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Chemistry Letters Vol.38, No.7 (2009)
Synthesis of Cyano-substituted Through-space Poly(p-arylenevinylene)
Yasuhiro Morisaki,ꢀ Lin Lin, and Yoshiki Chujoꢀ
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University,
Katsura, Nishikyo-ku, Kyoto 615-8510
(Received April 27, 2009; CL-090421; E-mail: ymo@chujo.synchem.kyoto-u.ac.jp)
The Knoevenagel reaction of pseudo-p-bis(cyanomethyl)-
1) s-BuLi
Br
CHO
2) DMF
[2.2]paracyclophane with 2,5-dialkoxy-1,4-diformylbenzene
was carried out to yield the cyano-substituted through-space poly-
(arylenevinylene). Thermal, optical, and electrochemical proper-
ties are described.
Et2O / THF
–78°C to r.t.
50%
Br
OHC
2
1
Tos CH2 NC
CN
t-BuOK
Over the past two decades, conjugated poly(p-arylene-
vinylene)s (PAVs) have been intensively investigated for their
unique electrical and optical properties.1 The first report of the
application of a PAV in light-emitting diodes (LEDs)2 triggered
extensive research on the development of high performance con-
jugated polymers such as poly(p-aryleneethynylene)s and poly-
(p-arylene)s in addition to PAVs. The introduction of cyano
groups into vinylene units in the PAV backbone lowers its
HOMO–LUMO band gap energy, leading to bathochromic shifts
of absorption and emission spectra. Thus, cyano-substituted poly-
(p-phenylenevinylene)s (PPVs) are observed to emit red light.3
Recently, we reported the synthesis of through-space conju-
gated polymers containing cyclophane moieties as the key re-
peating unit.4,5 Most of the through-space conjugated polymers
were prepared by palladium-catalyzed coupling reactions.5 We
found that the conjugation length of the polymers increased as
a result of ꢀ–ꢀ stacking, and that they emitted intense fluores-
cence despite the presence of the ꢀ-stacked structures in the
polymer chain. The expansion of the substrate scope of the new-
ly developed through-space conjugated polymers is an important
subject, and investigating their properties is necessary for mak-
ing advances in the field of the conjugated polymers. Thus,
in this study, we synthesized a new type of cyano-substituted
through-space PAV by the Knoevenagel reaction of pseudo-p-
bis(cyanomethyl)[2.2]paracyclophane with 2,5-dialkoxy-1,4-di-
formylbenzene and elucidated its properties.
1,2-Dimethoxyethane-CH2Cl2
NC
40°C, 3 h
40%
3
C8H17
OHC
O
3
+
CHO
OC8H17
4
C8H17
O
t-BuOK
OC8H17
CN
toluene-t-BuOH
100°C, 10.5 h
96%
NC
n
5
Mn = 8000, Mw/Mn = 2.3
Scheme 1. Synthesis of monomer 3 and polymer 5.
dicated that insoluble polymer formation was due to high molec-
ular weight rather than crosslinking. Polymer 5 was found to be
thermally stable (Figure S10),7 and the soluble component of 5
could be processed into an amorphous thin film by casting or
spin-coating from its toluene solution. The number-average mo-
lecular weight (Mn) of the soluble component of 5 was estimated
to be 8000 with Mw=Mn of 2.3 by gel permeation chromatogra-
phy (GPC) in eluent CHCl3 (polystyrene standards).
Pseudo-p-diformyl[2.2]paracyclophane (2) has been synthe-
sized;6 however, its synthetic procedure and analytical data are
unknown. Thus, we synthesized 2 by our own method, as shown
in Scheme 1. The synthesis of monomer 3 was carried out by
the reaction of 2 with 4-toluenesulfonylmethyl isocyanide
(Scheme 1). Then, cyano-substituted through-space PAV was
synthesized by carrying out the typical Knoevenagel reaction
of 3 with 2,5-dioctyloxy-1,4-diformylbenzene (4) in the pres-
ence of t-BuOK in toluene-t-BuOH at 100 ꢁC for 10.5 h. The
reaction was quenched with AcOH, and toluene was added to
obtain the crude polymer in the form of an orange powder, which
included an insoluble component in common organic solvents.
The insoluble component was washed with water and THF,
and the soluble component was purified by reprecipitation from
MeOH to obtain the corresponding polymer 5 in total 96% iso-
lated yield. FTIR spectra of both the insoluble and soluble poly-
mers confirmed that they had the same structure (Figure S7).7
Absorption intensities of stretching vibrations of CH and CN in-
The absorption spectra of polymer 5 and model compound 6
were measured in CHCl3 solution, as shown in Figure 1. Poly-
mer 5 exhibited absorption peaks at 436 and 360 nm in CHCl3
at room temperature, which can be attributed to the ꢀ–ꢀꢀ tran-
sition bands of the conjugated PAV backbone.8a The absorption
maximum (ꢁmax ¼ 436 nm) and the absorption edge (onset
ꢁ ¼ 525 nm) of 5 exhibited clear bathochromic shifts relative
to those of 6 (ꢁmax ¼ 413 nm, onset ꢁ ¼ 500 nm), as shown in
Figure 1. This result indicates the extension of the conjugation
length of 5 as a result of ꢀ–ꢀ stacking. The band gap energy
(Eg) of 5 was estimated to be approximately 2.4 eV on the basis
of the absorption edge. On the other hand, it is reported that
through-bond conjugated PPV exhibits ꢁmax of around 465
nm8 and Eg ¼ 2:2 eV,8c indicating through-bond conjugated
PPV is more effective for the extension of the conjugation length
than the through-space conjugation.
Fluorescence emission spectrum of polymer 5 was obtained
from sufficiently diluted CHCl3 solution (2:6 ꢂ 10ꢃ6 M)9 at
Copyright Ó 2009 The Chemical Society of Japan