Ladder oligo(p-phenylenevinylene)s with silicon and carbon bridges

Article abstract of DOI:10.1021/ja042964m

A general and versatile synthetic method for ladder oligo(p-phenylenevinylene)s (LOPVs) and related π-electron systems, having annelated π-conjugated structures with silicon and carbon bridges, has been developed on the basis of the combination of two cyc

Full text of DOI:10.1021/ja042964m

Published on Web 01/22/2005
Ladder Oligo(p-phenylenevinylene)s with Silicon and Carbon Bridges
Caihong Xu,‡ Atsushi Wakamiya,† and Shigehiro Yamaguchi*,†,‡
Department of Chemistry, Graduate School of Science, Nagoya UniVersity, and PRESTO, Japan Science and
Technology Agency (JST), Chikusa, Nagoya 464-8602, Japan
Received November 22, 2004; E-mail: yamaguchi@chem.nagoya-u.ac.jp
Scheme 1
Ladder molecules with annelated π-conjugated frameworks are
promising materials for broad applications in organic-based devices,
including light-emitting diodes, thin film transistors, and optically
pumped lasers.1 Their rigid coplanar structures promise enhanced
π-conjugation, which leads to a set of desirable properties, such as
intense luminescence and high carrier mobility. The recent persistent
interest in this field has led to considerable progress in the synthesis
of various types of molecules, such as ladder oligo- or poly(p-
phenylene)s with carbon2 or heteroatom bridges,3 polyacenes and
heteroacenes,4 and related π-conjugated systems.5 However, only
limited attention has been paid to phenylenevinylene-based ladder
molecules, probably due to the lack of efficient synthetic routes.6
As an example, Barton recently reported the synthesis of bis-silicon-
bridged stilbene 1 by the elegant rearrangement of 5,6-disiladibenzo-
[c,g]cyclooctynes and installed this skeleton as a pendant group
on polymeric systems.7 We also independently reported the efficient
route to the silicon-bridged stilbene derivatives based on a newly
developed cyclization.8 However, tetrakis-silicon-bridged bis(styryl)-
benzene 2 thus prepared was the longest example of the ladder
oligo(phenylenevinylene)s (LOPVs) reported thus far. Therefore,
the exploring of more efficient and general routes to the longer
ladder systems has been a compelling subject in our research. We
now report the first versatile synthetic methodology for the LOPVs
and related π-electron systems annelated with silicon and carbon
bridges. This methodology allows us to synthesize a homologous
series of molecules up to a 13-ring-fused system.
of 4, the cyclization immediately took place as indicated by the
appearance of a strong fluorescence, and the desired Si,C,C,Si-
bridged bis(styryl)benzene 5 was obtained in 96% yield as a bright
yellow solid (Scheme 1). No other regioisomers were detected.
Similarly, starting from 6, the successive two-step procedures
afforded the C,Si,Si,C-bridged 8 in 64% overall yield via a diol 7.
Advantageously, the substituents on the carbon bridges can be easily
modified by changing the ketones. For example, the use of
fluorenone instead of benzophenone resulted in producing fluorene-
bound bis(styryl)benzene 9 (Chart 1) that has an interesting spiro
structure (Supporting Information).
Our strategy for the construction of the Si,C-bridged LOPV
skeleton is to combine two types of cyclization, that is the intra-
molecular reductive cyclization of mono(o-silylphenyl)acetylene
derivatives and the Friedel-Crafts-type cyclization with a Lewis
acid. We recently reported the former reaction as an efficient method
for the preparation of various functionalized silaindenes,9 while the
These results indicate that (o-silylphenyl)acetylene A and 2,5-
latter electrophilic cyclization has been broadly employed to
bis(silyl)-1,4-diethynylbenzene B can serve as synthetic equivalents
synthesize the ladder oligo- or poly(p-phenylene)s.2
of the 3-ring-fused terminating unit A′ and 5-ring-fused spacer unit
We first examined the reactions of two kinds of starting materials,
B′, respectively, since the starting acetylenic materials of the present
i.e., 1,4-bis(phenylethynyl)benzenes 3 and 6 that have two silyl
methodology are readily obtainable by the Sonogashira coupling
groups at the terminal and central benzene rings, respectively. Thus,
of A or B with appropriate halogenated π-systems. On the basis of
the treatment of 3 with 4 mol amounts of lithium naphthalenide
this idea, we carried out the synthesis of more extended ladder
(LiNaph) followed by trapping the produced dianionic species with
molecules as shown in Chart 1. Thus, the 9-ring-fused compound
excess benzophenone gave a diol 4 in 84% yield. The next
10, consisting of two terminating units A′ and a fluorene spacer,
electrophilic cyclization of 4 was carried out using BF3‚OEt2 as a
was prepared from compound A and 2,7-diiodofluorene by the
Lewis acid. Upon the addition of BF3‚OEt2 to a CH2Cl2 solution
three-step reaction, which included the Sonogashira coupling of
these compounds, the reductive cyclization, and the electrophilic
cyclization. Similarly, the use of 2-iodofluorene and compound B
† Nagoya University.
‡ PRESTO, JST.
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10.1021/ja042964m CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Chart 1
Table 1. Photophysical Data for LOPVs and Related Compounds
UV
−
vis absorptiona
fluorescencea
max/nmb
φF
c
cmpd
λ
max/nmb
log
ꢀ
λ
5
8
425
438
447
433
476
502
4.49
4.41
4.36
4.80
4.72
4.88
443
464
473
445
499
523
0.73
0.56
0.50
0.84
0.40
0.59e
2d
10
11
13
a In THF. b All compounds show vibronic absorption and emission
spectra. Only the longest absorption maxima and the shortest emission
maxima are reported in this table. c Determined with perylene as a standard,
unless otherwise stated. The (ΦF is the average values of repeated
measurements within (5% errors. d Reference 8. e Determined with fluo-
rescein as a standard.
nm with a slight decrease in ΦF. As a consequence, the emission
colors varies from blue 10 to green 11 to yellow 13. As for the
electrochemical properties, the cyclic voltammetry of the LOPV
13 has been investigated as a representative example. This
compound shows two quasi-reversible redox processes both for the
oxidation (Epa 0.34, 0.64 V vs Fc/Fc+) and reduction (Epc -2.51,
-2.84 V vs Fc/Fc+), indicative of its potential as an ambipolar
carrier transporting material. Further studies on the solid-state
electronic properties, such as the carrier mobility and luminescence
properties, for this series of ladder molecules are currently in
progress.
as the starting materials resulted in producing the 11-ring-fused
system 11 in which two fluorene skeletons were connected with
the 5-ring ladder unit B′. Finally, we have succeeded in the
preparation of the 13-ring-fused LOPV 13 by connecting the two
terminating units A′ and the spacer unit B′ with two benzene
linkages. Thus, the iterative Sonogashira coupling reactions of
compound B with p-iodobromobenzene and then with A produced
an oligo(phenyleneethynylene) derivative 12. The reductive cy-
clization of 12 proceeded at the four acetylene moieties to produce
the corresponding tetraol in 44% yield, and the subsequent
electrophilic cyclization gave the fully annelated 13 as a bright-
orange solid in 52% yield. This compound has a good solubility
toward common solvents such as THF (0.3 g/mL) and has a
substantial thermal stability (T-d5 455 °C, under N2). Figure 1 shows
the crystal structure of 13. Notably, this compound has a nearly
flat π-conjugated framework with a length of ca. 2.9 nm.
Acknowledgment. This work was supported by Grants-in-Aid
(No. 12CE2005 for Elements Science and No. 15205014) from the
Ministry of Education, Culture, Sports, Science, and Technology
of Japan, and PRESTO, Japan Science and Technology Agency
(JST).
Supporting Information Available: Experimental details and
spectral and analytical data for the ladder molecules, a cyclic voltam-
mogram of 13, ORTEP drawings and CIF files of 9 and 13. This
material is available free of charge via the Internet at http://pubs.acs.org.
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Figure 1. ORTEP drawing of 13 (50% probability for thermal ellipsoids).
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All the ladder molecules show intense fluorescence in the visible
region, as their data are summarized in Table 1. The high quantum
yields as well as the relatively small Stokes shifts (12-26 nm) are
their notable common features. A comparison among the bis(styryl)-
benzene derivatives 2, 5, and 8 shows that the ΦF value increases
in the order of 2 < 8 < 5, while the emission maxima shift to
shorter wavelengths over 30 nm in the same order. These results
demonstrate a distinct effect of the silicon bridges on the fluores-
cence. The character of the disilaindacene moiety may be predomi-
nant relative to that of the terminal silaindene moiety. In the series
of extended systems, the extension of the π-conjugation from 10
to 11 to 13 effectively red-shifts the emission maxima by about 80
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