Fluorescence quenching of betacarboline (9H-pyrido[3,4-b]indole) induced by intermolecular hydrogen bonding with pyridines

The influence of the hydrogen bond acceptor properties of pyridine derivatives on the fluorescence quenching mechanism of the betacarboline (9H-pyrido[3,4-b]indole) (BC) has been investigated. Absorption measurements suggest the sequential formation of two BC-pyridine hydrogen-bonded complexes: a covalent hydrogen-bonded complex (HBC) and its proton transfer tautomer (PTC). Steady-state and time-resolved fluorescence measurements reveal that the BC fluorescence quenching has static and dynamic components due to the ground and the excited state formation of the very weakly fluorescent PTC. Since the excited state HBC formation is a diffusion-controlled process, the efficiency of dynamic quenching is mainly determined by the relative magnitudes of the HBC backward reaction and the PTC formation rate constants.