Excimer Emission of Naphthalene, Anthracene, and Phenanthrene Crystals Produced by Very High Pressures

Abstract

Fluorescence spectra of ultrapure crystals of naphthalene, anthracene, and phenanthrene have been studied at pressures as great as 50 kbar. These spectra exhibit complex irreversible effects superimposed upon the expected reversible shifts in the energies of the normal fluorescence spectra. The irreversible effects include a loss of the 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 crystals continue to exhibit this excimer fluorescence after the pressure is reduced to atmospheric pressure, but the irreversible effects can be removed by thermal annealing of the crystal at atmospheric pressure. The irreversibility is attributed to trapping of pairs of molecules in an excimer-like orientation as a crystal defect after they lose the excitation energy of the excimer. These defects then act as traps for excitation energy in the crystal. Other possible interpretations of the irreversible effects are discussed.