Modern Polymer Spect..

4 Vibrational Spectroscopy of Intact and Doped
Conjugated Polymers and Their Models
Y. Firrukawa and M. Tusumi
4.1 Introduction
Most organic polymers are electrical insulators, a property which distinguishes
them from metals. However, the development of a new class of organic polymers
with quasimetallic electrical conductivities has been actively pursued during the
past 18 years, following the discovery in 1977 of high electrical conductivities
for doped polyacetylenes [ 1, 21. The novel concept of a conducting organic polymer
has aroused the interest of a large number of researchers in various areas such as
chemistry, physics, electrical engineering, inaterial science, etc. Detailed discussions
in each area can be found in many review articles [3-171. In particular, a new field of
physics has been opened for the purpose of understanding their electrical properties.
A goal of basic research on conducting polymers is to understand the metallic
properties of such polymers. Since conducting organic polymers have conjugated
7c-electrons in common, chemists intuitively believe that studies on the physical and
chemical properties of conjugated systems would lead to an elucidation of the
mechanism of charge transport. On the other hand, new concepts, solitoris [18, 191,
yolui.oizs [20-221, and bipolaroils [21-231 have been proposed by solid-state physicists
as elementary excitations in conjugated polymers, in order to explain the
physical properties of these polymers. They are collectively called self-loculized
e-xcitutions. These concepts and terminologies are unfamiliar to molecular spectroscopists.
Thus, it seems desirable to build a bridge between solid-state physicists and
molecular spectroscopists.
The structures and properties of conducting polymers have been studied by
various spectroscopic techniques. Ainong them, vibrational ( Rainaii and infrared)
spectroscopy is a powerful tool for elucidating the molecular and electronic structures
of conducting polymers as described in previous reviews [13-151. In spite of
numerous spectroscopic studies, discussions on polarons, bipolarons, and solitons
were not based upon reliable evidence from vibrational spectroscopy until very
recently. Thus, the aims of this review are: (i) to provide a general introduction to
the concepts of polarons, bipolarons, and solitons from the standpoint of molecular
spectroscopists; (ii) to describe the methodology of Raman studies on these selflocalized
excitations; (iii) to review the results of studies on poly( p-phenylene) and
other polymers; and (iv) to discuss the mechanism of charge transport in conducting
polymers.