Steady state and time-resolved spectroscopic studies of α- and β-naphthanilides

Abstract

Using chemical substitution and polarity perturbation effects, three competitive fluorescence emissions from a common initial excited singlet state of α and β-naphthanilide are observed in non-polar solvents at 298 K. The fluorescence emissions are assigned as the normal fluorescence from a locally excited state (S 1(LE) → S 0), the proton-transfer tautomer emission (S′ 1(PT) → S′ 0(PT)) and the intramolecular charge-transfer fluorescence (S″ 1(TICT) → S 0(FC)). The picosecond time-resolved absorption spectra confirm the presence of excited species in different electronic states, e.g. S 1(LE) and S′ 1(PT) with comparable decay constants and S″ 1(TICT) and T 1(LE) with larger decay constants. At 77 K, the luminescence spectra appear as normal fluorescence (for N-methyl-α-naphthanilide and N-methyl-β-naphthanilide) or a fluorescence emission from microcrystals (for α- and β-naphthanilide) and very strong phosphorescence emission.