Electronic effect on the optical properties and sensing ability of AIEgens with ESIPT process based on salicylaldehyde azine

Two novel AIE-active salicylaldehyde azine (SAA) derivatives with a typical excited-state intramolecular proton transfer (ESIPT) process are prepared by introducing electron-withdrawing and donating groups at para-position of phenolic hydroxyl group (CN-SAA and TPA-SAA). The effect of the proton activity in SAA framework on their optical behaviors is investigated spectroscopically. The results from NMR and solvation measurements show that the proton of phenolic hydroxyl group has higher activity when there are electron-withdrawing groups, and the absorption and fluorescence spectra in buffers with different pH also provide the same results. After inviting F- as a nucleophilic probe, this proton activity difference in CN-SAA and TPA-SAA becomes more obvious. The potential application of both molecules is investigated. TPA-SAA exhibits good quantitative sensing ability towards F- with a fluorescence turn-on mode, whereas the aggregates of TPA-SAA can selectively and sensitively detect Cu2+ in aqueous solution. From these results, a structure-property relationship is established: the occurrence of ESIPT process will become much easier when linking electron-withdrawing groups at the para-position of phenolic hydroxyl group (e.g., CN-SAA), and it is better to introduce electron-donating groups to enhance the sensing ability towards ions (e.g., TPA-SAA). This work will provide guidance for further design and preparation of AIE-active luminogens with ESIPT process for sensing applications.