Rapid Intersystem Crossing Induced by Ultrafast Excited-State Intramolecular Proton Transfer in 3-Mercaptopyran-4-one

Investigation into the photoinduced processes of 3-mercaptopyran-4-one is carried out using trajectory-based surface hopping simulations. Excitation into the near-degenerate higher singlet excited states reveals rapid internal conversion (IC) into S 1 on a sub-50 fs timescale. Excited-state intramolecular proton transfer (ESIPT) also takes place simultaneously with IC. We observe that following tautomerization, the molecule has multiple relaxation pathways. A channel exists for it to nonradiatively decay into the tautomer ground-state or undergo rapid intersystem crossing (ISC) into the close-lying higher triplet state, which ultimately decays into T 1 . The simulations show that ISC is significantly enhanced after ESIPT, which is studied by tracking the changes in energy gaps and associated spin–orbit coupling elements.