Spontaneous site reorientation in a mixed molecular crystal: Tetracene in benzoic acid

Absorption and fluorescence spectra of tetracene in a benzoic acid host crystal at 1.5 K are presented. The fluorescence zero-phonon line is shifted by more than 800 cm−1 to the red of the maximum of the 120 cm−1 broad absorption origin. This shift is attributed to a lateral site reorientation of the guest upon excitation, permitted by the difference in size between the tetracene and the benzoic acid dimer it replaces. In addition, other features in the fluorescence spectrum are ascribed to proton tautomerization occurring in the host dimers in the vicinity of the guest. These features disappear upon deuteration of the host acid protons, while the magnitude of the red shift is virtually unchanged.