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_<i>nnn</i>H⁺) (<i>n</i> = 2, 4, or 8) and hexanoate (C₅H₁₁COO^-) 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_<i>nnn</i>H][C₅H₁₁COO], fluorescence from the hydrogen-bonding complex (AHBX^-*) of a proton-dissociated form (RO^<i>-*</i>) 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 [N₂₂₂H][CF₃COO] (Fujii et al., <i>J. Phys. Chem B</i>, <b>2017</b>, 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 [N₂₂₂H][CF₃COO] and depended on the number of alkyl carbons in the cation. The formation of AHBX^-* was quite fast compared to the case in [N₂₂₂H][CF₃COO] 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_<i>nnn</i>H][C₅H₁₁COO]. 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.