Regioselective alkyne polyhydrosilylation: Synthesis and photonic properties of poly(silylenevinylene)s

Article abstract of DOI:10.1021/ma201203w

Alkyne polyhydrosilylations of 1,2-bis(4-dimethylsilanylphenyl)-1,2- diphenylethene with 4,4′-diethynylbiphenyl, bis(4-ethynyl-2-methylphenyl) dimethylsilane, 3,6-diethynyl-9-heptylcarbazole, 1,1-dimethyl-2,5-bis(4- ethynylphenyl)-3,4-diphenylsilole, and

Full text of DOI:10.1021/ma201203w

ARTICLE
pubs.acs.org/Macromolecules
Regioselective Alkyne Polyhydrosilylation: Synthesis and Photonic
Properties of Poly(silylenevinylene)s
Ping Lu,†,‡,§ Jacky W. Y. Lam,†,‡ Jianzhao Liu,†,‡ Cathy K. W. Jim,†,‡ Wangzhang Yuan,†,‡ Carrie Y. K. Chan,†,‡
Ni Xie,†,‡ Qin Hu,†,‡ Kevin K. L. Cheuk,*,|| and Ben Zhong Tang*,†,‡,^
†Fok Ying Tung Research Institute, The Hong Kong University of Science & Technology, Nansha, Guangzhou, China
‡Department of Chemistry, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China
§State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, China
The Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Hunghom, Kowloon, Hong Kong, China
^Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China
S Supporting Information
b
ABSTRACT: Alkyne polyhydrosilylations of 1,2-bis(4-dimethylsilanyl-
phenyl)-1,2-diphenylethene with 4,40-diethynylbiphenyl, bis(4-ethy-
nyl-2-methylphenyl)dimethylsilane, 3,6-diethynyl-9-heptylcarbazole,
1,1-dimethyl-2,5-bis(4-ethynylphenyl)-3,4-diphenylsilole, and 2,5-bis-
(2-trimethylsilylethynyl)thiophene were mediated by Rh(PPh3)3Cl in
THF in an regioselective fashion, furnishing poly(silylenevinylene)s
with high molecular weights (Mw up to 36 500) and stereoregularities (E
content up to 100%) in satisfactory yields. All the polymers were
processable and thermally stable, losing little of their weights when
heated to g330 ꢀC. Whereas the polymers were weakly emissive in the
solutions, they became strong emitters when aggregated in poor
solvents or fabricated as thin films in the solid state, demonstrating a
phenomenon of aggregation-enhanced emission. The emissions of the
polymers were quenched exponentially by picric acid with quenching constant up to 8.48 ꢁ 105 L molꢀ1, making them as highly
sensitive chemosensors for explosive detection. Thin films of the polymers exhibited high refractive indices (RI = 1.7180ꢀ1.6102) in
the wavelength region of 400ꢀ1700 nm, high modified Abbꢀe numbers (vD0 up to 2303.9), and low optical dispersion (D0 down to
0.0004). Their RI values could be tuned to a large extent (Δn = 0.09), and their emissions could be faded by UV irradiation, enabling
ready generation of fluorescent patterns without development.
’ INTRODUCTION
compounds, thus providing an important synthetic tool for the
synthesis of organosilicon compounds. In 1957, Speier devel-
oped transition-metal-catalyzed hydrosilylation, which pro-
moted a widespread interest in the synthesis of polycarbo-
silanes as functional materials.5
Hydrosilylations of alkynes and silanes with two or more triple
bonds and silicon hydride groups will generate poly(silylene-
vinylene)s with linear and hyperbranched architectures. The
silicon-containing polymers may exhibit unique properties due
to the σ*ꢀπ* conjugation between the σ orbitals of the silicon
atoms and the π orbitals of the double bonds along the polymer
backbones.6 The polymers may be electrically conductive, light
emissive, and photo-cross-linkable, enabling them to find high-
technological applications in electronics, optics, and photolitho-
graphy.7 Such possibilities, although attractive, are less well studied.
Polyacetylene is the archetypal conjugated polymer and the
seminal discovery of the metallic conductivity of its doped form
has triggered a huge surge of interest in utilizing alkynes as
building blocks to construct functional polymers.1 Thanks to the
enthusiastic efforts of synthetic polymer chemists, a large number
of acetylenic polymers have been prepared.2 In most of these
polymers, their repeating units are strung together by carbonꢀ
carbon linkages. Polymerizations of acetylene monomers via
carbonꢀheteroatom hookups are of interest because the resul-
tant heteroatom-containing acetylenic polymers may show un-
ique properties, which are inaccessible by the polymers with pure
carbon-based skeletons.
Silanes can be oxidatively added to unsaturated hydrocarbons.3
This reaction is termed “hydrosilylation” and was first reported
by Sommer in 1947: admixing trichlorosilane and 1-octene in
the presence of dimethyl peroxide resulted in the formation of
n-octyltrichlorosilane in excellent yield.4 This reaction is ap-
plicable to a wide variety of silicon hydrides and unsaturated
Received: May 27, 2011
Revised:
June 30, 2011
Published: July 15, 2011
r
2011 American Chemical Society
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Scheme 1
Using Wilkinson’s catalyst Rh(PPh3)3Cl, Luh and co-workers
had recently prepared some poly(silylenevinylene)s by hydro-
silylations of diynes and disilanes.8 While the intrastrand
chromophoric interactions in the polymers had been investi-
gated, little efforts had been placed on the exploration of their
practical applications. All the diynes used possessed terminal
triple bonds at both ends. In contrast, those with internal
acetylene functionalities were virtually not utilized as mono-
mers for the polymerization.
refractive indices in the wavelength region of 400ꢀ1700 nm,
which could be tuned to a great extent by simple UV irradiation.
’ EXPERIMENTAL SECTION
Materials and Instrumentation. Tetrahydrofuran (THF) was
distilled under normal pressure from sodium benzophenone ketyl under
nitrogen immediately prior to use. Triethylamine was distilled and dried
over potassium hydroxide. Other solvents of high purities were used
without further purification. Bis(4-iodo-2-methylphenyl)dimethylsilane
(7), trimethylsilylacetylene (8), 2,5-diiodothiophene (10), 4-bromo-
benzophenone (11), zinc dust, titanium(IV) chloride, chlorodimethyl-
silane, copper(I) iodide, triphenylphosphine, and dichlorobis(triphenyl-
phosphine)palladium(II) were purchased from Aldrich and used as
received. Chlorotris(triphenylphosphine)rhodium(I) was synthesized
following the literature method.12 Diyne monomers, named 4,40-diethy-
nylbiphenyl (1),13 3,6-diethynyl-9-heptylcarbazole (3),14 and 1,1-di-
methyl-2,5-bis(4-ethynylphenyl)-3,4-diphenylsilole (4),15 were synthe-
sized according to our previous published procedures.
IR spectra were recorded on a Perkin-Elmer 16 PC FT-IR spectro-
photometer. 1H and 13C NMR spectra were measured on a Bruker ARX
300 spectrometer using chloroform-d as solvent and tetramethylsilane
(TMS; δ = 0 ppm) as internal standard. Mass spectra (MS) were
recorded on a GCT Premier CAB048 mass spectrometer operating in a
TOF mode. UVꢀvis absorption spectra were measured on a Varian
CARY 100 Bio spectrophotometer. Thermogravimetric analysis (TGA)
of the polymers was evaluated on a TA TGA Q5000 instrument under
nitrogen at a heating rate of 20 ꢀC/min. Photoluminescence (PL)
spectra were recorded on a Perkin-Elmer LS 55 spectrofluorometer. The
PL quantum yields (ΦF’s) were estimated using quinine sulfate (ΦF =
54% in 0.1 M sulfuric acid) as reference in THF. Weight- (Mw) and
number-average (Mn) molecular weights and polydispersity indices
(Mw/Mn) of the polymers were estimated by a Waters Associates gel
permeation chromatography (GPC) system equipped with refractive
index and UV detectors. THF was used as eluent at a flow rate of 1.0 mL/
min. A set of monodisperse polystyrene standards covering molecular
weight range of 103ꢀ107 was used for the molecular weight calibration.
Our group had been working on the development of con-
jugated polymers using acetylenes as building blocks. Employing
monoynes, diynes, and triynes as monomers, we had successfully
synthesized a large variety of polyacetylenes, polyarylenes, poly-
diynes, and polytriazoles with linear and hyperbranched archi-
tectures by metathesis, cyclotrimerization, coupling, and click
polymerizations.9 Recently, we had developed a thiol-click
polymerization technique for the synthesis of heteroatom-con-
taining acetylenic polymers.10 The alkyne polyhydrothiolations
of dithiols and bipropiolates proceeded smoothly in the presence
of diphenylamine or rhodium complexes, producing sulfur-rich
polymers with high molecular weights in high yields. Encouraged
by such success, in this study, we widened the applicability of
alkyne hydrosilylation to the construction of functional poly-
(silylenevinylene)s. We utilized tetraphenylethene (TPE)-con-
taining disilane and terminal and internal diynes as monomers
and succeeded in polymerizing them into high molecular weight
polymers in satisfactory yields (Scheme 1). All the poly-
(silylenevinylene)s were soluble and film-forming. Thanks to
the TPE unit, which exhibited a phenomenon of aggregation-
induced emission,11 all the polymers emitted intensely in the
solid state though their solutions were only weakly emissive.
Such effect had endowed the polymer nanoaggregates with a
superamplification effect in the explosive detection process. UV
irradiation of the thin films of the poly(silylenevinylene)s
through copper masks photo-oxidized the exposed regions,
generating fluorescent patterns. The polymers showed high
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Scheme 2
Refractive indices (RI’s) of the polymers were measured on a J.A.
Woollam variable angle ellipometry system with a wavelength tunability
from 300 to 1700 nm. To fit the acquired Ψ and Δ curves with the data
obtained from the three-layer optical model consisting of crystalline
silicon substrate, 2 nm SiO2 layer, and a uniform polymer film, the
LevenbergꢀMarquardt regression algorithm was employed. The Cau-
chy dispersion law was applied to describe the polymer layer from the
visible to IR spectral region.
linked with CH3), 138.76 [aromatic carbons linked with Si(CH3)2],
135.35 (aromatic carbons ortho to CH3), 133.88 (aromatic carbons
meta to CH3), 129.36 (aromatic carbons para to CH3), 123.71 (aromatic
carbons linked with CtCH), 84.38 (tCꢀAr), 78.32 (tCH), 23.36,
ꢀ0.45 [Si(CH3)2]. MS (TOF): m/z 289.14 [(M + 1)+, calcd 289.13].
λmax/εmax: 259 nm/1.64 ꢁ 105 molꢀ1 L cmꢀ1
.
2,5-Bis(2-trimethylsilylethynyl)thiophene (5). This compound was
synthesized from 2.2 g (8.1 mmol) of 10, 28.1 mg (0.04 mmol) of
Pd(PPh3)2Cl2, 19 mg (0.1 mmol) of CuI, 26 mg (0.1 mmol) of PPh3,
and 2.7 mL (1.9 g, 19.2 mmol) of 8 in dry THF/Et3N mixture (1:1 v/v).
The procedure was similar to that for the synthesis of 9. Brown powder;
yield 95.3%. IR (KBr), υ (cmꢀ1): 2942, 2901, 2141 (CtC stretching),
1502, 1438, 1406, 1252, 1164 (SiꢀCH3 bending), 848 (SiꢀC
Monomer Synthesis. Diyne 1, 3, and 4 were synthesized accord-
ing to our previous published procedures, whereas compounds 2, 5, and
6 were prepared according to the synthetic routes shown in Scheme 2.
Typical experimental procedures for their syntheses are shown below.
Preparation of Bis(4-ethynyl-2-methylphenyl)dimethylsilane (2).
To a 500 mL round-bottom flask equipped with a septum and a stirring
bar were added 2.3 g (8.0 mmol) of 7, 28.1 mg (0.04 mmol) of
Pd(PPh3)2Cl2, 19 mg (0.1 mmol) of CuI, and 26 mg (0.1 mmol) of
PPh3. Dry Et3N (100 mL) and THF (100 mL) and 2.7 mL (1.9 g,
19.2 mmol) of 8 were then injected. After stirring for 12 h, the mixture
was filtered and the precipitates were washed with diethyl ether. The
filtrates were collected, and after solvent evaporation under reduced
pressure, the crude product was purified by silica gel column chroma-
tography using hexane/chloroform (1:1 v/v) as eluent. Compound 9
was then placed into a 500 mL round-bottom flask followed by addition
of 200 mL of methanol and 1 g of KOH. After heating at 50 ꢀC for 3 h,
the solution was concentrated and poured into 100 mL of 1 M HCl
solution. The solution was extracted with chloroform four times. The
organic layers were collected, dried over magnesium sulfate, and
concentrated by a rotary evaporator. The obtained product was purified
by silica gel column chromatography using hexane/chloroform (3:1 v/v)
as eluent to give 2 as white solid in 86% yield (2.02 g). IR (KBr), υ
(cmꢀ1): 3279 (tCꢀH stretching), 3049, 2955, 2955, 2108 (CtC
stretching), 1905, 1783, 1589, 1533, 1446, 1382, 1252 (SiꢀCH3
1
stretching). H NMR (300 MHz, CDCl3), δ (TMS, ppm): 6.95 (s,
2H, thiophene protons), 0.08 [s, 18H, Si(CH3)3]. 13C NMR (300 MHz,
CDCl3), δ (ppm): 133.0 (thiophene carbons at 3 and 4 positions), 125.2
(thiophene carbons at 2 and 5 positions), 100.6 (ꢂCꢀAr), 97.6
[ꢂCꢀSi(CH3)3]. MS (TOF): m/z 276.08 (M+, calcd 276.08]. λmax
/
εmax: 316 nm/2.6 ꢁ 105 molꢀ1 L cmꢀ1
.
1,2-Bis(4-dimethylsilanylphenyl)-1,2-diphenylethene (6). 4-Bromo-
benzophenone (11, 5 g, 19.2 mmol) and zinc dust (2.5 g, 38.2 mmol)
were placed into a 250 mL two-necked round-bottom flask equipped
with a reflux condenser. After evacuated under vacuum and flushed with
dry nitrogen three times, 100 mL of THF was added. The mixture was
cooled to ꢀ5 to 0 ꢀC, and 2.1 mL (19.2 mmol) of TiCl4 was added
slowly using a syringe. The mixture was slowly warmed to room
temperature, and after stirring for 0.5 h, it was refluxed for 12 h.
Compound 12 was isolated in 76% yield (3.6 g) after purification by
silica gel column chromatography using hexane as eluent. Into another
round-bottom flask was added dropwise 7.6 mL (12 mmol) of
n-butyllithium (1.6 M in hexane) to a solution of 12 (1.5 g, 3 mmol)
in THF at ꢀ78 ꢀC. After being stirred for 8 h, chlorodimethylsilane
(1.32 mL, 12 mmol) was then injected slowly to the solution
at ꢀ78 ꢀC. The solution was warmed gradually to room temperature
and stirred overnight. The reaction was terminated by adding aqueous
sodium hydrogen carbonate solution. The organic layer was extracted
with petroleum ether and dried over anhydrous sodium sulfate. After
1
bending), 1130 (SiꢀPh stretching), 1064, 807 (SiꢀC stretching). H
NMR (300 MHz, CDCl3), δ (TMS, ppm): 7.45 (s, 2H, ArꢀH ortho to
CH3), 7.31 (d, 2H, ArꢀH para to CH3), 7.25 (s, 2H, ArꢀH meta to
CH3), 3.08 (s, 2H, tCH), 2.14 (s, 6H, CH3), 0.59 [s, 6H, Si(CH3)2].
13C NMR (300 MHz, CDCl3), δ (ppm): 144.51 (aromatic carbons
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solvent evaporation under reduced pressure, the residue was purified by
silica gel column chromatography using hexane as eluent. Yield 45%
(0.62 g). IR (KBr), υ (cmꢀ1): 3055, 3006, 2958, 2109, 1593, 1494, 1438,
1382, 1252 (SiꢀCH3 bending), 1115 (SiꢀPh stretching), 872 (SiꢀC
Table 1. Synthesis of Poly(silylenevinylene)sa
b
b
no.
polymer
yield (%)
Mw
Mw/Mn
1
2
3
4
5
P1/6
P2/6
P3/6
P4/6
P5/6
71.2
66.9
68.2
78.1
55.9
36 500
25 000
20 600
11 300
14 900
3.03
2.52
2.29
2.34
2.39
1
stretching), 767, 693. H NMR (300 MHz, CDCl3), δ (TMS, ppm):
7.10 (d, 4H, ArꢀH), 7.02 (m, 6H, ArꢀH), 6.98 (m, 8H, ArꢀH), 4.31
(q, 2H, SiꢀH), 0.28 [d, 12H, Si(CH3)2]. 13C NMR (75 MHz, CDCl3),
δ (ppm): 145.21, 144.3, 141.69, 135.9, 133.9, 132.0, 131.4, 128.4, 127.1,
ꢀ3.15. MS (TOF): m/z 448.10 (M+, calcd 448.75). λmax/εmax: 317 nm/
a Carried out in refluxed THF under nitrogen using Rh(PPh3)3Cl as
catalyst. [1ꢀ5] = [6] = 0.05 M, [cat.] = 0.001 M. b Determined by GPC
in THF on the basis of a polystyrene calibration.
1.90 ꢁ 105 molꢀ1 L cmꢀ1
.
Polymerization. All the polymerization reactions and manipula-
tions were carried out under nitrogen, except for the purification of the
polymers, which was done in open air. A typical experimental procedure
for the polymerization of diyne 1 with disilane 6 is given below as an
example. Into a baked 50 mL flask was added 1 (70.7 mg, 0.245 mmol), 6
(110 mg, 0.245 mmol), and 3.8 mg of Rh(PPh3)Cl. The flask was
evacuated under vacuum for half an hour and then flushed with nitrogen.
Freshly distilled THF was injected into the tube to dissolve the
monomers. After reflux under nitrogen for 24 h, the solution was cooled
to room temperature, diluted with 3 mL of chloroform, and added
dropwise to 150 mL of methanol through a cotton filter under stirring.
The precipitate was allowed to stand overnight and filtered with a Gooch
crucible. The polymer was washed with methanol and dried in a vacuum
oven at 40 ꢀC to a constant weight. Yield 71.2%. Mw 36 500; Mw/Mn
3.03 (GPC, polystyrene calibration). IR (KBr), υ (cmꢀ1): 3051, 2958,
2924, 2856, 1725, 1596, 1492, 1442, 1389, 1249 (SiꢀCH3 bending),
1107, 987 (SiꢀPh stretching), 817, (SiꢀC stretching), 774. 1H NMR
(300 MHz, CDCl3), δ (TMS, ppm): 7.59, 7.27, and 7.10 (ArꢀH), 6.87
(E-ArCH=), 6.57 (E-SiCH=), 6.17 (Z-ArCH=), 5.98 (Z-SiCH=), 0.38
[Si(CH3)2]. 13C NMR (300 MHz, CDCl3), δ (TMS, ppm): 144.63,
143.77, 141.16, 140.32, 137.43, 136.27, 133.29, 131.47, 130.74, 127.75,
1252 (SiꢀCH3 bending), 1179 (SiꢀPh stretching), 848, (SiꢀC
1
stretching), 759. H NMR (300 MHz, CDCl3), δ (TMS, ppm): 7.04
and 6.90 (ArꢀH), 6.45 (E olefin proton), 6.24 (Z olefin proton), 0.39
(SiCH3). 13C NMR (300 MHz, CDCl3), δ (TMS, ppm): 147.56,
145.74, 144.90, 144.24, 141.82, 138.26, 134.10, 132.06, 131.28,
128.33, 127.07, 2.36, ꢀ0.26. λmax/εmax: 325 nm/4.2 ꢁ 105 molꢀ1
L cmꢀ1
.
Fluorescent Photopatterning and RI Tuning. Photooxidation
and RI tuning of the polymer films were conducted in air or nitrogen at
room temperature using 365 nm light obtained from a Spectroline ENF-
280C/F UV lamp (diameter = 5 cm) at a distance of 1 cm as light source.
The intensity of the incident light was ∼18.5 mW/cm2. The films were
prepared by spin-coating the polymer solutions (5 mg/mL in 1,2-
dichloroethane) at 1000 rpm for 1 min on silicon wafers. The films
were dried in vacuum at 40 ꢀC for 1 h. The photopatterns were
generated on silicon wafers using Cu-negative masks. After UV exposure
for 5 min, the images of the resultant patterns were taken on an Olympus
BX41 fluorescence optical microscope.
127.05, 126.53, ꢀ2.41. λmax/εmax: 318 nm/4.90 ꢁ 105 molꢀ1 L cmꢀ1
.
Other polymers were prepared by a similar procedure and their
characterization data are given below.
’ RESULTS AND DISCUSSION
P2/6: Yield 66.9%. Mw 25 000; Mw/Mn 2.52 (GPC, polystyrene
calibration). IR (KBr), υ (cmꢀ1): 3063, 2958, 1592, 1446, 1382, 1252
(SiꢀCH3 bending), 1107 (SiꢀPh stretching), 808, (SiꢀC stretching),
693. 1H NMR (300 MHz, CDCl3), δ (TMS, ppm): 7.47, 7.25, and 7.08
(ArꢀH), 6.94 (E-ArCH=), 6.70 (E-SiCH=), 6.07 (Z olefin proton),
0.58, 0.41. 13C NMR (300 MHz, CDCl3), δ (TMS, ppm): 145.73,
145.07, 144.76, 144.00, 141.73, 139.71, 137.95, 136.87, 135.91, 133.87,
Monomer Synthesis. To explore the alkyne hydrosilylation, a
versatile tool for the synthesis of poly(silylenevinylene)s with
novel materials properties, we designed the molecular structures
of a group of functional diynes (1ꢀ5) and disilane (6) as shown
in Scheme 1. While some monomers (1, 3, and 4) were prepared
according to our previous published papers,13ꢀ15 diyne 2 and 5
and disilane 6 were synthesized by the procedures given in
Scheme 2. The Sonogashira coupling of 7 and 8 was catalyzed by
Pd(PPh3)2Cl2 and CuI in THF/Et3N mixture, which gave
compound 9 that converted into product 2 by base-catalyzed
hydrolysis followed by acidification. Utilizing the same proce-
dure, the Pd-catalyzed reaction of 10 with 8 furnished 5.
McMurry coupling of 11 in the presence of TiCl4 and Zn dust
produced 12, whose lithiation with n-butyllithium followed by
reaction with chlorodimethylsilane gave 6. All the monomers
were characterized by standard spectroscopic methods, from
which satisfactory data corresponding to their molecular struc-
tures were obtained.
Polymerization. After obtaining the monomers, we then tried
to polymerize them by Rh(PPh3)3Cl, a commonly used catalyst
for alkyne hydrosilylation.8 Reaction of 1 with 6 in the presence
of Rh(PPh3)3Cl in refluxed THF gave a high molecular weight
polymer in a reasonably high yield (Table 1, no. 1). Under the
same conditions, 6 reacted respectively with 2ꢀ5, producing P2/
6ꢀP5/6 with Ms’s from 25 000 to 14 900 in 56ꢀ78% yields.
Analysis of their filtrates by GPC showed that they were
unreacted monomers and oligomeric species with molecular
weights of several thousands.
132.04, 131.34, 128.52, 125.69, 123.93, ꢀ0.22, ꢀ0.57, ꢀ1.82. λmax/εmax
:
323 nm/1.87 ꢁ 105 molꢀ1 L cmꢀ1
.
P3/6: Yield 68.2%. Mw 20 600; Mw/Mn 2.29 (GPC, polystyrene
calibration). IR (KBr), υ (cmꢀ1): 3063, 3014, 2958, 2862, 1600, 1487,
1389, 1252 (SiꢀCH3 bending), 1107 (SiꢀPh stretching), 816, (SiꢀC
1
stretching), 703. H NMR (300 MHz, CDCl3), δ (TMS, ppm): 8.25,
7.59, and 7.35 (ArꢀH), 7.07 (ArꢀH and E-ArCH=), 6.71 (E-SiCH=),
4.26, 1.85, 1.32, 0.86, 0.38. 13C NMR (300 MHz, CDCl3), δ (TMS,
ppm): 144.50, 145.00, 144.45, 141.76, 137.40, 135.91, 133.93, 132.09,
131.33, 128.34, 127.10, 125.74, 125.32, 124.52, 123.91, 119.40, 109.49,
43.98, 32.38, 30.40, 27.89, 27.10, 23.25, 14.73, ꢀ2.36. λmax/εmax
:
304 nm/5.1 ꢁ 105 molꢀ1 L cmꢀ1
.
P4/6: Yield 78.1%. Mw 11 300; Mw/Mn 2.34 (GPC, polystyrene
calibration). IR (KBr), υ (cmꢀ1): 3063, 3021, 2958, 1600, 1494, 1446,
1252 (SiꢀCH3 bending), 1108 (SiꢀPh stretching), 823 (SiꢀC
1
stretching), 693. H NMR (300 MHz, CDCl3), δ (TMS, ppm): 7.02
(ArꢀH), 6.43 (E olefin proton), 5.90 (Z olefin proton), 0.36 (SiCH3).
13C NMR (300 MHz, CDCl3), δ (TMS, ppm): 145.55, 145.02, 144.35,
141.74, 139.67, 137.95, 136.96, 133.86, 133.11, 132.05, 131.33, 130.66,
130.09, 128.80, 129.00, 128.33, 127.12, ꢀ1.84, ꢀ3.10. λmax/εmax
:
310 nm/4.16 ꢁ 104 molꢀ1 L cmꢀ1
.
P5/6: Yield 55.9%. Mw 14 900; Mw/Mn 2.39 (GPC, polystyrene
calibration). IR (KBr), υ (cmꢀ1): 2958, 2901, 2149, 1511, 1442, 1414,
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Figure 1. IR spectra of (A) 1, (B) 6, and (C) its polymer P1/6.
Figure 3. 13C NMR spectra of (A) 1, (B) 6, and (C) its polymer P1/6 in
chloroform-d.
Figure 2. 1H NMR spectra of (A) 1, (B) 6, and (C) its polymer P1/6 in
chloroform-d.
Structural Characterization. The polymeric products were
characterized by IR and NMR spectroscopies, and all gave
satisfactory analysis data corresponding to their expected molec-
ular structures. An IR spectrum of P1/6 is shown in Figure 1 as an
example; for comparison, the spectra of its monomer 1 and 6 are
also provided in the same figure. The ꢂCꢀH and CtC
stretching vibrations of 1 occur at 3274 and 2100 cmꢀ1, respec-
tively, which are not observed in P1/6. The spectrum of P1/6
Figure 4. TGA thermograms of P1/6ꢀP5/6 recorded under nitrogen
at a heating rate of 20 ꢀC/min.
E-vinylene structure are observed at δ 6.57 and 6.87. The
associated absorptions of the Z-vinylene protons are observed
at 6.17 and 5.98, but their intensities are much weaker. Using
their integrals, an E/Z ratio of 91/9 is calculated. No unexpected
peaks are found, and all the peaks can be readily assigned. Thus,
the polymeric product is indeed P1/6 with a molecular structure
as shown in Scheme 1. Similar observations were found in the
spectra of P2/6ꢀP5/6, and their E contents are calculated to be
77.1, 100, 75, and 50%, respectively.
also displays no SiꢀH stretching vibration of 6 at 2117 cmꢀ1
.
Meanwhile, the peak at 1600 cmꢀ1 associated with CdC
stretching vibration becomes broader and stronger after the
polymerization, suggesting that the triple bonds of 1 and the
dimethylsilyl groups of 6 have been transformed into vinyl
silylene units in P1/6.
The 13C NMR spectra of P1/6, 1, and 6 are shown in Figure 3.
The acetylene carbon atoms of 1 resonate at δ 84.07 and 78.8,
which are absent in the spectrum of P1/6. The transformation of
the dimethylsilyl group of 6 to vinyl silylene units in P1/6 by the
polymerization has downfield shifted the methyl carbon reso-
nance, which is now observed at δ ꢀ2.41. The absorptions of the
Similar results were obtained from the NMR analyses. Figure 2
shows the 1H NMR spectra of P1/6 and its monomer 1 and 6.
The spectrum of P1/6 shows no acetylene and SiꢀH proton
resonances of 1 and 6 at δ 3.9 and 4.7, respectively. On the other
hand, new peaks assigned to the proton absorptions of the linear
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olefin carbons of the vinylene structures of P1/6 are hard to
distinguish, presumably due to their overlapping with those of
the aromatic carbons.
Thermal Stability. The thermal stability of the polymers was
evaluated by TGA analysis. As shown in Figure 4, all the polymers
are thermally stable, losing 5% of their weights (Td) at
330ꢀ378 ꢀC (Table 1). The high Td values of the polymers
are understandable because they are constructed from aromatic
rings, which possess a high resistance to thermolysis.
Absorption and Emission. Figure 5 shows the UV spectra of
the poly(silylenevinylene)s in their dilute solutions in THF. The
polymers exhibit absorption peaks inherent to their monomers.
Their maxima are, however, located at longer wavelengths. For
example, P2/6 absorbs at 270 and 330 nm, which are red-shifted
from those of 2 and 6 by 11 and 13 nm, respectively. This
indicates a more extended electronic conjugation in the polymer
system, thanks to the electronic communication between the
TPE and the aromatic chromophoric units.
in the solid state. The photoluminescence (PL) spectra of P1/6
in THF and THF/water mixture are given in Figure 6 as an
example. In THF, P1/6 emits faintly at 452 nm. Addition of
water into its THF solution has enhanced its PL accompanying
with a red shift in the peak maximum. The higher the water
content, the stronger is the light emission. From the molecular
solution in THF to the aggregate state in 90% aqueous solution,
the emission intensity of P1/6 has increased by 15-fold
(Figure 6B). Since P1/6 is not soluble in water, its chains thus
must have aggregated in THF/water mixtures with high water
contents. Clearly, the PL of P1/6 is enhanced by aggregate
formation, demonstrating a phenomenon of aggregation-en-
hanced emission (AEE). The AEE effect was not an isolated
case observed only in P1/6 but was also found in other polymers.
As shown in Figure S1ꢀS4, all the polymers are AEE-active
although their emissions vary with their molecular structures.
While the ΦF values of the polymers in the solution state were
low in absolute terms (ΦF = 0.24ꢀ3.17%, Table 2), in relative
All the poly(silylenevinylene)s were weakly emissive when
molecularly dissolved in the solutions but became strong emit-
ters when aggregated in poor solvents or fabricated as thin films
terms most of them were higher than that of TPE (ΦF
=
0.24%).16 In the solution of TPE, the active intramolecular
rotations of its phenyl rings nonradiatively deactivate its excitons,
making it weakly luminescent in the solution.17 The TPE units in
poly(silylenevinylene)s are, however, knitted by covalent bond,
which makes the phenyl blades less free to undergo intramole-
cular rotations, and as a result the polymers are somewhat
luminescent in the solutions. The solid thin films of the polymers
Table 2. Thermal and Optical Properties of
Poly(silylenevinylene)sa
polymer
Td (ꢀC)
λabs
λem
Φsoln
Φfilm
P1/6
P2/6
P3/6
P4/6
P5/6
330
366
372
377
378
318
323
304
310
325
452
450
450
502
495
3.17
0.67
0.79
1.00
0.24
31.0
23.6
24.1
22.1
13.1
a Abbreviation: Td = temperature for 5% weight loss, λabs = absorption
maximum in THF, λem = emission maximum in THF, Φsoln = quantum
yield in THF using quinine sulfate as standard (Φsoln = 54% in 0.1 M
sulphuric acid), Φfilm = quantum yield of the solid film measured by an
calibrated integrating sphere.
Figure 5. Absorption spectra of THF solutions of P1/6ꢀP5/6. Con-
centration: 10ꢀ5 M.
Figure 6. (A) PL spectra of P1/6 in THF and THF/water mixtures with different water fractions. (B) Change in the relative PL intensity (I/I0) with the
composition of the THF/water mixture. I0 = PL intensity in pure THF solution. Concentration: 10ꢀ5 M; excitation wavelength: 340 nm.
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Figure 7. (A) Fluorescence quenching of P1/6 in (A) THF and (B) THF/water mixture (1:9 v/v) by picric acid (PA). Concentration of P1/6: 10ꢀ5 M;
excitation wavelength: 345 nm. (C, D) SternꢀVolmer plots for fluorescence quenching of P1/6 by PA in (C) THF and (D) THF/water mixture (1:9 v/v).
I0 = PL intensity in the absence of PA in THF or THF/water mixture (1:9 v/v).
emitted at wavelengths similar to those in the solutions with
higher ΦF values (13ꢀ31%), duly verifying their AEE features.
Explosive Detection. Fluorescent probes for explosive detec-
tion are in great demand because the threat from the increased
use of explosives in terrorism acts has become a global concern.18
Promoted by the AEE characteristics of P1/6ꢀP5/6, we ex-
plored their utility as chemosensors for sensitive detection of
explosives. The nanoaggregates of the polymers in THF/water
mixtures with 90% water contents were utilized as probes. For
comparison, the detection was also carried out in pure THF
solutions. Picric acid (PA) was employed as a model compound
due to its commercial availability.
As shown in Figure 7, the PL spectrum of P1/6 is progressively
weakened when an increasing amount of PA is added into its
THF solution or nanoaggregates in the aqueous mixture. The PL
quenching can be clearly discerned at a PA concentration as low
as 1 ppm. The PL of the polymer aggregates drops at a much
faster rate than that of its isolated species in THF. When the PA
concentration is increased to 54 μg/mL, virtually no light is
emitted from the polymer nanoaggregates in the 90% aqueous
mixture. The SternꢀVolmer plot for the emission quenching of
isolated species of P1/6 in THF solution by PA is shown in
Figure 7C, from which the SternꢀVolmer constant (Ksv) or
quenching efficiency can be determined. Intriguingly, the plot is
composed by two stages. In stage I or in the low PA concentra-
tion (e0.2 mM), the plot is linear with a Ksv,I value of 2.7 ꢁ 104
M
ꢀ1. When the PA concentration is increased to >0.2 mM, the
SternꢀVolmer plot is deviated from the linear one for the low
[PA] region and enters stage II, where the plot follows another
linear relationship with a 3.8-fold higher quenching efficiency
(Ksv,II = 1.04 ꢁ 105 Mꢀ1). A similar curve was obtained for the
polymer nanoaggregates suspended in 90% aqueous mixture.
The Ksv,I and Ksv,II values are determined to be 1.5 ꢁ 105 and
8.48 ꢁ 105 Mꢀ1, which are 5.5- and 8.1-fold higher than those in
pure THF. The quenching constants are also much higher than
those of polysiloles and polygermoles reported previously
(6710ꢀ11 000 Mꢀ1 in the concentration range of 0ꢀ0.2 mM),
demonstrative of a superamplification effect in the emission
quenching process.19 Generally, intrinsic autoaggregation of
chains of fluorescent conjugated polymers and/or their ana-
lyte-induced aggregation cause self-quenching problems that
greatly reduce the sensing performance. However, aggregation
is beneficial to the PL of P1/6. Moreover, due to their three-
dimensional topological structures, the nanoaggregates offer
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Table 3. Refractive Indices and Chromatic Dispersions of
Poly(silylenevinylene)sa
0
polymer
n632.8
νD
νD
D
D0
P1/6
P1/6*b
1.6530
1.5625
1.6196
1.5733
1.6383
1.5994
1.6798
1.6265
1.6275
1.5385
30.51
29.68
23.89
57.83
18.36
33.08
35.83
47.86
33.08
28.42
150.31
336.95
2303.86
1896.67
119.61
131.45
142.97
191.66
131.63
112.36
0.0328
0.0337
0.0419
0.0173
0.0545
0.0302
0.0279
0.0209
0.0302
0.0352
0.0067
0.0030
0.0004
0.0005
0.0084
0.0076
0.0070
0.0052
0.0076
0.0089
P2/6
P2/6*b
P3/6
P3/6*b
P4/6
P4/6*b
P5/6
P5/6*b
Figure 8. Two-dimensional photopatterns generated by photo-oxida-
tion of (A, B) P1/6 and (C, D) P5/6 films in air at room temperature for
5 min. The photographs were taken under (A, C) normal room and (B,
D) UV illumination.
a All data taken from Figure 9 and Figures S9ꢀS12, unless otherwise
specified. Abbreviation: n = refractive index (at 632.8 nm), νD = Abbꢀe
number [defined as (nD ꢀ 1)/(nF ꢀ nC), where nD, nF, and nC are the
refractive indices at wavelengths of Fraunhofer D, F,0and C spectral lines
of 589.2, 486.1, and 656.3 nm, respectively], νD = modified Abbꢀe
number [defined as (n1319 ꢀ 1)/(n1064 ꢀ n1599), where n1319, n1064, and
n
1599 are the refractive indices at the nonabsorbing wavelengths at 1064,
b
0
1319, and 1550 nm], D0 = optical dispersion = 1/νD . For the thin films
of P1/6 and P5/6 after UV irradiation.
units of P1ꢀP3 may be well shielded by the phenyl rings and the
bulky dimethylsilyl groups, thus reducing the likelihood of the
polymers to cross-link by UV-induced olefin polymerization.
However, UV irradiation of the thin films of P1/6 and P5/6, for
example, through copper masks quenches the light emissions of
the exposed regions, while the unexposed squares remain
emissive. Two-dimensional fluorescent patterns are thus gener-
ated without development (Figure 8).
Light Refraction. The poly(silylenevinylene)s are comprised
of polarizable aromatic rings and heteroatoms and may show
high refractive indices (RI). Indeed, as can be seen in Figure 9,
P1/6 displays high RI values (n = 1.6985ꢀ1.6333) in a wide
wavelength region (400ꢀ1600 nm). Others polymers show
similarly high RI values (Figure S9ꢀS12). The RI values of
the polymers at 632.8 nm are 1.6196ꢀ1.6798 (Table 3), which
are much higher than those of the commercially important
optical plastics such as poly(methyl methacrylate) (n ∼1.49),
polycarbonate, and polystyrene (both n ∼1.589).21 Thus, all the
poly(silylenevinylene)s are highly refractive polymers, which
may find high-tech applications as coating materials in the
advanced display systems such as microlens components for
charge-coupled devices and high performance CMOS image
sensors.22
Figure 9. Wavelength dependence of refractive index of thin films of
P1/6. Data for the thin film of P1/6 after UV irradiation under nitrogen
for 15 min at room temperature are denoted as P1/6*.
many cavities to bind with the quencher molecules and provide
additional interbranch diffusion pathways for excitons to
migrate.20 The PA molecules in the inner cores and on the outer
shells of the aggregates work cooperatively to facilitate the PL
annihilation via electron and/or energy transfers, thus resulting
in the observed superquenching effect. The light emissions of
other poly(silylenevinylene)s are also quenched efficiently by PA
with large Ksv values (Figure S5ꢀS8). Again, the nanoaggregates
of the polymers work better as chemosensors than their isolated
species for explosive detection.
Photopatterning. Since the polymers are emissive in the solid
state, we thus explored their potential use as fluorescent imaging
materials. The polymers can form uniform, tough films by spin-
coating their 1,2-dichloroethane solutions onto silicon wafers.
Unlike previously prepared poly(vinylene sulfide)s,10 the poly-
(silylenevinylene)s render soluble even when they are exposed to
UV light from a UV lamp in air at room temperature for 45 min.
The polymers may be decomposed upon UV irradiation, but
GPC analysis shows that the molecular weights of the polymers
hardly change after the UV treatment. We speculate that the vinyl
The RI’s of the poly(silylenevinylene)s can be tuned by UV
irradiation. For example, upon exposure to UV light of 365 nm
from a UV lamp under nitrogen for 15 min, the n values of P1/6
decrease dramatically. The difference in the refractivity is as large
as 0.090, which is very unusual and very remarkable.
For a material to be useful for technological applications, its
optical dispersion, which is a measure of variation in its RI value
with wavelength, should be small.23 To evaluate the application
0
potential of an optical material, a modified Abbꢀe number (νD ),
using its RI values at the nonabsorbing wavelengths of 1064,
1319, and 1550 nm, has been proposed.24 The first two wave-
lengths are chosen in view of the practical interest of commercial
laser wavelengths (Nd:YAG), while the last one is the wavelength
for telecommunication. The νD values of P1/6ꢀP5/6 range
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from 18.36 to 35.83, which are similar to those of conjugated
polymers with absorptions in the visible spectral region (their
νD values are in the range0of 9ꢀ38, with majority being between
10 and 20).25 Their vD values are, however, much higher
(119.61ꢀ2303.86), particularly for P2/6, which exhibits the
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In this work, functional poly(silylenevinylene)s were synthe-
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Supporting Information. PL spectra of P2/6ꢀP5/6 in
b
THF and THF/water mixtures with different water fractions,
fluorescence quenching of P2/6ꢀP5/6 in THF and THF/water
mixture (1:1 v/v) by picric acid, and wavelength dependence of
refractive index of thin films of P2/6ꢀP5/6 before and after UV
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’ AUTHOR INFORMATION
Corresponding Author
*E-mail: tckevin@polyu.edu.hk (K.K.L.C.), tangbenz@ust.hk
(B.Z.T.). Phone: +852-2358-7375 (8801). Fax: +852-2358-
1594.
’ ACKNOWLEDGMENT
This project was partially supported by the grant from the
Niche Areas Funding Scheme of the Hong Kong Polytechnic
University (J-BB6K), the Research Project Competition of
HKUST (RPC10SC13), the Research Grants Council of Hong
Kong (5136/07E, 604711, 603509, 601608, HKUST13/CRF/
08, and HKUST2/CRF/10), the Innovation and Technology
Commission (ITP/008/09NP), the University Grants Commit-
tee of Hong Kong (AoE/P-03/08), and the National Science
Foundation of China (20974028 and 20634020). B.Z.T. thanks
the support from the Cao Guangbiao Foundation of Zhejiang
University.
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