Strong ion pair charge transfer interaction of 1,8-naphthalimide-bipyridinium conjugates with basic anions-towards the development of a new type of turn-on fluorescent anion sensors

Abstract

The electron transfer processes of two conjugates consisting of an N,N′-dimethyl-bipyridinium unit and an 1,8-naphthalimide or 4-piperidinyl-1,8-naphthalimide fluorophore have been investigated. The two dyads show only weak fluorescence due to the intramolecular photoinduced electron transfer (PET) from the naphthalimide to the bipyridinium moiety. The dyads form radical pairs with the Lewis-base anions such as fluoride, acetate and benzoate via intermolecular electron transfer from the anions to the bipyridinium units. The electron transfer from the anions is indicated by an intense coloration-due to the absorption of the bipyridinium radicals-and a dramatic enhancement of the fluorescence intensity due to the cancellation of intramolecular PET in the dyads. These results demonstrate the efficient operation of bipyridinium-naphthalimide conjugates as signal transduction units from which a new type of turn-on fluorescent anion sensors may be created.