Solvent-Assisted, Thermally Triggered Structural Transformation in Flexible Mesoporous Metal–Organic Frameworks

Metal–organic frameworks (MOFs) have been excellent platforms for the research of solid-state porous materials in response to external stimuli. Herein, we have designed and synthesized two mesoporous, flexible MOFs on the basis of trigonal-prismatic [M3O­(RCOO)6] clusters and 4-connected tetratopic carboxylate ligands, which show remarkable structural transformation upon heating. The intermediate states of these stimuli-responsive processes can be visualized by in situ X-ray diffraction analyses, and the mechanisms of the structural transformations can be elucidated unambiguously. The flexibility of the indium MOF PCN-167 mainly derives from the twisting of the ligands. However, for the iron analogue PCN-168, the elasticity of the inorganic clusters in addition to the deformation of the organic ligands play an important role in the significant increase of axial expansion/contraction coefficients. Furthermore, guest molecules in the pores are also very important for the structural transformation of the frameworks.