Metal-free organic phosphorescent materials are attractive alternatives to the predominantly used organometallic phosphors but are generally dimmer and are relatively rare, as, without heavy-metal atoms, spin–orbit coupling is less efficient and phosphorescence usually cannot compete with radiationless relaxation processes. Here we present a general design rule and a method to effectively reduce radiationless transitions and hence greatly enhance phosphorescence efficiency of metal-free organic materials in a variety of amorphous polymer matrices, based on the restriction of molecular motions in the proximity of embedded phosphors. Covalent cross-linking between phosphors and polymer matrices via Diels–Alder click chemistry is devised as a method. A sharp increase in phosphorescence quantum efficiency is observed in a variety of polymer matrices with this method, which is ca. two to five times higher than that of phosphor-doped polymer systems having no such covalent linkage.

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 stable supercooled liquid is a subtle force balance between aromatic interactions among the core units and van der Waals interactions among the aliphatic side chains acting in opposite directions. Moreover, by applying shear force to a supercooled liquid DPP8 film at different temperatures, we demonstrated direct writing of fluorescent patterns and propagating fluorescence amplification, respectively. Shear-triggered crystallization of DPP8 is further achieved even by living cell attachment and spreading, demonstrating the high sensitivity of the shear-triggered crystallization which is about 6 orders of magnitude more sensitive than typical mechanochromism observed in organic materials.