The Influence of Disorder in the Synthesis, Characterization and Applications of a Modifiable Two-Dimensional Covalent Organic Framework

Computationally directed manipulation of cross-linked covalent organic frameworks for membrane applications

Two-dimensional covalent organic frameworks (2D-COFs) exhibit characteristics ideal for membrane applications, such as high stability, tunability and porosity along with well-ordered nanopores. However, one of the many challenges with fabricating these materials into membranes is that membrane wetting can result in layer swelling. This allows molecules that would be excluded based on pore size to flow around the layers of the COF, resulting in reduced separation. Cross-linking between these layers inhibits swelling to improve the selectivity of these membranes. In this work, computational models were generated for a quinoxaline-based COF cross-linked with oxalyl chloride (OC) and hexafluoroglutaryl chloride (HFG). Enthalpy of formation and cohesive energy calculations from these models show that formation of these COFs is thermodynamically favorable and the resulting materials are stable. The cross-linked COF with HFG was synthesized and characterized with Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), thermogravimetric analysis with differential scanning calorimetry (TGA-DSC), and water contact angles. Additionally, these frameworks were fabricated into membranes for permeance testing. The experimental data supports the presence of cross-linking and demonstrates that varying the amount of HFG used in the reaction does not change the amount of cross-linking present. Computational models indicate that varying the cross-linking concentration has a negligible effect on stability and less cross-linking still results in stable materials. This work sheds light on the nature of the cross-linking in these 2D-COFs and their application in membrane technologies.

Advanced Covalent Organic Framework-Based Membranes for Recovery of Ionic Resources

Membrane technology has shown a viable potential in conversion of liquid-waste or high-salt streams to fresh waters and resources. However, the non-adjustability pore size of traditional membranes limits the application of ion capture due to their low selectivity for target ions. Recently, covalent organic frameworks (COFs) have become a promising candidate for construction of advanced ion separation membranes for ion resource recovery due to their low density, large surface area, tunable channel structure, and tailored functionality. This tutorial review aims to analyze and summarize the progress in understanding ion capture mechanisms, preparation processes, and applications of COF-based membranes. First, the design principles for target ion selectivity are illustrated in terms of theoretical simulation of ions transport in COFs, and key properties for ion selectivity of COFs and COF-based membranes. Next, the fabrication methods of diverse COF-based membranes are classified into pure COF membranes, COF continuous membranes, and COF mixed matrix membranes. Finally, current applications of COF-based membranes are highlighted: desalination, extraction, removal of toxic metal ions, radionuclides and lithium, and acid recovery. This review presents promising approaches for design, preparation, and application of COF-based membranes in ion selectivity for recovery of ionic resources.

Two-Dimensional Polymers and Polymerizations

Synthetic chemists have developed robust methods to synthesize discrete molecules, linear and branched polymers, and disordered cross-linked networks. However, two-dimensional polymers (2DPs) prepared from designed monomers have been long missing from these capabilities, both as objects of chemical synthesis and in nature. Recently, new polymerization strategies and characterization methods have enabled the unambiguous realization of covalently linked macromolecular sheets. Here we review 2DPs and 2D polymerization methods. Three predominant 2D polymerization strategies have emerged to date, which produce 2DPs either as monolayers or multilayer assemblies. We discuss the fundamental understanding and scope of each of these approaches, including: the bond-forming reactions used, the synthetic diversity of 2DPs prepared, their multilayer stacking behaviors, nanoscale and mesoscale structures, and macroscale morphologies. Additionally, we describe the analytical tools currently available to characterize 2DPs in their various isolated forms. Finally, we review emergent 2DP properties and the potential applications of planar macromolecules. Throughout, we highlight achievements in 2D polymerization and identify opportunities for continued study.

Synthesis, Postsynthetic Modifications, and Applications of the First Quinoxaline-Based Covalent Organic Framework

We report a new synthetic protocol for preparing highly ordered two-dimensional nanoporous covalent organic frameworks (2D-COFs) based on a quinoxaline backbone. The quinoxaline framework represents a new type of COF that enables postsynthetic modification by placing two different chemical functionalities within the nanopores including layer-to-layer cross-linking. We also demonstrate that membranes fabricated using this new 2D-COF perform highly selective separations resulting in dramatic performance enhancement post cross-linking.

Electrochemical and Spectroelectrochemical Studies on the Reactivity of Perimidine-Carbazole-Thiophene Monomers towards the Formation of Multidimensional Macromolecules versus Stable π-Dimeric States

During research on cross-linked conducting polymers, double-functionalized monomers were synthesized. Two subunits potentially able to undergo oxidative coupling were used—perimidine and, respectively, carbazole, 3,6-di(hexylthiophene)carbazole or 3,6-di(decyloxythiophene)carbazole; alkyl and alkoxy chains as groups supporting molecular ordering and 14H-benzo[4,5]isoquinone[2,1-a]perimidin-14-one segment promoting CH⋯O interactions and π–π stacking. Electrochemical, spectroelectrochemical, and density functional theory (DFT) studies have shown that potential-controlled oxidation enables polarization of a specific monomer subunit, thus allowing for simultaneous coupling via perimidine and/or carbazole, but mainly leading to dimer formation. The reason for this was the considerable stability of the dicationic and tetracationic π-dimers over covalent bonding. In the case of perimidine-3,6-di(hexylthiophene)carbazole, the polymer was not obtained due to the steric hindrance of the alkyl substituents preventing the coupling of the monomer radical cations. The only linear π-conjugated polymer was obtained through di(decyloxythiophene)carbazole segment from perimidine-di(decyloxythiophene)-carbazole precursor. Due to the significant difference in potentials between subsequent oxidation states of monomer, it was impossible to polarize the entire molecule, so that both directions of coupling could be equally favored. Subsequent oxidation of this polymer to polarize the side perimidine groups did not allow further crosslinking, because rather the π–π interactions between these perimidine segments dominate in the solid product.