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

Molecular\ncrystals that can display stimuli-responsive\nbehavior\nare a fascinating class of materials that can be used in various advanced\ntechnologies. However, designing and developing molecular crystal\npolymorphs that could respond differently to a single stimulus are\nstill a challenging task. In this article, we report remarkably different\nphotoresponses of the two polymorphs (<b>DMA-α</b> and <b>DMA-β</b>) of a simple donor–acceptor-type organic\nmolecule (<i>E</i>)-3(4-(dimethylamino)phenyl)-2(4-(trifluoromethyl)phenyl)acrylonitrile\n(<b>DMA</b>). Upon 456 nm light irradiation, <b>DMA-α</b> crystals undergo topochemical [2 + 2] cycloaddition reaction due\nto favorable stacking of CC double bonds in the crystal lattice,\nwhile <b>DMA-β</b> crystals exhibit <i>E</i>- to <i>Z</i>-isomerization reaction. Furthermore, the\nemission maxima (λ<sub>max,em</sub>) of both crystals shifted\ndifferently due to photoreaction. The origin of the distinct photoreactivities\nof two polymorphs could be rooted to different molecular packing and\nintermolecular interactions in the respective crystal lattices. Our\nwork provides an effective strategy to tune the photoreactivity of\norganic crystalline materials and their potential applications in\nsensors, actuators, and other optoelectronic devices.