Mixed-Ligand Metal–Organic Framework for Two-Photon Responsive Photocatalytic C–N and C–C Coupling Reactions

Modifying Parallel Excitations into One Framework for C(sp3)─H Bond Activation with Energy Combined More Than Two Photons

Natural photosynthesis enzymes utilize energies of several photons for challenging oxidation of water, whereas artificial photo-catalysis typically involves only single-photon excitation. Herein, a multiphoton excitation strategy is reported that combines parallel photo-excitations with a photoinduced electron transfer process for the activation of C(sp3)─H bonds, including methane. The metal-organic framework Fe3-MOF is designed to consolidate 4,4',4″-nitrilotrisbenzoic units for the photoactivation of dioxygen and trinuclear iron clusters as the HAT precursor for photoactivating alkanes. Under visible light irradiation, the dyes and iron clusters absorbed parallel photons simultaneously to reach their excited states, respectively, generating 1O2 via energy transfer and chlorine radical via ligand-to-metal charge transfer. The further excitation of organic dyes leads to the reduction of 1O2 into O2 •- through a photoinduced electron transfer, guaranteeing an extra multiphoton oxygen activation manner. The chlorine radical abstracts a hydrogen atom from alkanes, generating the carbon radical for further oxidation transformation. Accordingly, the total oxidation conversion of alkane utilizing three photoexcitation processes combines the energies of more than two photons. This new platform synergistically combines a consecutive excited photoredox organic dye and a HAT catalyst to combine the energies of more than two photons, providing a promising multiphoton catalysis strategy under energy saving, and high efficiency.

Theoretical study on photoinduced charge transfer in one-photon and two-photon absorption of branching oligofluorenes

Oligofluorenes have been identified as very promising two-photon absorption (TPA) materials and present great application potential for the fabrication of nonlinear optical devices, but the TPA mechanism and corresponding electron excitation properties have not been studied. Here, the photoinduced charge transfer characteristics of V-shaped and Y-shaped branching oligofluorenes that consist of two and three fluorene units in each branch during one-photon absorption (OPA) and TPA processes are analyzed theoretically using the density functional theory and visualization sum-over-states model. The calculated results show that the OPA intensity and TPA cross-section are significantly enhanced by increasing the branch length or changing the structure from V-shaped to Y-shaped. The long-distance charge transfer only occurs on the second transition of TPA at high excited states. Compared to Y-shaped molecules, V-shaped structures exhibit a stronger cooperative effect among the different branches.

Near-Infrared Light-Driven Photocatalysis with an Emphasis on Two-Photon Excitation: Concepts, Materials, and Applications

Efficient utilization of sunlight in photocatalysis is widely recognized as a promising solution for addressing the growing energy demand and environmental issues resulting from fossil fuel consumption. Recently, there have been significant developments in various near-infrared (NIR) light-harvesting systems for artificial photosynthesis and photocatalytic environmental remediation. This review provides an overview of the most recent advancements in the utilization of NIR light through the creation of novel nanostructured materials and molecular photosensitizers, as well as modulating strategies to enhance the photocatalytic processes. A special focus is given to the emerging two-photon excitation NIR photocatalysis. The unique features and limitations of different systems are critically evaluated. In particular, it highlights the advantages of utilizing NIR light and two-photon excitation compared to UV-visible irradiation and one-photon excitation. Ongoing challenges and potential solutions for the future exploration of NIR light-responsive materials are also discussed.

Harnessing Radicals in Confined Supramolecular Environments Made Possible by MOFs

Researching and utilizing radical intermediates in organic synthetic chemistry have innovated discoveries in methodology and theory. Reactions concerning free radical species opened new pathways beyond the frame of the two-electron mechanism while commonly characterized as rampant processes lacking selectivity. As a result, research in this field has always focused on the controllable generation of radical species and determining factors of selectivity. Metal-organic frameworks (MOFs) have emerged as compelling candidates as catalysts in radical chemistry. From a catalytic point of view, the porous nature of MOFs entails an inner phase for the reaction that could offer possibilities for the regulation of reactivity and selectivity. From a material science perspecti ve, MOFs are organic-inorganic hybrid materials that integrate functional units in organic compounds and complex forms in the tunable long-ranged periodic structure. In this account, we summarized our progress in the application of MOFs in radical chemistry in three parts: (1) The generation of radical species; (2) The weak interactions and site selectivity; (3) Regio- and stereo-selectivity. The unique role of MOFs play in these paradigms is demonstrated in a supramolecular narrative through the analyses of the multi-constituent collaboration within the MOF and the interactions between MOFs and the intermediates during the reactions.

Unleashing the photocatalytic potential of a noble-metal-free heteroleptic copper complex-based nanomaterial for an enhanced aza-Henry reaction

In this work, we fabricated a versatile and noble metal free copper-based heterogeneous photocatalyst, representing a green shift away from precious group metals such as Ir, Ru, Pt, which have been widely utilized as photocatalysts. The successfully synthesized and characterized copper photocatalyst was employed to establish a cross dehydrogenative coupling via C-H activation between tertiary amines and carbon nucleophiles. The highly efficient copper-based photocatalyst was characterized by numerous physico-chemical techniques, which confirmed its successful formation as well as its high activity. Inductively coupled plasma (ICP-OES) analysis revealed that the composite Cu@Xantphos@ASMNPs had a very high loading of 0.423 mmol g-1 of copper. The magnetic Cu@Xantphos@ASMNPs were utilized as a potential heterogeneous photocatalyst for the very facile and regioselective conversion of aryl tetrahydroqinoline to the respective nitroalkyl aryl tetrahydroisoquinoline in high yield using air as an oxidant and methanol as a green solvent with irradiation with visible light under mild reaction conditions. Additionally, the catalyst shows exceptional chemical stability and reusability without any agglomeration even after several cycles of use, which is one of the key features of this material, rendering it a potential candidate from economic and environmental perspectives.

Physical Mechanisms of Intermolecular Interactions and Cross-Space Charge Transfer in Two-Photon BDBT-TCNB Co-Crystals

Co-crystal materials formed by stacking different molecules with weak interactions are a hot research topic. In this work, we theoretically investigate the intermolecular interactions and charge transfer properties of the supramolecular BDBT-TCNB co-crystal (BTC). The π-π bonds, hydrogen bonds, and S-N bonds in the BTC bind the BDBT and TCNB molecules together to form a highly ordered co-crystal and lead to the co-crystal's excellent two-photon absorption (TPA) properties. The intermolecular interactions of the BTC are discussed in detail by the independent gradient model based on Hirshfeld partition (IGMH), atoms in molecules (AIM), electrostatic overlay diagram, and symmetry-adapted perturbation theory (SAPT) energy decomposition; it is found that there is a strong interaction force along the stacking direction. The charge transfer properties of the one-photon absorption (OPA) and TPA of the BTC were investigated by charge density difference (CDD) and transition density matrix (TDM). It is found that the dominant charge transfer mode is the cross-space charge transfer along the stacking direction. Therefore, strong intermolecular interactions will promote intermolecular cross-space charge transfer. This work is of great significance for the design of organic optoelectronic supramolecular materials.

Anthraquinone-Based Metal-Organic Frameworks as a Bifunctional Photocatalyst for C-H Activation

Metal-organic frameworks (MOFs) have gained attention as multifunctional catalytic platforms, allowing us to gain important insights into synergistically activating both C-H bonds and oxygen for improving oxidation. Herein, by ingenious incorporation of anthraquinone, we report an anthraquinone-based MOF as a bifunctional heterogeneous photocatalytic platform to simultaneously activate inert C(sp³)-H bonds and oxygen for C-H bond oxidation. Making use of the rigid framework with the fixation and isolation effect, both a great chemical stability and bifunctional synergistic photocatalytic effects were obtained through the immobilization of anthraquinone into a MOF. Importantly, while decorating two carboxyl groups on anthraquinone, the carbonyl groups of anthraquinone photosensitizers were not involved in coordinating the self-assembly and orderly arranged on the wall of channels that were constructed through a π-π interaction between the anthraquinone moieties in the adjacent layers, which was beneficial to form and stabilize the excited-state radical intermediates in the molecule-fenced channels, and the close proximity between the catalytic sites and the substrates to abstract a hydrogen atom from the substrate through the hydrogen atom transfer process aimed at activating the inertness of C-H bonds. Moreover, high-density-distributed anthraquinone dyes in the confined channels would activate oxygen to form singlet oxygen (¹O₂) through an energy transfer pathway, further promoting inert C(sp³)-H bond oxidation efficiency. Under visible light irradiation, this anthraquinone-based MOF was successfully applied to explore activation and oxidation of a series of substrates containing benzylic C(sp³)-H bonds in the presence of air or oxygen to produce the corresponding carbonyl products. This bifunctional photocatalytic platform based on a heterogeneous MOF provides an available catalytic avenue to develop a scalable and sustainable synthetic strategy using green and sustainable oxygen as the potent oxidant.

Visible-Light-Driven Oxidation of Amines to Imines in Air Catalyzed by Polyoxometalate-Tris(bipyridine)ruthenium Hybrid Compounds

The development of visible-light photocatalysts for the selective oxidative coupling of amines to imines is an area of great interest. Herein, four hybrid compounds based on polyoxometalate anions and tris(bipyridine)ruthenium cations, Ru(bpy)₃[M₆O₁₉] (M = Mo, W) 1-2, [Ru(bpy)₃]₂[Mo₈O₂₆] 3, [Ru(bpy)₃]₂[W₁₀O₃₂] 4, are prepared and characterized by X-ray diffraction (single-crystal and powder), elemental analysis, energy-dispersive X-ray spectroscopy (EDS) analysis, infrared (IR) spectroscopy, and solid diffuse reflective spectroscopy. Single-crystal structural analysis indicates that polyoxometalate anions and tris(bipyridine)ruthenium cations interact with each other through extensive hydrogen bonds in these compounds. These hybrid species with strong visible-light-harvesting abilities and suitable photocatalytic energy potentials show excellent photocatalytic activity and selectivity for the oxidation of amines to imines at room temperature in air as an oxidant. Among them, compound 1 with the [Mo₆O₁₉]^2- anion has the highest catalytic activity, which can swiftly convert >99.0% of benzylamine into N-benzylidenebenzylamine with a selectivity of 98.0% in 25 min illumination by a 10 W 445 nm light-emitting diode (LED). Its turnover frequency reaches 392 h^-1, which is not only better than the homogeneous catalyst [Ru(bpy)₃]Cl₂ but also much superior to those achieved over most of reported heterogeneous catalysts. Moreover, it shows a wide generality for various aromatic amines, accompanied by the advantages of good recyclability and stability. The photocatalytic oxidation mechanism of amines to the corresponding imines over polyoxometalate-based hybrid compounds was fully investigated.

Eosin Y-Containing Metal-Organic Framework as a Heterogeneous Catalyst for Direct Photoactivation of Inert C-H Bonds

Xanthene dyes as a class of ideal organic homogeneous photocatalyst have received significant attention in C-H bond activation; however, the inherent nature of fast carrier recombination/deactivation and low stability limits their practical applications. Herein, by the ingenious decoration of eosin Y into a porous metal-organic framework (MOF), a high-performance heterogeneous MOF-based photocatalyst was prepared to efficiently activate inert C-H bonds on the reactants via the hydrogen atom transfer pathway for the functionalization of the C-H bonds. Taking advantage of the fixation effect of a rigid framework, the incorporation of eosin Y into MOF leads to great enhancement of their chemical durability. More importantly, by the introduction of the second auxiliary ligand, the carbonyl groups of xanthene on the eosin Y dyes were perfectly retained and periodically aligned within the confined channels of this rigid framework, which could effectively form excited state radicals to prompt inert C-H bond activation, promoting reaction efficiency by the host-guest supramolecular interaction. New eosin Y-based MOFs were recyclable for six times without reducing photocatalytic activity. This eosin Y functionalized MOF-based heterogeneous photocatalytic system provides an availably catalytic avenue to develop a scalable and sustainable synthetic strategy for the practical application of organic dyes.

Two-photon-absorbing ruthenium complexes enable near infrared light-driven photocatalysis

One-photon-absorbing photosensitizers are commonly used in homogeneous photocatalysis which require the absorption of ultraviolet (UV) /visible light to populate the desired excited states with adequate energy and lifetime. Nevertheless, the limited penetration depth and competing absorption by organic substrates of UV/visible light calls upon exploring the utilization of longer-wavelength irradiation, such as near-infrared light (λ_irr > 700 nm). Despite being found applications in photodynamic therapy and bioimaging, two-photon absorption (TPA), the simultaneous absorption of two photons by one molecule, has been rarely explored in homogeneous photocatalysis. Herein, we report a group of ruthenium polypyridyl complexes possessing TPA capability that can drive a variety of organic transformations upon irradiation with 740 nm light. We demonstrate that these TPA ruthenium complexes can operate in an analogous manner as one-photon-absorbing photosensitizers for both energy-transfer and photoredox reactions, as well as function in concert with a transition metal co-catalyst for metallaphotoredox C-C coupling reactions.

Promoted Electron Transfer and Surface Absorption by Single Nickel Atoms for Photocatalytic Cross-Coupling of Aromatic Alcohols and Aliphatic Amines under Visible Light

The preparation of imines has drawn increasing attention as they are fundamental intermediates in the production of pharmaceuticals, agricultures, and fine chemicals. Nevertheless, current approaches for imines synthesis mainly focus on thermally driven reactions which always involve the consumption of high price noble metal catalysts, expensive ligands, strong base, and harsh reaction conditions. Herein, we demonstrate single atom nickel anchored on polymeric carbon nitride (Ni-SA@PCN) in Ni-N₄ structure for visible light-promoted crossed coupling between aromatic alcohols and aliphatic amines. As expected, the Ni atoms dispersed carbon nitride demonstrates an obviously improved charge separation and transfer as reflected in UV-vis, fluorescence intensity and lifetime, photocurrent density, and electrochemical impedance characterizations. More impressively, the density functional theory (DFT) calculations also reveals that the presence of Ni atoms can dramatically accelerate the absorption of reactive substrates on the surface of PCN. The decreased absorption energy from -0.51 to -3.35 eV, associated with increased O═O bond length from 1.226 to 1.371 Å indicates a huge advantage of single Ni atom on oxygen activation. As a result, the obtained Ni-SA@PCN photocatalyst shows a prominent catalytic efficiency in imines formation with a reaction conversion of 73% and selectivity of >99%. Lastly, the photocatalytic reactions displays an excellent compatibility with various imines being achieved with high yield.

Near-infrared light photocatalysis enabled by a ruthenium complex-integrated metal–organic framework via two-photon absorption

Photocatalysis under UV/visible light irradiation has emerged as one of the green methodologies for solar energy utilization and organic synthesis. These photocatalytic processes are typically initiated by one-photon-absorbing metal complexes or organic dyes. Nevertheless, the intrinsic restrictions of UV/visible light irradiation, such as shallow penetration in reaction solutions, competing absorption by substrates, and limited coverage of the solar spectrum, call for the development of innovative photocatalysts functioning under longer wavelength irradiation. Herein, we report a ruthenium complex containing a metal-organic framework, MOF-Ru1, which can drive diverse organic reactions under 740 nm light irradiation following the two-photon absorption (TPA) process. Various organic transformations such as energy transfer, reductive, oxidative, and redox neutral reactions were realized using this heterogeneous hybrid photocatalyst. Overall, MOF-Ru1 represents an intriguing TPA photocatalyst active under near-infrared light irradiation, paving a way for the efficient utilization of low-energy light and convenient photocatalyst recycling because of phase separation.

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Ru complexes with π-conjugation ligands show two-photon absorption of NIR photons

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Hybrid MOF-Ru has NIR light-driven photocatalytic performance with recyclability

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A variety of organic reactions were photocatalyzed by MOF-Ru under 740 nm irradiation

Linear and Nonlinear Photon-Induced Cross Bridge/Space Charge Transfer in STC Molecular Crystals

In this work, we theoretically studied the optical absorption properties of a layer-stacked cocrystal heterogeneous material Spe-TCNB cocrystal (STC) which is produced by supramolecular self-assembly of organic conjugated monomers SPE and TCNB. The highly ordered aggregate structure in the cocrystal STC will lead to intermolecular interactions such as π∼π, hydrogen bonds and van der Waals forces, resulting in significant charge transfer characteristics and large cross-sectional two-photon absorption characteristics. The physical mechanism of one-photon and two-photon charge transfer of cocrystal molecules is specifically discussed and the interaction between molecules and their role in charge transfer are quantitatively analyzed. We found that the charge transfer between molecular junctions composed of hydrogen bonds is mainly cross-bridge charge transfer, while the charge transfer between molecular junctions caused by accumulation is mainly cross-space charge transfer. This discovery is of great significance to the design of organic photoelectric functional materials.

Synergy of Oxygen Vacancies and Acid Sites on N-Doped WO3 Nanobelts for Efficient C-C Coupling Synthesis of Benzoin Isopropyl Ether

The surface property of a photocatalyst, including surface acid sites and oxygen vacancies, plays a pivotal role in photocatalytic organic synthesis reactions. Benzoin isopropyl ether (BIE) is usually produced via polycondensation of benzaldehyde and catalyzed with highly toxic cyanide. Here, we report a green photocatalytic approach for the selective synthesis of BIE over WO₃ driven by a green-light-emitting diode. The improved photocatalytic activity can be attributed to the synergy of oxygen vacancies (V_Os) and acid sites over N-doped WO₃ nanobelts. The results revealed that reactant molecules were predominantly adsorbed and activated on surface oxygen vacancies (V_OSs) and the Brønsted acid promoted the etherification reaction; the introduction of V_Os and nitrogen altered the band structure and electronic properties, resulting in improved photocatalytic activity. Our work provides an efficient approach to the selective photocatalytic synthesis of organics over photocatalysts with finely tuned surface properties and band structures via defect and doping engineering.

Ligand regulated metal–organic frameworks for synergistic photoredox and nickel catalysis

Synergistic photoredox and nickel catalytic cross-coupling systems have created a great attraction as a promising methodology to produce the aryl C−N bonds under mild conditions as well as extreme challenge,...

Metal–organic framework (MOF)-, covalent-organic framework (COF)-, and porous-organic polymers (POP)-catalyzed selective C–H bond activation and functionalization reactions

The review summarizes the state-of-the-art of C–H active transformations over crystalline and amorphous porous materials as new emerging heterogeneous (photo)catalysts.

De novo design and synthesis of dipyridopurinone derivatives as visible-light photocatalysts in productive guanylation reactions

Described here is the de novo design and synthesis of a series of 6H-dipyrido[1,2-e:2',1'-i]purin-6-ones (DPs) as a new class of visible-light photoredox catalysts (PCs). The synthesized DP1-5 showed their λ _Abs(max) values in 433-477 nm, excited state redox potentials in 1.15-0.69 eV and -1.41 to -1.77 eV (vs. SCE), respectively. As a representative, DP4 enables the productive guanylation of various amines, including 1°, 2°, and 3°-alkyl primary amines, secondary amines, aryl and heteroaryl amines, amino-nitrile, amino acids and peptides as well as propynylamines and α-amino esters giving diversities in biologically important guanidines and cyclic guanidines. The photocatalytic efficacy of DP4 in the guanylation overmatched commonly used Ir and Ru polypyridyl complexes, and some organic PCs. Other salient merits of this method include broad substrate scope and functional group tolerance, gram-scale synthesis, and versatile late-stage derivatizations that led to a derivative 81 exhibiting 60-fold better anticancer activity against Ramos cells with the IC₅₀ of 0.086 μM than that of clinical drug ibrutinib (5.1 μM).

Cuprous Cluster-Based Coordination Sheets as Photocatalytic Regulators to Activate Oxygen, Benzoquinone, and Thianthrenium Salts

Cuprous clusters are well known for their important fluorescent properties and tunable redox behavior, but the coordinated protecting groups restrict their application in photocatalysis, in particular, the inner-sphere activation of substrates. By modifying fluorescent cuprous clusters with terminal iodides into two-dimensional coordination sheets, we report a photocatalytic regulator to synergistically combine electron transfer and energy transfer for the oxidative coupling of benzoquinone and terminal alkynes. Under visible light irradiation, the well-modified excited state of the cuprous clusters in the coordination sheets reduces benzoquinones to generate aoxy radicals through electron transfer and activates oxygen through energy transfer. The aoxy radicals interact with copper-coordinated phenylacetylene to form an active intermediate, which is further oxidized by the in situ formed active oxygen species and aryl ketones are obtained. The warranted potential of the excited coordination sheets enables the reductive activation of thianthrenium salts as radical precursors, facilitating radical capture and further C-N coupling via an inner-sphere activation mechanism. The new catalytic approach optimizes the redox properties and excited-state lifetime, shortens the electron transfer steps, and promotes the potential collision of a low concentration of active species in tandem catalytic cycles, thus paving a new way to develop ecologically benign, cost-effective, multipurpose, and flexible catalytic systems.

A Metal-Organic Framework as a Multiphoton Excitation Regulator for the Activation of Inert C(sp3 )-H Bonds and Oxygen

The activation and oxidization of inert C(sp³ )-H bonds into value-added chemicals affords attractively economic and ecological benefits as well as central challenge in modern chemistry. Inspired by the natural enzymatic transformation, herein, we report a new multiphoton excitation approach to activate the inert C(sp³ )-H bonds and oxygen by integrating the photoinduced electron transfer (PET), ligand-to-metal charge transfer (LMCT) and hydrogen atom transfer (HAT) events together into one metal-organic framework. The well-modified nicotinamide adenine dinucleotide (NAD⁺ ) mimics oxidized Ce^III -OEt moieties to generate Ce^IV -OEt chromophore and its reduced state mimics NAD^. via PET. The in situ formed Ce^IV -OEt moiety triggers a LMCT excitation to form the alkoxy radical EtO^. , abstracts a hydrogen atom from the C(sp³ )-H bond, accompanying the recovery of Ce^III -OEt and the formation of alkyl radicals. The formed NAD^. activates oxygen to regenerate the NAD⁺ for next recycle, wherein, the activated oxygen species interacts with the intermediates for the oxidization functionalization, paving a catalytic avenue for developing scalable and sustainable synthetic strategy.

Photocatalytic C-H Activation with Alcohol as a Hydrogen Atom Transfer Agent in a 9-Fluorenone Based Metal-Organic Framework

Hydrogen atom transfer (HAT) has become an attractive strategy for the activation of hydrocarbon feedstocks. Alcohols, as inexpensive and efficient hydrogen transfer reagents, have limited application in C-H functionalization due to the difficulty in the alkoxy radical acquisition. 9-Fluorenone moieties were incorporated into the metal-organic framework (MOF) as a photocatalyst; through the formation of hydrogen bonds between the carbonyl group of a ligand and alcohol, alkoxy radicals could be obtained by the visible-light-driven oxidation of 2,2,2-trichloroethanol via proton-coupled electron transfer (PCET). Effectively photocatalyzed intermolecular coupling reactions between phenyl vinyl sulfone and aldehyde or cyclic ether were realized through the HAT pathway. Compared to homogeneous catalysts, the heterogeneous MOF photocatalyst improved the catalytic efficiency and could be recycled at least five times. The microenvironment of the Zn-OFDC channel was beneficial for the formation of hydrogen bonds and stability of alkoxy radicals.

Two porous Ni-MOFs based on 2,4,6-tris(pyridin-4-yl)-1,3,5-triazine showing solvent determined structures and distinctive sorption properties toward CO2 and alkanes

By regulating the solvent used for synthesis, two porous Ni-MOFs, namely {[Ni3(BTC)2(TPT)2/3(H2O)4.08(MeOH)0.92]·2DMF·0.5H2O·0.5MeOH}n (1) and {[Ni3(BTC)2(TPT)2(H2O)6]·6DMF}n (2) (H3BTC = 1,3,5-benzenetricarboxylic acid, TPT = 2,4,6-tris(pyridin-4-yl)-1,3,5-triazine, DMF = N,N-dimethylformamide, and MeOH = methanol) were obtained. Compound 1 reveals a rigid 3D framework, while compound 2 shows a flexible 3-fold interpenetrated framework. Compound 1 exhibits a selective adsorption of CO2 due to the sieving effect of the rigid framework containing two types of cages with small apertures. Noteworthily, the flexible compound 2 displays an obviously guest-induced structural transformation. The desolvated compound 2 reveals a much higher capacity toward CO2 and n-C4H10 than those of N2 CH4, C2H6 and C3H8.

Photocatalytic Visible-Light-Induced Nitrogen Insertion via Dual C(sp3)-H and C(sp2)-H Bond Functionalization: Access to Privileged Imidazole-based Scaffolds

Here we have demonstrated a visible-light-mediated metal-free organic-dye-catalyzed dehydrogenative N-insertion leading to highly substituted imidazoles and privileged dihydroisoquinoline-based imidazole derivatives via C(sp³)-H and C(sp²)-H bond functionalization. A sustainable, convenient, metal-free azidation/C-H aminative cyclization approach in the absence of stoichiometric oxidants is presented. This protocol involves a rare photoinduced iminyl radical as a key intermediate for the "N" insertion.

Photocatalyst- and additive-free decarboxylative alkylation of N-aryl tetrahydroisoquinolines induced by visible light

A photocatalyst- and additive-free visible light induced decarboxylative alkylation of N-aryl tetrahydroisoquinolines has been developed using tetrachloro-N-hydroxyphthalimide esters as alkylation agents.

Solid-state NMR studies of the acidity of functionalized metal-organic framework UiO-66 materials

The acid strength of metal-organic frameworks plays a key role in their catalytic performance such as activity and selectivity during catalytic reactions. Solid-state nuclear magnetic resonance in combination with probe molecules including 2-¹³ C-acetone and pyridine-d₅ was employed to characterize the acid strength of UiO-66-X (X = -H, -2COOH, -SO₃ H). It was found that after introduction of the functional groups, the acid strength of UiO-66-2COOH and UiO-66-SO₃ H is considerably enhanced compared with that of parent UiO-66, with that of the former being similar to that of zeolite H-ZSM-5, and with that of the latter being slightly stronger than that of the former. Even though the acid density can efficiently be modified through changing the relative ratio in multivariate functionalized UiO-66-X, no significant alternation for the acid strength could be discerned in the MTV-UiO-66-X compared with acidic same-link counterpart. Theoretical calculations were employed to further confirm the acid strength of UiO-66-SO₃ H and UiO-66-2COOH.

Metal–Organic Frameworks (MOFs) and Covalent Organic Frameworks (COFs) Applied to Photocatalytic Organic Transformations

Among the different alternatives for catalysis using metal–organic frameworks (MOFs) or covalent organic frameworks (COFs), photocatalysis has remarkably evolved during the last decade. Photocatalytic reticular materials allowed recyclability and easy separation of catalyst from the product, also reaching the activity and selectivity commonly observed for molecular systems. Recently, photocatalytic MOFs and COFs have been applied to synthetic applications in order to obtain organic molecules of different complexity. However, although a good number of works have been devoted to this issue, an updated comprehensive revision on this field is still needed. The aim of this review was to fill this gap covering the following three general aspects: (1) common strategies on the design of reticular photocatalytic materials, (2) a comprehensive discussion of the photocatalytic organic reactions achieved by the use of COFs and MOFs, and (3) some critical considerations highlighting directions that should be considered in order to make advances in the study of photocatalytic COFs and MOFs.

CO2-triggered reversible phase transfer of graphene quantum dots for visible light-promoted amine oxidation

Carbon dots, emerging as novel photoluminescent nanomaterials, have attracted increasing attention for photocatalytic applications such as hydrogen evolution, carbon dioxide reduction, pollutant degradation and organic synthesis. However, the separation of carbon dots from the reaction system is always a cumbersome process, which may limit their applications in photocatalysis. Herein, we report the synthesis of a graphene quantum dot with CO2-switchable reversible phase transfer performance via a facile surface functionalization approach. The mechanism of this hydrophilicity and hydrophobicity alteration involves the protonation-deprotonation transformation and reversible formation of hydrophilic bicarbonate salts when CO2 is bubbled and removed. Then, the obtained graphene quantum dot was utilized as a visible-light photocatalyst for the oxidative coupling of amines. Our photocatalyst demonstrates excellent catalytic efficiency with both high reaction conversion and selectivity. Furthermore, the proposed graphene quantum dot could be recycled via a simple CO2 bubbling method.

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.

AIEE Active Nanoassemblies of Pyrazine Based Organic Photosensitizers as Efficient Metal-Free Supramolecular Photoredox Catalytic Systems

Pyrazine derivatives DIPY, TETPY and CNDIPY have been designed and synthesized which form fluorescent supramolecular assemblies in mixed aqueous media due to their AIEE and ICT characteristics. Among all the derivatives, the assemblies of TETPY and CNDIPY show strong absorption in the visible region with high absorption coefficients, low HOMO-LUMO gap, and high photostability. Further, the supramolecular nanoassemblies of TETPY and CNDIPY show excellent potential to generate reactive oxygen species (ROS) under the visible light irradiation. Owing to their strong absorption in the visible region and ROS generation ability, the supramolecular nanoassemblies of TETPY and CNDIPY act as efficient photoredox catalytic systems for carrying out (a) oxidative amidation of aromatic aldehydes (b) hydroxylation of boronic acid and (c) oxidative homocoupling of benzylamines under mild conditions such as aqueous media, aerial environment, and natural sunlight as a source of irradiation. All the mechanistic investigations suggest the participation of in-situ generated ROS in the organic transformations upon light irradiation.