Band Alignment as the Method for Modifying Electronic Structure of Metal-Organic Frameworks

External Electric Field Control of Exciton Motion in Porphyrin-Based Metal Organic Frameworks

Porphyrins are excellent light-harvesting complexes. Presently they are unsuitable for photovoltaic applications, as their excellent light absorbance is compensated to a large extent by their poor transport properties, where most excitons are lost by recombination. Arranging porphyrins in regular, strongly bound, lattices of surface-anchored metal-organic frameworks (PP-SURMOFs) may facilitate charge carrier dissociation, but does not significantly enhance the conductive properties. In most cases, photogenerated excitons traverse undirected, Brownian motion through a hopping process, resulting in a substantial diffusion length to reach electrodes, leading to significant exciton loss through recombination. Here, we propose to guide exciton diffusion indirectly by an external electric field. We show that electric fields, even as strong as 1 V nm-1, do not affect the HOMO-LUMO gap of the porphyrins. However, fields of 0.1 V nm-1 and even less demonstrate a notable Stark effect, with slight band gap reductions, for some PP-SURMOFs. When applied as an electric field gradient, for instance, via the substrate, it creates a unidirectional hopping pathway for the excitons. Consequently, we expect a significant reduction of exciton diffusion length leading to increased utilization of photogenerated excitons as they reach the electrodes. This strategy holds promise for integrating photoactive molecules in photovoltaic and photocatalytic applications.

Elucidating Structural Stability, Bandgap, and Photocatalytic Hydrogen Evolution of (H2O/DMF)@HKUST-1 Host-Guest Systems

The H2O@HKUST-1 and DMF@HKUST-1 systems were experimental and computationally assessed, employing XRD/TGA/FT-IR/DFT-calculations, evidencing that H2O or DMF coordinated to Cu, modulating HKUST-1 photocatalytic properties. DMF@HKUST-1 has narrower bandgap promoting higher-crystallinity and light-harvesting. H2O@HKUST-1 showed smaller particle sizing and sharp morphology. Theoretical models, (H2O)1@HKUST-1 and (DMF)1@HKUST-1, containing one coordinated molecule, elucidated bandgap modulation associated with infiltration. H2O@HKUST-1/DMF@HKUST-1 presented bandgaps [eV] of 3.6/3.4, by Tauc plots, and 3.55/3.26, by theoretical calculations, narrowing bandgap, compared with non-solvated HKUST-1(HKUST-1NS). Both composites raised the valence band (VB) and lowered the conduction band (CB), but DMF@HKUST-1 most raised VB. Topological analysis revealed that guests i) with higher electronic density, raised VB, and ii) induced π-backbonding, lowering CB. DMF@HKUST-1 presented a higher photocatalytic hydrogen evolution (μmol), 26.45, in the first 30 min of the reaction, nevertheless, H2O@HKUST-1 presented a competitive activity, of 17.32. In large periods, H2O@HKUST-1/DMF@HKUST-1 showed practically the same hydrogen evolution, 45.50/49.03.

Rapid, Selective Extraction of Silver from Complex Water Matrices with a Metal-Organic Framework/Oligomer Composite Constructed via Supercritical CO2

Every year vast quantities of silver are lost in various waste streams; this, combined with its limited, diminishing supply and rising demand, makes silver recovery of increasing importance. Thus, herein, we report a controllable, green process to produce a host of highly porous metal-organic framework (MOF)/oligomer composites using supercritical carbon dioxide (ScCO2 ) as a medium. One resulting composite, referred to as MIL-127/Poly-o-phenylenediamine (PoPD), has an excellent Ag+ adsorption capacity, removal efficiency (>99 %) and provides rapid Ag+ extraction in as little as 5 min from complex liquid matrices. Notably, the composite can also reduce sliver concentrations below the levels (<0.1 ppm) established by the United States Environmental Protection Agency. Using theoretical simulations, we find that there are spatially ordered polymeric units inside the MOF that promote the complexation of Ag+ over other common competing ions. Moreover, the oligomer is able to reduce silver to its metallic state, also providing antibacterial properties.

Ligand field tuning of d-orbital energies in metal-organic framework clusters

Linker functionalization is a common route used to affect the electronic and catalytic properties of metal-organic frameworks. By either pre- or post-synthetically installing linkages with differing linker moieties the band gap, workfunction, and exciton lifetimes have been shown to be affected. One overlooked aspect of linker functionalization, however, has been the impact on the metal d-orbital energies to which they are bound. The ligand field differences should result in substantial changes in d-splitting. In this study we use density functional theory (DFT) to study the energetics of d-orbital energy tuning as a function of linker chemistry. We offer a general descriptor, linker pK_a, as a tool to predict resultant band energies in metal-organic frameworks (MOFs). Our calculations reveal that simple functionalizations can affect the band energies, of primarily metal d lineage, by up to 2 eV and illustrate the significance of this band modularity using four archetypal MOFs: UiO-66, MIL-125, ZIF-8, and MOF-5. Together, we show that linker functionalization dramatically affects d-energies in MOF clusters and highlight that linker functionalization is a useful route for fine-tuning band edges centered on the metals, rather than linkers themselves.

A customized MOF-polymer composite for rapid gold extraction from water matrices

With the fast-growing accumulation of electronic waste and rising demand for rare metals, it is compelling to develop technologies that can promotionally recover targeted metals, like gold, from waste, a process referred to as urban mining. Thus, there is increasing interest in the design of materials to achieve rapid, selective gold capture while maintaining high adsorption capacity, especially in complex aqueous-based matrices. Here, a highly porous metal-organic framework (MOF)-polymer composite, BUT-33-poly(para-phenylenediamine) (PpPD), is assessed for gold extraction from several matrices including river water, seawater, and leaching solutions from CPUs. BUT-33-PpPD exhibits a record-breaking extraction rate, with high Au³⁺ removal efficiency (>99%) within seconds (less than 45 s), a competitive capacity (1600 mg/g), high selectivity, long-term stability, and recycling ability. Furthermore, the high porosity and redox adsorption mechanism were shown to be underlying reasons for the material's excellent performance. Given the accumulation of recovered metallic gold nanoparticles inside, the material was also efficiently applied as a catalyst.

Spin-Resolved Band Structure of Hoffman Clathrate [Fe(pz)2Pt(CN)4] as an Essential Tool to Predict Optical Spectra of Metal-Organic Frameworks

Paramount spin-crossover properties of the 3D-Hoffman metalorganic framework (MOF) [Fe(pz)₂Pt(CN)₄] are generally described on the basis of the ligand field theory, which provides adequate insight into theoretical and simulation analysis of spintronic complexes. However, the ligand field approximation does not take into account the 3D periodicity of the actual complex lattice and surface effects and therefore cannot predict a full-scale periodic structure without utilizing more advanced methods. Therefore, in this paper, the electronic properties of the exemplar MOF were analyzed from the band structure perspective in low-spin (LS) and high-spin (HS) states. The density-of-states spectra determined for both spin-up and spin-down electrons of Fe d⁶ orbitals indicate spin-orbital splitting and delocalization for HS due to spin polarization in the iron atom ligand field. Presence of the surface states in the real crystal causes a red shift of the metal-metal charge transfer (MMCT) and metal-ligand charge transfer (MLCT) peaks for both HS and LS states. The addition of residual water molecules and disorder among the pyrazine rings reveal additional influences on the positions of the pyrazine band and, therefore, on the absorption spectra of the crystal. The results show a magnification of the peak correlated with the MLCT in the HS state and a significant red shift of the LS characteristic absorption band. The presented approach involving band structure analysis delivers a more complete image of the electronic properties of the [Fe(pz)₂Pt(CN)₄] crystalline network and can be a landmark for insightful studies of other MOFs.

Molecular Regulation Based on Functional Trimetallic Metal-Organic Frameworks for Efficient Oxygen Evolution Reaction

Metal-organic frameworks (MOFs) as classic crystalline porous materials have attracted great interest in the catalytic field. However, how to realize molecular regulation of the MOF structure to achieve a remarkable oxygen evolution reaction (OER) electrocatalyst is still a challenge. Herein, we designed several series of special MOF materials to explore the relationship between the structure and properties as well as the related reactive mechanism. First, various metal centers, including Fe, Co, Ni, Zn, and Mg, were utilized to construct the first series of trimetallic MOF materials, namely, M₃-MOF-BDC, where BDC = 1,4-benzenedicarboxylic acid, also known as terephthalic acid. Among of them, Fe₃-MOF-BDC shows the best OER performance and only needs an overpotential of 312 mV at 10 mA cm^-2. Then, functional BDC-X ligands (X = NH₂, OH, NO₂, DH) with various characteristic groups were selected to construct a new series, namely, Fe₃-MOF-BDC-X, to further improve its OER electrocatalytic performance. As expected, Fe₃-MOF-BDC-NH₂ exhibited a greatly enhanced OER performance with ultralow Tafel slopes of 45 mV dec^-1 and overpotentials of 280 mV at 10 mA cm^-2 when the BDC-NH₂ ligand was adopted, even superior to commercial IrO₂ (323 mV) and most of the reported pristine MOFs as OER electrodes. Much higher structural stability was proven. The detailed structure-property relationship and mechanism are discussed. In a word, this work provides a very important theoretical basis for the design and exploration of new MOF electrocatalysts.

Nonadiabatic Molecular Dynamics with Extended Density Functional Tight-Binding: Application to Nanocrystals and Periodic Solids

In this work, we report a new methodology for nonadiabatic molecular dynamics calculations within the extended tight-binding (xTB) framework. We demonstrate the applicability of the developed approach to finite and periodic systems with thousands of atoms by modeling "hot" electron relaxation dynamics in silicon nanocrystals and electron-hole recombination in both a graphitic carbon nitride monolayer and a titanium-based metal-organic framework (MOF). This work reports the nonadiabatic dynamic simulations in the largest Si nanocrystals studied so far by the xTB framework, with diameters up to 3.5 nm. For silicon nanocrystals, we find a non-monotonic dependence of "hot" electron relaxation rates on the nanocrystal size, in agreement with available experimental reports. We rationalize this relationship by a combination of decreasing nonadiabatic couplings related to system size and the increase of available coherent transfer pathways in systems with higher densities of states. We emphasize the importance of proper treatment of coherences for obtaining such non-monotonic dependences. We characterize the electron-hole recombination dynamics in the graphitic carbon nitride monolayer and the Ti-containing MOF. We demonstrate the importance of spin-adaptation and proper sampling of surface hopping trajectories in modeling such processes. We also assess several trajectory surface hopping schemes and highlight their distinct qualitative behavior in modeling the excited-state dynamics in superexchange-like models depending on how they handle coherences between nearly parallel states.

Lewis acid-triggered photocatalytic hydrogen peroxide production in an aluminum-based metal–organic framework

Al-based MIL-101-NH2, which was previously regarded as having silent photo-features, exhibits photocatalytic H2O2 production via O2 reduction accompanied by efficient suppression of undesired H2O2 decomposition.

Efficient Computation of Nonadiabatic Coupling Coefficients for Modeling Charge Carrier Recombination in Extended Systems: The Case of Metal-Organic Frameworks

Modeling excited state charge carrier dynamics and recombination in extended systems, such as metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and other hybrid organic-inorganic materials, by surface-hopping approaches is a challenging task due to the high computational cost. In this work, the steps of the simulations and the bottlenecks for such systems are analyzed. In particular, the bottlenecks related to computation of the nonadiabatic coupling coefficients (NACs) are considered. A simple, inexpensive, and portable scheme for computing scalar NACs employing a grid representation of the wave functions is presented and implemented in a Python code. It is tested for the simulation of the electron-hole nonradiative recombination in the MIL-125-NH₂ model system. The proposed approach allows for an on-the-fly estimation of the NACs alongside the simulation of the molecular dynamics trajectory and enables a straightforward interface between the Python libraries for nonadiabatic molecular dynamics and the majority of the existing quantum chemical codes.

Fabrication of Zn-MOF/ZnO nanocomposites for room temperature H2S removal: Adsorption, regeneration, and mechanism

Zn-MOF/ZnO nanocomposites with different organic linkers were fabricated by a rapid ultrasonication method using freshly prepared Zn(OH)₂ precipitate. The high metal-to-ligand ratio led to the simultaneous formation of MOFs and ZnO nanoparticles in the MOFs. The surface area was in the range of 12-21 m² g^-1. The nanocomposites were tested for H₂S adsorption at room temperature, where the maximum adsorption capacity of 14.2 mg g^-1 was recorded for ZnBTC/ZnO in dry conditions. The spent adsorbents were regenerated using methanol and UV irradiation as individual and combined strategies. The successive effect of methanol and UV radiation led to an increased adsorption capacity in the second cycle. The spectroscopic investigation of spent ZnBDC/ZnO confirmed the chemisorption of H₂S over Zn-sites via Zn²⁺-S^2- interaction. The XPS analysis of regenerated ZnBDC/ZnO confirmed a decreased sulfur content and decreased Zn ionic character. The regeneration work in this study is one of the first attempts and could be extrapolated to well-studied Zn-MOFs like MOF-5 for the desulfurization process.

Designing MOF Nanoarchitectures for Electrochemical Water Splitting

Electrochemical water splitting has attracted significant attention as a key pathway for the development of renewable energy systems. Fabricating efficient electrocatalysts for these processes is intensely desired to reduce their overpotentials and facilitate practical applications. Recently, metal-organic framework (MOF) nanoarchitectures featuring ultrahigh surface areas, tunable nanostructures, and excellent porosities have emerged as promising materials for the development of highly active catalysts for electrochemical water splitting. Herein, the most pivotal advances in recent research on engineering MOF nanoarchitectures for efficient electrochemical water splitting are presented. First, the design of catalytic centers for MOF-based/derived electrocatalysts is summarized and compared from the aspects of chemical composition optimization and structural functionalization at the atomic and molecular levels. Subsequently, the fast-growing breakthroughs in catalytic activities, identification of highly active sites, and fundamental mechanisms are thoroughly discussed. Finally, a comprehensive commentary on the current primary challenges and future perspectives in water splitting and its commercialization for hydrogen production is provided. Hereby, new insights into the synthetic principles and electrocatalysis for designing MOF nanoarchitectures for the practical utilization of water splitting are offered, thus further promoting their future prosperity for a wide range of applications.

Effect of Ligand Functionalization on the Rate of Charge Carrier Recombination in Metal-Organic Frameworks: A Case Study of MIL-125

Ligand functionalization is a powerful approach for modifying the electronic structure of metal-organic frameworks when targeting the optimal electronic properties for photocatalysis and photovoltaics. However, its effect on the charge carrier lifetimes and recombination pathways remains unexplored. In this work, first-principles simulations, including nonadiabatic molecular dynamics, are performed for the representative TiO₂-based metal-organic framework systems MIL-125-X to unravel the impact of ligand functionalization on the nonradiative electron-hole recombination process, decoherence rates, and phonon modes giving the largest contribution to the nonradiative decay. Nonradiative recombination rates, simulated using the PBE0 density functional, are in excellent agreement with experiment. The ligand functionalization in MIL-125-X influences the recombination rates, unraveling the trend opposite to the evolution of the band gap and affecting the nonadiabatic coupling coefficients. Ligand modification impacts the phonon modes, which contribute most to the recombination process, altering the distribution between soft phonon modes and vibrational modes associated with specific structural motifs.

Chemisorption of hydrogen sulfide over copper-based metal–organic frameworks: methanol and UV-assisted regeneration

H₂S adsorption and regeneration of Cu-based MOFs.

A data-driven perspective on the colours of metal-organic frameworks

Colour is at the core of chemistry and has been fascinating humans since ancient times. It is also a key descriptor of optoelectronic properties of materials and is often used to assess the success of a synthesis. However, predicting the colour of a material based on its structure is challenging. In this work, we leverage subjective and categorical human assignments of colours to build a model that can predict the colour of compounds on a continuous scale. In the process of developing the model, we also uncover inadequacies in current reporting mechanisms. For example, we show that the majority of colour assignments are subject to perceptive spread that would not comply with common printing standards. To remedy this, we suggest and implement an alternative way of reporting colour—and chemical data in general. All data is captured in an objective, and standardised, form in an electronic lab notebook and subsequently automatically exported to a repository in open formats, from where it can be interactively explored by other researchers. We envision this to be key for a data-driven approach to chemical research.

Colour is at the core of chemistry and has been fascinating humans since ancient times.

Current status and future prospects of metal–organic frameworks at the interface of dye-sensitized solar cells

In this Frontier Article recent progresses and challenges at the interface of metal–organic frameworks and dye-sensitized solar cells are highlighted and discussed.