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.