Solvent-Free Ground Thiadiazole Schiff Base Fluorescent Probe for Selective Detection of Dichromate with DFT Mechanism Investigation.

(E)-1-Phenyl-N-(5-phenyl-1,3,4-thiadiazol-2-yl)methanimine (probe Y) was synthesised via solvent-free, solid-phase grinding, using 2-amino-5-phenyl-1,3,4-thiadiazole and benzaldehyde as the starting materials. The probe exhibits excellent selectivity towards the dichromate anion (Cr2O72-) and generates dual-channel responses, including fluorescence quenching for quantitative detection and a visible colour change of the solution from light green to bright yellow for qualitative naked-eye detection. The probe's sensing performance remains stable over a wide pH range (2-12), with a rapid equilibrium response time (< 5 min). Fourier-transform infrared (FT-IR) and proton nuclear magnetic resonance (¹H NMR) titration experiments suggest that Probe Y interacts with Cr2O72- with an apparent stoichiometric ratio of 2:1. Density functional theory (DFT) calculations were performed to reveal the probe's intrinsic recognition mechanism, and frontier molecular orbital (FMO) analyses suggest that photoinduced electron transfer (PET) may contribute to the fluorescence quenching process. The fluorescence quenching constant (Ksv) of the probe towards Cr2O72- is 1.87 × 104 M- 1, and the fluorescence-based limit of detection is 4.02 µM, demonstrating competitive sensing performance among reported thiadiazole Schiff-base Cr2O72- probes.