Nonadiabatic dynamics of photoexcited Thiopyridone isomers: An interplay between El-Sayed’s conditions and energy gap law

The nonadiabatic dynamics of photoexcited thiopyridone systems was studied across their ortho-, meta-, and para-isomeric forms. Surface hopping dynamics, utilizing time-dependent density functional theory, elucidates the relaxation pathways of the three constitutional isomers in both gas-phase and solvent environments. Particular attention is given to intersystem crossing (ISC) and internal conversion in the triplet manifold, as the lowest triplet state is known to play a critical role in driving the excited-state proton transfer process in the ortho-isomer. The efficiency of ISC was analyzed by evaluating electronic features and energy gaps derived from molecular dynamics simulations. The investigations suggest a systematic reduction in the rate of ISC from ortho- to meta- to para-isomer. Furthermore, a comparison in the gas phase with simulations based on multiconfigurational wavefunction methods highlights how variations in electronic structure influence the dynamical pathways.