Dual fluorescence of naphthylamines in alkaline aqueous solution

Dual fluorescence was observed with N-(1-naphthyl)aminoacetate (1-NAA) in aqueous solution of pH 13.0 in the presence of cationic surfactants, cetyltrimethylammonium bromide (CTAB) and chloride (CTAC), below and after the critical micelle concentration (CMC). Similar dual fluorescence was also found with 1- and 2-naphthylamine (1-NA, 2-NA), N-(2-naphthyl)aminoacetate (2-NAA) and (1-naphthyl)ethylenediamine (1-NEDA), in the presence and absence of the cationic surfactants, but not with N,N-disubstituted 1- and 2-NAs. We concluded that the dual fluorescence was due to the excited-state deprotonation of the amino group in these NAs. The pKa*s of the dual fluorescent NAs were estimated to be around 14 from the dual fluorescence pH titrations. No clear correlation was found for pKa* with the amino substitution and the presence of cationic micelle.