Photophysical, NMR and density functional study on the ion interaction of norharmane: proton transfer vs. hydrogen bonding

Abstract

Interactions of norharmane (S), with different ions have been studied using spectroscopy, NMR and density functional studies. A significant gradual change both in absorption and emission spectra was observed upon addition of fluoride anions. The spectral change in the absorption and emission bands of norharmane (S) is found to be specific to fluoride ion; it is unaffected by the presence of other ions. Hydrogen bond mediated proton transfer from norharmane to fluoride is mainly attributed to the fluoride selective signaling behavior. Calculations of the transition energies of the norharmane (S), anionic species of norharmane (S−) and hydrogen bonded complexes (S⋯F−) show that the added fluoride anion could capture the proton in the free N–H moiety instead of the hydrogen-bonding one. Experimental results reveal that the long-wavelength absorption band in the presence of fluoride ion is due to the formation of anion.