Transition metal ion induced fluorescence enhancement of 4-(N, N-dimethylethylenediamino)-7-nitrobenz-2-oxa-1, 3-diazole

Abstract

The photophysical properties of a structurally simple fluorophore-spacer-receptor supramolecular system involving nitrobenzoxadiazole (NBD) fluorophore, NEA, and its fluorescence response toward the quenching transition metal ions are reported. It is shown that efficient through-space intramolecular photoinduced electron transfer (PET) between the NBD fluorophore and the amino group, used here as a receptor for metal ions and protons, is responsible for extremely low fluorescence quantum yield and lifetime of NEA compared to the system NAM which contains the same fluorophore but does not possess the receptor moiety. The interaction between the NBD moiety and the metal ions in the ground and excited state is evident from the metal ion induced changes in the absorption spectra and fluorescence quenching of NAM. It is shown that despite a strong interaction between the NBD fluorophore and the quenching metal ions, NEA exhibits considerable enhancement of fluorescence rather than quenching in the presence of the transition metal ions. The results show that for an efficiently PET-quenched supramolecular system, the quenching influence of the transition metal ions is not important at moderate concentrations of the ions. This implies that efficient and structurally simple fluorosensors for the quenching metal ions can simply be developed by maximizing PET in the supramolecular systems without paying much attention to the quenching influence of the ions.