Excited-state proton-dissociation of N-alkylated anilinium ions in aqueous solution studied by picosecond fluorescence measurements

Abstract

The proton-dissociation reactions from the excited state of protonated aniline (AN) and its derivatives in aqueous solution were investigated by using a femtosecond laser combined with a time-correlated single-photon counting system. The proton-dissociation rate of protonated AN at 298 K was obtained to be 1.4 × 10¹⁰ s⁻¹, which is the fastest of those reported so far for protonated aromatic amines. The rate decreased markedly on alkylation of the amino group, being 6.2 × 10⁹ s⁻¹, 3.7 × 10⁹ s⁻¹, 7.6 × 10⁸ s⁻¹, and 4.4 × 10⁷ s⁻¹, for N-methylaniline (NMeA), N-ethylaniline (NEtA), N,N-dimethylaniline (DMA), and N,N-diethylaniline (DEA), respectively, despite the larger exothermic enthalpy changes for the proton dissociation of alkylated compounds. The activation energies for the proton-dissociation reactions of dialkylated anilines, DMA (19 kJ mol⁻¹) and DEA (56 kJ mol⁻¹), were much larger than those of the other anilines, AN (11 kJ mol⁻¹), NMeA (8 kJ mol⁻¹), and NEtA (11 kJ mol⁻¹), suggesting that hydrophobic hydration around the alkyl amino group and steric hindrance due to the alkyl group cause an activation barrier for the movement of proton to water.