2D Conjugated Metal–Organic Frameworks: Defined Synthesis and Tailor-Made Functions

Abstract

Conspectus 2D conjugated metal–organic frameworks (2D -MOFs) have emerged as a class of graphene-like materials with fully π-conjugated aromatic structures. Their unique structural characteristics provide abundant physiochemical features, including regular nanochannels, high electrical conductivity, and customizable band gaps. Recent intensive research has significantly advanced this field, yet the exploration of 2D c-MOFs with enhanced features is limited by the availability of organic linkages and topologies. Designing novel ligands is essential for the construction of new 2D c-MOFs with high crystallinity, excellent conductivity, and tailor-made functions. In this Account, we summarize our recent contributions in fine-tuning the topology of 2D c-MOFs through precise ligand design, thereby giving them fantastic structures and tailor-made functions. First, we propose the concept of replacing planar ligands by nonplanar ligands on conductive MOF skeletons. The incorporation of nonplanar ligands improves the solubility of large π-conjugated organic molecules without interfering with the interlayer π-stacking. Our investigation discovered that conjugate polycyclic aromatics-based ligands can be synthesized through in situ Scholl reactions by means of oxidative cyclodehydrogenation of a nonplanar precursor ligand during the solvothermal synthesis process. Hence, fully conjugated 2D c-MOFs can be directly synthesized from nonplanar organic ligands, simplifying and diversifying the preparation of 2D c-MOFs. Accordingly, the design flexibility of the ligands expands the topological structures and pore types. By controlling the synthesis conditions, we can successfully induce either a rhombus or a kagome topology from a nonplanar D 2 symmetric ligand. Moreover, by employing a ligand engineering strategy, we incrementally increase the number of coordination functional groups on a twisted hexabenzocoronene core, resulting in the formation of three distinct symmetric hydroxyl ligands. These ligands elicit diverse target topologies and pore sizes, resulting in variances in the coordination node density on the skeletons. This, in turn, leads to differences in electron transfer abilities, ultimately causing variations in the electrical conductivity and mobility. In addition, we employ a straightforward coupling method to incorporate redox components, such as salphen and pyrazine, into nonplanar ligands, facilitating the synthesis of 2D c-MOFs with highly active centers. This strategy confers upon the resulting frameworks substantial capacity for catalysis and energy storage, offering a good platform for elucidating the structure–property relationships at the molecular level. Moreover, the well-defined synthesis of 2D c-MOFs imparts them with specific properties, particularly in the fields of electrical, electrochemical, and spintronic applications. At the end, the primary challenges facing 2D c-MOFs in achieving tailor functions and their practical applications are proposed. This account is expected to evoke new inspirations ...

Advancing Metal–Organic Framework Designs for Room-Temperature Chemiresistive Gas Sensors

“…They are built by assembling metal ions and strategically designed organic ligands into the layered structure that facilitates electron transport (Figure ). This design provides 2D conductive MOFs with unique electrical properties and a wide range of potential applications. , Moreover, in contrast to SMO-based sensors, which depend on chemisorbed oxygen species requiring high temperatures for activation, 2D conductive MOFs interact directly with gas molecules through their abundant adsorption sites and smaller bandgap. , Thus, unlike conventional chemiresistive materials, a conductive MOF can exhibit a sufficient response even at RT and in low-gas-concentration environments. , …”

“…Beyond the ongoing design and synthesis of novel 2D conductive MOFs, an alternative approach leverages the potential of established 2D conductive MOFs as room-temperature chemiresistors. This can be achieved by combining them with M-NPs or metal oxides, aiming to enhance the sensing performance, stability, and selectivity of the resulting sensors . Moreover, the composite preparation process can directly confront macroscopic organization challenges in 2D MOFs by using preordered substrates such as metal oxides to enhance sensor gas transfer efficiency.…”

Fabricating Large-Area Thin Films of 2D Conductive Metal–Organic Frameworks

“…Therefore, securing the interface between these layers is crucial to making large-area thin films suitable for this purpose. Meanwhile, the NM electronic structure and high electrical conductivity, along with the potential for long-range spin ordering in 2D MOFs and their composition of light elements resulting in minimal spin–orbit coupling effects, suggest their potential to serve as NM layers. , Research conducted by Chen et al utilized 2D MOF thin films to explore spin valve device behavior, achieving a magnetoresistance of −25% at 10 K in an organic spin valve (OSV) utilizing the Cu 3 (HHTP) 2 thin film as the NM layer, with the negative MR effect persisting to up to 200 K (Figure e, f) . This research highlights the potential of 2D MOF thin films in spintronics applications …”

Rational Construction of Two-Dimensional Conjugated Metal–Organic Frameworks (2D c-MOFs) for Electronics and Beyond

“…Two-dimensional conjugated metal–organic frameworks (2D c-MOFs) are an emerging class of electrically conductive organic 2D crystals with efficient in-plane conjugation and strong interlayer coupling that has recently garnered increasing attention and shown promise for various applications. − 2D c-MOFs share structural similarities with graphite, consisting of layered structures with in-plane π-extended conjugation and out-of-plane π-orbital overlap, facilitating efficient charge transport within the 2D plane and along the π-stacking direction, as well as mass transport through ordered and aligned pores. , These unique characteristics endow 2D c-MOFs with significantly enhanced electrical conductivities compared to conventional MOFs. Consequently, over the past decade, significant applications have arisen from the conductivity and porosity of 2D c-MOFs, including catalysis, energy storage, chemiresistive sensors, , optoelectronic devices, and more.…”