Is there an excited state proton transfer in phenol (or 1-naphthol)-ammonia clusters?: Hydrogen detachment and transfer to solvent:: a key for non-radiative processes in clusters

This contribution highlights recent advances in the understanding of excited state dynamics in aromatic enols. This review will be mainly focused on the experimental and theoretical work performed on two model systems, l-naphtholammonia and phenol-ammonia, and most particularly on cluster studies. These systems have long been thought to be prototypes for the famous 'solvent-induced excited state proton transfer reaction', but recent results contradict this mechanism. The dynamics of these systems, excited in the S-1 (pipi*) state, is not governed by couplings with ion pair states, inducing intracluster proton transfer, but rather linked to a crossing with a higher excited singlet state of the pisigma* Rydberg state character, dissociative along the OH coordinate, leading to a hydrogen transfer from the hydroxyl group to the ammonia cluster via a non-adiabatic process, a mechanism also referred to as 'concerted electron and proton transfer'. The crossing of the higher (1)pisigma* state with lower excited (1)pipi* and ground states seems to be a general property in aromatic enols and azines (indole derivatives) and can help to understand the non-radiative decays in amino acid molecules as well as in DNA bases.