D–A-Type fluorophores with efficient dual-state emission for imaging at ultralow concentration

Herein we demonstrate the synthesis and investigation of organic D-A type fluorophores, named Np-TPA and Np-2TPA, comprising of twisted triphenylamine (TPA) as electron-donor and planar 1,8-naphthalenimide (Np) as electron-acceptor. These fluorophores emit strong fluorescence not only in dilute solutions but also in the solid state with quantum yields up to 0.74 and 0.92, respectively. From theoretical and structural analysis, the conformation planarization of Np-TPA and Np-2TPA in the excited state could effectively diminish the molecular motions thus allowing intense emission in solution, while highly restricted π-π stacking permits their bright emission in crystalline state. Efficient yellow bioimaging in the cytoplasm of multiple of various cells can be achieved after the incubation with even extremely low concentrations of Np-2TPA (about 30 nM). All the results imply that the dual-state emissive fluorophore represents a new paradigm for bioimaging.