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.