Crystallization of a luminogen normally red-shifts and weakens its emission. During investigation of some luminogens exhibiting aggregation-induced emission (AIE), we first noticed that some AIE active luminogens display higher photoluminescence (PL) intensity and bluer emission in the crystalline state than in their amorphous phase, which is termed as crystallization induced emission enhancement (CIEE). The twisted conformations of CIEE active molecules afford loose packing patterns and rule out any detrimental strong interaction. Molecular conformations are locked more tightly in crystals than those in amorphous solid due to the regular weak interactions in crystals, which affords the CIEE effect. The loose packing patterns facilitate emission switching through morphology modulation. Many AIE active molecules have been developed based on restriction of intramolecular rotation (RIR) mechanism, and we obtained many CIEE compounds from those AIE active luminogens and investigated their utilities as stimuli responsive functional materials, especially as chemosensors, thermosensor, mechanosensors and solid-state emitters. Thus, we provide a possible design strategy for the stimuli responsive solid luminescent materials.
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