Thermodynamics drive crystalline organic molecules to be crystallized at temperatures below their melting point. Even though molecules can form supercooled liquids by rapid cooling, crystalline organic materials readily undergo a phase transformation to an energetically favorable crystalline phase upon subsequent heat treatment. Opposite to this general observation, here, we report molecular design of thermally stable supercooled liquid of diketopyrrolopyrrole (DPP) derivatives and their intriguing shear-triggered crystallization with dramatic optical property changes. Molten DPP8, one of the DPP derivatives, remains as stable supercooled liquid without crystallization through subsequent thermal cycles. More interestingly, under shear conditions, this supercooled liquid DPP8 transforms to its crystal phase accompanied by a 25-fold increase in photoluminescence (PL) quantum efficiency and a color change. By systematic investigation on supercooled liquid formation of crystalline DPP derivatives and their correlation with chemical structures, we reveal that the origin of this thermally stablesupercooled liquid is a subtle force balance between aromatic interactionsamong the core units and van der Waals interactions among the aliphaticside chains acting in opposite directions. Moreover, by applying shearforce to a supercooled liquid DPP8 film at different temperatures,we demonstrated direct writing of fluorescent patterns and propagatingfluorescence amplification, respectively. Shear-triggered crystallizationof DPP8 is further achieved even by living cell attachment and spreading,demonstrating the high sensitivity of the shear-triggered crystallizationwhich is about 6 orders of magnitude more sensitive than typical mechanochromismobserved in organic materials.
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