Polyparaphenylene under pressure: Optical absorption and vibrational modes

Optical absorption, Raman, and infrared (IR) spectra of undoped polyparaphenylene (PPP) have been studied under pressure up to 20 GPa at 300 K. PPP shows stable behavior, in marked contrast to the reactivity observed in trans-polyacetylene under pressure. The absorption edge of PPP shifts to lower energy with a total shift of 0.55 eV at 20 GPa. Raman spectra reveal a new band at about 820 cm−1 which becomes active above ∼5 GPa. At high pressure we find a considerable broadening of the infrared-active out-of-plane mode at 805 cm−1. The results are in part interpreted in terms of the larger interchain coupling induced by pressure.