Modern Polymer Spect..

230 4 Vilwntional Slwctvoscopjl of Intact mitl Doped Conjzrgutecl Poljwievs
4.9 Electronic Absorption and ESR Spectroscopies and
Theory
It is widely accepted [7, 101 that the major species generated by chemical doping it?
nondegenerate polymers is bipolarons, except for polyaniline. However, the result^
obtained from Raman spectroscopy indicate the existence of polarons at heavily
doped polymers. These results are inconsistent with the established view. The
experimental basis for claiming the existence of bipolarons has been electronic
absorption and ESR spectra [7, 9, lo]. We will comment on these experimental
results .
The electronic absorption spectra of p-type-doped polypyrrole have been interpreted
in terms of polarons and bipolarons for the first time 11121. Electronic
absorption spectra of doped polypyrrole in the range from visible to NIR show
three bands due to intragap transitions at low dopant contents and two bands at
high dopant contents [ 1131. The three bands are attributed to polarons and the two
bands to bipolarons [112], because a polaron is expected to have three intragap
transitions and a bipolaron is expected to have two intragap transitions (see Section
4.4.2). A quantitative ESR study [114] has confirmed the interpretation of the electronic
absorption experiments. This rule of thumb for band assignment has so fiiialso
been applied to other conducting polymers.
However, the two electronic absorption bands of doped poly( p-phenylene) are
not explained by bipolarons, as demonstrated in Section 4.7.3. Furthermore,
p-type-doped polythiophene shows two absorptions at about 12000 and 5200 cnir ’
[72, 73, 115, 1161 and p-type-doped poly( p-phenylenevinylenej shows two broad
absorptions at about 19000 and 7300 cmP1 [75, 117, 1181. In these two systems,
only positive polarons are detected by Raman spectroscopy (see Table 4-5). These
results of Rainan spectroscopy suggest that the two-band pattern in the electronic
absorption spectrum is attributed to polarons. As discussed in Section 4.7.1.2, it is
expected that a polaron has two intense intragap transitions and a bipolaron one
intense transition. From these findings, we propose a new rule: a two-hmd pattern
correspondy to yolavons mid one-barid puttern to hipolarons [79], when these species
do not coexist. When polaroils and bipolarons coexist, and their electronic absorptions
are overlapped, a more careful examination of the observed spectrum is of
course necessary.
Recently, Hill et al. [119] have proposed the existence of a singlet radical-cation
dimer (i.e., polaron dimer) as an alternative to a spinless bipolaron to explain weak
ESR signals from doped polymers. Their proposal is based on the observed dimerization
of the radical cation of 2,5”-dimethylterthiophene. They have found by
electronic absorption and ESR measurements that the above radical cation is
dinierized at low temperature even at a low concentration (4 x lor4 mol/l). Singlet
intrachain polaron pail-s and interchain polaron pairs give no ESR signals. Thus,
the absence of ESR signals or observation of weak ESR signals does not directly
lead to the conclusion that the doped polymer contains only bipolarons.
The stability of polarons, bipolarons, and solitons has been studied theoretically