We report a unique example of polymorphism-dependent and piezochromic fluorescence, based on 2-(anthracen-9-yl)-4,5-diphenyl-1H-imidazole.

A highly solid-emissive boron-difluoride dye exhibiting polymorph-dependent and mechanochromic luminescence has been synthesized. Three single crystal X-ray structures of the polymorphic dye molecule, having different fluorescence emissions, are elucidated. Each polymorph displays different stacking orientations among the chromophore moieties and several different intermolecular interactions, which could be related to their different crystal emissions. The molecule displays piezochromic characteristics, with the fluorescence and quantum yield of both the solid powder and two kinds of crystals being reversibly adaptable to mechanical force and heating. The conversion between monomer and π-π stackinginduced aggregate has been proposed as the mechanism behind mechanochromism, according to X-ray diffraction measurements and differential scanning calorimetry analyses.

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