Simple synthesis, outstanding thermal stability, and tunable light-emitting and optical-limiting properties of functional hyperbranched polyarylenes

Article abstract of DOI:10.1021/ma020180s

The synthesis, thermal stability and optical properties of functional hyperbranched polyarylenes were reported. The molecular structure of the polymers was determined by nuclear magnetic resonance (NMR) spectroscopy. The spectra showed that the hyperbranched polymers possessed a random molecular composition, an irregular stereostructure and a rigid core architecture. The optical limiting and tunable light emitting properties of the polymers make them suitable to be used for high-tech applications.

Full text of DOI:10.1021/ma020180s

Macromolecules 2002, 35, 5349-5351
5349
Sim p le Syn th esis, Ou tsta n d in g Th er m a l
Sta bility, a n d Tu n a ble Ligh t-Em ittin g a n d
Op tica l-Lim itin g P r op er ties of F u n ction a l
Hyp er br a n ch ed P olya r ylen es
Ha n P en g, Lin Ch en g, J in gd on g Lu o, Ka itia n Xu ,
Qu n h u i Su n , Yu p in g Don g, F ou a d Sa lh i,
P r iscilla P . S. Lee, J u n w u Ch en , 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 of the Institute of Molecular
Technology for Drug Discovery and Synthesis, Hong Kong
University of Science & Technology, Clear Water Bay,
Kowloon, Hong Kong, China
F igu r e 1. 1H NMR spectrum of 7 in dichloromethane-d2. Cz
) carbazolyl.
Received February 4, 2002
Revised Manuscript Received April 14, 2002
In their relentless pursuit of advanced materials,
polymer scientists have devoted much effort to the
construction of conjugated polymers using aromatic
rings as building blocks, in expectation that the result-
ant polyarylenes would exhibit novel materials proper-
ties. A typical example of polyarylenes is polyphenylene,
which was first prepared in 1886 and is still under
active investigation, with the research frontier now
moving from low (linear) to high (ladder, dendritic, and
hyperbranched) dimensionalities.1,2 Our group has
worked on the synthesis of hyperbranched polyarylenes
by acetylene polycyclotrimerizations initiated by tan-
talum and niobium halides3 and have succeeded in the
preparation of a completely soluble hyperbranched
polyphenylene by the copolycyclotrimerization of 1,4-
diethynylbenzene with phenylacetylene.4 While effective
to nonpolar monomers, the Ta and Nb catalysts are
intolerant of functional groups. In this work, we ex-
tended our efforts to the search for robust catalysts and
to the creation of functional polyarylenes. In our previ-
ous work, we were able to polymerize 3,9-dodecadiyne
using CpCo(CO)2 5 as catalyst at 120 °C, but the product
was only partially soluble.3a Instead of heating to
the high temperature, in this work, we used UV irradia-
tion to activate the Co catalyst at a lower temperature
(65 °C). The photoactivated catalyst initiated the poly-
cyclotrimerizations of functional acetylenes (Scheme 1),
and the resultant hyperbranched polyarylenes6 exhib-
ited unique solution, thermal, and optical properties.
The diyne monomers (1) containing aromatic rings of
molecular electronics interest were prepared by palla-
dium-catalyzed coupling of silylacetylene with dihalo-
arenes followed by base-catalyzed desilylation.7 The
liquid crystalline (LC) monoyne 2c was prepared by
our published procedure.8 The polycyclotrimerizations
were initiated by CpCo(CO)2 with UV irradiation under
nitrogen. The experimental details on the synthesis and
characterization of the monomers and polymers can be
found in the Supporting Information.
F igu r e 2. Absorption and emission spectra of 4 and 5 in
dichloromethane. λex (nm): 343 (4) and 334 (5). Polymer
concentration: 2 mM.
tion of 1a with 2c gave 5 in a high yieldsthis is
remarkable because our early attempts in using 2c as
a comonomer all failed when the Ta and Nb catalysts
were used. Similarly, while the Ta and Nb catalysts
were ineffective, the CpCo(CO)2-hν system initiated
polycyclotrimerizations of the functional carbazolyl
diyne 1c, again demonstrating the robustness of the Co
catalyst. The polymers all gave unimodal GPC peaks,
with their Mw up to ∼29 000 (5)9 and Mw/Mn down to
1.58 (7). Similar to the hyperbranched polyphenylenes
prepared by Suzuki coupling,2b the polyarylenes exhib-
ited low intrinsic viscosities when dissolved in good
solvents such as toluene.
The molecular structures of the polymers were char-
acterized by spectroscopic methods, and all the polymers
gave satisfactory spectral data (see Supporting Informa-
1
tion for details). The H NMR spectrum of 7 is given in
Figure 1 as an example. The spectrum shows no
unexpected signals, and all the resonance peaks can be
readily assigned. The peaks of the aromatic protons are
broad, suggesting that the hyperbranched polymer
possesses a random molecular composition, an irregular
stereostructure, and a rigid core architecture. This latter
point is supported by the sharper peaks of the flexible
alkyl chains not directly attached to the rigid aromatic
cores. The small peak at δ ∼3.4 is due to the absorption
of the acetylene protons at the peripheral ends.
We investigated the optical and thermal properties
of the soluble hyperbranched polymers using the meth-
ods described in our early reports.10 As shown in Figure
2, 4 absorbs at 309 nm and emits a strong UV light of
398 nm, whose ΦF (0.49) is higher that of poly(1-phenyl-
1-octyne) (0.43), a highly luminescent disubstituted
polyacetylene.11 The emission from 5 was much weaker
(ΦF ) 0.09), which was possibly quenched by its own
absorption in the same spectral region. The lumines-
The polymerization results are summarized in Table
1. The copolycyclotrimerization of 4,4′-diethynylbiphenyl
(1a ) with 1-phenylacetylene (2a ) gave an all-aromatic
hyperbranched polyarylene (3), which was, however,
partially soluble. Replacing the aromatic monoyne with
an aliphatic one solved the solubility problem: a com-
pletely soluble polymer 4 was produced by the copolym-
erization of 1a with 1-heptyne (2b). The copolymeriza-
10.1021/ma020180s CCC: $22.00 © 2002 American Chemical Society
Published on Web 06/05/2002

5350 Communications to the Editor
Macromolecules, Vol. 35, No. 14, 2002
Sch em e 1
Ta ble 1. Syn th esisa a n d P r op er ties of Hyp er br a n ch ed P olya r ylen es
c
c
d
e
g
h
i
no. feed ratio [2]/[1]b polymer yield (wt %)
Mw
Mw/Mn
λem (nm) ΦF
Td f (°C) Wr (wt %) FL (mJ /cm2) Ft,m/Fi,m
1
2
3
4
5
6
1.0 (2a /1a )
1.3 (2b/1a )
1.2 (2c/1a )
2.2 (2b/1b)
1.0 (2a /1c)
1.0 (2b/1c)
73.6 (3)
76.0 (4)
84.9 (5)
83.5 (6)
91.8 (7)
72.6 (8)
4 290j
20 350
28 570
16 240
6 370
1.58j
2.24
5.78
3.40
1.58
2.15
398
397
400
399
398
0.49
0.09
0.31
0.20
0.10
585
412
467
474
404
86.0
50.3
75.0
63.7
83.0
343
900
126
577
635
0.21
0.53
0.11
0.35
0.36
13 420
a
By copolycyclotrimerizations of diynes (1) with monoynes (2) catalyzed by CpCo(CO)2-hν in toluene under nitrogen; [1] ) 0.1 M,
[cat.] ) 10 mM, 65 °C, 6 h. b Molar ratio. c By GPC in THF (polystyrene calibration). Emission maximum (in dichloromethane). e Quantum
d
yield of fluorescence (9,10-diphenylanthracene standard). f Temperature for 5% weight loss (TGA, under nitrogen, heating rate: 20 °C/
g
h
min). Weight of residue at 800 °C in the TGA analysis. Optical limiting threshold (incident fluence at which nonlinear transmittance
is 50% of initial linear one). Signal suppression (ratio of saturated transmitted fluence to maximum incident fluence). j For soluble fraction
i
of the partially soluble polymer.
cence efficiency of other polymers also changed with the
change in the combination of monomer/comonomer pairs
(cf. Table 1); that is, the emission property of the
polymers can be tuned by altering their molecular
structures.
Polymer 4 lost little weight at 585 °C and carbonized
in high yield (86%) upon further heating (Figure 3A and
Table 1, no. 2). All its structural congeners 5-8 were
also thermally stable. Their stabilities are similar to
that of poly(p-phenylene) (stable up to ∼550 °C)1 but
different from those of polyacetylenes (e.g., PPA and PH
respectively started to lose their weights at 220 and 150
°C).12 This is obviously because the polyarylenes are
structurally similar to polyphenylene (stable aromatic
constituents) but different from polyacetylene (labile
polyene backbone).
Molecules of fused aromatic rings may limit optical
power,13 and we checked whether our thermally stable
polyarylenes would act as optical limiters. PPA photo-
degraded under the attack of harsh laser shots,14 but
4 strongly attenuated the power of intense 532 nm
optical pulses (Figure 3B). The optical limiting power
of 4 is comparable to that of C60, a best-known optical
limiter.13-15 Noticing that 4 is a copolymer of 1a and
F igu r e 3. (A) TGA thermograms of hyperbranched poly-
2b, we changed one of its two monomers at one time to
arylenes. (B) Optical responses to 8 ns, 10 Hz pulses of 532
see how this change will affect its limiting power. When
nm laser light, of dichloromethane solutions (0.86 mg/mL) of
1a and 2b were respectively changed to 1b and 2c, the
optical limiting performance of the corresponding co-
polymers 6 and 5 were respectively improved and
4-6 [linear transmittance (%): 92 (5), 79 (C60), 66 (4, 6)]. Data
for poly(phenylacetylene) (PPA; panels A and B), poly(1-hexyne)
(PH; panel A), and C60 (panel B) are shown for comparison.

Macromolecules, Vol. 35, No. 14, 2002
Communications to the Editor 5351
F.; Dong, Y. ACS Symp. Ser. 2000, 760, 146. (c) Lam, J . W.
Y.; Luo, J .; Peng, H.; Xie, Z.; Xu, K.; Dong, Y.; Cheng, L.;
Qiu, C.; Kwok, H. S.; Tang, B. Z. Chin. J . Polym. Sci. 2000,
19, 585. (d) Mi Y.; Tang, B. Z. Polym. News 2001, 26, 170.
worsened, indicating that the nonlinear optical property
can be manipulated by molecular engineering. This is
further verified by the data shown in Table 1: the
limiting power of the polymers varied in a large range
with their molecular structure. Among the polymers,
6 worked best. It limited the laser pulses at a low
threshold (126 mJ /cm2) and suppressed the optical
signals to a great extent (0.11; Table 1, no. 4), which
are respectively 2.8- and 2.5-fold better than those
achievable by C60 under comparable conditions.
In summary, in this study, we successfully synthe-
sized soluble functional hyperbranched polyarylenes in
high yields by alkyne polycyclotrimerizations initiated
by CpCo(CO)2-hν,5 thanks to the functionality-toler-
ance of the robust Co catalyst. The polyarylenes emitted
UV light and limited intense optical pulses. The UV
emission is of technological value and may be utilized
in the fabrication of full-color light-emitting devices.16
The excellent optical limiting properties, coupled with
their thermal stability and processing advantages, make
the hyperbranched polyarylenes promising candidate
materials for high-tech applications.
(4) Xu, K.; Peng, H.; Tang, B. Z. Polym. Prepr. 2001, 42 (1),
555.
(5) While Co2(CO)8 was used in the synthesis of benzene-based1g
and -cored2c dendrimers, we were unaware of any previous
attempt in utilizing CpCo(CO)2 as a catalyst to initiate
alkyne polycyclotrimerization before our early work reported
in ref 3a.
(6) Hyperbranched polyphenylenes were prepared by Pd(0)-
and Ni(II)-catalyzed aryl-aryl coupling reactions at high
temperatures.2b Our approach is, however, conceptually
different and offers a new synthetic route to hyperbranched
polyarylenes. The simplicity of our methodology and the
richness of acetylene chemistry7 facilitate systematic varia-
tions in the structures and properties of the polyarylenes.
(7) (a) Modern Acetylene Chemistry; Stang, P. J ., Diederich, F.,
Eds.; VCH: New York, 1995. (b) Brandsma, L.; Verkruijsse,
H. D. Synthesis of Acetylenes, Allenes and Cumulenes;
Elsevier: Amsterdam, 1981.
(8) Lam, J . W. Y.; Kong, X.; Dong, Y.; Cheuk, K. K. L.; Xu, K.;
Tang, B. Z. Macromolecules 2000, 33, 5027.
(9) GPC often underestimates molecular weights of branched
polymers;2b the difference between the relative and absolute
molecular weights can be as big as ∼40 times: Muchtar,
Z.; Schappacher, M.; Deffieux, A. Macromolecules 2001, 34,
7595. The true molecular weights of our hyperbranched
polymers thus could be much higher than those given in
Table 1.
(10) (a) Tang, B. Z.; Xu, H.; Lam, J . W. Y.; Lee, P.; Xu, K.; Sun,
Q.; Cheuk, K. K. L. Chem. Mater. 2000, 12, 1446. (b) Huang,
Y. M.; Ge, W.; Lam, J . W. Y.; Tang, B. Z. Appl. Phys. Lett.
1999, 75, 4094. (c) Kong, X.; Tang, B. Z. Chem. Mater. 1998,
10, 3352.
Ack n ow led gm en t. This work was supported by the
Hong Kong Research Grants Council (HKUST 6187/99P
and 6121/01P) and the Area of Excellence Scheme of
the University Grants Committee of Hong Kong (AoE/
P-10/01-1-A).
Su p p or tin g In for m a tion Ava ila ble: Synthetic proce-
dures and structural characterization data. This material is
available free of charge via the Internet at http://pubs.acs.org.
(11) Huang, Y. M.; Ge, W.; Lam, J . W. Y.; Tang, B. Z. Appl. Phys.
Lett. 2001, 78, 1652.
Refer en ces a n d Notes
(12) Masuda, T.; Tang, B. Z.; Higashimura, T.; Yamaoka, H.
Macromolecules 1985, 18, 2369.
(1) (a) Schluter, A. D.; Wegner, G. Acta Polym. 1993, 44, 59.
(b) Tour, J . M. Adv. Mater. 1994, 6, 190. (c) J ohnen, N. A.;
Kim, H. K.; Ober, C. K. ACS Symp. Ser. 1994, 579, 298. (d)
Kumar, U.; Neenan, T. X. ACS Symp. Ser. 1995, 614, 518.
(e) Kim, Y. H. J . Polym. Sci., Polym. Chem. Ed. 1998, 36,
1685. (f) Scherf, U. Top. Curr. Chem. 1999, 201, 163. (g)
Watson, M. D.; Fechtenkotter, A.; Mullen, K. Chem. Rev.
2001, 101, 1267.
(2) (a) Goldschmiedt, G. Monatsh. Chem. 1886, 7, 40. (b) Kim,
Y.; Webster, O. W. Macromolecules 1992, 25, 5561. (c) Hecht,
S.; Frechet, J . M. J . J . Am. Chem. Soc. 1999, 121, 4084. (d)
Wiesler, U. M.; Berresheim, A. J .; Morgenroth, F.; Lieser,
G.; Mullen, K. Macromolecules 2001, 34, 187.
(13) (a) Tutt, L. W.; Kost, A. Nature (London) 1992, 356,
225. (b) Sun, Y. P.; Riggs, J . E. Int. Rev. Phys. Chem. 1999,
18, 43.
(14) Tang, B. Z.; Xu, H. Macromolecules 1999, 32, 2569.
(15) (a) Tang, B. Z.; Leung, S. M.; Peng, H.; Yu, N.-T.; Su, K. C.
Macromolecules 1997, 30, 2848. (b) Peng, H.; Lam, J . W.
Y.; Leung, F. S. M.; Poon, T. W. H.; Wu, A. X.; Yu, N.-T.;
Tang, B. Z. J . Sol-Gel Sci. Technol. 2001, 22, 205.
(16) 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.
(3) (a) Xu, K.; Tang, B. Z. Chin. J . Polym. Sci. 1999, 17, 397.
(b) Tang, B. Z.; Xu, K.; Sun, Q.; Lee, P. P. S.; Peng, H.; Salhi,
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