FRET versus PET: ratiometric chemosensors assembled from naphthalimide dyes and crown ethers

Novel bi-chromophoric naphthalimide derivatives containing benzo-15-crown-5 and N-phenyl-aza-15-crown-5 receptor moieties BNI2 and BNI3 were designed and prepared. Significant Förster resonance energy transfer (FRET) from donor (D) amido-naphthalimide to acceptor (A) amino-naphthalimide chromophores as well as photoinduced electron transfer (PET) between the N-aryl receptor and amido-naphthalimide fragment was revealed by the steady-state and time-resolved UV/Vis absorption and fluorescence spectroscopy. Upon the addition of alkaline-earth metal perchlorates to an acetonitrile solution of ligands, FRET mediated fluorescence enhancement was observed, which was a result of inhibition of the PET competitive deactivation pathway. The studied compounds provide an opportunity to register a two-channel fluorescence response upon selective excitation of either of the photoactive units and, thus, might be of interest as ratiometric probes.