Excitation Spectrum of Crystalline Tetracene Fluorescence: A Probe for Optically-Induced Singlet-Exciton Fission

Abstract

The intrinsic fluorescence efficiency of crystalline tetracene depends on the energy of the exiciting light. It decreases for E_exc > 2.48 eV displaying a minimum at E_exc = 2.51 eV which is identical with the energy of the triplet pair state. It is suggested that an optically populated, vibronically excited S^v ₁-state. which is iso-energetic with the triplet pair state can undergo fission into two triplet states. However the effect is at least one order of magnitude smaller than theoretically predicted, strongly temperature dependent and only noticeable with light polarized perpendicular to the crystallographic b-axis (weak Davydov component). It is therefore concluded that thermally induced singlet exciton fission must proceed via a different vibrationally excited intermediate state, which is optically inactive.