Among the recent developments in metal-organic frameworks (MOFs), porous layered coordination polymers (CPs) have garnered attention due to their modular nature and tunable structures. These factors enable a number of properties and applications, including gas and guest sorption, storage and separation of gases and small molecules, catalysis, luminescence, sensing, magnetism, and energy storage and conversion. Among MOFs, two-dimensional (2D) compounds are also known as 2D CPs or 2D MOFs. Since the discovery of graphene in 2004, 2D materials have also been widely studied. Several 2D MOFs are suitable for exfoliation as ultrathin nanosheets similar to graphene and other 2D materials, making these layered structures useful and unique for various technological applications. Furthermore, these layered structures have fascinating topological networks and entanglements. This review provides an overview of different aspects of 2D MOF layered architectures such as topology, interpenetration, structural transformations, properties, and applications.

Multicomponent reactions (MCRs) can be used to introduce different functionalities into highly stable covalent organic frameworks (COFs). In this work, the irreversible three-component Doebner reaction is utilized to synthesize four chemically stable quinoline-4-carboxylic acid DMCR-COFs (DMCR-1−3 and DMCR-1NH) equipped with an acid−base bifunctionality. These DMCR-COFs show superior photocatalytic H 2 O 2 evolution (one of the most important industrial oxidants) compared to the imine COF analogue (Imine-1). This is achieved with sacrificial oxidants but also in pure water and under an oxygen or air atmosphere. Furthermore, the DMCR-COFs show high photostability, durability, and recyclability. MCR-COFs thus provide a viable materials' platform for solar to chemical energy conversion.

In this work, we have designed two new 3D metal–organic frameworks (MOFs), {Zn4(TPOM)­(1,4-NDC)4} n (1) and {Ni2(TPOM)­(1,4-NDC)2(H2O)2} n (2), utilizing both semi-rigid tetrapodal neutral linker, tetrakis­(4-pyridyloxymethylene)­methane (TPOM) and rigid ditopic anionic linker, 1,4-naphthalene dicarboxylic acid (H2(1,4-NDC)). On the basis of the single-crystal X-ray diffraction, 1 has a 3D structure with star-shaped pores arising from four-fold symmetry due to the presence of a paddle-wheel core [Zn2(O2CC12H6)4(C6H4N)2] as a subunit, whereas 2 consists of a zig-zag 3D framework with strong hydrogen bonding between the coordinated water molecules and coordinated carboxylate groups. Their thermogravimetric analysis indicates an extraordinary thermal stability: 1 up to 400 °C and 2 up to 350 °C. In addition to elemental microanalysis and spectroscopic characterization (UV–vis and infra-red spectroscopy), the bulk phase purity of 1 and 2 as well as hydrolytic stability of 1 are established by powder X-ray diffraction. Exploiting the luminescence nature of 1, both solvent-dependent fluorescence properties and sensing of various amines in aqueous medium are demonstrated. It exhibits good sensing ability toward 4-nitroaniline (4-NA) and 2,6-dichloro-4-nitroaniline (2,6-DCNA; a broad spectrum pesticide belonging to toxicity class III) with the lowest detection limit of 88 ppb and 0.28 ppm, respectively. The mechanism of action has been established through Stern–Volmer plots, time-resolved fluorescence studies, spectral overlap, and density functional theory calculations. The recyclability and stability of 1 after sensing experiments also reveal no change in its crystallinity. Furthermore, selectivity test and time-dependent detection for 4-NA have been successfully demonstrated. For practical applications, naked eye detection of 4-NA using test paper strips is also displayed.

A rational combination of an oxygen-rich pyridyl substituted tetrapodal ligand, tetrakis­(4-pyridyloxymethylene)­methane (TPOM), and a polar sulfone-functionalized conjugated bent dicarboxylate linker, dibenzothiophene-5,5′-dioxide-3,7-dicarboxylic acid (H2(3,7-DBTDC)), with d10 metal centers, Zn­(II) and Cd­(II), has led to the construction of two new three-dimensional (3D) metal–organic frameworks,{[Zn2(TPOM)­(3,7-DBTDC)2]·7H2O·DMA} n (1) and {[Cd2(TPOM)­(3,7-DBTDC)2]·6H2O·3DMF} n (2). Single-crystal X-ray analysis indicates that 1 is a 3D framework with a dinuclear repeating unit having two different Zn­(II) centers (tetrahedral and square pyramidal) and 2 is a 3D framework comprised of a dinuclear repeating unit with one crystallographically independent distorted pentagonal bipyramidal Cd­(II) coordinated to chelating/bridging carboxylates and nitrogen atoms of the TPOM ligand. In both cases, the pores are aligned with oxygen atoms of the TPOM ligand and decorated with polar sulfone moieties. On the basis of the stability established by thermogravimetric analysis and powder X-ray diffraction (PXRD) and the presence of large solvent accessible voids (25.4% for 1 and 40.6% for 2), gas sorption studies of different gases (N2, CO2, and CH4) and water vapor have been explored for both 1 and 2. The CO2 sorption isotherm depicts type I isotherm with an uptake of 93.6 cm3 g–1 (for 1) and 100.6 cm3 g–1 (for 2) at 195 K. Additionally, sorption of CO2 is highly selective over that of N2 and CH4 for both 1 and 2 due to the strong quadrupolar interactions between sulfone moieties and CO2 molecules. Configurational bias Monte Carlo (CBMC) molecular simulation has further justified the highly selective CO2 capture. On the other hand, the luminescence nature of 1 and 2 has been employed for highly selective detection of acetylacetone in aqueous methanol with a limit of 59 ppb in 1 and 66 ppb in 2, which are among the best reported values so far in the literature. The Stern–Volmer plots, spectral overlap, density functional theory calculations, CBMC simulation, and time-resolved lifetime measurements have been utilized for an extensive mechanistic study. The exclusive selectivity for acetylacetone in 1 and 2 have been confirmed by competitive selectivity test. Both exhibited good recyclability and stability after sensing experiments analyzed by fluorescence, PXRD, and field emission scanning electron microscopy studies.

Utilizing flexible bis(tridentate)polypyridyl ligands, the two new luminescent 2D metal organic frameworks {Zn(tpbn)(2,6-NDC)} (1) and {[Zn(tphn)(2,6-NDC)]·4HO} (2), where tpbn = N,N',N″,N‴-tetrakis(2-pyridylmethyl)-1,4-diaminobutane, tphn = N,N',N″,N‴-tetrakis(2-pyridylmethyl)-1,6-diaminohexane, and 2,6-HNDC = 2,6-naphthalenedicarboxylic acid, have been isolated in good yields under solvothermal conditions. Their solid-state molecular structures have been determined by single-crystal X-ray diffractometry. Both 1 and 2 have pentacoordinated Zn(II) centers with an NO environment from three nitrogen atoms of the tpbn or tphn ligand and two carboxylate oxygen atoms from two different 2,6-NDC linkers. However, the binding modes of the tridentate part of polypyridyl ligands to the Zn(II) center are different in 1 and 2-meridional (tpbn) vs facial (tphn) due to an increase (1.5 times) in the methylene chain length. Thus, the binding mode of 2,6-NDC to the Zn(II) center differs: bis(monodentate) syn-anti in 1 and bis(monodentate) syn-syn in 2. This difference in binding modes of the components has a profound effect on the conformation of the six-membered ring (metal centers are considered as the vertices in it) within the 2D framework: honeycomb vs chair form for 1 and 2, respectively. In addition to further characterization by elemental analysis and UV-vis and FT-IR spectroscopy, their framework stabilities in water and thermal properties have been studied by powder X-ray diffraction and thermogravimetric analysis, respectively. On the basis of thermodiffractometry, 1 and 2 retain their crystallinity and overall structure up to 350 and 325 °C, respectively. Their luminescent properties have been utilized to demonstrate sensing of various solvents as well as nitro-aromatic compounds in water, which correlate well with their structural differences. Through the spectral overlap, lifetime measurements, and nature of the Stern-Volmer plots, the fluorescence quenching pathway for the nitro-analytes, particularly 2,4,6-trinitrophenol (TNP), is established for 1 and 2. Their recyclability and stability after sensing experiments are found to be excellent.