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Excited-state intramolecular proton transfer of naphthalene-fused 2-(2'-hydroxyaryl)benzazole family.

Intramolecularly hydrogen-bonded organic compounds often exhibit fluorescence emission at considerably longer wavelengths than typical fluorescence as a result of excited-state intramolecular proton transfer (ESIPT). The structure-property relationship of such ESIPT molecules, however, remains obscure. The present article reports the excited-state dynamics of a new family of ESIPT molecules, 2-(2'-hydroxynaphthyl)benzazoles 1-3, based on steady-state and time-resolved spectroscopy measurements. In comparison with the parent compound HBO, all three compounds 1-3 exhibited absorption bands at longer wavelengths and emitted fluorescence from the excited keto-tautomer K* at shorter wavelengths, indicating that the introduction of a naphthalene ring increases the energy gap between the ground and excited states for the keto-tautomer despite the expansion of the aromatic ring. Time-resolved fluorescence spectra revealed dual emission for compounds 1 and 3, consisting of two distinct fluorescence bands originating from K* and the excited rotamer E'*, whereas 2 exhibited fluorescence only from the K* state. In the transient absorption spectra, both the T-T absorption band and the ground state absorption band of the Z-keto tautomer were observed for 1, whereas only the T-T absorption band was observed for 2 and only the Z-keto tautomer band was observed for 3.