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.