Behavior of Fused-Ring Aromatic Hydrocarbons at Very High Pressure

Abstract

The effect of very high pressure has been studied on the electronic structure of some fused‐ring aromatic hydrocarbons. The material most thoroughly studied is pentacene. Observations include optical absorption of the low‐energy peaks and electrical resistance. The optical absorption peaks of tetracene and pentacene shift red at a rate which increases with density. Azulene peaks shift first blue, then red. The dipole moment of the excited state increases with increasing pressure. The electrical resistance decreases rapidly with increasing pressure, as a result of the decrease in activation energy for carrier production. Single‐crystal pentacene becomes metallic at 270 kbar. The fused powder exhibits metallic behavior at somewhat higher pressures. At pressures above about 180–200 kbar and temperatures above 180°—190°K an irreversible transition takes place in pentacene involving resistance increase and, at higher pressure, a transformation from metal to semiconductor. The low‐energy optical absorption peaks have largely disappeared. It is proposed that this is a cross linking between adjacent molecules. A similar phenomenon is observed in violanthrene. The very sharp irreversible rise in resistance in single‐crystal graphite above 150 kbar and 180°K may be an analogous cross‐linking transformation.