Proximity Effect in Crystalline Framework Materials: Stacking‐Induced Functionality in MOFs and COFs

Abstract

Metal-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.

Topological two-dimensional polymers

“…We have covered this subject in a recent perspective article, to which we refer the interested reader. 55…”

Carbon Dots Based Nanoscale Covalent Organic Frameworks for Photodynamic Therapy

“…[9][10][11] Most of the reported COFs are made from aromatic molecules with several functional groups, such as amino, aldehyde, or boric acid. [12][13][14][15][16][17][18][19][20][21][22] Actually, some molecular clusters or aggregates could also be excitation. The quantum yield of CDs is 2%.…”

“…[ 9–11 ] Most of the reported COFs are made from aromatic molecules with several functional groups, such as amino, aldehyde, or boric acid. [ 12–22 ] Actually, some molecular clusters or aggregates could also be used for constructing the porous materials. And this idea has been documented in making MOFs.…”

Acid-triggered interlayer sliding of two-dimensional copper(i)–organic frameworks: more metal sites for catalysis

“…1-11 The 3D structures of these framework materials may be constructed from 2D monolayers or sheets, stacking each other by non-covalent interactions (e.g., π-π and van der Waals interactions) to form porous channels orthogonal to the planes. The alteration of interlayer interactions and stacking can greatly affect not only structure such as crystallinity and porosity, 12 but also the electronic and optic properties including interlayer electron or charge transport, and excitation states, 13 which has been extensively demonstrated by graphene 14, 15 and transitionmetal dichalcogenides (TMDs). 16,17 In contrast, few examples and strategies are explored to control the interlayer interactions and realize the interlayer sliding of 2D COFs, such as incorporation of complementary donor-acceptor interactions, solvation and Scheme 1.…”