Linear tetraphenylethene-appended bis-imidazolium salts for sensing of ATP

Five tetraphenylethene (TPE) functionalized linear bis-imidazolium salts with different chain length spacers, i.e., methylene (BT-1), 1,2-ethanediyl (BT-2), 1,3-propanediyl (BT-3), 1,5-pentanediyl (BT-4) and 1,10-decanediyl (BT-5), have been designed and synthesized. The anion recognition ability of all of the probes to recognize anions was the investigated by emission spectra, 1H NMR titration and DLS experiments. All the bis-imidazolium salts show a very weak fluorescence in aqueous solution. However, the weak fluorescence of the free probes can be affected by ATP anion, with a significant fluorescence enhancement of the tetraphenylethene-based aggregation-induced emission at 471 nm. Importantly, the binding affinity of probes toward ATP can be modulated by adjusting the spacer length, with the binding constants being in the order of BT-1 > BT-2 > BT-3 > BT-4 > BT-5. The 1H NMR titration and the DLS experiments revealed the binding mechanisms between probes and ATP. For practical application, probe BT-3 was selected and successfully used for fluorescence imaging of intracellular ATP in live cells.