Edge-to-Center Propagation of Photochemical Reaction during Single-Crystal-to-Single-Crystal Photomechanical Transformation of 2,5-Distyrylpyrazine Crystals

Photomechanical molecular crystals are promising materials as photon-powered artificial actuators. To design and control the photomechanical responses, the spatiotemporal distribution of photoproducts in crystals could be a key factor in addition to molecular structures, molecular packings, illumination conditions, crystal morphology, crystal size, and so on. In this study, we have found that single crystals of 2,5-distyrylpyrazine show a smooth single-crystal-to-single-crystal photomechanical expansion, and the photochemical reaction propagates from the edge to the center of the single crystal. Moreover, we have performed a numerical simulation to reproduce the experimental results and revealed that both the cooperativity effect and the surface effect in the crystal are essential for the edge-to-center propagation of the photochemical reaction. Our results would provide a framework for analyzing the photochemical reaction dynamics in photoresponsive single crystals and a benchmark for future studies of photomechanical molecular crystals.