Excited-State Proton Transfer Dynamics of Cyanonaphthol in Protic Ionic Liquids: Concerted Effects of Basicity of Anions and Alkyl Carbons in Cations

Excited-state proton transfer (ESPT) reactions of 5-cyano-2-naphthol (5CN2) and 5,8-dicyano-2-naphthol (DCN2) were investigated in protic ionic liquids (PILs) composed of quaternary ammonium (N nnn H + ) ( n = 2, 4, or 8) and hexanoate (C 5 H 11 COO – ) using time-resolved fluorescence spectroscopy. The effects of the number of alkyl carbons in the cation and the basicity of the anion on the reaction yield and dynamics were examined. In a series of [N nnn H][C 5 H 11 COO], fluorescence from the hydrogen-bonding complex (AHBX –* ) of a proton-dissociated form (RO –* ) with a solvent acid in the electronic excited state was observed between the fluorescence bands of an acidic form (ROH * ) and an anionic form (RO –* ) as in the case of [N 222 H][CF 3 COO] (Fujii et al., J. Phys. Chem B, 2017, 121, 6042). The yield, formation rate, and decay rate of AHBX –* were assessed by the steady-state fluorescence intensity ratio of AHBX –* to RO –* and by analysis of the time-resolved fluorescence spectra within several tens of nanoseconds. The stability of AHBX –* was significantly different from that in [N 222 H][CF 3 COO] and depended on the number of alkyl carbons in the cation. The formation of AHBX –* was quite fast compared to the case in [N 222 H][CF 3 COO] and almost close to the excitation pulse width. Excitation wavelength dependence of the fluorescence dynamics was observed within several hundred picoseconds for the series of [N nnn H][C 5 H 11 COO]. The origin was ascribed to the complex with solvent IL formed in the electronic ground state, as suggested by the density-functional theory (DFT) calculations.