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.