Water-Triggered Reversible Fluorescence Switching in 2-Methylbenzimidazole/TCNB Cocrystals: Dynamic Structural Transformation and Hydrogen Bond-Regulated Mechanism

This study uncovers the molecular mechanism of water-triggered reversible fluorescence switching through cocrystalline assemblies of 2-methylbenzimidazole and 1,2,4,5-tetracyanobenzene. Single-crystal analysis reveals that in the hydrated phase (2METC-2), water molecules bridge donor–acceptor units via O–H···N/N–H···O hydrogen bonds, inducing substantial molecular distortion (dihedral angle change Δθ = 50.67°) and π–π compression (interplanar spacing reduction Δd = 0.209 Å). These structural rearrangements enhance exciton coupling and activate nonradiative decay pathways, quenching the fluorescence quantum yield from 13.70% (anhydrous phase, 2METC-1) to 0.48%. Variable-temperature PXRD, DSC, TGA, and cycling experiments confirm >5 reversible transitions at 80 °C/solvent with <8% intensity decay, demonstrating exceptional thermal-humidity dual responsiveness. Theoretical analyses establish that methyl-directed steric effects and cooperative C–H···O interactions construct dynamic hydrogen-bonding networks, providing an alternative strategy to conventional hydrophilic group dependency. This work establishes a new design paradigm for highly stable stimuli-responsive luminescent materials.