Nanoscale Metal–Organic Frameworks and Metal–Organic Layers with Two-Photon-Excited Fluorescence

Postsynthetic Modification of Metal-Organic Layers

ConspectusMetal-organic layers (MOLs), as a subclass of two-dimensional (2D) metal-organic frameworks (MOFs), have gained prominence in materials science by combining the structural versatility of MOFs with the unique physical and chemical properties of 2D materials. MOLs consist of metal oxide clusters connected by organic ligands, forming periodically extended 2D architectures with tunable properties and large surface areas. These characteristics endow MOLs with significant potential for applications in catalysis, sensing, energy storage, and biomedicine.The synthesis of MOLs predominantly follows two key pathways: top-down exfoliation of bulk layered MOFs and bottom-up assembly from molecular building units. The exfoliation method allows for the isolation of ultrathin MOL sheets from bulk precursors, but scalability and structural defects present ongoing challenges. In contrast, the bottom-up assembly offers more precise control over structural design, enabling the formation of MOLs with tailored chemical functionalities and morphologies. By carefully selecting linkers and synthetic conditions, researchers have successfully constructed MOLs with diverse geometric configurations including linear, triangular, and rectangular ligand motifs. Nevertheless, achieving consistent monolayer formation and controlling lateral dimensions remain critical challenges for the widespread application of these materials.A defining advantage of MOLs is their exceptional amenability to postsynthetic modification (PSM). PSM strategies enable fine-tuning of MOL properties and the introduction of novel functionalities without compromising the integrity of the underlying framework. Four principal approaches to PSM have been established: (1) linker modification, where additional coordination sites facilitate selective metalation or functional group incorporation; (2) secondary building unit (SBU) modification, using replaceable sites perpendicular to the MOL plane for targeted functionalization; (3) dual modification, integrating linker and SBU functionalization to achieve complex multifunctional platforms; and (4) multilevel assembly, incorporating MOLs into larger hierarchical architectures such as biomimetic systems and composite materials.These versatile modification strategies have unlocked novel applications of MOLs, including single-site catalysis, photocatalysis, and artificial photosynthetic systems. For instance, MOLs functionalized with transition metal complexes have more accessible reactive sites than conventional MOFs for faster substrate transport. Additionally, MOLs interfaced with biomimetic systems, such as liposomes and proteins, have demonstrated significant promise in photochemical energy conversion and selective oxidation processes.Despite these advancements, several key obstacles persist. Achieving uniform monolayer thickness while preventing multilayer aggregation remains a formidable task, necessitating deeper insights into the thermodynamic and kinetic factors governing MOL growth. Furthermore, the behavior of MOLs during drying, adsorption, and structural modification often deviates from classical models, suggesting the involvement of complex interfacial phenomena that warrant further investigation. Addressing these challenges will be crucial for harnessing the full potential of MOLs in next-generation functional materials.In summary, MOLs represent a versatile and dynamic class of materials that offer opportunities for innovation across diverse scientific disciplines. By advancing synthetic methodologies and deepening our understanding of postsynthetic modification strategies, researchers can continue to expand the functional landscape of MOLs, paving the way for transformative applications in catalysis, energy conversion, and beyond.

Multiphoton Excited Fluorescence Imaging over Metal-Organic Frameworks

Multiphoton excited fluorescence (MPEF) imaging has emerged as a powerful tool for visualizing biological processes with high spatial and temporal resolution. Metal-organic frameworks (MOFs), a class of porous materials composed of metal ions or clusters coordinated with organic ligands, have recently gained attention for their unique optical properties and potential applications in MPEF imaging. This review provides a comprehensive overview of the design, synthesis, and applications of multiphoton excited fluorescence imaging using MOFs. We discuss the principles behind the fluorescence behavior of MOFs, explore strategies to enhance their photophysical properties, and showcase their applications in bioimaging. Additionally, we address the current challenges and future prospects in this rapidly evolving field, highlighting the potential of multiphoton excited fluorescence imaging by MOFs for advancing our understanding of complex biological processes.

Preparation of a Nanosheeted Uranyl-Organic Framework for Enhanced Photocatalytic Oxidation of Toluene

Preparation of ultrathin metal-organic framework (MOF) nanosheets is an effective way to improve the catalytic efficiency of MOF photocatalysts owing to their superiority in reducing the recombination rate of photogenerated electrons and holes and enhancing charge transfer. Herein, a light-sensitive two-dimensional uranyl-organic framework named HNU-68 was synthesized. Due to its interlayer stacking structure, the corresponding ultrathin nanosheets with a thickness of 4.4 nm (HNU-68-N) can be obtained through ultrasonic exfoliation. HNU-68-N exhibited an enhanced ability to selectively oxidize toluene to benzaldehyde, with the value of turnover frequency being approximately three times higher than that of the bulk HNU-68. This enhancement is attributed to the smaller size and interface resistance of the layered HNU-68-N nanosheets, which facilitate more thorough substrate contact and faster charge transfer, leading to an improvement in the photocatalytic efficiency. This work provides a potential candidate for the application of ultrathin uranyl-based nanosheets.

Two anthracene-based zirconium metal-organic frameworks with fcu and hcp topologies as versatile fluorescent sensors for detection of inorganic ions and nitroaromatics

Two anthracene-based zirconium metal-organic frameworks (UiO-68-AN-fcu and UiO-68-AN-hcp) with blue emission were synthesized by the solvothermal reaction of ZrCl₄ with anthracene-based ligand 4,4'-(9,10-anthracenediyl)dibenzoic acid. The two MOFs inherited the luminescence properties of anthracene-based ligand and exhibited different topologies due to the change of connection mode of Zr-O clusters. The two stable anthracene-based zirconium MOFs served as luminescent sensors for selectively detecting 2-nitrophenol, Fe³⁺ and Cr₂O₇^2-. UiO-68-AN-hcp with flower morphology exhibited stronger quenching effect for 2-nitrophenol, Fe³⁺ and Cr₂O₇^2- by comparing to UiO-68-AN-fcu. Adsorption tests, fluorescence lifetime and spectroscopy studies demonstrated that the fluorescence responses of MOFs for analytes can be primarily attributed to the dynamic quenching mechanism involving energy and electron transfer. These results revealed that the combination of luminescent anthracene-based ligand and Zr-O clusters is a feasible strategy to construst MOFs-based fluorescent sensors.

Preparation of Eu0.075Tb0.925-Metal Organic Framework as a Fluorescent Probe and Application in the Detection of Fe3+ and Cr2O72

Luminescent Ln-MOFs (Eu0.075Tb0.925-MOF) were successfully synthesised through the solvothermal reaction of Tb(NO3)3·6H2O, Eu(NO3)3·6H2O, and the ligand pyromellitic acid. The product was characterised by X-ray diffraction (XRD), TG analysis, EM, X-ray photoelectron spectroscopy (XPS), and luminescence properties, and results show that the synthesised material Eu0.075Tb0.925-MOF has a selective ratio-based fluorescence response to Fe3+ or Cr2O72-. On the basis of the internal filtering effect, the fluorescence detection experiment shows that as the concentration of Fe3+ or Cr2O72- increases, the intensity of the characteristic emission peak at 544 nm of Tb3+ decreases, and the intensity of the characteristic emission peak at 653 nm of Eu3+ increases in Eu0.075Tb0.925-MOF. The fluorescence intensity ratio (I653/I544) has a good linear relationship with the target concentration. The detection linear range for Fe3+ or Cr2O72- is 10-100 μM/L, and the detection limits are 2.71 × 10-7 and 8.72 × 10-7 M, respectively. Compared with the sensor material with a single fluorescence emission, the synthesised material has a higher anti-interference ability. The synthesised Eu0.075Tb0.925-MOF can be used as a highly selective and recyclable sensing material for Fe3+ or Cr2O72-. This material should be an excellent candidate for multifunctional sensors.

Snapshots of Postsynthetic Modification in a Layered Metal-Organic Framework: Isometric Linker Exchange and Adaptive Linker Installation

Terminal ligand exchange and framework linker exchange have been frequently practiced as powerful tools to functionalize reticular structures such as metal-organic frameworks (MOFs). Herein, we report the postsynthetic modification (PSM) of a 6-connected layered MOF (hxl topology) to achieve a 12-connected fcu framework. In the PSM process, isometric linker exchange in the layers and linker installation between adjacent layers by the substitution of modulating ligands happen simultaneously. Snapshots of PSM at different time points reveal that the hxl domain is adaptively reorganized to create sites for new linker installation, and gradually the fcu domain dominates the crystal. Detailed kinetic analysis suggests that, although adaptive linker installation requires interlayer expansion of stackings in situ, it is kinetically faster than isometric linker exchange in the layers.

Metal-Organic Frameworks Based on Group 3 and 4 Metals

Metal-organic frameworks (MOFs) based on group 3 and 4 metals are considered as the most promising MOFs for varying practical applications including water adsorption, carbon conversion, and biomedical applications. The relatively strong coordination bonds and versatile coordination modes within these MOFs endow the framework with high chemical stability, diverse structures and topologies, and interesting properties and functions. Herein, the significant progress made on this series of MOFs since 2018 is summarized and an update on the current status and future trends on the structural design of robust MOFs with high connectivity is provided. Cluster chemistry involving Y, lanthanides (Ln, from La to Lu), actinides (An, from Ac to Lr), Ti, and Zr is initially introduced. This is followed by a review of recently developed MOFs based on group 3 and 4 metals with their structures discussed based on the types of inorganic or organic building blocks. The novel properties and arising applications of these MOFs in catalysis, adsorption and separation, delivery, and sensing are highlighted. Overall, this review is expected to provide a timely summary on MOFs based on group 3 and 4 metals, which shall guide the future discovery and development of stable and functional MOFs for practical applications.