Frontier Orbital Changes Induce Slow-down of Excited State Population Dynamics in Thiopyridone Isomers

The non-adiabatic dynamics of photoexcited thiopyridone systems were studied across their ortho-, meta-, and para-isomeric forms. Surface hopping dynamics, utilizing TDDFT based electronic structure calculations, elucidate the relaxation pathways of the three constitutional isomers in both gas-phase and solvation environments. Particular attention is given to intersystem crossing (ISC) and non-adiabatic dynamics in triplet states, as the lowest triplet state plays a critical role in driving the excited-state proton transfer process in the ortho isomer. The efficiency of singlet-triplet crossings was analyzed by evaluating electronic features and energy gaps derived from dynamic simulations. The investigations suggest a systematic reduction in the rate of ISC from ortho to meta to para isomer. Furthermore, a comparison with a multireference approach in the gas phase highlights how variations in electronic structure influence the observed dynamics.