Optical properties of heat-treated polyparaphenylene

Abstract

The optical properties of heat-treated polyparaphenylene (PPP) were investigated by means of Raman and photoluminescence (PL) spectroscopy. Special attention is given to PPP heat-treated to temperatures (THT) near the carbonizing temperature region (THT ≈ 700°C) since polymer-based carbonaceous compounds with low-THT (<1000 °C) have been found to exhibit electrochemical properties that strongly contrast both the as-prepared polymer and fully carbonized samples. The Raman spectra show that for THT in the range 650–725°C, several Raman bands near 1300 cm−1 can be correlated with both ground-state benzenoid and excited-state quinoid PPP Ag modes. An increase in quinoid character is observed with increasing THT, which is consistent with the theoretically predicted stabilization of the quinoid form in the presence of a high density of defects. The smaller energy bandgap for π – π* transitions in the quinoid conformation relative to that for the benzenoid form allows for a resonance condition to be present for laser excitation wavelengths (λexc) near the visible (∼1–2 eV). We also report a small dispersion effect in the observed 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 samples heat-treated up to 650°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 analysis shows an increase in the Huang–Rhys parameter (S) with increasing THT which can be related to changes in the electron-phonon coupling of valence and conduction band states.