Modern Polymer Spect..

4.3 Georrietvy oj Intuct Po1vniev.r 209
and all are either insulators or semiconductors in their intact states, and they
become conductors upon doping. Metallic properties are observed for poly( p-
phenylene) [28], polythiophene 129-331, polypyrrole [3 11, and polyaniline [34, 351 at
heavy doping levels, although reported data depend on samples and preparation
methods. The origin of the metallic states of heavily doped polymers is one of the
major unresolved problem in the field of conducting polymers.
4.3 Geometry of Intact Polymers
The question of whether the C-C bonds in an infinite polyene chain are equal or
alternate in length has been discussed by quantum chemists since the 1950s [36, 371.
Various experimental results on oligoenes have shown the existence of alternating
single and double bonds. The bond alternation in rrnns-polyacetylene has been
confirmed by X-ray [38] and nutation NMR [39] studies. However, it is difficult to
determine exact structure parameters for conducting polymers from X-ray diffraction
studies, because single crystals of the polymers are unavailable. It is then useful
to examine the geometries of the polymers and model oligomers by the molecular
orbital (MO) method, especially at nb biitio Hartree-Fock levels or in higher
approximations.
Conducting polymers can be divided into two types, degenevnte and nondegenerate,
according to the structure of the ground-states of intact polymers. The
total energy curve of a degenerate system in the ground state is shown schematically
as a function of a structural defoiination coordinate, R, in Figure 4-23, and that
for a nondegenerate system in Figure 4-21 Let us consider an infinite transpolyacetylene
chain as a prototype of the degenerate ground-state polymers. There
are two degrees of freedom in bond lengths: rc-c and YC-C, which correspond, respectively,
to the lengths of the alternating single and double CC bonds. The coordinate
for structural deformation is simply expressed by the following equation.
This coordinate reflects the degree of bond alternation. This coordinate is used in the
effective-conjugation-coordinate model proposed by Zerbi et al. [15] for the purpose
Figtire 4-2. Total energy as a function
of a structural deformation coordinate.
(a) Trans-polyacetylene
>
u
LT
(degenerate ground-state system); ib) +
poly( p-plienylenei (nondegenerate
ground-state system).
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