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.