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Organic single crystals with elastic bending flexibility are rare because they are generally brittle. We report here fluorescent organic single crystals based on thiophene-tetrafluorobenzene-thiophene derivatives, mainly 1,4-bis(thien-2-yl)-2,3,5,6-tetrafluorobenzene. Three derivatives were synthesized by Pd-catalyzed cross-coupling reactions (Stille or direct arylation pathways). The crystallization of the derivatives gave large (mm- or cm-scale) crystals. Two crystals of 1,4-bis(thien-2-yl)-2,3,5,6-tetrafluorobenzene, 1, and 1,4-bis(4-methylthien-2-yl)-2,3,5,6-tetrafluorobenzene, 3, bent under applied stress and quickly recovered its original shape upon relaxation. The other crystal of 1,4-bis(5-methylthien-2-yl)-2,3,5,6-tetrafluorobenzene, 2, showed brittle breakage under applied stress (normal behavior). Fibril lamella crystal structure based on criss-cross packed slip-stacked molecular wires and its structural integrity are important factors for the design and production of next generation crystal materials with elastic bending flexibility. Furthermore, mechanical bending–relaxation resulted in reversible change of the morphology and fluorescence (mechanofluorochromism). Such bendable crystals would lead to the next generation solid-state fluorescent and/or semiconducting materials.

Commercially available 4,7-dibromo-2,1,3-benzothiadiazole can be used as a building block for the organic synthesis of high performance optoelectronics devices. Herein, we report that a single crystal of 4,7-dibromo-2,1,3-benzothiadiazole displays elastic bending flexibility and crystalline-state fluorescence. A large needle-shaped single crystal (over 1 cm long) exhibited a fibril lamella type structure based on slip-stacked (Jaggregated) molecular wires. The crystal was capable of bending under applied stress and quickly reverts to its original shape upon relaxation. The crystal shows a reversible fluorescence change via mechanical bending−relaxation that can be performed many times.