[Structural features and regulation of adsorption/separation properties of photoresponsive metal-organic frameworks].

Abstract

Metal-organic frameworks （MOFs） are attractive porous materials with periodic network structures， which are constructed via the self-assembly of metal ions （or metal clusters） and bridging organic ligands. MOFs have some unique advantages of designability， ordered pore structure， high specific surface area and modifiability. Photoisomerization refers to the phenomenon in which， under light of specific wavelengths， the molecular structures of certain compounds undergo reversible interconversion due to photoexcitation. Both isomeric forms can exist stably under certain conditions and exhibit distinct chemical and physical properties. On this basis， photoresponsive MOFs （PMOFs） are proposed as a class of smart materials constructed by integrating photoactive molecules into MOFs. PMOFs combine the inherent superior porous structure of MOFs with the dynamic tunability enabled by photo-induced isomerization and exhibit unique advantages in many fields， including catalysis， drug release， separation， anti-counterfeiting， etc. PMOFs， for instance， can precisely regulate adsorption/separation behaviors and enable green， energy-efficient recovery of adsorbents under photoinduction， which has attracted widespread attention. This review summarizes common types and properties of photoactive molecules， including azobenzene， spiropyran， diarylethene， spirooxazine， fulgide， Schiff base， etc.， as well as three main integration strategies of photosensitive units/molecules with MOFs frameworks. Backbone incorporation involves using the entire photoactive organic molecule as an organic ligand within the MOF framework. For side-group grafting， the photoactive moiety is attached as a side group or through post-synthetic modification of the organic ligand， effectively grafting it onto the MOF backbone. Guest encapsulation is referred to embedding photoactive molecules as independent guest species within the cavities or pores of the MOF. By analyzing the differences in isomerization of various photoactive groups and the structural features of different integration modes， the multidimensional modulations of MOF are illustrated in pore size， surface properties， host-guest interaction， "hiding" and "exposure" of adsorption active sites， and framework geometry via photoisomerization. Furthermore， from a structure-performance perspective and with representative case studies， this review discusses the regulatory mechanisms and application potential of PMOFs in the adsorption and separation of small-molecule gases， metal ions， and organic dyes. The key limitation of PMOFs lies in whether the MOF framework is robust enough to tolerate the molecular mechanical forces produced by photoswitch configurational rearrangements. The cyclic photo‑switching performance of PMOFs is often unsatisfactory， which is primarily attributed to the molecular mechanical forces generated during repeated photoisomerization， which may cause local microscopic distortion， phase transitions， or even framework collapse in MOFs. In addition， the photo‑response efficiency of PMOFs is inadequate， which is obviously lower than photosensitive molecules in solution. This is likely due to limited optical penetration depth， steric hindrance within the pores， and framework‑induced quenching of the excited‑state configuration. The above challenges are summarized in this paper， with the aim of facilitating discussions among researchers and promoting their multidimensional exploration and development.