Morphology-dependent fluorescence ON/OFF of a beryllium complex: ACQ in amorphous solids, AEE in crystalline powders and the dark/bright fluorescence switch

Here we report a bis(2'-hydroxychalcone) beryllium complex Be(HC)(2) that displays yellow fluorescence (lambda(em) = 557 nm; Phi(f) = 0.10) in solution. Notably, the solution of this complex produces a non-emissive amorphous thin film (ACQ effect; fluorescent "OFF" state) but brightly emissive crystalline powders (AEE-active; fluorescent "ON" state) with deep red (lambda(em) = 678 nm; Phi(f) = 0.27) or near infrared (lambda(em) = 700 nm; Phi(f) = 0.20) emission colors and the fluorescent "ON" and "OFF" states can be smoothly transformed into each other by simple engineering processes: mechanical grinding and solvent fuming.