24 Solid-State Polymerization of Conjugated …
503
polymerization occurs from one crystal surface to the other surface on the opposite
side [6, 7].
Due to the formation of the π-conjugated backbones after the polymerization,
the resulting polydiacetylenes usually have the excitonic absorption bands with the
maxima in the wavelength region approximately between 500 and 700 nm, showing
characteristic blue to red colors. As mentioned later, when the materials after the
solid-state reaction of butadiyne derivatives do not show such characteristic bands,
the reaction may not proceed in the 1,4-addtion manner to form the polydiacetylene structure. Although the backbone structure of polydiacetylene is fixed to the
ene-yne alternating sequence, its color varies depending on the structural conditions.
Generally, it is believed that blue color corresponds to planer extended π-conjugated
backbone and red color corresponds to slightly disordered or undulated π-conjugated
backbone, respectively. Thus, the polymer color can be altered through conformational changes of the substituents triggered by external stimuli, which result in conformational changes of the π-conjugated backbone. Based on this mechanism, a variety
of bio- and chemo-sensors have been developed [8, 9].
Meanwhile, applications of polydiacetylenes for electronics and photonics have
been started first as third-order nonlinear optical materials. In 1969, the third-order
nonlinear optical susceptibilities (χ
(3) s) of polydiacetylenes evaluated by thirdharmonic generation (THG) were reported to be comparable to or more than those
of semiconductors, which have been known as the materials with large χ
(3) s [10].
On the other hand, although many of π-conjugated polymers have been investigated
as conducting polymers, electrical conductivities of polydiacetylenes are generally
poor. This is because high crystallinity of the polymer prevents dopant immersion
and effective chemical doping of the π-conjugated backbones cannot be achieved.
Semiconducting electrical conductivities in the crystalline states were reported, for
example, for the polydiacetylene obtained by polymerization of the doped monomer
crystals or doping during polymerization [11].
In this article, we review some polydiacetylenes prepared to improve their physical properties. The first topic contains polydiacetylenes substituted by electrondonating aromatic rings directly bound to the backbone. They may be used as
hole-transport materials. Also, if they form charge-transfer complexes by combining
with appropriate electron-accepting species, electrically conducting monomers and
polymers may be obtained. The second topic is related to polydiacetylenes with
fibrous morphology. When the π-conjugated backbones are formed along the fiber
direction, the resulting fibrous crystals may be used as conducting wires for molecular electronics. The third topic is related to polymerization of the monomers with
two butadiyne moieties, which may give ladder-type polydiacetylenes with unique
properties.
503
polymerization occurs from one crystal surface to the other surface on the opposite
side [6, 7].
Due to the formation of the π-conjugated backbones after the polymerization,
the resulting polydiacetylenes usually have the excitonic absorption bands with the
maxima in the wavelength region approximately between 500 and 700 nm, showing
characteristic blue to red colors. As mentioned later, when the materials after the
solid-state reaction of butadiyne derivatives do not show such characteristic bands,
the reaction may not proceed in the 1,4-addtion manner to form the polydiacetylene structure. Although the backbone structure of polydiacetylene is fixed to the
ene-yne alternating sequence, its color varies depending on the structural conditions.
Generally, it is believed that blue color corresponds to planer extended π-conjugated
backbone and red color corresponds to slightly disordered or undulated π-conjugated
backbone, respectively. Thus, the polymer color can be altered through conformational changes of the substituents triggered by external stimuli, which result in conformational changes of the π-conjugated backbone. Based on this mechanism, a variety
of bio- and chemo-sensors have been developed [8, 9].
Meanwhile, applications of polydiacetylenes for electronics and photonics have
been started first as third-order nonlinear optical materials. In 1969, the third-order
nonlinear optical susceptibilities (χ
(3) s) of polydiacetylenes evaluated by thirdharmonic generation (THG) were reported to be comparable to or more than those
of semiconductors, which have been known as the materials with large χ
(3) s [10].
On the other hand, although many of π-conjugated polymers have been investigated
as conducting polymers, electrical conductivities of polydiacetylenes are generally
poor. This is because high crystallinity of the polymer prevents dopant immersion
and effective chemical doping of the π-conjugated backbones cannot be achieved.
Semiconducting electrical conductivities in the crystalline states were reported, for
example, for the polydiacetylene obtained by polymerization of the doped monomer
crystals or doping during polymerization [11].
In this article, we review some polydiacetylenes prepared to improve their physical properties. The first topic contains polydiacetylenes substituted by electrondonating aromatic rings directly bound to the backbone. They may be used as
hole-transport materials. Also, if they form charge-transfer complexes by combining
with appropriate electron-accepting species, electrically conducting monomers and
polymers may be obtained. The second topic is related to polydiacetylenes with
fibrous morphology. When the π-conjugated backbones are formed along the fiber
direction, the resulting fibrous crystals may be used as conducting wires for molecular electronics. The third topic is related to polymerization of the monomers with
two butadiyne moieties, which may give ladder-type polydiacetylenes with unique
properties.