We synthesized a two-dimensional (2D) crystalline covalent organic framework (sp2c-COF) that was designed to be fully π-conjugated and constructed from all sp2 carbons by C C condensation reactions of tetrakis(4-formylphenyl)pyrene and 1,4-phenylenediacetonitrile. The C C linkages topologically connect pyrene knots at regular intervals into a 2D lattice with π conjugations extended along both x and y directions and develop an eclipsed layer framework rather than the more conventionally obtained disordered structures. The sp2c-COF is a semiconductor with a discrete band gap of 1.9 electron volts and can be chemically oxidized to enhance conductivity by 12 orders of magnitude. The generated radicals are confined on the pyrene knots, enabling the formation of a paramagnetic carbon structure with high spin density. The sp2 carbon framework induces ferromagnetic phase transition to develop spin-spin coherence and align spins unidirectionally across the material.
Unlike graphene and transition-metal dichalcogenides, two-dimensional transition-metal carbides (MXenes) have many surface sites that can be chemically modified. Etching of the aluminum layer of a parent MAX phase Ti3AlC2 layered material with hydrofluoric acid leads to the MXene Ti3C2 with various surface terminations. Molten salts can achieve uniform chloride terminations, but these are difficult to further modify. Kamysbayev et al. show that etching of MAX phases in molten cadmium bromide leads to bromide-terminated MXenes that can then be substituted with oxygen, sulfur, selenium, tellurium, and NH groups as well as with vacancy sites. The surface groups can alter electronic transport. For example, the Nb2C MXenes exhibit surface group–dependent superconductivity. Science , this issue p.  Versatile chemical transformations of surface functional groups in two-dimensional transition-metal carbides (MXenes) open up a previously unexplored design space for this broad class of functional materials. We introduce a general strategy to install and remove surface groups by performing substitution and elimination reactions in molten inorganic salts. Successful synthesis of MXenes with oxygen, imido, sulfur, chlorine, selenium, bromine, and tellurium surface terminations, as well as bare MXenes (no surface termination), was demonstrated. These MXenes show distinctive structural and electronic properties. For example, the surface groups control interatomic distances in the MXene lattice, and Ti n +1C n ( n = 1, 2) MXenes terminated with telluride (Te2−) ligands show a giant (>18%) in-plane lattice expansion compared with the unstrained titanium carbide lattice. The surface groups also control superconductivity of niobium carbide MXenes. : /lookup/doi/10.1126/science.aba8311
Covalent organic framework (COF) materials have been difficult to characterize structurally and to exploit because they tend to form powders or amorphous materials. Ma et al. studied a variety of three-dimensional COFs based on imine linkages (see the Perspective by Navarro). They found that the addition of aniline inhibited nucleation and allowed the growth of crystals large enough for single-crystal x-ray diffraction studies. Evans et al. describe a two-step process in which nanoscale seeds of boronate ester–linked two-dimensional COFs can be grown into micrometer-scale single crystals by using a solvent that suppresses the nucleation of additional nanoparticles. Transient absorption spectroscopy revealed superior charge transport in these crystallites compared with that observed in conventional powders.
Diamonds consist of an ordered array of tetrahedral carbon atoms, held together by strong covalent bonds. Despite this simple structure, making diamond crystals is difficult, requiring extremely high pressures and temperatures that can only be achieved deep in Earth's crust or through sophisticated experiments. Building covalent organic frameworks (COFs) similarly involves the ordered spatial arrangement, in a two- or three-dimensional crystalline network, of organic units linked together by strong covalent bonds. Growing single crystals of robust COFs is challenging because it requires the dynamic formation and cleavage of strong covalent bonds. On page 48 of this issue, Ma et al. (1) report a simple method for growing large, high-quality single crystals of COF networks held together by strong imine covalent bonds.
The need to use wavefunction or density functional theory (DFT) calculations to determine electron densities has been bypassed by a machine learning model. It will allow chemists to quickly determine properties that depend on the electron density of large systems such as van der Waals forces, halogen bonding and C-H–π interactions. These non-covalent interactions can hold insight into the binding of host–guest systems or favoured enantiomers within reaction pathways where intermediates and transition states may be stabilised by subtle attractions. The electron density distribution is one of the most powerful tools at the disposal of a computational chemist. From the electron density, properties such as charges, dipoles and electrostatic interaction energies can be determined.