Crystallographic Observation of Reversible [2 + 2] Photocycloaddition and Photosalient Behavior in a Zinc(II) Coordination Polymer

The relationship between single-crystal-to-single-crystal (SCSC) transformations in solid-state [2 + 2] photocycloaddition and light-induced motion of crystals, known as the photosalient effect, remains poorly understood. Here, we report a two-dimensional zinc(II) coordination polymer (1) that exhibits both reversible SCSC [2 + 2] photocycloaddition reactivity and macroscopic photomechanical motions (jumping, splitting, and other effects). 1 contains three crystallographically distinct olefin pairs, enabling stepwise [2 + 2] cycloaddition under ultraviolet light, and the reverse reaction proceeds by heating. Single-crystal X-ray diffraction was used to capture the intermediate states during stepwise photodimerization and thermal ring cleavage, revealing the preferred reactivity of the olefin pairs in the structure. This work provides an example of fully reversible, photosalient SCSC [2 + 2] photocycloaddition, demonstrating that macroscopic dynamics and precise molecular-scale transformations can coexist in dynamic crystals.