Excited-state proton transfer to solvent of protonated aniline derivatives in aqueous solution: a remarkable effect of ortho alkyl group on the proton-dissociation rate

Proton dissociation from the lowest excited singlet state of protonated aniline (AN) derivatives in aqueous solution has been studied by picosecond time-resolved fluorescence measurements. The proton-dissociation rate is significantly influenced by introduction of ortho alkyl group(s) to AN as 1.4×1010 s−1 for AN, 4.1×109 s−1 for 2-toluidine and 1.7×109 s−1 for 2,6-xylidine. The remarkable decrease of the proton-dissociation rate by alkylation is attributed partly to change in exothermicity of the reactions. The activation barrier for the proton-dissociation reaction is increased in the alkylated ANs, suggesting that the hydrophobic effects of the alkyl group on the water structure in the vicinity of the amino group influences the rate of proton transfer to solvent.