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 b inding 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 syst ems an found to be very weakly fluorescent compared to their constituent fl uorophores (3 and 4) and this observation has been ascribed to photoinduced intramolecular electron transfer (PIET) between the electron rich amino mo iety (donor) and relatively electron deficient fluorophore component (accep tor). Spectral and electrochemical data indicate the thermodynamic feasibil ity of PIET (exergonic free energy changes) in these multicomponent systems and PIET is found to be most efficient in systems where the fluorophore an d the amino moiety are separated by two methylene groups. Fluorescence deca y behavior of the systems suggest that PIET occurs by a through-space mecha nism. In the presence of the transition metal ions, well-known for their fl uorescence quenching abilities, the present systems exhibit significant flu orescence enhancement (FE). Moreover, it has been observed that guest-induc ed 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 fluoropho re by the water molecules of the hydrated metal salts could be partially re sponsible for the high FE values.