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

Abstract

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, 1-naphthol± ammonia 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 S1 (ºº*) 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 º¼* 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 º¼* state with lower excited 1 ºº* 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.

Revealing the role of excited state proton transfer (ESPT) in excited state hydrogen transfer (ESHT): systematic study in phenol–(NH3)n clusters

“…When the zwitterionic cluster PhO À -NH 4 + (NH 3 ) nÀ1 is excited, there is no driving force to dissociate the cluster, since the most stable NH 4 + structure is already formed and this system relaxes in the excited state by evaporation of NH 3 fragments. 12,15 This is consistent with the absence of the fragments NH 4 + (NH 3 ) nÀ1 .…”

“…There was, however, a notable difference between PhOH-(NH 3 ) n and NpOH-(NH 3 ) n clusters: free NH 4 + (NH 3 ) nÀ1 clusters formed aer photoexcitation were detected only for PhOH-(NH 3 ) n clusters and not for NpOH-(NH 3 ) n clusters. 12 Moreover, a ps rise time of the NH 4 + (NH 3 ) nÀ1 signal was similar to the decay signal of the parent PhOH-(NH 3 ) n . The formation of NH 4 + (NH 3 ) nÀ1 clusters aer the ionization of the PhOH-(NH 3 ) n cluster was observed, and the energy dependence of the ps dynamics was probed.…”

Molecular recognition in the gas phase: benzocaine–phenol as a model of anaesthetic–receptor interaction

“…The existence of a hydrogen donor reaction in the ground state of phenol-ammonia and of a competition between hydrogen and proton donation in the excited electronic state is well known. 38,39 Substitution of one hydrogen atom of ammonia by ethyl-p-benzoate decreases the basicity of the -NH 2 group because of the charge transfer from the amino moiety to the ester substituent, hampering the ground state hydrogen abstraction reaction. However, the reaction may still proceed in the excited state, but not on the v 0 = 0 vibronic level.…”

S1-State Decay Dynamics of Benzenediols (Catechol, Resorcinol, and Hydroquinone) and Their 1:1 Water Clusters

“…Actually, the excitation of the intermolecular stretching mode often modifies the tunneling barrier of ESHT in the way of expediting the H-atom transfer process. 14,19,21,22 Accordingly, the experimental fact that the S 1 lifetime of the catechol• water cluster decreases rather sharply with the increase of the S 1 internal energy, as well as its strong mode dependence, indicates that the ESHT could be significantly responsible for the S 1 -state relaxation, similarly found in the case of the oaminophenol•water complex. 53 There are two different conformational isomers of resorcinol: the C S asymmetric isomer (cis) or the C 2V symmetric isomer (trans).…”