EXCITED-STATE PROTON-TRANSFER IN BETA-CARBOLINE

Abstract

Steady state and time-resolved fluorescence techniques were used to investigate the excited state acid-base equilibria behaviour of β-carboline and 2-methyl-β-carboline in aqueous solutions. In acid conditions the β-carboline cation is responsible for light absorption and fluorescence (λmax = 450 nm; τF = 22.0 ns). However, in alkaline solutions excitation of the neutral species leads to the formation of the β-carboline cation by a rapid proton transfer with the solvent. A diffusion-controlled quenching of the excited state cation by OH- ions produces a zwitterion species which fluoresces with λmax at 510 nm (τF = 1.6 ns). The zwitterion is also formed after excitation of the ground state an..