Absorptions‐ und Fluoreszenzspektren sowie Dissoziationskonstanten der Grund‐ und ersten angeregten Singulettzustände von 4′‐Hydroxyflavon und 4′‐Methoxyflavon [1]

The absorption and fluorescence spectra of 4′‐hydroxyflavone and 4′‐methoxyflavone have been studied as a function of the acidity (pH or Ho) of the solution. Dissociation constants have been determined for the ground and first excited singlet state. The values are compared with those obtained from Förster‐Weller calculations.—The pKa's obtained by fluorimetry are rather similar to the ground state pKa's, which indicates a rapid deactivation of the S1‐state, before prototropic equilibration is complete.—On changing from protic to deuterated solvents partially strong effects are observed with respect to the fluorescence intensity, but also to the shape of the curves. It is evident, that proton transfer processes in the excited state significantly contribute to fluorescence quenching, thus making the fluorescence quantum yields strongly acidity dependent.