Schiff bases constructed from 2-hydroxy-4-methoxybenzophenone hydrazone and aromatic aldehydes: Synthesis, characterization, structure-photophysics relationship, HCl and NH3 stimuli-responsive solid fluorescence, selective detection of Cu2+

A series of fluorescent Schiff bases were skillfully designed and efficiently synthesized through the condensation reaction of 2-hydroxy-4-methoxybenzophenone hydrazone with various aromatic aldehydes (benzaldehyde, p -chlorobenzaldehyde, p -tolualdehyde, and p -dimethylaminobenzaldehyde). Their structures were rigorously confirmed using a combination of spectroscopic techniques, including FT-IR, HR-ESI-MS, and 1 H/ 13 C NMR spectroscopy. Notably, photophysical investigations revealed that p -dimethylaminobenzaldehyde-functionalized Schiff base featured with excited-state intramolecular proton transfer (ESIPT) upon photoexcitation in various organic solvents, whereas the other derivatives did not. In particular, this derivative demonstrated remarkable fluorescence enhancement due to synergistic effect of ESIPT and aggregation-induced emission (AIE) during aggregation process. Conversely, the emission intensities of the other Schiff bases gradually quenched in THF/H 2 O mixed solvent as the water fraction ( f w ) increased from 0 to 90 %, attributed to the typical aggregation-caused quenching (ACQ) effect. Additionally, this derivative demonstrated superior solid-state fluorescence and dyeing performance compared to its counterparts. Furthermore, this derivative showed reversible HCl/NH 3 stimuli-responsive solid fluorescence and a selective fluorescence turn-off response to Cu 2+ . This study would provide valuable insights into the molecular engineering of functional fluorescence materials based on benzophenone hydrazone derivative.