A Mechanochromic Luminescent Dye Exhibiting On/Off Switching by Crystalline-Amorphous Transitions.

A mechanochromic luminescent dye based on a simple aminomaleimide skeleton was readily synthesized in a one-pot process. It exhibited an on/off mechanochromic luminescent switching property dependent on external stimuli, unlike a traditional mechanochromic color change. The green emission was turned on by grinding in a mortar and turned off by heating or treatment with dichloromethane. In the crystalline state, two molecules were stacked by cofacial π-π interactions, which caused concentration self-quenching. The crystalline-to-amorphous transition induced by grinding removed cofacial π-π stacking, which led to intensive emission. Crystallizing processes recovered the cofacial π-π stacking, resulting in elimination of the emission. Theoretical calculations and X-ray diffraction analyses revealed that the dye molecule was distorted in the crystalline state; thus even a mechanical stimulus caused the crystalline-to-amorphous transition.