Protonation of aromatic carboxylic acids in the first excited singlet state

Abstract

Previous methods for determining the excited state acid-base characteristics of molecules from kinetic data are applicable only to molecules which undergo dissociation upon excitation. This paper presents a method of obtaining kinetic data from fluorescence measurements for compounds, which, upon excitation, take on a proton. The kinetic data so obtained can be used to calculate the excited state acidity constants of the protonated molecules. Comparison, for two naphthoic acids, of the results so obtained with data from the spectral method due to Förster and with data from the pH at which the fluorescence spectrum undergoes a transformation reveals considerable discrepancies: these are discussed briefly.