Photophysical and transition metal ion signaling properties of some 4-amino-1,8-naphthalimide derivatives

Photophysical and transition metal ion signaling behaviour of several 4-amino-1,8-naphthalimide derivatives in aprotic solvents, tetrahydrofuran and acetonitrile, are reported. The weakly fluorescent nature of the fluorophore–spacer–receptor systems (1a, 1b) compared to the parent fluorophore is shown to be due to photoinduced electron transfer (PET) between the terminal moieties of the systems. PET is found to be more efficient in polar medium. These systems show reasonably good ‘off–on’ fluorescence signaling in the presence of the transition metal ions. On the other hand, 2, 3a and 3b show unusual absorption and fluorescence behavior in acetonitrile in the presence of Cu²⁺. These spectral changes have been found to be particularly useful for specific signaling of Cu²⁺ in acetonitrile. The present study also reveals that reduction of Cu²⁺ to Cu⁺ in this solvent aided by the hydrogen atom of the 4-amino group of the fluorophore leads to the observed spectral changes.