Aggregation-induced emission of cis,cis-1,2,3,4-tetraphenylbutadiene from restricted intramolecular rotation

Article abstract of DOI:10.1021/jp048475q

cis,cis-1,2,3,4-Tetraphenylbutadiene (TPBD) exhibits aggregation-induced emission (AIE) in the UV-blue band: the photoluminescence (PL) quantum yield of TPBD aggregates can differ from that of molecularly dissolved species by 2 orders of magnitude (>200).

Full text of DOI:10.1021/jp048475q

7522
J. Phys. Chem. A 2004, 108, 7522-7526
Aggregation-Induced Emission of cis,cis-1,2,3,4-Tetraphenylbutadiene from Restricted
Intramolecular Rotation
Junwu Chen,*,† Bin Xu,† Xiaoying Ouyang,† Ben Zhong Tang,‡ and Yong Cao*,†
Institute of Polymer Optoelectronic Materials & DeVices, South China UniVersity of Technology,
Guangzhou 510640, People’s Republic of China, and Department of Chemistry, Hong Kong UniVersity of
Science & Technology, Clear Water Bay, Kowloon, Hong Kong, People’s Republic of China
ReceiVed: April 6, 2004; In Final Form: June 16, 2004
cis,cis-1,2,3,4-Tetraphenylbutadiene (TPBD) exhibits aggregation-induced emission (AIE) in the UV-blue
band: the photoluminescence (PL) quantum yield of TPBD aggregates can differ from that of molecularly
dissolved species by 2 orders of magnitude (>200). When the isolated molecules in solutions are cooled to
extremely low temperature, they also emit intense light comparable to that in the solid state. TPBD thin layer
shows on-off fluorescence switching behavior that can be utilized for the sensing of organic vapors. The
phenyl substituents in TPBD are twisted in the solid state, and excimer formation is greatly prohibited. The
cooling-enhanced emission of the TPBD solution and the fluorescence switching behavior suggest that the
aggregation-induced emission is caused by restricted intramolecular rotation of the phenyl groups. The
intramolecular phenyl rotations of TPBD can be regarded as rotational relaxations around their equilibrium
positions, from which mean relaxation time is defined based on an Arrhenius equation. All the PL behaviors
of TPBD can be well explained qualitatively by the magnitude of the relaxation time.
Introduction
active materials are promising emitters for the fabrications of
highly efficient LEDs.9,10 AIE-active molecules are normally
highly emissive in their crystal forms; therefore, they are
promising candidates for the realization of electrically pumped
lasers11 since crystals of conjugated molecules possess higher
carrier mobilities to afford the high current injection, compared
to their amorphous forms.12
Many conjugated organic emitters are highly emissive in their
dilute solutions but become weakly luminescent when fabricated
into thin films.1 This is believed to be caused by aggregate
formation: in the solid state, the molecules aggregate to form
less emissive species such as excimers, leading to a reduction
in the luminescence efficiency.2 The conjugated materials are
practically utilized in the solid state commonly as thin films,
and mitigation of the aggregation quenching has been a thorny
problem in the development of light-emitting diodes (LED).
Many groups have attempted to obstruct aggregate formation
through elaborate chemical, physical, and engineering ap-
proaches, which have, however, met with only limited success.
Aggregation is in some sense inherent in film formation,3 and
it would be ideal if a molecule can emit intense light in its
aggregation state. Swager and co-workers elegantly observed
an unusual phenomenon that the photoluminescence (PL)
quantum yield (ΦF) of an aggregated film of a poly(p-phenyl-
enethynylene) (PPE) was 3.5 times that of its solution.4 The
same phenomenon with ΦF of neat film of about 8-10 times
higher compared to its solution was also found for four diphenyl-
substituted phenylenevinylene and diphenylenevinylene poly-
mers (PPVs).5 Interestingly, 2,3,4,5-tetraphenylsiloles6 and
1-cyano-trans-1,2-bis(4′-methylbiphenyl)ethylene (CN-MBE)7
were virtually nonemissive in solutions, but their aggregates or
solid films are highly luminescent; a several hundreds of times
change of ΦF of the aggregated phase to the solution was found.
Thus a concept of aggregation-induced emission (AIE) was
proposed.6a Very recently, AIE effects were realized in silole-
containing polyacetylenes8 and also found for bis(4-(N-(1-
naphthyl)phenylamino)phenyl)fumaronitrile (NPAFN).9 AIE-
Several models, such as exciton diffusion, rotational deactiva-
tion, and noncoplanarity, have been proposed for the lower PL
quantum yield or PL quenching in solutions, compared with
the enhanced emission in the solid state.5-8 Belton et al. reported
that the PPV chains had greater spatial freedom in solutions to
explore more planar conformations that reduced the energy
barriers for exciton diffusion along the polymer backbone,
increasing the number of nonradiative routes and decreasing
the PL yield.5a Holzer et al. reported that both the vibrational,
librational, and rotational freedoms of motion of the side chain
substituents of PPVs and the distortions of the main chain units
in solutions seemed to be responsible for the fast nonradiative
relaxation and reduced fluorescence quantum yield.5b They
further pointed out that the lower PL quantum yield in solution
was due to rotational deactivation.13 Park et al.7 proposed that
twisted structure or noncoplanarity was responsible for the PL
quenching of CN-MBE in solution while coplanarization in the
solid state caused their aggregation-induced emission. The
crystal structures of siloles, however, are not consistent with
the former coplanarity hypothesis:6a the phenyl peripheries of
siloles are twisted to a great extent (up to 87°) in the solid
state.6b,14 Based on further experimental results of enhanced
emissions of silole solutions upon cooling and thickening, we
pointed out that deactivations of excitons by phenyl rotations
were responsible for the PL quenching of the siloles in
solutions.6b,8,14
* To whom correspondence should be addressed. Phone: 8620-8711-
4609. Fax: 8620-8711-4535. E-mail: psjwchen@scut.edu.cn (J.C.); poycao@
scut.edu.cn (Y.C.).
To enlarge the family of materials with enhanced emissions
in the solid state, it is necessary to carry out more extensive
studies in the field, particularly to discover the structural
† South China University of Technology.
‡ Hong Kong University of Science & Technology.
10.1021/jp048475q CCC: $27.50 © 2004 American Chemical Society
Published on Web 08/12/2004

Aggregation-Induced Emission of TPBD
J. Phys. Chem. A, Vol. 108, No. 37, 2004 7523
Figure 1. (A) PL spectra of TPBD in acetone, TPBD in water/acetone mixture (90:10 by volume), and its thin solid film (80 nm). Concentration
of TPBD 10 µM; excitation wavelength 345 nm. (B) PL quantum yield of TPBD vs solvent composition of the water/acetone mixture.
SCHEME 1
characteristics of AIE-active molecules. In this paper, we report
the aggregation-induced UV-blue emission of cis,cis-1,2,3,4-
tetraphenylbutadiene (TPBD) and its origin from the restrictions
of the rotations of the phenyl substituents on the butadiene core.
We also demonstrate that the intense emission in aggregation
state can be quenched upon fuming with solvent vapor, showing
on-off fluorescence switching behavior. We point out that
phenyl rotations are rotational relaxations around their equilib-
rium positions, and the mathematical description for the
rotational relaxations is established and discussed.
solution shows a noisy spectrum, peaking at about 408 nm.
When water, the nonsolvent of TPBD, is added to the acetone
solution, TPBD molecules form aggregates in the water-acetone
mixture. The aggregates are highly fluorescent and emit an UV-
blue light with a maximum at 390 nm and a vibronic shoulder
at 410 nm (Figure 1A), indicating that TPBD is AIE-active.
Compared to the TPBD aggregates, its thin solid film (80 nm)
prepared by thermal evaporation under high vacuum also emits
an intense light, with a 3 nm red shift of the emission peak and
the same position of the vibronic shoulder. The emission
maxima of the neat films of the reported PPE and PPVs with
the enhanced solid-state emissions are around 520 nm.4,5
Tetraphenylsiloles, silole-containing linear and hyperbranched
polymers, and CN-MBE normally emit blue-green light while
NPAFN emits light in the red region.6-9 Therefore, the UV-
blue emission of TPBD is unique. Highly efficient UV-blue
emission is valuable for full color display based on color
converters.19
Results and Discussion
Two synthetic methods can be used to prepare TPBD
(Scheme 1). According to a similar procedure by Yamaguchi
et al.,15 TPBD was readily obtained in a yield of 76% by the
desilylation of 1,1-dimethyl-2,3,4,5-tetraphenylsilole16 (route 1
in Scheme 1). Alternatively, according to a classic report by
Smith and Hoehn,17 quenching of 1,4-dilithio-1,2,3,4-tetra-
phenylbutadiene6b with methanol first gave complicated isomers;
further heating of the isomers in toluene at 95 °C for 10 min
and then recrystallization from toluene-heptane mixture also
afforded the same TPBD in 70% yield (route 2 in Scheme 1).
The spectroscopic data of TPBD are listed in the Experimental
Section.
Compared to 1,1,4,4-tetraphenylbutadiene,18 a typical blue
emitter, TPBD derivatives are normally not regarded as
fluorescent materials.15 Indeed, upon excitation at 345 nm, the
fluorescence of TPBD is very faint in common organic solvents
such as acetone (Figure 1A), chloroform, tetrahydrofuran, etc.
The 50 times magnified data of the emission of the acetone
To have a quantitative picture of the AIE process, we
estimated the PL quantum yields (ΦF) of TPBD in water-
acetone mixtures, using 9,10-diphenylanthracene as the refer-
ence.20 The ΦF of the acetone solution is as low as 0.0011.
The ΦF is almost unchanged upon addition of water up to
50 vol %, but starts to swiftly increase afterward (Figure 1B).
When the volume fraction of water in the water/acetone
mixture is increased to 90%, ΦF rises to 0.28, which is about
250 times higher than that of the acetone solution. The trajectory
of the ΦF change suggests that TPBD molecules start to
aggregate at a water fraction of >50% and that the size and

7524 J. Phys. Chem. A, Vol. 108, No. 37, 2004
Chen et al.
Figure 3. (A) Photoluminescence spectra of TPBD in 1,4-dioxane at
different temperatures. (B) Effect of temperature on the peak intensity
of the photoluminescence of TPBD in dioxane and THF. Concentration
of TPBD 10 µM; excitation wavelength 345 nm.
Figure 2. UV absorption spectra of TPBD thin solid film (80 nm)
and TPBD in water/acetone mixtures with different volume fractions
of water.
butadiene core is 75° while that between Ph1 and the chain is
34° in the TPBD crystal (Ph1 and Ph2 are labeled in Scheme
1). Similar to 2,3,4,5-tetraphenylsiloles,6b coplanarity is not
responsible for the AIE mechanism of TPBD. The twisted
structures of phenyl substituents in TPBD and 2,3,4,5-tetraphen-
ylsiloles should be very important for their intense emissions
in the solid state and probably prohibit excimer formations in
the solid state. The absorption and emission of the thin solid
film of TPBD are both blue shifted compared with those of
TPBD solution. This is evidence that excimer hardly forms in
the aggregates of TPBD.
population of the aggregates continue to increase as the water
fraction is increased.
The UV absorption spectra of TPBD thin solid film and
TPBD in water/acetone mixtures with different volume fractions
of water are shown in Figure 2. The absorption spectrum of the
thin solid film peaks at 322 nm, with two vibronic shoulders at
336 and 354 nm. The difference between the absorption peak
and the first vibronic shoulder is 14 nm, which is close to that
in the emission spectrum of the thin solid film. The UV
absorption spectrum of TPBD in acetone peaks at 334 nm, which
is practically identical to the first vibronic shoulder of the thin
solid film. The absorption maxima for the 50, 60, and 90%
mixtures red shift to 342, 344, and 344 nm, respectively, and
similar results were found for siloles in nonsolvent/solvent
mixtures.6 For the 90% mixture, a new peak at 356 nm can be
observed. Notably, the UV absorptions for 60 and 90% mixtures
in the long-wavelength region are greatly intensified. This is a
signature of the aggregate formation, due to light scattering of
aggregates,21 which effectively decreases the light transmission
through the TPBD mixtures. The abrupt increase in the
absorbance from >50% water fraction agrees well with the
sudden jump in the quantum yield shown in Figure 1B, doubly
confirming that TPBD molecules start to aggregate when >50%
of poor solvent (water) is added to the acetone solution. The
large influence of the light scattering of the aggregates on the
absorption results in the big absorption difference between the
aggregates and the thin solid film.
Twisted intramolecular charge transfer (TICT) has been
known to greatly affect the emission properties of some chromo-
phoric molecules containing donor and acceptor groups.22,23 In
such systems, the polarity of the solvent has been found to alter
both the ground and excited states of the molecules. Increasing
the polarity of the solvents of 4-N,N′-dimethylaminoflavone
solutions, for example, red-shifted their absorption and emission
maxima by up to >100 nm and enhanced their ΦF by up to
∼143 times.24 The TPBD is, however, not a push-pull
molecule. The UV absorption maxima of TPBD in cyclohexane,
ethyl acetate, and acetonitrile are at 330, 330, and 328 nm,
respectively; the solvent polarity exerts little effect on the
absorption transitions. TPBD is also virtually nonemissive in
the solutions of the three solvents. The TICT mechanism does
not contribute to the AIE process of TPBD.
Cooling-enhanced emission was found for isolated species
of siloles, demonstrating that phenyl rotations of siloles are
thermally activated.6b We check the temperature effect on the
photoluminescence of TPBD in dilute solutions. Two solvents,
dioxane (mp ) 11.8 °C) and THF (mp ) -108 °C), were
selected as solvents for low-temperature PL measurements.
When a dilute dioxane solution of TPBD (10 µM) was cooled,
the intensity of its PL spectrum was increased (Figure 3A).
When cooled from room temperature to below the melting point
of dioxane, the liquid solution changed to a solid “glass”. The
PL intensity progressively increased when the temperature was
successively decreased from 25 to -78 °C, indicating that the
cooling is gradually limiting the thermally activated intramo-
lecular rotations of phenyls of TPBD. Further decreasing the
temperature from -78 to -196 °C caused little change in the
peak intensity, implying that the intramolecular rotations may
have already been frozen at -78 °C. When THF was used as
the solvent, no significant enhancement of PL intensity was
found from 25 to -18 °C (Figure 3B) since THF possesses a
higher solvating power and much lower melting point compared
to dioxane. Restriction of the thermally activated intramolecular
rotations began to be effective at -78 °C, although the PL
intensity was much lower than that of the frozen dioxane at
-78 °C. Further cooling the THF solution to a frozen temper-
ature (-196 °C) brought the PL intensity to a level comparable
to that of dioxane. The PL intensity at -196 °C is ∼280-fold
higher than that at room temperature, which is comparable to a
ΦF increase of aggregates at 90% water fraction in the acetone
solution, indicating that the restrictions of the rotations of the
phenyl groups of isolated TPBD molecules in the solid “glasses”
at much lower temperature (-196 °C) are similar to those of
TPBD aggregates.
Although the intramolecular phenyl rotations of TPBD are
restricted in the solid state, they can still be activated by solvent
vapors, causing fluorescence quenching like that in solutions.
As shown in Figure 4, a TPBD thin layer developed on a TLC
silica plate emits bright and deep blue light under illumination
with a 365 nm UV lamp at room temperature; the fluorescence
The crystal structure of TPBD, as reported by Karle and
Dragonette,25 shows that TPBD is a centrosymmetric molecule,
and the butadiene core of TPBD is almost coplanar while the
four phenyls are twisted. The angle between Ph2 and the

Aggregation-Induced Emission of TPBD
J. Phys. Chem. A, Vol. 108, No. 37, 2004 7525
Experimental Section
Materials. THF and 1,4-dioxane (Aldrich) were predried over
4 Å molecular sieves and distilled from sodium benzophenone
ketyl immediately prior to use. Diphenylacetylene and lithium
wire were purchased from Aldrich and used as received without
further purification. 1,1-Dimethyl-2,3,4,5-tetraphenylsilole was
prepared according to a previous report.16
Instrumentation. The 1H and 13C NMR spectra were
obtained on a Bruker DRX 400 spectrometer. The UV-visible
absorption spectra were recorded on an HP 8453. Gas chroma-
tography-mass spectrometry (GC-MS) was performed on a
Finnigan Trace 2000. Melting points were measured on a Model
X-4 of Beijing Taike Instruments. The fluorescence display of
a TPBD thin layer on a TLC plate was photographed on a Sony
DSC-5P digital camera.
The PL spectra of the silole solutions and the nanoaggregate
mixtures were recorded on a Jobin Yvon Fluorolog-3 spectro-
fluorometer. The sample chamber of the spectrofluorometer was
dehumidified with P2O5. The nanoaggregate mixtures were
freshly prepared by adding poor solvents into the TPBD
solutions with vigorous shaking. For example, a nanoparticle
mixture of TPBD was prepared by adding 8 mL of water into
2 mL of an acetone solution in a 10 mL volumetric flask. The
concentrations of all the aggregate mixtures were adjusted to
10 µM.
Figure 4. On-off fluorescence switching of TPBD thin layer
developed on the TLC plate without vapor (left) and in vapor
(dichloromethane) (right) under UV light (365 nm) illumination at room
temperature.
turns off reversibly in the atmosphere of dichloromethane vapor.
The on-off fluorescence switching behavior of TPBD thin layer
can thus be utilized for the sensing of organic vapors. The same
behavior was found for CN-MBE, an AIE-active molecule.7
Thus the on-off fluorescence switching behavior could be a
common property for AIE-active chromophores.26
It is evident that the extent of intramolecular phenyl rotation
plays a dominant role in the PL behaviors of TPBD and
tetraphenylsiloles. Here we point out that the intramolecular
rotations can be regarded as rotational relaxations around their
equilibrium positions and the mean relaxation time (τ) can be
expressed with the following well-known Arrhenius equation:
PL characterization of TPBD on a TLC plate was measured
according to a previous report,6b performed on the Jobin Yvon
Fluorolog-3 spectrofluorometer. Merck TLC plates (Silica 60
F254) were used for TLC-fluorescence measurement. The TPBD
thin layer was developed from its chloroform solution (2 mg/
mL).
τ ) τ0 e(∆E/RT)
(1)
Synthesis of TPBD by Desilylation (Route 1). According
to a similar preparation method by Yamaguchi et al.,15 a mixture
of 1,1-dimethyl-2,3,4,5-tetraphenylsilole (1.24 g, 3 mmol) and
potassium hydroxide (1.68 g, 30 mmol) in toluene/water 2:1
mixed solvent (15 mL) was heated to reflux for 20 h. After
dilution with an aqueous solution of HCl (0.1 N), the mixture
was extracted with ether several times. The combined extract
was washed with brine, dried over anhydrous MgSO4, and
concentrated. Recrystallization from a toluene/heptane mixture
afforded 0.81 g of TPBD in 76% yield. White solids; mp )
where τ0 is a constant, ∆E is the activation energy for the
rotational relaxation, R is the gas constant, and T is the absolute
temperature (K).
All the PL behaviors of TPBD can be well explained
qualitatively with eq 1. For TPBD molecularly dissolved in a
solvent at room temperature, the activation energy ∆E for the
rotational relaxation is very small due to the strong solvation
power of the solvent, giving a very small τ which opens the
nonradiative channel. For the case of TPBD aggregates, ∆E
can be very high due to the big intramolecularly steric
restrictions to the intramolecular phenyl rotations in the solid
state, giving a large τ which effectively closes the nonradiative
channel and populates the radiative decay. The activation energy
∆E in the solid state decreases again when a TPBD thin layer
on a TLC plate is fumed by a solvent vapor, which causes the
on-off fluorescence switching behavior. The τ increases with
decreasing of temperature, and this causes the cooling-enhanced
emission.
In summary, TPBD is an AIE-active molecule: virtually
invisible in common organic solvents while highly emissive in
the UV-blue band in the aggregation state. Upon cooling, the
isolated TPBD molecules in frozen glasses (-196 °C) also emit
intense light comparable to that in the solid state. The on-off
fluorescence switching behavior of TPBD thin layer can be
utilized for the sensing of organic vapors. Restricted intramo-
lecular rotation contributes to the AIE behavior. The intramo-
lecular phenyl rotations of TPBD can be regarded as rotational
relaxations around their equilibrium positions, from which mean
relaxation time is defined based on an Arrhenius equation. All
the PL behaviors of TPBD can be well explained qualitatively
by the magnitude of the relaxation time.
1
183-184 °C. H NMR (400 MHz, CDCl3) δ [ppm]: 7.43-
7.33 (m, 10H), 7.04 (m, 6H), 6.76 (m, 4H), 6.33 (s, 2H); 13C
NMR (100 MHz, CDCl3) δ [ppm]: 145.6, 139.8, 137.3, 131.7,
130.4, 129.5, 128.8, 127.8, 127.4, 126.6; GC-MS: m/z 358
(M+); UV (CHCl3) λmax (nm)/ꢀmax (dm3 mol-1 cm-1): 334/
3.05 × 104.
Synthesis of TPBD by Route 2. Under dry nitrogen, 177
mg (25.4 mmol) of freshly cut lithium shavings were added to
a solution of diphenylacetylene (5 g, 28.0 mmol) in 25 mL of
THF. The mixture was stirred for 12 h at room temperature.
Methanol (2 mL) was dropwise added to the resultant THF
solution of 1,4-dilithio-1,2,3,4-tetraphenylbutadiene. The mixture
was further stirred for 1 h. The crude product was purified on
a silica gel column using hexane/chloroform mixture (15:1 by
volume) as eluent. Heating of the product (TPBD isomers) in
toluene at 95 °C for 10 min and then recrystallization from
toluene-heptane mixture afforded 3.18 g of pure TPBD in 70%
yield (based on lithium).
Acknowledgment. Financial support from the Natural Sci-
ence Foundation of China (No. 50303006) and the Ministry of
Science and Technology of China (No. 2002CB613404) is
gratefully acknowledged.

7526 J. Phys. Chem. A, Vol. 108, No. 37, 2004
Chen et al.
(14) Chen, J.; Peng, H.; Law, C. C. W.; Dong, Y.; Lam, J. W. Y.;
Williams, I. D.; Tang, B. Z. Macromolecules 2003, 36, 4319.
(15) Yamaguchi, S.; Endo, T.; Uchida, M.; Izumizawa, T.; Furukawa,
K.; Tamao, K. Chem. Eur. J. 2000, 6, 1683.
(16) Tang, B. Z.; Zhan, X.; Yu, G.; Lee, P. P. S.; Liu, Y.; Zhu, D. J.
Mater. Chem. 2001, 11, 2874.
(17) Smith, L. I.; Hoehn, H. H. J. Am. Chem. Soc. 1941, 63, 1184.
(18) See, for example: (a) Kim, J. H.; Noh, S.; Kim, K.; Lim, S. T.;
Shin, D. M. Synth. Met. 2001, 117, 227. (b) Huang, J. S.; Yang, K. X.;
Xie, Z. Y.; Liu, S. Y.; Jiang, H. J. Opt. Quantum Electron. 2001, 33, 165.
(c) Gardecki, J. A.; Maroncelli, M. Appl. Spectrosc. 1998, 52, 1179. (d)
Wen, W. K.; Jou, J. H.; Chiou, J. F.; Chang, W. P.; Whang, W. T. Appl.
Phys. Lett. 1997, 71, 1302.
(19) Qiu, C. F.; Wang, D.; Chen, H. Y.; Wong, M.; Kwok, H. S. Appl.
Phys. Lett. 2001, 79, 2276.
(20) Morris, J. V.; Mahaney, M. A.; Huber, J. R. J. Phys. Chem. 1976,
80, 969.
(21) (a) Tang, B. Z.; Xu, H.; Lam, J. W. Y.; Lee, P. P. S.; Xu, K.; Sun,
Q.; Cheuk, K. K. L. Chem. Mater. 2000, 12, 1446. (b) Tang, B. Z.; Geng,
Y.; Lam, J. W. Y.; Li, B.; Jing, X.; Wang, X.; Wang, F.; Pakhomov, A. B.;
Zhang, X. X. Chem. Mater. 1999, 11, 1581.
(22) (a) Rettig, W.; Baumann, W. In Progress in Photochemistry and
Photophysics; Rabek, J. F., Ed.; CRC Press: Boca Taton, FL, 1990; Vol.
6, Chapter 3, p 79. (b) Suppan, P. Chemistry and Light; Royal Society of
Chemistry: London, 1994.
References and Notes
(1) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.;
Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A.; Bredas, J.
L.; Logdlund, M.; Salaneck, W. R. Nature 1999, 397, 121.
(2) Jenekhe, S. A.; Osaheni, J. A. Science 1994, 265, 765.
(3) Film Formation in Coatings: Mechanisms, Properties, and Mor-
phology; Provder, T., Urban, M. W., Eds.; American Chemical Society:
Washington, DC, 2001.
(4) Deans, R.; Kim, J.; Machacek, M. R.; Swager, T. M. J. Am. Chem.
Soc. 2000, 122, 8565.
(5) (a) Belton, C.; O’Brien, D. F.; Blau, W. J.; Cadby, A. J.; Lane, P.
A.; Bradley, D. D. C.; Bryne, H. J.; Stockmann, R.; Horhold, H. H. Appl.
Phys. Lett. 2001, 78, 1059. (b) Holzer, W.; Penzkofer, A.; Stockmann, R.;
Meysel, H.; Liebegott, H.; Horhold, H. H. Polymer 2001, 42, 3183.
(6) (a) Luo, J.; Xie, Z.; Lam, J. W. Y.; Cheng, L.; Chen, H.; Qiu, C.;
Kwok, H. S.; Zhan, X.; Liu, Y.; Zhu, D.; Tang, B. Z. Chem. Commun.
2001, 1740. (b) Chen, J.; Law, C. C. W.; Lam, J. W. Y.; Dong, Y.; Lo, S.
M. F.; Williams, I. D.; Zhu, D.; Tang, B. Z. Chem. Mater. 2003, 15, 1535.
(7) An, B. K.; Kwon, S. K.; Jung, S. D.; Park, S. Y. J. Am. Chem.
Soc. 2002, 124, 14410.
(8) Chen, J.; Xie, Z.; Lam, J. W. Y.; Law, C. C. W.; Tang, B. Z.
Macromolecules 2003, 36, 1108.
(9) Yeh, H. C.; Yeh, S. J.; Chen, C. T. Chem. Commun. 2003, 2632.
(10) Chen, H. Y.; Lam, J. W. Y.; Luo, J. D.; Ho, Y. L.; Tang, B. Z.;
Zhu, D. B.; Wong, M.; Kwok, H. S. Appl. Phys. Lett. 2002, 81, 574.
(11) Though Scho¨n et al. claimed that electrically pumped lasers were
realized (Scho¨n, J. H.; et al. Science 2000, 289, 599; Science 2000, 290,
963), the papers have since been retracted by the journal (Science 2002,
298, 961).
(12) (a) Funahashi, M.; Hanna, J. Phys. ReV. Lett. 1997, 78, 2184. (b)
Schein, L. B.; McGhie, A. R. Phys. ReV. B 1979, 20, 1631.
(13) Holzer, W.; Penzkofer, A.; Stockmann, R.; Meysel, H.; Liebegott,
H.; Horhold, H. H. Synth. Met. 2002, 125, 343.
(23) (a) Morozumi, T.; Anada, T.; Nakamura, H. J. Phys. Chem. B
2001, 105, 2923. (b) Bajorek, A.; Paczkowski, J. Macromolecules 1998,
31, 86. (c) Soujanya, T.; Fessenden, R. W.; Samanta, A. J. Phys. Chem.
1996, 100, 3507. (d) Grabowski, Z. R. Pure Appl. Chem. 1993, 65,
1751.
(24) Wang, P.; Wu, S. J. Lumin. 1994, 62, 33.
(25) Karle, I. L.; Dragonette, K. S. Acta Crystallogr. 1965, 19, 500.
(26) We have found that siloles also possess the same property.