Synthesis, light emission, explosive detection, fluorescent photopatterning, and optical limiting of disubstituted polyacetylenes carrying tetraphenylethene luminogens

Article abstract of DOI:10.1021/ma502341j

Tetraphenylethene-functionalized acetylenes [(C₆H₅)₂C=C(C₆H₅)(C₆H₄C ≡CR), R = C₈H₁₇ and C₆H₅] were synthesized, and their polymerizati

Full text of DOI:10.1021/ma502341j

Article
pubs.acs.org/Macromolecules
Synthesis, Light Emission, Explosive Detection, Fluorescent
Photopatterning, and Optical Limiting of Disubstituted
Polyacetylenes Carrying Tetraphenylethene Luminogens
Carrie Y. K. Chan,†,‡ Jacky W. Y. Lam,†,‡ Chunmei Deng,†,‡ Xiaojun Chen,‡ Kam Sing Wong,‡
and Ben Zhong Tang*,†,‡,§
†HKUST-Shenzhen Research Institute, No. 9 Yuexing 1st RD, South Area, Hi-tech Park, Nanshan, Shenzhen 518057, China
‡Department of Physics, 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
S
* Supporting Information
ABSTRACT: Tetraphenylethene-functionalized acetylenes
[(C6H5)2CC(C6H5)(C6H4CCR), R = C8H17 and C6H5] were
synthesized, and their polymerizations were effected by WCl6−
Ph4Sn at elevated temperatures in toluene under nitrogen, furnishing
polymers P1 and P2. The polymers possessed good solubility and
degraded at high temperatures of up to ∼400 °C under nitrogen.
Both polymers emitted weakly in the solution state. Whereas the
emission of P1 was enhanced when aggregated, that of P2 was
quenched, demonstrating that the light emission of polyacetylenes
could be varied readily by changing their molecular structure. The
polymers could serve as fluorescent chemosensors for explosive
detection with an amplification effect. UV irradiation of their films
in air photo-oxidized and bleached the fluorescence of the exposed
parts, generating fluorescent photopatterns. The polymers exhibited optical nonlinearity and could limit laser pulses.
developed so far are low molecular weight molecules and thus are
not suitable for the manufacture of large-area flat-panel devices.
To solve this problem, one of the best ways is to make their
counterparts with high molecular weights, i.e., polymers, which
can fabricate into large-area thin solid films through simple
processes such as spin coating, static casting, and inkjet printing.
Polyacetylene is a well-known conjugated polymer and ex-
hibits metallic conductivity upon doping. Such discovery has
created a new area of research on “synthetic metals”.5,6 Our
group has worked on acetylene research for many years and has
synthesized many substituted polyacetylenes with advanced
materials properties.7 With the expectation that the resulting
polymers will show unique light-emitting properties, we are
interested in incorporating AIE molecules into the polyacetylene
structures. We had prepared monosubstituted polyacetylenes
with silole and TPE luminogens and systematically investigated
their optical properties (Chart 1). Whereas the direct attachment
of silole pendant to the rigid polyacetylene chain afforded AIE-
inactive polymer P3 due to the poor packing in the aggregated
INTRODUCTION
■
Development of efficient luminophores has attracted consid-
erable interest for their potential applications in organic light-
emitting diodes (OLEDs), chemosensors, bioprobes, etc.1 Many
luminophores have been prepared and found to show strong
light emission in solutions. However, they become weak emitters
or completely nonluminescent when aggregated due to the
aggregation-caused quenching (ACQ) effect.2 In the aggregated
state, the luminogenic molecules are in close proximity. This
favors the formation of excimers and exciplexes by strong inter-
molecular interactions and hence led to nonradiative relaxation.
This problem must be properly tackled as luminophores are used
as solid thin films in their real-world applications. We observed a
phenomenon of aggregation-induced emission (AIE), in which
aggregate formation is beneficial to the light emission of some
molecules such as silole and tetraphenylethene (TPE).3 Because
such a phenomenon is of academic importance and practical
signature, many scientists around the world are now doing AIE
research. Thanks to their enthusiastic efforts, many new AIE dyes
have been created and used for the fabrication of OLEDs with
outstanding device performances and served as sensory materials
for detecting VOCs, explosives, and biomolecules in high sensi-
tivities and selectivities.4 However, almost all the AIE luminogens
Received: November 20, 2014
Revised: January 20, 2015
© XXXX American Chemical Society
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Chart 1. Molecular Structures of Silole and Tetraphenylethene-Containing Monosubstituted Polyacetylenes
Scheme 1. Synthetic Routes to Tetraphenylethene-Functionalized Acetylenes and Their Polymers
state,8 polyacetylene P5 carrying directly linked TPE units
exhibited aggregation-enhanced emission (AEE) characteristic.9
When a flexible alkyl spacer was inserted between the polymer
backbone and the pendant group, the motion of the latter was
decoupled from the former and hence endowed the resulting
polymers P4 and P6 with AIE features.8,9
communication between the emissive polyacetylene backbone
and the AIE unit may confer the resulting polymers with intrigu-
ing optical properties. We herein report the synthesis of TPE-
functionalized disubstituted polyacetylenes P1 and P2 (Scheme 1)
and present their properties and their utilization as chemosensors
for explosive detection and optical limiters.
Compared with monosubstituted polyacetylenes, disubstituted
polyacetylenes generally show higher thermal stability, better film
forming and stronger mechanical strength.10 Although poly-
acetylene itself is not luminescent, its substituted counterparts can
be emissive, with disubstituted polyacetylenes showing stronger
light emission than their monosubstituted congeners.11 Although
disubstituted polyacetylenes possess such advantages, they are
difficult to prepare, especially those with bulky substituents, due to
the comparative low reactivity of their corresponding monomers.
In this paper, we take the challenge and intend to prepare disub-
stituted polyacetylenes carrying AIE luminogens. The electronic
EXPERIMENTAL SECTION
■
Materials and Instrumentation. Tetrahydrofuran (THF) and
toluene were freshly distilled from sodium benzophenone and calcium
hydride, respectively, under nitrogen. All the chemicals and reagents
were purchased from Aldrich and used as received. Weight-average
molecular weight (Mw) and polydispersity (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.12
1H and 13C NMR spectra were measured on a Bruker AV 300 spectro-
meter using tetramethylsilane (TMS; δ = 0) as internal reference.
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High-resolution mass spectra (HRMS) were recorded on a GCT
premier CAB048 mass spectrometer operating in MALDI-TOF mode.
UV spectra were measured on a Milton Ray Spectronic 3000 Array
spectrophotometer. Photoluminescence (PL) spectra were recorded on
a PerkinElmer LS 55 spectrofluorometer. Fluorescence quantum yields
(ΦF) of thin films of the polymers were measured on a calibrated
integrating sphere. The particle sizes of the polymer aggregates were
measured on a Beckman Coulter Delsa 440SX Zeta potential analyzer.
Thermogravimetric analysis (TGA) was carried on a TA TGA Q5000
under nitrogen at a heating rate of 10 °C/min. The thermal transitions of
the polymers were investigated on a differential scanning calorimeter
(DSC) using a TA DSC Q1000 under nitrogen at a heating rate of
10 °C/min. The ground-state geometries were optimized using the
density functional (DFT) with B3LYP hybrid functional at the basis set
level of 6-31G*. All the calculations were performed using the Gaussian
03 package.
Fluorescence decay curves were recorded on an Edinburgh Instru-
ments FLS920. A femtosecond titanium−sapphire oscillator was used as
excitation laser source. The second harmonic (358 nm) of the oscillator
output at 716 nm was used for the PL measurement. Time-resolved PL
measurements were carried out on a Hamamatsu model C4334 streak
camera coupled to a spectrometer. The PL signals were collected at 490
or 510 nm. The decay in the PL intensity with time was fitted by a
double-exponential function.13 The optical nonlinearity of the polymers
was investigated by using a frequency-doubled, Q-switched, mode-locked
continuum ns/ps Nd:YAG laser. Detailed procedures are described in our
previous publication.14
layer was dried over anhydrous MgSO4 and concentrated. The crude
product was purified on a silica gel column using hexane as eluent.
Characterization Data for 1. White solid; yield 77% (2.3 g). H
1
NMR (300 MHz, CDCl3), δ (ppm): 7.14−7.09 (m, 11H), 7.04−7.01
(m, 6H), 6.93 (d, 2H), 2.37 (t, 2H), 1.44−1.30 (m, 12H), 0.88 (t, 3H).
13C NMR (75 MHz, CDCl3), δ (ppm): 144.28, 144.19, 144.14, 143.76,
141.98, 141.06, 132.04, 131.98, 131.89, 131.95, 131.55, 128.46, 128.35,
128.31, 127.23, 127.19, 122.57, 91.38 (≡C−Ph), 81.31 (≡C−C8H17),
32.53, 29.88, 29.81, 29.61, 29.45, 23.34, 20.12, 14.79. HRMS (MALDI-
TOF): m/z 468.2380 [M+, calcd 468.2817].
1
Characterization Data for 2. White solid; yield 90% (2.5 g). H
NMR (300 MHz, CDCl3), δ (ppm): 7.49 (t, 2H), 7.30−7.28 (m, 6H),
7.12−7.02 (m, 16H). 13C NMR (75 MHz, CDCl3), δ (ppm): 144.20,
143.73, 143.65, 143.56, 141.87, 140.87, 140.48, 131.76, 131.59, 131.55,
131.51, 131.17, 128.55, 128.39, 128.06, 127.95, 127.88, 126.90, 126.82,
126.77, 123.53, 121.26, 89.82 (C−TPE), 89.79 (C−Ph). HRMS
(MALDI-TOF): m/z 432.2629 [M+, calcd 432.1878].
Polymerization. All the polymerization reactions were carried out
under nitrogen. Experimental procedures for the polymerization of 1 are
given below as an example.
To a dry Schlenk tube were placed 29 mg (0.05 mmol) of WCl6 and
22 mg (0.05 mmol) of Ph4Sn under nitrogen. Dry toluene (3 mL) was
injected into the tube, and the tube was then aged at 60 °C for 15 min.
Monomer 1 (128.3 mg, 0.18 mmol) was dissolved in 2 mL of anhydrous
toluene, and this solution was then transferred to the catalyst solu-
tion using a syringe. The resulting mixture was stirred at 60 °C under
nitrogen. After 24 h, the mixture was diluted with chloroform and added
to a large amount of methanol under stirring via cotton filter to filter, if
possible, any insoluble substances. The precipitates were filtered with a
Gooch crucible, washed with methanol, and dried under vacuum to a
constant weight.
Monomer Synthesis. Monomers 1 and 2 were synthesized according
to Scheme 1. Detailed procedures are shown below.
Synthesis of 1-(4-Bromophenyl)-1,2,2-triphenylethene (9).
To a solution of diphenylmethane (7, 5 g, 29.7 mmol) in dry THF
(40 mL) was added 17.7 mL (28.3 mmol) of n-butyllithium (2 M
solution in hexane) at 0 °C under nitrogen. After stirring for 30 min, 7.4 g
(28.3 mmol) of 4-bromobenzophenone (8) was added. The reaction
mixture was warmed to room temperature. After stirring for another 6 h,
the reaction was terminated by addition of an aqueous solution of
ammonium chloride. The mixture was extracted with dichloromethane.
The organic layer was washed with water and dried over anhydrous
magnesium sulfate. After solvent evaporation, the crude alcohol with
excess diphenylmethane was then subjected to acid-catalyzed dehydra-
tion. The crude alcohol was dissolved in toluene (∼50 mL) in a two-
necked round-bottom flask equipped with a condenser. After addition of
a catalytic amount of p-toluenesulfonic acid, the mixture was refluxed for
3−4 h. Afterward, the mixture was washed with 10% aqueous NaHCO3
solution. The organic layer was separated and dried over anhydrous
magnesium sulfate. After filtration followed by solvent evaporation, the
crude product was purified by silica gel column chromatography using
hexane/dichloromethane (9:1 v/v) as eluent. White solid; yield 68%
(7.9 g). 1H NMR (300 MHz, CDCl3), δ (ppm): 7.22 (d, 2H), 7.14−7.09
(m, 9H), 7.05−7.00 (m, 6H), 6.89 (d, 2H). 13C NMR (75 MHz,
CDCl3), δ (ppm): 143.99, 143.37, 142.27, 140.32, 133.66, 131.95,
131.52, 128.45, 127.35, 121.11. HRMS (MALDI-TOF): m/z 412.0681
[(M + 2)+, calcd 412.0670].
1-[4-(2-Octylethynyl)phenyl]-1,2,2-triphenylethene (1) and
1-[4-(2-Phenylethynyl)phenyl]-1,2,2-triphenylethene (2). 3.8 mL
(6 mmol) of n-butyllithium (1.6 M in hexane) was added into a THF
solution (50 mL) of 9 (2 g, 5 mmol) at −78 °C. After stirring at −78 °C
for 2 h, 1.4 g (5.5 mmol) of iodine was added. After stirring at room
temperature for another 2 h, water was added and the mixture was
extracted with dichloromethane. The organic layer was separated, washed
with saturated sodium thiosulfate solution and water, and dried over
magnesium sulfate. The mixture was filtrated. After solvent evaporation,
the crude product 10 was purified by silica gel column chromatography
using hexane as eluent. Into a new 250 mL round-bottom flask was
dissolved 3 g (6.5 mmol) of 10 in 50 mL of triethylamine at room
temperature. After addition of Pd(PPh3)2Cl2 (208 mg, 0.3 mmol),
triphenylphosphine (160 mg, 0.6 mmol), CuI (113 mg, 0.6 mmol), and
1-decyne (1.4 g, 10.0 mmol) or phenylacetylene (1.0 g, 10.0 mmol), the
mixture was refluxed for 12 h under nitrogen. After solvent evaporation,
the residue was extracted with dichloromethane and water. The organic
Characterization Data for P1. Yellow solid; yield 82.2% (Table 1,
no. 4). Mw 10 000; Mw/Mn 1.7 (GPC, polystyrene calibration). IR
a
Table 1. Polymerization of 1 and 2
b
b
entry
catalyst
temp (°C) yield (%)
monomer 1
Mw
Mw/Mn
1
2
NbCl5−Ph4Sn
TaCl5−Ph4Sn
TaCl5−n-Bu4Sn
WCl6−Ph4Sn
WCl6−Ph4Sn
WCl6−Ph4Sn
80
trace
8.8
80
123000
20700
10000
9200
2.3
3.4
1.7
1.6
2.1
c
3
80
25.4
82.2
46.0
40.5
4
5
6
60
80
100
6700
monomer 2
7
TaCl5−Ph4Sn
TaCl5−n-Bu4Sn
TaCl5−n-Bu4Sn
WCl6−Ph4Sn
WCl6−Ph4Sn
WCl6−Ph3SiH
80
80
trace
c
8
3.3
6.1
18000
4600
3800
5000
3.0
1.5
1.8
2.5
c
9
100
80
10
11
12
28.2
32.0
0
100
80
a
Carried out under nitrogen in toluene for 24 h. [M]0 = 0.2 M;
b
[cat.] = [cocat.] = 10 mM. Determined by GPC in THF on the basis
of a polystyrene calibration. [cocat.] = 20 mM.
c
(KBr), υ (cm−1): 3077, 3055, 3023, 2924, 2853, 2068, 1943, 1598, 1492,
1251, 1075, 1030. 1H NMR (300 MHz, CDCl3), δ (TMS, ppm): 7.07,
(broad peak), 1.27, 0.90. 13C NMR (75 MHz, CDCl3), δ (TMS, ppm):
144.55, 141.41, 132.06, 128.27, 127.02, 32.60, 30.01, 23.45, 14.87.
Characterization Data for P2. Green solid; yield 28.2% (Table 1,
no. 10). Mw 3800; Mw/Mn 1.8 (GPC, polystyrene calibration). IR (KBr),
υ (cm−1): 3076, 3052, 3021, 2067, 1946, 1490, 1445, 1251, 1074, 1030.
1H NMR (300 MHz, CDCl3), δ (TMS, ppm): 7.05 (broad peak). 13
C
NMR (75 MHz, CDCl3), δ (TMS, ppm): 144.44, 141.16, 132.01,
128.26, 126.97.
Fluorescent Photopatterning. Photopatterning was conducted
in air at room temperature using 365 nm UV light from a Spectroline
ENF-280C/F UV lamp at a distance of 1 cm. The intensity of the
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incident light was ∼18.5 mW/cm2. The film was prepared by spin
coating the polymer solution (∼5 wt % in 1,2-dichloroethane) on
a silicon wafer. The polymer film was dried in a vacuum oven and
UV-irradiated for 10 min through a copper mask. The fluorescent image
of the resulting photopattern was taken on an Olympus BX41 fluo-
rescence optical microscope.
RESULTS AND DISCUSSION
■
Monomer Synthesis. Two TPE-containing acetylenes
(1 and 2) were synthesized according to the synthetic routes
shown in Scheme 1. Compound 9 was first prepared by coupling
reaction of diphenylmethane with 4-bromobenzophenone
followed by acid-catalyzed dehydration. It was then converted
into its iodinated congener (10) by the halogen-exchange
reaction. Sonogashira coupling of 10 with 1-decyne and
phenylacetylene was catalyzed by Pd(PPh3)2Cl2, CuI, and PPh3
in Et3N, which furnished the target products 1 and 2 in satis-
factory to high yield. All the monomers were carefully purified.
Their structures were characterized by standard spectroscopic
methods with good results. The 1H and 13C NMR spectra of 1
and 2 are shown in Figures S1−S4 of the Supporting Information.
Polymer Synthesis. Since NbCl5 and TaCl5 are good cata-
lysts for the polymerization of 1-phenyl-1-alkynes15 and sterically
bulky diphenylacetylene derivatives,16 we thus first tried to use
them for the polymerization of 1. Reaction of 1 in the presence of
NbCl5−Ph4Sn in toluene at 80 °C, however, gave only a small
amount of product (Table 1, no. 1). Although a polymer with a
high molecular weight was produced using TaCl5−Ph4Sn as
catalyst, the yield was low. Changing the cocatalyst to n-Bu4Sn
increased the polymer yield but decreased the molecular weight.
In our previous investigation, we found that WCl6−Ph4Sn
worked well for the polymerization of diphenylacetylene deriv-
atives.8 Thus, we tested whether monomer 1 could be poly-
merized by such catalytic system. Delightfully, after stirring
a toluene solution of 1 at 60 °C with WCl6−Ph4Sn, a yellow
powdery polymer with a satisfactory molecular weight of 10 000
was obtained in a high yield. A further attempt to obtain a better
polymerization result by increasing the reaction temperature
failed: the isolated yield and molecular weight of the obtained
polymer decreased progressively when the polymerization was
carried out at 80 and 100 °C.
Figure 1. 1H NMR spectra of (A) monomer 1 and (B) its polymer P1 in
chloroform-d.
Figure 2. 13C NMR spectra of (A) monomer 1 and (B) its polymer P1 in
chloroform-d.
to the absorption of the methylene protons next to the triple bond
(C−CH2) of 1. Its absorption peaks were much broader than
those of 1, which was suggestive of its polymeric character. On the
other hand, the acetylenic carbon atoms of 1 absorbed at δ 91.4 and
81.3, which disappeared completely in the spectrum of P1. The
absorption peaks of the olefinic carbon atoms of the polyacetylene
backbone may be overlapped with those of the TPE pendants and
are thus not observed.
Monomer 2 displays a polymerization behavior similar to that
of 1. While TaCl5−Ph4Sn and TaCl5−n-Bu4Sn were generally
inactive for the polymerization of 2, the monomer could be
polymerized by WCl6−Ph4Sn at 80 °C. It is noteworthy that the
molecular weight of P2 is much lower than P1. This is
understandable because 2 is sterically more bulky than 1, which
makes the coordination of its molecules to the catalytic active
sites a daunting task. Raising the reaction temperature to 100 °C
did not help much on the isolated yield and the polymer molec-
ular weight. Further attempt to polymerize 2 by WCl6−Ph3SiH
was unsuccessful. All the obtained polymers were completely
soluble in common organic solvents such as chloroform, dichlo-
romethane, and THF but insoluble in water, hexane, and methanol.
Structural Characterization. Both P1 and P2 gave good
spectroscopic data corresponding to their molecular structures.
Examples of 1H and 13C NMR spectra of P1 are shown in Figures 1
and 2, respectively, while those of P2 are given in Figures S5 and
S6. For comparison, the spectrum of its monomer 1 was also
provided in the same figure. The NMR analysis proved that the
acetylene triple bonds of 1 had been converted to the polyene
double bonds of P1 by the polymerization reaction. For example,
the 1H NMR spectrum of P1 showed no peak at δ 2.4 corresponding
Thermal Properties. The thermal properties of P1 and P2
are investigated by TGA and DSC analyses under nitrogen.
Because the majority of the polymers were constructed from
aromatic rings, they thus showed outstanding thermal stability
(Figure 3). The temperatures for 5% weight loss (Td) were up to
∼400 °C, which were higher than those of P3−P5 (Td = 350−
360 °C), thanks to their more rigid polymer backbone. P2
showed a higher thermal stability, which was somewhat expected
because the aliphatic octyl chain in P1 had a lower resistance than
the phenyl ring in P2 toward thermolysis. The DSC thermo-
grams of both P1 and P2 detected no peaks corresponding to
glass transition from room temperature to 300 °C, presumably
due to the high rigidity of their disubstituted polyacetylene
backbones.
Optical Properties. Figure 4 shows the absorption spectra of
P1 and P2 as well as their monomers 1 and 2 in THF. Whereas 1
and 2 showed no peaks at wavelengths longer than 410 and
440 nm, the spectra of P1 and P2 were well extended to the
visible light region up to 530 nm. Clearly, the peaks at the longer
wavelengths are due to the absorptions of the double-bond
backbones of the polymers. In other words, the polymers are
more conjugated than their monomers, thanks to the electronic
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be due to the presence of an additional phenyl ring in the polymer
backbone. This also demonstrates that the effective conjugation
length of the polymers is rather short, probably owing to the
twisting of the polyacetylene chain by the steric repulsion between
neighboring substituents.
To better understand their photophysical properties, theoreti-
cal calculations on the energy levels of 1 and 2 were carried
out. Their highest occupied (HOMO) and lowest unoccupied
(LUMO) molecular orbitals plots are given in Figure 5. The
HOMO and LUMO of 1 mainly originated from the orbitals of
the TPE unit, revealing that its absorption originated mainly
from the π−π* transition of the TPE component. On the con-
trary, the orbitals of the HOMO and LUMO of 2 were localized
on the whole molecule. This results in an extended conjugation
in 2. The band gap for 1 was calculated to be 3.91 eV, which was
wider than that of 2 (3.67 eV). Thus, the theoretical calculation
nicely explains the hypsochromic shift in the absorption of 1
from that of 2.
TPE is practically nonluminescent in solution but becomes
highly emissive in the aggregated state. Are the TPE-containing
disubstituted polyacetylenes also AIE-active? To answer this, the
photoluminescence (PL) of P1 and P2 as well as their monomers
in THF and THF/H2O mixtures was investigated. As shown
in Figure 6, P1 was somewhat luminescent at 493 nm in THF
(10 μM). The ΦF was estimated to be 5.2% (Table 2). On the
contrary, its monomer 1 is nonemissive under the same mea-
surement conditions. We previously proposed that the AIE effect
is caused by the restriction of intramolecular motion, which
blocks the nonradiative relaxation channel. Since the TPE units
are directly bonded to a rigid polymer skeleton in P1, their intra-
molecular motions are, to some extent, restricted, thus allowing
the polymer to emit in the solution state. With a gradual addition
of water into the THF solution of P1, the PL intensity was
progressively enhanced, while the spectral pattern remained
almost the same. At 90 vol % water content, the intensity was
4.5-fold higher than that in THF. Since P1 is insoluble in water,
aggregation of its chains should readily occur in THF/H2O
mixtures with high water contents. Clearly, aggregate formation
has enhanced the PL of P1, or in other words, P1 is AEE-active.
Only a small (6 nm) red-shift in the emission maximum was
observed when P1 was fabricated as solid thin film, which was
indicative of weak interactions between polymer strands.
Conversely, it exhibited a higher ΦF value of 7% as measured
by a calibrated integrating sphere.
Figure 3. TGA thermographs of P1 and P2 recorded under nitrogen at a
heating rate of 10 °C/min.
Figure 4. Absorption spectra of 1, 2, P1, and P2 in THF solutions.
Solution concentration: 10 μM.
communication between the TPE pendants and the polyacetylene
backbone. Although P2 possesses a much lower molecular weight
than P1, it shows a higher conjugation, as revealed by its longer
onset wavelength (470 nm in P1 and 530 nm in P2). This is
further substantiated by their appearance: while P1 is a yellow
solid, P2 appears green in color. The higher conjugation in P2 may
Interestingly, P2 exhibits a different emission behavior. Like
P1, its diluted THF solution was emissive and emitted at 511 nm
with a ΦF value of 19.4%. No discernible signals, however, were
recorded in the PL spectrum of its monomer. When a small
Figure 5. Molecular orbital amplitude plots of HOMO and LUMO levels of 1 and 2 calculated using the B3LYP/6-31G* basis set.
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Figure 6. (A) PL spectrum of P1 in THF and THF/H2O mixtures with different water fractions. Inset: emission spectrum of 1 in THF solution.
Concentration: 10 μM; excitation wavelength: 330 nm. (B) Plot of I/I0 values versus the compositions of the THF/H2O mixtures of P1.
π−π stacking interactions and restrict their intramolecular
motions. While the former effect shifts the PL spectrum to the
redder region and quenches the light emission, the latter boosts
the emission intensity. The competition between the two effects
determines the PL behavior of a luminogen. In P2, the former
effect is prevailed at high water fractions, which thus makes it
weakly emissive at the longer wavelength under such circum-
stance. The ΦF of its thin film was measured to be 14.1%, which
was smaller than its solution value. Thus, unlike P1, P2 exhibits
the ACQ effect.
To confirm the formation of polymer particles in THF/H2O
mixtures with high water contents, particle size analyses are
carried out. An average size of ∼300 nm was detected in 50%
aqueous solution of P1 (Figure 8A). When the amount of water
was progressively increased to 70% and then 90%, the sizes of
the particles were finally decreased to ∼143 nm. At low water
fraction, not all the polymer chains are aggregated, and this may
allow the dissolved chains to cluster slowly to form “larger”
aggregates. On the other hand, the polymer chains may aggregate
quickly at high water content, which finally give nanoparticles of
“smaller” sizes. A similar phenomenon was also found in P2.
Explosive Detection. Detection of explosives has attracted
much current interest because of its antiterrorism implications.17
Among various materials, sensors based on fluorescent conju-
gated polymers have received much attention, thanks to their
signal-amplifying effect and high binding capability to analytes.18
Because of this, we thus investigated the possibility of the present
polymers as fluorescent chemosensors for efficient detection
of explosives. Picric acid (PA) was used as a model explosive
because of its commercial availability. The polymer aggregates in
THF/H2O mixtures with 50 and 90 vol % water fractions were
employed as fluorescent sensors, while the performance of their
isolated chains in pure THF was also studied for comparison.
When PA was gradually added into the nanoparticle sus-
pensions of P1 and P2 in aqueous mixtures, the PL intensity
was progressively decreased but without causing any spectral
profile change (Figure 9A and Figure S7 in the Supporting
Information). The PL quenching could be recognized at a PA
concentration of 1 μg/mL or 1 ppm. At [PA] of 28−50 μg/mL,
the emission from the aqueous mixtures was almost quenched
completely.
Figure 7. (A) PL spectrum of P2 in THF and THF/H2O mixtures with
different water fractions. Inset: emission spectrum of 2 in THF solution.
Concentration: 10 μM; excitation wavelength: 330 nm. (B) Plot of I/I0
values versus the compositions of the THF/H2O mixtures.
Table 2. Absorption and Emission of P1 and P2 in Solution
a
b
(soln) , Aggregate (aggr), and Amorphous (amor) States
c
d
λab (nm)
λem (nm)
polymer
soln
soln (ΦF)
aggr
amor (ΦF)
P1
P2
326
341
493 (5.2)
496
529
502 (7.0)
511 (19.4)
526 (14.1)
a
b
In dilute THF solution (10 μM). In THF/H2O mixtures (1:9 by
c
d
volume). Absorption maximum. Emission maximum with quantum
yields (ΦF, %) given in the parentheses. The ΦF values were deter-
mined using 9,10-diphenylanthracene (ΦF = 90% in cyclohexane) as
standard (soln) or by a calibrated integrating sphere (amor).
amount of water (<50 vol %) was added to the polymer solution,
the emission became stronger progressively. The maximum
enhancement was observed at 50% water content. Afterward,
the PL intensity dropped along with a red-shift in the emission
maximum to 529 nm. The shrinkage in the volume of polymer
chain due to the aggregate formation in aqueous mixtures has
physically brought the phenyl rings closer. This will increase their
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Figure 8. Particle size distributions of aggregates of (A) P1 and (B) P2 suspended in THF/water mixtures with water fractions (fw) of 50, 70, and
90 vol %. Abbreviation: de = effective diameter, dm = mean diameter, PD = polydispersity.
Figure 9. PL spectra of P1 in (A) THF solution and (B, C) THF/H2O mixtures with (B) 50 and (C) 90 vol % water contents containing different
amounts of picric acid (PA). (D) Plots of I0/I values versus PA concentrations. I0 = intensity at [PA] = 0 mM.
The Stern−Volmer plots of relative PL intensity (I0/I) versus
PA concentration reflected the sensing performance and showed
upward bent curves instead of straight lines (Figure 9D and
Figure S7D). This indicates that the quenching process becomes
more efficient or amplified at higher quencher concentration.
The PL of the polymers in THF solutions also became weaker
upon addition of PA. However, at the same PA concentration,
the THF solutions showed stronger emission than the aqueous
mixtures, which was indicative of their poorer sensing perfor-
mance. In our recent publications, the static quenching model is
more adequate than the diffusion-controlled dynamic mecha-
nism to describe the PL quenching behaviors by PA.19 In this
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Figure 10. Two-dimensional photopatterns generated by photo-oxidation of films of (A, B) P1 and (C, D) P2 on silicon wafers. The photographs were
taken under (A, C) normal light and (B, D) UV illuminations.
work, our fluorescent polymers can act as electron donors. The
interaction of the PA quenchers with the fluorescent polymers
will lead to the formation of nonemissive ground-state dark
complexes. The unbound polymers, however, will exhibit their
natural lifetimes. On the contrary, in the diffusion-controlled
dynamic quenching model, the PL lifetime is shortened with
increasing the quencher concentration. To validate which
quenching model is involved in the present system, the effect
of PA on the lifetime of P1 and P2 is investigated. As depicted in
Figure S8 and Table S1 of the Supporting Information, the life-
times of the polymers remained almost unchanged in response to
the presence of PA, suggesting that the PL quenching process
operated through the static mechanism.
P2 THF:
I0
I
= 2.41e4764[PA] − 1.39
(5)
(6)
(7)
THF/water (5/5, v/v):
I0
I
= 3.07e6369[PA] − 2.07
THF/water (1/9, v/v):
I0
I
= 3.17e9020[PA] − 2.20
Generalization from eqs 2−7 gave eq 8, which was similar to eq 1
but with two extra constants (A and B).
In the static mechanism, the PL annihilation can be described
by eq 1.
I0
I
= Aek[PA] + B
I0
I
= eV [PA]
q
(8)
(1)
If these two terms are neglected, the static quenching constants
(k) for the solution and aggregated systems of P1 and P2 were
determined and are summarized in Table S2. In 90% aqueous mix-
tures, the k values for P1 and P2 were 10 089 and 9020 L mol−1,
respectively. These values were much higher than those
(1−185 L mol−1) of iptycene-containing poly(p-phenyl-
enebutadiynylene)s and poly(p-phenyleneethynylene)s, which
were widely studied fluorescent conjugated polymers for explosive
detection.20 At [PA] → 0, eq 8 can be transformed to eq 9, which
can be rearranged to give eq 10 with K = Ak and C = A + B = 1.
where I0 and I are the PL intensities without and with PA, respec-
tively, and Vq is the static quenching constant. Mathematical
fitting of the Stern−Volmer plots in Figure 9D and Figure S7D
gave eqs 2−7 for P1 and P2 in pure THF and THF/H2O
mixtures with 50 and 90% water fractions, respectively:
P1 THF:
I0
I
= 2.20e5513[PA] − 1.20
(2)
THF/water (5/5, v/v):
⎛
⎞
I0
I
1
2
kn[PA]n
n!
= A 1 + k[PA] + k2[PA]2 + ...+
+ ... + B
I0
I
= 3.87e6539[PA] − 2.99
⎜
⎟
⎝
⎠
(3)
(9)
THF/water (1/9, v/v):
I0
I
= A(1 + k[PA]) + B = Ak[PA] + A + B = K[PA] + C
I0
I
= 3.06e10089[PA] − 2.15
(4)
(10)
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Thus, the static quenching constant (K) of P1 in 90% aqueous
mixture at low PA concentration was 30 872 L mol−1, which
was 2.5 times larger than the value in pure THF solution
(12 129 L mol−1). The K value for P2 in 90% aqueous mixture was
also reasonably high (28 593 L mol−1), being also 2.5-fold larger
than that in THF (11 481 L mol−1). All these values were much
higher than those of chemosensors based on polymetalloles,
whose K values fell in the range from 6710 to 11 000 L mol−1.21
The higher quenching constants in aqueous mixtures revealed a
amplification effect in the nanoaggregated system. Such an effect is
presumably due to the AEE characteristics of the polymers and/or
the presence of many internal voids for capturing PA molecules
and more migration channels available to excitons in the polymer
aggregates. These factors work cooperatively to endow the system
with sensitiveresponse toPA. Conjugated polymersP1 and P2 are
good promising materials for the construction of fluorescent
chemosensors with amplified responses and extremely low
detection limits.
Fluorescent Photopatterning. Light-driven techniques
are more convenient due to their simple operation and precise
control on the curing reaction and patterning process. On
the other hand, creation of fluorescent patterns is important to
the construction of photonic and electronic devices and the
development of biological sensing and probing systems. Both P1
and P2 possess good processability and can form thin films
by spin-coating of their solutions. They are also emissive in the
aggregated state. These enable the fabrication of fluorescent
photopattern by photolithography process. Indeed, when the
thin films of P1 and P2 were exposed to UV light through copper
masks in air for 10 min, the exposed parts were photo-oxidized
and their emission was quenched, while the masked regions
(emissive squares) remained emissive. Without going through
the development process, two-dimensional fluorescent patterns
were thus generated (Figure 10). The nonirradiated parts
of the patterned film of P1 emitted blue light, whose intensity
was so strong that it appeared white under UV illumination
(Figure 10B).
properties. Figure 11 shows the optical limiting performance
of P1 and P2 in chloroform at similar linear transmittance
(T = ∼30%). The transmitted fluence of the polymers increased
initially with increasing the incident fluence but started to deviate
from linearity at a value of ∼1 J/cm2. When the incident fluence
was further strengthened, the transmittedfluencereacheda plateau
and saturated at 0.4 J/cm2. Clearly, both P1 and P2 can limit laser
pulses. Together with their novel properties described before, they
are expected to find many high-technological applications.
CONCLUSIONS
■
In this work, disubstituted acetylenes with directly linked TPE
pendants were synthesized and polymerized by WCl6−Ph4Sn in
toluene at elevated temperatures, producing polymers P1 and
P2. The polymers possessed good solubility and showed high
thermal stability. Both polymers were weak emitters in the
solution state. Whereas the PL of P1 was enhanced by aggregate
formation, that of P2 was quenched. The polymers could detect
explosives sensitively with an amplification effect. They were
sensitive to UV irradiation and were promising materials for
fabricating fluorescent photopatterns. The polymers exhibited
optical nonlinearity and could limit laser pulses.
ASSOCIATED CONTENT
■
S
* Supporting Information
1H and 13C NMR spectra of 1, 2, and P2, PL spectra of P2, PL
decay curves, fluorescence decay parameters, and quenching
constants for explosive detection. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*(B.Z.T.) E-mail tangbenz@ust.hk; Ph +852-2358-7375 (8801);
Fax +852-2358-1594.
■
Notes
The authors declare no competing financial interest.
Optical Limiting. Previous studies found that polymers
constructed from aromatic acetylenic monomers can limit optical
power.22 P1 and P2 contain numerous aromatic rings, and
they are thus anticipated to exhibit interesting optical limiting
ACKNOWLEDGMENTS
■
This work has been partially supported by the National Basic
Research Program of China (973 Program; 2013CB834701),
the National Science Foundation of China (21490570 and
21490574), the Research Grants Council of Hong Kong
(604913, 604711, 602212 and N_HKUST620/11), 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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