Polymorphism and Multicolor Mechanofluorochromism of a D‑π‑A Asymmetric 4H‑Pyran Derivative with Aggregation-Induced Emission Property

A D-π-A 4H-pyran derivative (AMP) containing dicyanomethylene, benzene, and 4-hydroxybenzene units was designed and synthesized, which exhibited an aggregation-induced emission property. This asymmetric compound had three polymorphs, namely, yellow-emitting AMP-y, orange-emitting AMP-o, and red-emitting APM-r. AMP-y and APM-r showed very similar twisted conformations, and their different solid-state emissions depended not only on the molecular conformations but also on the zig-zag packing arrangements containing the coexistence of J-aggregation and H-aggregation with a subtle difference, whereas the solid-state emission of AMP-o mainly depended on the complete planar conformation. The polymorphs exhibited different stimulus-responsive fluorescence color changes upon grinding. AMP-y and APM-r displayed bathochromic and hypsochromic mechanofluorochromic (MFC) characteristics, respectively, and were both converted into orange-emitting solids, which were ascribed to the destruction of π···π interactions in the packing arrangements and the resultant planarization of molecular conformations and the change of packing arrangements. However, although a crystal-to-amorphous transformation occurred upon grinding, crystal AMP-o was MFC inactive because the grinding treatment could not result in the enhanced π···π stacking interactions owing to the dislocation arrangements in the molecules with planar conformations. Our results indicate that the combination of the polymorphic and MFC properties is beneficial to enrich the fluorescence color changes of MFC-active materials. Furthermore, the different MFC properties of the polymorphs are helpful for understanding the mechanism of the mechanofluorochromism.