Modern Polymer Spect..

m
0
m
I
1500 1000 5
WAVENUMBER icrn~l
Figure 4-10. Vibrational spectra of intact poly(yphenylene).
(a) Raman spectrum taken with the
1064-nni line in powder sample; (b) infrared spectrum
in a KBr disk [83].
inter-ring CC stretch. The 1222 cm-' mode (Figure 4-12c) is a mixture of CC
stretch and CH in-plane bend. The 798 cm-' mode (Figure 4-12d) is a mixture of
CC stretch and CCC deformation. The intensity of the 1278-1275 cn-' band relative
to that of the 1221-1220 c1n-l band decreases as the chain length becomes
longer. Thus, this ratio is a marker of the length of conjugated segments [81, 82, 881.
The wavenumbers of the Raman bands of poIy( p-phenylene) and y-oligophenyls
are almost independent on the excitation wavelength [Sl]. It is well known that the
wavenumbers of the two strong Rainan bands of tmm-polyacetyleiie depend greatly
on the wavelength of the excitation laser. These large dispersions are explained by
the existence of segments of various conjugation lengths that give rise to the Raman
bands at different wavenumbers [13, 141. However, the Raniail spectra of other
conducting polymers do not show such dependence on the excitation wavelength.
The (resonance) Raman intensities of conjugated molecules have been analyzed
by the classical vibronic theory due to Tang and Albrecht [89]. Inagaki et al. 1901
have shown that resonance Ranian intensities of totally symmetric modes of p-
carotene (an oligoene with eleven conjugated C=C bonds) arise from the A term
(Franck-Condon term) of the Albrecht theory. Probably, the Franck-Condon
factor plays an important role in determining the resonance Rainan intensities of
almost all conjugated molecules. The resonance Ranian intensity of each totally
symmetric mode is proportional to the square of the shift of the potential minimum
in the resonant excited electronic state from that in the ground electronic state along
the normal mode giving rise to the resonance Raman band, when the shift is small
[91-941. The effective conjugation coordinate represents approximately the change
of equilibrium geometry between the ground electronic state and the first dipoleallowed
excited electronic state [15]. Thus, the resonance Raman intensity of a
normal mode is determined by the contribution of the effective conjugation coordinate
to the mode. Details are described in [15]. It has been demonstrated that the