Photoresponsive metal-organic polyhedra in metal-organic frameworks: Achieving “real” responsiveness

Photoresponsive metal-organic polyhedra (PMOPs) show controllable properties in a broad range of applications, such as adsorption, catalysis, and molecule inclusion. However, the aggregation of bulk PMOPs leads to their inaccessibility of inside nanocages and low regulatory efficiency by light. Herein, a new PMOP (PM₂L₄) with pendant azobenzene units was synthesized and dispersed into the pores of the metal-organic framework (MOF, PCN-333). The obtained PM₂L₄@MOF composites show improved CO₂ uptake and photoresponsive efficiency. Upon visible-light irradiation, the azobenzene groups stay in the trans state where CO₂ molecules can freely enter the nanospace of PM₂L₄. Nevertheless, upon ultraviolet (UV)-light irradiation, the azobenzene groups transform to the cis state, which hinders the entrance of CO₂ to the nanospace of PM₂L₄. In addition, UV/visible light irradiation can facilitate the reversible cis-/trans-isomerization of the azobenzene groups of PM₂L₄. The adsorption variation of CO₂ captured by PM₂L₄@MOF composite under light is 15.5%, which is much higher than that of bulk PM₂L₄ (5.9%). We believe that the findings of this study will provide insights into the potential of PMOPs and may inspire the development of exquisite strategies to efficiently control adsorption processes. [Figure not available: see fulltext.].