Modern Polymer Spect..

is worthwhile pointing out that the cu3 and (03' transitions are symmetry forbidden.
According to a continuum electroii-phonon-coupled model reported by Fesser et
al. [48], the (01 and 032 transitions are dominant among the three expected for a
polaron, and the 031' transition is more intense between the two expected for a
bipolaron. Finally, it is expected that a polaron hus two intense intrugup tramitions
and a bipolavoir one intense tmnsition [79].
The absorption spectrum of Bu4Nf-doped poly( p-phenylene) can now be
explained as follows. Two broad bands centered at about 5600 and 19400 or1
(Figure 4-6) are attributable to the 031 and cc)? transitions of negative yolarons,
respectively. These assignments will be discussed again in Section 4.7.3 on the basis
of the Raman results.
4.7.2 Raman Spectra
4.7.2.1 Intact Poly(p-phenylene) and p-Oligophenyls
The observed infrared and Raman spectra of intact poly( p-phenylene) [80-841 have
been analyzed by normal coordinate calculations [43, 84-87]. The factor group of a
coplanar polymer is isomorphous with the point group &. When the JZ plane is
taken in the plienylene-ring plane (s axis along the polymer chain) and the x axis
perpendicular to the phenylene-ring plane, the irreducible representation at the zone
center (k = 0) is as follows:
rvlb
rvlb
in-pldne = 5ug(R) + 4b1u(IR) + 4hu(IR) + 5hg(R)
out-of-plane = 2 ~u + lbl,(R) + 3hg(R) + 2hu(IR)
where R and IR denote Raman- and infrared-active vibrations, respectively. The
1064-nm excited Raman spectrum and infrared absorption spectrum of intact
poly( p-phenylene) prepared by dehalogenation polycoiidensation of y-dihalogenobenzene
are shown in Figure 4-10a, b, respectively. The observed and calculated
vibrational frequencies of poly( p-phenylene), "C-substituted analog, and perdeuterated
analog are listed in Table 4-3, except for CH (CD) stretches [q(ug), i'g(hlu),
v16(bzu), and v20(03~)]. Major Ranian and infrared bands have been assigned.
In Figure 4-10b, the 809 cm-' infrared band is assigned to the CH out-of-plane
bend of the phenylene rings, and the 765 and 694 cm-' bands are assigned to the
CH out-of-plane bends of the terniinal phenyl rings.
The Raman spectrum of poly( p-phenylene) has been compared with those of
p-oligophenyls. The 1064-nm excited Rainan spectra of p-oligophenyls in the solid
state [88] are shown in Figure 4-11. The bands at 1605-1593, 1278-1275, 1221-
1220, and 795-774 cmrl correspond to those at 1595, 1282, 1222, and 798 ,nip' of
intact poly( y-phenylene), respectively. These bands are assigned to the ug vibrations
(v2-vg). The atomic displacements of these modes obtained by normal coordinate
calculations based on the PM3 method [84] are depicted in Figure 4-12a-d. The
1595 cm-' mode (Figure 4-12a) is contributed mainly by the CC stretch of the
phenylene ring. The 1282 cm-' mode (Figure 4-12b) is mainly contributed by the