Approaches for synthesizing breathing MOFs by exploiting dimensional rigidity

A subclass of metal-organic frameworks (MOFs) undergoes a "breathing" phenomenon which is a reversible flexing of the framework as a function of adsorbed guest. The existence of multiple stable states in a single framework leads to the modification of the shape and size of the pores as a function of guest, and has encouraged researchers to consider innovative applications for these flexible materials. To assist in improving the designs of these breathing MOFs, a thorough understanding of the different mechanisms by which frameworks breathe, must be established. This review will classify current breathing mechanisms by designating which part of the framework will not flex. By classifying the dimensional rigidity within a framework, a more in-depth discussion of each breathing mechanism can be established. Additionally, strategies for the design and synthesis of the next generation of breathing MOFs based on the reviewed research are discussed. (C) 2013 Elsevier B.V. All rights reserved.