Control of Redox Events by Dye Encapsulation Applied to Light-Driven Splitting of Hydrogen Sulfide

Host-Guest Approach to Promoting Photocatalysis Based on Consecutive Photo-Induced Electron-Transfer Processes via Efficient Förster Resonance Energy Transfer

Supramolecular artificial light-harvesting system with highly efficient host-guest energy transfer pathway provides an ideal platform for optimizing the photochemistry process. The consecutive photo-induced electron transfer (conPET) process overcomes the energy limitation of visible-light photocatalysis, but is often compromised by mismatching between the absorption of ground state dye and its radical, weakening the efficiency of photoredox reaction. By encapsulating a conPET photocatalyst rhodamine 6G into metal-organic cage, the supramolecular approach was undertaken to tackle the intrinsic difficulty of matching the light absorption of photoexcitation between rhodamine 6G and its radical. The highly efficient Förster resonance energy transfer from the photoexcited cage to rhodamine 6G forced by host-guest encapsulation facilitates the conPET process for the single-wavelength light-driven activation of aryl halides by stabilizing and accelerating the production and accumulation of the rhodamine 6G radical intermediate. The tunable and flexible nature of the supramolecular host-guest complex renders the cage-based encapsulation strategy promising for the development of ideal photocatalysts toward the better utilization of solar energy.

Encapsulated Dye in Coordination-Assembled Octahedron for Visible-Light-Driven Proton Reduction and Nitroaromatic Hydrogenation

To mimic the finely tuned natural photosynthetic systems, a large metal-organic octahedron was synthesized by one-pot self-assembly with modified triphenylamine ligands and redox-active cobalt ions. By encapsulating an organic dye, fluorescein (Fl), within the inner cavity of the octahedron, the host-guest supramolecular system was provided for light-driven hydrogen production. The intimate distance between the redox site and the photosensitizer in the supramolecular metal-organic cage allowed the photoinduced electrons to transfer from the excited state Fl* to the redox cobalt center in a pseudo-intramolecular pathway. The supramolecular system showed good performance in light-driven hydrogen production and the reduction of nitroaromatic compounds. Control experiments based on a mononuclear compound resembling a cobalt corner of the octahedron and inhibitor competition provided evidence of enzyme-like catalytic behavior. The supramolecular reaction pathways within confined spaces contribute to the superior activity of the host-guest system.

Modifying the Oxidative Potentials of Imines in a Dye Loaded Capsule for Photocatalytic Cyclization with Hydrogen Evolution

Modifying redox potential of substrates and intermediates to balance pairs of redox steps are important stages for multistep photosynthesis but faced marked challenges. Through co-clathration of iridium photosensitizer and imine substrate within one packet of a metal-organic capsule to shift the redox potentials of substrate, herein, we reported a multiphoton enzymatic strategy for the generation of heterocycles by intramolecular C-X hydrogen evolution cross-couplings. The cage facilitated a pre-equilibrium substrate-involving clathrate that cathodic shifts the oxidation potential of the substrate-dye-host ternary complex and configuration inversion of substrate via spatial constraints in the confined space. The new two photon excitation strategy enabled the precise control of the multistep electron transfer between each pair (photosensitizer, substrate and the capsule), endowing the catalytic system proceeding smoothly with an enzymatic fashion. Three kinds of 2-subsituted (-OH, -NH2 , and -SH) imines and N-aryl enamines all give the corresponding cyclization products efficiently under visible light irradiation, demonstrating the promising of the microenvironment driven thermodynamic activation in the host-dye-substrate ternary for synergistic combination of multistep photocatalytic transformations.

A Platinum(II)-Based Molecular Cage with Aggregation-Induced Emission for Enzymatic Photocyclization of Alkynylaniline

Enzymes facilitate chemical conversions through the collective activity of aggregated components, but the marriage of aggregation-induced emission (AIE) with molecular containers to emulate enzymatic conversion remains challenging. Herein, we report a new approach to construct a Pt^II -based octahedral cage with AIE characteristics for the photocyclization of alkynylaniline by restricting the rotation of the pendant phenyl rings peripheral to the Pt^II corner. With the presence of water, the C-H⋅⋅⋅π interactions involving the triphenylphosphine fragments resulted in aggregation of the molecular cages into spherical particles and significantly enhanced the Pt^II -based luminescence. The kinetically inert Pt-NP chelator, with highly differentiated redox potentials in the ground and excited states, and the efficient coordination activation of the platinum corner facilitated excellent catalysis of the photocyclization of alkynylaniline. The enzymatic kinetics and the advantages of binding and activating substrates in an aqueous medium provide a new avenue to develop mimics for efficient photosynthesis.

Binding of Dual-Function Hybridized MetalOrganic Capsules to Enzymes for Cascade Catalysis

The combination of chemo- and biocatalysis for multistep syntheses provides attractive advantages in terms of evolvability, promiscuity, and sustainability striving for desirable catalytic performance. Through the encapsulation of flavin analogues by both NADH and heme mimics codecorated heteroleptic metal-organic capsules, herein, we report a progressive host-guest strategy to imitate cytochrome P450s catalysis for cascade oxidative coupling catalysis. Besides the construction of stable dual-function metal-organic capsules and the modification of cofactor-decorated capsules at the domain of enzymes, this supramolecular strategy involves multistage directional electron flow, affording reactive ferric peroxide species for inducing oxygenation. Under light irradiation, the metal-organic capsule selectively converts stilbene to oxidative coupling products (including 2-oxo-1,2-diphenylethyl formate, 2-alkoxy-1,2-diphenylethanone) in tandem with enzymatic reactions respectively, at the domain of natural enzymes. The ingenious combination of capsules and enzymes with the in situ-regenerated capsule-loaded NADH cofactor promises non-native coupling reactions by forming regional cooperation and division. This abiotic-biotic conjugated host-guest strategy is conducive to the de novo creation of multifunctional components approaching active enzymatic sites for reinforced matter and energy transporting, demonstrating a key role of multicomponent supramolecular catalysts for one-pot integrated catalytic conversions.

Modifying electron injection kinetics for selective photoreduction of nitroarenes into cyclic and asymmetric azo compounds

Modifying the reactivity of substrates by encapsulation is essential for microenvironment catalysts. Herein, we report an alternative strategy that modifies the entry behaviour of reactants into the microenvironment and substrate inclusion thermodynamics related to the capsule to control the electron injection kinetics and the selectivity of products from the nitroarenes photoreduction. The strategy includes the orchestration of capsule openings to control the electron injection kinetics of electron donors, and the capsule’s pocket to encapsulate more than one nitroarene molecules, facilitating a condensation reaction between the in situ formed azanol and nitroso species to produce azo product. The conceptual microenvironment catalyst endows selective conversion of asymmetric azo products from different nitroarenes, wherein, the estimated diameter and inclusion Gibbs free energy of substrates are used to control and predict the selectivity of products. Inhibition experiments confirm a typical enzymatic conversion, paving a new avenue for rational design of photocatalysts toward green chemistry.

Direct Z-Scheme Heterojunction Catalysts Constructed by Graphitic-C3N4 and Photosensitive Metal-Organic Cages for Efficient Photocatalytic Hydrogen Evolution

The demand for improving the activity, durability, and recyclability of metal-organic cages (MOCs) that work as photocatalytic molecular devices in a homogeneous system has promoted research to combine them with other solid materials. An M₂L₄ type photosensitive metal-organic cage MOC-Q2 with light-harvesting ligands and catalytic Pd²⁺ centers has been synthesized and further heterogenized with graphitic carbon nitride to prepare a robust direct Z-scheme heterojunction photocatalyst for visible-light-driven hydrogen generation. The optimized g-C₃N₄/MOC-Q2 (0.7 wt%) sample exhibits a high H₂ evolution activity of 6423 μmol g⁻¹ h⁻¹ in 5 h, and a total turnover number of 39,695 after 10 h, significantly superior to the bare MOC-Q2 used in the homogeneous solution and the comparison sample Pd/g-C₃N₄/L-4. The enhanced performances of g-C₃N₄/MOC-Q2 can be ascribed to its direct Z-scheme heterostructure, which effectively improves the charge separation and transfer efficiency. This work presents a rational approach of designing a binary photocatalytic system through combing micromolecular MOCs with heterogeneous semiconductors for water splitting.

Dye-loaded metal-organic helical capsules applied to the combination of photocatalytic H2S splitting and nitroaromatic hydrogenation

Two dye-loaded metal-organic capsules constructed with different spatial sizes and functional groups simulated the enzymatic substrate activation for hydrogenation of nitroarenes with the kinetics obeying the Michaelis-Menten mechanism.

Photooxidase Mimicking with Adaptive Coordination Molecular Capsules

One important feature of enzyme catalysis is the induced-fit conformational change after binding substrates. Herein, we report a biomimetic water-soluble molecular capsule featuring adaptive structural change toward substrate binding, which offers an ideal platform for efficient photocatalysis. The molecular capsule was coordination-assembled from three anthracene-bridged bis-TPT [TPT = 2,4,6-tris(4-pyridyl)-1,3,5-triazine] ligands and six (bpy)Pd(NO3)2 (bpy = 2,2'-bipyridine). Once substrates bind to its hydrophobic cavity, this capsule would undergo quantitative capsule-to-bowl transformation. Visible-light absorption brought about by both the anthracene units and the charge-transfer absorption on the late-formed quintuple π-π stacked host-guest complex efficiently facilitates aerobic photooxidation for the sulfide guests by visible-light irradiation under mild conditions. Desired turnover numbers and product selectivity (sulfoxide over sulfone) have been achieved by the transformable nature of the catalyst and the hydrophilicity of the sulfoxide product. Such a photocatalytic process enabled by an adaptive coordination capsule and substrates as the allosteric effector paves the way for constructing artificial systems to mimic enzyme catalysis.

Cobalt-Based Metal-Organic Cages for Visible-Light-Driven Water Oxidation

Water oxidation to molecular oxygen is indispensable but a challenge for splitting H₂O. In this work, a series of Co-based metal-organic cages (MOCs) for photoinduced water oxidation were prepared. MOC-1 with both bis(μ-oxo) bridged dicobalt and Co-O (O from H₂O) displays catalytic activity with an initial oxygen evolution rate of 80.4 mmol/g/h and a TOF of 7.49 × 10^-3 s^-1 in 10 min. In contrast, MOC-2 containing only Co-O (O from H₂O) in the structure results in a lower oxygen evolution rate (40.8 mmol/g/h, 4.78 × 10^-3 s^-1), while the amount of oxygen evolved from the solution of MOC-4 without both active sites is undetectable. Isotope experiments with or without H₂¹⁸O as the reactant successfully demonstrate that the molecular oxygen was produced from water oxidation. Photophysical and electrochemical studies reveal that photoinduced water oxidation initializes via electron transfer from the excited [Ru(bpy)₃]²⁺* to Na₂S₂O₈, and then, the cobalt active sites further donate electrons to the oxidized [Ru(bpy)₃]³⁺ to drive water oxidation. This proof-of-concept study indicates that MOCs can work as potential efficient catalysts for photoinduced water oxidation.

Coal-Tar Dye-based Coordination Cages and Helicates

A strategy to implement four members of the classic coal-tar dye family, Michler's ketone, methylene blue, rhodamine B, and crystal violet, into [Pd2 L4 ] self-assemblies is introduced. Chromophores were incorporated into bis-monodentate ligands using piperazine linkers that allow to retain the auxochromic dialkyl amine functionalities required for intense colors deep in the visible spectrum. Upon palladium coordination, ligands with pyridine donors form lantern-shaped dinuclear cages while quinoline donors lead to strongly twisted [Pd2 L4 ] helicates in solution. In one case, single crystal X-ray diffraction revealed rearrangement to a [Pd3 L6 ] ring structure in the solid state. For nine examined derivatives, showing colors from yellow to deep violet, CD spectroscopy discloses different degrees of chiral induction by an enantiomerically pure guest. Ion mobility mass spectrometry allows to distinguish two binding modes. Self-assemblies based on this new ligand class promise application in chiroptical recognition, photo-redox catalysis and optical materials.

Supramolecular strategies in artificial photosynthesis

Artificial photosynthesis is a major scientific endeavor aimed at converting solar power into a chemical fuel as a viable approach to sustainable energy production and storage. Photosynthesis requires three fundamental actions performed in order; light harvesting, charge-separation and redox catalysis. These actions span different timescales and require the integration of functional architectures developed in different fields of study. The development of artificial photosynthetic devices is therefore inherently complex and requires an interdisciplinary approach. Supramolecular chemistry has evolved to a mature scientific field in which programmed molecular components form larger functional structures by self-assembly processes. Supramolecular chemistry could provide important tools in preparing, integrating and optimizing artificial photosynthetic devices as it allows precise control over molecular components within such a device. This is illustrated in this perspective by discussing state-of-the-art devices and the current limiting factors - such as recombination and low stability of reactive intermediates - and providing exemplary supramolecular approaches to alleviate some of those problems. Inspiring supramolecular solutions such as those discussed herein will incite expansion of the supramolecular toolbox, which eventually may be needed for the development of applied artificial photosynthesis.

Self-Assembly of a Ti4(embonate)6 Cage toward Silver

Herein we report a variety of supramolecular architectures that are self-assembled by the highly charged anionic Ti₄L₆ (L = embonate) cages and noble-metal Ag⁺ ions in the presence of the different ligands, including six Ti₄L₆-Ag(PPh₃) cages in whose structures the Ti₄L₆ cage catches various in situ formed [Ag(PPh₃)]⁺ moieties by a coordination bond and one cocrystal superstructure of a Ti₄L₆ cage with an in situ generated [Ag₂(Ph₂P(CH₂)₅PPh₂)₃]²⁺ cage via supramolecular interactions. In addition, the third-order nonlinear-optical properties of these compounds are investigated in detail.

The Covalent and Coordination Co-Driven Assembly of Supramolecular Octahedral Cages with Controllable Degree of Distortion

Discovering and constructing novel and fancy structures is the goal of many supramolecular chemists. In this work, we propose an assembly strategy based on the synergistic effect of coordination and covalent interactions to construct a set of octahedral supramolecular cages and adjust their degree of distortion. Our strategy innovatively utilizes the addition of sulfur atoms of a metal sulfide synthon, [Et₄N][Tp*WS₃] (A), to an alkynyl group of a pyridine-containing linker, resulting in a novel vertex with low symmetry, and of Cu(I) ions. By adjusting the length of the linker and the position of the reactive alkynyl group, the control of the deformation degree of the octahedral cages can be realized. These supramolecular cages exhibit enhanced third-order nonlinear optical (NLO) responses. The results offer a powerful strategy to construct novel distorted cage structures as well as control the degree of distortion of supramolecular geometries.

A host-guest approach to combining enzymatic and artificial catalysis for catalyzing biomimetic monooxygenation

Direct transfer of protons and electrons between two tandem reactions is still a great challenge, because overall reaction kinetics is seriously affected by diffusion rate of the proton and electron carriers. We herein report a host–guest supramolecular strategy based on the incorporation of NADH mimics onto the surface of a metal-organic capsule to encapsulate flavin analogues for catalytic biomimetic monooxygenations in conjunction with enzymes. Coupling an artificial catalysis and a natural enzymatic catalysis in the pocket of an enzyme, this host–guest catalyst–enzyme system allows direct proton and electron transport between two catalytic processes via NADH mimics for the monooxygenation of both cyclobutanones and thioethers. This host–guest approach, which involves the direct coupling of abiotic and biotic catalysts via a NADH-containing host, is quite promising compared to normal catalyst–enzyme systems, as it offers the key advantages of supramolecular catalysis in integrated chemical and biological synthetic sequences.

Combining artificial and natural enzymes is a strategy to mimic biocatalytic processes with high efficiency and selectivity. This study reports a dual catalytic system composed of flavin adenine dinucleotide model and NADH mimics to catalyze the monooxygenation of cyclobutanones and thioethers.

Fundamental Aspects of Xanthene Dye Aggregation on the Surfaces of Nanocluster Polyoxometalates: H- to J-Aggregate Switching

The induced aggregation of the xanthene dye rhodamine B (RhB) on metal oxide centers belonging to the highly symmetric surfaces of precise nanoscale templates with Keplerate (Mo₁₃₂ ) or toroidal (Mo₁₃₈ ) structures has been studied. With the joint use of the Langmuir isotherm and full Stern-Volmer models, the thermodynamic reasons for dye adsorption on the nanocluster surface, such as a mixture of monomer, H-aggregate (H-dimer), and J-aggregate forms (which can coexist or switch one into another under the exact conditions), were established: this was shown through UV/Vis and fluorescence spectroscopies. By using the framework of the exciton model, it is shown that the angle (α) between the transition dipole moments of RhB is very sensitive to surface strain inside the dye sub-monolayer. As a result, it is possible to switch from H- to J-aggregates by the post-functionalization of polyoxometalate (POM)-RhB associates by the surfactant bilayer shell, which allows the surface strain to grow. Recommendations are provided for managing the appearance of H- or J-aggregates on metal oxide (or polyelectrolyte) surfaces during photovoltaic or bioimaging applications.

Electron transfer in the confined environments of metal–organic coordination supramolecular systems

In this review, we overview regulatory factors and diverse applications of electron transfer in confined environments of supramolecular host–guest systems.

Metal-Organic Capsules with NADH Mimics as Switchable Selectivity Regulators for Photocatalytic Transfer Hydrogenation

Switchable selective hydrogenation among the groups in multifunctional compounds is challenging because selective hydrogenation is of great interest in the synthesis of fine chemicals and pharmaceuticals as a result of the importance of key intermediates. Herein, we report a new approach to highly selectively (>99%) reducing C═X (X = O, N) over the thermodynamically more favorable nitro groups locating the substrate in a metal-organic capsule containing NADH active sites. Within the capsule, the NADH active sites reduce the double bonds via a typical 2e^- hydride transfer hydrogenation, and the formed excited-state NAD⁺ mimics oxidize the reductant via two consecutive 1e^- processes to regenerate the NADH active sites under illumination. Outside the capsule, nitro groups are highly selectively reduced through a typical 1e^- hydrogenation. By combining photoinduced 1e^- transfer regeneration outside the cage, both 1e^- and 2e^- hydrogenation can be switched controllably by varying the concentrations of the substrates and the redox potential of electron donors. This promising alternative approach, which could proceed under mild reaction conditions and use easy-to-handle hydrogen donors with enhanced high selectivity toward different groups, is based on the localization and differentiation of the 2e^- and 1e^- hydrogenation pathways inside and outside the capsules, provides a deep comprehension of photocatalytic microscopic reaction processes, and will allow the design and optimization of catalysts. We demonstrate the advantage of this method over typical hydrogenation that involves specific activation via well-modified catalytic sites and present results on the high, well-controlled, and switchable selectivity for the hydrogenation of a variety of substituted and bifunctional aldehydes, ketones, and imines.

Multicomponent Porphyrin-Based Tetragonal Prismatic Metallacages and their Photophysical Properties

Multicomponent coordination-driven self-assembly has proved to be a convenient approach to prepare advanced supramolecular coordination complexes (SCCs), especially for those with three-dimensional structures. Herein, we report the preparation of three tetragonal prismatic cages via the self-assembly of Pt(PEt₃)₂(OTf)₂, three different linear dipyridyl ligands and porphyrin-based sodium benzoate ligands. Due to the efficient charge separation in the coordination process of Pt(PEt₃)₂(OTf)₂ with pyridine and carboxylic acid and the directionality of metal-coordination bonds, these cages were prepared in high isolated yields (more than 90%). The absorption and emission properties as well as the singlet oxygen quantum yields of these cages were also studied, showing their potential applications as contrast agents for bio-imaging and photosensitizers for photodynamic therapy.

Photochemical Properties of Host-Guest Supramolecular Systems with Structurally Confined Metal-Organic Capsules

Inspired by natural photosynthesis, researchers have designed symmetric metal-organic hosts with large inner pockets that are spontaneously generated through preorganized ligands and functionalized metallocorners to construct dye-containing host-guest systems. The abundant noncovalent interaction sites in the pockets of the hosts facilitated substrate-catalyst interactions for possible enrichment, fixation, and activation of substrates/reagents, providing special electron transfer pathways for regio- or stereoselectively photocatalytic chemical transformations. In this Account, we focus our attention on metal-organic hosts that contain photoactive or redox-active units to evaluate electron transfer and charge separation between host and guest units in these supramolecular systems and elucidate the related photoinduced chemical reactions controlled by these electron transfer processes within the structurally confined pockets of these interesting metal-organic hosts. We have been engaged in developing methods to isolate a series of chromophores for charge separation in supramolecular systems, incorporating organic dyes as photosensitizers in metal-organic hosts with electron acceptor/donor guests is a promising way to enable typical enzyme-like photocatalytic transformations within a confined microenvironment. Related to these inter- and intramolecular photoinduced electron transfer (PET) processes, the formation of host-guest supramolecular systems to fix and isolate the donor-acceptor pair with a short through-space distance provided a new PET pathway to stabilize the charge-separated ion pair. Highly efficient photosynthetic systems can be obtained when charge transfer to electron donors/acceptors occurs faster than the charge recombination. This Account starts with a brief summary of the potential approaches for constructing photoactive metal-organic hosts through the incorporation of dye molecules within ligand backbones or as a part of the metal nodes of the architecture. Following the methodological summary is a discussion on the mechanisms governing the photoinduced charge separation and electron transfer pathways within the dye-incorporated metal-organic hosts. We also searched for strategies for constructing photoactive supramolecular systems through encapsulating dye molecules within the inner space of redox-active hosts. The photochemistry of these systems demonstrated the following advantages due to the structural confinement: avoiding excited state quenching caused by other chemical species, including aggregated dyes, stabilizing the radical intermediate and tuning the absorption or emission of the guest through electron/energy transfer pathways. The photoinduced dye to redox-active host electron transfer is a new and efficient pathway that is meaningful for chemists to realize and understand many important enzymatic processes and to reveal the secrets of a substance and energy metabolism in biological systems. The confined interactions between the host and the guest have shown fascinating effects of promoting and controlling light-induced chemical transformations.

Synergistic catalysis for light-driven proton reduction using a polyoxometalate-based Cu–Ni heterometallic–organic framework

Cu–Ni–POM employing bimetallic Cu–Ni and POM, which allows a concerted catalysis mechanism, hydrogen evolution reaction exhibited higher activity.

Synthesis and Characterization of Self-Assembled Chiral FeII 2 L3 Cages

We present here the synthesis of chiral BINOL-derived (BINOL=1,1'-bi-2-naphthol) bisamine and bispyridine-aldehyde building blocks that can be used for the self-assembly of novel chiral FeII 2 L3 cages when mixed with an iron(II) precursor. The properties of a series of chiral cages were studied by NMR and circular dichroism (CD) spectroscopy, cold-spray ionization MS, and molecular modeling. Upon formation of the M2 L3 cages, the iron corners can adopt various isomeric forms: mer, fac-Δ, or fac-Λ. We found that the coordination geometry around the metal centers in R-Cages 1 and 2 were influenced by the chiral BINOL backbone only to a limited extent, as a mixture of cages was formed with fac and mer configurations at the iron corners. However, single cage species (fac-RR-Cage and fac-RS-Cage) that are enantiopure and highly symmetric were obtained by generating these chiral M2 L3 cages by using the bispyridine-aldehyde building blocks in combination with chiral amine moieties to form pyridylimine ligands for coordination to iron. Next to consistent NMR spectra, the CD spectra confirm the configurations fac-(Λ,Λ) and fac-(Δ,Δ) corresponding to RR- and RS-Cage, respectively.

Luminescent Tetrahedral Molecular Cages Containing Ruthenium(II) Chromophores

We have designed linear metalloligands which contain a central photoactive [Ru(N^∧N)₃]²⁺ unit bordered by peripheral metal binding sites. The combination of these metalloligands with Zn(II) and Fe(II) ions leads to heterometallic tetrahedral cages, which were studied by NMR spectroscopy, mass spectrometry, and photophysical methods. Like the parent metalloligands, the cages remain emissive in solution. This approach allows direct incorporation of the favorable properties of ruthenium(II) polypyridyl complexes into larger self-assembled structures.

Tetrahedral metal–organic cages with cube-like cavities for selective encapsulation of fullerene guests and their spin-crossover properties

Selective encapsulation of fullerene guests and solid state spin-crossover behaviors were observed in iron(ii) tetrahedral metal–organic cages with cube-like cavities.