Ground and singlet excited state hydrogen bonding interactions of betacarbolines

Abstract

To study the ground and singlet excited state hydrogen bonding donor/acceptor properties of the betacarboline ring, 9H-pyrido[3,4-b]indole, we have carried out a spectroscopic study of the interactions of harmane, 1-methylbetacarboline, HN, and its N₉-methyl derivative, MHN, with different hydrogen bonding acceptor/donor molecules in the non-polar solvent cyclohexane. UV–visible, steady-state and time-resolved fluorescence measurements show that HN and MHN form fluorescent 1:1 ground state hydrogen bonded pyridinic complexes with the hydrogen bond donors tert-butanol, 2-chloroethanol and hexafluoropropan-2-ol. At high concentrations, the strongest hydrogen bond donors chloroethanol and hexafluoropropan-2-ol form additional proton transfer ground-state 1:2 hydrogen-bonded complexes which, upon photoexcitation, give phototautomers of zwitterionic structures. The aromatic donor phenol also forms hydrogen bonded pyridinic complexes with HN, but zwitterionic species are not observed. Furthermore, the hydrogen bonding HN–phenol interaction quenches the HN fluorescence. On the other hand, the interactions of HN with the proton acceptors tetrahydrofuran, N,N-dimethylformamide and hexamethylphosphoramide also give fluorescent 1:1 hydrogen bonded pyrrolic complexes which do not form phototautomeric zwitterions. These results conclusively show that the formation of zwitterionic phototautomers involves the initial attack of a hydrogen bonding donor molecule on the pyridinic nitrogen atom of the betacarboline and the formation of a 1:2 proton transfer complex.