The optical properties of heat-treated polyparaphenylene (PPP) were investi
gated by means of Raman and photoluminescence (PL) spectroscopy. Special at
tention is given to PPP heat-treated to temperatures (T-HT) near the carbon
izing temperature region (T-HT approximate to 700 degrees C) since polymer-
based carbonaceous compounds with low-T-HT (<1000 degrees C) have been foun
d to exhibit electrochemical properties that strongly contrast both the as-
prepared polymer and fully carbonized samples, The Raman spectra show that
for T-HT in the range 650-725 degrees C, several Raman bands near 1300 cm(-
1) can be correlated with both ground-state benzenoid and excited-state qui
noid PPP A(g) modes. An increase in quinoid character is observed with incr
easing T-HT which is consistent with the theoretically predicted stabilizat
ion of the quinoid form in the presence of a high density of defects. The s
maller energy bandgap for pi - pi* transitions in the quinoid conformation
relative to that for the benzenoid form allows for a resonance condition to
be present for laser excitation wavelengths (lambda(exc)) near the visible
(similar to 1-2 eV). We also report a small dispersion effect in the obser
ved quinoid breathing mode band which can be compared to dispersion effects
previously reported for the case of trans-PA. The decrease in bandgap for
the defect-induced quinoid form is also evidenced in the PL spectra of samp
les heat-treated up to 650 degrees C, which show vibronic structure in the
blue-green emission data in the energy range 2.4-3.0 eV, with well-resolved
peaks separated by quinoid phonon energies of 0.165 eV, Franck-Condon anal
ysis shows an increase in the Huang-Rhys parameter (S) with increasing T-HT
which can be related to changes in the electron-phonon coupling of valence
and conduction band states.