Fluorescence of Doped Crystals of Anthracene, Naphthalene, and Phenanthrene under High Pressures: The Role of Excimers in Energy Transfer to the Guest Molecules

Abstract

Fluorescence spectra of crystals of anthracene, naphthalene, and phenanthrene doped with other polycyclic aromatic hydrocarbons have been studied at pressures as great as 50 kbar. The most significant effects of pressure on the spectra of the host crystals are a loss of intensity of the normal fluorescence with increasing pressure and the almost simultaneous appearance of a broad, featureless emission at energies about 3000–6000 cm−1 lower than that of the normal fluorescence. This emission is assigned as the fluorescence of excimers formed upon optical excitation of the crystals under high pressure. The growth of the intensity of the excimer emission with increasing pressure, relative to the intensity of the normal fluorescence, is the same in doped crystals as in pure crystals. The intensities of the guest molecule fluorescence relative to the intensity of the normal host fluorescence in the various doped crystals increase with pressure at about the same rate as the intensity of the excimer emission. These observations are interpreted as indicating that excimers are the lowest-energy excited singlet species of aromatic molecular crystals at these pressures and that when the excimer concentration is appreciable, the path of energy transfer is from the host via excimers to the guest.