Hydrogen-bonding interactions of norharmane in mixtures of acetic acid with benzene, p-dioxane and acetonitrile

Abstract

Spectroscopic studies on the hydrogen-bonding interactions of norharmane with acetic acid (AcH), a proton acceptor/donor molecule, have been carried out in two non-polar solvents (benzene and p-dioxane) and in a polar solvent (acetonitrile). The steady and non-steady photophysical results show three different kinds of norharmane/AcH hydrogen-bonded complexes in the first excited state depending on the norharmane nitrogen implied in the complex formation: 1 ∶ 1 pyridinic nitrogen complex, 1 ∶ 1 pyrrolic nitrogen complex and 1 ∶ 2 pyridinic–pyrrolic nitrogen complex. In the three solvents and from the dependence of the reciprocal of norharmane lifetime on the acetic acid concentration, the quenching deactivation constants have been obtained. These constants are much higher than the diffusion constant indicating that the interaction between norharmane and AcH exists in the ground state. The 1 ∶ 1 stoichiometry of norharmane–AcH complexes in the ground state has been confirmed by the absorbance change at 354 nm. Benzene is the solvent in which complex formation is more favoured. Single, double and tri-exponential fluorescence decays were recorded, depending on the AcH concentration and the emission wavelength. Decay times of the neutral form and the 1 ∶ 1 complexes are quite close, ca. about 1 ns. The cation form has a lifetime of about 17 ns and the complex with two acetic molecules has a decay time of about 3 ns. With the formation of these complexes, it is possible to explain the fact that the emission around 500 nm disappears for high acetic acid concentrations.