Heterogeneous Photochemical Reaction on Optical Length Scale in Photomechanical Molecular 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 crystal 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 shows a smooth photomechanical expansion, and the photochemical reaction proceeds heterogeneously on the optical length scale by observation of the changes in color, fluorescence, and birefringence. Moreover, we have performed a numerical simulation to reproduce the experimental results and revealed that both the cooperativity effect and the surface effect in crystal are essential for the heterogeneous progress of the photochemical reaction. Our results would provide a framework for analyzing the heterogeneous reaction dynamics on optical length scale in photoresponsive molecular crystals and a benchmark for future studies of photomechanical molecular crystals.