Excited-State and Ground-State Proton-Transfer Reactions in 5-Aminoindole

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Abstract
Dual fluorescence emissions are observed when the electron-donating group (–NH2) is present at the position 5 of indole moiety. From the study of solvent effect and the large blue shift observed in the fluorescence spectra at 77 K, it is concluded that the long wavelength fluorescence band (480 nm) is assigned to the more polar state and the short wavelength band (350 nm) to the less polar state of 5-aminoindole (AI). Monocation is formed by protonating the amino group and no proton-induced fluorescence quenching of the neutral species is observed prior to its being protonated. This is mainly due to the shorter life of the conjugated acid–base pair. CNDO/S calculations indicate that dication is formed by protonating the C-3 position of the monocation of AI in the S0 state but position 4 of monocation of AI in the S1 state. Monoanion is formed by the deprotonation of >NH group in the S0 state, but from the deprotonation of –NH2 group in the S1 state.