Visible-Light-Induced Photoredox Catalysis of Dye-Sensitized Titanium Dioxide: Selective Aerobic Oxidation of Organic Sulfides

Integrating Polyoxometalates and Silver Clusters into Atomically Precise Molecular Heterojunction

The Ag cluster-POM assemblies have been shown to possess interesting and potentially useful properties. However, there is no precedent example of atomically precise Ag cluster-POM assemblies showing heterojunction effects in photocatalysis. Herein, the synthesis and total structure determination of the periodically distributed molecular heterojunction [Ag12(SCy)6(CH3CN)12(PW12O40)]n (Ag12-PW12) are reported. The assembly of Ag/W clusters into 3D network can endow the resulting binary structure with an aesthetic topology and unique physicochemical properties. More remarkably, the incorporation of Ag12 cluster with PW12 can efficiently facilitate the separation of photogenerated electrons and holes, thus significantly promoting the catalytic efficiency in selective oxidation of sulfides. The Ag12-PW12 heterojunction can be recovered and reused five times with no drastic change in the catalytic performance. This research is expected to assist in the rational design of cluster-based heterojunction catalysts. The increase of catalytic activity of the Ag12-PW12 assembly in comparison with the unassembled Ag12 and PW12 clusters is attributed to the synergistic effect of Ag12 and PW12 clusters, offering the splendid opportunity for deciphering structure-reactivity relationship of heterostructure-coupled photosystem.

Rational Design of Highly Porous Donor-Acceptor Based Conjugated Microporous Polymer for Photocatalytic Benzylamine Coupling Reaction

Conjugated microporous polymers (CMPs) are an important class of organic materials with several useful features like, inherent nanoscale porosity, large specific surface area and semiconducting properties, which are very demanding for various sustainable applications. Carbazole building blocks are extensively used in designing photocatalysts due to easy electron donation and hole transportation. In the current study, a new CMP material CBZ-CMP containing carbazole unit used for photocatalytic C═N coupling reaction under blue light irradiation is designed. The CBZ-CMP framework is made through the polycondensation of 4,4'-di(9H-carbazol-9-yl)-1,1'-biphenyl using FeCl3 as a catalyst. The CBZ-CMP shows very high BET surface area of 1536 m2 g-1 together with unimodal porosity (ca. 1.7 nm supermicropore), nanowire-like particle morphology (16-18 nm diameter), and low band gap property. The bi-phenyl moiety functions as the electron accepting center and the carbazole unit acts as the donor center, which accounts for the low band gap energy of CBZ-CMP. This nanoporous semiconducting CBZ-CMP material for photocatalytic benzylamine coupling reaction is explored, where it shows good conversion together with high selectivity under mild reaction conditions. This study offers simple method of preparation of a D-A-D-based porous photocatalyst for sustainable synthesis of value-added organics.

Competitive photooxidation of small colorless organics controlled by oxygen vacancies under visible light

Visible-light photooxidation sensitized by surface attachment of small colorless organics on semiconductor photocatalysts has emerged as an economical method for photocatalytic synthesis or degradation. In particular, heteroatom (X = N and Cl)-containing substrates could undergo either C-N coupling or dechlorination degradation via sensitizing TiO2, but the mechanism in conducting the competitive visible-light sensitized photooxidations is still vague. Herein, the visible-light photooxidation of colorless 4-chlorobenzene-1,2-diamine (o-CAN) on TiO2 was revealed, contributing to selective C-N coupling rather than dechlorination. Oxygen vacancies (OVs) were in situ generated on the TiO2 surface, which could be dominant in weakening the Cl-Ti adsorption of o-CAN and regulating the activation of O2 for selective C-N coupling. The C-N coupling product, functionalized as the sensitizer, further promoted the visible-light photooxidation upon N-Ti and Cl-Ti coordination. This process was then confirmed by on-line mass spectrometric analysis, and the intermediates as well as their kinetics were determined. Thereby, theoretical calculations were employed to verify the roles of OVs in competitive photooxidation and lowering the energy barriers as well. Based on the comprehensive characterizations of both the catalysts and intermediates, this work has provided insights into competitive photooxidations under visible light.

Photocatalytic fixation and oxygenation of NAD+/NADP+ and sulfides using solar light: Exploring mechanistic investigations and their impact on synthetic applications

Sulfur-doped Eosin-B (SDE-B) photocatalysts were synthesized for the first time utilizing sublimed sulfur (S8) as a dopant in an in situ thermal copolymerization technique. Sulfur doping not only increased Eosin-B (E-B) absorption range for solar radiation but also improved fixation and oxygenation capabilities. The doped sulfur bridges the S-S bond by substituting for the edge bromine of the E-B bond. The improved photocatalytic activity of SDE-B in the fixation and oxygenation of NAD+/NADP+ and sulfides using solar light is attributed to the photo-induced hole of SDE-B's high fixation and oxygenation capacity, as well as an efficient suppression of electron and hole recombination. The powerful light-harvesting bridge system created using SDE-B as a photocatalyst works extremely well, resulting in high NADH/NADPH regeneration (79.58/76.36%) and good sulfoxide yields (98.9%) under solar light. This study focuses on the creation and implementation of a sulfur-doped photocatalyst for direct fine chemical regeneration and organic transformation.

Embracing heterogeneous photocatalysis: evolution of photocatalysts in annulation of dimethylanilines and maleimides

Recent advances in visible-light-promoted construction of tetrahydroquinolines from dimethylanilines and maleimides are documented. Homogeneous and heterogeneous photocatalytic systems, as well as the reaction mechanism, are emphasized. The mechanism of this photocatalytic annulation reaction is quite clear, i.e., dimethylanilines and maleimides serve as the radical precursors and radical acceptors, respectively. This annulation reaction could serve as an excellent platform for evaluating novel oxidative heterogeneous photocatalytic systems, which could further inspire chemists in this field to develop more efficient photocatalytic systems. Significant opportunities are expected in the future for heterogeneous photocatalysis strategies.

Single-Atom palladium engineered cobalt nanocomposite for selective aerobic oxidation of sulfides to sulfoxides

Developing efficient catalysts for the selective oxidation of sulfides to sulfoxides using molecular oxygen as the oxidant is a challenging task. Here, we report a novel catalyst comprising a single atom palladium engineered cobalt nanocomposite (denoted as PdCo@NC-800-0.01) for this reaction. The incorporation of single atom palladium effectively transforms an originally inactive cobalt nanocomposite into a highly efficient and selective catalyst for the oxidation of sulfides. This catalyst PdCo@NC-800-0.01 exhibited outstanding performance in the selective oxidation of sulfides to sulfoxides using O2 as the oxidant in the presence of isobutyraldehyde (IBA) under mild conditions, demonstrating high activity and excellent selectivity for a broad spectrum of sulfides with good tolerance toward various functional groups, including those susceptible to oxidation. Furthermore, the catalyst could be easily recovered and reused up to 10 times without any significant loss in activity and selectivity. Comprehensive characterizations and theoretical calculations revealed that the engineering of cobalt nanocomposite with single atom Pd greatly enhanced the ability to adsorb and activate IBA, leading to the generation of the key acyl radical. This radical then reacted with singlet oxygen 1O2 derived from molecular oxygen, producing reactive oxygen species peroxy radical, which ultimately promoted the catalytic performance.

Expanding the Use of Benzothioxanthene Imides to Photochemistry: Eco-Friendly Aerobic Oxidation of Sulfides to Sulfoxides

The sulfoxide moiety is one of the most commonly utilized groups in pharmaceutical and industrial chemistry. The need for sustainability and easy accessibility to sulfoxide moieties is deemed necessary, due to its ubiquity in natural products and potentially pharmaceutically active compounds. In this context, we report herein a sustainable, aerobic and environmentally friendly photochemical protocol based on the use of a benzothioxathene imide as the photocatalyst to selectively oxidize sulfides under mild irradiation (456 nm), in very low catalyst loading (0.01 mol %) and on water. In addition, to demonstrate the compatibility of our protocol with wide scope of substrates, the latter was successfully applied to the synthesis of the biologically-active Sulforaphane and Modafinil.

Recyclable and Stable Porphyrin-Based Self-Assemblies by Electrostatic Force for Efficient Photocatalytic Organic Transformation

Development of efficient, stable, and recyclable photocatalysts for organic synthesis is vital for transformation of traditional thermal organic chemistry into green sustainable organic chemistry. In this work, the study reports an electrostatic approach to assemble meso-tetra (4-sulfonate phenyl) porphyrin (TPPS)tetra (4-sulfonate phenyl) porphyrin (TPPS) as a donor and benzyl viologen (BV) as an acceptor into stable and recyclable photocatalyst for an efficient organic transformation reaction - aryl sulfide oxidation. By use of the electrostatic TPPS-BV photocatalysts, 0.1 mmol aryl sulfide with electron-donating group can be completely transformed into aryl sulfoxide in 60 min without overoxidation into sulfone, rendering near 100% yield and selectivity. The photocatalyst can be recycled up to 95% when 10 mg amount is used. Mechanistic study reveals that efficient charge separation between TPPS and BV results in sufficient formation of superoxide which further reacts with the oxidized sulfide by the photocatalyst to produce the sulfoxide. This mechanistic pathway differs significantly from the previously proposed singlet oxygen-dominated process in homogeneous TPPS photocatalysis.

A Porous Carbazolic Al-MOF for Efficient Aerobic Photo-Oxidation of Sulfides into Sulfoxides under Air

A robust, microporous, and photoactive aluminum-based metal-organic framework (Al-MOF, LCU-600) has been assembled by an in situ-formed [Al3O(CO2)6] trinuclear building unit and a tritopic carbazole ligand. LCU-600 shows a high water stability and permanent porosity for N2 and CO2 adsorption. Notably, the incorporation of photoresponsive carbazole moieties into LCU-600 makes it a highly efficient and recyclable photocatalyst for aerobic photo-oxidation of sulfides into sulfoxides under an air atmosphere at room temperature. Mechanism investigations unveil that photogenerated holes (h+), superoxide radical anion (O2•-), and singlet oxygen (1O2) are critical active spices for the photo-oxidation reaction performed in an air atmosphere.

Remarkable Enhancement of Photocatalytic Activity of High-Energy TiO2 Nanocrystals for NO Oxidation through Surface Defluorination

The superior photocatalytic activity of TiO2 nanocrystals with exposed high-energy (001) facets, achieved through the use of hydrofluoric acid as a shape-directing reagent, is widely reported. However, in this study, we report for the first time the detrimental effect of surface fluorination on the photoreactivity of high-energy faceted TiO2 nanocrystals towards NO oxidation (resulting in a NO removal rate of only 5.9%). This study aims to overcome this limitation by exploring surface defluorination as an effective strategy to enhance the photocatalytic oxidation of NO on TiO2 nanocrystals enclosed with (001) facets. We found that surface defluorination, achieved through either NaOH washing (resulting in an improved NO removal rate of 23.2%) or calcination (yielding an enhanced NO removal rate of 52%), leads to a large increase in the photocatalytic oxidation of NO on TiO2 nanocrystals with enclosed (001) facets. Defluorination processes stimulate charge separation, effectively retarding recombination and significantly promoting the production of reactive oxygen species, including superoxide radicals (·O2-), singlet oxygen (1O2), and hydroxyl radicals (·OH). Both in situ diffuse reflectance infrared Fourier-transform spectroscopy and density functional theory calculations confirm the higher adsorption of NO after defluorination, thus facilitating the oxidation of NO on TiO2 nanocrystals.

Green approach for the fabrication of a ternary nanocatalyst (Ag-ZnONPs@Cy) for visible light-induced photocatalytic reduction of nitroarenes to aminoarenes

In recent times, the incorporation of metal oxide nanoparticles with organic dyes has piqued the interest of numerous researchers due to their diverse applications under visible light instead of UV radiation. This investigation employed a three-step methodology to fabricate cyanidin-sensitized silver-doped zinc oxide nanoparticles (Ag-ZnO@Cy). Initially, cyanidin dye was extracted from fresh black mulberry fruit, followed by the eco-friendly synthesis of Ag-ZnO nanoparticles (Ag-ZnONPs). The successful integration of the prepared cyanidin dye with Ag-ZnONPs was achieved through a straightforward, environmentally benign, and cost-efficient procedure. The resultant ternary composite underwent comprehensive characterization and confirmation utilizing various techniques, such as SEM, FT-IR, EDX, DRS, elemental mapping, and XRD. The experimental results for Ag-ZnONPs@Cy demonstrated that the nanocrystalline wurtzite exhibited spherical shapes with an average crystal size of 27.42 nm. Moreover, the photocatalytic activity of the synthesized Ag-ZnONPs@Cy was meticulously investigated under blue LED light irradiation. This inquiry encompassed examinations of catalyst amount, regeneration, stability, reusability, and the influence of light source on the hydrogenation of nitroarenes to the corresponding aminoarenes. The findings shed light on the potential of this composite for diverse photocatalytic applications.

In Situ Probing the Short-Lived Intermediates in Visible-Light Heterogeneous Photocatalysis by Mass Spectrometry

Visible-light-mediated heterogeneous photocatalysis has recently emerged as an environmentally friendly and energy-sustainable alternative for organic transformations. Despite the advancements in developing wide varieties of photocatalysts during the past decades, the accurate probing and identification of the photogenerated species, especially the short-lived radical intermediates, are still challenging. In this work, we reported a hybrid ion emitter that integrated with a pico-liter heterogeneous photocatalytic reactor, which was fabricated by depositing the photocatalyst (e.g., TiO2) into the front tip of a quartz micropipette. Benefited from the dual-function feature of the hybrid micropipette (i.e., a clog-free tip-confined pico-liter reactor for heterogeneous photocatalysis and an ion emitter for nanoelectrospray ionization), sensitized photoredox reactions at the catalyst-solution interface can be triggered upon visible-light irradiation using a cheap LED laser (453 nm), and the newly produced transient radical intermediates can be rapidly transformed into gaseous ions for mass spectrometric identification. Using this novel low-delay coupling device, photogenerated intermediates, including the cationic radicals produced during the photooxidation of anilines and the anionic radicals produced during the photoreduction of quinones, were successfully captured by mass spectrometry. We believe that our hybrid photochemical microreactor/ion emitter has provided a new and powerful tool for exploring the complicated heterogeneous photochemical processes, especially their ultrafast initial transformations.

Emerging developments in dye-sensitized metal oxide photocatalysis: exploring the design, mechanisms, and organic synthesis applications

In the present day, the incorporation of environmentally conscious practices in the realm of photocatalysis holds a prominent position within the domain of organic synthesis. The imperative to tackle environmental issues linked to catalysts that cannot be recycled, generation of waste, byproducts, and challenges in achieving reaction selectivity during organic synthesis are more crucial than ever. One potential solution involves the integration of recyclable nanomaterials with light as a catalyst, offering the possibility of achieving sustainable and atom-efficient transformations in organic synthesis. Metal oxide nanoparticles exhibit activation capabilities under UV light, constituting a small percentage (4-8%) of sunlight. However, this method lacks sufficient environmental friendliness, and the issue of electron-hole recombination poses a significant hurdle. To tackle these challenges, multiple approaches have been proposed. This comprehensive review article focuses on the efficacy of dyes in enhancing the capabilities of heterogeneous photocatalysts, offering a promising avenue to overcome the constraints associated with metal oxides in their role as photocatalysts. The article delves into the intricate design aspects of dye-sensitized photocatalysts and sheds light on their mechanisms in facilitating organic transformations.

Organic Synthesis Using Nitroxides

Nitroxides, also known as nitroxyl radicals, are long-lived or stable radicals with the general structure R1R2N-O•. The spin distribution over the nitroxide N and O atoms contributes to the thermodynamic stability of these radicals. The presence of bulky N-substituents R1 and R2 prevents nitroxide radical dimerization, ensuring their kinetic stability. Despite their reactivity toward various transient C radicals, some nitroxides can be easily stored under air at room temperature. Furthermore, nitroxides can be oxidized to oxoammonium salts (R1R2N═O+) or reduced to anions (R1R2N-O-), enabling them to act as valuable oxidants or reductants depending on their oxidation state. Therefore, they exhibit interesting reactivity across all three oxidation states. Due to these fascinating properties, nitroxides find extensive applications in diverse fields such as biochemistry, medicinal chemistry, materials science, and organic synthesis. This review focuses on the versatile applications of nitroxides in organic synthesis. For their use in other important fields, we will refer to several review articles. The introductory part provides a brief overview of the history of nitroxide chemistry. Subsequently, the key methods for preparing nitroxides are discussed, followed by an examination of their structural diversity and physical properties. The main portion of this review is dedicated to oxidation reactions, wherein parent nitroxides or their corresponding oxoammonium salts serve as active species. It will be demonstrated that various functional groups (such as alcohols, amines, enolates, and alkanes among others) can be efficiently oxidized. These oxidations can be carried out using nitroxides as catalysts in combination with various stoichiometric terminal oxidants. By reducing nitroxides to their corresponding anions, they become effective reducing reagents with intriguing applications in organic synthesis. Nitroxides possess the ability to selectively react with transient radicals, making them useful for terminating radical cascade reactions by forming alkoxyamines. Depending on their structure, alkoxyamines exhibit weak C-O bonds, allowing for the thermal generation of C radicals through reversible C-O bond cleavage. Such thermally generated C radicals can participate in various radical transformations, as discussed toward the end of this review. Furthermore, the application of this strategy in natural product synthesis will be presented.

Visible-Light-Driven Photocatalytic Dehydrogenation of Alcohols on TiO2 via Ligand-to-Metal Charge Transfer for Coproduction of H2 and Aldehydes

Developing visible-light-driven photocatalysts for the catalytic dehydrogenation of organics is of great significance for sustainable solar energy utilization. Here, we first report that aromatic alcohols could be efficiently split into H₂ and aldehydes over TiO₂ under visible-light irradiation through a ligand-to-metal charge transfer (LMCT) mechanism. A series of TiO₂ catalysts with different surface contents of the hydroxyl group (-OH) have been synthesized by controlling the hydrothermal and calcination synthesis methods. An optimal H₂ production rate of 18.6 μmol h^-1 is obtained on TiO₂ synthesized from the hydrothermal method with a high content of surface -OH. Experimental characterizations and comparison studies reveal that the surface -OH markedly influences the formation of LMCT complexes and thus changes the visible-light-driven photocatalytic performance. This work is anticipated to inspire further research endeavors in the design and fabrication of visible-light-driven photocatalyst systems based on the LMCT mechanism to realize the simultaneous synthesis of clean fuel and fine chemicals.

Enhanced Photocatalytic Coupling of Benzylamine to N-Benzylidene Benzylamine over the Organic-Inorganic Composites F70-TiO2 Based on Fullerenes Derivatives and TiO2

The organic-inorganic composites F70-TiO₂, based on fullerene with carboxyl group derivatives and TiO₂ semiconductor, have been designed and constructed to become an optical-functional photocatalyst via the facile sol-gel method. The composite photocatalyst obtained shows excellent photocatalytic activity for the high-efficiency conversion of benzylamine (BA) to N-benzylidene benzylamine (NBBA) with air pressure at a normal temperature under visible light irradiation. By optimizing the composition, the composites with the 1:15 mass ratio of F70 and TiO₂, denoted as F70-TiO₂(1:15), demonstrated the highest reaction efficiency for benzylamine (>98% conversion) to N-benzylidene benzylamine (>93% selectivity) in this study. However, pure TiO₂ and fullerene derivatives (F70) exhibit decreased conversion (56.3% and 89.7%, respectively) and selectivity (83.8% and 86.0%, respectively). The UV-vis diffuse reflectance spectra (DRS) and Mott-Schottky experiment's results indicate that the introduction of fullerene derivatives into anatase TiO₂ would greatly broaden the visible light response range and adjust the energy band positions of the composites, enhancing the sunlight utilization and promoting the photogenerated charge (e^--h⁺) separation and transfer. Specifically, a series of results on the in situ EPR tests and the photo-electrophysical experiment indicate that the separated charges from the hybrid could effectively activate benzylamine and O₂ to accelerate the formation of active intermediates, and then couple with free BA molecules to form the desired production of N-BBA. The effective combination, on a molecular scale, between fullerene and titanium dioxide has provided a profound understanding of the photocatalysis mechanism. This work elaborates and makes clear the relationship between the structure and the performance of functional photocatalysts.

Photocatalytic Aryl Sulfide Oxidation Using 4-Nitrophenylacetylene-Modified Cu2O Crystals

4-Nitrophenylacetylene-functionalized Cu₂O rhombic dodecahedra and cubes have been used to photocatalyze aryl sulfide oxidation generating aryl sulfoxides. With an oxygen supply and light from a blue light-emitting diode (LED), the reaction can be completed in 12 h with a water and methanol mixed solution. Generally high product yields and excellent product selectivity of sulfoxides over sulfones were achieved. In particular, a thioanisole to methyl phenyl sulfoxide yield of 98% was obtained. A mechanistic study has revealed that photogenerated electrons, holes, and superoxide radicals are involved in the oxidation reaction. The benefit of simple photocatalyst preparation and molecular functionalization to boost catalytic performance shows that surface-controlled ionic solids can be very effective photocatalysts for some organic transformations.

Visible-Light-Induced Oxidative Coupling of Phenols and Alkenylphenols with a Recyclable, Solid Photocatalyst

A photocatalytic method for phenol and alkenylphenol oxidative coupling is reported using an inexpensive heterogeneous titanium dioxide photocatalyst with air and visible light. During the coupling process, the Ti-substrate complex is activated under visible light through a ligand to metal charge transfer effect, and the diphenol adduct is proposed to form through a radical cation. The heterogeneous TiO2 catalyst remains stable throughout the reaction and can be easily removed and reused multiple times.

Cucurbit[10]uril-based supramolecular radicals: Powerful arms to kill facultative anaerobic bacteria

Two water-soluble supramolecular complexes (CB[10]⊃PSA and CB[10]⊃TPE-cyc) are constructed based on the host-guest interaction between cucurbit[10]uril (CB[10]) and perylene diimide derivative (PSA) or tetracationic cyclophane (TPE-cyc). Attributing to the matched redox potential, both supramolecular complexes can be specifically reduced into corresponding supramolecular radical cations or anions by facultative anaerobic E. coli. Benefiting from the strong near-infrared (NIR) absorption, CB[10]⊃PSA radical anions and CB[10]⊃TPE-cyc radical cations act as efficient NIR photosensitizers and perform an excellent antimicrobial activity (close to 100%) via PTT. In addition, the biocompatibility of TPE-cyc is notably improved under the protection of CB[10], guaranteeing its biosafety for in vivo application. CB[10]⊃PSA radical anions and CB[10]⊃TPE-cyc radical cations are in situ generated in the E. coli-infected abscess of mice and effectively inhibit the bacterial infection without obvious system toxicity. It is anticipated that this supramolecular strategy may pave a new way for the selective bacteria inhibition to regulate the balance of different bacterial flora.

A Photoredox/Sulfide Dual Catalysis System That Uses Sulfide Radical Cations to Promote Alkene Chlorotrifluoromethylation

Sulfides and their derivatives are among the most important class of reagent in synthetic chemistry. Despite the importance of such compounds, the use of sulfide radical cations in synthetic chemistry is underdeveloped. To address this issue, herein, we describe alkene chlorotrifluoromethylation reactions promoted by photoredox/sulfide dual catalysis systems, which involves sulfide radical cations generated through the oxidation of sulfides by a photoredox catalyst. The high functional group tolerance of this chemistry was demonstrated using natural products and drug molecules as substrate alkenes.

Visible light-induced photoredox catalyzed C-N coupling of amides with alcohols

A visible-light-mediated method for the construction of N-monoalkylated products from easily available benzamides and benzyl alcohol in the presence of eosin Y has been developed. The reaction proceeded smoothly, for a wide range of derivatives of benzamides and benzyl alcohols, to give the desired products in good to excellent yields. Biological studies, such as those on drug-likeness and molecular docking, are carried out on the molecules.

Co(NO3)2/covalent organic framework nanoparticles for high-efficiency photocatalytic oxidation of thioanisole

Herein, we demonstrated a highly efficient photocatalytic sulfide oxidation reaction at ambient conditions without a sacrificial reagent or redox mediator, by using Co(NO₃)₂/covalent organic framework nanoparticles as a photocatalyst.

Highly Effective Photocatalytic Radical Reactions Triggered by a Photoactive Metal-Organic Framework

On account of their inherent reactive properties, radical reactions play an important role in organic syntheses. The booming photochemistry provides a feasible approach to trigger the generation of radical intermediates in organic reaction processes. Thus, developing effective photocatalysts becomes the key step in radical reactions. In this work, the triphenylamine moiety with photoactivity is successfully embedded in a highly porous and stable metal-organic framework (MOF), and the obtained MOF, namely, Zr-TCA, naturally displays a photoactive property derived from the triphenylamine-based ligand. In photocatalytic studies, the triphenylamine-based Zr-TCA not only exhibits a high catalytic activity on the aerobic oxidation of sulfides via the generation of the superoxide radical anion (O2•-) under light irradiation but also shows good efficiency in the trifluoromethylation of arenes and heteroarenes by the formation of the trifluoromethyl radical (CF3•) as an intermediate. Moreover, the high performance of Zr-TCA can be well maintained over a wide range of substrates in these radical reactions, and the recycled Zr-TCA still retains its excellent photocatalytic activity. The high recyclability and catalytic efficiency to various substrates make the constructed triphenylamine-based Zr-TCA a promising photocatalyst in diverse radical reactions.

Nitrogen-Centered Radicals in Functionalization of sp2 Systems: Generation, Reactivity, and Applications in Synthesis

The chemistry of nitrogen-centered radicals (NCRs) has plentiful applications in organic synthesis, and they continue to expand as our understanding of these reactive species increases. The utility of these reactive intermediates is demonstrated in the recent advances in C-H amination and the (di)amination of alkenes. Synthesis of previously challenging structures can be achieved by efficient functionalization of sp² moieties without prefunctionalization, allowing for faster and more streamlined synthesis. This Review addresses the generation, reactivity, and application of NCRs, including, but not limited to, iminyl, aminyl, amidyl, and aminium species. Contributions from early discovery up to the most recent examples have been highlighted, covering radical initiation, thermolysis, photolysis, and, more recently, photoredox catalysis. Radical-mediated intermolecular amination of (hetero)arenes can occur with a variety of complex amine precursors, generating aniline derivatives, an important class of structures for drug discovery and development. Functionalization of olefins is achievable in high anti-Markovnikov regioselectivity and allows access to difunctionalized structures when the intermediate carbon radicals are trapped. Additionally, the reactivity of NCRs can be harnessed for the rapid construction of N-heterocycles such as pyrrolidines, phenanthridines, quinoxalines, and quinazolinones.

Highly Crystalline Ag-based Coordination Polymers for Efficient Photocatalytic Oxidation of Sulfides

Coordination polymers (CPs) display great potential for the development of highly active photocatalysts. Herein, we report the fabrication of a highly crystalline CP, [Ag₂ BTT]_n (BTT=benzene-1,2,4,5-tetrathiol). The crystal structure of [Ag₂ BTT]_n was resolved and its performance for photocatalytic oxidation of thioanisole was explored. [Ag₂ BTT]_n is highly active and selective for the photo-oxidation of sulfides to sulfoxides under mild conditions, that is, in air, at room temperature and in the absence of a sacrificial reagent, co-catalyst or redox mediator.

Acetylene/Vinylene-Bridged π-Conjugated Covalent Triazine Polymers for Photocatalytic Aerobic Oxidation Reactions under Visible Light Irradiation

Solar-driven photocatalytic chemical transformation provides a sustainable strategy to produce valuable feedstock, but designing photocatalysts with high efficiency remains challenging. Herein, two acetylene- or vinylene-bridged π-conjugated covalent triazine polymers, A-CTP-DPA and V-CTP-DPE, were successfully fabricated toward metal-free photocatalytic oxidation under visible light irradiation. Compared to the one without acetylene or vinylene bridge, both resulting polymers exhibited superior activity in photocatalytic selective oxidation of sulfides and oxidative coupling of amines; in particular, A-CTP-DPA delivered an optimal photocatalytic performance. The superior activity was attributed to the broadened spectral response range, effective separation, rapid transportation of photogenerated charge carriers, and abundant active sites for photogenerated electrons due to the existence of the acetylene bridge in the framework. This work highlights the potential of acetylene and vinylene bridges in tuning catalytic efficiency of organic semiconductors, providing a guideline for the design of efficient photocatalysts.

Aerobic Photocatalysis: Oxidation of Sulfides to Sulfoxides

Sulfoxides constitute one of the most important functional groups in organic chemistry found in numerous pharmaceuticals and natural products. Sulfoxides are usually obtained from the oxidation of the corresponding sulfides. Among various oxidants, oxygen or air are considered the greenest and most sustainable reagent. Photochemistry and photocatalysis is increasingly applied in new, as well as traditional, yet demanding, reaction, like the aerobic oxidation of sulfides to sulfoxides, since photocatalysis has provided the means to access them in mild and effective ways. In this review, we will summarize the photochemical protocols that have been developed for the oxidation of sulfides to sulfoxides, employing air or oxygen as the oxidant. The aim of this review is to present: i) a historical overview, ii) the key mechanistic studies and proposed mechanisms and iii) categorize the different catalytic systems in literature.

[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...

Z-Scheme In2 S3 /NU-1000 Heterojunction for Boosting Photo-Oxidation of Sulfide into Sulfoxide under Ambient Conditions

Photocatalytic oxidation of sulfide into sulfoxide has attracted extensive attention as an environmentally friendly strategy for chemical transformations or toxic chemicals degradation. Herein, we construct a series of In₂ S₃ /NU-1000 heterojunction photocatalysts, which can efficiently catalyze the oxidation of sulfides to form sulfoxides as the sole product under LED lamp (full-spectrum) illumination in air at room temperature. Especially, the sulfur mustard simulant, 2-chloroethyl ethyl sulfide (CEES), can also be photocatalytically oxidized with In₂ S₃ /NU-1000 to afford nontoxic 2-chloroethyl ethyl sulfoxide (CEESO) selectively and effectively. In contrast, individual NU-1000 and In₂ S₃ show very low catalytic activity on this reaction. The significantly improved photocatalytic activity is ascribed to the constructing of an efficient Z-scheme photocatalysts In₂ S₃ /NU-1000, which exhibits the enhancement of light harvesting, the promotion of photogenerated electron-hole separation, and the retention of high porosity of the parent MOF. Moreover, mechanism studies in photocatalytic oxidation reveal that the superoxide radical (^. O₂ ^- ) and singlet oxygen (¹ O₂ ) are the main oxidative species in the oxidation system. This work exploits the opportunities for the construction of porous Z-scheme photocatalysts based on the photoactive MOFs materials and inorganic semiconductors for promoting catalytic organic transformations. More importantly, it provides a route to the rational design of efficient photocatalysts for the detoxification of mustard gas.

Visible-Light-Driven Oxidative Cleavage of Alkenes Using Water-Soluble CdSe Quantum Dots

The oxidative cleavage of C=C bonds is an important chemical reaction, which is a popular reaction in the photocatalytic field. However, high catalyst-loading and low turnover number (TON) are general shortcomings in reported visible-light-driven reactions. Herein, the direct oxidative cleavage of C=C bonds through water-soluble CdSe quantum dots (QDs) is described under visible-light irradiation at room temperature with high TON (up to 3.7×10⁴ ). Under the same conditions, water-soluble CdSe QDs could also oxidize sulfides to sulfoxides with 51-84 % yields and TONs up to 3.4×10⁴ . The key features of this photocatalytic protocol include high TONs, wide substrates scope, low catalyst loadings, simple and mild reaction conditions, and molecular O₂ as the oxidant.

Sulfoxide and Sulfone Synthesis via Electrochemical Oxidation of Sulfides

The oxidation of diaryl sulfides and aryl alkyl sulfides to the corresponding sulfoxides and sulfones under electrochemical conditions is reported. Sulfoxides are selectively obtained in good yield under a constant current of 5 mA for 10 h in DMF, while sulfones are formed as the major product under a constant current of 10 or 20 mA for 10 h in MeOH. The oxygen of both the sulfoxide and sulfone function is derived from water.

Synthesis of Two-Dimensional C-C Bonded Truxene-Based Covalent Organic Frameworks by Irreversible Brønsted Acid-Catalyzed Aldol Cyclotrimerization

The synthesis of new C-C bonded two-dimensional (2D) covalent organic frameworks (COFs) is highly desirable. Here, a simple but effective synthetic strategy has been developed using an irreversible Brønsted acid-catalyzed aldol cyclotrimerization reaction by virtue of truxene as a linkage. Nonolefin C-C bonded 2D truxene-based covalent organic frameworks (Tru-COFs) were constructed by polymerization of 1,3,5-triindanonebenzene (TDB). The structure formation was confirmed by wide-angle X-ray scattering, Fourier-transform infrared spectroscopy, and solid-state ¹³C CP/MAS NMR. The results showed that the Tru-COFs were porous (645 m²/g) and chemically stable. Benzyl methylene in conjugated Tru-COFs more effectively produced photoinduced radicals than the model truxene compound. Due to the radical photoresponsiveness, Tru-COFs were efficient catalysts for photocatalytic oxidation of sulfides. We expect that this will provide a new synthetic methodology to obtain C-C bonded functional 2D COFs.

Iron-Catalyzed Thiolation and Selenylation of Cycloalkyl Hydroperoxides via C-C Bond Cleavage

A cheap iron-catalyzed C-C bond cleavage/thiolation and selenylation of cycloalkyl hydroperoxides are presented. This redox-neutral protocol provides efficient access to diverse distal keto-functionalized thioethers and selenium compounds. Remarkably, only some amounts of disulfides are required for this transformation.

Grape Seed Extract Assisted Synthesis of Dual-Functional Anatase TiO2 Decorated Reduced Graphene Oxide Composite for Supercapacitor Electrode Material and Visible Light Photocatalytic Degradation of Bromophenol Blue Dye

The grape extract is a potential natural reducing agent because of its high phenolic content. The extracts of seeds, skin, and pulp of grape were prepared by digestion, grinding, and soxhlet methods and used for reducing graphene oxide (GO). The reduced GO made using the soxhlet extract of grape seed (GRGO) was hydrothermally treated with titanium dioxide (TiO₂) for the synthesis of GRGO-TiO₂ nanocomposite. The X-ray diffraction (XRD), thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR), UV-vis, photoluminescence, and Raman spectra studies further confirmed the formation of GRGO and the GRGO-TiO₂ hybrid. Scanning electron microscope and transmission electron microscope studies showed the decoration of spherical TiO₂ particles (<100 nm) on the few-layered GRGO sheets. The GRGO-TiO₂ hybrid was explored as a working electrode for supercapacitors and visible light photocatalyst for water decontamination. GRGO-TiO₂ showed higher specific capacitance (175 F g^-1) than GRGO (150 F g^-1) and TiO₂ (125 F g^-1) in an aqueous electrolyte. GRGO-TiO₂ exhibited 83.6% capacitance retention even after 2000 cycles, indicating the good stability of the material. Further, under visible light irradiation (λ > 400 nm), GRGO-TiO₂ showed ∼30% higher photo-oxidation of the bromophenol blue (BPB) dye than TiO₂. Also, GRGO-TiO₂ decreased the total organic carbon content of BPB from 92 to 18 ppm. Overall, the soxhlet extract of grape seed was found to be a cost-effective reducing agent for the preparation of GRGO, which is a suitable material to be used in supercapacitors and photocatalysis.

A Donor-Acceptor [2]Catenane for Visible Light Photocatalysis

Colored charge-transfer complexes can be formed by the association between electron-rich donor and electron-deficient acceptor molecules, bringing about the narrowing of HOMO-LUMO energy gaps so that they become capable of harnessing visible light. In an effort to facilitate the use of these widespread, but nonetheless weak, interactions for visible light photocatalysis, it is important to render the interactions strong and robust. Herein, we employ a well-known donor-acceptor [2]catenane-formed by the mechanical interlocking of cyclobis(paraquat-p-phenylene) and 1,5-dinaphtho[38]crown-10-in which the charge-transfer interactions between two 4,4'-bipyridinium and two 1,5-dioxynaphthalene units are enhanced by mechanical bonding, leading to increased absorption of visible light, even at low concentrations in solution. As a result, since this [2]catenane can generate persistent bipyridinium radical cations under continuous visible-light irradiation without the need for additional photosensitizers, it can display good catalytic activity in both photo-reductions and -oxidations, as demonstrated by hydrogen production-in the presence of platinum nanoparticles-and aerobic oxidation of organic sulfides, such as l-methionine, respectively. This research, which highlights the usefulness of nanoconfinement present in mechanically interlocked molecules for the reinforcement of weak interactions, can not only expand the potential of charge-transfer interactions in solar energy conversion and synthetic photocatalysis but also open up new possibilities for the development of active artificial molecular shuttles, switches, and machines.

Incorporating Photochromic Triphenylamine into a Zirconium-Organic Framework for Highly Effective Photocatalytic Aerobic Oxidation of Sulfides

A zirconium-based metal-organic framework (MOF) was successfully constructed via solvothermal assembly of a triphenylamine-based tricarboxylate ligand and Zr(IV) salt, the structure simulation of which revealed that it possesses a two-dimensional layered framework with a relatively rare dodecnuclear Zr₁₂ cluster as the inorganic building unit. The inherent photo-responsive property derived from the incorporated photochromic triphenylamine groups combined with its high stability makes the constructed MOF an efficient heterogeneous photocatalyst for the oxidation of sulfides, which is a fundamentally important reaction type in both environmental and pharmaceutical industries. The photocatalytic activity of the constructed MOF was first investigated under various conditions with thioanisole as a representative sulfide substrate. The MOF exhibited both high efficiency and selectivity on aerobic oxidation of thioanisole in methanol utilizing molecular oxygen in air as the oxidant under blue light irradiation for 10 h. Its high photocatalytic performance was also observed when extending the sulfide substrate to diverse thioanisole derivatives and even a sulfur-containing nerve agent simulant (2-chloroethyl ethyl sulfide). The high photocatalytic efficiency and selectivity to a broad set of sulfide substrates make the triphenylamine-incorporating zirconium-based MOF a highly promising heterogeneous photocatalyst.

Observation of intermediates by online mass spectrometry to demonstrate the multiple mechanisms of dye-sensitized photocatalysis

Online mass spectrometry was applied to reveal multiple mechanisms of visible-light irradiated dye-sensitized photocatalysis for o-phenylenediamine oxidation. The reactants, products and short-lived intermediates were recorded and dynamically tracked. Dimer and unexpected trimer intermediates were observed to deduce the stepwise aerobic photooxidation mechanism with multiple routes, which was supported by theoretical calculations.

Fullerene soot and a fullerene nanodispersion as recyclable heterogeneous off-the-shelf photocatalysts

Metal-free heterogeneous photocatalysis, which requires no prior catalyst immobilization or chemical modification and can operate in green solvents, represents a highly-sought after, yet currently still underdeveloped, synthetic method. In this report we present a comparative study which aims to evaluate the use of unmodified fullerene soot and a fullerene nanodispersion as non-soluble and quasi-soluble carbon-based photocatalysts, respectively, for sulfide oxidation and other transformations using oxygen as an oxidant in ethanol. A wide range of sulfoxides were successfully prepared with good yields and chemoselectivity using a very low catalyst loading. The fullerene soot photocatalyst is easily recovered and shows excellent stability of the catalytic properties. The reaction was shown to proceed via a singlet oxygen pathway and has a high selectivity for aliphatic sulfides, whereas the oxidation of thioanisoles can be accomplished using an amine mediated electron transfer mechanism. The applicability of the fullerene nanodispersion as a general purpose photocatalyst was demonstrated in radical cyclization, boronic acid oxidation and imine formation reactions.

Vinylene-bridged donor–acceptor type porous organic polymers for enhanced photocatalysis of amine oxidative coupling reactions under visible light

Two vinylene-bridged D–A structural POPs are constructed by the electron-rich triarylamine and electron-deficient tricyanomesitylene, which exhibited highly effective photocatalytic activities for aerobic oxidative coupling of amines to imine.

Tandem-like vanadium cluster chains in a polyoxovanadate-based metal–organic framework for efficient catalytic oxidation of sulfides

The vanadium cluster chain in V-Ni-MOF can efficiently catalyze the oxidation of sulfides with hydrogen peroxide as the oxidant, achieving the complete conversion from sulfides to sulfones within 1 hour at 40 °C.