Metal-Organic Coordination Network Thin Film by Surface-Induced Assembly

Influence of Humidity on Layer-by-Layer Growth and Structure in Coordination Networks

Metal-organic frameworks (MOFs) are promising materials because of their high designability of pores and functionalities. Especially, MOF thin films and their properties have been investigated toward applications in nanodevices. Typically, MOF thin films are fabricated by using a bottom-up method such as layer-by-layer (LbL) growth in air. Because the water molecules can coordinate and be replaced with organic linkers during synthesis, humidity conditions will be expected to influence the LbL growth processes. In this study, we fabricated MOF thin films composed of Zn2+, tetrakis-(4-carboxyphenyl)-porphyrin (TCPP), and 4,4'-bipyridyl (bpy) at 10 and 40% relative humidity (RH) conditions. Then, we investigated the humidity effects on chemical compositions of TCPP and bpy, periodic structure, orientation, and surface morphology. At high RH, coordination replacement of water with the organic linkers becomes more competitive than that at low RH, resulting in a different TCPP/bpy composition ratio between the two RH conditions. Also, more frequent coordination replacements of water with the organic linkers at high RH led to the formation of phases other than that observed at low RH, loss of growth orientation, and rough surface. The findings clarified the importance of controlling the RH condition during LbL growth to obtain the desired coordination networks.

Noncovalent Self-Assembly of Single-Layer MoS2 Nanosheets and Zinc Porphyrin into Stable SURMOF Nanohybrids with Multimodal Photocatalytic Properties

A noncovalent integration of nanosheets of molybdenum disulfide (MoS2) and the zinc porphyrin complex Zn(II) 5,10,15,20-tetrakis(4-carboxyphenyl)porphine (ZnTCPP) through coordination bonding with metal clusters of zinc acetate (Zn[OAc]2) was applied for synthesis of stable hybrid nanomaterial avoiding surface prefunctionalization. The X-ray powder diffraction in combination with the BET nitrogen adsorption method confirms formation of a ZnTCPP-based surface-attached metal-organic framework (SURMOF) with micropores of 1.63 nm on the MoS2 nanosheets. Fluorescence spectroscopy confirmed Forster resonance energy transfer (FRET) between MoS2 and ZnTCPP without contact quenching. Fluorescent trapping with terephthalic acid for hydroxyl radicals and Sensor Green for singlet oxygen was applied for studying the pathways of photodegradation of model organic pollutant 1,5-dihydroxynaphthalene (DHN) in the presence of SURMOF/MoS2. Visible light initiates sensitization through the excitation of ZnTCPP generating singlet oxygen, whereas UV-light promotes either aerobic FRET-mediated "Z scheme" or anaerobic "Type II heterojunction" mechanisms. Owing to its multimodal photochemistry, the SURMOF/MoS2 hybrid showed comparatively high photocatalytic activity in UV-assisted degradation of DHN (keffUV = 4.0 × 10-2 min-1) as well as the antibacterial activity confirmed by E. coli survival test under visible light. Noncovalent self-assembly utilizing coordination bonding in SURMOFs as supramolecular adhesive to avoid surface premodification provides a basis for new types of multicomponent nanosystems with switchable functionalities by combining different 2D materials and chromophores in one hybrid structure.

Functional supramolecular systems: design and applications

The interest in functional supramolecular systems for the design of innovative materials and technologies, able to fundamentally change the world, is growing at a high pace. The huge array of publications that appeared in recent years in the global literature calls for systematization of the structural trends inherent in the formation of these systems revealed at different molecular platforms and practically useful properties they exhibit. The attention is concentrated on the topics related to functional supramolecular systems that are actively explored in institutes and universities of Russia in the last 10–15 years, such as the chemistry of host–guest complexes, crystal engineering, self-assembly and self-organization in solutions and at interfaces, biomimetics and molecular machines and devices.

The bibliography includes 1714 references.

Docking MOF crystals on graphene support for highly selective electrocatalytic peroxide production

Tailoring the reaction kinetics is the central theme of designer electrocatalysts, which enables the selective conversion of abundant and inert atmospheric species into useful products. Here we show a supporting effect in tuning the electrocatalytic kinetics of oxygen reduction reaction (ORR) from four-electron to two-electron mechanism by docking metalloporphyrin-based metal-organic frameworks (MOFs) crystals on graphene support, leading to highly selective peroxide production with faradaic efficiency as high as 93.4%. A magic angle of 38.1° tilting for the co-facial alignment was uncovered by electron diffraction tomography, which is attributed to the maximization of π-π interaction for mitigating the lattice and symmetry mismatch between MOF and graphene. The facilitated electron migration and oxygen chemisorption could be ascribed to the supportive effect of graphene that disperses of the electron state of the active center, and ultimately regulates rate-determining step.

ELECTRONIC SUPPLEMENTARY MATERIAL

Supplementary material (synthesis protocols for control samples, morphological and structural characterizations, porosity, electrochemical properties and activities including SEM, TEM, XPS, Raman, AFM investigations) is available in the online version of this article at 10.1007/s12274-021-3382-3.

Facile Orientational Control of M2L2P SURMOFs on ⟨100⟩ Silicon Substrates and Growth Mechanism Insights for Defective MOFs

Layer-by-layer growth of Cu₂(bdc)₂(dabco) surface-mounted metal-organic frameworks (SURMOFs) was investigated on silicon wafers treated with different surface anchoring molecules. Well-oriented growth along the [100] and [001] directions could be achieved with simple protocols: growth along the [100] direction was achieved by substrate pretreatment with 80 °C piranha, while growth along the [001] direction was enabled by only rinsing silicon with absolute ethanol. Growth along the [001] direction produced more homogeneous SURMOF films. Optimization to enhance [001]-preferred orientation growth revealed that small changes in the SURMOF growth sequence (the number of rinse steps and linker concentrations) have a noticeable impact on the final film quality and the number of misaligned crystals. This new straightforward protocol was used to successfully grow other layer pillar-type SURMOFs, including the growth of Cu₂(bdc)₂(bipy) with simultaneous suppression of framework interpenetration.

Metal-salen molecular cages as efficient and recyclable heterogeneous catalysts for cycloaddition of CO2 with epoxides under ambient conditions

A salen based molecular cage, salen@cage, was synthesized and complexed with Co and Al to yield metal-salen molecular cages, Co(ii)@cage, Co(iii)@cage and Al(iii)@cage. These cages were demonstrated to be efficient heterogeneous catalysts for the cycloaddition of CO2 with styrene oxide, achieving full conversion at 25 °C and 1 atm CO2. Good to excellent yields of various cyclic carbonates were also achieved under mild conditions. Al(iii)@cage can be reused up to five times without any significant loss of its high catalytic activity. The capability to access a variety of heterogeneous organometallic catalysts with salen@cage offers new prospects for practical CO2 utilization and chemical manufacturing.

Graphitic carbon nitride/CoTPP type-II heterostructures with significantly enhanced photocatalytic hydrogen evolution

CoTPP inhibits the recombination of electron-hole pairs through extracting holes from g-C₃N₄ thus dramatically enhancing photocatalytic hydrogen production under visible light irradiation.

Understanding Self-Assembly of Porphyrin-Based SURMOFs: How Layered Minerals Can Be Useful

Porphyrin-based metal-organic frameworks on surfaces are a new class of planar materials with promising features for applications in chemical sensing, catalysis, and organic optoelectronics at nanoscale. Herein, we studied systematically a series of the SURMOFs assembled from variously meso-carboxyphenyl/pyridyl-substituted porphyrins and zinc acetate on template monolayers of graphene oxide via layer-by-layer deposition. This microscopically flat template can initiate the growth of macroscopically uniform SURMOF films exhibiting well-resolved X-ray diffraction. By applying the D'yakonov method, which has been previously used for the extraction of self-convolution of electron density in clay minerals, to the analysis of the experimental diffraction patterns of the SURMOFs, we determined the relation between the structure of porphyrin linkers and the geometry of packing motives in the films. We showed that the packing of the SURMOFs differs significantly from that of bulk powders of similar composition because of steric limitations imposed on the assembly in 2D space. The results of microscopic examination of the SURMOFs suggest that the type of metal-to-linker chemical bonding dictates the morphology of the films. Our method provides an enlightening picture of the interplay between supramolecular ordering and surface-directed assembly in porphyrin-based SURMOFs and is useful for rationalizing the fabrication of various classes of layered metal-organic frameworks on solids.

Development of Isostructural Porphyrin-Salen Chiral Metal-Organic Frameworks through Postsynthetic Metalation Based on Single-Crystal to Single-Crystal Transformation

The development of well-defined multimetallic porous metal-organic frameworks (MOFs) will add a new dimension to the application of MOF catalysis. From this perspective, the understanding and tailoring of the catalytic metal sites in MOFs are key fundamental challenges that could reveal the intrinsic potential of these materials. In this work, a series of porphyrin-salen chiral MOFs (ps-CMOFs 2-7) have been synthesized through postsynthetic metalation (PSMet) of the parent ps-CMOF via single-crystal to single-crystal transformation. Crystal structures of these ps-CMOF analogues revealed the same topological structure but varied metal entities compared to those of the parent framework. Note that the PSMet process involves three methods involving cation exchange at the nodes, cation exchange at the metalated porphyrin, and cation addition at the free porphyrin, which has been systematically investigated using single-crystal X-ray diffraction and other physicochemical methods. The N₂ adsorption tests, thermogravimetric analysis, and powder X-ray diffraction of 2-7 showed curves or patterns similar to those of 1, indicating the maintenance of the crystallinity, porosity, and thermal stability of the framework during the PSMet process. In addition, 2-7 showed distinctly improved adsorption capacities and isosteric heats of adsorption (Q_st) for CO₂ compared to those of their parent counterpart. Lastly, as a representative example of the ps-CMOF catalytic platform, 5 proved to be an efficient recyclable heterogeneous catalyst for the asymmetric addition reaction of CO₂ with epoxides under mild conditions. Furthermore, because of the constrained chiral environment within ps-CMOF, the enantioselectivity of this reaction appears to be dependent on substrate size.

Multilayer Growth of Porphyrin-Based Polyurea Thin Film Using Solution-Based Molecular Layer Deposition Technique

Controllable synthesis of organic thin film materials on solid surfaces is a challenging issue in the research field of surface science, as it is affected by several physical parameters. In this work, we demonstrated a solution-based molecular layer deposition (MLD) approach to prepare porphyrin-based covalent organic molecular networks on a 3-aminopropyl trimethoxysilane (APTMS) modified substrate surface using the urea coupling reaction between 1,4-phenylene diisocyanate (1,4-PDI) and 5,10,15,20-tetrakis-(4-aminophenyl) porphyrin (H₂TAPP) at room temperature (22 ± 2 °C). Multilayer growth was investigated under different relative humidity (RH) conditions of the reaction chamber. Sequential molecular growth at low relative humidity (≤10% RH) was observed using UV-vis absorption spectroscopy and atomic force microscopy (AFM). The high-RH condition shows limited film growth. Infrared spectroscopy (IR) and X-ray photoelectron spectroscopy (XPS) revealed the polyurea bond formation in sequential multilayer thin films, demonstrating that stepwise multilayer film growth was achieved using the urea coupling reaction.

Interactions on External MOF Surfaces: Desorption of Water and Ethanol from CuBDC Nanosheets

The external surfaces of metal-organic framework (MOF) materials are difficult to experimentally isolate due to the high porosities of these materials. MOF surface surrogates in the form of copper benzenedicarboxylate (CuBDC) nanosheets were synthesized using a bottom-up approach, and the surface interactions of water and ethanol were investigated by temperature-programmed desorption (TPD). A method of analysis of diffusion-influenced TPD was developed to measure the desorption properties of these porous materials. This approach also allows the extraction of diffusion coefficients from TPD data. The transmission Fourier transform infrared spectra, powder X-ray diffraction patterns, and TPD data indicate that water desorbs from CuBDC nanosheets with activation energies of 44 ± 2 kJ/mol at edge sites and 58 ± 1 kJ/mol at external surface and internal and pore sites. Ethanol desorbs with activation energies of 58 ± 1 kJ/mol at internal pore sites and 66 ± 0.4 kJ/mol at external surface sites. Co-adsorption of water and ethanol was also investigated. The presence of ethanol was found to inhibit the desorption of water, resulting in a water desorption process with an activation energy of 68 ± 0.7 kJ/mol.

Monolayer of Hydrazine Facilitates the Direct Covalent Attachment of C60 Fullerene to a Silicon Surface

The development of oxygen-free organic-inorganic interfaces has led to new schemes for the functionalization of silicon surfaces with nitrogen-based chemical groups. However, building layers of large structures directly on this functionalized surface has remained elusive. This work confirms the path to form a stable interface between silicon and buckminsterfullerene C₆₀ based on covalent chemical bonds. The starting point for this modification is the hydrazine-reacted Si(111) surface with the diamine functionality, which is further reacted directly with the C₆₀ molecules. The chemistry of this process is confirmed spectroscopically and microscopically and can be used to form organic-inorganic interfaces separated by a single layer of nitrogen.

Epitaxial Growth of Oriented Metalloporphyrin Network Thin Film for Improved Selectivity of Volatile Organic Compounds

This study reports an oriented and homogenous cobalt-metalloporphyrin network (PIZA-1) thin film prepared by liquid phase epitaxial (LPE) method. The thickness of the obtained thin films can be well controlled, and their photocurrent properties can also be tuned by LPE cycles or the introduction of conductive guest molecules (tetracyanoquinodimethane and C₆₀ ) into the PIZA-1 pores. The study of quartz crystal microbalance adsorption confirms that the PIZA-1 thin film with [110]-orientation presents much higher selectivity of benzene over toluene and p-xylene than that of the PIZA-1 powder with mixed orientations. These results reveal that the selective adsorption of volatile organic compounds highly depends on the growth orientations of porphyrin-based metal-organic framework thin films. Furthermore, the work will provide a new perspective for developing important semiconductive sensing materials with improved selectivity of guest compounds.

Layer-by-layer assembly of porphyrin-based metal–organic frameworks on solids decorated with graphene oxide

Planar MOFs grow on graphene oxide templates.

Recent advances in hybrid measurement methods based on atomic force microscopy and surface sensitive measurement techniques

This review summaries the recent progress of the combination of optical and non-optical surface sensitive techniques with the atomic force microscopy.