Hyperbranched poly(phenylenesilolene)s: Synthesis, thermal stability, electronic conjugation, optical power limiting, and cooling-enhanced light emission

Article abstract of DOI:10.1021/ma034012r

Silole-containing hyperbranched polyphenylenes (1) are synthesized, which exhibit high thermal stability, extended electronic conjugation, excellent optical power limiting performance, and novel cooling-enhanced photoluminescence. The homopolycyclotrimeri

Full text of DOI:10.1021/ma034012r

Macromolecules 2003, 36, 4319-4327
4319
Hyperbranched Poly(phenylenesilolene)s: Synthesis, Thermal Stability,
Electronic Conjugation, Optical Power Limiting, and Cooling-Enhanced
Light Emission
J u n w u Ch en , Ha n P en g, Ch a r les C. W. La w , Yu p in g Don g, J a ck y W. Y. La m ,
Ia n D. Willia m s, a n d Ben Zh on g Ta n g*
Department of Chemistry, Center for Display Research, Institute of Nano Science and Technology, and
†
Open Laboratory of Chirotechnology, Hong Kong University of Science & Technology,
Clear Water Bay, Kowloon, Hong Kong, China
Received J anuary 5, 2003; Revised Manuscript Received May 1, 2003
ABSTRACT: Silole-containing hyperbranched polyphenylenes (1) are synthesized, which exhibit high
thermal stability, extended electronic conjugation, excellent optical power limiting performance, and novel
cooling-enhanced photoluminescence. The homopolycyclotrimerization of 1,1-diethynyl-2,3,4,5-tetraphe-
nylsilole (2) and its copolycyclotrimerizations with 1-octyne catalyzed by TaCl
5
4
-Ph Sn proceed smoothly
at room temperature and produce completely soluble polymers in high yields (up to ∼85%). The molecular
structures of 1 are characterized by spectroscopic analyses. The thermal stability of 1 is evaluated by
thermogravimetric analyses, which detect virtually no weight losses when the polymers are heated to
∼
300 °C. The hyperbranched polyphenylenes are electronically conjugated, as suggested by their strong
absorption in the visible spectral region (λmax ∼ 520 nm). Because of this extended electronic conjugation,
polymers 1 are nonlinear optically active and strongly attenuate the optical power of intense laser pulses,
whose optical limiting performances are superior to that of C60, the best-known optical limiter. The
photoluminescence of the polymers is dramatically enhanced by cooling their solutions to low temperatures.
This unique phenomenon of cooling-enhanced emission is probably caused by the restricted intramolecular
rotations of the phenyl rings upon the axes of the single bonds linked to the silole cores at the cryogenic
temperatures.
In tr od u ction
Ch a r t 1
Silacyclopentadienes or siloles are a group of orga-
nometallic molecules that possess novel electronic and
optical properties. The five-membered silacyclics exhibit,
1
for example, high electron acceptability and fast elec-
tron mobility.2 We have recently observed a novel
phenomenon of aggregation-induced emission (AIE) in
this group of molecules: the siloles are practically
nonluminescent when molecularly dissolved in good
solvents but become highly emissive when aggregated
Ch a r t 2
3
-5
into nanoclusters or fabricated into thin solid films.
The AIE effect greatly boosts the quantum yields of
photoluminescence of the siloles (by more than 300-fold),
turning them from faint fluorophores into strong emit-
ters. Utilizing this AIE property, we have fabricated
silole-based light-emitting diodes, which exhibit truly
outstanding electroluminescence performance. A diode
of 1,1,2,3,4,5-hexaphenylsilole (HPS; Chart 1), for ex-
ample, radiates brilliantly (luminance up to ∼60 000 cd/
2
m ), while that of 1-methyl-1,2,3,4,5-pentaphenylsilole
(
MPS) shows an extremely high external quantum
3
,4
efficiency (ηEL ) 8%), reaching the theoretical limit
for a light-emitting diode based on a singlet emitter.
6
,7
We are interested in incorporating the silole chro-
mophores into conjugated polymers in an effort to create
new materials that are easily processable and have
useful optoelectronic properties. In our previous work,
we designed and synthesized silole-containing poly-
of the poly(silolylacetylene)s are completely soluble in
common solvents and are readily processable. Like their
silole parents, the polymers are nonemissive in molecu-
larly dissolved solutions. When poor solvents are added
into the dilute solutions, the polymers aggregate into
nanosize particles. The nanoaggregates of poly{11-
8
acetylenes, examples of which are given in Chart 2. All
[
(1,2,3,4,5-pentaphenylsilolyl)oxy]-1-phenyl-1-undec-
yne} (PS9PA) and poly{11-[(1,2,3,4,5-pentaphenylsilol-
yl)oxy]-1-undecyne} (PS9A) are highly luminescent
†
An Area of Excellence (AoE) Scheme administrated by the
University Grants Committee of Hong Kong.
*
358-7375; Fax +852-2358-1594; e-mail tangbenz@ust.hk.
(
AIE-active); in contrast, those of poly(1,2,3,4,5-pen-
taphenylsilolylacetylene) (PSA) are nonradiative (AIE-
To whom correspondence should be addressed: phone +852-
2
1
0.1021/ma034012r CCC: $25.00 © 2003 American Chemical Society
Published on Web 05/23/2003

4
320 Chen et al.
Macromolecules, Vol. 36, No. 12, 2003
Sch em e 1
F igu r e 1. ORTEP drawing of 2 with atom-labeling scheme
for Table 2.
Ta ble 1. Su m m a r y of Cr ysta l Da ta a n d In ten sity
Collection P a r a m eter s for
1
,1-Dieth yn yl-2,3,4,5-tetr a p h en ylsilole (2)
empirical formula
mol wt
C32H22Si
434.59
Sch em e 2
crystal dimensions, mm
crystal system
0.35 × 0.25 × 0.15
triclinic
space group
P-1
unit cell constants
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
11.1276(8)
12.5644(9)
18.1209(13)
106.305(3)
99.974(3)
90.573(3)
2390.3(3)
4
-
3
Dcalcd, g cm
F000
1.208
912
inactive). The photoluminescence efficiencies of PS9(P)A
can be boosted by cooling their molecularly dissolved
solutions (cooling-enhanced emission).
temp, K
radiation, λ, Å
µ(Mo KR) mm
2θmax, deg [completeness]
no. of collected reflns
no. of unique reflns [R(int)]
data/restraints/parameters
R1, wR2 [obs I > 2σ(I)]
R1, wR2 (all data)
100(2)
Mo KR, 0.710 70
0.116
50 [98.5%]
12 129
8283 [0.0329]
8283/0/599
0.0474, 0.0746
0.0718, 0.0816
0.883
In the work discussed above, the silole-containing
linear polyacetylenes are synthesized by metathesis
-1
8
,9
polymerization of acetylenes (or monoynes). In this
work, we intend to generate nonlinear or three-
dimensional silole polymers by incorporating the silole
rings into hyperbranched polyarylene structures via
cyclotrimerization of diacetylenes (or diynes). We uti-
goodness of fit, S
lized the diyne polycyclotrimerization route we recently
residual peak/hole e- Å-3
0.333/-0.319
1.0/0.81
developed1
0,11
and succeeded in the synthesis of hyper-
transmission ratio
branched poly(phenylenesilolene)s 1 (Scheme 1). All of
the polymers are completely soluble in common organic
solvents due to their highly branched molecular struc-
tures. The polymer solutions are somewhat luminescent
but are completely AIE-inactive. The luminescence of
the hyperbranched polymers can, however, be enhanced
by cooling their solutions to low temperatures.
sium bromide, leads to the formation of the desired
monomer 2.
The silolyldiyne (2) gives satisfactory analysis data
see Experimental Section for details). Its crystal struc-
(
ture was studied by X-ray diffraction, and the crystal
and diffraction data are summarized in Table 1. Crys-
tals of 2 belong to the triclinic system, space group P-1,
with two molecules per asymmetric unit. The structure
refinement proceeded smoothly to final discrepancy
indices R1 ) 4.7% and wR2 ) 8.16% for data to 2θ )
50°. The two independent molecules in the asymmetric
unit have similar molecular geometries but differ slightly
in the dispositions of the twist angles of the phenyl rings
with respect to the silacyclopentadiene plane. A plot of
one molecule in the asymmetric unit with labeling
scheme is displayed in Figure 1, and the selected
geometric parameters are listed in Table 2.
Resu lts a n d Discu ssion
Mon om er a n d P olym er Syn th eses. To make silole
susceptible to diyne polycyclotrimerization, we attached
two ethynyl groups to the 1,1-position of the silacyclo-
pentadiene ring. Following the synthetic route shown
in Scheme 2, we prepared a silolyldiyne monomer, 1,1-
diethynyl-2,3,4,5-tetraphenylsilole (2). Lithiation of to-
lane (4) gives 1,4-dilithiotetraphenylbutadiene (5), the
1
2
cyclization of which with silicon(IV) chloride yields 1,1-
8
dichloro-2,3,4,5-tetraphenylsilole (6). Its nucleophilic
Search of the Cambridge Structural Database (CSD
5.23) found 51 records for silacyclopentadienes, and the
substitution with a Grignard reagent,13 ethynylmagne-

Macromolecules, Vol. 36, No. 12, 2003
Silole-Containing Hyperbranched Polyphenylenes 4321
Ta ble 2. Geom etr ica l P a r a m eter s for Cr ysta l of 1,1-Dieth yn yl-2,3,4,5-tetr a p h en ylsilole (2)
Bond Length (Å)
1.853(2)
Si1-C10
C10-C20
Si1-C1
C1-C2
1.862(2)
1.353(3)
1.824(2)
1.182(3)
Si1-C40
C20-C30
Si1-C3
C3-C4
Si1-C1
C(30)-C(40)
1.824(2)
1.365(3)
Si1-C3
1.808(2)
1.503(3)
1.808(2)
1.185(3)
Bond Angle (deg)
106.45(15)
125.69(15)
106.99(10)
176.3(2)
C10-Si1-C40
C10-C20-C30
C10-C20-C21
Si1-C1-C2
93.86(9)
Si1-C10-C20
Si1-C10-C11
C1-Si1-C3
Si1-C3-C4
116.68(18)
123.49(19)
176.7(2)
Torsion Angle (deg)
Si1-C10-C20-C30
C16-C11-C10-C20
C32-C31-C30-C20
3.3(2)
-44.6(4)
-66.8(3)
C10-C20-C30-C40
C42-C41-C40-C30
C26-C21-C20-C30
0.5(3)
-31.8(3)
-58.2(3)
Ta ble 3. Syn th esisa a n d P r op er ties of Hyp er br a n ch ed P oly(p h en ylen esilolen e)s 1
feed ratio
[3]/[2]
polymer yield
(%)
Tde
(°C)
FLf
(mJ /cm )
λemh
(nm)
b
c
Mw/Mnc
Apen/Aphd
2
Ft,m/Fi,mg
ΦFi
no.
Mw (Da)
1
2
3
4
0
83.0 (1a )
85.3 (1b)
67.6 (1c)
34.0 (1d )
5320
5820
3610
3530
1.6
1.7
1.4
1.4
0 (0)
395
378
355
343
185
182
190
0.19
0.24
0.28
0.32
505
493
504
499
0.01
0.01
0.01
0.01
0.5
1.0
1.5
11/45 (11/45)
11/34 (11/23)
11/28 (11/16)
1140
a
By the homopolymerization of diyne 2 or the copolymerizations of 2 with 1-octyne 3 catalyzed by TaCl5-Ph4Sn in toluene at room
temperature in an atmosphere of dry nitrogen for 24 h; [2] ) 0.072 M, [TaCl5] ) [Ph4Sn] ) 20 mM. Molar ratio. c Estimated by GPC in
THF relative to polystyrene. Integrated areas of proton absorption peaks of pentyl (Apen) and phenyl groups (Aph); data given in the
b
d
parentheses are the theoretic ratios of the pentyl protons to the aromatic and acetylenic protons calculated from the monomer feed ratios.
e
f
Temperature for 5% weight loss (TGA, under nitrogen, heating rate: 20 °C/min). Optical limiting threshold (incident fluence at which
g
the nonlinear transmittance is 50% of the linear one). Signal suppression (ratio of the saturated transmitted fluence to the maximum
incident fluence). Emission maximum (in THF). Quantum yield of fluorescence (using 9,10-diphenylanthracene as standard).
h
i
Sch em e 3
geometry of the silacyclopentadiene ring in 2 matches
those found in the CSD database.14 The silole ring
carbons C10-C20 and C30-C40 have characteristic
double-bond lengths of 1.353(3)-1.365(3) Å. The exocy-
clic ethynyl groups have typical triple-bond lengths of
1
.182(3)-1.185(2) Å, whose geometry is essentially
linear, with Si1-C1(3)-C2(4) angles close to 180°. Silole
has a planar silacyclopentadiene ring, with torsion
2
angles e3.3°. The phenyl substituents on the silole ring
are typically twisted out of the silacyclopentadiene
plane, with smaller torsion angles for the phenyl groups
next to the silicon [from -44.6(4)° to -31.8(3)°] and
larger angles for the groups at the C20 and C30
positions [from -66.8(3)° to -58.2(3)°].
Polycyclotrimerization of 2 is effected by TaCl5-Ph4-
Sn in toluene at room temperature, which gives a
hyperbranched polyarylene 1a in a high yield (83.0%;
Table 3, no. 1). Polymer 1a is completely soluble in such
common solvents as toluene, THF, chloroform, and
dichloromethane, thanks to its highly branched molec-
ular structure. The gel permeation chromatography
ization of an active branch carrying a triple-bond
functional group with two triple bonds of two monoyne
molecules would produce a “dead” benzene ring that
11
ceases to propagate. We envision that further increas-
ing the molar feed ratio will further decrease the
molecular weight of the polymer as well as its yield; this
is confirmed by the polymerization data given in Table
3
(no. 4). Like the homopolymer (1a ), all of the copoly-
(
GPC) analysis (relative to linear polystyrene) gives a
mers (1b-1d ) are also soluble and processable, forming
thin films upon spin-coating their solutions onto solid
substrates.
weight-average molecular weight of 5320. This value is
probably considerably underestimated because of the
hyperbranched nature of the polymer, coupled with its
rigid molecular structure. In our previous studies on
Ch a r a cter iza tion of Molecu la r Str u ctu r es. The
structures of the polymers were characterized by spec-
troscopic methods. An example of the IR spectrum of
polymer 1b is shown in Figure 2; in the same figure,
the spectrum of its monomer 2 is also given for com-
parison. The tC-H and CtC stretching vibrations of
1
0,11,15
hyperbranched polymers,
the underestimation was
as large as 7-fold,11 and the actual or real molecular
weight of 1a thus could be much higher than the relative
value estimated from the GPC analysis.
-
1
Copolycyclotrimerization of diyne 2 with monoyne 3
in a molar feed ratio [3]/[2] ) 0.5 produces a copolymer
2 occur at 3252 and 2038 cm , respectively. These
acetylenic bands are not observed in the spectrum of
1b, indicating that the hyperbranched polymer contains
no acetylene groups. The triple bonds may have been
exhausted by the cyclotrimerization reactions, or in
other words, the alkyne building blocks have been
consumed in the construction processes of the aromatic
cores (Scheme 1) and peripheries (Scheme 3).
(
1b) with a similar molecular weight (5820), again in a
high yield (85.3%; Table 3, no. 2). Increasing the feed
ratio to 1.0 decreases the molecular weight of the
resultant polymer, possibly due to the increased pos-
sibility of end-capping of the propagating branches by
the monoyne molecules (Scheme 3). The cyclotrimer-

4
322 Chen et al.
Macromolecules, Vol. 36, No. 12, 2003
F igu r e 2. IR spectra of (A) silolyldiyne 2 and (B) its polymer
b.
1
F igu r e 4. 13C NMR spectra of (A) 1-octyne 3 (in CDCl
silolyldiyne 2 (in CD Cl ), and (C) their polymer 1b (in CD
Cl ). The solvent peaks are marked with *.
), (B)
3
2
2
2
-
2
The two partially overlapping peaks in the spectral
region of δ 0.5-2 (c′ and d′) are unambiguously due to
the resonance of the aliphatic protons of the pentyl
group originally from 1-octyne (3), while the strong
single peak in the downfield region of δ ∼ 6-8 is the
sum of the absorptions of the aromatic protons of the
“
old” phenyl rings of diyne 2 and the “new” ones formed
by the cyclotrimerizations. The ratio of the integrated
areas of absorption peaks of the pentyl (Apen) and phenyl
protons (Aph) for 1b is 11/45, coinciding with the
theoretical value calculated from the molar ratio ([3]/
[2] ) 0.5 or 1/2) of the monomer feed (Table 3, no. 2).
This suggests that in this case the cyclization process
involves an average of one (1) molecule of monoyne 3
and two (2) molecules of diyne 2. When the feed ratio is
increased, the theoretical Apen/Aph value increases rap-
idly, although the experimental one increases much
slowly. When the feed ratio is increased to 1.5, the
experimental value (11/28) becomes very different from
the theoretical one (11/16). This is not surprising
because if the reaction had proceeded according to the
exact stoichiometry of the monomer feed mixture, no
polymer products would have been formed. The differ-
ence between the experimental and theoretical values
suggests that many monoyne molecules have been
consumed by their self-cyclotrimerizations into low
molecular weight benzene derivatives of 1,2,4- and/or
F igu r e 3. 1H NMR spectra of (A) 1-octyne 3 (in CDCl
silolyldiyne 2 (in CD Cl ), and (C) their polymer 1b (in CD
Cl ). The solvent peaks are marked with *.
), (B)
3
2
2
2
-
2
The acetylenic protons of monoyne 3 (a; Figure 3A)
and diyne 2 (e; Figure 3B) resonate at δ 1.92 and 2.60,
respectively. These peaks are not observed in the
spectrum of polymer 1b (Figure 3C). The alkyne poly-
cyclotrimerization converts the acetylenic triple bonds
to aromatic rings, giving a strong absorption peak of
aryl protons in the downfield region. The aryl absorption
peak is, however, totally structureless, preventing us
1
0,11
1,3,5-trihexylbenzenes,
which were washed away
from estimating the isomeric ratio of the 1,2,4- and
during the purification processes of the polymeric
product. The higher relative ratio of the monoyne moiety
in polymer 1d (11/28 or 39%), in comparison to that in
1b (11/45 or 24%), indicates that a larger amount of
1-octyne 3 has been incorporated into the structure of
1d . In other words, the end-capping reaction (Scheme
3) is more active when the feed ratio is increased. The
active termination should decrease the molecular weight
of the polymer as well as its yield, in agreement with
the experimental results.
,3,5-conformers of the polymer.1
0,11
Accompanying the
1
polycyclotrimerization, the propargyl protons (b) of
1
-octyne (3) are transformed to the benzyl ones (b′) of
b and the proton resonance is shifted downfield ac-
1
1
6
cordingly. The resonance signals of the benzyl protons
are very weak, spreading over a wide spectral range (δ
2
-4), because the protons are located in the immediate
10,11,16
neighborhood of the rigid aryl rings.
Similar
phenomena have been observed in our linear polyacet-
ylene and hyperbranched poly(ferrocenylenesilyne) sys-
tems: the absorption “peaks” of the allylic protons
in the vicinity of the stiff polyacetylene backbones
Figure 4 shows the 13C NMR spectra of monoyne 3,
diyne 2, and polymer 1b. The acetylenic carbons of
1-octyne (3) absorb at δ 84.6 and 68.0, while those of 2
resonate at δ 97.6 and 81.4. All of these acetylenic peaks
disappear in the spectrum of 1b, again confirming that
the triple bonds have been consumed by the cyclotrim-
(
dC-CH2),17 and the methylene protons next to the
15
rigid ferrocene rings (Fc-CH2) are also very weak and
broad and sometimes undetectable.

Macromolecules, Vol. 36, No. 12, 2003
Silole-Containing Hyperbranched Polyphenylenes 4323
F igu r e 5. UV spectra of THF solutions (10 µg/mL) of
hyperbranched poly(phenylenesilolene)s.
F igu r e 7. Optical responses to 8 ns, 10 Hz pulses of 532 nm
laser light, of dichloromethane solutions (0.70 mg/mL) of
hyperbranched poly(phenylenesilolene)s. Data for poly(phe-
nylacetylene) (PPA) and C60 solutions are shown for compari-
son. Linear transmittance (%): 70 (1a ), 80 (1b), 82 (1c), 85
(1d ), 79 (C60), 75 (PPA).
moieties, is as high as 395 °C, while the Td of 1d , which
has the highest content of monoyne moieties, decreases
to 343 °C. This is easy to understand: the incorporation
of the weak aliphatic moieties into the copolymers
should enhance their thermolytic susceptibility, and the
copolymers should thus decompose at lower tempera-
tures.
The hyperbranched polymers are also optically stable.
Figure 7 shows the optical responses of the polymer
solutions to 532 nm laser pulses, along with the data
1
9
for the solutions of poly(phenylacetylene) (PPA) and
2
0
C60. The transmitted fluence of PPA, a linear acety-
lenic polymer, linearly increases with an increase in the
incident influence (linear transmittance T ) 75%). The
hyperbranched polyarylenes show comparable linear
transmittance in the low-fluence region (T ) 70-85%)
but become opaque in the high-fluence region. The
polymers strongly attenuate the power of the intense
laser pulses, whose optical limiting performances are
F igu r e 6. TGA thermograms of hyperbranched poly(phe-
nylenesilolene)s.
erization reactions. The propargyl carbon of 1-octyne
exhibits a strong, sharp resonance peak (c) in Figure
4
A, which is, however, absent in the spectrum of 1b.
The propargyl peak is replaced by a weak, broad
benzylic peak (c′) in the downfield region (Figure 4C)
due to cyclization.1
20-22
superior to that of C60, the best-known optical limiter.
Among the hyperbranched polymers, 1a exhibits the
0,11
best performance, which starts to limit the optical power
The siloles absorb in the UV spectral region (λmax ∼
2
at a low threshold (185 mJ /cm ) and suppresses the
,4,8
3
78 nm).3
All of the hyperbranched polymers, how-
optical signals to a great extent (19%; Table 3, no. 1).
The optical limiting properties decrease when the
content of the nonconjugated 1-alkyne moieties is
increased in the hyperbranched polymer (although their
performances are still better than that of C60). The high
degree of electronic conjugation in 1a (cf. Figure 5) may
be responsible for its high optical nonlinearity.
Coolin g-En h a n ced Ligh t Em ission . The dilute
solutions (10 µg/mL) of the hyperbranched polymers
emit blue-green light peaked at ∼500 nm when excited
at 407 nm (Figure 8). The photoluminescence spectrum
of the polymer blue shifts from 1a to 1d , probably due
to the difference in the degree of conjugation of the
polymers. In our previous study, we found that the
polyacetylenes prepared from the metathesis polymer-
izations of silolylmonoynes (Chart 2) were nonemissive
when molecularly dissolved in dilute solutions.8 As
shown in Figure 8, polymers 1a -1d are photolumines-
cent, even when they are molecularly dissolved. This
difference in photoluminescence behavior between poly-
mers 1a -1d and the polyacetylenes rules out the
possibility that silolyldiyne 2 underwent alkyne me-
ever, absorb in the visible region (Figure 5). The
polymers thus must possess extended conjugation, pos-
sibly due to the synergistic interplay of the σ-π conju-
gation of the silole rings and the electronic communi-
1
,15,18
cation of the aromatic rings via the silicon bridges.
Homopolymer 1a is the most conjugated on the basis of
its strongest absorption in the visible (λmax ∼ 520 nm).
All three copolymers (1b-1d ) are less conjugated,
probably due to the incorporation of nonconjugating
1
-alkyne moieties into the macromolecular structures.
Th er m a l a n d Op tica l Sta bilities. All of the silole-
containing hyperbranched polymers are thermally
stable: no weight losses are detected by the TGA
analyses when the polymers are heated to ∼300 °C
(Figure 6). About half of their original weights remain
when the polymers are heated to ∼800 °C; that is, the
polymers graphitize or ceramize upon pyrolysis. The
temperatures for 5% weight loss (Td’s) for the polymers
are given in Table 3. The thermal stability of the
polymers decreases with increasing monoyne compo-
nent. Thus, the Td of 1a , which contains no monoyne

4
324 Chen et al.
Macromolecules, Vol. 36, No. 12, 2003
F igu r e 8. Photoluminescence spectra of hyperbranched poly-
phenylenesilolene)s 1a -1d in THF and 1b in a methanol/
chloroform mixture (9:1 by volume) at room temperature; [1]
10 µg/mL, λex ) 407 nm.
F igu r e 10. Photoluminescence spectra of 1b in dioxane at
different temperatures; [1b] ) 10 µM, λex ) 407 nm.
(
)
F igu r e 11. Effect of temperature on the peak intensity of the
photoluminescence of 1b in dioxane and THF; [1b] ) 10 µM,
F igu r e 9. Peak intensity of photoluminescence of 1b vs
solvent composition of methanol/chloroform mixture at room
temperature; [1b] ) 10 µM, λex ) 407 nm. Data for 1,2,3,4,5,6-
hexaphenylsilole (HPS) in water/acetone mixtures are shown
for comparison.
λex ) 407 nm.
rods and the hard polyarylene balls may have randomly
piled up when they aggregate in the poor solvent
environments. Such chaotic stacks of the rigid polymers
may generate large free volumes in the aggregates,2
in which the phenyl rings may still be able to rotate
round the axes of the single bonds linked to the
silacyclopentadiene core. Such intramolecular interac-
tions may have nonradiatively deactivated the excited
5,26
tathesis polymerization or that polymers 1a -1d possess
a linear polyacetylene structure.23
Compared to low molecular weight silole analogues,
the hyperbranched polymers are more luminescent [ΦF
2
7
)
1% for the polymer (Table 3) vs ΦF e 0.1% for the
species and hence quenched the silole emission, as
3
,4,14,24
8,14
siloles
]. In general, however, the polymers are still
discussed in our previous papers.
weak emitters. As discussed in the Introduction section,
the emission of the silole molecules can be enhanced by
We then checked whether the photoluminescence of
polymers could be enhanced by cooling. When a dilute
dioxane solution of polymer 1b is cooled, its photolu-
minescence intensity increases (Figure 10). This is
because the intramolecular rotations of the phenyl rings
round the axes of the silacyclopentadiene core are
impeded at the low temperatures, which blocks the
radiationless channel and populates the radiative decay
aggregation (AIE)3 and cooling (CCE).
,8
8,14
We first
checked whether the luminescence of the polymers could
be enhanced by aggregate formation by adding nonsol-
vents to their dilute solutions. The polymer emission
is, however, gradually weakened, instead of being
intensified, when methanol is progressively added to the
polymer solutions, as exemplified by the data shown in
Figures 8 and 9 for polymer 1b. In contrast, when
similar amounts of a nonsolvent (water) are added to
an acetone solution of HPS (Chart 2), a silole analogue
of 1b, its photoluminescence intensity is enhanced by
up to ∼400 times (Figure 9)! In our previous study, we
found that when the silole pendants were directly
attached to the rigid linear polyacetylene backbone, the
8
,14
of the excitons.
The spectral profile hardly changes
with temperature, suggesting that the photolumines-
cence is still associated with the radiative decay of the
4
singlet excitons but not their triplet counterparts. The
photoluminescence intensity increases rapidly when the
polymer solution is cooled from 23 to -78 °C but levels
off when further cooled to -196 °C (Figure 11). Dioxane
has a melting point of 11.8 °C, and decreasing temper-
ature would rapidly increase the solvent viscosity and
vitrify the polymer solution. The void spaces of the
polymer aggregates would be filled by the frozen solvent
molecules and the intramolecular rotations of the silole
8
resultant polymer (PSA in Chart 2) was AIE-inactive.
In this study, the silole rings are put into the rigid
hyperbranched polyphenylene structure, and the result-
ant polymers are again AIE-inactive. The stiff polyene

Macromolecules, Vol. 36, No. 12, 2003
Silole-Containing Hyperbranched Polyphenylenes 4325
moieties would be hampered by the glassy surrounding,
thus leading to the rapid increase in the photolumines-
cence intensity. At -78 °C, the whole solution may have
benzophenone ketyl immediately prior to use. Triethylamine
(RdH) was distilled from KOH. Tolane (98%), silicon(IV)
8
chloride (99%), lithium metal wire (99.9%), 1-octyne (3; 97%),
ethynylmagnesium bromide (0.5 M in THF), tantalum(V)
chloride (99.8%), and tetraphenyltin (97%) were all purchased
from Aldrich and used as received without further purification.
already become a densely packed solid, and further
cooling thus causes little change in the light emission.
The cooling effect on the THF solution of 1b is
different from that on its dioxane solution (Figure 11).
Upon cooling, the photoluminescence intensity again
increases but at a slower rate. THF is a liquid with a
high solvating power but a low melting point (-108 °C),
which can keep the polymer in solution at much lower
temperatures. When cooled to -78 °C, the solvent is still
in the liquid state and the solute should still be
molecularly dissolved. The photoluminescence enhance-
ment with temperature thus should be caused by the
restricted intramolecular rotations due to pure cooling
effect but no “glassy” effect. Because of this, the photo-
luminescence intensities of the THF solution are weaker
than those of the dioxane solution or glass in the
temperature region of 23 to -78 °C. Further cooling to
In str u m en ta tion . The molecular weights of the polymers
were estimated by GPC using a Waters Associates liquid
chromatograph equipped with a Waters 510 HPLC pump, a
Rheodyne 7725i injector with a stand kit, a set of three
Styragel columns [HT3 (effective molecular weight range:
3
3
3
500-30 × 10 ), HT4 (5 × 10 -600 × 10 ), and HT6 (200 ×
3
6
10 -10 × 10 )], a column temperature controller, a Waters
4
86 wavelength-tunable UV-vis detector, a Waters 410 dif-
ferential refractometer (RI detector), and a system DMM/
scanner with eight-channel scanner option. All 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 main-
tained at 40 °C, and the working wavelength of the UV
detector was set at 254 nm. A set of 12 monodisperse
polystyrene standards (Waters Associates) covering a molec-
ular weight range of 982-1945300 was used as calibration
references.
-
196 °C freezes the THF solvent and brings the pho-
toluminescence intensity of the polymer in the THF
glass to a level comparable to that in the dioxane glass.
The H and 13C NMR spectra were obtained on a J EOL 400
or a Varian Mercury 300 using TMS (tetramethylsilane) as
internal reference. The IR spectra were recorded on a Perkin-
Elmer 16 PC FT-IR spectrometer using KBr pellets. The mass
spectra were measured on a Finnigan TSQ 7000 triple quad-
rupole mass spectrometer operating in a chemical ionization
1
Con clu d in g Rem a r k s
The silole ring, an organometallic chromophore with
novel luminescence properties, is successfully incorpo-
rated into a hyperbranched polyphenylene structure
through the facile polycyclotrimerizations of alkynes
catalyzed by a transition-metal mixture of TaCl5-Ph4-
Sn under mild reaction conditions. The silole units are
interconnected through the formation of trisubstituted
benzenes, or the silolyl rings are linked together by the
phenyl rings in the three-dimensional space. This
unique molecular structure endows 1 with an array of
interesting properties: the hyperbranched polymers are
readily processable (completely soluble), thermally stable
(CI) mode using methane as carrier gas. The UV-vis spectra
were obtained on a Milton Roy Spectronic 3000 array spec-
trophotometer. The thermal stability of the polymers was
evaluated on a Perkin-Elmer thermogravimetric analyzer TGA
7 under nitrogen at a heating rate of 20 °C/min. The photo-
luminescence spectra of the silole solutions and aggregates
were recorded on a SLM 8000C spectrofluorometer.
The X-ray diffraction intensity data were collected from a
suitable specimen of 0.35 × 0.25 × 0.15 mm dimension on a
Bruker SMART CCD area detector with graphite-monochro-
mated Mo KR radiation at 100 K. The crystal structure was
solved by direct methods, expanded by difference Fourier
syntheses, and refined by full-matrix least-squares methods
(
Td up to ∼400 °C), electronically conjugated (λmax ∼ 520
2
nm), nonlinear optically active (FL ∼ 180 mJ /cm ), and
highly emissive upon excitation at low temperatures
(
cooling-enhanced emission).
The linear optical transmittance of the polymer solu-
2
on F by using the Bruker SHELXTL (version 6.10) program
package. Non-hydrogen atoms were refined anisotropically,
and hydrogen atoms were placed in ideal positions and refined
as riding atoms.
tions is comparable to, or higher than, that of the C60
solutions, but the optical limiting thresholds and signal
suppression ratios of the former are superior to those
of the latter. In other words, the hyperbranched polyphe-
nylenes are better optical limiters than C60. Noting that
the fullerene is a three-dimensionally conjugated bucky-
ball, the three-dimensionally conjugated electronic struc-
ture of the hyperbranched polyphenylenes may be
responsible for their optical nonlinearity, although the
detailed optical limiting mechanism awaits further
investigation.
The light emission of 1 is sensitive to temperature
change: the photoluminescence intensities of the poly-
mer solutions increase when temperatures are de-
creased. The cooling-enhanced emission of 1 is probably
caused by the restricted intramolecular rotations of the
phenyl rings upon the axes of the single bonds linked
to the silole cores. This is a thermally modulated
photonic process and can be regarded as a special type
of thermochromism,28 offering a versatile means for
tuning the photoluminescence of the hyperbranched
polymers.
The optical limiting experiments were carried out at 532
nm, using 8 ns optical pulses generated from a frequency-
doubled Q-switched Nd:YAG laser (Quanta Ray GCR-3) op-
erating in a near-Gaussion transverse mode with a repetition
rate of 10 Hz. The pulsed laser beam was focused onto a 1 cm
square quartz cell filled with a polymer solution in dichlo-
romethane. The incident and transmitted energies were
measured by an OPHIR detector (30-A-Diff-SH), and every
point of the optical limiting data was the average of at least
15 laser shots. The detector was connected to a computer before
and after the optical limiting data were taken, and the output
stability of the laser equipment was double checked by taking
a series of output data by the energy meter every 10 s for an
extended period of time.
Syn th esis. The silolyldiyne monomer (2) was prepared by
the reaction of 1,1-dichloro-2,3,4,5-tetraphenylsilole (6) with
ethynylmagnesium bromide (Scheme 2). The detailed synthetic
procedures are given below.
1
,1-Dichloro-2,3,4,5-tetraphenylsilole (6). Under dry nitro-
gen, 180 mg of freshly cut lithium shavings (25.9 mmol) was
added to a solution of tolane (4; 5.99 g, 33.6 mmol) in 25 mL
of THF. The mixture was stirred for 12 h at room temperature
and was then added dropwise to a solution of silicon(IV)
chloride (1.33 mL, 11.6 mmol) in 120 mL of THF. The solution
was stirred for 2 h at room temperature and then refluxed for
5 h at evaluated temperature. The product (6) was used in
Exp er im en ta l Section
Ma ter ia ls. Tetrahydrofuran and 1,4-dioxane (Aldrich) were
predried over 4 Å molecular sieves and distilled from sodium

4
326 Chen et al.
Macromolecules, Vol. 36, No. 12, 2003
situ as a starting material for the preparation of 2 without
isolation.
Commun. 2001, 1740-1741. (b) Tang, B. Z.; Zhan, X.; Yu,
G.; Lee, P. P. S.; Liu, Y.; Zhu, D. J . Mater. Chem. 2001, 11,
2
874-2878.
1
,1-Diethynyl-2,3,4,5-tetraphenylsilole (2). Into the above
THF solution of 6 at room temperature was dropwise added
6.4 mL of 0.5 M THF solution of ethynylmagnesium bromide
23.2 mmol). After stirring for 2 h, the mixture was filtered,
(
4) Chen, H.; Lam, J . W. Y.; Luo, J .; Ho, Y. L.; Tang, B. Z.; Zhu,
D.; Wong, M.; Kwok, H. S. Appl. Phys. Lett. 2002, 81, 774-
4
(
7
76.
(
(
5) Freemantle, M. Chem. Eng. News 2001, 79 (41), 29.
6) The highest ηEL possible for a singlet emitter has been
theoretically predicted to be ∼5.5%, but recent research in
and the crude product was purified on a silica gel column using
a hexane/chloroform mixture (2:1 by volume) as eluent. A
yellow-greenish solid of 2 was isolated in 30% yield (1.5 g)
based on silicon(IV) chloride. Recrystallization from toluene/
heptane solution gave 1.2 g of pure product (24% yield based
7
the area suggests that this limit may be uplifted to ∼8-9%.
(7) (a) Cao, Y.; Parker, I. D.; Yu, G.; Zhang, C.; Heeger, A. J .
Nature (London) 1999, 397, 414-417. (b) Kim, J . S.; Ho, P.
K.; Greenham, N. C.; Friend, R. H. J . Appl. Phys. 2000, 88,
1073-1081. (c) Wohlgenannt, M.; Tandon, K.; Mazumdar, S.;
Ramaesha, S.; Vardeny, Z. V. Nature (London) 2001, 409,
-
1
on silicon(IV) chloride); mp 199-201 °C. IR (KBr), υ (cm ):
1
3
252 (tC-H stretch), 2038 (CtC stretch). H NMR (300 MHz,
CDCl ), δ (TMS, ppm): 7.20-7.01 (m, br, 16H, Ar), 6.81 (m,
H, Ar), 2.60 (s, 2H, tCH). C NMR (75 MHz, CDCl
ppm): 157.4 (CdC-Si), 138.0 (ipso-Ar), 137.2, 133.5, 129.5,
3
4
94-497.
1
3
4
3
), δ (TMS,
(
8) Chen, J .; Xie, Z.; Lam, J . W. Y.; Law, C. C. W.; Tang, B. Z.
Macromolecules 2003, 36, 1108-1117.
1
(
4
1
29.4, 128.0, 127.7, 126.9 (m-, p-, o-Ar), 126.5 (dC-Si), 97.6
(9) (a) Mi, Y.; Tang, B. Z. Polym. News 2001, 26, 170-176. (b)
Xu, K.; Peng, H.; Lam, J . W. Y.; Poon, T. W. H.; Dong, Y.;
Xu, H.; Sun, Q.; Cheuk, K. K. L.; Salhi, F.; Lee, P. P. S.; Tang,
B. Z. Macromolecules 2000, 33, 6918-6924. (c) Tang, B. Z.;
Chen, H.; Xu, R.; Lam, J . W. Y.; Cheuk, K. K. L.; Wong, H.
N. C.; Wang, M. Chem. Mater. 2000, 12, 213-221. (d) Kong,
X.; Lam, J . W. Y.; Tang, B. Z. Macromolecules 1999, 32,
+
3
tCH), 81.5 (Si-Ct). MS (CI): m/e 435 [(M + 1) ]. UV (CHCl ,
-
5
-1
-1
.0 × 10 mol/L), λmax (nm)/ꢀmax (mol L cm ): 247/2.35 ×
4
3
0 , 378/9.27 × 10 .
P olym er iza tion . All of the polymerization reactions and
manipulations were carried out under dry nitrogen using a
standard Schlenk technique in a vacuum line system or an
inert-atmosphere glovebox (Vacuum Atmospheres), except for
the purification of the polymers, which was conducted in air.
A typical procedure for the copolymerization of 2 with 1-octyne
1
722-1730.
(10) (a) Tang, B. Z.; Xu, K.; Peng, H.; Sun, Q.; Luo, J . US Patent
Appl. No. 10/109,316, 2002. (b) Peng, H.; Luo, J .; Cheng, L.;
Lam, J . W. Y.; Xu, K.; Dong, Y.; Zhang, D.; Huang, Y.; Xu,
Z.; Tang, B. Z. Opt. Mater. 2002, 21, 315-320. (c) Peng, H.;
Cheng, L.; Luo, J .; Xu, K.; Sun, Q.; Dong, Y.; Salhi, F.; Lee,
P. P. S.; Chen, J .; Tang, B. Z. Macromolecules 2002, 35, 5349-
3
is given below.
Into a thoroughly baked and moisture excluded Schlenk
tube were placed 18.0 mg of TaCl
Ph Sn (0.05 mmol) in a glovebox. The catalyst mixture was
5
(0.05 mmol) and 21.3 mg of
5
351. (d) Lam, J . W. Y.; Luo, J .; Peng, H.; Xie, Z.; Xu, K.;
4
Dong, Y.; Cheng, L.; Qiu, C.; Kwok, H. S.; Tang, B. Z. Chin.
J . Polym. Sci. 2001, 19, 585-590. (e) Xu, K.; Tang, B. Z. Chin.
J . Polym. Sci. 1999, 17, 397-402.
dissolved in 1.0 mL of toluene and aged at room temperature
for 15 min. A solution of 78.1 mg (0.18 mmol) of 2 and 13.3
mL (0.09 mmol) of 3 in 1.5 mL toluene was then added
dropwise into the catalyst solution. After stirring at room
temperature for 24 h, the reaction was quenched by the
addition of a small amount of methanol. The polymer solution
was dropped into 250 mL of methanol via a cotton filter while
(11) Xu, K.; Peng, H.; Sun, Q.; Dong, Y.; Salhi, F.; Luo, J .; Chen,
J .; Huang, Y.; Zhang, D.; Xu, Z.; Tang, B. Z. Macromolecules
2
002, 35, 5821-5834.
(
(
12) Curtis, M. D. J . Am. Chem. Soc. 1969, 91, 6011-6018.
13) Corriu, R. J .-P.; Douglas, W. E.; Yang, Z.-X. J . Organomet.
Chem. 1993, 456, 35-39.
stirring. A deep purple powder of 1b was isolated in 85.3%
(
14) 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,
1
yield (75.1 mg). M
NMR (300 MHz, CD
w
5820; M
Cl
), 1.30 (CH ), 0.91 (CH
), δ (TMS, ppm): 156.8 (CdC-Si), 138.9 (ipso-Ar), 130.2-
), 32.1 (Ar-CH
-CH -CH ), 23.0
, 10 µM), λmax (nm)/ꢀmax
w
/M
n
1.7 (GPC, Table 1, no. 2). H
2
2
), δ (TMS, ppm): 6.90 (br, Ar), 2.54
1
5, 1535-1546.
13
(
Cl
br, Ar-CH
2
2
2
3 2
). C NMR (400 MHz, CD -
(
15) (a) Tang, B. Z.; Sun, Q.; Xu, K.; Peng, H.; Lam, J . W. Y.; Cha,
J . A. K.; Luo, J .; Zhang, X. US Patent Appl. No. 10/106,752,
2002. (b) Sun, Q.; Xu, K.; Lam, J . W. Y.; Cha, J . A. P.; Zhang,
X.; Tang, B. Z. Mater. Sci. Eng., C 2001, 16, 107-112. (c)
Sun, Q.; Lam, J . W. Y.; Xu, K.; Xu, H.; Cha, J . A. P.; Wong,
P. C. L.; Wen, G.; Zhang, X.; J ing, X.; Wang, F.; Tang, B. Z.
Chem. Mater. 2000, 12, 2617-2624. (d) Sun, Q.; Xu, K.; Peng,
H.; Zheng, R.; Chen, J .; Law, C. C. W.; H a¨ ussler, M.; Tang,
B. Z. Macromolecules 2003, 36, 2309-2320.
1
26.5 (m-, o-, p-Ar; dC-Si), 36.2 (Ar-CH
CH and CH -CH -CH ), 29.8 (CH -CH
-CH ), 14.3 (CH ). UV (CHCl
2
2
-
2
2
2
3
2
2
2
3
(CH
2
-
3
3
3
1
-1
(mol L cm ): 242/22 200, 368/3300 (sh), 521/567.
Ack n ow led gm en t. The work described in this paper
was partially supported by the Research Grants Council
HKUST 6187/99P, 6121/01P, and 6085/02P) and the
(16) Silverstein, R. M.; Webster, F. X. Spectrometric Identification
of Organic Compounds, 6th ed.; Wiley: New York, 1998.
17) (a) Lam, J . W. Y.; Dong, Y.; Cheuk, K. K. L.; Luo, J .; Xie, Z.;
Kwok, H. S.; Mo, Z.; Tang, B. Z. Macromolecules 2002, 35,
1229-1240. (b) Lam, J . W. Y.; Kong, X.; Dong, Y.; Cheuk, K.
K. L.; Xu, K.; Tang, B. Z. Macromolecules 2000, 33, 5027-
(
(
University Grants Committee (AoE/P-10/01-1-A) of Hong
Kong. We thank Prof. K. S. Wong of the Department of
Physics of our University for his helpful discussions.
5
3
040. (c) Kong, X.; Tang, B. Z. Chem. Mater. 1998, 10, 3352-
363. (e) Tang, B. Z.; Kong, X.; Wan, X.; Peng, H.; Lam, W.
Su p p or tin g In for m a tion Ava ila ble: Crystal structures
with atom labels, tables of crystal data and intensity data,
atomic coordinates, anisotropic displacement parameters, and
bond distances and angles for silolyldiyne 2 as well as its cif
file (CCDC 195947). This material is available free of charge
via the Internet at http//pubs.acs.org.
Y.; Feng, X.; Kwok, H. S. Macromolecules 1998, 31, 2419-
2
432.
(
18) Sun, Q.; Peng, H.; Xu, K.; Tang, B. Z. In Metal- and Metalloid-
Containing Macromolecules; Carraher, C., Pittman, C., Abd-
El-Aziz, A., Zeldin, M., Sheats, J ., Eds.; Wiley: New York,
2003; Vol. 1, in press.
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19) 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, 1446-1455.
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R., Hood, P., Hagan, D., Lewis, K., Perry, J . W., Eds.;
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(
(
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Macromolecules, Vol. 36, No. 12, 2003
Silole-Containing Hyperbranched Polyphenylenes 4327
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and liquid-permeating materials.26
2
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(
(
23) This conclusion is further supported by the experimental
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261-1272. (c) Masuda, T.; Iguchi, Y.; Tang, B. Z.; Higash-
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(
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(
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MA034012R