Influence of acidity on the fluorescence spectra of 2-pyridylbenzimidazoles in aqueous solution

Abstract

The fluorescence spectra of 2-(3-pyridyl)benzimidazole (3PBI) and the quaternary salt 2-(1-methyl-4-pyridinio)-benzimidazole iodide (4PBIQS) in aqueous solutions covering a wide range of acidities have been studied to investigate their photophysical behaviour and the acid–base processes occurring in the first excited singlet states of these species. Depending on the acidity, 3PBI exists in the excited state as the dication, the monocation protonated at the benzimidazole N(3) atom (C*), the neutral species and the anion. Of all the possible processes of interconversion among these species, only the protonation of C* is fast enough to compete with its deactivation. 4PBIQS too has the same species in excited and ground states: the dication D*, the monocation with a positive charge on the pyridyl nitrogen (T*) and the zwitterion Z*. Deprotonation of D*, protonation of T* and deprotonation of T* by the dihydrogen phosphate anion were all detected. 3PBI and 4PBIQS thus allow separate study of the photophysical properties of the two 2-pyridylbenzimidazole monocations, C* and T*, with no interference between the two.