Hydrogen-bonding interactions between harmane and pyridine in the ground and lowest excited singlet states

A spectroscopic (UV-visible, Fourier transform IR, steady-state and time-resolved fluorescence) study of hydrogen-bonding interactions between harmane (1-methyl-9H-pyrido/3,4-b/indole) and pyridine in the ground and lowest excited singlet state is reported, In low polar and weakly or nonhydrogen-bonding solvents, such as cyclohexane, chloroform, carbon tetrachloride, toluene and benzene, the analysis of the spectroscopic data indicates that harmane and pyridine form 1:1 stoichiometric hydrogen-bonded complexes in both the ground and singlet excited states. The formation constants of the complexes are greater in the excited than in the ground state. Hydrogen-bonding interaction in the excited state is essential for the quenching of the fluorescence of harmane by pyridine. The stabilities of the hydrogen-bonded complexes between harmane and pyridine diminish as the polarity and hydrogen-bonding ability of the solvent increase.