Proton-Transfer Tautomerism of β-Carbolines Mediated by Hydrogen-Bonded Complexes

The carboxylic acid catalyzed excited-state amino-imino tautomerism for β-carboline (β-CB) and its analogues has been investigated. Thermodynamics and microsolvation (i.e., stoichiometry of the complex formation) of various β-CB/acetic acid complexes in nonpolar solvents have been studied by means of absorption and emission titration experiments. Supplementary support of the stoichiometric ratio and structure for the hydrogen-bonding formation was provided by molecular design and syntheses of various β-CB analogues incorporating either only one hydrogen bonding site or dual hydrogen bonding sites where interplay between two sites are sterically prohibited. The results in combination with time-resolved measurements and theoretical approaches suggest the 1:2 β-CB/acetic acid complex with a structure of triple hydrogen bonding formation to be responsible for the excited-state proton-transfer tautomerism in cyclohexane. The proton transfer time is beyond the response limit of the detecting system of 15 ps, indicating that only a negligibly small geometry adjustment is required for the guest molecule (i.e., acetic acid) to a correct geometry, i.e., a hydrogen-bond relay configuration, for the triple proton transfer to proceed. In comparison, for the 1:1 β-CB/acetic acid non-hydrogen-bond relayed complexes, amino−imino tautomerism is prohibited during the excited-state lifetime, giving rise to a normal Stokes shifted emission. The results provide detailed ground-state thermodynamics of β-CB HB complexes as well as the dynamics of proton-transfer tautomerism mediated by the hydrogen-bonding structures.