Physical Phenomena in Porous Frameworks

Abstract

RecommendationsP orous materials are characterized by a high internal surface area and significant pore volume.Natural examples, such as microporous zeolites, have been known to humanity since ancient times, but their systematic investigation only started in the 1930s and flourished with the discovery of synthetic zeolites and their critical role in modern catalysis, molecular sieving, and ion exchange.The broader family of framework compounds was later enriched with the development of coordination networks 1,2 and metal-organic frameworks (MOFs). 2 The latter are distinguished by their increased stability and permanent porosity. 3,4MOFs, along with their purely organic relatives, covalent organic frameworks (COFs) 5 and porous aromatic frameworks (PAFs), 6 are obtained by reticular chemistry, "the chemistry of linking molecular building blocks by strong bonds to make crystalline open frameworks". 7The vast structural complexity of molecular building blocks results in a plethora of crystal nets that describe these framework materials. 8Because the crystal structure significantly influences the physical properties (e.g., see ref 9 for a tutorial review on two-dimensional systems), targeting particular topologies can be used as a rational design element for new property-tailored materials.Traditional applications of framework materials take advantage of their porosity, for instance, in gas storage, molecular separation, and catalysis.However, reticular framework materials offer possibilities beyond these, which take advantage of the long-range order, the particular topology, crystallographic nets, and dimensionality of the extended structures.Indeed, framework materials can range from zerodimensional cages to one-dimensional chains or tubular networks, to two-and three-dimensional networks.Likewise, dimensionality can be thought of as structural or electronic, with structurally three-dimensional frameworks exhibiting exotic one-or two-dimensional electronic properties, for instance. 10Exotic, complex extended structures are often a prerequisite for exotic electronic structures, such as Dirac or Weyl points, van Hove singularities, and flat bands, which excite our fellow physicists.The combination of molecular functionality and crystalline order is beneficial for applications in light harvesting and optoelectronics.Concerted molecular flexibility can result in flexible framework materials that open and close upon external stimuli and which, hence, change the crystal properties dynamically.This special issue focuses on physical phenomena in framework materials that have emerged in recent years.A collection of 18 Accounts from experiment and theory cover MOFs, COFs, and PAFs, frameworks ranging from zero to three dimensions, as well as coordination polymer glasses.They feature mechanical flexibility, electrical conductivity,

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