Aggregation-Induced Emission and Photocyclization of Poly(hexaphenyl-1,3-butadiene)s Synthesized from 1 + 2 Polycoupling of Internal Alkynes and Arylboronic Acids

Article abstract of DOI:10.1021/acs.macromol.6b01148

A new polymerization method for in situ generation of luminescent polymers with aggregation-induced emission characteristics from readily prepared or commercially available reactants was developed in this work. The 1 + 2 polycoupling of internal alkynes and arylboronic acids took place under mild reaction conditions and was tolerant to moisture, air, and monomer stoichiometric imbalance, furnishing poly(hexaphenyl-1,3-butadiene)s (PHBs) with partially or wholly conjugated structures in satisfactory yields. All the PHBs showed good solubility, thermal stability, and film-forming ability. Their optical properties could be easily tuned by proper monomer design and external perturbation. The PHBs underwent active photocyclization in both solution and solid states, converting them to poly(dihydronaphthalene)s. Such a property made them promising materials for fabricating luminescent photopatterns. A new mechanistic pathway was proposed as supplementary to explain the aggregation-induced emission (AIE) phenomenon by studying the emission of PHBs and the corresponding model compound.

Full text of DOI:10.1021/acs.macromol.6b01148

Article
pubs.acs.org/Macromolecules
Aggregation-Induced Emission and Photocyclization of
Poly(hexaphenyl-1,3-butadiene)s Synthesized from “1 + 2”
Polycoupling of Internal Alkynes and Arylboronic Acids
Yajing Liu,†,‡ Jacky W. Y. Lam,†,‡ Xiaoyan Zheng,‡ Qian Peng,§ Ryan T. K. Kwok,†,‡ Herman H. Y. Sung,‡
Ian D. Williams,‡ 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, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration &
Reconstruction, Division of Life Science, Institute for Advanced Study, Institute of Molecular Functional Materials, State Key
Laboratory of Molecular Neuroscience and Division of Biomedical Engineering, The Hong Kong University of Science and
Technology, Clear Water Bay, Kowloon, Hong Kong, China
§Key Laboratory of Organic Solids, Beijing National Laboratory for Molecular Science, Institute of Chemistry, Chinese Academy of
Sciences, Beijing 100190, 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
S
* Supporting Information
ABSTRACT: A new polymerization method for in situ
generation of luminescent polymers with aggregation-induced
emission characteristics from readily prepared or commercially
available reactants was developed in this work. The “1 + 2”
polycoupling of internal alkynes and arylboronic acids took
place under mild reaction conditions and was tolerant to
moisture, air, and monomer stoichiometric imbalance, furnish-
ing poly(hexaphenyl-1,3-butadiene)s (PHBs) with partially or
wholly conjugated structures in satisfactory yields. All the
PHBs showed good solubility, thermal stability, and film-
forming ability. Their optical properties could be easily tuned
by proper monomer design and external perturbation. The
PHBs underwent active photocyclization in both solution and
solid states, converting them to poly(dihydronaphthalene)s. Such a property made them promising materials for fabricating
luminescent photopatterns. A new mechanistic pathway was proposed as supplementary to explain the aggregation-induced
emission (AIE) phenomenon by studying the emission of PHBs and the corresponding model compound.
fields such as organic light-emitting diodes,2 organic field-effect
INTRODUCTION
■
transistors,3 chemosensors,4 biosensors,5 and biomedical
Luminescent materials play a vital role in real-world
applications and high-tech innovations. Most of the lumines-
imaging agents.6 Compared with their small molecule counter-
parts, polymeric materials enjoy a wider scope of industrial
cent materials are used in the solid state, such as those for
applications due to their easy fabrication into large-area panel
lighting and display devices, or in physiological environment for
bioimaging agents or bioprobes. However, a majority of the
films by simple, cheap, and energy-saving techniques such as
spin-coating. Their structure, topology, and functionality can be
readily tuned by molecular engineering, which is sometimes
difficult with low molecular weight materials. Because of such
attractive perspectives, various synthetic strategies have been
proposed to prepare AIE polymers.7
luminescent materials are organic molecules composed of
hydrophobic aromatics, which normally form aggregates in
aqueous media. It is known that aggregate formation can
partially or completely quench the emission of luminophores.
Such aggregation-caused quenching (ACQ) effect has limited
their applications in some areas. Aggregation-induced emission
(AIE) is a photophysical phenomenon that is exactly opposite
of the ACQ effect. Luminogens with AIE features (AIEgens)
are weakly or nonemissive in the monodispersed state but are
induced to emit intensely in the aggregate state.1 Such a
property enables AIEgens to find many applications in various
Scheme 1 shows the commonly adopted methodologies for
synthesizing AIE polymers. Direct polymerization of AIEgen-
Received: May 30, 2016
Revised: July 22, 2016
© XXXX American Chemical Society
A
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Scheme 1. Polymerization Strategies toward AIE Polymers
Scheme 2. Polycoupling of Internal Diynes, Aryl Diiodides, and Arylboronic Acids
containing monomers or their copolymerization with other
monomers generate (co)polymers with AIEgens embedded in
the main chain (Scheme 1A). Alternatively, an AIEgen is
attached to a polymerizable monomer, whose homopolymeri-
zation or copolymerization produces polymers with AIEgens as
side chains (Scheme 1B). In addition to these routes, AIE-
inactive polymers can be endowed with AIE characteristics
through postmodification (Scheme 1C). While these methods
are versatile to generate AIE polymers with different structures,
properties, and applications, functional AIEgen-containing
monomers need to be presynthesized in time-consuming
synthetic steps,8 and polymers with irregular structures may
be generated as the polymer effect tends to prevent
postpolymerization from proceeding to completion. Thus, the
development of new synthetic tools for facile preparation of
AIE polymers is of great importance to polymer chemists and
materials scientists.
Recent studies have shown that AIEgens such as 1-
cyanotriarylethene and tetraarylethene with high regio- and
stereoregularity can be generated from commercially available,
non-AIE, functional reactants (aromatic internal alkynes,
aromatic halides, and potassium ferrocyanide or arylboronic
acids or aryl magnesium bromides) in the presence of
transition-metal catalyst under mild reaction conditions.9
Thus, these organic reactions may provide conceptually new,
operationally simple, and practical protocols for synthesizing
AIE polymers. However, such possibility is less explored by
polymer scientists. Tomita synthesized wholly aromatic poly-
(arylene vinylene)s containing tetra-aryl-substituted olefin units
in the main chain by the three-component coupling polymer-
ization of dihaloarenes, aryldiboronic acids, and diphenyl-
acetylene derivatives catalyzed by PdCl2(CN)2 and KHCO3 in
DMF/H2O at 100 °C.10 Although soluble polymers were
isolated in high yields, their molecular weights were low (Mw ∼
4000). We also made efforts to prepare polymers with AIEgens
generated in situ during the polymerization.11 In the presence of
PdCl2 and NaF, the reaction of diynes, aryl diiodides, and
arylboronic acids proceeded in a more efficient fashion,
generating triarylphenyethene-containing polymers with higher
molecular weights (Mw up to 18 000) and novel materials
properties (Scheme 2). Although such a method is versatile for
synthesizing AIE-active polymer, we would like to enrich the
synthetic toolbox by developing systems using more simple
reactants.
Recently, Miura and co-workers reported a palladium-
catalyzed oxidative coupling reaction of internal alkynes and
arylboronic acids to produce multisubstituted 1,3-butadienes
(Scheme 3).12 Since hexaaryl-1,3-butadiene derivatives were
reported as excellent AIE-active materials,13 we believed that
Scheme 3. Oxidative Coupling of Alkynes with Arylboronic
Acids To Produce 1,4-Diaryl-1,3-butadienes
B
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Scheme 4. Synthesis of Poly(hexaphenyl-1,3-butadiene)s by Palladium-Catalyzed Polycoupling of Internal Alkynes with
Arylboronic Acids
given in the Supporting Information. Photocyclization and photo-
patterning were conducted in air at room temperature using 365 nm
light obtained from a Spectroline ENF-280C/F UV lamp with a
working power of 180 W.14
this reaction could be utilized for in situ generation of AIE units
in the polymer chain. As this coupling occurs in a reactant ratio
of 2:2,11 polymers can be readily obtained by replacing one of
the monomers from mono- to difunctionality. As internal
alkynes and arylboronic acids can be readily functionalized, the
properties of their associated polymers can be easily tuned.
In this work, we developed two new systems for preparing
AIE polymers: (A) polycoupling of internal diynes with
arylboronic acid and (B) polycoupling of diphenylacetylene
with benzene-1,4-diboronic acid (Scheme 4). These polymer-
ization reactions can be performed under mild and monomer
stoichiometric imbalanced conditions in open air. They are also
tolerant to various functional groups in the monomers. The
obtained poly(hexaphenyl-1,3-butadiene)s (PHBs) possess
good solubility in common organic solvents and high thermal
stability. PHBs serve as ideal models for the mechanistic study
of the AIE phenomenon. Investigation on the photocyclization
reactions of PHBs let us gain further insight into the alternative
pathways of the AIE process. In addition to the mechanism of
restriction of intramolecular motion (RIM)1,4 proposed in our
previous reports, here we identified that the restriction of
intramolecular cyclization (RIC) is another elementary step in
the AIE process, especially for olefin-based AIE systems.
Polymer Synthesis. A typical procedure for the polymerization of
1a and 2a is given below as an example. Into a 25 mL tube with a
magnetic stirrer were placed 0.1 mmol of 1a, 0.3 mmol of 2a, 0.01
mmol of Pd(OAc)2, and 0.3 mmol of Ag2CO3 in 3 mL of solvent
mixture (1.4 mL of toluene, 1.5 mL of 1-butanol, and 0.1 mL of H2O).
The mixture was stirred in open air at 120 °C for 13 h. The
polymerization was terminated by pouring the mixture into a large
quantity of methanol through a dropper filled with neutral Al2O3 to
remove, if any, the catalyst residue and insoluble substrates. The
precipitate was collected by filtration through a sand-core funnel. The
crude product was then dissolved in THF, reprecipitated in methanol,
and washed repeatedly before being dried under vacuum to a constant
weight. Yellow powder of polymer P1a/2a was obtained in 56.9% yield
(Table 5, entry 1). Mw 10 580; Mw/Mn 1.57. IR (film), υ (cm−1): 3024
1
(CH3), 2940 (CH2), 2866, 1602, 1507, 1245, 1175, 1021. H NMR
(400 MHz, CD2Cl2), δ (ppm): 7.21−6.46 (26H, aromatic protons),
3.85 (4H, CH2 protons), 2.23 (6H, CH3 protons), 1.71 (4H, CH2
protons), 1.42 (4H, CH2 protons). 13C NMR (100 MHz, CD2Cl2), δ
(ppm): 157.69, 145.28, 142.76, 132.01, 131.71, 128.94, 124.14, and
115.12 (aromatic carbons), 68.55, 29.69, and 26.35 (CH2 carbons),
21.41 (CH3 carbons).
P1a/2b. Yellow powder; yield 56.4% (Table 5, entry 2). Mw 6410;
Mw/Mn 1.83. IR (film), υ (cm−1): 3054, 2930, 2857, 2730, 1670, 1560,
1508, 1387, 1285, 1210, 1174, 1015. 1H NMR (400 MHz, CD2Cl2), δ
(ppm): 9.87 (2H, CHO protons), 7.57−6.49 (26H, aromatic
protons), 3.86 (4H, CH2 protons), 1.71 (4H, CH2 protons), 1.44
(4H, CH2 protons). 13C NMR (100 MHz, CD2Cl2), δ (ppm): 192.25
(CHO carbons), 158.92, 141.68, 134.94, 132.94, 131.97, 129.29,
126.29, 126.96, and 115.11 (aromatic carbons), 68.32, 29.70, and
26.35 (CH2 carbons).
EXPERIMENTAL SECTION
■
Materials and Instruments. Palladium acetate, silver carbonate,
arylboronic acids, diphenylacetylene, and other reagents were
purchased from Aldrich and used as received without further
purification. Toluene, 1-butanol, tetrahydrofuran (THF), water, and
other solvents were distilled before use. Monomers 1a−1c were
synthesized according to the procedures reported previously.11
Details about the instruments for obtaining the high-resolution mass
spectra (HRMS), gel permeation chromatography (GPC) curves, IR
P1a/2c. Yellow powder; yield 62.6% (Table 5, entry 3). Mw 6420;
Mw/Mn 1.19. IR (film), υ (cm−1): 3054, 2925, 2855, 1659, 1633, 1505,
1
1
spectra, H and 13C NMR spectra, thermogravimetric analysis (TGA)
1489, 1242, 1174, 1027. H NMR (400 MHz, CD2Cl2), δ (ppm):
thermograms, UV spectra, and photoluminescence (PL) spectra are
7.50−6.48 (26H, aromatic protons), 3.86 (4H, CH2 protons), 1.71
C
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(4H, CH2 protons), 1.44 (4H, CH2 protons). 13C NMR (100 MHz,
CD2Cl2), δ (ppm): 160.91, 159.90, 158.83, 157.92, 142.20, 140.79,
133.51, 131.88, 130.29, 128.94, 128.32, 127.18, 126.42, 124.14, 115.52,
114.54, and 113.90 (aromatic carbons), 68.36, 29.73, and 26.36 (CH2
carbons).
Table 2. Time Course and Other Effects on the
Polymerization of 1a and 2a
a
entry
1
time (h)
yield (%)
Mw
Mw/Mn
1
4
15.0
36.2
14.7
48.5
31.1
1810
7140
1.11
2.85
1.43
3.83
1.83
4.11
P1b/2a. Yellow powder; yield 79.9% (Table 5, entry 4). Mw 8040;
Mw/Mn 1.30. IR (film), υ (cm−1): 3051, 3021, 2955, 2924, 2868, 1655,
1604, 1508, 1284, 1240, 1174, 1110. 1H NMR (400 MHz, CD2Cl2), δ
(ppm): 7.37−6.47 (26H, aromatic protons), 3.88 (4H, CH2 protons),
2.23 (6 H, CH3 protons), 1.81 (4H, CH2 protons). 13C NMR (100
MHz, CD2Cl2), δ (ppm): 158.90, 142.03.68, 136.60, 133.15, 131.64,
130.35, 128.82, 128.04, 126.15, 115.17, and 113.82 (aromatic
carbons), 67.96 and 26.56 (CH2 carbons), 21.44 (CH3 carbons).
P1c/2a. Yellow powder; yield 85.4% (Table 5, entry 5). Mw 6200;
Mw/Mn 1.43. IR (film), υ (cm−1): 3076, 2053, 3022, 2923, 2854, 1670,
1681, 1597, 1387, 1505, 1442, 1182, 1111. 1H NMR (400 MHz,
CD2Cl2), δ (ppm): 7.05−6.85 (36H, aromatic protons), 2.25 (6H,
CH3 protons). 13C NMR (100 MHz, CD2Cl2), δ (ppm): 149.53,
141.65, 134.94, 136.98, 131.79, 130.52, 128.41, 127.91, 126.74, 123.29,
and 122.52 (aromatic carbons), 21.43 (CH3 carbons).
b
2
c
3
4
2990
4
13
13
18
14720
6210
d
5
e
6
32.8
10240
a
Polymerization at 120 °C in open air in the presence of Pd(OAc)2
and Ag2CO3. [1a]:[2a]:[Pd(OAc)2]:[Ag2CO3] = 1:3:0.1:3. [1a] =
b
0.033 M. Solvent composition: THF/1-butanol/H2O = 4/5/1. Data
c
taken from Table 1, entry 1. Polymerization performed in a sealed
d
e
system. [1a] = 0.05 M. Soluble fraction.
Table 3. Temperature and Ligands Effect on the
a
Polymerization of 1a and 2a
P3/4. Yellow powder; yield 57.5% (Table 5, entry 6). Mw 2030;
Mw/Mn 1.25. Mw/Mn 1.25. IR (film), υ (cm−1): 3076, 3025, 2957,
2924, 2858, 1643, 1491, 1442. 1H NMR (400 MHz, CD2Cl2), δ
(ppm): 7.25−6.94 (aromatic protons). 13C NMR (100 MHz,
CD2Cl2), δ (ppm): 144.72, 143.84, 142.20, 141.13, 132.02, 131.78,
130.83, 129.39, 128.13, 127.92, 127.02, and 126.54 (aromatic
carbons).
Model Reaction. Model compound 5 was synthesized by the
oxidative coupling of diphenylacetylene with p-tolylboronic acid. The
experimental procedure was similar to the preparation of P1a/2a.
Yellow solid was obtained in 49.6% yield (after purification by column
chromatography). IR (film), υ (cm−1): 3078, 3052, 3021, 2921, 1597,
1509, 1489, 1443, 1346, 1181, 1028. 1H NMR (400 MHz, CD2Cl2), δ
(ppm): 7.22 (d, J = 7.2, 4H), 7.21 (s, 6H), 6.91 (m, 14H), 6.81 (d, J =
7.6, 4H), 2.24 (s, 6H). 13C NMR (100 MHz, CD2Cl2), δ (ppm):
144.92, 143.40, 142.38, 141.43, 140.44, 136.88, 131.77, 130.19, 128.47,
127.67, 126.84, and 126.30 (aromatic carbons), 21.41 (CH3 carbons).
HRMS (MALDI-ToF): m/z 538.2677 (M+, calcd 538.2661) (Figure
S1 in the Supporting Information).
entry
temp (°C)
ligand
yield (%)
Mw
Mw/Mn
1
2
65
95
54.3
42.3
48.5
47.2
7.5
4820
5480
2.07
2.52
3.83
1.79
1.19
b
3
120
120
120
14720
3910
4
5
L1
L2
2540
a
Polymerization for 13 h in open air in the presence of Pd(OAc)2 and
Ag2CO3. [1a]:[2a]:[Pd(OAc)2]:[Ag2CO3] = 1:3:0.1:3. [1a] = 0.033
M. Solvent composition: THF/1-butanol/H2O = 4/5/1. Data taken
from Table 2, entry 4.
b
Table 4. Solvent Composition Effect on the Polymerization
of 1a and 2a
a
entry THF/1-buOH/H2O conc (M) yield (%)
Mw
Mw/Mn
1
2
3
4
1/2/0
0.033
0.036
0.033
0.033
trace
51.0
48.5
56.9
0/10/1
8/10/2
14/15/1
7640
14720
10580
3.52
3.83
1.57
b
c
RESULTS AND DISCUSSION
Polymerization. To develop the oxidative coupling of
alkynes and arylboronic acids into a facile tool for the synthesis
■
a
Polymerization at 120 °C for 4 h in open air in the presence of
Pd(OAc)2 and Ag2CO3. [1a]:[2a]:[Pd(OAc)2]:[Ag2CO3] = 1:3:0.1:3.
[1a] = 0.033 M. Data taken from Table 2, entry 4. Solvent
b
c
composition: toluene/1-buOH/H2O = 14/15/1.
Table 1. Effect of Reactant Ratio on the Polymerization of 1a
and 2a
a
a
Table 5. Polymerization of Different Monomers
entry [1a]:[2a]:[Pd(OAc)2]:[Ag2CO3] yield (%)
Mw
Mw/Mn
entry
polymers
yield (%)
Mw
Mw/Mn
1
2
3
4
5
1.0:3.0:0.10:3.0
1.0:2.0:0.10:2.0
1.2:2.0:0.10:2.0
1.0:3.0:0.05:3.0
1.0:3.0:0.10:2.5
36.2
11.6
6.1
7140
1510
1480
4030
5980
2.85
1.09
1.07
2.16
2.37
b
1
P1a/2a
P1a/2b
P1a/2c
P1b/2a
P1c/2a
P3/4
56.9
56.4
62.6
79.9
85.4
57.5
10580
6410
6420
8040
6200
2030
1.57
1.83
1.19
1.30
1.43
1.25
2
3
4
5
36.8
39.9
a
c
Polymerization at 120 °C for 4 h in open air in the presence of
Pd(OAc)2 and Ag2CO3. [1a] = 0.033 M. Solvent composition: THF/
1-butanol/H2O = 4/5/1.
6
a
Polymerization at 120 °C for 13 h in open air in the presence of
Pd(OAc)2 and Ag2CO3. [1]:[2]:[Pd(OAc)2]:[Ag2CO3] = 1:3:0.1:3.
[1] = 0.033 M. Solvent composition: toluene/1-butanol/H2O = 14/
15/1. Data taken from Table 4, entry 5. [3]:[4]:[Pd(OAc)2]:
[Ag2CO3] = 2:1.5:0.1:1.5. [4] = 0.1 M.
b
c
of AIE polymers, we synthesized or purchased such molecules
1−4 with mono- or bifunctional groups and optimized the
reaction conditions with respect to reactant ratio, reaction time,
temperature, ligand, and solvent composition by using 1a and
2a as model monomers. The polymerization of 1a and 2a was
catalyzed by palladium acetate and silver carbonate in a mixture
of THF/1-butanol/H2O at 120 °C for 4 h in an open
atmosphere with varied monomer ratio, catalyst loading, and
oxidant content.12 The effects of an excess amount of 2a (entry
1) and 1a (entry 3) and their ratio are shown in Table 1, entries
1−3. Results show that an excess amount of boronic acid led to
the generation of a higher molecular weight polymer in a higher
yield. This suggests that the polymerization can be carried out
under stoichiometric imbalanced condition. Reducing the
catalyst loading from 10% (entry 1) to 5% (entry 4) had
lowered the molecular weight. Although lowering the oxidant
D
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Scheme 5. Synthesis of Model Compound from Diphenylacetylene and p-Tolylboronic Acid
Figure 1. Photographs of single crystals of 5 taken under (A) bright
field and (B) UV irradiation. (C) Molecular geometry of 5 in the
crystal state.
Figure 3. 13C NMR spectra of (A) 1a, (B) 2a, (C) 5, and (D) P1a/2a
in (A, C, and D) CD2Cl2 and (B) D2O. The solvent peaks are marked
with asterisks.
Figure 2. 1H NMR spectra of (A) 1a, (B) 2a, (C) 5, and (D) P1a/2a
in (A, C, and D) CD2Cl2 and (B) D2O. The solvent peaks are marked
with asterisks.
Figure 4. Absorption spectra of 5, P1a/2a, P1a/2b, P1c/2a, and P3/4
in THF solutions. Solution concentration: 10 μM.
content from 3.0 equiv (entry 1) to 2.5 equiv (entry 5) had
slightly improved the yield (39.9%), it had decreased the
molecular weight of the resulting polymer. Therefore, the
reactant ratio in entry 1 was adopted for the below
investigation.
At a fixed reactant ratio, we tracked the time course on the
polymerization (Table 2). When the reaction time was
prolonged from 1 h (entry 1) to 4 h (entry 2) and then 13 h
E
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a
Apparently, ligands are not necessary for this polymerization
reaction.
Table 6. Optical Properties of PHBs and 5
λabs (nm)
ε/104 (mol−1 L cm−1
)
λem (nm)
Solvent is another crucial factor affecting the polymerization
efficiency. According to the literature,11 this reaction performs
well in THF and is tolerant to moisture. Occasionally, water
can even assist the reaction. This also was true in the
polymerization system: only a trace amount of product was
generated in the absence of water (Table 4, entry 1). On the
other hand, THF possesses a high solvating power, which
allows the propagating polymer chain to grow longer. Indeed,
the final molecular weight of the resulting polymer will be
lowered without its presence (entry 2, Table 4). Interestingly, it
was later discovered that toluene outperformed THF as
polymerization solvent (Table 4, entry 5) because a polymer
with a narrower polydispersity was isolated in a higher yield
(56.9%) under the same conditions. As a result, further
experiments were carried out in a solvent mixture of toluene/1-
buOH/H2O at a volume ratio of 14/15/1.
On the basis of the optimization result, we explored the
monomer scope by using different internal diynes and
arylboronic acids to construct PHBs with various structures.
The results are summarized in Table 5. PHBs abbreviated as
P1a/2a−c were synthesized from internal diyne 1a and
arylboronic acids with an electron-donating or electron-
withdrawing group such as methyl (2a), aldehyde (2b), or
fluoride (2c) substituents (entries 1−3). Among them, P1a/2a
was obtained with the highest molecular weight, which was in
accordance with the previous observation that electron-
donating boronic acid was a preferred reactant for such
reaction.12 The obtained yields of all the PHBs were only
moderate, probably due to the formation of bis-phenyl
derivatives as byproducts. P1a−c/2a were prepared from
internal diynes with different conjugation degree (entries 1, 4,
and 5) and p-tolylboronic acid. The longer alkyl chain in P1a/
2a made its growing chain more soluble in the solvent mixture
during the polymerization, leading to its higher molecular
weight than P1b/2a. The polymerization of 1,2-diphenylethyne
(3) and 1,4-phenylenediboronic acid (4) generated P3/4,
which contained solely phenylene and butadiene moieties along
its backbone (entry 6). The close proximity of rings may lead to
severe steric congestion, preventing the polymer chain to grow
further and hence lead to low polymerization degree. On the
other hand, the GPC result is obtained based on a calibration
P1a/2a
P1a/2b
P1c/2a
P3/4
362
359
354
360
349
0.353
1.094
2.893
1.307
1.315
444
472
522
392
432
5
a
In THF solution. Abbreviation: λabs = absorption maximum, λem
emission maximum.
=
(entry 4), both the molecular weight and yield of the obtained
polymer were improved. Further lengthening the reaction time
to 18 h, surprisingly, gave rise to insoluble gel (entry 6). The
polymer may readily undergo intramolecular cyclization in such
a long reaction time, converting to poly(dihydronaphthalene)
with a poorer solubility. Details of photocyclization of P1a/2a
will be discussed in the later section. Therefore, all the
polymerization reactions were terminated at 13 h. Interestingly,
when the polymerization was performed in open air (entry 2)
rather than in a sealed tube (entry 3), the reaction efficiency
was much higher. Such difference may be attributed to oxygen
in air that facilitates the oxidation process in the reaction
mechanism. Thus, the air-insensitive feature of this reaction
largely simplifies the experimental operation. Increasing the
monomer concentration from 0.033 M (entry 4) to 0.05 M
(entry 5) had narrowed the polydispersity (Mw/Mn = 1.83) but
resulted in lower molecular weight and yield. Thus, the reaction
condition was set at a monomer concentration of 0.033 M in
open air and a reaction time of 13 h.
Temperature exerts a strong effect on the reaction efficiency
from a thermodynamic point of view. For energy conservation,
we tried to lower the reaction temperature from 120 to 95 °C
or 65 °C (Table 3). However, in doing so, the molecular weight
of the corresponding polymer (entries 1 and 2) was
progressively lowered. Obviously, a higher temperature was
more suitable for the polymerization. In previous studies,12
b
phosphite or phosphate ligands had been found to increase the
lifetime of catalysts and hence improve the polymerization
result for this reaction system. Thus, the effect of tri-p-tolyl
phosphite (L1) or triphenyl phosphate (L2) on the polymer-
ization was tested. From the data shown in entries 4 and 5,
neither of them could enhance the molecular weight and yield.
Figure 5. (A) PL spectra of 5 in THF and THF/H2O mixtures with different water fractions (fw). Concentration: 10−5 M; excitation wavelength:
349 nm. (B) Plot of relative PL intensity (I/I0) versus the composition of the THF/H2O mixture of 5. Inset: fluorescent photograph of THF/H2O
mixtures of 5 with water fractions of 70% and 80% taken under 365 nm UV irradiation from a hand-held UV lamp.
F
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Figure 6. (A) Normalized PL spectra of model compound 5 in THF solution, solid, and THF/H2O mixture with 99% water fraction. Excitation
wavelength: 349 nm. (B) Relative brightness (B/B0) of the fluorescent photographs of 99% aqueous mixture, THF solution, and powder of 5 taken
under 365 nm UV irradiation from a hand-held UV lamp. B0 = brightness of the THF solution. The relative brightness of the photographs was
analyzed by a MATLABV program. Camera parameters: f/5.6, 1/10s, ISO 800.
curve of polystyrene. Considering the quite large structural
difference between P3/4 and polystyrene, the actual molecular
weight of P3/4 should be higher than the present value.
the oxygen atom of 1a was splitted into a doublet at δ 3.86 and
δ 3.77 in P1a/2a. Similarly, the aromatic protons ortho to the
oxygen atom were now resonated at δ 6.58 and 6.44. The
methylene proton resonances “b” and “c” in 1a were less
affected as they were far away from the unsaturated moieties.
Likewise, the 13C NMR spectrum of P1a/2a inherited the
resonance peaks for the methylene carbon absorptions of 1a at
δ 68.85, 29.52, and 26.27 and the absorption peak of methyl
carbon of 2a at δ 21.54. The signals for the triple bond carbon
absorptions of 1a locating at δ 88.96 and 88.05 were absent in
the spectra of both 5 and P1a/2a. Thus, the IR and NMR
analyses have convincingly demonstrated the occurrence of the
polymerization and clarified the polymer structure. The IR and
NMR data for other PHBs are provided in the Experimental
Section, which all confirm their structures as illustrated in
Scheme 4.
Optical Properties. Measurement on the absorption and
photoluminescence (PL) of the polymers enables us to better
understand the photophysical properties of the PHBs and their
structural features. As shown in Figure 4, the absorption
maximum of the polymers fell in the range of 354−362 nm,
which was in accordance with the reported values (Table 6).15
P1a/2b showed a redder emission than P1a/2a, probably due
to the intramolecular charge transfer effect from the electron-
rich backbone to the electron-deficient aldehyde groups.
Although P3/4 possessed a conjugated structure, it emitted at
a shorter wavelength (392 nm) than other PHBs, presumably
owing to its twisted molecular conformation caused by the
steric hindrance between the phenyl rings.
1
Nevertheless, the H NMR spectrum of P3/4 shown in Figure
S6 exhibits broad aromatic proton resonances, which was
suggestive of its polymeric nature. All the polymers including
those with fully conjugated structures such as P1c/2a and P3/4
are completely soluble in common organic solvents such as
THF, toluene, chloroform, and dimethylformamide.
Model Reaction. Model reaction was carried out to assist
the structural characterization and property investigation of the
PHBs. By coupling diphenylacetylene (3) with p-tolylboronic
acid under conditions similar to those for the polymerization,
model compound 5 was obtained (Scheme 5). The crystals of 5
emitted intense green light under UV irradiation (Figure 1).
Analysis by X-ray crystallography revealed that the phenyl rings
in 5 twisted in different extents around the butadiene core,
suggesting a helical s-cis conformation (Figure 1C, Figure S2
and Table S2).
Structural Characterization. The structures of the PHBs
and the model compound were analyzed by IR and NMR
spectroscopies. The IR spectra of P1a/2a and its corresponding
monomers 1a and 2a and model compound 5 are given in
Figure S3 as an example. The absorption peaks at 2937 and
3078 cm−1 in the spectrum of P1a/2a were assigned to the
stretching vibration of CH2 and CH3 groups in 1a and 2a,
respectively. On the other hand, the absorption band at 1337
cm−1 in 2a ascribed to B−O stretching vibration disappeared in
the spectra of 5 and P1a/2a. More informative structural data
were obtained from the H NMR spectra (Figure 2). The
Since hexaphenyl-1,3-butadiene (HPB) is a typical AIEgen,12
model compound 5 and PHBs are anticipated to be AIE-active
as well. As shown in Figure 5, the dilute THF solution of 5 was
barely emissive under UV illumination. When its hydrophobic
molecules formed nanoaggregates in THF/water mixtures with
water fractions (fw) of 80% or higher,14 their emission was
turned on, with the intensity at 95% water content being ∼9-
fold higher than that in pure THF solution. The photographs of
THF solution, suspended aggregates, and solid powders of 5
taken under UV irradiation are given in Figure 6, in which the
emission of the solid powders captured by a camera and
analyzed by a MATLAB program15 is 10-fold brighter than its
solution. Likewise, P1a/2a, P1a/2b, and P1c/2a exhibited
1
methylene protons of 1a resonating at δ 3.99, 1.84, and 1.55
were all located at the same chemical shifts in the spectrum of
P1a/2a. The resonance peak at δ 2.44 for the methyl protons of
2a underwent an upfield shift to δ 2.23 in the spectra of 5 and
P1a/2a. Neither 5 nor P1a/2a showed the resonance peak at δ
4.79 associated with the resonance of the boronic acid protons,
demonstrating a complete consumption of such functional
group by the polymerization reaction. Since the polymerization
does not proceed in a regioselective fashion, regioisomeric units
thus exist in the resulting polymers (Scheme 4). The change in
the local electron environment will lead to peak shift.
Therefore, the resonance of the methylene protons next to
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Figure 7. PL spectra of (A) P1a/2a, (C) P1a/2b, and (E) P1c/2a in THF and THF/H2O mixtures with different water fractions (fw).
Concentration: 10−5 M; excitation wavelength (nm): (A) 362, (C) 359, and (E) 354. Plot of relative PL intensities (I/I0) versus the compositions of
the THF/H2O mixtures of (B) P1a/2a, (D) P1a/2b, and (F) P1c/2a. Insets: fluorescent photographs of THF/H2O mixtures of (B) P1a/2a, (D)
P1a/2b, and (F) P1c/2a with water fractions of 0% and 99% taken under 365 nm UV irradiation from a hand-held UV lamp.
similar AIE or aggregation-enhanced emission (AEE) phenom-
enon (Figure 7), demonstrating that this polymerization
method was highly efficient for in situ generation of AIE
polymers. Besides, the varied emission colors of the PHBs
proved the efficacy of the functional groups in tuning the
photophysical properties of the polymers (insets in Figure 7B,
D, F).
Notably, the emission of 5 (Figure 6A), P1a/2a (Figure 7A),
P1a/2b (Figure 7C), and P3/4 (Figure 8) all experienced a
bathochromic shift when aggregates formed. For example, 5
emitted at 432 and 489 nm in THF and 99% aqueous mixture,
respectively. Considering that the amorphous powder of HPB
was reported to emit at 493 nm,12 the much bluer emission
observed in the solution state seemed abnormal. Crystal
analysis and theoretical calculation (Table S2) suggested that
the molecules of 5 showed similar dihedral angles in the
solution and crystal states. Thus, such a phenomenon did not
likely result from their conformational difference. On the other
hand, P3/4 exhibited two distinct emission bands at about 392
and 527 nm in THF and THF/water mixtures, whose
intensities varied with the water content (Figure 8A). Since
the absorption maximum changed little with the composition of
H
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Figure 8. (A) PL spectra of P3/4 in THF and THF/H2O mixtures with different water fractions (fw). Concentration: 10−5 M; excitation
wavelength: 360 nm. (B) Plot of relative PL intensity (I/I0) at 528 nm versus the composition of the THF/H2O mixture of P3/4.
Scheme 6. Photocyclization of 5, P1/2a, and P3/4
the THF/water mixture (Figure S4), excimer formation seems
to be ruled out for such possibility.17 On the other hand, we
noticed that the PL spectrum of tetraphenylethene (TPE)
exhibited a similar phenomenon,18 and the emission at the
bluer region was proved to be caused by its photocyclization in
the presence of UV irradiation, producing 9,10-diphenyl-
phenanthrene as the product. We further noticed that HPB
derivatives could undergo photoinduced cyclization to generate
their cycloisomers, namely dihydronaphthalene (DHN).19
Thus, there is a large possibility that the shorter-wavelength
emission in the PL spectra of 5, P1a/2a, P1a/2b, and P3/4
arise from the DHN moieties in their backbones.
With such presumption, the PL spectra of 5, P1a/2a, P1a/
2b, and P1c/2a can be better explained. In the presence of UV
irradiation, 5 and the hexaphenylbutadiene moieties in PHBs
undergo photocyclization, generating DHN which is highly
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Figure 11. Conversion for the photocyclization of 5, P1a/2a, P1c/2a,
and P3/4 at different irradiation times.
Figure 9. 1H NMR spectra of 5 in CD2Cl2 before and after UV
irradiation for 0, 30, 60, 90, and 120 min. The solvent peaks are
marked with asterisks.
Figure 12. 1H NMR spectra of a solution of tetraphenylethene before
and after UV irradiation for different times in DMSO-d6. The
conversion was calculated from the peak integral and was determined
to be 0%, 3.5%, 4.5%, and 5.0% at 0, 30, 60, and 120 min, respectively.
reaction condition is fairly mild for this reaction (UV irradiation
in air at room temperature), it is possible that some
photocyclized products have already been formed during the
material preparation.
Figure 10. 1H NMR spectra of P1c/2a in CD2Cl2 before and after UV
irradiation at different times. The solvent peaks are marked with
asterisks.
1H NMR spectroscopy is a facile tool for monitoring the
structural change. The solutions of 5, P1a/2a, P1c/2a, and P3/
4 (∼20 mg/mL) were irradiated by UV light and analyzed by
emissive in the bluer region in solutions. During the gradual
addition of water into their THF solutions, their PL spectra
showed a superficial bathochromic shift due to the mathematic
superposition of the emission bands of both HPB and DNH. In
P1c/2a, the emission of the TPE units in the aggregate state
was so intense that the PL of DHN was neglected.
Photocyclization. In order to verify our hypothesis, we
further investigated the photocyclization of 5 and PHBs. The
proposed mechanism is illustrated in Scheme 6.18 Because of
the steric effect of the periphery phenyl rings, 5 took a twisted
s-cis conformation. The close proximity of the alkene and the
ortho phenyl carbons led to facile cyclization followed by a
subsequent irreversible [1,5]-hydrogen shift, affording penta-
phenyl DHN 6. Likewise, P1/2a and P3/4 could be cyclized
into poly(dihydronaphthalene)s (PDHNs, P7−9). Since the
1
NMR spectroscopy at different times. The time-dependent H
NMR spectra of 5, P1a/2a, P1c/2a, and P3/4 are given in
Figures 9 and 10 and Figures S5 and S6. In Figure 9, the newly
emerged resonance peak at δ 4.30 was attributed to the
methine protons of 6, whose intensity kept increasing with the
irradiation time. The peak located at δ 2.25 was assigned to the
methyl protons of 5, which shifted into several peaks after UV
irradiation and was indicative of the gradual conversion of 5 to
6. Similarly, the resonance peaks for the methine protons of the
DHN units in P7, P8, and P9 were located at δ 4.22, 4.19, and
4.41, respectively, and their intensities were gradually
augmented during the irradiation process.
By calculating the integration of the resonance peaks for the
methine and methyl/aromatic protons, we can estimate the
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Figure 13. (A) Normalized absorption and emission spectra of P1a/2a and P7 in THF solutions. Concentration: 10−5 M; excitation wavelength: 362
nm. Inset: fluorescent photos of (left) P1a/2a and (right) P7 in THF solutions taken under 365 nm UV irradiation from a hand-held UV lamp.
Concentration: 10−3 M. (B) PL spectra of THF solutions of P1a/2a and P7 and THF/water mixture of P7 (1:99, v/v). Inset: fluorescent photos of
(left) THF solution and (right) 99% aqueous mixture of P7 taken under 365 nm UV irradiation from a hand-held UV lamp.
at 120 min, 54.0% at 90 min, 66.8% at 240 min, and 61.5% at
160 min, respectively. Such difference is closely related to their
structures. Compared to the low molecular weight model
compound, the polymer backbones are more difficult to twist.
This has lowered the chance for the reaction sites to approach
each other for cyclization.
A control experiment was carried out to study the potential
photocyclization reaction of the TPE moieties in P1c/2a.
Analysis by 1H NMR spectroscopy of a solution of TPE (Figure
12) suggested that only 5.0% of TPE were photocyclized into
9,10-diphenylphenanthrene after UV irradiation for 120 min.
Such value was much lower than that achieved by HPB. The
characteristic resonance peak for 9,10-diphenylphenanthrene at
δ 8.96 in Figure 12 was absent in the spectra of P1c/2a (Figure
10), suggesting that the major photocyclized product was DHN
(Figure 10).
Owing to the structural change, the optical properties of the
PHBs are tuned after UV-induced cyclization. The normalized
1
Figure 14. H NMR spectra of P1a/2a (A) before and (B, C) after
UV irradiation for 90 min in the (B) solution and (C) solid thin film
absorption and emission spectra of P1a/2a before and after
photocyclization, for example, are obviously different (Figure
13A). The absorption band attributed to the HPB moiety was
located at 362 nm in P1a/2a, which became much weaker in
P7. The emission spectrum of P7, on the other hand, was 30
nm blue-shifted from that of P1a/2a with a narrower
bandwidth because the latter polymer was more conjugated
and its spectrum also combined the spectrum of DHN.
Notably, unlike P1a/2a, P7 was no longer AIE-active. As shown
in Figure 13B, the solution of P7 was much more emissive than
its suspended aggregates. Such aggregation-caused quenching
effect may be due to the π−π stacking of the DHN cores in the
aggregate state. Although the HPB units in P1c/2a also
underwent photocyclization upon UV light irradiation, the
emission of its photocyclized product P8 was still dominated by
the TPE units in the backbone. Thus, P8 preserved the AIE
characteristic of P1c/2a (Figure S7).
states for 90 min in (A, B) CD2Cl2 and (C) CDCl3.
The efficient photocyclization of PHBs’ solutions spurs us to
investigate whether such reaction can be carried out in the solid
state. Thus, a thin film of P1a/2a was spin-coated on silica
wafer, and after being UV irradiated in air for 90 min, NMR
analysis was carried out. The appearance of the resonance peak
for the methine protons in solution at δ 4.20 was also found in
the UV-irradiated film, indicating that the photocyclization
Figure 15. Fluorescent photo of P1a/2a dip-coated on a piece of filter
paper after UV irradiation for 2 h through a cardboard mask.
efficiency of the photocyclization. As shown in Figure 11, the
ultimate conversion of 5, P1a/2a, P1c/2a, and P3/4 are 76.2%
K
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Figure 16. Restriction of intramolecular cyclization: AIE mechanism in butadiene-containing materials.
illustrated in Figure 16, the butadiene derivatives could undergo
intramolecular twist to a cis conformation in the solution state,
subsequent photocyclization of which generated dihydro-
naphthalene. The photocyclized products may suffer π−π
stacking or excimer/exciplex formation in the aggregate state,
which quench their emission. However, the intramolecular
rotation of the butadiene core is much difficult in the aggregate
state. As a result, photocyclization is less likely to occur, leading
to the preservation of the strong emission. In PHBs, the
butadiene units may adopt a predominant trans conformation
to prevent steric hindrance between segments; coupled with
their high molecular weight and hence slow rotation, butadiene-
bearing polymers should be more difficult to be photocyclized,
which is consistent with our result on the different conversion
for molecules and polymers with different structures.
Thermal Stability. Polymers with high thermal stability are
good candidates as heat-resistant materials. As shown in Figure
17, all the PHBs lose merely 5% of their weight at high
temperatures of 380−445 °C. In particular, P1c/2a (445 °C)
showed a much higher heat resistance than other polymers,
thanks to its high aromatic content. The high thermal stability
of the PHBs makes them promising functional materials for
real-world applications.
Figure 17. TGA thermograms of P1/2 and P3/4.
reaction could readily took place in the solid state (Figure 14B).
The conversion estimated from the peak integral was
approximately 49.1% according to the integration, making
this reaction a simple tool to change the polymer structure and
property.
Photopatterning. Because of the high photosensitivity of
P1a/2a and the ACQ feature of P7, we explored the potential
application of P1a/2a as a photopatterning material. We
fabricated a thin film of P1a/2a on a piece of filter paper by dip-
coating and then shielded part of paper by black cardboard in
the letter form of “AIE”. After UV irradiation for 2 h, the
cardboard was removed. While the unexposed or covered
region exhibited blue emission, the irradiated part appeared
dark under UV illumination (Figure 15). This is because in the
presence of UV light the AIE-active P1a/2a is photocyclized
into P7 with ACQ effect. Such a result demonstrates a facile
approach for the microfabrication of photopatterns by UV
lithography.
Restriction of Intramolecular Cyclization. From the
study above and the literature reports,18,20 we can gain some
mechanistic insight into the AIE process. As we know, RIM,
including intramolecular rotation and vibration, is responsible
for the AIE or AEE of some molecules.1,4 Here, we propose
that for hexaarylbutadiene derivatives and polymers containing
butadiene skeletons RIC is the cause for their AIE feature. As
CONCLUSIONS
■
In this work, we have demonstrated an efficient method to
prepare PHBs by a palladium-catalyzed oxidative polycoupling
of internal alkynes and arylboronic acids. This polymerization
reaction realized in situ generation of AIEgens in the polymer
backbones from a wide monomer scope. PHBs with electron-
donating or -withdrawing pendants were readily obtained under
mild and monomer nonstoichiometric conditions in the
presence of moisture and oxygenic species. The PHBs enjoyed
good film-forming capability and high thermal stability. The
strong solid-state emission of the PHBs enabled their films to
serve as high-contrast photopatterning materials. The PHBs
could undergo facile photocyclization in air under UV
irradiation, during which the optical properties of the polymers
were tuned. By analysis of such process, deeper mechanistic
insights into the AIE phenomenon could be obtained.
L
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11508. (b) Wang, L.; Yang, L.; Cao, D. Application of Aggregation-
Induced Emission (AIE) Systems in Sensing and Bioimaging. Curr.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.6b01148.
■
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Details of the instrumental analysis, HRMS spectrum of
5, crystal data and structure refinement of 5, computa-
tional calculation of 5, dihedral angles of 5 in single
crystal, solution and gas states, IR spectra of 1a, 2a, 5,
and P1a/2a, absorption spectra of P3/4, 1H NMR
spectra of P1a/2a and P3/4 before and after photo-
cyclization, PL spectra of P8 (PDF)
AUTHOR INFORMATION
Corresponding Author
*(B.Z.T.) E-mail tangbenz@ust.hk; Tel +852-2358-7375; Fax
+852-2358-1594.
■
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 and
2013CB834702), the National Science Foundation of China
(21490570 and 21490574), the University Grants Committee
of Hong Kong (AoE/P-03/08), the Research Grants Council of
Hong Kong (16305014 and 16303815), the Innovation and
Technology Commission (ITC-CNERC14SC01 and ITCPD/
17-9), and the Nissan Chemical Industries, Ltd. B.Z.T.
expresses thanks for the support of the Guangdong Innovative
Research Team Program (201101C0105067115).
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