Flexibility in Metal-Organic Frameworks: A fundamental understanding

Expansion and contraction of structurally flexible Metal-Organic Frameworks (MOFs) or Porous Coordination Polymers (PCPs) have been extensively studied from different structural and application perspectives including crystal engineering, structural characterization, and gas adsorption-separation applications. The flexibility of the MOFs or PCPs depends on a number of factors including the nature of secondary building units (SBUs), organic linkers, pore geometry and/or solvent molecules. The flexibility can lead to unique properties, especially in gas-adsorption-separation related applications that is not observed in rigid frameworks. In this review, we offer a brief summary regarding a fundamental understanding of framework expansion-contraction in flexible porous MOFs in terms of their design and structure tunability by covering representative examples in the literature.