Hyperbranched conjugated polysiloles: Synthesis, structure, aggregation-enhanced emission, multicolor fluorescent photopatterning, and superamplified detection of explosives

Article abstract of DOI:10.1021/ma902432m

Hyperbranched poly(2,5-silole)s [hb-P1(m), m = 1, 6] are synthesized for the first time in this work. 1,1-Dialkyl-2,5-bis(4-ethynylphenyl)-3,4- diphenylsiloles [1(m)] were polymerized by TaBr₅, affording hb-P1(m) with high molecular weights (M<

Full text of DOI:10.1021/ma902432m

Macromolecules 2010, 43, 4921–4936 4921
DOI: 10.1021/ma902432m
Hyperbranched Conjugated Polysiloles: Synthesis, Structure,
Aggregation-Enhanced Emission, Multicolor Fluorescent
Photopatterning, and Superamplified Detection of Explosives
Jianzhao Liu,†,§ Yongchun Zhong,‡ Jacky W. Y. Lam,†,§ Ping Lu,†,§ Yuning Hong,†,§
Yong Yu,†,§ Yanan Yue,^ Mahtab Faisal,†,§ Herman H. Y. Sung,† Ian D. Williams,†
Kam Sing Wong,‡ and Ben Zhong Tang*,†,§,
†Department of Chemistry, Institute of Molecular Functional Materials, ‡Department of Physics, The Hong
Kong University of Science & Technology (HKUST), Clear Water Bay, Kowloon, Hong Kong, China,
§HKUST Fok Ying Tung Research Institute, Nansha, Guangzhou, China, ^Department of Chemistry,
The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China, and Department of
Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China
Received November 2, 2009; Revised Manuscript Received April 20, 2010
ABSTRACT: Hyperbranched poly(2,5-silole)s [hb-P1(m), m = 1, 6] are synthesized for the first time in this
work. 1,1-Dialkyl-2,5-bis(4-ethynylphenyl)-3,4-diphenylsiloles [1(m)] were polymerized by TaBr5, affording
hb-P1(m) with high molecular weights (Mw up to 2.5 ꢀ 105) in high yields (up to 98%). The structures of hb-
P1(m) were characterized by spectroscopic methods and the degree of branching of hb-P1(6) was determined
to be 0.55. The hyperbranched polymers are soluble and stable, with no changes in solubility observed after
they have been stored under ambient conditions for more than two years. Absorption and emission spectra of
hb-P1(m) are red-shifted from those of 1(m), indicating that the polymers are more conjugated than the
monomers. Both 1(m) and hb-P1(m) are nonemissive or weekly fluorescent when dissolved in their good
solvents but become highly emissive when aggregated in their poor solvents or fabricated into thin solid films,
showing unusual phenomena of aggregation-induced (AIE) and -enhanced emissions (AEE). Restriction of
intramolecular rotations in the aggregate state is rationalized to be the main cause for the AIE and AEE
effects. Photoluminescence (PL) of 1(m) and hb-P1(m) is tunable by varying their concentrations and
morphologies. The polymers are readily cured when heated to high temperatures or upon photoirradiation,
furnishing cross-linked networks with novel excitation wavelength-dependent emissions in the red spectral
region. Photolithography of hb-P1(m) generates fluorescent photopatterns, with the exposed and unexposed
areas emitting lights with different colors. The polymers function as sensitive fluorescent chemosensors for
the detection of explosives, with a superamplification effect observed in the emission quenching of the
polymer nanoaggregates by picric acid.
Introduction
“aggregation-induced emission” (AIE) for this novel phenomen-
on because the nonemissive luminogens were induced to emit
Development of efficient luminescent materials is of academic
interest and practical value. A thorny obstacle against the deve-
lopment is the aggregation-caused quenching (ACQ) effect: in the
condensed phase, the luminogenic molecules are located in the
immediate vicinity, which promotes the formation of such detri-
mental species as excimers and exciplexes, leading to undesired
radiationless transitions.1 Various chemical, physical, and engi-
neering approaches and processes have been taken to hamper
the aggregate formation and to mitigate the ACQ effect, but the
attempts have met with only limited success. In most cases,
aggregation is impeded only partially or temporarily. If a system
can be developed in which light emission is enhanced, rather
than quenched, by aggregation, it would be very rewarding. This
will make life much easier because no hard work is needed to
artificially interrupt the very natural process of luminogen
aggregation.
efficiently by aggregate formation. In the AIE system, the
luminogen aggregation plays a constructive, instead of destruc-
tive, role in the light-emitting process. Through experimental3,4
and theoretical studies,5 the restriction of intramolecular rota-
tions (RIR) in the aggregate state has been identified as a main
cause for the AIE effect. In solutions, active rotations of the
peripheral phenyl rings (rotors) against the central silacyclopen-
tadiene core (stator) effectively annihilate the excited states, thus
making the silole molecules weakly luminescent or nonemissive.
The activation of the RIR process by cooling, thickening,
pressurization, crystallization, etc. blocks the nonradiative re-
laxation channels and populates the radiative excitons, thereby
turning “on” the emission of the luminogens.3
Attracted by the novel AIE effect, our and other research
groups have worked on the exploration of technological applica-
tions of the AIE luminogens as chemosensors, bioprobes, im-
munoassay markers, stimuli-responsive materials, solid-state
emitters, and so on.6 Organic light-emitting diodes (OLEDs)
have been fabricated from the AIE luminogens, some of which
showed outstanding electroluminescence performances. For ex-
ample, an OLED constructed from 1,1,2,3,4,5-hexaphenylsilole
In 2001, our research groups discovered such a system in which
a series of silole molecules were found nonemissive in the solution
state but highly luminescent in the aggregate state.2,3 We coined
*Corresponding author. Telephone: þ852-2358-7375. Fax: þ852-
2358-1594. E-mail: tangbenz@ust.hk.
r 2010 American Chemical Society
Published on Web 05/12/2010
pubs.acs.org/Macromolecules

4922 Macromolecules, Vol. 43, No. 11, 2010
Liu et al.
(HPS; Chart 1) emits brilliantly (luminance up to ∼60000 cd/m2),4
while that from 1-methyl-1,2,3,4,5-pentaphenylsilole (MPPS)
shows an extremely high external quantum efficiency (8%),7
reaching the theoretical limit for an OLED based on a singlet
emitter.
A2-type monomers.13,14 The remarkable feature of the polycy-
clotrimerization is that it does not require stoichiometric balance
of the diyne monomers and can thus produce polymers with very
highMWs. Because aromatic rings are the basic constituent units,
the polymers are thermally very stable [with degradation tem-
perature (Td) up to ∼600 ꢀC] and can efficiently emit intense blue
light upon photoexcitation and function as excellent optical
limiters when irradiated with harsh laser pulses.9
Silole possesses a highly branched molecular structure and
exhibits the novel AIE effect and excellent solubility arising
from the large volume of its sp3-hybridized silicon bridge in
the silacyclopentadiene ring. Its highest occupied (HOMO) and
lowest unoccupied molecular orbitals (LUMO) are mainly loca-
lized on the silacyclopentadiene ring and two substituent groups
at the 2,5-positions,6g through which electronic conjugation can
be extended. These unique structural attributes make silole an
ideal building block for the construction of hyperbranched poly-
mers. In this work, we synthesized 2,5-diethynylsiloles 1(m) and
successfully transformed them into hyperbranched poly(2,5-
silole)s hb-P1(m) (Scheme 2) with three-dimensional topological
structures and extended electronic conjugations (Chart 2).
The polymers have high MWs, are completely soluble, very stable
and readily processable (film-forming), and show a novel
The silole luminogens are low molecular weight (MW) com-
pounds and have to be fabricated into thin solid films by
expensive techniques such as vacuum vapor deposition, which
are not well suited to the manufacture of large area devices. One
way to overcome this processing disadvantage is to prepare high
MW polymers, which can be readily processed from their solu-
tions into thin solid films over large areas by a simple process of
spin-coating. Several research groups have worked on the synth-
esis of silole-containing polymers and the resultant polysiloles
have been found to exhibit unique properties, such as narrow
energy band gap, efficient photoluminescence (PL), rapid elec-
trochemical switching, and high electron mobility.8 So far, most
of the polysiloles have been prepared by coupling polymeriza-
tions. Such reactions, however, require stringent stoichiometric
balances of comonomer feeds. This technical difficulty has made
the synthesis of high MW polysiloles a challenging task. Almost
all the polysiloles are linear macromolecules. The synthesis of
their hyperbranched counterparts with three-dimensional archi-
tectures remains to be virtually unexplored.
Scheme 2. Thermo- and Photo-Induced Cross-Linking of hb-P1(m)
Our research groups have been working on the synthesis of
conjugated polymers in the past few decades.9 Utilizing mono-
ynes and triynes as monomeric building blocks, we have prepared
a wide variety of linear polyacetylenes and hyperbranched poly-
triazoles and polydiynes by metathesis,10 click,11 and coupling12
polymerizations. We have also synthesized a large number of
hyperbranched polyarylenes by diyne polycyclotrimerizations of
Chart 1. HPS and MPPS: Two Archetypal AIE Luminogens
Scheme 1. Syntheses of 2,5-Diethynylsiloles 1(m) and Transformations of the Monomers into Hyperbranched Polysiloles P1(m) via Diyne
Polycyclotrimerization

Article
Macromolecules, Vol. 43, No. 11, 2010 4923
Chart 2. Structures of Dendritic, Linear, and Terminal Units in hb-P1(m)
Table 1. Polymerizations of 2,5-Diethynylsiloles 1(m)a
phenomenon of aggregation-enhanced emission (AEE). Utilizing
the AEE effect, we explored the high-tech applications of hb-P1(m)
as multicolor light-emitting photoresists and fluorescent chemo-
sensor for explosive detection.
[cat.] time
no. monomer (mM) (h)
yield
(%)
c
b
Mw,rb (Mw,a
)
Mw/Mn
1
2
3
4
5
1(1)
1(1)
1(6)
1(6)
1(6)
2.5
5
2.5
5
1
0.5
2
0.5
0.25
87.3
75.6
97.9
81.7
84.5
17 400
3.3
3.2
4.1
6.4
1.9
17 600 (238 000)
27600 (249 000)
52 600
Results and Discussion
Monomer Preparation. Since the synthesis of the first silole
compound, HPS, was reported in the late 1950s,15 studies on
the preparations, structures and properties of silole mole-
cules have been a topic of intense interest among scientists,
especially synthetic chemists.16,17 The synthetic schemes
developed by Tamao and Yamaguchi have become the
general routes to silole derivatives.16-18 In this work, we pre-
pared monomers 1(m) (m = 1, 6) in satisfactory yields, using
the published experimental procedures with some modifica-
tions (Scheme 1).18b In this multistep reaction scheme,
diethynylsilanes 2(m) undergo intramolecular reductive cy-
clization in an endo-endo mode by the treatment with
lithium 1-naphthalenide (LiNaph) to form 2,5-dilithiosiloles
3(m), which is transformed to 2,5-dizinc siloles 4(m) by the
7.5
22 200
a Carried out in toluene at room temperature under nitrogen using
TaBr5 as catalyst; [M]0 = 0.05 M. b Relative weight-average molecular
weight (Mw,r) and polydispersity index (Mw/Mn) determined by gel
permeation chromatography (GPC) in THF on the basis of linear
polystyrenes calibration. c Absolute weight-average molecular weights
(Mw,a) determined by static laser light-scattering technique.
the crystals with the X-ray diffractogram (Supporting In-
formation, Table 1).
Polymer Synthesis. In our previous work, we have system-
atically investigated the effects of reaction conditions on the
polycyclotrimerizations of diyne monomers catalyzed by
tantalum halides and found that high MW polymers could
be obtained in high yields when the polymerization reactions
were carried out in nonpolar solvents, at high temperatures
and catalyst and monomer concentrations for prolonged
periods of time.13 The study offers us constructive guidance
to polymerize 1(m) under optimal reaction conditions. We
first carried out the polymerization of 1(1) catalyzed by
TaBr5 in toluene at a catalyst concentration of 2.5 mM.
After stirring at room temperature for 1 h, a soluble polymer
product with a relative weight-average MW of 17400 was
isolated in a yield of ∼87% (Table 1, no. 1). Doubling the
catalyst concentration helps shorten the reaction time at no
cost of polymerization results (Supporting Information,
Figure S1). Similar polymerization behaviors were observed
for 1(6). It should be pointed out that MW of a hyper-
branched polymer measured by the GPC system calibrated
by linear polystyrene standards is often underestimated,
due to the rigid globular architecture of hyperbranched
transmetalation with ZnCl2 TMEDA (TMEDA = N,N,N0,
3
N0-tetramethylethylenediamine). The subsequent reaction of
4(m) with (4-bromophenylethynyl)trimethylsilane in the pre-
sence of a palladium catalyst yields 5(m). The base-catalyzed
desilylation of 5(m) in methanolic K2CO3 solution furnishes
the desired products 1(m).
The silole monomers were isolated, purified and charac-
terized by standard spectroscopic techniques, from which
satisfactory analysis data corresponding to their expected
chemical structures were obtained (see Experimental Section
for detailed spectroscopic data). Whereas the monomer
carrying long n-hexyl groups at the 1,1-positions [1(6)] is a
viscous liquid, its counterpart with short methyl substituents
[1(1)] is a solid. High-quality single crystals of 1(1) were
successfully grown from a mixture of THF/ethanol. The
molecular structure of 1(1) was determined by analyzing

4924 Macromolecules, Vol. 43, No. 11, 2010
Liu et al.
Figure 1. IR spectra of (A) monomer 1(6) and (B) its polymer hb-P1(6)
(sample taken from Table 1, no. 3).
polymer.19 Indeed, when MWs of hb-P1(m) were measured
by a static laser light-scattering method, their absolute Mw
values are 9-13 times higher than the polystyrene-calibrated
relative ones (Table 1, nos. 2 and 3). Evidently, the poly-
cyclotrimerizations of 1(m) can produce hb-P1(m) with high
MWs in high yields.
Figure 2. 1H NMR spectra of DCM-d2 solutions of (A) monomer 1(6)
and (B) its polymer hb-P1(6) (sample taken from Table 1, no. 3). Inset in
panel B: resolved peaks for the resonances oftheacetylenic protons. The
solvent peaks are marked with asterisks.
All the polymer products are highly soluble in common
organic solvents, such as THF, toluene, dichloromethane
(DCM), and chloroform, thanks to the large free volumes in
the polymers contributed from the twisted propeller-shaped
silole repeat units. The polymers remain completely soluble
and show excellent optical spectral stability after they have
been stored in air in vials under normal room lighting for
more than 2 years. This is very different from our previously
prepared hyperbranched polyarylenes, which are highly
soluble when they have been freshly prepared but become
poorly soluble after storage, possibly due to the postpoly-
merizations of the peripheral triple-bond functional groups
coupled with the strong intermolecular π-π stacking inter-
actions between the aromatic repeat units. The present study
thus provides a designing guidance to the synthesis of new
hyperbranched polymers with macroscopic processability.
Structural Characterizations. The polymer products were
fully characterized spectroscopically (see Experimental Sec-
tion for detailed analysis data). The IR spectrum of hb-P1(6)
is given in Figure 1 as an example. The spectrum of its
monomer 1(6) is also shown in the same figure for the sake of
comparison. The tC-H stretching vibration of 1(6) is
observed at 3294 cm-1, which becomes much weaker after
the diyne polycyclotrimerization reaction. The CtC stretch-
ing vibration at 2107 cm-1 is hardly discernible, indicating
that a large portion of the acetylene triple bonds of the
molecules of the diyne monomer have been consumed by the
polymerization reaction, with only a small portion of the
triple bonds left intact, existing as the linear and terminal
units in the polymer structure (cf., Chart 2).
Figure 3. 13C NMR spectra of DCM-d2 solutions of (A) monomer 1(6)
and (B) its polymer hb-P1(6) (sample taken from Table 1, no. 3). The
solvent peaks are marked with asterisks.
1.5-0.8 are greatly broadened after the polymerization
reaction. All these spectral data indicate that the acetylenic
triple bonds of monomer 1(6) have been cyclotrimerized to
form benzene rings in polymer hb-P1(6). From our previous
results on the cyclotrimerization of phenylacetylene as a
model reaction,13 the new resonance peaks correspond to
the absorption of the protons on the newly formed 1,2,4- and
1,3,5-trisubstituted benzene rings.
Similar results are obtained from the NMR analyses.
Figure 2 shows the H NMR spectra of hb-P1(6) and its
1
monomer 1(6) in DCM-d2. The acetylene protons of 1(6)
resonate at δ 3.07, whose intensity becomes much lower in
the spectrum of hb-P1(6). Most of these unreacted acetylenic
triple bonds exist as end groups on the peripheries of the
polymer, offering an opportunity to decorate the macromo-
lecule with other functional groups by end-capping reac-
tions. Additionally, new resonance peaks are emerged in the
aromatic region and the “old” peaks at δ 7.2-6.8 and
Similarly, 13C NMR spectrum of hb-P1(6) shows weak
resonance peaks of acetylenic carbon atoms at δ 84.3
and 77.5 and new resonance peaks of aromatic carbon atoms
in the downfield region (Figure 3). All the resonance peaks in
both the 1H and 13C NMR spectra can be readily identified
and assigned without difficulty, as marked in Figures 2 and 3.
No any strange peaks are observed, indicative of the
high purify of the isolated polymer products. The IR and

Article
Macromolecules, Vol. 43, No. 11, 2010 4925
NMR spectra exemplified here and the spectral data given in
the Experimental Section duly confirm that silole monomers
1(m) have undergone the diyne polycyclotrimerization
reactions to yield hyperbranched polysiloles hb-P1(m) with
expected structures (cf., Scheme 1).
large as about 6 times because of the globular architecture of
the hyperbranched polymer:
MnðNMRÞ
MnðGPCÞ
37700
¼
¼ 5:63
ð5Þ
6700
Determination of Degree of Branching (DB). From the 1H
NMR spectra shown in Figure 2, the extent of conversion (p)
of the acetylenic triple bonds that have been transformed to
benzene rings can be estimated using eq 1:
For the polycyclotrimerization of an A2-type diyne mono-
mer, if p < 75% and no loop is formed, eqs 6 and 7 hold:13
2fT þ fL
¼ DPnð1 - pÞ
ð6Þ
Aalkyl
fT - fD
APh
-
ꢀ 18
NRT
NTT
4m þ 2
p ¼
¼
Aalkyl
ꢀ 2
4m þ 2
fT þ fD þ fL ¼ 1
ð7Þ
ð2m þ 1ÞAPh - 9Aalkyl
where fD, fT, and fL are the fractions of dendritic (D),
terminal (T) and linear (L) units, respectively (Chart 2).
Because of the difficulty in differentiating the newly formed
benzene rings in the D, T, and L units from each other by the
spectroscopic techniques, we diverted our effort to analyze
the acetylenic proton signal. Magnification of the proton
signal at δ 3.07 reveals that it is actually comprised of two
peaks (cf., Figure 2). We assigned the sharper peak to the
resonance of protons of the peripheral T units because of the
faster exchange caused by their more rapid molecular motion
in comparison to that of the L units, which are embedded in
the hyperbranched sphere. Fitting the two peaks gives the
result that the amount of the acetylenic protons in the T units
is ∼1.34-fold higher than that in the L units. Because there
are two such protons in the T unit but only one in the L unit,
eq 8 can be established:
¼
ð1Þ
Aalkyl
where NRT is the number of the reacted triple bonds, NTT is
the total number of the triple bonds of the monomers,
APh is the integrated area of the resonance peaks for the
protons of all benzene rings in the polymer, Aalkyl is the
integral of the aliphatic resonance peaks, and m is the
number of the carbon atoms in the alkyl chain (m = 1, 6)
attached to the silicon atom at the 1,1-positions in the silole
ring.
Generally speaking, the polycyclotrimerization reaction
should be terminated before the critical extent of conversion
(pc) or gel point in order to obtain a soluble polymer. In our
previous study, from a pure structure point of view, we
have theoretically derived that if no loops are formed in
the propagation step of the diyne polycyclotrimerization,
the upper limit of the p value is equal to 75%.13 On the basis
of a kinetic model, Malkov et al. have performed the
theoretical studies of polycyclotrimerization and predicted
pc values of 0.50 and ∼0.62 with nonallowance and
allowance for the first shell negative substitution effect,
respectively.20 The monomer solution we normally used for
the polymerization is so dilute that it will result in a high pc
value (>0.62). Indeed, the p value of hb-P1(6) prepared
under the conditions shown in Table 1, no. 3 is theoretically
predicted to be 73.8%. We believe that when the p value is
less than 75%, the probability of loop formation is very low.
To simplify the structural analysis, we assume that no loop
structures are formed in the polymer at p < 75%. On
the basis of this hypothesis, we have established the relation-
ships between p and the number of newly formed benzene
rings (Nb) and between the degree of polymerization (DPn)
2fT
AT
AL
¼
¼ 1:34
ð8Þ
fL
where AT and AL are the integrated areas of the resonance
peaks for the protons of the T and L units, respectively. By
combining eqs 6-8, the values of fD, fT and fL are calculated
to be 0.24, 0.31 and 0.45, respectively. According to defini-
tion, DB is expressed as:21
fD þ fT
DB ¼
¼ 1 - fL ¼ 0:55
ð9Þ
fD þ fT þ fL
The DB value of our polymer (0.55) is comparable to the most
probable DB values for the conventional hyperbranched
polymers prepared by the condensation polymerization of
AB2-type monomers (∼0.5).22 This further confirms the
hyperbranched topological structure of our polymer.
13
and Nb:
Thermal Properties. Thermal stabilities of hb-P1(m) were
evaluated by thermogravimetric analysis (TGA). Polymer
hb-P1(1) is thermally stable and loses 5% of its original
weight when it is heated to a temperature of about 370 ꢀC
(Figure 4). More than 75% of its weight is still retained even
when the temperature is raised to as high as 500 ꢀC. Similar
thermal properties are observed in the case of hb-P1(6). The
high thermal stabilities of hb-P1(m) are in some sense ex-
pected because the hyperbranched polymers are basically
comprised of stable aromatic rings with high resistance to
thermolytic attacks. The thermally induced cross-linking of
the acetylenic triple bonds may have also contributed to the
high thermal stabilities of the polymers.23
3Nb
p ¼
ð2Þ
ð3Þ
4Nb þ 2
DPn ¼ 2Nb þ 1
Since p equals to 0.738, the values of Nb and DPn are
calculated to be 30.8 and 62.6, respectively. Thus, the
number-average MW of the polymer (Mn) can be derived
by eq 4:
Mn ¼ DPn ꢀ Mm ¼ 37700
ð4Þ
To examine whether the hyperbranched polymers are
indeed thermally curable, we used differential scanning
calorimetry (DSC) to follow their thermal transitions. In
the first heating scan, a very broad exothermic peak starts to
where Mm is the MW of monomer 1(6). Comparing the
calculated Mn value with that estimated by GPC relative to
the polystyrene standards, the difference is found to be as

4926 Macromolecules, Vol. 43, No. 11, 2010
Liu et al.
Figure 6. Normalized UVand PL spectraofTHFsolutions of1(m) and
hb-P1(m). Concentration: 10 μM. Excitation wavelength: 373 nm.
Figure 4. TGA thermograms of hb-P1(1) (sample taken from Table 1,
no. 2) and hb-P1(6) (Table 1, no. 3) recorded under nitrogen at a heating
rate of 10 ꢀC/min.
Figure 7. (A) ORTEP drawing of crystal structure of 1(1) and molec-
ular orbital amplitude plots of its (B) HOMO and (C) LUMO.
Figure 5. DSC thermograms of 1(1) and hb-P1(m) (samples taken
from Table 1, nos. 2 and 3) measured at a scan rate 10 ꢀC/min under
nitrogen.
Photophysical Properties. Figure 6 shows the UV and PL
spectra of 1(m) and their polymers in THF solutions. The
absorption spectrum of hb-P1(1) is peaked at 379 nm, while
the absorption maximum of 1(1) is located at 373 nm.
Similarly, the absorption peak of hb-P1(6) is 7 nm bath-
ochromically shifted from that of 1(6). Upon photoexcita-
tion, hb-P1(1) and hb-P1(6) emit green lights of 507 and
509 nm, respectively, while the PL spectra of their monomers
are peaked at a shorter wavelength of ∼490 nm. All the
absorption and emission data indicate a more extended
electronic conjugation in the polymer system. Evidently,
the hookup of the silole rings by the benzene rings formed
in the cyclotrimerization polymerization at the 2,5-positions
in three-dimensional space has helped increase the effective
length of electronic conjugation.
To prove that the electronic conjugation in a silole mole-
cule is affected by the substituents at the 2,5-positions,6g,24
we carried out theoretical calculation based on the density
functional theory (DFT) at the B3LYP/6-31G* level.25 The
molecular geometry of monomer 1(1) used in the calculation
is extracted from its single crystal structure (Figure 7). The
HOMO and LUMO of the isolated single molecule of 1(1)
are mainly localized in the silacyclopentadiene ring and the
two ethynylphenyl groups at the 2,5-positions.16f,24 This
verifies that the 2,5-aryl groups have a strong influence on
emerge from ∼120 ꢀC in the thermogram of hb-P1(1) (Figure 5).
Since the polymer does not loss its weight below 370 ꢀC, the
exothermic peak at ∼267 ꢀC is thus not due to the chain
scission but associated with the thermally induced alkyne
polymerization.23 The second heating scan of the DSC
analysis gives an even line in the same temperature region,
suggesting that all the acetylenic triple bonds have reacted
during the first heating scan. A similar thermogram is
obtained for hb-P1(6) and the cross-linking reaction also
starts from ∼120 ꢀC with an exothermic peak at ∼256 ꢀC.
If the thermal curing of the hyperbranched polymers is
really associated with the thermal cross-linking of their
triple-bond functional groups, their monomers should also
be thermally polymerizable. This is indeed the case. As can be
seen from the DSC thermogram of 1(1) shown in Figure 5, its
thermal curing commences from ∼130 ꢀC. The thermogram
recorded during the first cycle is featured with a series of
continuous exothermal peaks at 208, 218, 225, and 262 ꢀC
with the last peak being the broadest. The spectral profile
generally resembles those of the polymers, suggestive of
occurrence of the same reaction. Similarly, no thermal
transition peaks are recorded during the second heating scan.

Article
Macromolecules, Vol. 43, No. 11, 2010 4927
Figure 8. (A) Concentration-dependent PL spectra of hb-P1(6) in THF solutions. (B) PL spectra of hb-P1(6) in THF/water mixtures with different
water fractions (fw). PL spectrum of 1(6) in THF is shown for comparison. Concentration: 10 μM; excitation wavelength: 373 nm.
Figure 9. (A) Variations in quantum yields of1(m) and hb-P1(m) with water fractions (fw) in THF/water mixtures. Size distributions of nanoaggregates
of (B) 1(6) and (C) hb-P1(6) in a THF/water mixture with fw = 90%. Concentration: 10 μg/mL. Insets: fluorescent images of (B) 1(6) and (C) hb-P1(6)
taken under UV illumination in pure THF (fw = 0) and a THF/water mixture with fw = 90%.
the energy band gap of a silole molecule. Clearly, the elec-
tronic conjugation is extended when 1(1) is transformed to
hb-P1(1), in which the silole rings are connected by the new
benzene rings at the 2,5-positions.
Light emission of conjugated polymers often suffers from
concentration-quenching or ACQ effect due to the forma-
tion of nonradiative species such as excimers and exciplexes,
which significantly limits the scope of their technological
applications, especially in optoelectronic devices. In sharp
contrast, our hyperbranched polymers emit strongly upon
aggregation. As shown in Figure 8A, the PL of hb-P1(6) is
progressively intensified when its solution is thickened from
0.8 to 25 μg/mL. Addition of a nonsolvent such as water into
THF induces the molecules of hb-P1(6) to aggregate and
enhances its PL (Figure 8B). The higher is the water fraction,
the stronger is the light emission. Evidently, the emission of
hb-P1(6) is enhanced by aggregate formation; in other words,
the polymer shows a typical effect of AEE.
The mechanism for the AEE effect is similar to that for the
AIE system discussed above. Comparative studies of the
photophysical properties of the AIE-active monomers and
their AEE-active polymers from molecular solutions in their
good solvents to nanoparticle suspensions in their poor
solvents and to thin films in the solid state may help gain
further insights into the novel phenomena. The absorption
spectra of the monomer aggregates in THF/water mixtures
with 90 vol % water fraction are almost the same as those of
We have proposed the RIR process as the main cause for
the AIE effect of siloles.3-5 Fundamental physics teaches us
that any molecular motions consume energy. The peripheral
phenyl rings (rotors) surrounding the silole core (stator) can
rotate via the single-bond axes. In the solution state, the active
intramolecular rotation serves as a relaxation channel for the
excited state to decay, hence rendering the silole molecules
nonemissive. In the aggregated state, however, such rotation is
physically restricted, which blocks the nonradiative pathway
and populates the radiative excitons, thus enabling the
molecules to luminesce efficiently. Monomers 1(1) and 1(6)
are AIE luminogens and emit weakly in the dilute solutions of
their good solvents (e.g., THF; Figure 8B). Their hyper-
branched polymers, however, are somewhat luminescent in
their THF solutions, as exemplified by the fluorescent image
of the hb-P1(6) solution shown in Figure 9C. The silole units
are located within the stiff polymer spheres, which limit their
intramolecular rotations to some extent and thus render the
polymers somewhat luminescent in the solution state.

4928 Macromolecules, Vol. 43, No. 11, 2010
Liu et al.
Table 2. Absorption and Emission Properties of Monomers and Their Polymers in Solution (soln),a Aggregate (aggr),b Film,c Crystalline (cryst),
and Amorphous (amor) States
λ
ab (nm)d
aggr
λ
em (nm)e
film
Φ
F (%)f
g
soln
soln
aggr
cryst
494
amor
soln
aggr
RAIE
1(1)
hb-P1(1)
1(6)
373
379
373
380
371
372
490
507
491
509
492
508
497
510
484
489
480
490
507
523h
507i
526h
0.1
3.0
0.1
5.2
12.4
15.4 (25.6)
9.4
124.0
5.1
94.0
3.5
hb-P1(6)
18.4 (23.3)
a In THF solution (10 μM). b In THF/water (10:90 v/v) mixture (10 μM). c Prepared by spin-coating a 1,2-dichloroethane solution of a pure polymer or
a blend of monomer (2.5 wt %) and poly(methyl methacrylate) (PMMA). d Absorption maximum. e Emission maximum. f Fluorescence quantum yield
estimated using 9,10-diphenylanthrance as standard (ΦF = 90% in cyclohexane). Given in the parentheses is the absolute ΦF value in the solid state
determined by a calibrated integrating sphere. g AIE factor defined by eq 10. h Prepared by static casting of a polymer solution. i Viscous liquid.
their THF solutions. The spectra of the polymer aggregates
resemble those of their solid thin films and covered a wide
spectral region with no clearly discernible peaks (Supporting
Information, Figure S2). In THF, the fluorescence quantum
yield (ΦF) of 1(m) is 0.1%, whereas hb-P1(1) and hb-P1(6)
show relatively higher values of 3.0% and 5.4%, respectively
(Table 2). The ΦF values of 1(m) remain unchanged until
∼70 vol % water is added into THF (Figure 9A). Afterward,
the fluorescence efficiencies rise swiftly.
The ΦF values of the hyperbranched polymers, on the
other hand, are monotonically increased with an increase in
the water content (Figure 9A). In comparison to their
monomers, the polymers are more sensitive in morphology
to the change in their surrounding environments. Even when
a small amount of water is added into THF, the subtle
change in the solvating power of the solvent mixture can be
sensed by the macromolecules, causing them to aggregate. At
the same water contents, the ΦF values of the polymers are
always higher than those of their monomers. This may be
explained by the fact that the motions of 1(m) are only
restricted by physical aggregation, whereas both internal
Figure 10. Fluorescence decay curves of THF solutions (1 μg/mL) of
1(6) and hb-P1(6).
and external factors of chemical binding and physical ag-
gregation are involved in the RIR processes of the polymers.
In the THF/water mixtures with 90 vol % water fraction, the
ΦF values of 1(1) and 1(6) are 12.4% and 9.4%, respectively,
giving the corresponding RAIE values of 124 and 94 (Table 2),
where RAIE is the AIE factor defined by the following eq:26
Table 3. Fluorescence Decay Parameters of 1(m) and hb-P1(m) in
Dilute THF Solutionsa
A1 (%)
A2 (%)
τ1 (ns)
τ2 (ns)
Æτæ (ns)
1(1)
100
73
100
62
0
27
0
0.05
0.09
0.09
0.10
0.05
0.29
0.09
0.31
hb-P1(1)
1(6)
hb-P1(6)
0.82
0.66
ΦFðaggrÞ
ΦFðsolnÞ
RAIE
¼
ð10Þ
38
a The dynamic parameters (Ai, τi, and Æτæ) were calculated from eqs 20
and 21; see Experimental Section for details.
The ΦF values of hb-P1(1) and hb-P1(6) in the same solvent
mixture are 15.4% and 18.4%, respectively, which are
3.5-5.1 times higher than those in the THF solutions. The
fluorescence images shown in panels B and C of Figure 9
visually demonstrate the AIE and AEE phenomena of 1(6)
and hb-P1(6), respectively. Particle size analyses reveal the
existence of particles with average sizes of 101 and 150 nm in
the solvent mixture with 90 vol % water content for 1(6) and
hb-P1(6), respectively. The AEE effect enables the polymers
to emit efficiently in the solid state. The absolute ΦF values of
the thin solid films of hb-P1(1) and hb-P1(6) measured by an
integrating sphere are as high as 25.6% and 23.3%, respec-
tively (Table 2).
through the same pathway. The decay curve of hb-P1(6),
on the other hand, is better fitted by a double-exponential
function. This means that two relaxation pathways are
involved in its decay process: 62% (A1) and 38% (A2) of
the excitons decay via the fast and slow channels with life-
times of 0.10 ns (τ1) and 0.66 ns (τ2), respectively.
The ΦF value is known to be associated with the fluores-
cence lifetime by the following eq
τ
ΦF
¼
ð11Þ
τr
where τ is related to radiative (τr) and nonradiative (τnr)
lifetimes by the expressions of
Fluorescence Decay Dynamics. Lifetime is an important
kinetic parameter for the PL decay process. Study of emis-
sion dynamics of the monomers and their polymers by time-
resolved technique may help deepen our understanding on
the AIE and AEE mechanisms. Figure 10 shows the PL
decay curves of 1(6) and hb-P1(6) in THF solutions as
examples. The PL of monomer 1(6) decays faster than that
of its polymer hb-P1(6). The excited state of 1(6) decays in a
single-exponential fashion with a lifetime as short as ∼90 ps
(Table 3), suggesting that all the excited molecules relax
1
1
1
¼
þ
ð12Þ
ð13Þ
τ
τr τnr
k ¼ kr þ knr
where kr and knr are the radiative and nonradiative decay
rates, respectively.1,27 The τr term is an intrinsic property of a

Article
Macromolecules, Vol. 43, No. 11, 2010 4929
molecule. Consequently, the faster is the nonradiative decay
rate (knr), the lower is the ΦF value. The short-lived excited
state and the low ΦF value of 1(6) in the THF solution
indicate that there exist vigorous nonradiative paths leading
to effective quenching of its PL in the dilute solution.
Rotational energy relaxations are known to nonradiatively
deactivate the excited states. In the molecular solution of
1(6), the intramolecular rotations are active and the excited
states of the monomer molecules are readily annihilated
through the nonradiative deactivation process. However,
in the solution of hb-P1(6), the probability of intramolecular
rotations of the phenyl rotors in the silole units is decreased
due to the polymer effect discussed above, which partially
blocks the nonradiative relaxation channel. Thus, both the
emission lifetime and efficiency of the polymer in the solution
are enhanced. Similar behaviors are observed in the 1(1)/hb-
P1(1) pair (Table 2). These results strongly support the
validity of the RIR model for the AIE and AEE effects.
Solid-State Emissions. After studying the emissions of
1(m) and hb-P1(m) in the molecular solutions in good
solvents and the nanoparticle suspensions in poor solvents,
we investigated their PL behaviors in the condensed phase.
Both the monomers and polymers show strong solid-state
emissions, as can be seen from the examples of fluorescence
images shown in Figure 11. The crystals of 1(1) emit at 494 nm,
which is blue-shifted from that of its amorphous powder
by 13 nm (Figure 12). Crystallization generally red-shifts
fluorescence, but why is the opposite effect observed in 1(1)?
Scrutinization of the molecular conformation and packing
arrangement of 1(1) in the crystalline phase offers some
clue.3,28 As shown in Figure 7A, the molecule of 1(1) takes
a propeller-shaped conformation in the crystalline state. The
peripheral phenyl rings are twisted out of the silacyclopen-
tadiene plane to varying extents, with dihedral angels of ∼21ꢀ
and ∼37ꢀ for the phenyl rotors at the 2,5-positions and larger
twists of ∼52ꢀ and ∼57ꢀ at the 3,4-positions. The sp3
hybridization of the silicon atom makes the 1,1-methyl
groups completely out of plane of the silole core. The
nonplanarity caused by the steric repulsion between the
phenyl groups and the sp3 hybridization of the silicon atom
reduces the intermolecular interactions and the likelihood of
excimer formation. The physical confinement in the crystal
lattice rigidifies the conformation of the molecules of 1(1),
thereby enabling it to emit efficiently in the solid state.
The unusual blue shift observed in the crystalline phase is
attributable to the conformational twisting in the molecular
packing process, during which the molecules of 1(1) adjust
their conformations in order to fit into crystalline lattice
(Figure 13). The twisted conformation is stabilized by the
˚
C-H π hydrogen bonds (d1 = 2.721 A) between the
3 3 3
hydrogen atom in a phenyl group in one silole molecule
and the π electron cloud of a phenyl ring in another neigh-
boring molecule. Without these physical restraints, the silole
molecules in the amorphous powder may statistically assume
a more planar conformation and thus show a redder emis-
˚
sion. The large intermolecular distances (d2 = 7.346 A, d3 =
˚
˚
7.760 A, and d4 = 12.140 A) in the crystal packing structure
manifest the lack of strong chromophore interactions that
tend to induce nonradiative transitions and red shifts as seen
in the conventional luminophore crystals with strong π-π
stacking interactions. Additionally or alternatively, the con-
formation rigidification enforced by the crystal packing of
AIE luminogen molecules hampers vibrational relaxations
and favors vertical transitions, according to the Franck-
Condon principle. The bigger energy gap involved in the
vertical transition thus leads to the bluer emission in the
crystalline state.
Since the conformations of silole molecules can affect their
photophysical processes, it is envisioned that if a method for
conformational alteration can be developed, the similar
morphochromism should be observed. We prepared blend
films by spin-coating the solutions of 1(1)/PMMA mixtures
onto silicon wafers. The idea is that at a low doping con-
centration, the molecules of 1(1) should be well isolated or
segregated in the dense polymer matrix. The shearing effect
of the spin-coating process may force the silole molecules to
Figure 11. Photographs of (A and B) crystalline rods of1(1) and (C and
D) amorphous plates of hb-P1(6) taken under (A and C) normal
laboratory lighting and (B and D) UV illumination (330-380 nm).
Figure 12. (A) PL spectra of 1(1) dispersed in PMMA films and in powdery and crystalline states. Inset: PL spectra of spin-coated film and amorphous
powder of hb-P1(1). (B) PL spectra of viscous liquid of 1(6), spin-coated film of 1(6)/PMMA, and spin-coated film and amorphous powder of hb-P1(6).

4930 Macromolecules, Vol. 43, No. 11, 2010
Liu et al.
Figure 15. Photographs of thermal curing product of hb-P1(6) taken
(A) under normal room lighting and upon excitation with (B) UV
(330-380 nm), (C) blue (450-490 nm), and (D) green (510-560 nm)
lights.
Figure 13. Perspective view of packing arrangements in the crystals of
˚
1(1), with C-H π hydrogen-bonding (d = 2.721 A) and other
3 3 3
intermolecular distances marked by dashed lines.
the rapid evaporation of the solvent and the quick formation
of the glassy film.
Fluorescent Modulation and Photopatterning. The DSC
analyses reveal that 1(m) and hb-P1(m) can be thermally
cured and our previous studies have found that diynes and
their polymers prepared by polycyclotrimerization are
photocross-linkable (Scheme 2).9 The IR spectra of 1(1) after
thermolysis and photolysis show absorption bands in the
olefinic region of 1690-1600 cm-1 (Supporting Informa-
tion, Figure S3), suggesting that the triple bonds of 1(1) have
been transformed into double bonds of polyenes and that the
effective conjugation of hb-P1(m) can be lengthened by the
curing reaction. Indeed, the thermally cross-linked product
of hb-P1(6), abbreviated as X-hb-P1(6), emits in the red
spectral region (Figure 14). Remarkably, the PL spectrum
of X-hb-P1(6) is sensitive to the excitation wavelength, which
is not observed in its precursor hb-P1(6). The emission
peak is gradually red-shifted from 610 to 735 nm accompa-
nying with a decrease in the emission intensity when the
excitation wavelength is changed from 420 to 560 nm at an
interval of 20 nm (Figure 14B). This fact evinces that the
curing product possesses various conjugation lengths with
different populations, which response to the matched stimu-
li. This enables the cross-linked polymer to emit lights of
different colors in response to the variations in the excitation
wavelengths.
After thermal curing, the orange appearance of hb-P1(6)
changes to brown, with some parts becoming even dark, as
circled by the dotted lines in Figure 15A. The dark areas may
be rich in the polyene structure that can absorb lights of long
wavelengths. Under the illuminations of UV, blue and green
lights, X-hb-P1(6) shows green, yellow and red emissions,
respectively (Figure 15, panels B-D), which are consistent
with the PL spectral data discussed above. From these
results, it can be concluded that various conjugation lengths
or band gaps exist in X-hb-P1(6). Notably, many white light-
emitting dots are observed under the UV light illumination
(Figure 15B). When irradiated by the UV light, polymer
branches with appropriate conjugation lengths will be
pumped to their excited states and emit blue to green lights,
which can serve as excitation sources to induce yellow and
Figure 14. Excitation wavelength (λex)-dependent PL spectra of hb-
P1(6) (sample taken from Table 1, no. 3) (A) before and (B) after
thermal curing at 400 ꢀC for 1 min.
take a more twisted conformation and thus emit a bluer light.
As anticipated, the PMMA film containing 2.5 wt % of 1(1)
shows an emission maximum at 484 nm, which is 10 nm bluer
than the crystal emission. When the doping concentration
is increased to 5 wt %, the emission peak is red-shifted to
505 nm. At the high doping concentration, the silole mole-
cules may cluster together, in which free volumes are created
due to the large intermolecular distances. This allows the
molecules to structurally relax to comparatively more planar
conformations, thereby red-shifting the emission peak. Simi-
larly, a blue shift of 27 nm is observed in monomer 1(6) when
2.5 wt % of its viscous liquid is doped into the PMMA matrix
(Figure 12B).
We further investigated the PL behaviors of the hyper-
branched polymers. It is noteworthy that the light emissions
of the thin films of the polymers spin-coated from their 1,2-
dichloroethane solutions are hypsochromically shifted from
those of their amorphous powders by 34-36 nm, as can be
seen from the examples of PL spectra of hb-P1(1) shown in
the inset of Figure 12A and the emission data summarized in
Table 2. It can be rationalized that the shearing force in the
spin-coating process has helped populate the emitting species
in the polymers with more twisted molecular conformations
that have been instantly frozen in the dense matrices due to

Article
Macromolecules, Vol. 43, No. 11, 2010 4931
Figure 16. Two-dimensional photopatterns generated by UV irradiation of hb-P1(6) (sample taken from Table 1, no. 3). Photographs were taken by
fluorescence microscopy (A) under normal laboratory lighting and upon excitations with (B) UV (330-380 nm) and (C) green (510-560 nm) lights.
red emissions from the branches with longer conjugations.
Admixture of these emitted lights results in the emission of a
white light. It is envisioned that specific areas of a film of the
polymer can be cured or cross-linked by local heating with
laser or electron beam to modulate the band gap and hence to
obtain fluorescence patterns with different emission colors,
which may find an array of applications in information
storage and multicolor imaging or display systems.
Light-driven technique is more convenient, in terms of
operational simplicity and precise control of curing reaction
and patterning process. Because our polymers possess good
film-forming ability and are photosensitive and light-emit-
ting, they are promising candidates for fabrication of lumi-
nescent patterns by photolithography process. Creations of
fluorescent patterns is of great importance to the construc-
tion of photonic and electronic devices and to the develop-
ment of biological sensing and probing systems, such as
OLED, liquid-crystal display, and medicinal diagnostic
biochip.29 When a thin spin-coated film of hb-P1(6) is
irradiated by a UV light for 20 min through a copper negative
mask, the exposed region of the film is rendered cross-linked
and hence insoluble.13,14 A two-dimensional photopattern is
generated without development. The good quality of the
pattern (sharp line edges, uniform film thickness, etc.) is
clearly seen under normal laboratory lighting (Figure 16A),
although the photolysis process is yet to be optimized. Since
Figure 17. PL spectra of hb-P1(6) in (A) THF/water mixture (1:9 v/v)
and (B) THF containing different amounts of picric acid ([PA]).
the effective conjugation of hb-P1(6) can be lengthened by a
cross-linking reaction, the exposed and unexposed areas of
the film are expected to show different emission behaviors.
To verify this, the PL spectra of the thin solid film of X-hb-
P1(6) are taken (Supporting Information, Figure S4). In-
deed, the polymer film shows excitation wavelength-depen-
dent emission peaks with spectral profiles similar to those of
the thermal curing product (cf., Figure 14).
The unexposed squares of the patterned film emit intense
green light, which is so bright that it looks white visually,
under UV illumination (Figure 16B). This emission corre-
sponds to the inherent energy band gap of hb-P1(6). The
exposed cross-linked lines, however, show only faint emis-
sion, probably due to the mismatching with the excitation
wavelength, coupled with the oxidation during the pattern-
ing process performed in air (Supporting Information,
Figure S3). On the contrary, when excited by a green light,
the exposed areas or the patterned lines emit a red light
(Figure 16C), whereas the unexposed areas are almost none-
missive, owing to their weak absorption in the green spectral
region. Evidently, the emission colors of our polymers are
readily tuned by the photoinduced curing reaction. The
multicolor light-emitting patterning of the polymers may
allow them to be used in the constructions of wavelength-
dependent and -manipulable optical and photonic devices.
This exciting possibility is currently under active exploration
in our laboratories.
Polymer concentration: 0.17 μM. Plots of I0/I vs [PA] in (C) THF/
water mixture (1:9 v/v) and (D) THF. Inset of panel C: fluorescence
image of hb-P1(6) deposited on a filter paper. The letters “HKUST”
were written by dropping aliquots of a PA solution on the filter paper
using a capillary tube. Inset in panel D: chemical structure of PA.
Superamplified Explosive Detection. Nitroaromatic com-
pounds such as 2,4-dinitrotoluene (DNT), 2,4,6-trinitrotoluene
(TNT), and picric acid (PA) are warfare explosives, sensitive
detection of which is of antiterrorism implications. In recent
years, the detection of explosives using fluorescent probes
based on conjugated polymers has attracted much interest.30,31
Our hyperbranched polymers have the following unique fea-
tures: (i) extended electronic conjugation with AEE activity
and signal-amplifying effect,8,11,30,31 (ii) high binding capabil-
ity to analytes due to the presence of numerous molecular
cavities and the potential involvement of Lewis acid-base
interactions,30-32 and (iii) multidimensional excitonic migra-
tion pathways in the three-dimensional space.30 It is thus
expected that hb-P1(m) may function as sensitive fluorescent
chemosensors for explosive detection. Because of its commer-
cial availability, we used PA as a model explosive in this work.
The nanoaggregates of hb-P1(6) in the THF/water mixture
with 90 vol % water were employed as the fluorescent sensor,
while its isolated molecules in pure THF solution was studied
for comparison.
With gradual addition of PA to the nanoparticle suspension
of hb-P1(6) in the aqueous mixture, the emission intensity of

4932 Macromolecules, Vol. 43, No. 11, 2010
Liu et al.
These two eqs can be generalized as follows:
the polymer aggregates is progressively decreased, while the PL
spectral profile maintains unchanged (Figure 17A). The PL
quenching can be clearly recognized at a PA concentration as
low as 1 μg/mL or 1 ppm. Remarkably, the Stern-Volmer plot
of I0/I versus [PA] shows a upward bent curve instead of a
linear line (Figure 17C). The PL of hb-P1(6) in the THF
solution also becomes weaker in the presence of PA. Its corres-
ponding Stern-Volmer plot also bends upward (Figure 17D).
At a PA concentration of 2.4 ꢀ 10-4 M, the PL intensity of hb-
P1(6) is ∼60% of that in the absence of PA. In the presence of
the same amount of PA, however, the emission of the nano-
particle suspension in the aqueous mixture is almost totally
quenched, showing a 69-fold higher sensitivity, in comparison
to that of the polymer solution in THF. The nanoaggregates
can associate with more PA molecules due to the existence of
numerous cavities generated by the loose packing of the
globular spheres of hb-P1(6) containing bulky silole units with
twisted molecular conformations. Furthermore, the aggre-
gates offer additional intersphere exciton diffusion pathways.
Consequently, the nanoaggregates show a larger PL change or
higher sensitivity when exposed to the same amount of ex-
plosive analyte.
Different from the conventional linear Stern-Volmer
plot, the upward bent curves shown in panels C and D of
Figure 17 indicate that the PL quenching becomes more
efficient with increasing quencher concentration. As discussed
in our recent papers, in comparison to the dynamic quenching
model, the static quenching model with an effective quenching
sphere is more adequate to describe the quenching behaviors of
hyperbranched conjugated polymers,31,33 because the poly-
mers can function as molecular containers to capture small
molecule analytes due to the existence of internal cavities and
the potential involvement of Lewis acid-base interactions
between the silole moieties and the PA analytes. The preasso-
ciation of the PA quenchers with the silole polymers leads to
the formation of nonemissive ground-state dark complexes,
whereas the unbound polymers exhibit their natural lifetimes.
However, in the diffusion-controlled dynamic quenching mod-
el, the PL lifetime is shortened with an increase in the quencher
concentration.33 To know which type of the quenching models
is at work, the dependence of the lifetime of hb-P1(6) on the PA
concentration is studied. It is found that the lifetime of the
polymer is independent of the quencher concentration
(Supporting Information, Figure S5). This confirms that the
static quenching model is operating in this hyperbranched
polymer system, as previously observed in other hyperbranched
polymer systems.31
Io
¼ AeV ½PAꢁ þ c
ð17Þ
q
I
Compared with eq 14, eqs 15 and 16 show the same trend
of single-exponential growth but have two additional con-
stants A and c (eq 17). If we conservatively neglect these
two terms, the static quenching constants Vq for the solution
and nanoaggregate systems are approximately 333.3 and
16666.7 L mol-1, respectively, which are much higher than
those (1-185 L mol-1) reported in the literatures for the linear
conjugated iptycene-containing poly(p-phenylenebutadiyny-
lene)s and poly(p-phenyleneethynylene)s.34 At low quencher
concentrations ([PA] f 0), eqs 15 and 16 can be transformed
to eqs 18 and 19 using a mathematical treatment of Taylor
expansion:31
I0
- 1 ¼ 1500½PAꢁ
ð18Þ
I
I
0 - 1 ¼ 33333½PAꢁ
ð19Þ
I
The quenching constant for the nanoaggregate system
given by eq 19 is 33333 L mol-1, which is more than 22-fold
higher than that for its solution counterpart (1500 L mol-1).
It is also higher than those for the linear conjugated poly-
siloles reported in the literature (<20000 L mol-1).32
This extremely high quenching efficiency reveals a super-
amplification effect in the nanoaggregate system.
The superamplification effect is probably associated with
the AEE activity of the polymer, its three-dimensional
topological structure with numerous internal voids for cap-
turing analyte species and multidimensional migration
pathways available to excitons, and the potential involve-
ment of Lewis acid-base interactions between silole units
and PA quenchers. All these factors may have worked
cooperatively to endow the system with highly sensitive
responses to the analytes. It should be pointed out that most
conjugated polyelectrolytes unavoidably suffer from the
ACQ effect in aqueous media due to the strong π-π stacking
interaction between the hydrophobic conjugated polymer
backbones. This effect greatly decreases their sensitivities in
real detection systems and often generates false positive
signals. The AIE- and AEE-active conjugated polyelectro-
lytes are thus promising candidate materials for the devel-
opment of fluorescent chemosensors and bioprobes with
superamplified responses effects and extremely low detection
limits.
In the effective quenching sphere model, I0/I ratio nor-
mally shows an exponential growth dependence on quencher
concentration:33
In addition to the solution and suspension in the liquid
media, the explosive detection can also be done in the solid
state using a polymer-coated strip. For example, a prototype
of a solid-state chemosensor is prepared by simply dipping a
filter paper into a hb-P1(6) solution. After solvent evapora-
tion, a highly emissive test paper is obtained. When aliquots
of a PA solution are dropped onto the polymer-coated
filter paper using a capillary tube, the PA-covered parts
become nonemissive. As can be clearly seen from the exam-
ple shown in the inset of Figure 17C, under UV illumination,
the words of “HKUST” written by dropping the PA aliquots
onto the filter paper emit no light due to efficient PL
quenching by the PA quenchers. These results prompt us
to develop ultrasensitive and ultrafast chemosensor devices
with portability for on-site detections of explosive vapors
and particulates.
I0
¼ eV ½PAꢁ
ð14Þ
q
I
where I0 and I are the fluorescence intensities in the absence
and presence of a quencher, respectively, and Vq is the static
quenching constant in unit of L mol-1. Mathematical fitting
of the I0/I - [PA] plots shown in panels C and D of Figure 17
gives eqs 15 and 16, respectively:
I0
¼ 4:5e333:3½PAꢁ - 3:5
ð15Þ
ð16Þ
I
I0
¼ 2:0e16666:7½PAꢁ - 1:0
I

Article
Macromolecules, Vol. 43, No. 11, 2010 4933
Concluding Remarks
Crystals of 1(1) with quality suitable for X-ray diffraction
(XRD) analysis were grown from a 1:1 THF/ethanol mixture.
Single crystal XRD intensity data were collected at 173 K on a
Bruker-Nonius Smart Apex CCD diffractometer with graphite-
monochromated Mo KR radiation. The data were processed
using the SAINT and SADABS routines, and the structure
solution and refinement were carried out by the SHELXTL
suite of X-ray programs (version 6.10). Particle sizes of the
polymers in the THF/water mixtures were measured on a
Beckman Coulter Delsa 440SX ζ potential analyzer. IR spectra
were recorded on a Perkin-Elmer 16 PC FTIR spectrophot-
ometer. 1H and 13C NMR spectra were measured on a Bruker
ARX 400 NMR spectrometer using chloroform-d or DCM-d2
as solvent and tetramethylsilane (TMS) as internal reference.
UV absorption spectra were taken on a Milton Ray Spectronic
3000 array spectrophotometer. PL spectra were recorded on a
Perkin-Elmer LS 55 spectrofluorometer. Emission efficiencies
of thin films of the polymers were measured by a calibrated
integrating sphere. High-resolution mass spectra (HRMS) were
measured on a GCT Premier CAB 048 mass spectrometer
operating in MALDI-TOF model. Thermal stabilities of the
monomers and polymers were evaluated on TA Instruments
TGA Q5000 and DSC Q1000 under nitrogen at a heating rate of
10 ꢀC/min.
Fluorescence decay curves were recorded on an Edinburgh
Instruments FLS920 combined with steady-state and time-
resolved fluorescence spectrometers. A femtosecond Ti:sap-
phire oscillator was used as excitation laser source (200 fs pulse
width and 76 MHz repetition rate). The second harmonic (358 nm)
of the oscillator output at 716 nm was used for the PL measure-
ment. Time-resolved PL measurements were carried out on a
Hamamatsu model C4334 streak camera coupled to a spectro-
meter, with a time resolution of 20 ps. PL signals were collected at
the emission peak at 500 nm and the laser energy level for
excitation was 2 mW. Decay in the PL intensity (I) with time (t)
was fitted by a double-exponential function (eq 20) and the
weighted mean lifetime Æτæ was calculated according to eq 21:
In thiswork, we have succeeded in the first synthesisof a pairof
σ-π conjugated hyperbranched poly(2,5-silole)s hb-P1(m) by the
polycyclotrimerization of 2,5-diethynylsiloles 1(m) catalyzed by
TaBr5. Polymeric products with high MW (Mw up to ∼2.5 ꢀ 105)
and most probable DB (∼0.55) are obtained in high yields (up to
∼98%). The hyperbranched polysiloles are soluble, durable,
stable and processable (film-forming). The absorptions and
emissions of hb-P1(m) are red-shifted from those of 1(m) due to
the electronic communication between the silole rings and the
newly formed benzene rings. Whereas the monomers and the
polymers are nonemissive or weakly luminescent when molecu-
larly dissolved in their good solvents, they become highly emissive
when aggregated as nanoparticle suspensions in their poor
solvents and as thin films in the solid state, showing unique
AIE and AEE phenomena. The AIE and AEE effects signifi-
cantly boost their PL quantum yields by up to 2 orders of
magnitude. The RIR process is rationalized to be the main cause
for the novel photophysical phenomena.
The emissions of 1(m) and hb-P1(m) can be tuned by varying
their concentrations and morphologies. Large blue shifts in the
emission colors of the spin-coated films of 1(m)/PMMA (2.5 wt %)
blends are observed, implying an unusual shear-induced mech-
anochromism.35 Upon heating or photoirradiation, the hyper-
branched polymers cross-link into network gels that show ex-
citation wavelength-dependent PL in the red spectral region of
610-735 nm. Photolithography of hb-P1(m) generates fluores-
cent patterns, in which the exposed and unexposed areas emit
lights of different colors. The polymers function as sensitive
fluorescent chemosensors for the detection of explosives and a
novel superamplification effect is observed in the process of
emission quenching of polymer nanoaggregates by PA. These
polymers are multifaceted in functionalities and are expected to
find an array of technological applications, especially in the area
of OLED fabrications and explosive detections.
- t=τ2
I ¼ A1e- t=τ þ A2e
ð20Þ
1
Experimental Section
General Information. Toluene, hexane, and THF were dis-
tilled from sodium benzophenone ketyl immediately prior to
use. DCM was distilled over calcium hydride. Triethylamine was
distilled under nitrogen and stored over sodium hydroxide in a
dark, cold place before use. Tantalum(V) bromide, dichlorobis-
A1τ1 þ A2τ2
Æτæ ¼
ð21Þ
A1 þ A2
Thin solid films of hb-P1(m) polymers and 1(m)/PMMA
blends were prepared by spin-coating their 1,2-dichloroethane
solutions (1.5 wt %) at 800 rpm for 10 s and then 1500 rpm for
60 s on precleaned silicon wafers. The films were dried in a
vacuum oven at room temperature to constant weights. The
photopattern images of hb-P1(6) were taken by an OLYMPUS
BX41 fluorescence microscope equipped with a SPOT 7.2 color
mosaic camera.
Monomer Preparation. 2,5-Diethynylsiloles 1(m) were pre-
pared by a multistep reaction protocol according to the synthetic
routes shown in Scheme 1. Typical experimental procedures for
the synthesis of the monomers are given below.
Preparation of Dimethylbis(phenylethynyl)silane [2(1)]. To a
THF solution of phenylacetylene (4.0 mL, 36.4 mmol) was
added n-BuLi (25.0 mL, 40.1 mmol, 1.6 M solution in hexane)
at -78 ꢀC. After stirring at -78 ꢀC for 4 h, dichlorodimethylsi-
lane (2.2 mL, 18.2 mmol) was added. The mixture was warmed
to room temperature and stirred overnight. The solvent was
removed under reduced pressure. The mixture was dissolved in
DCM and washed with brine and water. The organic layer was
dried over magnesium sulfate. The crude product was purified
by a silica-gel column using hexane as eluent. A colorless solid
was obtained in 86.1%. 1H NMR (400 MHz, CDCl3), δ (ppm):
7.57 (m, 4H), 7.36 (m, 6H), 0.55 (s, 6H). 13C NMR (100 MHz,
CDCl3), δ (ppm): 132.1, 128.9, 128.2, 122.6, 105.9, 90.6, 0.45.
HRMS (MALDI-TOF), m/z 260.1013 (Mþ, calcd 260.1021).
(triphenylphosphine)palladium(II), ZnCl2 TMEDA, copper(I)
3
iodide, triphenylphosphine, and other chemicals and solvents were
all purchased from Aldrich and used as received without further
purification.
Mw,r and Mw/Mn values of the polymers were estimated by
GPC using a Waters Associates liquid chromatograph equipped
with a Waters 515 HPLC pump, a set of Styragel columns (HT3,
HT4, and HT6; MW range 102-107), a column temperature
controller, a Waters 486 wavelength-tunable UV-vis detector, a
Waters 2414 differential refractometer, and a Waters 2475
fluorescence detector. The polymer solutions were prepared in
THF (∼2 mg/mL) and filtered through 0.45-μm PTFE syringe-
type filters before being injected into the GPC system. THF was
used as eluent at a flow rate of 1.0 mL/min. The column
temperature was maintained at 40 ꢀC, and the working wave-
length of the UV-vis detector was set at 254 nm. A set of
monodisperse polystyrene standards (Waters) covering MW
range of 103-107 were used for MW calibration. Mw,a values
of hb-P1(m) were measured by commercial laser light scattering
(LLS) spectrometer (ALV/DLS/SLS-5022F) equipped with a
multi-τ digital time correlator (ALV5000) and a cylindrical
22 mW He-Ne laser (λ0 = 632 nm, Uniphase) as light source.
The (dn/dC)632.8 values at 25 ꢀC in THF for hb-P1(1) and hb-P1(6)
were 0.194 and 0.177 mL/g, respectively, determined by using the
Jianke differential refractometer.

4934 Macromolecules, Vol. 43, No. 11, 2010
Liu et al.
Preparation of Di(n-hexyl)bis(phenylethynyl)silane [2(6)]. This
compound was prepared using the procedures similar to those
for the preparation of 2(1). A colorless liquid was obtained in
80.3% yield. 1H NMR (400 MHz, CDCl3), δ (TMS, ppm): 7.52
(m, 4H), 7.34 (m, 6H), 1.60 (m, 4H), 1.50-1.20 (m, 12H),
0.8-1.0 (m, 10H). 13C NMR (100 MHz, CDCl3), δ (TMS,
ppm): 132.1, 128.7, 128.2, 122.9, 106.6, 89.5, 32.7, 31.5, 23.7,
22.6, 14.8, 14.1. HRMS (MALDI-TOF): m/z 400.2586 (Mþ,
calcd 400.2586).
16H), 0.99 (t, 4H), 0.83 (t, 6H). 13C NMR (100 MHz, CD2Cl2), δ
(TMS, ppm): 156.6, 141.9, 141.3, 139.3, 132.2, 130.4, 129.4,
128.0, 127.0, 119.4, 84.3, 77.5, 33.3, 32.0, 24.1, 23.1, 14.4, 12.4.
HRMS (MALDI-TOF), m/z 602.342 (Mþ, calcd 602.337).
Polymer Synthesis. All the polymerization reactions were
carried out under nitrogen using standard Schlenk technique
in a vacuum line system or an inert atmosphere glovebox. A
typical experimental procedure for the polycyclotrimerization
of 1(1) is given below.
To a thoroughly baked and carefully evacuated 15 mL Schlenk
tube with a three-way stopcock on the side arm was placed 8.7 mg
(0.015 mmol) of TaBr5 under nitrogen in a glovebox. Freshly
distilled toluene (2.0 mL) was then injected into the tube using a
hypodermic syringe. After aging for 5 min at 80 ꢀC, a solution of
69.4 mg (0.15 mmol) of 1(1) in 1.0 mL of toluene was injected. The
resultant mixture was stirred at room temperature under nitrogen
for 0.5 h and the polymerization was then quenched by the
addition of a small amount of methanol. The solution was added
dropwise to 300 mL of methanol through a cotton filter under
stirring. The precipitate was allowed to stand overnight and then
collected by filtration. The polymer was washed with methanol
and dried under vacuum at room temperature to a constant
weight.
Preparation of 1,1-Dimethyl-2,5-bis[4-(trimethylsilylethynyl)-
phenyl]-3,4-diphenylsilole [5(1)]. A mixture of lithium (0.056 g,
8 mmol) and naphthalene (1.04 g, 8 mmol) in 8 mL of THF was
stirred at room temperature under nitrogen for 3 h to form a
deep dark green solution of LiNaph. The viscous solution was
then added dropwise to a solution of 2(1) (0.52 g, 2 mmol) in
5 mL of THF over 4 min at room temperature. After stirring for
1 h, the mixture was cooled to 0 ꢀC and then diluted with 25 mL
THF. A black suspension was formed upon addition of
ZnCl2 TMEDA (2 g, 8 mmol). After stirring for an additional
3
hour at room temperature, a solution of [(4-bromophenyl)-
ethynyl]trimethylsilane (1.06 g, 4.2 mmol)13 and PdCl2(PPh3)2
(0.08 g, 0.1 mmol) in 25 mL of THF was added. The mixture was
refluxed overnight. After cooled to room temperature, 100 mL
of 1 M HCl solution was added and the mixture was extracted
with DCM. The combined organic layer was washed with brine
and water and then dried over magnesium sulfate. After solvent
evaporation under reduced pressure, the residue was purified by
a silica-gel column using hexane as eluent. The product was
obtained as a yellow solid in 71.2% yield. IR (thin film), ν
(cm-1): 2156 (CtC stretching). 1H NMR (400 MHz, CD2Cl2), δ
(TMS, ppm): 7.17 (m, 4H), 7.00 (m, 6H), 6.84 (m, 4H), 6.78 (m,
4H), 0.43 (s, 6H), 0.19 (s, 18H). 13C NMR (100 MHz, CD2Cl2), δ
(TMS, ppm): 155.0, 142.1, 140.7, 138.9, 131.7, 130.2, 129.0,
127.8, 126.7, 120.4, 105.5, 94.4, -0.1, -4.1. HRMS (MALDI-
TOF): m/z 606.272 (Mþ, calcd 606.259).
Characterization Data for hb-P1(1). Yellow powder; yield:
75.6%. Mw,r 17600; Mw/Mn 3.2 (Table 1, no. 2). IR (thin film), ν
1
(cm-1): 3291 (tC-H stretching), 2106 (CtC stretching). H
NMR (300 MHz, CD2Cl2), δ (TMS, ppm): 6.70-8.40 (Ar-H),
3.07 (tC-H), 0.45 [Si(CH3)2]. 13C NMR (75 MHz, CD2Cl2), δ
(TMS, ppm): 119.0-158.0 (aromatic carbons), 84.2 (CtCH),
77.5 (CtCH), -3.9 [Si(CH3)2].
Characterization Data for hb-P1(6). Yellow powder; yield:
97.9%. Mw,r 27600; Mw/Mn 4.1 (Table 1, no. 3). IR (thin film), ν
1
(cm-1): 3294 (tC-H stretching), 2107 (CtC stretching). H
NMR (400 MHz, CD2Cl2), δ (TMS, ppm): 6.70-8.00 (Ar-H),
3.07 (tC-H), 0.5-1.8 [Si(CH2CH2CH2CH2CH2CH3)2]. 13C
NMR (100 MHz, CD2Cl2), δ (TMS, ppm): 119.0-158.0
(aromatic carbons), 84.3 (CtCH), 77.5 (CtCH), 33.3, 32.0,
24.1, 23.1, 14.5, 12.5 [Si(CH2CH2CH2CH2CH2CH3)2].
Preparation of 1,1-Dihexyl-2,5-bis[4-(trimethylsilylethynyl)-
phenyl]-3,4-diphenylsilole [5(6)]. This compound was prepared
according to the procedures described above. A yellow solid was
obtained in 60.7% yield. IR (thin film), ν (cm-1): 2156 (CtC
1
stretching). H NMR (400 MHz, CD2Cl2), δ (ppm): 7.17 (m,
Acknowledgment. The work described in this paper was
partially supported by the Research Grants Council of Hong
Kong (603509 and 603008), the University Grants Committee of
Hong Kong (AoE/P-03/08), the Innovation and Technology
Fund of Hong Kong (ITP/008/09NP and ITS/168/09), the
National Natural Science Foundation of China (20634020 and
20974028), and the Ministry of Science and Technology of China
(2009CB623605). BZT thanks the support from Cao Gaungbiao
Foundation of Zhejiang University.
4H), 7.00 (m, 6H), 6.83 (m, 4H), 6.77 (m, 4H), 1.45-1.10 (m,
16H), 0.98 (t, 4H), 0.83 (t, 6H), 0.19 (s, 18H). 13C NMR (100
MHz, CD2Cl2), δ (ppm): 156.5, 141.6, 141.4, 139.4, 132.0, 130.5,
129.4, 128.0, 126.9, 120.5, 105.8, 94.6, 33.3, 32.0, 24.0, 23.1, 14.4,
12.4, 0.2. HRMS (MALDI-TOF), m/z 746.511 (Mþ, calcd
746.416).
Synthesis of 1,1-Dimethyl-2,5-bis(4-ethynylphenyl)-3,4-diphe-
nylsilole [1(1)]. Into a round-bottomed flask equipped with
septum and stirring bar was added 5(1) (1.21 g, 2 mmol), THF
(40 mL), methanol (60 mL), and K2CO3 (345 mg, 2.5 mmol).
The mixture was stirred at room temperature for 8 h and then
neutralized by 3 M HCl solution. The organic solvents were
evaporated and the reaction mixture was extracted by DCM
three times. The organic layers were combined and after solvent
evaporation, the crude product was purified by a silica-gel
column using hexane as eluent. A yellow solid was obtained in
96.4% yield. IR (thin film), ν (cm-1): 3291 (tC-H stretching),
2106 (CtC stretching). 1H NMR (400 MHz, CD2Cl2), δ (TMS,
ppm): 7.21 (m, 4H), 7.00 (m, 6H), 6.86 (m, 4H), 6.79 (m, 4H),
3.06 (s, 2H), 0.44 (s, 6H). 13C NMR (100 MHz, CD2Cl2), δ
(TMS, ppm): 155.3, 142.3, 141.3, 139.1, 132.2, 130.4, 129.3,
128.0, 127.0, 119.5, 84.2, 77.5, -3.9. HRMS (MALDI-TOF),
m/z 462.1821 (Mþ, calcd 462.1804).
Supporting Information Available: Table of crystal struc-
tural parameters of 1(1) and figures showing GPC traces of hb-
P1(m), UV absorption spectra of 1(m) and hb-P1(m) in THF/
water mixtures with 90 vol % water, IR spectra of 1(1) before
and after thermal treatment and UV irradiation, excitation
wavelength-dependent emission spectra of UV irradiated thin
film of hb-P1(6), and fluorescence decay curves of hb-P1(6) in the
presence of different amounts of picric acid in THF, and a .cif
file containg crystallographic resuts for 1(1). This material is
available free of charge via the Internet at http://pubs.acs.org.
References and Notes
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