An Electric Field Induced Breath for Metal–Organic Frameworks

One of the reasons for the enormous interest in the class of porous metal organic frameworks (MOFs) is their structural flexibility, which is in contrast to the rather rigid zeolites. Kitagawa et al. have coined the term “third generation porous coordination polymers” for systems that undergo structural transformations in response to external stimuli, such as guest adsorption. One of the most dramatic and intensively investigated transformation is the so-called “breathing” effect: an open pore contracts upon guest adsorption and reopens again at saturation load. The prototypical system for this breathing is the MIL-53 series of porous materials. The large volume change of the breathing motion can not only be triggered by guest molecule adsorption (adsorption stress) but equally by mechanical pressure or temperature and is found for different MOFs with a “wine-rack” type structure. In their recent work, Ghoufi et al. have added to this list an additional interesting stimulus to trigger the breathing of MIL-53: namely, a (strong) homogeneous electric field. In a theoretical molecular dynamics investigation, they used their well tested force field to study the influence of such a field on MIL-53(Cr). Surprisingly, at ambient conditions and at a field strength of about 1.75 GV/m, the system closes in the same fashion as it does under pressure or guest molecule load. In addition, a hysteresis (e.g., lag) is observed, since the pores reopen at about 1.0 GV/m when reducing the field strength. The field induced polarization initially saturates already at about 0.1 GV/m (Figure S4 of ref 4) but increases again around 1.7 GV/m when the transformation is observed. By guest molecule adsorption in the presence of a field, the stimuli could be combined. When the authors maintained a system with narrow pores via the electric field, a size selective separation of carbon dioxide and methane was shown. This demonstrates the possibility to tune adsorption properties by the electric field. This first theoretical observation of an electric field-driven breathing opens numerous new possibilities for application, butnaturallyalso raises new questions. In particular, the physical driving force behind the pore closing is not entirely clear, yet. Both hydrostatic pressure and host−guest interactions lead to opposing forces on neighboring pore walls, whereas for a centrosymmetric system like MIL-53(Cr) no direct interaction with the field is present in the first place, apart from an induced polarization in field direction. This polarization results from both electronic polarization and structural deformations. However, since a nonpolarizable force field (fixed atomic charges) was used in the present study, the former is not captured in the model, and the observed breathing must be thus due to the latter. It is instructive to consider the atomistic structure of MIL-53(Cr). In Figure 1, a schematic representation of the