Multi-color mechanochromic luminescence of three polymorphic crystals of a donor–acceptor-type benzothiadiazole derivative†
The past decade has witnessed rapid advances in the development of organic crystals that exhibit mechanochromic luminescence (MCL), i.e., the reversible color change of photoluminescence induced by mechanical force. However, a greater understanding of the correlation between the MCL properties and the molecular structures in the crystalline state is still necessary to rationally control the photophysical and mechanical properties of organic crystals. Herein, a thienyl-substituted benzothiadiazole derivative is described as a new donor–acceptor-type MCL dye that crystallizes in three polymorphic forms. Both bicolor and tricolor MCL have been realized by the single donor–acceptor-type dye. The green-emissive polymorph exhibits typical bicolor MCL between green and orange based on crystal-to-amorphous phase transitions. Meanwhile, the yellow and yellow-orange-emissive crystals show tricolor emission-color switching. Specifically, the emission color of the yellow-emissive crystals changed to yellowish orange and orange in response to thermal and mechanical stimuli, respectively. The yellow-orange-emissive crystals display bicolor MCL between yellow-orange and orange emission. The single-crystal X-ray diffraction analyses of the polymorphic crystals indicated that the emission color of these crystals should be determined by the dihedral angle between benzothiadiazole and thiophene rings as well as the mode of packing structures. This study reinforces the understanding of the MCL of polymorphic crystals, which should help to advance the possible applications of MCL crystals in multi-color mechanosensing systems.