Modern Polymer Spect..

228 4 Ii’Dratioizal Spectroscopy of Intact and Doped Coiijuyatecl Polytners
and G are strongly observed. Band B is assignable to a symmetric mode consisting
of the v,~(bl,,) mode (Figure 4-12e) of each benzene ring in the charged domain.
Band G is also attributed to a symmetric mode consisting of the i112(61~,) mode
(Figure 4-12f). Bands D and F are assignable to out-of-phase symmetric inodes
consisting of the I I ~ ( L ~ and ~ ) i ~ ~ ( modes ~ i ~ )(Figure 4-12b, c) of each benzene ring,
respectively. The appearance of bands B, D, F, and G indicates the loss of translational
symmetry of neutral polyi p-phenylene) chain, wliich is consistent with the
forniatioii of localized charged doniains upon doping. Bands B, D, F, and G are
not observed or weakly observed for negative polarons, but strongly observed for
negative bipolarons. These observations seem to reflect different geometric changes
occurring between polarons and bipolarons 011 going from their respective ground
electronic states to their respective excited electronic states.
4.7.3 Assignments of Electronic Absorption Spectra of Doped
Poly( p-phen ylene)
In Section 4.7.1, two broad bands centered at about 5600 and 19400 cm-l in the
electronic absorption spectrum of Bu4N+-doped poly( p-phenylene) have been
attributed to the QI and 02 transitions of negative polarons, respectively, on the
basis of the data of the radical anions and dianions of p-oligophenyls. Raman
spectroscopy provides information on the electronic levels associated with polarons
and bipolarons as well as on their geometries. The Ranian results indicate the coexistence
of polarons and bipolarons in Na-doped poly( p-phenylene). The Ranian
bands due to polarons are resoiiantly enhanced by the use of the 488.0 and 514.5
11111 laser lines, which are located within the electronic absorption around 19 400
cin-l (51 5 nin). These observations confirm the proposed assignment that the
19400 c1n-l band is due to the cuz transition of polarons. The Ranian bands due to
bipolarons are resonantly enhanced by the use of the 632.8, 720, and 1064 nm laser
lines. However, in the absorption spectruin of Bu4N+-doped poly( p-phenylene), no
peaks are observed in the 15800-9400 cm-’ (632.8-1064 nm) region. This fact can
be explained by considering that the content of the bipolaron is very small and the
QI’ absorption due to the bipolaron is overlapped by the strong to1 absorption band
due to polarons.
4.8 Other Polymers
The Raman spectra of doped states of polyaniline [99], poly( p-phenylenevinylene)
[75, 100-1021, and polythiophene [72, 731 have been also analyzed on the basis of
the data of charged oligoiners (charged oligomer approach). The detected selflocalized
excitations are suniniarized in Table 4-5. Some important conclusions can
be drawn from these Rainaii studies.