Photoresponsive Coordination Polymer Single Crystal Platforms: Design and Applications

The concept of photoresponsive coordination polymer (CP) single crystal platforms (CPSCPs) is based on photoresponsive olefin CP single crystals, which can undergo photocycloaddition reactions under light irradiation through a single-crystal-to-single-crystal (SCSC) transformation. Taking advantage of the coordination of olefin ligands to metal ions of Zn²⁺, Cd²⁺, etc., a pair of C═C double bonds is positioned adjacent to each other in space at a suitable distance and orientation to allow [2 + 2] photocycloaddition triggered by UV–vis irradiation, affording cyclobutanes in the CPs. The single crystal nature of CPs allows their structures to be determined by X-ray diffraction, providing details of the arrangements in space of the C═C double bonds. These CPs are promising platforms for the synthesis of organic molecules, such as cyclobutanes and derivatives, with high regioselectivity and stereoselectivity without any catalyst. The [2 + 2] photocycloaddition reactions may induce structural modifications like expansion or shrinking of unit cells, resulting in macroscopic changes (e.g., cracking, bending, etc.) of the whole CP single crystals and leading to changes in chemical and physical properties. Applications take advantage of their optical properties, including luminescence and absorption, and allow the detection of guest molecules and photomechanical motions. Although much effort has been devoted to such studies, it remains challenging to develop systematic investigations aiming at increasing the diversity of CPs and properties to meet practical needs. Moreover, more efficient methods are desirable to investigate the reaction mechanisms in the solid state and monitor the structural changes occurring during the process.

In this Account, we introduce our research on the design and applications of photoresponsive CPSCPs. It is divided into three parts. First, the design and construction of various CPs with different olefin ligands are discussed. Through a suitable and sometimes sophisticated choice of metal ions and auxiliary carboxylate ligands, these olefin ligands meet the requirements to undergo [2 + 2] photocycloaddition reactions in CP structures, allowing for the precise synthesis of cyclobutanes and their derivatives. These compounds could be subsequently extracted from the CPs to give pure organic products. Second, we introduce new strategies, such as a combination of single crystal X-ray diffraction (SCXRD) with thermal/phototreatments of CPs and in situ fluorescence spectroscopy, to monitor the structural changes occurring on the olefin ligands during the reaction. Furthermore, the fast stepwise photoreaction could also be visualized with high resolution. These data significantly strengthen our understanding of solid-state [2 + 2] photocycloaddition reactions in CPs. Third, applications of photoresponsive CPs are described, which focus on optical and photoinduced mechanical properties. Considering the optical properties, the conjugated structures of the olefin ligands change during the reactions, and circular dichroism (CD) and fluorescence were used for their detection and imaging. Furthermore, the photoinduced mechanical properties of CPs could be significantly expanded through the combination of CP crystals with polymers. Lastly, we point out the challenges and directions for future research in the field. We hope this Account will provide an overview of research on photoresponsive CPSCPs, attract more attention from the community, and inspire future research.