Abstract
Abstract— The acid‐base equilibria of the excited singlet and triplet states of acridine orange (AO) were studied by flash‐photolysis and fluorometric methods. The dye is a stronger base in the first excited singlet state (pKs= 13.3) than in the triplet and ground states (pKr= 10.3: pKc, = 10.2); acridine orange follows the trend observed with some other heterocyclic compounds, viz. pKs >pKr= pK,c. At room temperature, an anomalous fluorescence occurs from the dye in basic media: the assignment of this emission is discussed.The semi‐reduced dye was studied as a function of pH. In a large pH range (3–14), only the protolytic equilibrium between the cationic (AOH2+) and the neutral (AOH) radicals was observed; the pK value corresponding to this equilibrium was found to be in the range of pH 5–6.
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