Acidity constants in the triplet state

Abstract

Acidity constants of the lowest triplet state have been determined for seven aromatic mole­cules by two independent methods. In the first method the triplet state is populated by flash photolysis and the acid-base equilibrium is studied spectrophotometrically. In the second method the difference between acidity constants in the triplet and ground states is calculated from energy levels of the acid and base derived from phosphorescence spectra. For five of the molecules investigated—2-naphthol, 1- and 2-naphthoic acids, 2-naphthyl-amine and acridine—acidity constants are now known for three electronic states. It is found that acidity constants of the triplet and ground states are comparable while that of the first excited singlet state differs by a factor of about 10 6 in each case. The electron density distribution in the three states is briefly discussed.