Crescent-Shaped Carbazole Derivatives as Light-Up Fluorescence Probes for G-Quadruplex DNA and Live Cell Imaging**

In recent years, studies on the organic small molecule fluorescent probes targeting G-quadruplexes have attracted a wide attention, because G-quadruplexes play an important role in various biological functions. Herein, we reported three crescent-shaped carbazole derivatives (4a-4c) and studied their interactions with the single-stranded, duplex, G-quadruplex and i-motif DNA. Both 4b and 4c have an above 100 nm Stokes shift and low fluorescence intensity, moreover, they exhibit the stronger affinity to G-quadruplex than to the other DNA structures. 4b with a cyanovinyl pyridine salt group displays a specific light-up fluorescence response to G-quadruplexes. FRET and CD results suggest that both 4b and 4c are able to improve the stability of G-quadruplexes and maintain their topology, moreover, they induce G-rich sequences (bcl-2, HTG, and KSS) to fold into G-quadruplexes in Na+/K+ free buffer. In addition, CLSM images suggest that 4b and 4c are mainly distributed in the mitochondrion of living HepG2 cells, and a weak fluorescence signal is also observed in the nucleus for 4c. Given that the two compounds have the larger association constants to G-quadruplexes over to duplex and single-stranded DNA, we speculate that the fluorescence signals in cells may mainly be attributed to the compound/G-quadruplex DNA complexes.