Ultrafast dual-bond photodissociation of trifluorothioanisole: Mechanistic insights into the competing deactivation channels and environmental implications

Abstract

Fluorinated compounds play indispensable roles across pharmaceutical, agrochemical, and materials science due to fluorine’s unique electronegativity, small atomic radius, and high metabolic stability. Among them, trifluorothioanisole (Ph–S–CF3) serves as a representative functionally model system for environmental photochemistry, given the widespread use of the –SCF3 group in agrochemicals and its concerning persistence. However, the ultrafast photodynamics and non-radiative decay mechanisms of Ph–S–CF3 remain poorly understood, limiting predictive insight into its environmental fate. Here, we combine high-level static electronic structure calculations with excited-state non-adiabatic dynamics simulations to unravel the mechanism of Ph–S–CF3. Our results reveal that excitation to the S1 state initiates ultrafast internal conversion via two competitive bond-cleavage pathways mediated by distinct conical intersections: dissociation at either the S1–C3 or S1–CF3 bond, yielding ·CF3 (46%) and ·SCF3 (54%) radicals, with an overall S1 lifetime of ∼612 fs. These findings not only elucidate the photodegradation mechanism of a prominent fluorinated environmental contaminant but also provide a general theoretical framework for predicting the photostability and formation dynamics of persistent radical species (·CF3/·SCF3) derived from –SCF3-functionalized compounds, underscoring the regulatory role of fluorine substitution in excited-state dynamics. Thereby, this study provides a crucial basis for assessing the environmental persistence and ecotoxicity of fluorinated organics and offers strategic guidance for the rational design of low-persistence fluorinated functional molecules.