Ab initio molecular dynamics of excited-state intramolecular proton transfer around a three-state conical intersection in malonaldehyde

Excited-state potential energy surface (PES) characterization is carried out. at the CASSCF and MRSDCl levels, followed by ab initio dynamics simulation of excited-state intramolecular proton transfer (ESIPT) on the S-2(pi pi*) state in malonaldehyde. The proton-transfer transition state lies close to an S-2/S-1 conical intersection, leading to substantial coupling of proton transfer with electronic relaxation. Proton exchange proceeds freely on S,, but its duration is limited by competition with twisting out of the molecular plane. This rotamerization pathway leads to an intersection of the three lowest singlet states, providing the first detailed report of ab initio dynamics around a three-state intersection (3SI). There is a significant energy barrier to ESIPT on S-1, and further pyramidalization of the twisted structure leads to the minimal energy S-1/S-0 intersection and energetic terminal point of excited-state dynamics. Kinetics and additional mechanistic details of these pathways are discussed. Significant depletion of the spectroscopic state and recovery of the ground state is seen within the first 250 fs after photoexcitation.