Two 8-hydroxyquinoline-based fluorescent chemosensors for ultra-fast and sensitive detection of water content in strong polar organic solvents with large Stokes shifts

It is of great significance to detect the moisture in organic solvents before used in water-sensitive reactions. Herein, two Schiff base quinoline derivatives, 8-hydroxyquinoline-2-carboxaldehyde thiosemicarbazone (HQCT) and 8-hydroxyquinoline-2-carboxaldehyde (pyridine-2-carbonyl)-hydrazine (HQPH), were designed and synthesized by a simple one-step reaction, and used as fluorescent chemosensors for ultra-fast and sensitive detection of water content in strong polar organic solvents. Based on excited-state intramolecular proton transfer (ESIPT) process, HQCT and HQPH exhibited strong fluorescence emissions with large Stokes shifts in dimethyl sulfoxide (DMSO) and N, N-dimethylformamide (DMF) solvents compared to other various organic solvents, and their fluorescence quenching and fluorescent color changes were obviously observed with increasing water content. The experimental results revealed that the hydroxyl groups substituted at the 8-position of HQCT and HQPH played a key role in the fluorescence emission processes. Dynamic light scattering (DLS) and 1H NMR titration indicated that the sensing mechanism for the detection of water was based on inhibition of the ESIPT by H2O via forming hydrogen bonds. In the range of 0.0–1.8 wt%, the fluorescence intensity of chemosensors changed as a linear function of water content. The detection limits of water in DMSO by HQCT and HQPH were as low as 0.0220 wt% and 0.0274 wt%, respectively. Moreover, HQCT and HQPH are successfully applied for the detection of moisture content in real commercial organic solvents.