Internal Functionalization of Three‐Dimensional Covalent Organic Frameworks

Abstract

Covalent organic frameworks (COFs) represent an emerging class of porous crystalline materials composed of light elements, [1] typically C, N, O, and/or B, that crystallize into two-dimensional (2D) layered structures or three-dimensional (3D) networks. [2,3] The nearly eclipsed structures [4,5] of most 2D COFs give rise to high intrinsic charge mobilities [6] and their recent synthesis as oriented thin films [7] portends their use in optoelectronic and energy-storage devices. In contrast, few 3D COFs have been crystallized, and despite exhibiting exceptionally high surface areas (> 4000 m 2 g À1 ) and record low densities (0.17 g cm À3 ), these networks have no well-developed applications.[8] Functionalizing the interior of 3D COFs might harness these desirable properties to provide structurally precise platforms for catalysis, [9] separations, [10] and the storage and release of molecular payloads.[11] However, no functionalized 3D COFs have been reported, while the functionalization of 2D COFs has been limited to alkyl chains.[12] Postsynthetic functionalization of related metalorganic frameworks (MOFs) relies on incorporating reactive groups, [13] such as alkynes [14] or amines, [15] on the organic linkers, but these moieties are not readily incorporated onto symmetric, polyvalent 3D COF building blocks.Herein we report a general approach to functionalize 3D COFs using a new monomer-truncation strategy. A tetrahedral building block, which self-condenses to form the 3D network known as COF-102[2] (Scheme 1 a), was modified such that one of its four arylboronic acid moieties is replaced with an arbitrary functional group. The resulting trigonal tris(boronic acid) is co-condensed with the parent tetrahedral monomer to provide functionalized COF-102 (Scheme 1 b). The degree of functionalization is determined by the feed ratio of the two monomers and tolerates relatively high loadings of the truncated monomer (> 30 %), while the crystallinity, permanent porosity, and high surface area of the unfunctionalized material are maintained. This method also requires no modification of the solvothermal growth conditions used to crystallize COF-102. The truncated monomer is incorporated throughout the lattice, rather than on the crystallite surface, which might be unexpected given the reversible bond-forming conditions employed in COF synthesis.[16] However, growth conditions that produce crystalline materials are optimized empirically, and COF nucleation and growth processes are poorly understood. Our results indicate that boroxine hydrolysis is too slow to liberate truncated monomers from the COF-102 interior, thus their pendant functionality is distributed throughout the material.[17] Dodecyl-functionalized COF-102 (COF-102-C 12 ) was obtained by condensing mixtures of 1 and 2 under solvothermal conditions (mesitylene/1,4-dioxane 1:1 v/v, 90 8C, 24 h). Samples of COF-102-C 12 were isolated as microcrystalline powders by filtration and were activated under vacuum at 90 8C for 13 h. Fourier transform infrared (FTIR) spec...

Covalent organic frameworks (COFs): from design to applications

“…9,41,[48][49][50][51]65,71,72,[90][91][92][93][94][95][96][97][98] The dehydration reaction of boronic acid and diol would lead to the formation of five-membered C 2 O 2 B rings (boronate ester, reaction B in Fig. 2).…”

A novel benzimidazole-functionalized 2-D COF material: Synthesis and application as a selective solid-phase extractant for separation of uranium

“…COFs can be categorized into either two-dimensional (2D) or threedimensional (3D) ordered porous structure and be only composed of light elements (e.g. C, H, O, B, Si) through strong covalent bonds (CAC, CAO, CASi, BAO) [2,3], which make them possess many outstanding characteristics: (1) low mass densities, (2) high specific surface areas because of the porosity (the surface area of most COFs are larger than 1000 m 2 g À1 ), (3) flexible and adjustable structure in COFs molecular design caused by numerous building block combinations, (4) high thermal stabilities [4][5][6]. Based on the above advantages, especially regular, orderly, size-adjustable pore structure, as well as its high stability, COFs play a huge role in the fields of gas storage and separation, catalyst support, optical imaging, etc.…”

Chemically Delaminated Free‐Standing Ultrathin Covalent Organic Nanosheets

“…CONs are generally synthesized by exfoliating their bulk counterparts, such as covalent organic frameworks (COFs) [5] or covalent triazene frameworks (CTFs), [6] through mechanical delamination, [7] solvent-assisted exfoliation, [8] and self-exfoliation.…”