Crystal Spectra of Tetracene and Rubrene under Pressure

Pressure-induced spectral shifts (0–20 kbar) are compared for the absorption and fluorescence of tetracene and rubrene as crystals or embedded in polystyrene matrices. At 20 kbar the red shift of the tetracene crystal absorption is two to three times larger than either the red shift of tetracene in polystyrene or rubrene in both crystal and solid-solution environments. The crystal shifts and increased Davydov splitting at high pressure are related to calculations on compressed anthracene by Rice and Jortner. The spectral shifts indicate that crystal-field effects are negligible for rubrene due to steric factors. Strong specific interactions in the excited state between the phenyl substituents of rubrene and the phenyl groups of polystyrene are indicated from the fluorescence spectra at atmospheric pressure. The influence of pressure on tetracene crystal luminescence can be described in terms of excitation-energy transfer to tetracene dimers at defects. High pressures enhance the transformation of excitation energy into heat, probably by increasing non-radiative excition trapping at imperfections.