Photoresponsive Coordination Polymer Single Crystal Platforms: Design and Applications

Abstract

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.