Less is More: Tunable Polymorphs with Packing-Dependent Deep-Red to Near-Infrared Emission Based on a Geometrically Simple Molecule

Deep-red (DR) and near-infrared (NIR) luminogens have witnessed powerful applications in various aspects in recent decades. However, their inevitably complicated structures not only bring tedious and time-consuming synthesis, but also lead to inherent difficult in crystallization which hinder the understanding of structure-emission relationship. Polymorphism represents an ingenious strategy for revealing unambiguous structure-emission relationships. Therefore, designing simple molecules to construct DR and NIR polymorphic emission is challenging yet highly desirable. Herein, multiple DR and NIR emission ranges from 624 to 742 nm is achieved in polymorphisms of an extremely simple π-conjugated luminogen. Multiple packing modes are simultaneously integrated into this compound. Its unique packing-dependent emission facilitates the establishment of clear packing-emission relationship. Increased intermolecular exciton coupling between adjacent molecules leads to gradually red-shifted emission from orange to DR and finally to NIR. The aggregation-induced emission feature of the molecule renders the bulk crystals with visible crystal-to-crystal polymorphic transformation under external stimuli. This work not only breaks the limitations of the research of organic polymorphism materials with NIR emission, but also provides a reliable strategy to reveal the influence of molecular packing on NIR emission.