Prototropic equilibria of electronically excited molecules. Part III. Mono- and bi-cyclic aromatic hydrocarbons

Abstract

The expectation, based upon π-electron theory or spectroscopic data by use of the Förster cycle, that the acid–base properties of aromatic hydrocarbons in the lowest excited electronic state differ substantially from the corresponding ground state properties has been investigated by measuring the variation in the fluorescence spectrum of mono- and bi-cyclic molecules with changes in the acidity of the solvent and by hydrogen isotope exchange studies. It is found that prototropic equilibrium is not established during the lifetime of the excited state of benzene, toluene, naphthalene, azulene, and indolizine and that theoretical pK_a* values for the excited state of these molecules in aqueous solution do not correspond to observable processes. Changes in fluorescence spectrum with acidity follow only ground-state ionic dissociation processes, and the upper limits for the quantum efficiencies of photochemical hydrogen isotope exchange in aqueous 1 M-perchloric acid indicate that the radiative deactivation rate of an electronically excited aromatic hydrocarbon is faster than the rate of protonation by factor > 10⁵.