How Charge Redistribution Governs Photoreaction Pathways: Evidence from XMS-CASPT2 Studies of a S–H···O Intramolecular Hydrogen Bond

Understanding how photoinduced charge redistribution directs competing reaction pathways is crucial to the rational design of excited-state intramolecular proton transfer (ESIPT) materials. Here, we employ extended multistate complete active space second-order perturbation theory (XMS-CASPT2) calculations and electrostatic potential (ESP) analysis to uncover the reaction mechanism of 3-mercaptopyran-4-one (3MP), a model of a thiol-based S–H···O intramolecular hydrogen-bond system. Photoexcitation of the bright S 2 ( 1 ππ*) state triggers electron density redistribution from S to O, strengthening the O···H–S bond and driving subsequent proton transfer. After S 2 →S 1 internal conversion, the electronic character dictates the reaction pathways: population of S 1 ( 1 n O π*) reduces the O electron density, weakens the hydrogen bond, induces H out-of-plane motion, and enables efficient intersystem crossing to the triplet state due to large spin–orbit coupling. On the other hand, internal conversion to S 1 ( 1 ππ*) maintains the 1 ππ* character of the S 2 state and thus initiates the proton transfer to yield the enol tautomer, which exhibits weak fluorescence and promotes the reactions hereafter. Our findings establish a direct connection between electron density redistribution and reaction pathways, offering new mechanistic insights for designing advanced photofunctional materials.