Excited state intramolecular proton transfer in some tautomeric azo dyes and Schiff bases containing intramolecular hydrogen bond

Photophysical properties of several basically important aromatic azodyes (1-phenylazo-2-naphthol and 2-phenylazo-1-naphthol) and Schiff bases (N-(2-hydroxy-1-naphthylmethylidene) aniline and N-(1-hydroxy-2-naphthylmethylidene) aniline) all containing an intramolecular hydrogen bond were studied by both steady-state and time-resolved fluorescence spectroscopy with temperatures down to 98 K. It was found that the fluorescence results from the quinone form (H-form) only. The enol form (A-form) undergoes rapid excited state intramolecular proton transfer (ESIPT) resulting in the excited H-form. The compounds have relatively low quantum yields at room temperature, which increase considerably at low temperatures. Lifetime data at the different temperatures indicate that a substitution by both acceptor or donor groups on the para position in the phenyl ring decreases the deactivation rate and hence results in increased lifetime.