How important is the quenching influence of the transition metal ions in the design of fluorescent PET sensors?

The fluorescence response of a nitrobenzoxadiazole fluorophore, attached to an amine receptor through a spacer unit, in the presence of a strongly quenching metal ion Cr3+ is reported. It is shown that the supramolecular system exhibits considerable enhancement of fluorescence in the presence of the metal ion, despite the quenching nature of the metal ion. This observation is rationalised in terms of the excited state lifetime of the fluorophore and the possible implication of this result in the design of fluorosensors for the quenching transition metal ions is discussed

Modulation of metal-fluorophore communication to developstructurally simple fluorescent sensors for transition metal ions

Fluorophores 2a and 2b show significant fluorescence enhancement in the presence of transition metal ions

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

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

Fluorescence response of structurally simple fluorophore-spacer-receptor systems towards transition metal ions and protons

Two new multicomponent systems with fluorophore-spacer-receptor architecture employing 4-aminophthalimide and 4-amino-1,8-naphthalimide fluorophores and aniline receptor have been developed and their fluorescence response towards quenching transition metal ions and protons has been investigated. The low fluorescence quantum yields of the supramolecular systems compared with those of the free fluorophores have been interpreted in terms of photoinduced intramolecular electron transfer (PET) in the systems. In the presence of the d-block metal ions, known for their quenching abilities, the supramolecular systems exhibit fluorescence enhancement rather than quenching. This observation has been interpreted taking into consideration of PET-quenched short fluorescence lifetime of the fluorophores. The systems reported here can be considered as potential candidates as fluorosensors for the transition metal ions and protons

Unusually high fluorescence enhancement of some 1,8-naphthalimide derivatives induced by transition metal salts

Three-component systems, 1a-c and 2a,b, comprising 1,8-naphthalimide and 4-methoxy-1,8-naphthalimide as fluorophore, a dimethylamino moiety as guest binding site and a polymethylene group as spacer, have been synthesized and the fluorescence behavior of these systems has been studied in the absence and in the presence of the salts of several transition metal ions. The systems are found to be very weakly fluorescent compared to their constituent fluorophores (3 and 4) and this observation has been ascribed to photoinduced intramolecular electron transfer (PIET) between the electron rich amino moiety (donor) and relatively electron deficient fluorophore component (acceptor). Spectral and electrochemical data indicate the thermodynamic feasibility of PIET (exergonic free energy changes) in these multicomponent systems and PIET is found to be most efficient in systems where the fluorophore and the amino moiety are separated by two methylene groups. Fluorescence decay behavior of the systems suggest that PIET occurs by a through-space mechanism. In the presence of the transition metal ions, well-known for their fluorescence quenching abilities, the present systems exhibit significant fluorescence enhancement (FE). Moreover, it has been observed that guest-induced FE can even be severalfold higher than that expected from consideration of PIET in the system. It is suggested that a system can exhibit unusually high FE when the guest is capable of inducing FE by more than one means. In the present case, it is shown that preferential solvation of the fluorophore by the water molecules of the hydrated metal salts could be partially responsible for the high FE values