Fluorescence sensor design for transition metal ions: the role of the PIET interaction efficiency

Multicomponent systems 1, 2, and 3 having fluorophore-spacer-receptor architecture have been prepared with a view to understand the role of the photoinduced intramolecular electron transfer (PIET) interactions in transition metal ion sensing efficiency of these systems. Structurally similar compounds 4-amino-1,8-naphthalimide (4), 4-aminophthalimide (5), and 4-methoxy-1,8-naphthalimide (6) were used as the fluorophore moieties. Dimethylamino group (as in the case of 1a, 2a, and 3a, series A) and an aniline moiety (like in 1b, 2b, and 3b, series B) have been employed as the receptor components. A two-carbon ethylene chain serves as a spacer unit. The absorption and fluorescence spectral features of the systems have been studied in the absence and presence of various transition metal ions. All the multicomponent systems (except 1a) show weak fluorescence intensities compared to that of their constituent fluorophores (4, 5, and 6) in any given solvent. The reason for this low fluorescence quantum yield could be ascribed to the efficient PIET interaction between receptor moiety and the electron deficient fluorophore component of the systems. This has been corroborated with the estimated thermodynamic driving force (delta G*) for the PIET process in the multicomponent systems, calculated using electrochemical and spectral properties of individual components, is more negative for 2 and 3 than for 1 having electron deficient fluorophores 5, and 6, respectively. Especially, as evidenced by their low fluorescence quantum yield values, the PIET interaction is found to be more significant in the systems of series B (1b, 2b, and 3b) than the respective system of series A (1a, 2a, and 3a, respectively). The sensing capability of the systems is directly related to the efficiency of the PIET interaction in the unbound state. Accordingly, all these systems (except 1a) show significant fluorescence enhancement in the presence of transition metal ions, well known for their high fluorescence quenching behavior. The present paper describes, the feasibility of optimizing the PIET interaction in the multicomponent sensor system in unbound state, and thus transition metal ion signaling capability of the system.