A first direction of applied research has been the control of flexibility to optimize guest recognition and design materials with high gas separation performance. This can be achieved in various ways, in optimizing the strength of the host-guest interactions, activating shape recognition (i.e., optimizing the fit of the desired guest molecule inside the pores), or dynamic considerations. One example already detailed above is the optimization of acetylene adsorption and storage in Bonneau et al. (see subsection 2.3). In Wang et al. , the authors used a soft porous coordination polymer with "breathing" transitions to optimize the difficult separation of xylene isomers: the material can discriminate ortho-xylene from the other isomers by its specific guest-loading configuration, due to π-π stacking in the flexible framework providing shape recognition. Wu et al. provided another example of the use of flexibility to enhance selectivity: they synthesized a two-fold interpenetrated porous coordination polymer with a flexible propeller-like ligand, whose local flexibility specifically accommodates the CO 2 molecule -with high adsorption affinity due to strong confinement. This confers to the framework a very high selectivity for CO 2 trapping, and later promotes chemical reactions inside the pores of the material. Similarly, Chanut et al. demonstrated that mechanical control of the pore size aperture (through the application of external pressure) could optimize the molecular sieving separation of CO 2 /N 2 and CO 2 /CH 4 in MIL-53(Al).