A theoretical study of excited state proton transfer in 3-hydroxychromone and related molecules

Semiempirical, ab initio and DFT methods were applied to the study of proton transfer in the ground and some of the excited states of 3-hydroxyflavone, 3-hydroxychromone and the gamma-pyrone ring contained in both molecules. Geometry optimizations and reaction-path potential energy curves for the normal and proton-transferred isomers were performed employing AM 1 and B3LYP methodologies tin the last case both with the 3-21G and 6-31G(d,p) basis sets). Electronic transitions were calculated employing the ZINDO-S/CI method, while the reaction path for the proton transfer in the lowest singlet and triplet excited states were calculated employing the CIS/6-31G(d,p) method on the gamma-pyrone ring. The singlet-singlet excitation in 3HF is calculated at 302 nm (experimental 325 nm) while the fluorescence band is calculated at 526 nm (experimental 525 nm). The data calculated for 3HC and the gamma-pyrone explain the intervention of the triplet states in the dynamics of ESIPT in these molecules. (C) 1999 Elsevier Science B.V. All rights reserved.