The position of the N atom in the pentacyclic ring of heterocyclic molecules affects the excited-state decay: A case study of isothiazole and thiazole

Abstract

The radiationless decay of the isothiazole and thiazole molecules in the gas phase is investigated by a combination of static MS-CASPT2 computations with dynamics simulations, and some deactivation pathways are identified leading from the lowest excited singlet (S1) state back to the electronic ground (S0) state. The dominant relaxation pathway of the excited isothiazole is found to involve out-of-plane ring deformation (ring puckering) at the C4=C5(H8)S1 moiety, while the excited-state decay of thiazole can proceed through the ring puckering and S1C2 bond cleavage pathways both of which happen at the S1C2(H6) = N3 moiety. On the basis of the calculation results, we suggest that both ring-puckering and ring-opening routes should play rather important roles in the photophysics and photochemistry of heterocyclic molecules, and the position of the N atom in the heterocyclic ring has an effect on the excited state deactivation.