On the mechanism of rapid non-radiative decay in intramolecularly hydrogen-bonded π systems

Reaction paths and potential-energy profiles of excited singlet states of malonaldehyde have been investigated using the ab initio CASSCF method. The results confirm the barrierless nature of the hydrogen-transfer process in the ππ^* excited singlet state. A hitherto unknown ¹πσ^* state has been identified which is strongly repulsive with respect to in-plane detachment of the mobile hydrogen atom, leading to a low-lying conical intersection with the electronic ground state. Vibronic coupling of the ¹ππ^* with the ¹πσ^* state by out-of-plane modes connects the former, via a low barrier, to the conical intersection, providing a mechanism for efficient internal conversion to the ground state. It is argued that these findings reflect general properties of the H-chelate ring in intramolecularly hydrogen-bonded molecules and can explain the photostability of these systems.