Aggregation induced emission properties of naphthalimide–coumarin conjugates with various intermolecular linkages

Naphthalimide–coumarin conjugates (NCs 1–5), possessing various linkages such as ether, thioether, secondary amine, acetylene and C–C single bond between naphthalimide and coumarin moieties, have been synthesized. NCs 1–5 were fully characterized by 1H NMR, 13C NMR and mass spectroscopy. The effects of linkages in NCs 1–5 on the absorption, fluorescence and aggregation induced emission (AIE) were explored. The electronic absorption spectra of the NCs 1–5 show red shifted absorption and fluorescence as compared to 4–bromonaphthalimide. The secondary amine linked naphthalimide–coumarin (NC) 3 conjugate shows significant red shift in absorption and fluorescence, whereas other NCs show moderate red shifts. Among NCs conjugates, only thioether linked NC 2 clearly showed a significant AIE effect while the other NCs conjugates showed aggregation caused emission quenching.