Intense phototautomer emission of 2-(3′-pyridyl)benzimidazole encapsulated in Nafion membrane

Abstract

When encapsulated in the water nanochannels of a Nafion membrane, 2-(3′-pyridyl)benzimidazole is found to exhibit fluorescence from a monocationic species, T*, in which the pyridyl nitrogen is protonated. This kind of fluorescence has not been observed earlier for this molecule, except to a very small extent in strongly acidic aqueous solutions. Thus, Nafion is found to promote the formation of an uncommon excited state in this molecule. Two bands are observed in the T* fluorescence in Nafion. These are ascribed to fluorophores in two different kinds of environments: bulk water-like and interfacial. The band ascribed to the fluorophores residing near the interface is found to become the predominant one in partially dehydrated membranes, thereby fortifying our contention of the importance of electrostatic interactions at low hydration levels. The assignment of the fluorescence in native membranes to the phototautomer gains support from the experiments with cation-exchanged Nafion membranes, in which the fluorescence is blue shifted and is ascribed to the monocationic C* form, in which the benzimidazole nitrogen is protonated.