Covalent Organic Frameworks as a Platform for Multidimensional Polymerization

Abstract

The simultaneous polymerization and crystallization of monomers featuring directional bonding designs provides covalent organic frameworks (COFs), which are periodic polymer networks with robust covalent bonds arranged in two- or three-dimensional topologies. The range of properties characterized in COFs has rapidly expanded to include those of interest for heterogeneous catalysis, energy storage and photovoltaic devices, and proton-conducting membranes. Yet many of these applications will require materials quality, morphological control, and synthetic efficiency exceeding the capabilities of contemporary synthetic methods. This level of control will emerge from an improved fundamental understanding of COF nucleation and growth processes. More powerful characterization of structure and defects, improved syntheses guided by mechanistic understanding, and accessing diverse isolated forms, ranging from single crystals to thin films to colloidal suspensions, remain important frontier problems.

Figure 1

COF topologies are set by the symmetries of their monomers. (A) A 2D square lattice derived from square planar and linear comonomers. The insets show a prospective view (top) and side view (bottom) that depict van der Waals stacking between layers. (B) A 3D diamondoid net derived from tetrahedral and linear comonomers. The inset shows the interpenetration of congruent networks.

Figure 2

Structure and composition of notable COFs and their composites: imine-linked electrocatalyst COF-366-Co (top left); imine-linked tartaric acid derived catalyst CCOF-1 functionalized with Ti(OiPr)₄ (top right); β-ketoenamine-linked redox-active DAAQ-TFP COF containing PEDOT within its pores (middle left); phenazine-linked, hole-conductive CS-COF functionalized with C₆₀ (middle right); β-ketoenamine-linked, proton-conductive Tp-Azo COF loaded with aqueous phosphoric acid (bottom left); and various derivatives of imine-linked TAPB-based COFs reported for catalysis (pyrrolidine derivative) and proton conduction (−OMe derivative) when loaded with nitrogen-containing heterocycles (bottom right).

Figure 3

Various interlayer stacking modes possible for 2D square lattice COFs when considering a 3-layer system. Adapted with permission from ref (62). Copyright 2016 Elsevier.

Figure 4

Solved 3D COF crystal structures. (A) The structure of an azodioxy-linked COF (left) solved from macroscopic single crystals (right). Adapted with permission from ref (63). Copyright 2013 Nature Publishing Group (NPG). (B) The structure of an imine-inked COF (left) solved by rotational electron diffraction of the crystallite (right) for which a representative diffraction pattern for a single rotation is shown. Adapted with permission from ref (64). Copyright 2013 American Chemical Society (ACS).

Figure 5

Mechanistic study of COF-5 synthesis where initially homogeneous solution (A) becomes turbid as COF-5 precipitates (B). (C) Turbidity measurements of COF formation in which, following an induction period, an initial rate is measured. (D) Reaction scheme for COF-5 formation in the presence of a catechol competitor. Adapted with permission from ref (71). Copyright 2014 ACS.

Figure 6

Contrasting mechanisms proposed for the crystallization of boronate ester-linked (top) and imine-linked (bottom) COFs.

Figure 7

A geometric monomer design strategy to facilitate COF crystallization. (A) The stacking of a propeller shaped monomer on a 2D COF layer and the steep, singular-welled potential energy surface that necessitates its eclipsed stacking. (B) Docking sites for fusing together of neighboring island domains nucleated on a layer of COF using this strategy. (C) Similar sites in 2D COFs may be left as vacancies or result in strain upon fusion due to incongruent interlayer stacking offsets as indicated by the yellow arrows. Adapted with permission from ref (75). Copyright 2016 NPG.

Figure 8

Oriented thin film formation of 2D COFs. (A) The solvothermal growth of 2D boronate ester-linked COFs on single layer graphene. Adapted with permission from ref (81). Copyright 2011 AAAS. (B) Change in Sauerbrey mass as boronate ester-linked COF thin film-grown either solvothermally (red) or from heated flow (blue). Adapted with permission from ref (86). Copyright 2016 ACS.

Figure 9

Colloidal COF systems. (A) Boronate ester-linked COFs form as colloids in the presence of nitrile cosolvents and were processed into free-standing films (inset). Adapted with permission from ref (89). Copyright 2017 ACS. (B) The formation of an imine COF shell on an Fe₃O₄ nanoparticle and demonstrations of the hybrid’s Tyndall effect and magnetism (C, D). Adapted with permission from ref (90). Copyright 2016 John Wiley & Sons, Inc.

Figure 10

(A) Formation of a boroxine-linked COF as a monolayer when confined to a surface and annealed in the presence of H₂O to yield large crystalline domains. (B) STM image of a BPDA COF monolayer grown by this method. (Inset is the Fourier transform of the STM image.) Adapted with permission from ref (107). Copyright 2011 The Royal Society of Chemistry.