Titanium Hydroxide Secondary Building Units in Metal–Organic Frameworks Catalyze Hydrogen Evolution under Visible Light

Metalation of metal-organic frameworks: fundamentals and applications

Metalation of metal-organic frameworks (MOFs) has been developed as a prominent strategy for materials functionalization for pore chemistry modulation and property optimization. By introducing exotic metal ions/complexes/nanoparticles onto/into the parent framework, many metallized MOFs have exhibited significantly improved performance in a wide range of applications. In this review, we focus on the research progress in the metalation of metal-organic frameworks during the last five years, spanning the design principles, synthetic strategies, and potential applications. Based on the crystal engineering principles, a minor change in the MOF composition through metalation would lead to leveraged variation of properties. This review starts from the general strategies established for the incorporation of metal species within MOFs, followed by the design principles to graft the desired functionality while maintaining the porosity of frameworks. Facile metalation has contributed a great number of bespoke materials with excellent performance, and we summarize their applications in gas adsorption and separation, heterogeneous catalysis, detection and sensing, and energy storage and conversion. The underlying mechanisms are also investigated by state-of-the-art techniques and analyzed for gaining insight into the structure-property relationships, which would in turn facilitate the further development of design principles. Finally, the current challenges and opportunities in MOF metalation have been discussed, and the promising future directions for customizing the next-generation advanced materials have been outlined as well.

Selective Reduction of Nitro Compounds by Organosilanes Catalyzed by a Zirconium Metal-Organic Framework Supported Salicylaldimine-Cobalt(II) Complex

Reducing nitro compounds to amines is a fundamental reaction in producing valuable chemicals in industry. Herein, the synthesis and characterization of a zirconium metal-organic framework-supported salicylaldimine-cobalt(II) chloride (salim-UiO-CoCl) and its application in catalytic reduction of nitro compounds are reported. Salim-UiO-Co displayed excellent catalytic activity in chemoselective reduction of aromatic and aliphatic nitro compounds to the corresponding amines in the presence of phenylsilane as a reducing agent under mild reaction conditions. Salim-UiO-Co catalyzed nitro reduction had a broad substrate scope with excellent tolerance to diverse functional groups, including easily reducible ones such as aldehyde, keto, nitrile, and alkene. Salim-UiO-Co MOF catalyst could be recycled and reused at least 14 times without noticeable losing activity and selectivity. Density functional theory (DFT) studies along with spectroscopic analysis were employed to get into a comprehensive investigation of the reaction mechanism. This work underscores the significance of MOF-supported single-site base-metal catalysts for the sustainable and cost-effective synthesis of chemical feedstocks and fine chemicals.

Symmetry-Breaking Charge Transfer in Metal-Organic Frameworks

High quantum-yield charge carrier generation from the initially prepared excitons defines a key step in the light-harvesting and conversion scheme. Photoinduced charge transfer in molecular electron donor-acceptor assemblies is driven by a sizable ΔG0, which compromises the potential of the generated carriers. Reminiscent of the special pair at the reaction center of the natural light-harvesting complex, symmetry-breaking charge transfer (SBCT) within a pair of identical struts of metal-organic framework (MOF) will facilitate the efficient generation of long-lived charge carriers with maximized potentials without incorporating any foreign redox species. We report SBCT in pyrene-based zirconium metal-organic framework (MOF) NU-1000 that leads to efficient generation of radical ions in a polar solvent and bound CT states in a low-polar solvent. The probe unveils the role of the low-lying non-Franck-Condon excitonic states as intermediates in the formation of the SBCT state from the initially prepared Franck-Condon S1 states. Ultrafast and transient spectroscopy─probed over 200 fs-30 μs time scale─evinces a kSBCT = (110 ps)-1 in polar media (εs = 37.5) forming solvated radical ions with recombination rate kCR = (∼45 ns)-1. A slower rate with kSBCT = (203 ps)-1 was recorded in low-polar (εs = 7.0) solvent manifesting a bound [TBAPy•+ TBAPy•-] state with kCR ≈ (17 μs)-1. This discovery, along with other unique photophysical features relevant to light harvesting, should define a MOF-based platform for developing heterogeneous artificial photon energy conversion systems.

Sulfur atom-directed metal-ligand synergistic catalysis in zirconium/hafnium-oxo clusters for highly efficient amine oxidation

The performance applications (e.g., photocatalysis) of zirconium (Zr) and hafnium (Hf) based complexes are greatly hindered by the limited development of their structures and the relatively inert metal reactivity. In this work, we constructed two ultrastable Zr/Hf-based clusters (Zr9-TC4A and Hf9-TC4A) using hydrophobic 4-tert-butylthiacalix[4]arene (H4TC4A) ligands, in which unsaturated coordinated sulfur (S) atoms on the TC4A4- ligand can generate strong metal-ligand synergy with nearby active metal Zr/Hf sites. As a result, these two functionalized H4TC4A ligands modified Zr/Hf-oxo clusters, as catalysts for the amine oxidation reaction, exhibited excellent catalytic activity, achieving very high substrate conversion (>99%) and product selectivity (>90%). Combining comparative experiments and theoretical calculations, we found that these Zr/Hf-based cluster catalysts accomplish efficient amine oxidation reactions through synergistic effect between metals and ligands: (i) The photocatalytic benzylamine (BA) oxidation reaction was achieved by the synergistic effect of the dual active sites, in which, the naked S sites on the TC4A4- ligand oxidize the BA by photogenerated hole and oxygen molecules are reduced by photogenerated electrons on the metal active sites; (ii) in the aniline oxidation reaction, aniline was adsorbed by the bare S sites on ligands to be closer to metal active sites and then oxidized by the oxygen-containing radicals activated by the metal sites, thus completing the catalytic reaction under the synergistic catalytic effect of the proximity metal-ligand. In this work, the Zr/Hf-based complexes applied in the oxidation of organic amines have been realized using active S atom-directed metal-ligand synergistic catalysis and have demonstrated very high reactivity.

Electronic Tuning of Photoexcited Dynamics in Heteroleptic Cu(I) Complex Photosensitizers

Photoexcited dynamics of heteroleptic Cu(I) complexes as noble-metal-free photosensitizers are closely intertwined with the nature of their ligands. By utilizing ultrafast optical and X-ray transient absorption spectroscopies, we characterized a new set of heteroleptic Cu(I) complexes [Cu(PPh3)2(BPyR)]+ (R = CH3, H, Br to COOCH3), with an increase in the electron-withdrawing ability of the functional group (R). We found that after the transient photooxidation of Cu(I) to Cu(II), the increasing electron-withdrawing ability of R barely affects the internal conversion (IC) (e.g., Jahn-taller (JT) distortion) between singlet MLCT states. However, it does accelerate the dynamics of intersystem crossing (ISC) between singlet and triplet MLCT states and the subsequent decay from the triplet MLCT state to the ground state. The associated lifetime constants are reduced by up to 300%. Our understanding of the photoexcited dynamics in heteroleptic Cu(I) complexes through ligand electronic tuning provides valuable insight into the rational design of efficient Cu(I) complex photosensitizers.

Construction of Hexameric Ru-Substitution POMs to Improve Photocatalytic H2 Evolution

Converting solar energy into storable hydrogen energy by employing green photocatalytic technology offers a reliable alternative for meeting the energy crisis. The polyoxometalates are a promising candidate for hydrogen production photocatalysts because of their unique electronic and structural properties and controllable design at the molecular level. Introducing noble metals was proven to be an effective method to greatly enhance the photocatalytic efficiency of polyoxometalates. Herein, two unprecedented compounds of hexameric Ru-POMs, Na4H10[As2RuIV2W11O18(OH)4(H2O)6{AsW8RuIVO31(OH)Cl}2(B-β-AsW9O33)4]·93H2O (1) and Na2H19[AsRuIII2W11O20(OH)2(H2O)6(RuIIICl3)(B-β-AsW9O33)6]·90H2O (2), were successfully self-assembled. The H2 evolution rates of 1 and 2 under optimal conditions were 3578.75 and 3027.69 μmol h-1 g-1 with TONs of 255 and 205, respectively. The stability of 1 was demonstrated by a series of characterizations. Besides, a possible photocatalytic mechanism was proposed.

Isoreticular Expansion and Linker-Enabled Control of Interpenetration in Titanium-Organic Frameworks

Titanium-organic frameworks offer distinctive opportunities in the realm of metal-organic frameworks (MOFs) due to the integration of intrinsic photoactivity or redox versatility in porous architectures with ultrahigh stability. Unfortunately, the high polarizing power of Ti4+ cations makes them prone to hydrolysis, thus preventing the systematic design of these types of frameworks. We illustrate the use of heterobimetallic cluster Ti2Ca2 as a persistent building unit compatible with the isoreticular design of titanium frameworks. The MUV-12(X) and MUV-12(Y) series can be all synthesized as single crystals by using linkers of varying functionalization and size for the formation of the nets with tailorable porosity and degree of interpenetration. Following the generalization of this approach, we also gain rational control over interpenetration in these nets by designing linkers with varying degrees of steric hindrance to eliminate stacking interactions and access the highest gravimetric surface area reported for titanium(IV) MOFs (3000 m2 g-1).

Confining charge-transfer complex in a metal-organic framework for photocatalytic CO2 reduction in water

In the quest for renewable fuel production, the selective conversion of CO₂ to CH₄ under visible light in water is a leading-edge challenge considering the involvement of kinetically sluggish multiple elementary steps. Herein, 1-pyrenebutyric acid is post-synthetically grafted in a defect-engineered Zr-based metal organic framework by replacing exchangeable formate. Then, methyl viologen is incorporated in the confined space of post-modified MOF to achieve donor-acceptor complex, which acts as an antenna to harvest visible light, and regulates electron transfer to the catalytic center (Zr-oxo cluster) to enable visible-light-driven CO₂ reduction reaction. The proximal presence of the charge transfer complex enhances charge transfer kinetics as realized from transient absorption spectroscopy, and the facile electron transfer helps to produce CH₄ from CO₂. The reported material produces 7.3 mmol g^-1 of CH₄ under light irradiation in aqueous medium using sacrificial agents. Mechanistic information gleans from electron paramagnetic resonance, in situ diffuse reflectance FT-IR and density functional theory calculation.

Mesoporous Mixed-Metal-Organic Framework Incorporating a [Ru(Phen)3]2+ Photosensitizer for Highly Efficient Aerobic Photocatalytic Oxidative Coupling of Amines

[Ru(Phen)₃]²⁺ (phen = phenanthroline) as a very classical photosensitizer possesses strong absorption in the visible range and facilitates photoinduced electron transfer, which plays a vital role in regulating photochemical reactions. However, it remains a significant challenge to utilize more adequately and exploit more efficiently the ruthenium-based materials due to the uniqueness, scarcity, and nonrenewal of the noble metal. Here, we integrate the intrinsic advantages of the ruthenium-based photosensitizer and mesoporous metal-organic frameworks (meso-MOFs) into a [Ru(Phen)₃]²⁺ photosensitizer-embedded heterometallic Ni(II)/Ru(II) meso-MOF (LTG-NiRu) via the metalloligand approach. LTG-NiRu, with an extremely robust framework and a large one-dimensional (1D) channel, not only makes ruthenium photosensitizer units anchored in the inner wall of meso-MOF tubes to circumvent the problem of product/catalyst separation and recycling of catalysts in heterogeneous systems but also exhibits exceptional activities for the aerobic photocatalytic oxidative coupling of amine derivatives as a general photocatalyst. The conversion of the light-induced oxidative coupling reaction for various benzylamines is ∼100% in 1 h, and more than 20 chemical products generated by photocatalytic oxidative cycloaddition of N-substituted maleimides and N,N-dimethylaniline can be synthesized easily in the presence of LTG-NiRu upon visible light irradiation. Moreover, recycling experiments demonstrate that LTG-NiRu is an excellent heterogeneous photocatalyst with high stability and excellent reusability. LTG-NiRu represents a great potential photosensitizer-based meso-MOF platform with an efficient aerobic photocatalytic oxidation function that is convenient for gram-scale synthesis.

Zr- and Ti-based metal-organic frameworks: synthesis, structures and catalytic applications

Recently, Zr- and Ti-based metal-organic frameworks (MOFs) have gathered increasing interest in the field of chemistry and materials science, not only for their ordered porous structure, large surface area, and high thermal and chemical stability, but also for their various potential applications. Particularly, the unique features of Zr- and Ti-based MOFs enable them to be a highly versatile platform for catalysis. Although much effort has been devoted to developing Zr- and Ti-based MOF materials, they still suffer from difficulties in targeted synthesis, especially for Ti-based MOFs. In this Feature Article, we discuss the evolution of Zr- and Ti-based MOFs, giving a brief overview of their synthesis and structures. Furthermore, the catalytic uses of Zr- and Ti-based MOF materials in the previous 3-5 years have been highlighted. Finally, perspectives on the Zr- and Ti-based MOF materials are also proposed. This work provides in-depth insight into the advances in Zr- and Ti-based MOFs and boosts their catalytic applications.

Charge transfer in metal-organic frameworks

Metal-organic frameworks (MOFs, also known as porous coordination polymers or PCPs) are a novel class of crystalline porous material. The tailorable porous structure, in terms of size, geometry and function, has attracted the attention of researchers across all disciplines of materials science. One of the many exciting aspects of MOFs is that through directional and reversible coordination bonding, organic linkers (chromophores with metal-coordinating functional groups) and metal ions (and clusters) can be spatially organized in a preconceived geometry. The well-defined spatial geometry of the metals and linkers is very advantageous for optoelectronic functions (solar cells, light-emitting diodes, photocatalysts) of the materials. This feature article evaluates the scope of charge transfer (CT) interactions in MOFs, involving the organic linkers and metal ion or cluster components. Irrespective of the type (size, shape, electronic property) of organic chromophores involved, MOFs provide an insightful path to design and make the CT process efficient. The selected examples of MOFs with CT characteristics do not only illustrate the design principles but render a pathway towards understanding the complex photophysical processes and implementing those for future optoelectronic and catalytic applications.

Modulating Uranium Extraction Performance of Multivariate Covalent Organic Frameworks through Donor-Acceptor Linkers and Amidoxime Nanotraps

Covalent organic frameworks (COFs) can be designed to allow uranium extraction from seawater by incorporating photocatalytic linkers. However, often sacrificial reagents are required for separating photogenerated charges which limits their practical applications. Herein, we present a COF-based adsorption-photocatalysis strategy for selective removal of uranyl from seawater in the absence of sacrificial reagents. A series of ternary and quaternary COFs were synthesized containing the electron-rich linker 2,4,6-triformylphloroglucinol as the electron donor, the electron-deficient linker 4,4'-(thiazolo[5,4-d]thiazole-2,5-diyl)dibenzaldehyde as the acceptor, and amidoxime nanotraps for selective uranyl capture (with the quaternary COFs incorporating [2,2'-bipyridine-5,5'-diamine-Ru(Bp)₂]Cl₂ as a secondary photosensitizer). The ordered porous structure of the quaternary COFs ensured efficient mass transfer during the adsorption-photocatalysis capture of uranium from seawater samples, with photocatalytically generated electrons resulting in the reduction of adsorbed U(VI) to U(IV) in the form of UO₂. A quaternary COF, denoted as COF 2-Ru-AO, possessed a high uranium uptake capacity of 2.45 mg/g/day in natural seawater and good anti-biofouling abilities, surpassing most adsorbents thus far. This work shows that multivariate COF adsorption-photocatalysts can be rationally engineered to work efficiently and stably without sacrificial electron donors, thus opening the pathway for the economic and efficient extraction of uranium from the earth's oceans.

Inter-Network Charge-Transfer Excited State Formation Within a Two-fold Catenated Metal-Organic Framework

Charge-transfer excited state (CTES) defines the ability to split photon energy into work producing redox equivalents suitable for photocatalysis. Here, we report inter-net CTES formation within a two-fold catenated crystalline metal-organic framework (MOF), constructed with two linkers, N,N'-di(4-pyridyl)-1,4,5,8-naphthalenetetracarboxydiimide (DPNDI) and 2,6-dicarboxynaphthalene (NDC). The structural flexibility puts two complementary linkers from two nets in a proximal position to interact strongly. Supported by the electrochemical and steady-state electronic spectroscopic data, this ground-state interaction facilitates forming CTES that can be populated by direct excitation. We map the dynamics of the CTES which persists over a few nanoseconds and highlight the utilities of such relatively long-lived CTES as enhanced conductivity of the MOF under light over that measured in dark and as a proof-of-the-principle test, photo-reduction of methyl viologen under white light.

Photoelectrochemical water oxidation by a MOF/semiconductor composite

Herein, we report the development of a MOF-semiconductor composite film active for water oxidation at a thermodynamic underpotential.

Metal-Organic Framework-Based Photocatalysis for Solar Fuel Production

Metal-organic frameworks (MOFs) represent a novel class of crystalline inorganic-organic hybrid materials with tunable semiconducting behavior. MOFs have potential for application in photocatalysis to produce sustainable solar fuels, owing to their unique structural advantages (such as clarity and modifiability) that can facilitate a deeper understanding of the structure-activity relationship in photocatalysis. This review takes the photocatalytic active sites as a particular perspective, summarizing the progress of MOF-based photocatalysis for solar fuel production; mainly including three categories of solar-chemical conversions, photocatalytic water splitting to hydrogen fuel, photocatalytic carbon dioxide reduction to hydrocarbon fuels, and photocatalytic nitrogen fixation to high-energy fuel carriers such as ammonia. This review focuses on the types of active sites in MOF-based photocatalysts and discusses their enhanced activity based on the well-defined structure of MOFs, offering deep insights into MOF-based photocatalysis.

Anisotropic phenanthroline-based ruthenium polymers grafted on a titanium metal-organic framework for efficient photocatalytic hydrogen evolution

Conjugated polymers and titanium-based metal-organic framework (Ti-MOF) photocatalysts have demonstrated promising features for visible-light-driven hydrogen production. We report herein a strategy of anisotropic phenanthroline-based ruthenium polymers (PPDARs) over Ti-MOF, a tunable platform for efficient visible-light-driven photocatalytic hydrogen evolution reaction (HER). Several analytical methods including X-ray absorption spectroscopy (XAS) revealed the judicious integration of the surface-active polymer over the Ti-MOF reinforcing the catalytic activity over the broad chemical space. PPDAR-4 polyacrylate achitecture led to a substantial increase in the H₂ evolution rate of 2438 µmolg^-1h^-1 (AQY: 5.33%) compared to pristine Ti-MOF (238 µmol g^-1 h^-1). The separation of photogenerated charge carriers at the PPDAR-4/Ti-MOF interface was confirmed by the optical and electrochemical investigations. The experimental, as well as theoretical data, revealed their physical and chemical properties which are positively correlated with the H₂ generation rate. This offers a new avenue in creating polymer-based MOF robust photocatalysts for sustainable energy.

Ultrasensitive dual-signal electrochemical ratiometric aptasensor based on Co-MOFs with intrinsic self-calibration property for Mucin 1

The concentration of tumor biomarker Mucin 1 (MUC 1) is highly related with many diseases, which can be employed for the early diagnosis of cancer. In this paper, an electrochemical ratiometric aptasensor with intrinsic self-calibration property for the detection of MUC 1 is presented. In this paper, Co-MOFs themselves were employed as signal substances. This strategy was fabricated by using gold nanoparticles@black phosphorus (BP) as the substrate on the electrode, followed by modification of DNA nanotetrahedrons (DTN) via Au-S bond. The terminal of DTN contains MUC 1 aptamer. In the presence of MUC 1, the signal of DNA-labeled Co-MOFs can be detected. The current signal of Co-MOFs increased and that of thionine (as reference) was unchanged upon the addition of MUC 1. Thus, an intrinsic self-calibration aptasensor was achieved. In order to simplify the modification procedure, the electrolyte solution thionine was employed as an inner reference probe. Moreover, coupling of the hybridization chain reaction (HCR) with these MOFs signal tags presents an enzyme-free method for signal amplification, endowing the proposed ratiometric biosensor detection with high reproducibility and high sensitivity. The current ratio (I_IR/I_SP) remained stable over 30 individual measurements performed on ten different working electrodes. Even ten repeated scans performed on a single electrode exhibited a constant current ratio. The electrochemical ratiometric aptasensor is highly sensitivity for MUC 1 with the detection limit of 1.34 fM. Our proposed ratiometric sensor has great potential for the detection of cancer-related biomarkers.

Enhanced Photocatalytic Efficiency in Visible-Light-Induced NADH Regeneration by Intramolecular Electron Transfer

Inspired by natural photosynthesis, photocatalytic NADH regeneration has drawn increasing interest in the recent decade as it provides a perfect approach for NAD⁺ reduction into NADH, which can be further consumed by oxidordeuctase for enzymatic redox reactions. However, two issues still remain unsolved in this procedure. First, the photocatalytic efficiency in NAD⁺ hydrogenation requires further improvement. Second, the rhodium electron mediator [Cp*Rh(bpy)H₂O]²⁺ (M), which is always required for selective 1,4-NADH regeneration, is difficult to recover because of its good solubility in aqueous solution. Given the high price of M, it is highly wasteful and inefficient if it only spends once. Here, we report a Cp*Rh(bpy)Cl implanted conjugated microporous polymer DTS/Rh@CMPs which can be employed as a highly effective visible light photocatalysts for in situ NADH regeneration without using additional M. In addition, the insertion of Rh complex into a polymer skeleton, as demonstrated in UV-vis, fluorescence, photocurrent and electrochemical impedance, dramatically improves the light absorption capacity and the electron separation and transfer efficiency. Compared with that of DTS@CMP-1 with M, an enhanced reaction yield of 33% was determined in DTS/Rh@CMP-1 suggesting that intramolecular electron transfer has a better activity than that of intermolecular electron transfer in photocatalytic NAD⁺ reduction. Moreover, as the Rh complex is rooted firmly in a polymer framework, negligible Rh loss and conversion decrease in NADH regeneration are observed. When the DTS/Rh@CMP-1 was coupled with yeast alcohol dehydrogenase (YADH, from Saccharomyces cerevisiae), 1.36 mM of methanol was accumulated, implying an excellent biocompatibility of DTS/Rh@CMP-1 and a high feasibility of photobiocatalysis for formaldehyde hydrogenation.

Metal-Organic Frameworks Can Photocatalytically Split Water-Why Not?

The opinion is provided about the stability and photocatalytic capability of metal-organic frameworks in photocatalytic overall water splitting.

Visible light-driven efficient palladium catalyst turnover in oxidative transformations within confined frameworks

Palladium catalyst turnover by reoxidation of a low-valent Pd species dominates the proceeding of an efficient oxidative transformation, but the state-of-the-art catalysis approaches still have great challenges from the perspectives of high efficiency, atom-economy and environmental-friendliness. Herein, we report a new strategy for addressing Pd reoxidation problem by the fabrication of spatially proximate Ir^III photocatalyst and Pd^II catalyst into metal-organic framework (MOF), affording MOFs based Pd/photoredox catalysts UiO-67-Ir-PdX₂ (X = OAc, TFA), which are systematically evaluated using three representative Pd-catalyzed oxidation reactions. Owing to the stabilization of single-site Pd and Ir catalysts by MOFs framework as well as the proximity of them favoring fast electron transfer, UiO-67-Ir-PdX₂, under visible light, exhibits up to 25 times of Pd catalyst turnover number than the existing catalysis systems. Mechanism investigations theoretically corroborate the capability of MOFs based Pd/photoredox catalysis to regulate the competitive processes of Pd⁰ aggregation and reoxidation in Pd-catalyzed oxidation reactions.

[Ru(NꓥNꓥN)2-Ce]-based Framework for Photocatalytic Sulfide Oxidation and Hydrogen Production

Using photosensitized Ru(NꓥNꓥN)2-metalloligand, a series of Ce-frameworks were synthesized. The incorporation of visible-light-responsive Ru(NꓥNꓥN)2-unit endows the Ce-frameworks with photocatalytic activities in both sulfide oxidation and hydrogen production. The doping of...

Syntheses of New Zeolitic Imidazolate Frameworks in Dimethyl Sulfoxide

Presented here are the syntheses of ZIFs in dimethyl sulfoxide (DMSO). A series of new ZIFs with various topologies such as ACO, coi, zni, ANA, GIS, even new topology can...

Photoinduced Charge Transfer with a Small Driving Force Facilitated by Exciplex-like Complex Formation in Metal-Organic Frameworks

Photoinduced charge transfer (PCT) is a key step in the light-harvesting (LH) process producing the redox equivalents for energy conversion. However, like traditional macromolecular donor-acceptor assemblies, most MOF-derived LH systems are designed with a large ΔG⁰ to drive PCT. To emulate the functionality of the reaction center of the natural LH complex that drives PCT within a pair of identical chromophores producing charge carriers with maximum potentials, we prepared two electronically diverse carboxy-terminated zinc porphyrins, BFBP(Zn)-COOH and TFP(Zn)-COOH, and installed them into the hexagonal pores of NU-1000 via solvent-assisted ligand incorporation (SALI), resulting in BFBP(Zn)@NU-1000 and TFP(Zn)@NU-1000 compositions. Varying the number of trifluoromethyl groups at the porphyrin core, we tuned the ground-state redox potentials of the porphyrins within ca. 0.1 V relative to that of NU-1000, defining a small ΔG₀ for PCT. For BFBP(Zn)@NU-1000, the relative ground- and excited-state redox potentials of the components facilitate an energy transfer (EnT) from NU-1000* to BFBP(Zn), forming BFBP(Zn)_S1* which entails a long-lived charge-separated complex formed through an exciplex-like [BFBP(Zn)_S1*-TBAPy] intermediate. Various time-resolved spectroscopic data suggest that EnT from NU-1000* may not involve a fast Förster-like resonance energy transfer (FRET) but rather through a slow [NU-1000*-BFBP(Zn)] intermediate formation. In contrast, TFP(Zn)@NU-1000 displays an efficient EnT from NU-1000* to [TFP(Zn)-TBAPy], a complex that formed at the ground state through electronic interaction, and thereon showed the excited-state feature of [TFP(Zn)-TBAPy]*. The results will help to develop synthetic LHC systems that can produce long-lived photogenerated charge carriers with high potentials, i.e., high open-circuit voltage in photoelectrochemical setups.

Tunable Band Gaps in MUV-10(M): A Family of Photoredox-Active MOFs with Earth-Abundant Open Metal Sites

Titanium-based metal-organic frameworks (Ti-MOFs) have attracted intense research attention because they can store charges in the form of Ti³⁺ and they serve as photosensitizers to cocatalysts through heterogeneous photoredox reactions at the MOF-liquid interface. Both the charge storage and charge transfer depend on the redox potentials of the MOF and the molecular substrate, but the factors controlling these energetic aspects are not well understood. Additionally, photocatalysis involving Ti-MOFs relies on cocatalysts rather than the intrinsic Ti reactivity, in part because Ti-MOFs with open metal sites are rare. Here, we report that the class of Ti-MOFs known as MUV-10 can be synthetically modified to include a range of redox-inactive ions with flexible coordination environments that control the energies of the photoactive orbitals. Lewis acidic cations installed in the MOF cluster (Cd²⁺, Sr²⁺, and Ba²⁺) or introduced to the pores (H⁺, Li⁺, Na⁺, K⁺) tune the electronic structure and band gaps of the MOFs. Through the use of optical redox indicators, we report the first direct measurement of the Fermi levels (redox potentials) of photoexcited MOFs in situ. Taken together, these results explain the ability of Ti-MOFs to store charges and provide design principles for achieving heterogeneous photoredox chemistry with electrostatic control.

Metal-Organic Frameworks: Molecules or Semiconductors in Photocatalysis?

In the realm of solids, metal-organic frameworks (MOFs) offer unique possibilities for the rational engineering of tailored physical properties. These derive from the modular, molecular make-up of MOFs, which allows for the selection and modification of the organic and inorganic building units that construct them. The adaptable properties make MOFs interesting materials for photocatalysis, an area of increasing significance. But the molecular and porous nature of MOFs leaves the field, in some areas, juxtapositioned between semiconductor physics and homogeneous photocatalysis. While descriptors from both fields are applied in tandem, the gap between theory and experiment has widened in some areas, and arguably needs fixing. Here we review where MOFs have been shown to be similar to conventional semiconductors in photocatalysis, and where they have been shown to be more like infinite molecules in solution. We do this from the perspective of band theory, which in the context of photocatalysis, covers both the molecular and nonmolecular principles of relevance.

Neighboring Zn-Zr Sites in a Metal-Organic Framework for CO2 Hydrogenation

ZrZnO_x is active in catalyzing carbon dioxide (CO₂) hydrogenation to methanol (MeOH) via a synergy between ZnO_x and ZrO_x. Here we report the construction of Zn²⁺-O-Zr⁴⁺ sites in a metal-organic framework (MOF) to reveal insights into the structural requirement for MeOH production. The Zn²⁺-O-Zr⁴⁺ sites are obtained by postsynthetic treatment of Zr₆(μ₃-O)₄(μ₃-OH)₄ nodes of MOF-808 by ZnEt₂ and a mild thermal treatment to remove capping ligands and afford exposed metal sites for catalysis. The resultant MOF-808-Zn catalyst exhibits >99% MeOH selectivity in CO₂ hydrogenation at 250 °C and a high space-time yield of up to 190.7 mg_MeOH g_Zn^-1 h^-1. The catalytic activity is stable for at least 100 h. X-ray absorption spectroscopy (XAS) analyses indicate the presence of Zn²⁺-O-Zr⁴⁺ centers instead of Zn_mO_n clusters. Temperature-programmed desorption (TPD) of hydrogen and H/D exchange tests show the activation of H₂ by Zn²⁺ centers. Open Zr⁴⁺ sites are also critical, as Zn²⁺ centers supported on Zr-based nodes of other MOFs without open Zr⁴⁺ sites fail to produce MeOH. TPD of CO₂ reveals the importance of bicarbonate decomposition under reaction conditions in generating open Zr⁴⁺ sites for CO₂ activation. The well-defined local structures of metal-oxo nodes in MOFs provide a unique opportunity to elucidate structural details of bifunctional catalytic centers.

H-Bond-Mediated Selectivity Control of Formate versus CO during CO2 Photoreduction with Two Cooperative Cu/X Sites

It is highly desirable to achieve solar-driven conversion of CO₂ to valuable fuels with controlled selectivity. The existing catalysts are mainly explored for CO production but rarely for formate generation. Herein, highly selective photoreduction of CO₂ to formate (99.7%) was achieved with a high yield of 3040 μmol g^-1 in 10 h by hierarchical integration of photosensitizers and monometallic [bpy-Cu/ClX] (X = Cl or adenine) catalysts into a stable Eu-bpy metal-organic framework. However, replacing X with pyridine in [bpy-CuCl/X] significantly reduced formate production while increasing the CO yield to 960 μmol g^-1. Systematic investigations revealed that the catalytic process is mediated by the H-bond synergy between Cu-bound X and CO₂-derived species, and the selectivity of HCOO^- can be controlled by simply replacing the coordination ligands. This work provides a molecularly precise structural model to provide mechanistic insights for selectivity control of CO₂ photoreduction.

Efficient detection of Fe(iii) and chromate ions in water using two robust lanthanide metal–organic frameworks

Two novel robust Ln-MOFs feature a 3D highly porous pillared-layer framework and demonstrate selective sensing of Fe(iii) and chromate ions in aqueous solution.

Tuning the photoelectric response of pyrene-based coordination polymers by optimizing charge transfer

Three π–π stacked CPs were designed and synthesized for application of photoelectric response. The effect of charge transfer on the photoelectric properties is explored by adjusting the composition and π-stacking fashion of the CPs.

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.

Porphyrin Grafting on a Mercapto-Equipped Zr(IV)-Carboxylate Framework Enhances Photocatalytic Hydrogen Production

We employ facile aromatic nucleophilic substitution between the mercapto (-SH) and arylfluoro (Ar-F) groups to achieve extensive and robust cross-linking of a coordination host by porphyrin guests that also serve the purpose of versatile postsynthetic functionalization. For this, a tritopic linker with three trident-like thiol-flanked carboxyl units are reacted with ZrOCl₂·8H₂O to afford a two-dimensional (3,6-connected) net. The wide aperture of the porous framework solid, together with its stability in both air and boiling water, facilitates the entry of bulky metalloporphyrin guests and the subsequent property studies. On the porphyrin side, four pentafluorophenyl (C₆F₅-) groups offer multiple fluoro groups to facilitate their replacement by the thiol groups from the host net. The inserted metalloporphyrin bridges impart to the metal-organic framework (MOF) host stable and recyclable activities for photocatalytic hydrogen production. We also disclose an improvement in synthetic methodology, in which BBr₃ is used to simultaneously cleave the ester and benzyl thioether groups to more efficiently access thiol-equipped carboxylic acid building block.

Self-assembly of a cobalt(II)-based metal-organic framework as an effective water-splitting heterogeneous catalyst for light-driven hydrogen production

The solar photocatalysis of water splitting represents a significant branch of enzymatic simulation by efficient chemical conversion and the generation of hydrogen as green energy provides a feasible way for the replacement of fossil fuels to solve energy and environmental issues. We report herein the self-assembly of a Co^II-based metal-organic framework (MOF) constructed from 4,4',4'',4'''-(ethene-1,1,2,2-tetrayl)tetrabenzoic acid [or tetrakis(4-carboxyphenyl)ethylene, H₄TCPE] and 4,4'-bipyridyl (bpy) as four-point- and two-point-connected nodes, respectively. This material, namely, poly[(μ-4,4'-bipyridyl)[μ₈-4,4',4'',4'''-(ethene-1,1,2,2-tetrayl)tetrabenzoato]cobalt(II)], [Co(C₃₀H₁₆O₈)(C₁₀H₈N₂)]_n, crystallized as dark-red block-shaped crystals with high crystallinity and was fully characterized by single-crystal X-ray diffraction, PXRD, IR, solid-state UV-Vis and cyclic voltammetry (CV) measurements. The redox-active Co^II atoms in the structure could be used as the catalytic sites for hydrogen production via water splitting. The application of this new MOF as a heterogeneous catalyst for light-driven H₂ production has been explored in a three-component system with fluorescein as photosensitizer and trimethylamine as the sacrificial electron donor, and the initial volume of H₂ production is about 360 µmol after 12 h irradiation.

Metal-Organic Framework-Based Catalysts with Single Metal Sites

Metal-organic frameworks (MOFs) are a class of distinctive porous crystalline materials constructed by metal ions/clusters and organic linkers. Owing to their structural diversity, functional adjustability, and high surface area, different types of MOF-based single metal sites are well exploited, including coordinately unsaturated metal sites from metal nodes and metallolinkers, as well as active metal species immobilized to MOFs. Furthermore, controllable thermal transformation of MOFs can upgrade them to nanomaterials functionalized with active single-atom catalysts (SACs). These unique features of MOFs and their derivatives enable them to serve as a highly versatile platform for catalysis, which has actually been becoming a rapidly developing interdisciplinary research area. In this review, we overview the recent developments of catalysis at single metal sites in MOF-based materials with emphasis on their structures and applications for thermocatalysis, electrocatalysis, and photocatalysis. We also compare the results and summarize the major insights gained from the works in this review, providing the challenges and prospects in this emerging field.

Adjustment of the performance and stability of isostructural zeolitic tetrazolate-imidazolate frameworks

Presented here are two isostructural SOD-type zeolitic tetrazolate-imidazolate frameworks (ZTIFs), Zn(etz)0.86(mim)1.14 (ZTIF-9, Hetz = 5-ethyltetrazole, Hmim = 2-methylimidazole) and Zn(vtz)0.63(mim)1.37 (ZTIF-10, Hvtz = 5-vinyltetrazole). The adjustment of the ligand ratios within these ZTIFs was realized through changing the substituent groups of tetrazole ligands. Remarkably, ZTIF-9 with a suitable ligand ratio perfectly balances gas uptake and stability, exhibiting 6-fold improvement of C2H2 uptake compared to the prototype ZIF-8 (Zn(mim)2).

Highly Stable Phosphonate-Based MOFs with Engineered Bandgaps for Efficient Photocatalytic Hydrogen Production

Photoactive metal-organic frameworks (MOFs) represent one of the most promising materials for photocatalytic hydrogen production, but phosphonate-based MOFs have remained largely underdeveloped compared to other conventional MOFs. Herein, a photocatalyst of 1D titanium phosphonate MOF is designed through an easy and scalable stirring hydrothermal method. Homogeneous incorporation of organophosphonic linkers can narrow the bandgap, which is due to the strong electron-donating ability of the OH functional group that can efficiently shift the top of the valence band, moving the light absorption to the visible portion of the spectrum. In addition, the unique 1D nanowire topology enhances the photoinduced charge carrier transport and separation. Accordingly, the titanium phosphonate nanowires deliver remarkably enhanced photocatalytic hydrogen evolution activity under irradiation of both visible light and a full-spectrum simulator. Such concepts of engineering both nanostructures and electronic states herald a new paradigm for designing MOF-based photocatalysts.

Visible-light-induced controlled radical polymerization of methacrylates mediated by zirconium-porphryinic metal–organic frameworks

Visible-light-induced controlled radical polymerization of methacrylates by using zirconium-porphryinic metal–organic frameworks.

Conjugated nanoporous polycarbazole bearing a cobalt complex for efficient visible-light driven hydrogen evolution

A conjugated nanoporous polycarbazole (CNP) cross-linked by pyridine and coordinated to Co(iii) displays high catalytic performance for visible light-driven H₂ generation.

The role of photoinduced charge transfer for photocatalysis, photoelectrocatalysis and luminescence sensing in metal–organic frameworks

Understanding PCT taking place within MOFs is crucial for designing porous photo/electrocatalysts and luminescent sensors. Unique features of PCT in MOFs and recent progress along with state-of-the-art characterization methods are discussed in the context of its applications.

Hydroxides Ni(OH)2&Ce(OH)3 as a novel hole storage layer for enhanced photocatalytic hydrogen evolution

In this work, a novel photocatalyst Ni(OH)₂&Ce(OH)₃@P-CdS was synthesized successfully by phosphorization of CdS and in situ loading of Ni(OH)₂&Ce(OH)₃ on the surface of P-CdS.