Polymorphism-dependent fluorescence of bisthienylmaleimide with different responses to mechanical crushing and grinding pressure

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Abstract
A herringbone structured compound, 3,4-bisthienylmaleimide (BTM), with reversible four-color and on/off switching upon external stimuli is reported here. Three kinds of BTM crystals with strong red (RC), orange (OC) and yellow (YC) fluorescence, as well as a brown solid (BS) with weak orange luminescence are obtained in the experiments. Heating, solvent vapor and pressure, including crushing and grinding, can reversibly switch BTM's emission between RC, OC, YC and BS. Base vapor (NEt3)-heating cycle treatment of BTM film in polystyrene can induce emission switching between bright/dark (on/off) states. The results of crystal structural analysis and photophysical property studies demonstrate that although OC and YC have the same crystal structure, they exhibit different fluorescent properties, due to their different surface structures. Heat induces metastable RC, with orderly π-stacking, to turn into stable OC or YC, with brick stone stacking. Crushing can damage the surface structure of OC and transform it into YC without altering the crystal structure. Grinding OC or YC destroys the orderly stacking to yield amorphous BS.

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