Guest-Induced Emergent Properties in Metal–Organic Frameworks

Abstract

Metal-organic frameworks (MOFs) are crystalline nanoporous materials comprised of organic electron donors linked to metal ions by strong coordination bonds. Applications such as gas storage and separations are currently receiving considerable attention, but if the unique properties of MOFs could be extended to electronics, magnetics, and photonics, the impact on material science would greatly increase. Recently, we obtained "emergent properties," such as electronic conductivity and energy transfer, by infiltrating MOF pores with "guest" molecules that interact with the framework electronic structure. In this Perspective, we define a path to emergent properties based on the Guest@MOF concept, using zinc-carboxylate and copper-paddlewheel MOFs for illustration. Energy transfer and light harvesting are discussed for zinc carboxylate frameworks infiltrated with triplet-scavenging organometallic compounds and thiophene- and fullerene-infiltrated MOF-177. In addition, we discuss the mechanism of charge transport in TCNQ-infiltrated HKUST-1, the first MOF with electrical conductivity approaching conducting organic polymers. These examples show that guest molecules in MOF pores should be considered not merely as impurities or analytes to be sensed but also as an important aspect of rational design.

Intrinsically conducting metal–organic frameworks

“…Through-bond mechanisms take advantage of covalent bonding between appropriately matched metals and ligands; through-space mechanisms rely on noncovalent interactions. While not covered in this article, it is also important to note that post-synthesis modulation (such as via redox-state changes 10 or intercalation of guest molecules 9 ) has also been shown to modulate the conductivities of some MOFs.…”

“…5 -10 In this article, we discuss the exciting research frontier of conducting MOFs, with an emphasis on experimental reports of systems that display intrinsic conductivity, wherein conductivity is an inherent property, rather than being induced by augmentations such as the exchange of guest molecules 9 or ionic conduction through the framework pores. We also provide a brief historical perspective of the fi eld, given that conductivity in the broader context of coordination polymers has been of interest for more than two decades.…”

Guest molecules as a design element for metal–organic frameworks

“…The role of other "bystander" guests, such as solvent molecules, is not understood, although we have evidence that these can affect the conductivity of TCNQ@HKUST-1. 5 Additionally, the range of guest-framework interaction energies is considerably wider than that typically seen in the storage of light gases by MOFs, in which weak dispersion forces dominate. As a result, conventional force-fi eld approaches to modeling Guest@MOF materials are unlikely to be useful.…”

“…This phenomenon represents a new radiation-detection mechanism that has advantages of simplicity and speed over conventional particle discrimination mechanisms that rely on analysis of the time signatures of the emitted radioluminescence. In addition to MOFs, 5 this approach can also be implemented in polymers. 40 …”

“…5 First, the MOF structure itself is, to fi rst order, determined by strong coordination bonds between the linker and metal ions. This strong bonding can be extended in some cases to a guest molecule, if unsaturated coordination sites at the metal nodes are present in the pores.…”