New insight into the excited-state proton-transfer reactions of 1-naphthylamine in solution

Proton-transfer reactions from the excited state of 1-naphthylamine (RNFb) have been investigated in aqueous solutions of various HClO₄ concentrations. The system is found to contain three species rather than simply the acidic and the neutral form of RNH2 as previously believed. The new species X, can be clearly detected at [HClO₄] > 5 M, in both absorption and emission. X is proposed to be an adduct of RNH2 and an unhydrated HC1O₄, i.e., X = RNH₂- - -H⁺- - -ClO₄^-. Absorption and fluorescence spectra of X are shown. Quantum yields of the three excited species RNH2, RNH³⁺*, and X* have been found to be 0.49, 0.327, and 0.065, respectively. A kinetic scheme is presented indicating that the proton transfer from RNH₃⁺* depends on the availability of free water, i.e., 'Water that can accept protons. All rate constants have been determined. A theoretical model for the hydration of protons shows that as long as there is excess free water an average of 9 water molecules are used for the hydration of one proton.