Excited-State Intramolecular Proton Transfer and Global Aromaticity.

A general survey of excited-state intramolecular proton transfer (ESIPT) processes was made from the viewpoint of global aromaticity. For most ESIPT processes studied, a tautomeric product in the first excited electronic state was found to have a larger topological resonance energy (TRE) than the reactant in the same excited state. Conversely, if a transient tautomer is much less aromatic in the excited state than the reactant, an appreciable aromaticity-imposed energy barrier to the reaction will result. Thus, excited-state aromaticity is a very important factor, although not a definitive one, in determining the allowedness of ESIPT.