Covalent Organic Frameworks: Chemical Approaches to Designer Structures and Built‐In Functions

Abstract

A new approach has been developed to design organic polymers using topology diagrams. This strategy enables covalent integration of organic units into ordered topologies and creates a new polymer form, that is, covalent organic frameworks. This is a breakthrough in chemistry because it sets a molecular platform for synthesizing polymers with predesignable primary and high‐order structures, which has been a central aim for over a century but unattainable with traditional design principles. This new field has its own features that are distinct from conventional polymers. This Review summarizes the fundamentals as well as major progress by focusing on the chemistry used to design structures, including the principles, synthetic strategies, and control methods. We scrutinize built‐in functions that are specific to the structures by revealing various interplays and mechanisms involved in the expression of function. We propose major fundamental issues to be addressed in chemistry as well as future directions from physics, materials, and application perspectives.

2D and 3D Porphyrinic Covalent Organic Frameworks: The Influence of Dimensionality on Functionality

“…To study the dimensionality effect, the major concern is how to rationally select the proper topology diagram and then design the functional precursors to construct the desired 2D and 3D COFs. However, unlike the well‐established 2D COFs, the construction of 3D COFs have been proved to be a big challenge, and only a few topology diagrams for 3D COFs has been reported until now . After careful consideration, we decided to choose the reported [ C 4 + C 2 ] diagram to synthesize 2D COFs, whereas for 3D COFs we used our reported topology strategy starting from tetrahedral ( T d ) and quadrilateral ( C 2 or C 4 ) building blocks .…”

“…However, unlike the well‐established 2D COFs, the construction of 3D COFs have been proved to be a big challenge, and only a few topology diagrams for 3D COFs has been reported until now . After careful consideration, we decided to choose the reported [ C 4 + C 2 ] diagram to synthesize 2D COFs, whereas for 3D COFs we used our reported topology strategy starting from tetrahedral ( T d ) and quadrilateral ( C 2 or C 4 ) building blocks . Since we aimed to construct porphyrinic COFs, we accordingly designed and synthesized the functional precursor, [5,10,15,20‐tetrakis(4‐benzaldehyde)porphyrin]palladium ( p ‐PdPor‐CHO), which can form 2D or 3D COFs by condensation with p ‐phenylenediamine (PPDA) or tetra( p ‐aminophenyl)methane (TAPM), respectively.…”

Combined Intrinsic and Extrinsic Proton Conduction in Robust Covalent Organic Frameworks for Hydrogen Fuel Cell Applications

“…pore size, shape and exposed functional groups) is difficult to achieve, thereby leading to a lack of thorough understanding of the structure–property relationships involved . In the past decade, covalent organic frameworks (COFs) have emerged as a new class of crystalline porous organic polymer materials and demonstrated great potential to overcome the limits of conventional organic polymers owing to their well‐defined structures, high surface areas, fine‐tunable pore environments, and custom‐design functionalities . These advantages render COFs a promising platform for proton conductivity and PEMs.…”

Confining H3PO4 network in covalent organic frameworks enables proton super flow

“…We sought to explore an anhydrous proton-conducting system as we anticipate synthetic channels with large pore size could become a freeway for proton super flow. We focus on porous molecular frameworks, specifically covalent organic frameworks (COFs) [11][12][13][14][15][16] as predesignable yet stable materials for proton transport. We reasoned that COFs could potentially achieve both stability and proton conductivity: (i) the framework stability can be assumed using strong covalent bond as linkage, while designing building units with a capability of triggering inter and intralayer interactions can further improve stability; 17,18 (ii) the stability of proton network loaded in channels can be realised by anchoring the network on pore walls, which can be preset with specific sites to enable interactions with the proton network; 6 (iii) the frameworks can be constructed via topology-guided growth of polygonal backbones to achieve aligned yet dense 1D channels together with a simultaneous manipulation of pore shape, size and connectivity; 11,12,15 (iv) using pure organic units with lightweight elements such as H, C and N offers the possibility to synthesise highly porous material that enables exceptional loading of proton network in the channels 11,13 .…”