Primary Photochemical Processes of Cationic Acridine Orange in Aqueous Solution Studied by Flash Photolysis

Abstract—Photochemical reactions of cationic acridine orange (AO+) in aqueous solution were studied using the flash photolysis technique. Complete conversion to the triplet state was achieved. The triplet dye decays by first‐ and second‐order reactions, which partly regenerate the dye in the ground state and partly produce semi‐oxidized and semi‐reduced acridine orange. These radical species are formed by an electron‐transfer mechanism between two triplet molecules. The radicals recombine rapidly to give the dye in the ground state. An efficient photooxidation reaction is observed in acridine orange solution containing ferric ions. Semi‐oxidized dye is formed in high yield by reaction between the triplet and the oxidant; the semi‐oxidized radicals disappear, yielding permanent photoproducts. Tetra‐methylparaphenylene diamine (TMPD) reacts efficiently with the acridine orange triplet to give the semi‐reduced dye and the semi‐oxidized form of TMPD; this reduction was found to be reversible. Absorption spectra of the transient species were determined and rate constants for the observed reactions measured. The results are compared with results from a previous study of neutral acridine orange.