Tetraphenylethene-capped aminobenzopyranoxanthene hydrazone probes for colorimetric recognition of Hg²⁺ and fluorescent sensing of HCl gas

Two sensing molecules, A-T1 and A-T2 with bichromophore were synthesized by linking different numbers of TPE units to a xanthene platform (ABPX). There was a big difference in symmetry and sensing features between the two molecules. The asymmetric molecule A-T1 has hydrazide and TPE-capped hydrazone ligands, while the symmetric molecule A-T2 with two TPE units showed brighter AIE fluorescence in solid state and aqueous solution. In heavy metal ion analysis, A-T1 showed obvious absorption increase centered at 499 and 533 nm in the presence of Hg2+, and the color changed to pink; A-T2 showed weak response to Cu2+ due to the steric hindrance of TPE. The Job's Plot and ESI-MS results indicated that a 1 : 1 ligand-metal complex was formed between the A-T1 hydrazide site and Hg2+, and Cu2+ can coordinate with the TPE-capped hydrazone site. Furthermore, A-T1 powder could respond to HCl gas via the spiro-opening process, and this can be restored by NH3. The H-1 NMR chemical shifts of the xanthene at 6.16 to 6.56 ppm vanished and reappeared, demonstrating the protonation of diethylamine groups by HCl gas. These probes could be applied as fluorescence indicators in vivo imaging with high resolution.