A TDDFT study on the hydrogen-bonding effect on ESIPT mechanism for [2,2′-bipyridyl]-3,3′-diol-(H2O)n (n =0, 1, 2) clusters: single or double?

The excited-state intramolecular proton transfer (ESIPT) mechanisms of [2,2 '-bipyridyl]-3,3 '-diol (BP(OH)(2)) in gas are studied by using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. The intramolecular hydrogen bond (H-bond) is strengthened in the first excited-state in view of the structural parameters and infrared (IR) vibrational frequencies. The enhanced intramolecular H-bond is favourable for ESIPT process. The effect of the extra intermolecular H-bond between BP(OH)(2) and water on ESIPT is considered. The potential energy surfaces, molecular electrostatic potential, topological analysis, frontier molecular orbitals, absorption and fluorescence spectra are investigated. Our calculated results show that the intermolecular H-bond enhances the intramolecular H-bond, changes the mechanism and decreases the barrier height of ESIPT process.