Oligo(fluorenyleneethynylenegermylene)s and their metallopolymers

Article abstract of DOI:10.1039/b409580d

Oligo(fluorenyleneethynylenegermylene)s and their polyplatinynes are synthesized and photophysically characterized; inclusion of heavy germylene bridges greatly boosts the phosphorescence decay rate in metallopolymers.

Full text of DOI:10.1039/b409580d

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Oligo(fluorenyleneethynylenegermylene)s and their metallopolymers{
Wai-Yeung Wong,*a Suk-Yue Poon,a Albert W.-M. Lee,a Jian-Xin Shia and Kok-Wai Cheahb
a Department of Chemistry and Centre for Advanced Luminescence Materials, Hong Kong Baptist
University, Waterloo Road, Hong Kong, P. R. China. E-mail: rwywong@hkbu.edu.hk
b Department of Physics, Hong Kong Baptist University, Waterloo Road, Hong Kong, P. R. China
Received (in Cambridge, UK) 24th June 2004, Accepted 17th August 2004
First published as an Advance Article on the web 23rd September 2004
Oligo(fluorenyleneethynylenegermylene)s and their poly-
OFEGs are very organic-soluble and are fully characterized by
NMR, IR and MS. The structures of 1a and 2a were confirmed by
X-ray analyses (Fig. 1).6 Stable polyplatinynes 3a–c and 4a–c were
obtained by the dehydrohalogenative polymerization between
trans-[PtCl2(PBu3)2] and the respective OFEG and their excellent
film-forming properties and GPC data (Table 2) suggest a
macromolecular nature for these materials. All the new compounds
display excellent thermal stability in the TGA curves (Tables 1
and 2). The onset decomposition temperatures (Tdec) are almost
invariant of the chain length and the R group. While trans-
[-Pt(PBu3)2CMC(9,9-dihexylfluorene-2,7-diyl)CMC-]n 5 commences
decomposition at 349 uC,7 addition of a GeR2 unit into the aryl-
acetylene segment significantly increases the thermal stability of
these Pt polyynes.
platinynes are synthesized and photophysically characterized;
inclusion of heavy germylene bridges greatly boosts the
phosphorescence decay rate in metallopolymers.
The challenge of developing molecular systems composed of main
group heteroatoms and acetylene units has had aesthetic appeal in
recent years.1 These acetylene-based scaffolds and their metal-
capped congeners play a vital role in molecular optoelectronics.2
While the study of oligo- and poly(aryleneethynylenesilylene)s
represents an active research area,3 little is understood on related
systems with the heavier group 14 element germanium, and no
oligo(aryleneethynylenegermylene)s of precise length and constitu-
tion have ever been described. We report here the unprecedented
synthesis and photophysics of stable blue-emitting oligo(fluoreny-
leneethynylenegermylene)s (OFEG)s with alternating Ge and
diethynylfluorene units. The first examples of Ge-bridged Pt(II)
metallopolymers, derived from these oligomers, are also presented
which display very fast phosphorescence decay rates. The triplet
states in light-emitting diodes can be utilized in metal–organics
through light-harvesting techniques.4 To optimise this, work
should focus on polymers with high energy triplets to avoid
competition with nonradiative decay. Up till now, no reports on
polymetallaynes with comparable radiative and nonradiative decay
rate constants are known and it is highly desirable to achieve this
goal for practical applications. The key feature of this study is the
inclusion of a conjugation-interrupting sp3-Ge linker that can limit
the effective conjugation length (ECL) and trigger the triplet light
emission by taking advantage of the heavy-atom effect of Ge atoms.
Lithiation of I by nBuLi in THF at 278 uC followed by
germylation with R2GeCl2 (R ~ Me, Ph) gave a separable mixture
of monodisperse OFEG 1a–c and 2a–c (Scheme 1). In both series,
the yields of the dimers (m ~ 1) and trimers (m ~ 2) were increased
The absorption spectra of OFEG consist of organic 1(pp*)
transitions in the near-UV region (Table 1), and a slight
bathochromic shift in lmax with increased extinction coefficients
being observed for both series as m increases. But, the magnitude of
red shifts rapidly decreases with increasing m value and there is no
further significant shift in going from 2c to the longer-chain
polymer [-CMC(9,9-dihexylfluorene-2,7-diyl)CMCGePh2-]n (lmax
~
346 nm). So, there would be little benefit for m w 3 in the control of
spectral properties. The R groups on Ge exert minimal effect on the
absorption energies. The transition energies of the Pt polymer are
lowered relative to those of the OFEG, suggesting a well extended
singlet excited state in the polymers. Examination of the absorption
behavior of 1–4 in CH2Cl2 and in the solid state precludes any
solid-state aggregates in thin films.
Our OFEGs are good light emitters and provide the best finite
models for the Ge-containing poly(fluorenyleneethynylene)s. They
emit remarkably strong purple-blue fluorescence at room tem-
perature. The fluorescence quantum yields (WF) of these oligomers
are found to be chain length dependent and they increase down
each series from a to c (i.e. m ~ 2 w 1 w 0). We also note that the
Me compounds exhibit higher WF than the Ph congeners and the
emissions from 1b and 1c are highly efficient with WF up to 0.92.
These observations suggest that intrachain aggregation of
luminophores may exist in these oligomers, and this interaction
seemed to be more important as the chain length increases.3a,8
While the presence of Ge can make the molecules more flexible, the
opportunity for one chromophore unit to be located proximal to
the other in space would increase with the molecular weight. The
fluorescence radiative decay rate, (kr)F, is found to increase down
each series or when passing from Ph to Me substituents. However,
such fluorescence enhancement by increasing the m value tends to
reach saturation up to m ~ 3 and WF converges to a limiting value
in their polymeric homologue (nECL ~ 3). For the Pt polymers,
n
at the expense of the monomers (m ~ 0) when more BuLi was
used. The merit of these short-chain oligomers over their polymeric
counterparts lies in their precise molecular architectures, better
solubility and their predetermined chemical composition.5 These
Scheme 1 Reagents and conditions: (i) nBuLi, 278 uC, R2GeCl2, THF; (ii)
trans-[PtCl2(PBu3)2], CuI, NEt3 CH2Cl2.
{ Electronic supplementary information (ESI) available: experimental
procedures and spectral data of all new compounds. See http://
www.rsc.org/suppdata/cc/b4/b409580d/
Fig. 1 ORTEP drawings of (a) 1a and (b) 2a.
2 4 2 0
C h e m . C o m m u n . , 2 0 0 4 , 2 4 2 0 – 2 4 2 1
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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Table 1 Photophysical data (at 293 K) and thermal analyses of OFEG
abs/nma (CH2Cl2)
labs/nm (film)
Egap/eV
lem/nm (CH2Cl2)
WF
tF/ns
(kr)F/s21
Tdec/uC
b
OFEG
l
1a
1b
1c
2a
2b
2c
337 (12.8)
343 (15.6)
344 (20.7)
339 (13.0)
345 (16.9)
346 (22.3)
339
343
345
341
347
348
3.57
3.51
3.49
3.54
3.49
3.47
362, 379
368, 384
369, 386
364, 379
371, 386
371, 386
0.78
0.90
0.92
0.35
0.66
0.75
1.18
1.18
1.16
1.09
1.08
1.05
6.6 6 108
7.6 6 108
7.9 6 108
3.2 6 108
6.1 6 108
7.1 6 108
447
442
442
442
446
443
a e (104 dm3 mol21 cm21) values are shown in parentheses. b Measured in CH2Cl2 relative to quinine sulfate in 0.1 N H2SO4 (WF ~ 0.54),
lex ~ 313 nm.
Table 2 Structural, thermal stability and emission data (at 20 K) of metallopolymers 3a–c and 4a–c
(knr)P/s21
(kr)P/s21
a
a
Polymer
Mw
Mn
Tdec/uC
lem/nm (film)
tP/ms
WP
3a
3b
3c
4a
4b
4c
42 970
22 950
22 790
23 970
28 350
22 600
13 580
11 150
16 150
10 400
12 830
17 550
410
414
407
404
407
418
412, 547
411, 544
410, 544
414, 548
411, 545
411, 545
1.27
2.08
1.41
1.32
1.21
1.16
0.45
0.43
0.45
0.17
0.18
0.20
4.3 6 105
2.7 6 105
3.9 6 105
6.3 6 105
6.8 6 105
6.9 6 105
3.5 6 105
2.1 6 105
3.2 6 105
1.3 6 105
1.5 6 105
1.7 6 105
a Calibration against polystyrene standards.
both fluorescent and phosphorescent emissions arise from ligand-
centred (pp)* transitions. In dilute fluid solutions, we observe an
harvesting from the T1 state and one can take benefit from the large
yield and radiative decay rate of triplet excitons.
Financial support from the Hong Kong Research Grants
Council (HKBU2054/02P and 2022/03P) and HKBU (FRG/01-02/
II-48) is gratefully acknowledged.
1
intense (pp*) fluorescence peak near 400 nm for 3a–c and 4a–c,
which do not display a large shift at high concentrations, excluding
an excimer origin. At low temperatures, lower-lying spin-forbidden
phosphorescence bands emerge at around 545 nm and the
substantial Stokes shifts of these peaks from the dipole-allowed
absorptions, plus the long emission lifetimes in the microsecond
range are suggestive of their triplet parentage, and they are assigned
to the 3(pp*) states of the organic chromophores. The triplet energy
does not vary much with oligomer chain length, i.e. the lowest T1
state is confined to a single repeat unit. Variation of the R group
does not seem to alter this strong confinement. Insertion of a
conjugation hindered GeR2 group in 3 and 4 shifts the
phosphorescence bands to the blue relative to 5.7 Values of
DE(So–T1) (energy gap between So and T1) are found to be ca.
2.27–2.28 eV for both series. This corresponds to S1–S0 gaps of
y3.0 eV. The DE(S1–T1) values lie within the narrow range of
0.73–0.75 eV, in line with the S1–T1 energy gap of 0.7 ¡ 0.l eV for
metal polyynes of group 10–12 elements.9
Notes and references
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1515; (b)C.D.Entwistle,A.S.Batsanov,J.A.K.Howard,M.A.Foxand
T. B. Marder, Chem. Commun., 2004, 702; (c) Modern Acetylenic
Chemistry, ed. P. J. Stang and F. Diederich, Wiley-VCH, Weinheim, 1995.
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2586; (b) U. H. F. Bunz, Chem. Rev., 2000, 100, 1605; (c) I. Manners,
Synthetic Metal-Containing Polymers, Wiley-VCH, Weinheim, 2004.
3 (a) R. J. P. Corriu, Angew. Chem., Int. Ed., 2000, 39, 1376;
(b) W. E. Douglas, D. M. H. Guy, A. K. Kar and C. Wang, Chem.
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4 (a) X. Gong, M. R. Robinson, J. C. Ostrowski, D. Moses, G. C. Bazan
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M. E. Thompson and S. R. Forrest, Nature, 2000, 403, 750; (c) A. Ko¨hler,
J. S. Wilson and R. H. Friend, Adv. Mater., 2002, 14, 701.
The phosphorescence lifetimes (tP), quantum yields (WP), and
radiative ((kr)P) and nonradiative ((knr)P) decay rates at 20 K are
listed in Table 2. Although the measured WP values are relatively
insensitive to the value of m, they are found to vary with the ER2
group (E ~ Si, Ge). The GeMe2 systems give more efficient
phosphorescence than the GePh2 congeners by over 2 times. But,
replacement of GePh2 by SiPh2 reduces WP by almost half (WP y
10–13% for a SiPh2 system) that can be correlated to the heavy-
atom effect associated with the Ge atoms in the former case. The
markedly different properties exhibited by these germylene
polymers compared with their silylene analogues are mirrored by
differences in other pairs of Ge and Si systems.10 The (kr)P values at
20 K are (2.1–3.5) 6105 s21 for 3a–c and (1.3–1.7) 6105 s21 for 4a–
c. Relative to 5 ((kr)P y 4.4 6 104 s21), insertion of the germylene
component can increase (kr)P by about 1 order of magnitude. Now,
we are able to get comparable orders of magnitude for (knr)P and
(kr)P which have never been observed in polymetallaynes reported so
far. For phosphorescence in aromatic hydrocarbons, (kr)P lies
5 (a) R. E. Martin and F. Diederich, Angew. Chem., Int. Ed., 1999, 38,
1350; (b) Electronic Materials: The Oligomer Approach, ed. K. Mu¨llen
and G. Wegner, Wiley-VCH, Weinheim, 1998.
6 Crystal data for 1a: C60H72Ge, Mw ~ 865.82, monoclinic, space group
˚
C2/c, a ~ 33.747(2), b ~ 10.2041(5), c ~ 31.427(1) A, b ~ 91.242(1)u,
3
V ~ 10819.8(9) A , Z ~ 8, m(Mo–Ka) ~ 0.600 mm21. 65722 reflections
˚
measured, 26369 unique, R(int) ~ 0.106. Final R ~ 0.0860 and Rw ~
0.1050 for 4866 observed reflections with I w 1.5s(I). For 2a: C70H76Ge,
Mw ~ 989.96, monoclinic, space group P2/c, a ~ 9.1566(7), b ~
3
˚
˚
12.0396(9), c ~ 26.852(2) A, b ~ 97.021(2)u, V ~ 2938.0(4) A , Z ~ 2,
m(Mo–Ka) ~ 0.560 mm21.17699 reflections measured, 7122 unique,
R(int) ~ 0.038. Final R ~ 0.0510 and Rw ~ 0.0530 for 4172 observed
reflections with I w 1.5s(I). CCDC 237762–237763. See http://
www.rsc.org/suppdata/cc/b4/b409580d/ for crystallographic data in .cif
or other electronic format.
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2002, 35, 3506.
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M. S. Khan, P. R. Raithby, A. Ko¨hler and R. H. Friend, J. Am. Chem.
Soc., 2001, 123, 9412; (b) H.-Y. Chao, W. Lu, Y. Li, M. C. W. Chan,
C.-M. Che, K.-K. Cheung and N. Zhu, J. Am. Chem. Soc., 2002, 124,
14696; (c) W.-Y. Wong, L. Liu and J.-X. Shi, Angew. Chem., Int. Ed.,
2003, 42, 4064.
10 (a) A. C. Spivey, D. J. Turner and S. Yeates, Org. Lett., 2002, 4, 1899;
(b) J. L. Bre´fort, R. J. P. Corriu, Ph. Gerbler, C. Gue´rin, B. J. L. Henner,
A. Jean, Th. Kuhlmann, F. Garnier and A. Yassar, Organometallics,
1992, 11, 2500.
typically between 0.1 and 1 s21 11
So, heavy-atom derivatization
.
using Pt and Ge atoms together with conjugation interruption by the
latter can greatly boost (kr)P values by ca. 5 orders of magnitude. It is
likely that the high energy benzene stretching modes of the Ph group
is efficient at promoting (knr)P for 4a–c, making WP and (kr)P values
smaller than those in 3a–c.
In summary, we have developed a novel approach based on the
heavy Ge conjugation-interrupter in metallopolymers that can limit
ECL and result in much larger (kr)P values. The present work has
great potential to excel in optoelectronics that demand light energy
11 (a) N. J. Turro, Modern Molecular Photochemistry, University Science
Books, Mill Valley, CA, USA, 1991.
C h e m . C o m m u n . , 2 0 0 4 , 2 4 2 0 – 2 4 2 1
2 4 2 1