Host-guest MOFs

In a subsequent study, the synthesis of urea and thiourea derivatives of types 3a and 3b were undertaken in an attempt to obtain new hosts that might yield more stable complexes with glutamic acid (of types 4a and 4b) and that might, in fact, hold together in polar solvents. The rationale for the design was based on the previously documented observation that both 1,3-dimethylurea and 1,3-dimethyl-thiourea form stable host-guest complexes with acetate in deutero-dimethyl sulfoxide the K values for the latter are 45 dm moF and 340 dm moF , respectively. 

As schematically shown by Fig. 46a, ferrierite contains two mutually intersecting arrays of channels. In comparison with the strictly one-dimensional MOF crystals considered in the previous section, their analysis is additionally complicated by the existence of two rooflike parts on either side of the platelike main crystal body. It turned out, however, that these features did in no way complicate the method of analysis. Contrary to the MOFs, which required an additional activation step after each uptake experiment, methanol in ferrierite proved to be an ideal host-guest system, where one and the same crystal could alternately be subjected to adsorption and desorption without any perceptible change in the sorbate profiles. It were these special conditions under which interference microscopy could be developed to a technique of diffusion measurement in nanoporous materials of unprecedented power [63,65,70,71,88,89]. 

These results suggest that MOFs as extended metal-organic assemblies could show a similar host-guest chemistry. In uncharged MOFs however, space confinement should be the most effective stabilizing effect on these species. 

Fleterogeneous catalytic activity was one of the first proposed and demonstrated host-guest properties of MOFs, with subsequent research providing a range of different catalytic activities across a diverse array of framework systems. Most notable among these systems are cases in which catalysis arises due to chemical activation at specific surface binding sites. 

Homochiral MOFs are able to create an environment generating chiral induction and allowing molecular recognition from weak host-guest interactions. Lin [133] and Rosseinky [134] described the separation of chiral alcohols from a racemic... 

To monitor the structure and crystallinity change of MOFs, powder X-ray diffraction is a convenient technique. More precise host-guest structural information could be obtained by synchrotron diffraction and single-crystal X-ray diffraction [97, 98]. Combination of other in-situ spectroscopic techniques can give more insightful structural information [99], especially for non-periodic stmctural alterations. 

However, the host-guest interaction between carbon dioxide and MOFs are generally too weak to effectively capture the low-concentration carbon dioxide. Nevertheless, this problem can be very likely solved by introducing coordinatively unsaturated metal centers, amine groups, etc. on the pore surfaces. Of course, high adsorption enthalpy will always produce desorption problems, which may be solved by using multiple weak interactions [114, 115]. 

Host-Guest Reactions in Pillared-Layer MOFs. 133... 

Fig. 31 Radical polymerization on the MOF surfaces, (a) Molecular structure of the DVTP ligand, (b) Monomers used in the host-guest cross-polymerizations, (c) Cioss-iinkabie MOF [Cu(DVTpi(terephthalate)i (triethylenediamine)o.5] (l t shown in light blue) prepared in solid solution fashion, (d) Host accommodates vinyl monomers in its nanocavities, (e) Heat triggers radical cross-polymerization in the presence of AIBN initiator, yielding polymer nanocomposites, (f) Selective decomposition of the MOF matrix generates highly ordered cross-linked polymers. This figure is reproduced from [57] with permissirai...

These characteristic features allow the exploitation of luminescent MOFs as chemical sensors. The rational design of MOFs for specific sensing applications and the investigation of their luminescent response is an emerging research field. The purpose of this chapter is to present an overview of the recent development on host-guest chemistry for the preparation of luminescent MOFs (Fig. 1). 

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