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, namely Np-TPA and Np-2TPA, comprising twisted triphenylamine (TPA) as the electron-donor and planar 1,8-naphthalenimide (Np) as the 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 the theoretical and structural analyses, the conformation planarization of Np-TPA and Np-2TPA in the excited state could effectively diminish the molecular motions, allowing intense emission in solution, while the highly restricted pi-pi stacking permits their bright emission in the crystalline state. Efficient yellow bioimaging in the cytoplasm of various cells can be achieved even after incubation with extremely low concentrations of Np-2TPA (about 30 nM). These results imply that dual-state emissive fluorophores represent a new paradigm for bioimaging.