Abstract
AbstractMetal–organic frameworks (MOFs) and covalent organic frameworks (COFs) consist of molecular building blocks being stitched together by strong bonds. They are well known for their porosity, large surface area, and related properties. The electronic properties of most MOFs and COFs are the superposition of those of their constituting building blocks. If crystalline, however, solid‐state phenomena can be observed, such as electrical conductivity, substantial dispersion of electronic bands, broadened absorption bands, formation of excimer states, mobile charge carriers, and indirect band gaps. These effects emerge often by the proximity effect caused by van der Waals interactions between stacked aromatic building blocks. Herein, it is shown how functionality is imposed by this proximity effect, that is, by stacking aromatic molecules in such a way that extraordinary properties emerge in MOFs and COFs. After discussing the proximity effect in graphene‐related materials, its importance for layered COFs and MOFs is shown. For MOFs with well‐defined structure, the stacks of aromatic building blocks can be controlled via varying MOF topology, lattice constant, and by attaching steric control units. Finally, an overview of theoretical methods to predict and analyze these effects is given, before the layer‐by‐layer growth technique for well‐ordered surface‐mounted MOFs is summarized.
Highlights
Molecular framework materials, including metal–organic frameworks (MOFs),[1,2,3] coordination polymers,[4] and covalent organic frameworks (COFs),[5,6] provide an intriguing bridge between chemistry and solid-state physics. They are composed of molecular units that may carry the whole range of functional groups known to chemoften by the proximity effect caused by van der Waals interactions between stacked aromatic building blocks
The intrinsic exciton diffusion length should reach the micrometerregime, which would make this thin layer MOF one of the best exciton transport materials. This Progress Report shows that, in addition to molecular functional groups, undercoordinated metal sites, porosity, and large surface areas, a further possibility of property control can be incorporated into crystalline framework materials, such as COFs and MOFs: If aromatic molecules are placed in well-controlled stacks, the proximity effect gives raise to strong electronic effects
If the intermolecular distance between the basal planes of the aromatic molecules is in the range of the interlayer distance of graphene (≈3–3.5 Å), disperse electronic bands emerge, resulting in a ballistic charge carrier transport with appreciable mobilities
Summary
In order to understand the potential impact of stacking of aromatic moieties in molecular framework materials, it is useful to www.afm-journal.de. The difference between flat, conjugated bands, and dispersionless bands should be noted here: the former results from the symmetry of the lattice (e.g., kagome) with extended π conjugation, while the latter indicates localized, non-interacting states without electron conjugation This short summary of electronic properties of graphene, bilayer graphene, and graphite illustrates the importance of the proximity effect: Even though the graphene layers are subject to only weak van der Waals forces, which leave the atomistic structure essentially unaffected, the interlayer interaction has significant impact on the electronic structure, which is so strong that it can even cause the electronic phase transition from a zerogap semiconductor to a superconductor.[31] Similar stackings are present in layered COFs and in MOFs with suitable geometries. The impact of the proximity effect in these materials will be discussed in the chapters
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