Theoretical study of the radiationless deactivation mechanisms of photo-excited thiophene

Highlights: ► The radiationless deactivation mechanisms of photo-excited thiophene are investigated. ► Excited state depletion is found to be possible by C–S cleavage as well as puckering of the aromatic ring. ► The proposed mechanisms are good candidates to explain why thiophene is non-fluorescent. - Abstract: The radiationless deactivation mechanisms of photo-excited thiophene have been studied using the multi-reference second-order perturbation theory and linear response coupled cluster methods. The electronic spectrum has been established and various minimum energy structures and conical intersections involving the ground and lowest singlet excited states have been characterized. Simplified reaction paths connecting the optimized geometries have been calculated as well. Based on these investigations, several deactivation mechanisms have been identified leading from the lowest bright 1ππ∗ states back to the electronic ground state. The excited state depletion in each case is possible due to the existence of low-lying conical intersections formed by either cleavage of one of the CS bonds or out-of-plane deformations of the aromatic ring. The deactivation mechanisms suggested in this work should provide some very efficient decay channels after excitation into the first UV absorption band of thiophene, and are good candidates to explain why this compound is non-fluorescent.