high molecular
weight PDAs in high yields. The obtained polymers are soluble
and thermally stable and can be fabricated into good films.
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