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 an 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.