Ground and singlet excited state hydrogen bonding interactions of betacarbolines

To study the ground and singlet excited state hydrogen bonding donor/accept or properties of the betacarboline ring, 9H-pyrido[3,4-b]indole, we have ca rried out a spectroscopic study of the interactions of harmane, 1-methylbet acarboline, HN, and its N-9-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 com plexes with the hydrogen bond donors tert-butanol, 2-chloroethanol and hexa fluoropropan-2-ol. At high concentrations, the strongest hydrogen bond dono rs chloroethanol and hexafluoropropan-2-ol form additional proton transfer ground-state 1:2 hydrogen-bonded complexes which, upon photoexcitation, giv e phototautomers of zwitterionic structures. The aromatic donor phenol also forms hydrogen bonded pyridinic complexes with HN, but zwitterionic specie s are not observed. Furthermore, the hydrogen bonding HN-phenol interaction quenches the HN fluorescence. On the other hand, the interactions of HN wi th the proton acceptors tetrahydrofuran, N,N-dimethylformamide and hexameth ylphosphoramide also give fluorescent 1:1 hydrogen bonded pyrrolic complexe s which do not form phototautomeric zwitterions. These results conclusively show that the formation of zwitterionic phototautomers involves the initia l attack of a hydrogen bonding donor molecule on the pyridinic nitrogen ato m of the betacarboline and the formation of a 1:2 proton transfer complex.