Tunable Photoresponsive Behavior of Organic Materials by Polymorphic Variation: Topochemical [2 + 2] Cycloaddition vs <i>E</i>–<i>Z</i> Isomerization

Molecular crystals that can display stimuli-responsive behavior are a fascinating class of materials that can be used in various advanced technologies. However, designing and developing molecular crystal polymorphs that could respond differently to a single stimulus are still a challenging task. In this article, we report remarkably different photoresponses of the two polymorphs (DMA-α and DMA-β) of a simple donor–acceptor-type organic molecule (E)-3(4-(dimethylamino)phenyl)-2(4-(trifluoromethyl)phenyl)acrylonitrile (DMA). Upon 456 nm light irradiation, DMA-α crystals undergo topochemical [2 + 2] cycloaddition reaction due to favorable stacking of C═C double bonds in the crystal lattice, while DMA-β crystals exhibit E- to Z-isomerization reaction. Furthermore, the emission maxima (λmax,em) of both crystals shifted differently due to photoreaction. The origin of the distinct photoreactivities of two polymorphs could be rooted to different molecular packing and intermolecular interactions in the respective crystal lattices. Our work provides an effective strategy to tune the photoreactivity of organic crystalline materials and their potential applications in sensors, actuators, and other optoelectronic devices.