Acid–base properties in the triplet state of aromatic ketones in H2O–CH3CN (4:1) mixtures have been studied by means of nanosecond laser flash photolysis. The acidity constants pKa(T) in the triplet state were determined by the Tn ← T1 absorbance titration curve, the Ware plot, and the Rayner–Wyatt plot. Good agreement among them shows that the acid–base equilibrium in the T1 state of aromatic ketones is established during the triplet lifetime. A linear relation between pKa(T) and the Taft σ* values was obtained for phenyl alkyl ketones.

The excited-state proton-transfer reactions of 4-(9-anthryl)-7V,/V-dimethylaniline (A) in Et0H-H20 (4:1 by weight) mixtures at 300 K have been studied by fluorimetry and the single-photon-counting method. It is found that there is no excited-state protqtropic equilibrium of A. The excited acid and base forms of A decay independently with single exponential functions. The pAfa* value (2.4 ± 0.2) of A obtained from the usual fluorescence titration curves is, therefore, an apparent value, reflecting the pK3 value (2.5 ± 0.2) in the ground state. This is mainly caused by the extremely slow proton-dissociation rate of 'A+H* compared to its decay rate (2 X 10® s"1). No proton-induced quenching of A is observed, though the intramolecular CT state 'Act* is produced very rapidly via *A* under the experimental conditions.Proton-transfer reactipns in the excited state of aromatic compounds are elementary processes in chemistry. The acid-base properties in the excited state of aromatic compounds are closely related to the corresponding electronic structure, which is sig-