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 S2(1ππ*) state triggers electron density redistribution from S to O, strengthening the O···H-S bond and driving subsequent proton transfer. After S2→S1 internal conversion, the electronic character dictates the reaction pathways: population of S1(1nOπ*) 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 S1(1ππ*) maintains the 1ππ* character of the S2 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.