Crystal Structure of a 4,7-Bis(phenylethynyl)-2,1,3-benzothiadiazole-Based Cyclophane and the Mechanoresponsive Luminescence

The photophysical properties of luminescent cyclophanes have been traditionally examined in an isolated state in solution. In contrast, the behavior of luminescent cyclophanes in response to external stimuli in the solid state has not yet been fully understood. Herein, we report on the crystal structure of a cyclophane featuring one 4,7-bis(phenylethynyl)-2,1,3-benzothiadiazole group and its mechanochromic luminescence. Mechanical grinding of the crystals results in a phase transition to an amorphous state, which is concomitant with an emission color change from green to yellow. Annealing the amorphous phase at 100 °C induces a phase transition to another crystalline state, and green emission is observed again. On subsequent heating, the benzothiadiazole-based cyclophane forms a nematic liquid-crystalline phase above 191 °C. These results indicate that the introduction of π-extended planar luminophores within cyclic molecular structures is a promising approach for the design of stimuli-responsive luminescent materials.