Electronically Excited States of Neutral, Protonated α-Naphthol and Their Water Clusters: A Theoretical Study

The RI-MP2 and RI-CC2 methods have been employed to determine the potential energy profiles of neutral and protonated α-naphthol, in their individual forms and microhydrated with 1 and 3 water molecules, at different electronic states. According to calculated results, it has been predicted that dynamics of nonradiative processes in protonated α-naphthol is essentially different from that of its neutral homologue. In protonated α-naphthol, the calculations reveal that (1)σπ* state, is the most important photophysical state, having a bound nature with a broad potential curve along the OH coordinate of isolated system, while it is dissociative in monohydrated homologue. In neutral system, similar to phenol, the (1)πσ* state, plays the fundamental relaxation role along the O-H stretching coordinate. Moreover, microhydration strongly affects the photophysical properties of α-naphthol, mostly by alteration of the (1)ππ* PE profile, from a bound state in an isolated analogue to a dissociative state in hydrated systems. Furthermore, it has been found that three water molecules are necessary for ground state proton transfer between protonated α-naphthol and water; with a small barrier; (ΔE< 0.1 eV).