Organic Molecules of Intrinsic Microporosity

Published online: 2020-01-23 20 Organic Materials McKeown Short Review Organic Molecules of Intrinsic Microporosity Neil B. McKeown*a a EaStCHEM School of Chemistry, University of Edinburgh, David Brewster Road, Edinburgh EH9 3FJ, United Kingdom [email protected] porous molecular crystals.6 A large proportion of molec- Received: 08.10.2019 ular crystals are based on cages7 or macrocycles, both of Accepted after revision: 20.11.2019 which act as prefabricated pores,8 but others are simply DOI: 10.1055/s-0039-3402512; Art ID: om-19-0016-rev organic molecules that pack inefficiently but with a 9 License terms: crystalline order. For all of these crystalline materials, the porosity is only revealed on the removal of the solvent © 2020. The Author(s). This is an open access article published by Thieme under the terms of the Creative Commons Attribution-NonDerivative-NonCommercial-License, of crystallization, a process often termed activation, permitting copying and reproduction so long as the original work is given appropriate which needs to occur without the structural collapse of credit. Contents may not be used for commercial purposes, or adapted, remixed, transformed or built upon. (https://creativecommons.org/licenses/by-nc-nd/4.0/). the crystal. Despite the understandable fascination with well-ordered porous materials—many of which have Abstract Organic molecules of intrinsic microporosity (OMIMs) are aesthetically appealing crystal structures—wholly amor- rigid molecules with an awkward shape that are designed to pack space phous materials can also be highly porous as demon- inefficiently in the solid state maximizing free volume and thereby stratedbythecommerciallyubiquitousactivated generating apparent microporosity as determined by gas adsorption. In this perspective article, the origin of the OMIM concept is explained and carbons. In parallel to the PCP/MOF/COF revolution, there the progress in its realization both by synthesis and packing simulation has also been increasing interest in making amorphous is reviewed. porous organic polymers (POPs), usually via the irrevers- ible formation of three or more bonds between molecular Key words Intrinsic microporosity, organic molecules, gas adsorption, packing simulations components to form a rigid network polymer. Some of these network POPs, such as hypercrosslinked polymers (HCPs)10 and network polymers of intrinsic microporosity (PIMs),11 can be highly porous with HCPs prepared from the Yamamoto coupling reaction of tetrakis(4- Introduction bromophenyl)methane, in particular, demonstrating porosity that rivals most MOFs and COFs.12 The porosity of Over the last two decades, there has been an intense most POPs is ensured by a three-dimensional (3D) global research effort to prepare porous crystalline network of covalent bonds so that they are stable but materials from organic molecular components to meet intractable solids and therefore share the difficulties of the requirements for improved catalysis, adsorption, processing from solution with conventional porous molecular storage, and separations.1 This research effort materials. In contrast, PIMs generate porosity from the has led to the discovery of a variety of materials known by inefficient amorphous packing of their rigid and con- an ever-increasing number of acronyms including porous torted macromolecular chains.13 Many PIMs do not have a coordination polymers (PCPs),2 metal–organic frame- 3D network structure and are therefore soluble in works (MOFs),3 covalent organic frameworks (COFs),4 common organic solvents. PIMs are prepared via step- and hydrogen-bonded organic frameworks.5 As their growth polymerizations based on the formation of names imply, these materials involve the formation of ladder-like benzodioxin13a or Tröger’sbaselinkages.14 coordination or covalent bonds between the molecular It was noted that oligomeric by-products that were components to ensure spatial separation and, hence, the removed during the purification of PIMs, by reprecipita- creation of an open-ordered framework. The bond- tion from a good solvent into a nonsolvent, showed formingreactionsarereversiblesothatstructuralerrors similar microporosity, via gas adsorption, to the desired can be corrected, which is necessary to obtain crystallin- PIM product, despite their relatively low molecular mass. ity. However, such a coordination or covalent framework This observation suggested the development of the is not a prerequisite for stable porosity within crystalline related concept of organic molecules of intrinsic micro- materialsasnowdemonstratedbymanyexamplesof porosity (OMIMs). Organic Materials 2020, 2,20–25 Georg Thieme Verlag KG, Rüdigerstraße 14, 70469 Stuttgart, Germany ! ~ 21 Organic Materials McKeown Short Review Biographical Sketch Neil B. McKeown, FRSC, FRSE,has sity (Canada) and the University of chemistry with particular emphasis occupied the Crawford Tercentenary Toronto, he obtained his first indepen- on the synthesis of nanoporous mo- Chair of Chemistry at The University of dentacademicpostat theUniversityof lecular crystals and polymers of intrin- Edinburgh since 2014. He received his Manchester in 1991, then he moved to sic microporosity (PIMs). For the latter, PhD in organic chemistry from the Cardiff University as a professor he was awarded the Beilby Medal University of East Anglia in 1987. After (2004–2014). His interests span (2008) and the Tilden Prize (2017) by postdoctoral research at York Univer- many aspects of organic materials the Royal Society of Chemistry. Theoretical Considerations Underpinning the between the geometric shapes restricts their motion.15 OMIM Concept Importantly, jamming can be correlated directly with the glass transition temperature (Tg) of a molecular material, at Organic molecules, on cooling from their melt or a which point concerted molecular motions cease. Using a saturated solution, tend to form solids in which they pack random jamming modeling technique, it was found that the space efficiently so as to minimize the amount of void space. maximum packing density of concave two-dimensional Although it has long been stated that “nature abhors a (2D) superdisks16 and 3D superballs17 decreases with vacuum” (horror vacui), the molecular imperative is to increasing concave faces, as space filling by mutual maximize attractive intermolecular interactions. For the interpenetration (or interdigitation) becomes more diffi- great majority of small organic molecules, the optimum cult. It follows that molecules with large concavities will packing efficiency is provided by a crystal structure, which pack highly inefficiently leading to microporosity, which is typically gives a packing density (Φ) in the range of 0.67–0.77, the conceptual basis for the OMIMs. Cages and macrocycles a value close to 0.74 obtained for an ordered array of close- represent an extreme example of a concavity whereby the packed spheres. However, organic molecules that possess internal space defined by the cage or ring is protected from “awkward” shapes and larger molecules, including many the interpenetration of other molecules.18 Most cage- and polymers, often crystallize slowly so that a solid amorphous macrocycle-based porous materials are crystalline in glass forms preferentially on cooling the melt. structure7; however, Cooper et al. showed that suitably Such molecular glasses are similar in structure to designed cages, provided by scrambled peripheral sub- noncrystalline polymers below their glass transition temper- stituents, produce amorphous materials with significant ature (e.g., atactic polystyrene or a PIM). Translational porosity as determined by gas adsorption.19 The porosity of molecular movement is frozen within a glass and so the cages as amorphous solids has also been investigated by molecules are kinetically trapped and are unable to rearrange packing simulations.20 themselves into the more thermodynamically stable crystal. Typically, the space efficiency for the packing of an organic molecule in a glass is around 5–10% less than that of a densely Molecules with Concavities packed crystal but, in most cases, this does not generate sufficient free volume to be considered microporous. The archetypal molecule with obvious concavities is However, if a molecule is designed to have a shape that is triptycene, for which Swager et al. introduced the concept of awkward to pack space, when it self-associates to form an internal free volume (IFV; Figure 1).21 This concept was amorphous solid it may trap sufficient free volume so that it exploited for the design of triptycene-based dyes that acts as a microporous material—i.e., it possesses intercon- orientate in liquid crystals, due to the rod-like molecules nected pores of less than 2 nm in diameter. filling the triptycene concavities, or for generating high- The most efficient arrangement for packing solid performance dielectricmaterials from enhancing freevolume geometric shapes into a defined space has fascinated and in various classes of polymers such as polyimides.22 MacLa- challenged mathematicians for centuries and in recent years chlan et al. also cited the concept of IFV in the design of rigid space-inefficient packing has also been considered by oligomeric triptycenes linked via metal-containing sal- theory. In recent years, Torquato and coworkers have linked phens23 and shape-persistent triptycene-based oligomers.24 the various mathematical “packing problems” with the This concept of creating extended molecular structures using behavior of real particles and

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