Enhanced emission efficiency and excited state lifetime due to restricted intramolecular motion in silole aggregates

Article abstract of DOI:10.1021/jp046659z

The aggregation-induced emission (AIE) properties of 1,1,2,3,4,5- hexaphenylsilole (HPS) and poly{11-[(1,2,3,4,5-pentaphenylsilolyl)oxy]-1-phenyl- 1-undecyne} (PS9PA) were studied by time-resolved fluorescence technique. The enhanced fluorescence and long

Full text of DOI:10.1021/jp046659z

J. Phys. Chem. B 2005, 109, 1135-1140
1135
Enhanced Emission Efficiency and Excited State Lifetime Due to Restricted Intramolecular
Motion in Silole Aggregates
Yan Ren,†,‡ Jacky W. Y. Lam,§ Yongqiang Dong,§ Ben Zhong Tang,§ and Kam Sing Wong*,†
Departments of Physics and Chemistry, The Hong Kong UniVersity of Science and Technology,
Clear Water Bay, Kowloon, Hong Kong, China, and State Key Laboratory of Crystal Materials,
Shandong UniVersity, Jinan, 250100, People’s Republic of China
ReceiVed: July 27, 2004; In Final Form: October 30, 2004
The aggregation-induced emission (AIE) properties of 1,1,2,3,4,5-hexaphenylsilole (HPS) and poly{11-
[(1,2,3,4,5-pentaphenylsilolyl)oxy]-1-phenyl-1-undecyne} (PS9PA) were studied by time-resolved fluorescence
technique. The enhanced fluorescence and long fluorescent lifetime were obtained for the sample in an aggregate
state as compared to the sample in solution. The time-decay of fluorescence of HPS and PS9PA in high
viscosity solvents and low-temperature glasses has also been measured in detail to further investigate the
possible mechanism for AIE. Enhanced light emission and long fluorescence lifetime were detected for both
HPS and PS9PA in the solution-thickening and -cooling experiments. These results provided direct evidence
that the enhanced photoluminescence (PL) efficiency is due to restricted intramolecular motion, which ascribes
AIE to the deactivation of nonradiative decay caused by restricted torsional motions of the molecules in the
solid state or aggregate form.
Introduction
strongly depend on temperature and viscosity of the solvent,
we expect strong modification of PL efficiency and emission
lifetime as a function of these parameters.
The emission efficiency is a key factor to evaluate the
performance of an organic electroluminescent device.1,2 In these
devices, the luminescent materials are normally used in a form
of thin film prepared by spin coating its solution onto the glass
substrate or by the vacuum vapor deposition technique. But
frequently, we find that highly fluorescent organic materials in
solution become nonluminescent or weakly luminescent in the
solid state. Therefore, how to mitigate the aggregation quenching
has always been an attractive issue.3,4 Recently, silole-based
organic light-emitting materials have attracted much attention
because of their unusual highly luminescent properties in the
aggregate state.5-9 An intriguing phenomenon was found in that
a spot of HPS solution just dropped on the TLC plate was
nonluminescent under the UV lamp, while it became highly
luminescent after the solvent evaporated. Thereafter, the en-
hanced PL and high fluorescence quantum yield of this kind of
silole-based material was found in the ethanol solution after
the addition of excessive water, a poor solvent of this material.
Nanoparticles of aggregate siloles were detected in the mixed
solvents with the size of a few tens to 100 nm.6 Recently, a
number of other compounds were also found to show strong
aggregation-induced emission.10,11 Measurements showed that
PL efficiency of the silole-based compounds increases several
orders of magnitude in the solid or aggregate form as compared
to the solution state.5 It is suggested that the possible mechanism
of the AIE is the nonradiative channel via the vibrational/
torsional energy relaxation processes which is blocked in the
aggregate state, thus populating the radiative state of excitons,
and turns on the emission for this kind of compound.6 Since
vibrational, rotational, and torsional motions of a molecule
The fluorescence lifetime and quantum yield are perhaps the
most important characteristics of light-emitting materials. The
lifetime determines the time available for the fluorophore to
interact with its environment, and the information can be
available from its emission. Since the steady-state fluorescence
is an average of the time-resolved phenomena over the intensity
decay of the sample, much of the molecular information
available from fluorescence is lost during the time-averaging
process. By contrast, time-resolved fluorescence can provide
valuable information about the interaction of molecule in the
excited state with its environment.12,13 In this paper, to further
investigate the possible mechanism for AIE, we studied the PL
enhancement behaviors of siloles in detail by using a time-
resolved fluorescence technique with a Ti:sapphire femtosecond
pulse as the excitation source. Solution-thickening and -cooling
experiments were performed for HPS and PS9PA. The evidence
obtained from these experiments strongly supported the re-
stricted intramolecular rotation model for the AIE.
Experimental Procedures
Materials.
HPS was synthesized by first reacting diphenylacetylene with
an appropriate amount of lithium metal and then reacting the
resultant 1,4-dilithiotetraphenylbutadiene with dichlorodiphe-
nylsilane. Monomer of PS9PA was prepared by nucleophilic
substitution of 1-chloro-1,2,3,4,5-pentaphenylsilole with 11-
phenyl-10-undecyn-1-ol. Polymerization of the monomer was
effected by the WCl6-Ph4Sn catalyst in toluene at 60 °C, which
gave PS9PA in ∼60% yield. Detailed procedures for the
synthesis of HPS and PS9PA can be found in our previous
publications.6,7,14 The polymeric PS9PA in addition to mono-
meric HPS is studied because PS9PA can be spin-coated into
* Corresponding author. E-mail: phkswong@ust.hk. Tel: (852) 2358
7475. Fax: (852) 2358 1652.
† Department of Physics, The Hong Kong University of Science.
‡ Shandong University.
§ Department of Chemistry, The Hong Kong University of Science.
10.1021/jp046659z CCC: $30.25 © 2005 American Chemical Society
Published on Web 01/04/2005

1136 J. Phys. Chem. B, Vol. 109, No. 3, 2005
Ren et al.
Figure 2. UV-vis absorption (0.002 wt %) and fluorescence (2 wt
Figure 1. SEM photomicrograph of HPS particles in solution
containing 90% volume fraction of water and 10% DMF imaged two
weeks after preparation. The scale shown in the figure is 1 µm under
the magnification of 8500.
%) spectra of benzene solutions of HPS and PS9PA.
and fluorescence peaks are a little red-shifted as compared to
those of HPS. Laser pulses at 267 nm were used as the excitation
laser source for the PL measurements. This was produced by
the frequency summation of 800 nm with its 400 nm frequency-
doubling from a Ti:sapphire regenerative amplifier (200 fs pulse
width and 1 kHz repetition rate). For time-resolved fluorescence
measurements, the fluorescence signals were collimated and
focused onto the entrance slit of a monochromator with the
output plane connected to a synchroscan streak camera (Hamamat-
su Model C4334, 20 ps resolution). The PL signals were
measured near the emission peak at ∼500 nm. In the optical
setup, a flip mirror was inserted before the monochromator-
streak camera system to reflect the fluorescence to a fiber
spectrometer (Ocean Optics Inc.) to record the time-integrated
PL spectra before each run of the lifetime measurement. The
laser energy level for excitation is ∼400 µW. In the experiment,
all samples used were freshly prepared.
smooth thin solid films so that the polymer-based light-emitting
diode can be fabricated. The development of PS9PA thus will
extend the silole-based compounds for organic optoelectronic
applications using both evaporation and spin-coating techniques.
Three groups of samples were prepared for the PL lifetime
measurements. The mixed solutions of HPS were prepared by
different volume ratios of water into the HPS/DMF solution
being added. The final solutions obtained by this method have
the identical concentration of 1.3 × 10-5 mol/L. DMF was used
as the solvent because of its good miscibility with water. The
solution with 30% water was clear when mixing was finished,
while those containing 70 and 90% water became slightly milky.
This indicates that some particles with large sizes are formed
after excessive water is added into the DMF solution, and the
scattering of the visible light by these particles makes the
solutions misty. Despite all this, all the solutions prepared were
homogeneous without visible deposits observed after mixing.
Moreover, the solution with 90% water was very stable even
after it was kept for more than two months. There was no trace
of visible deposits of HPS in the solution. The SEM image of
HPS particles obtained from the mixed solution (two weeks after
preparation) containing 90% volume fractions of water are
shown in Figure 1. This shows that HPS forms particles of a
few hundred nanometers in size.
Results and Discussion
In the previous measurements,5 the fluorescence quantum
yields (ΦF) of silole aggregates were dramatically enhanced by
up to several hundred times of those in pure solutions with
identical concentrations. In our experiment, HPS in pure solution
was also weakly emissive even though it was excited with a
relatively high peak power focused laser. By contrast, very bright
fluorescence was observed in the sample containing a 70 and
90% volume fraction of water.
1,2-Dichloroethane, ethylene glycol monoethyl ether, and
benzyl alcohol were used for preparing the other group of
samples for the solution-thickening experiment. We selected
these solvents because they possess different viscosities but
similar polarity to avoid the possible effect of polarity on the
PL of the materials. The samples used in the solution-cooling
measurement are 2 wt % HPS and PS9PA/DMF solutions.
Instrumentation.
Before the PL lifetime measurement, the UV-vis absorption
and time-integrated PL spectra were measured for HPS and
PS9PA by using a UV-2401 (Shimadzu) spectrophotometer and
fiber spectrometer (Ocean Optics Inc.), respectively, to select
the optimal excited laser source and wavelength for the PL signal
collection. Benzene solution (0.002 wt %) was used in the UV-
vis absorption measurement, and 2 wt % benzene solution was
used in the time-integrated PL spectrum measurement.
The absorption and fluorescence spectra are shown in Figure
2. The maximum absorption and fluorescence of HPS is located
at ∼366 and ∼500 nm, respectively. For PS9PA, the absorption
The strongly enhanced fluorescence of HPS in solution with
an increased water volume fraction also showed corresponding
dramatic changes in emission lifetime as illustrated by the time-
resolved fluorescence curves in Figure 3. It can be shown that
ΦF ) τ/τr, where PL lifetime τ is related to radiative (τr) and
nonradiative (τnr) lifetimes by the expression 1/τ ) 1/τr + 1/τnr.
τr is the intrinsic property of the molecule; therefore, it is a
constant. The low ΦF and the PL lifetime τ are essentially
limited in system resolution for HPS in pure DMF solvent and
in mixtures with low volume fractions of water (few tens of
picoseconds), indicative of strong nonradiative recombination
process. Increasing the water fraction caused an enhanced
emission and increased the PL lifetime to nanoseconds. The
lifetime data are shown in Table 1. The fluorescence decay
behavior of HPS in pure DMF solvent with the same concentra-
tion as that of the DMF/H2O mixture is well-fitted by the single-
exponential function. It shows that the excited state relaxation
of HPS in pure solvent is actually a single-exponential decay
(i.e., all the excited molecules of HPS decay through the same

Intramolecular Motion in Silole Aggregates
J. Phys. Chem. B, Vol. 109, No. 3, 2005 1137
Figure 3. Time-resolved fluorescence of HPS in solution with different
volume fractions of water and DMF. The identical concentration for
the mixtures is 1.3 × 10-5 mol/L.
TABLE 1: PL Lifetime of HPS in DMF/H2O Mixtures with
Identical Concentrations of 1.3 × 10-5 mol/L
H2O (%)
A1/A2
τ1 (ns)
τ2 (ns)
0
30
70
90
1/0
0.04
0.10
0.82
1.27
0.80/0.20
0.50/0.50
0.43/0.57
3.75
4.98
7.16
pathway). Its fluorescence lifetime is only as short as 40 ps,
which is close to the resolution limit of the streak camera system
(∼25 ps). The fluorescence lifetime data for HPS in the mixture
are also given in Table 1, which were obtained by fitting the
time-resolved fluorescence curves based on the following
double-exponential function:
Figure 4. (A) Fluorescence peak intensities of DMF solution of HPS
(2 wt %) at different temperatures and (B) its PL spectra at 295, 135,
and 30 K.
-t/τ2
y ) A1e-t/τ + A2e
drastically the radiative/nonradiative recombination processes
of the excited state.15-19 For HPS in dilute solution, since its
peripheral benzene rings rotate actively around the single bonds
linked to the central silole ring, the excitons are believed to be
effectively annihilated by this kind of intramolecular torsional
motion. Therefore, siloles are nonluminescent or weakly lumi-
nescent in solution. While in aggregate state, the intramolecular
motion is restricted. According to the previous mechanism, the
nonradiative decay channel is blocked, which populates the
radiative decay and makes the siloles luminescent. We then
performed the time-resolved fluorescence measurements of HPS
and PS9PA in viscous solution and at low temperature because
both approaches are expected to effectively hamper the in-
tramolecular motion.6 Through the previous two experiments,
we try to investigate the real role of restricted intramolecular
motion on the PL properties of siloles.
In the low-temperature experiment, the fluorescence became
stronger when the sample temperature was decreased. This was
especially apparent when the sample was cooled to a temperature
below the melting point of DMF (mp ) -60.4 °C;20 however,
the melting point of the siloles/DMF solution could be much
lower than that of pure DMF), and the fluorescence signal was
so strong that we had to use an attenuator to decrease the
intensity to avoid saturation of the signal in the streak camera.
Figure 4A shows the fluorescence peak intensity of HPS/DMF
solution at different temperatures. The emission intensity peaks
at about 135 K, and at this temperature, the emission intensity
is over 2 orders of magnitude stronger than that at room
temperature. Further cooling of the sample below 135 K caused
it to decrease in emission intensity. The decrease in intensity at
1
Fitting based on this function gave better fitting results than
that on the single-exponential function for the sample with water.
It indicates that the molecules in the mixed solvent may exist
in two different environments, or they decay through two
relaxation pathways. The values of A1 and A2 represent the
fractional amount of molecules in each environment. The decay
in one pathway is short, and that in another is relatively long.
For HPS in 30% water mixture, 80% of the molecules decay
through the fast pathway. The lifetime is only 0.1 ns for the
fast decay component. With the volume fraction of water in
the mixture getting higher, the decay time of molecules through
the fast pathway is over 20 times longer than that in pure solvent.
In the mixture containing 90% water, the molecular radiative
decay is mainly through the slow pathway. The fluorescence
lifetime of the slow decay rises to ∼7 ns. Although measurement
on PL quantum yield as a function of volume fraction of water
shows a threshold at ∼50% water fraction,5,6 the time-resolved
measurement is able to separate short- and long-lived compo-
nents and observation of small slow PL decay component at
<50% water fraction, indicating that there are already some
aggregates formed in the solutions with low water fractions.
The amount of aggregates is small at the low water volume
fractions, so that it has a small contribution to the overall
quantum yields as compared to the large water volume fraction
case.
Restricted intramolecular motion has been suggested as the
possible mechanism of AIE previously described,6,7 considering
that vibrational/torsional motion of the molecule can affect

1138 J. Phys. Chem. B, Vol. 109, No. 3, 2005
Ren et al.
state relaxation is a single-exponential decay), and the decay is
very fast due to efficient nonradiative process via vibrational/
torsional molecular motion. Their lifetimes are as short as about
40 ps for HPS in 1,2-dichloroethane and DMF solvents as shown
in Figures 5 and 6, respectively. As the viscosities of the solvents
become high and the sample temperature decreased, the in-
tramolecular motion was restricted to some degree due to a
change in the environment. The time-decay process of HPS is
characterized by radiative/nonradiative depopulation of the
excited state through two pathways. Moreover, more than 50%
of silole molecules recombined through the slow pathway as
shown in Table 2. In a high viscosity solution, the decay time
of HPS through the fast pathway rises to 10 times of that in the
low viscosity solution. The time-decay behaviors of HPS are
distinctive with the solution cooling, especially when the
temperature was decreased to below the melting point of DMF
where the intramolecular motion of HPS is effectively restricted.
For example, at 150 K, the fast decay of HPS takes 1.23 ns,
and its slow decay takes 7.19 ns, which is up to ∼30-fold of
that at room temperature (Table 3). The lifetime of HPS becomes
longer at even lower temperature. As shown in the inset of
Figure 6, the excited state decay at 30 K is well over 10 ns,
indicating that this is the intrinsic radiative recombination
lifetime of the HPS excited state. The fluorescence enhancement
in restricted intramolecular vibrational/torisional state can be
obtained not only from the small silole molecule but also from
the silole-based polymer. It is unavoidable that the inherent
structural difference between PS9PA and HPS will have an
effect on recombination dynamics; hence, the measured lifetime
and temperature effect are slightly different from each other.
However, as shown in Tables 2 and 3, the general behaviors
are essentially the same (i.e., the PS9PA solution exhibits a
substantial increase in PL decay time at low temperatures and
in high viscosity solvents).
Figure 5. Time-resolved fluorescence of HPS solutions (0.2 wt %)
with different solvent viscosities at room temperature.
Figure 6. Time-resolved fluorescence of DMF solution of HPS (2 wt
%) at different temperatures. The inset is the PL decay at 30 K at 5 ns
time scale, showing the slow decay component of PL at the low
temperature.
It has been suggested that planarization of the molecule upon
formation of aggregate state21 may have caused this enhanced
emission.5 However, in this case, we would expect to see red
shifts and well-resolved vibronic features in the emission spectra
in the aggregate or solid samples as compared to the solution
case. In our case, both silole solution and aggregate emissions
are broad and featureless, and there are essentially no shifts in
the emission peak positions. Furthermore, recent X-ray mea-
surement showed that the ground state structure of the silole
crystal is actually also nonplanar.6 The silole crystal structure
is shown in Figure 7, which clearly indicates that the silole
molecule is highly nonplanar; thus, this model of planarization
in the aggregation state giving rise to high fluorescence yield
can be ruled out. However, planarization may take place at the
excited state since there is large red-shift in the emission with
respect to absorption. This cannot explain the large enhancement
in the PL yield in aggregate as compared to solution. The PL
spectra are sharp, and peak positions are the same for the silole
solution and aggregate, which indicates that the degree of
excited-state planarization is about the same for both cases. We
also note that planar conformation of the excited state will
normally emit fluorescence with well-resolved vibronic fea-
tures,22 which is in contrast to our case where the fluorescence
is featureless. Furthermore, interaction with the solvent mol-
ecules is expected to progressively reduce for growing thermal
activation so that planarization becomes more effective at higher
temperatures.22 This would yield stronger PL at higher temper-
atures; however, our temperature-dependent measurements in
Figure 4 show an opposite trend.
low temperature below the melting point of the solution was
caused by the scattering loss because when the solution freezes
into solid, it is highly scattered. This is further confirmed by
lifetime measurements where the PL decay time continues to
increase from 150 to 30 K as shown in the next section. The
PL spectra at three different temperatures are shown in Figure
4B. One can see that the spectra shape and peak positions of
the PL spectra are not shifted with sample cooling. Similar to
the cooling experiment, an enhancement in fluorescence ef-
ficiency was also observed from the samples with higher solvent
viscosity.
The time-resolved fluorescence spectra of HPS in solutions
with different viscosities and at different temperatures are shown
in Figures 5 and 6, respectively. The fitted decay lifetimes from
the time-resolved PL data are summarized in Tables 2 and 3.
One can see that their fluorescence decay behavior in solution-
thickening and -cooling experiments can be well-interpreted by
the restricted intramolecular motion model, which deactivates
the nonradiative pathways and hence enhances the fluorescence
of siloles for high viscosity solvents or at low temperatures.
For HPS in low viscosity solvent and at room temperature, since
the molecules are basically in a completely free state and little
restriction imposed on intramolecular motion, A1 is close to 1.
Almost all molecules are in the same environment and decay
through the same pathway (i.e., A1 is close to 1 so that excited-

Intramolecular Motion in Silole Aggregates
J. Phys. Chem. B, Vol. 109, No. 3, 2005 1139
TABLE 2: Room Temperature PL Lifetime of 0.2 wt % HPS and 0.17 wt % PS9PA in Solvents with Different Viscosities
solventa
HPS
PS9PA
ηb
ꢀc
A1/A2
τ1 (ns)
τ2 (ns)
A1/A2
τ1 (ns)
φ2 (ns)
DMFd
DCE
EGME
BA
0.794
0.799
1.85
38.2520
10.4220
13.3825
11.9230
1/0
0.04
0.05
0.09
0.24
0.86/0.14
0.79/0.21
0.53/0.47
0.49/0.51
0.16
0.08
0.13
0.42
0.58
0.84
1.53
2.47
0.96/0.04
0.63/0.37
0.60/0.40
2.17
1.38
3.40
5.47
a Abbreviations: DMF ) dimethylformamide, DCE ) 1,2-dichloroethane, EGME ) ethylene glycol monoethyl ether, and BA ) benzyl alcohol.
b Viscosity at 25 °C. c Dielectric constant at given temperature. d Concentration: 2 wt %.
TABLE 3: PL Lifetime of DMF Solutions of HPS and
PS9PA (2 wt %) at Different Temperatures
HPS
PS9PA
τ1 (ns) τ2 (ns)
T (K)
A1/A2
1/0
1/0
0.51/0.49
0.43/0.57
0.34/0.66
τ1 (ns) τ2 (ns)
A1/A2
295
250
200
150
30
0.04
0.11
0.31
1.23
2.49
0.86/0.14
0.72/0.38
2.89 0.58/0.42
7.19 0.44/0.56
10.39 0.26/0.74
0.16
0.69
0.79
0.80
1.18
0.58
3.43
9.24
8.26
7.44
The previous results provide strong evidence that the en-
hanced PL efficiency of siloles is due to restricted intramolecular
motion. The phenomenon observed was also consistent with
the previous report about the enhancement of time-integrated
fluorescence in a restricted intramolecular rotation state of
siloles.6,7 Therefore, we can understand the AIE of HPS in the
following way by analyzing the packing state of HPS molecules
in nanoparticles in the DMF/water mixture: we consider that
HPS molecules existed in different aggregate morphologies in
the system as different volume fractions of water were added.
In the dilute solution without water added, molecules exist
completely in a free state without a restriction imposed on the
intramolecular torsional motions. Therefore, molecules of HPS
are nonluminescent or weakly luminescent due to depopulation
of the excited state mainly by nonradiative decay; after a small
fraction of water was added in the DMF solution, because water
is a poor solvent of HPS, some molecules began to pack in the
aggregate state. However, most of the molecules are still in the
free state, and aggregate particles formed are small in size. In
one particle, there may be only a few molecules to aggregate.
In addition, the molecules in the aggregate state are less in
quantity as compared to that of the molecules in the free state.
Consequently, the fluorescence enhancement effect caused by
restricted intramolecular motion is not distinct. However, after
a large volume fraction of water was added in the system, for
example, in a mixture containing 70 or 90% water, the aggregate
particles with large sizes predominated in morphology of HPS
molecules in the mixture. The particle size is large enough to
be easily observed under SEM. From Figure 1, one can see
that the size of particles formed in the mixture containing 90%
water is at the range of few tens to few hundreds of nanometers
in diameter. Obviously, in one particle, there are approximately
a few hundreds of packed molecules. In this situation, HPS is
characterized by the enhanced emission because of the intramo-
lecular vibrational/torsional motions restricted in these packed
nanoparticles. Although it is well-known that well-packed
aggregates with π-π stacking such as in an H-aggregate tend
to prevent radiative recombination, the silole molecule has
distinct nonplanar structure in the solid state with an interplane
molecular distance of ∼10 Å and virtually no π-π stacking as
shown in Figure 7. This suppresses the intermolecular interaction
that tends to induce nonradiative recombination as seen in
H-aggregates. The interplane and intermolecular distances
indicated in Figure 7 are the separation between the silicon
atoms in the molecules; however, the neighboring phenyl rings
Figure 7. Crystal structure of silole clearly shows that the silole
molecule is nonplanar. The interplane distance is 10.04 Å, and
intermolecular distance within the unit cell is 7.61 Å.
are very close to each other, and this will restrict the torsional/
vibrational motions of these molecules in the solid state due to
steric hindrances.
It has been proposed that the enhanced emission in the film
or solid state is a result of arresting the geometry relaxation of
the excited fluorescent state to a nonfluorescent state located
between HOMO and LUMO levels.11 This model is entirely
consistent with our result that the torsional motion is restricted
in the aggregate so that this prevents or at least partially restricts
geometry relaxation in the excited state. Currently, there is no
experimental evidence for this nonfluorescent state in silole
compounds. However, photoinduced absorption experiments are
now underway to verify the existence of this nonfluorescent
state (i.e., triplet state). Nevertheless, our temperature depen-
dence and solvent viscosity measurements, irrespective of
whether this dark state model is correct, indicate clearly that
restricted motion of the phenyl rings of the silole molecules
enhances the radiative recombination.
Conclusions
We studied the PL enhancement behaviors of siloles in the
aggregate state in detail by using a time-resolved fluorescence
technique. The time-decay behaviors of siloles in solution-
thickening and -cooling experiments provide direct evidence
for the restricted intramolecular motion model. Our conclusion
is that the AIE of silole is attributed to the deactivation of
nonradiative decay by restricted intramolecular vibrational and
torsional motions.
Acknowledgment. This research was supported in part by
the Research Grant Council of Hong Kong under Grants
N_HKUST610/02 and HKUST6085/02P. The optical experi-
ment was performed in the Joyce M. Kuok Laser and Photonics
Laboratory at The Hong Kong University of Science and
Technology.

1140 J. Phys. Chem. B, Vol. 109, No. 3, 2005
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