Asymmetric reversible structural switching of a diene coordination polymer promoted by UV-visible light

In natural and artificial systems, reversible reactions are commonly asymmetric with respect to the time scale and nature of the stimuli which drive the forward and backward processes. In applications for which switching behavior is required, it is desirable that the reversible reaction goes as close to symmetric as possible; however, such systems are uncommon. Herein, we report an example of ultraviolet (UV)-visible light-regulated asymmetric reversible structural switching involving a diene-based coordination polymer, CP1 and its monocyclobutane product, CP1a. It is possible to cycle at least ten times through a forward [2 + 2] photocycloaddition reaction and the reverse, photocleavage reaction. A single cycle can be completed within a few minutes. The transformation is accompanied by fast and distinct fluorescence changes, arising from optimisation of the reaction conditions. Density functional theory calculations allow rationalisation of the asymmetric reversible transformation between CP1 and CP1a rather than between CP1 and its dicyclobutane product CP1b. This work provides a clear illustration of reversible structural switching which approaches symmetric behaviour with respect to reaction rate and stimuli. The insights gained from this work also assist in the design of fast, reversible switching materials.