Organic Higher-Order Topological Insulators: Heterotriangulene-Based Covalent Organic Frameworks

Abstract

The ability to design and control the chemical characteristics of covalent organic frameworks (COFs) offers a new avenue for the development of functional materials, especially with respect to topological properties. Based on density functional theory calculations, by varying the core units through the choice of bridging groups [O, CO, CH2, or C­(CH3)2] and the linker units [acetylene, diacetylene, or benzene], we have designed heterotriangulene-based COFs that are predicted to be two-dimensional higher-order topological insulators (TIs). The higher-order TI characteristics of these COFs are identified via their topological invariants and the presence of in-gap topological corner modes and gapped edge states. The frontier molecular orbital energies of the building moieties play an important role in determining the size of the higher-order TI gap, which we find to be highly dependent on linker units. We also examined the deposition of the COFs on a boron nitride substrate to assess the feasibility of experimental observation of a higher-order TI phase in the organic layer. This work thus provides new insights into heterotriangulene-based COFs and guidance for the exploration of purely organic topological materials.

Magnetic Coupling Control in Triangulene Dimers

“…The electronic coupling strength between the fragment orbitals in the dimers can be quantified by the hopping or transfer integral (Table ). This can be approximated by the HOMO–LUMO gap of the dimer (Figure a): , E gap = 2 t In addition to the OSS ground state, the high-spin and closed-shell states are also required, as their energy difference indicates the strength of the electronic correlation in spin-dimer systems, which is related to the on-site Coulomb repulsion term, U , in the Hubbard model (Figure a) . In general, according to the Goodenough–Kanamori rule, , the magnetic coupling J is dominated by both potential exchange and kinetic exchange, as formulated by J = K − 4 t 2 / U …”

Combination of covalent organic frameworks (COFs) and polyoxometalates (POMs): the preparation strategy and potential application of COF–POM hybrids

“…26,27 In recent studies, scientists have thoroughly investigated the topological semimetal and higher-order topological insulator properties of COFs through quantum chemical calculations, which are important for a deeper understanding of COFs and for the design and development of their properties. 28–30 In order to expand the usage of COFs and improve their performance, they are also mixed with other materials, such as graphene, 31 carbon nanotubes, 32 metal oxides, 33 metal nanoparticles, 34 conductive polymers, 35 MOFs (metal–organic frameworks), 36 MXenes 37 and so on. Recently, POMs were introduced into COFs to construct COF–POM hybrids.…”

Recent advancements of covalent organic frameworks (COFs) as proton conductors under anhydrous conditions for fuel cell applications

“…In general, COFs are considered as electrical insulators. 27 So, COFs are doped with proton sources like H 3 PO 4 , H 2 SO 4 , ionic liquids, etc. to ensure the availability and efficient conduction of protons in their skeletons.…”