Photocatalytic Hydroxylation and Oxidative Coupling Reactions Mediated by Multinuclear Au(I) Supramolecular Clusters

Polynuclear Au(I) cluster photocatalysts, known for their high activity and stability, hold substantial potential in organic synthetic chemistry. This study synthesized two Au(I) supramolecular cluster catalysts with different nuclearities: a tetranuclear cluster, C1 ([(dppmAu2)2L1] • PF6 -), and a hexadecanuclear cluster, C2 [(dppmAu2)6(Au4)(L1)4] • 4PF6 -, through a multicomponent stepwise self-assembly approach. Both cluster structures feature aurophilicity interaction motifs that endow them with exceptional photocatalytic performance, exhibiting optical band gaps of 2.27 eV and 2.41 eV, respectively. Upon photoexcitation, these clusters efficiently generate reactive oxygen species, significantly enhancing their photocatalytic efficacy for the oxidative hydroxylation of phenylboronic acids and oxidative coupling of benzylamines under mild conditions. Catalytic efficiencies exceeding 90 % were achieved. Turnover frequencies for C2 and C1 were measured at 52.045 h-1 and 6.030 h-1, respectively, representing the highest efficiencies reported for photocatalysts to date. Compared to C1, C2 exhibited superior photocatalytic activity, attributed to its higher photoelectric sensitivity and greater exposure of active metal sites. Using a combination of experimental data and density functional theory calculations, the plausible mechanisms were proposed for two photocatalytic reactions. This study demonstrates that the use of multicomponent cooperative self-assembly strategy to synthesize high-nuclearity Au(I) clusters offers innovative pathways for the development of efficient, green, light-driven organic synthesis.

Harnessing Solar Power for Oxidation of Organic Compounds by Re(I)Dipyrrinato Complexes

Metal dipyrrinato complexes of 4d and 5d metals have distinctive features such as high absorption coefficients in the visible section and room temperature phosphorescence in the red region. This work demonstrates the light-assisted oxidation of organic compounds employing rhenium(I)dipyrrinato complexes as catalysts. The heavy atom effect in rhenium(I)dipyrrinato complexes leads to the formation of long-lived triplet excited states, and these complexes can generate singlet oxygen in excellent yields (up to 84 %). A method was developed for photocatalytic aerobic oxidation of sulfides and amines using only 0.05 mol % and 0.025 mol % of the rhenium(I)dipyrrinato complexes, respectively. The method is efficient, and within 2 h, a variety of substrates were oxidized to produce sulfoxides and imines in high yields (up to 97 %). Rhenium(I)dipyrrinato complexes work very well both in visible light and sunlight, making them promising candidates for photocatalytic applications.

Functional molecular models of photosynthesis

This perspective focuses on functional models of photosynthesis to achieve molecular photocatalytic systems that mimic photosystems I and II (PSI and PSII). A long-lived and high-energy electron-transfer state of 9-mesityl-10-methylacridinium ion (Acr+-Mes) has been attained as a simple and useful model of the photosynthetic reaction center. Acr+-Mes has been used as an effective photoredox catalyst for photocatalytic hydrogen evolution and regioselective reduction of NAD(P)+ from plastoquinone analogs as a molecular functional model of PSI. A functional molecular model system to mimic the function of PSII has also been developed to oxidize water by plastoquinone analogs to produce O2 and plastoquinol analogs. The PSI molecular models have finally been integrated with the PSII molecular models to achieve production of a solar fuel (hydrogen) and NAD(P)H and its analogs from water by use of solar energy as a molecular artificial photosynthesis.

Pioneering Metal-Free Late-Stage C-H Functionalization Using Acridinium Salt Photocatalysis

Using organic dyes as photocatalysts is an innovative approach to photocatalytic organic transformations. These dyes offer advantages such as widespread availability, adaptable absorption properties, and diverse chemical structures. Recent progress has led to the development of organic photocatalysts that can utilize visible light to modify chemically inert C-H bonds. These catalysts are sustainable, selective, and versatile, enabling mild reactions, late-stage functionalization, and various transformations in line with green chemistry principles. As catalysts in photoredox chemistry, they contribute to the development of efficient and environmentally friendly synthetic pathways. Acridinium-based organic photocatalysts have proved valuable in late-stage C-H functionalization, enabling transformative reactions under mild conditions. This review emphasizes their innovative features, such as organic frameworks, efficient light absorption properties, and their applications in modifying complex molecules. It provides an overview of recent advancements in the use of acridinium-based organic photocatalysts for late-stage C-H bond functionalization without the need for transition metals, showcasing their potential to expedite the development of new molecules and igniting excitement about the prospects of this research in the field.

Synthesis, structural characterization, DFT and molecular dynamics simulations of dinuclear (μ-hydroxo)-bridged triethanolamine copper(II) complexes: efficient candidates towards visible light-mediated photo-Fenton degradation of organic dyes

Multinuclear (di/tri) copper(II) complexes bridged through hydroxyl groups are very interesting coordination complexes owing to their potential applications in various fields. In this work, three novel dinuclear (μ-hydroxo)-bridged copper(II) complexes in the crystal form, namely, [Cu2(3,5-DIFLB)2(H2tea)2](H2O) (1), [Cu2(4-ClB)2(H2tea)2](H2O) (2), and [Cu2(4-ETHB)2(H2tea)2](H2O)2 (3) (where DIFLB = difluorobenzoate, CLB = chlorobenzoate, ETHB = ethoxybenzoate, and H3tea = triethanolamine), were isolated at room temperature using methanol and water in a 4 : 1 v/v ratio as a solvent. Furthermore, all three complexes (1-3) were characterised using spectroscopic (UV-vis, DRS, and FT-IR), electrochemical (CV) and single-crystal X-ray diffraction techniques. Structural insights gained by packing analysis revealed the role of steric constraints of substituents and various non-covalent interactions in lattice stabilization, which were indeed supported by theoretical and molecular electrostatic potential illustrations. Hirshfeld surface analysis provided quantitative verification about various non-covalent interactions (interatomic contacts) involved in the packing of molecules. Interestingly, as a potential application, complexes 1-3 all exhibited remarkable visible light-mediated photo-Fenton degradation of approximately 98% for 50 ppm concentration of organic dyes (fuchsin basic (FB) and methyl orange (MO)) in 90 minutes with the optimized conditions of 1 mg mL-1 of dye solution. In all the cases, dye degradation by these materials was ascribed to the symbiotic relations among the molecular structures of complexes 1-3, which were endowed with various electron-withdrawing and electron-releasing substituents and ionic strength, with respect to the structure, shape and interacting patterns of dye molecules. The adsorption mechanism indicates that various weak interactions between the donor and acceptor groups of complexes and dyes, such as electrostatic, hydrogen bonding, and direct coordination to metal sites, play a crucial role, which is confirmed by molecular dynamics (MD) simulations. Theoretical studies by DFT-based descriptors, molecular electrostatic potentials, and band gaps provided deep insights into various electronic and reactivity parameters. For subsequent processes of dye degradation, complexes 1-3 were stable and recoverable. The successful integration of experimental and theoretical approaches sheds light on copper-based dinuclear stable coordination complexes, showcasing a significant step towards the development of novel heterogeneous photo-Fenton catalysts.

Organophotocatalytic access to C-glycosides: multicomponent coupling reactions using glycosyl bromides

Photochemical multi-component coupling reactions initiated by the activation of glycosyl bromides in the presence of 1,4-bis(diphenylamino)benzene (BDB) as an organic photocatalyst were developed. C-glycosides accompanied by olefin (di)functionalization were obtained. This method allows us to access various C-glycosides with alkene, carbonyl, alcohol, ether, and amide functionalities.

Dehydrogenative Coupling of Alkylamines with Primary Alcohols Forming α-Amino Ketones

Acceptorless dehydrogenative coupling reactions between C-H bonds offer straightforward and atom-economical methods connecting readily available materials while liberating gaseous hydrogen as the sole byproduct. Despite the growing interest in such transformations, their realization still poses a significant challenge. Here we report a photoinduced dehydrogenative coupling reaction of alkylamines with primary alcohols. C-H bonds adjacent to nitrogen and oxygen are site-selectively cleaved, and a C-C bond is created between the carbon atoms in a cross-selective manner to produce α-amino ketones. Diverse polar functionalities such as esters, amides, and carboxylic acids survived, demonstrating the broad applicability of the present method.

Leuco Ethyl Violet as Self-Activating Prodrug Photocatalyst for In Vivo Amyloid-Selective Oxygenation

Aberrant aggregates of amyloid-β (Aβ) and tau protein (tau), called amyloid, are related to the etiology of Alzheimer disease (AD). Reducing amyloid levels in AD patients is a potentially effective approach to the treatment of AD. The selective degradation of amyloids via small molecule-catalyzed photooxygenation in vivo is a leading approach; however, moderate catalyst activity and the side effects of scalp injury are problematic in prior studies using AD model mice. Here, leuco ethyl violet (LEV) is identified as a highly active, amyloid-selective, and blood-brain barrier (BBB)-permeable photooxygenation catalyst that circumvents all of these problems. LEV is a redox-sensitive, self-activating prodrug catalyst; self-oxidation of LEV through a hydrogen atom transfer process under photoirradiation produces catalytically active ethyl violet (EV) in the presence of amyloid. LEV effectively oxygenates human Aβ and tau, suggesting the feasibility for applications in humans. Furthermore, a concept of using a hydrogen atom as a caging group of a reactive catalyst functional in vivo is postulated. The minimal size of the hydrogen caging group is especially useful for catalyst delivery to the brain through BBB.

Unraveling the mechanisms of triplet state formation in a heavy-atom free photosensitizer

Triplet excited state generation plays a pivotal role in photosensitizers, however the reliance on transition metals and heavy atoms can limit the utility of these systems. In this study, we demonstrate that an interplay of competing quantum effects controls the high triplet quantum yield in a prototypical boron dipyrromethene-anthracene (BD-An) donor-acceptor dyad photosensitizer, which is only captured by an accurate treatment of both inner and outer sphere reorganization energies. Our ab initio-derived model provides excellent agreement with experimentally measured spectra, triplet yields and excited state kinetic data, including the triplet lifetime. We find that rapid triplet state formation occurs primarily via high-energy triplet states through both spin-orbit coupled charge transfer and El-Sayed's rule breaking intersystem crossing, rather than direct spin-orbit coupled charge transfer to the lowest lying triplet state. Our calculations also reveal that competing effects of nuclear tunneling, electronic state recrossing, and electronic polarizability dictate the rate of non-productive ground state recombination. This study sheds light on the quantum effects driving efficient triplet formation in the BD-An system, and offers a promising simulation methodology for diverse photochemical systems.

Guest-Regulated Generation of Reactive Oxygen Species from Porphyrin-Based Multicomponent Metallacages for Selective Photocatalysis

The development of novel materials for highly efficient and selective photocatalysis is crucial for their practical applications. Herein, we employ the host-guest chemistry of porphyrin-based metallacages to regulate the generation of reactive oxygen species and further use them for the selective photocatalytic oxidation of benzyl alcohols. Upon irradiation, the sole metallacage (6) can generate singlet oxygen (1O2) effectively via excited energy transfer, while its complex with C70 (6⊃C70) opens a pathway for electron transfer to promote the formation of superoxide anion (O2⋅-), producing both 1O2 and O2⋅-. The addition of 4,4'-bipyridine (BPY) to complex 6⊃C70 forms a more stable complex (6⊃BPY) via the coordination of the Zn-porphyrin faces of 6 and BPY, which drives fullerenes out of the cavities and restores the ability of 1O2 generation. Therefore, benzyl alcohols are oxidized into benzyl aldehydes upon irradiation in the presence of 6 or 6⊃BPY, while they are oxidized into benzoic acids when 6⊃C70 is employed as the photosensitizing agent. This study demonstrates a highly efficient strategy that utilizes the host-guest chemistry of metallacages to regulate the generation of reactive oxygen species for selective photooxidation reactions, which could promote the utilization of metallacages and their related host-guest complexes for photocatalytic applications.

Dicationic Acridinium/Carbene Hybrids as Strongly Oxidizing Photocatalysts

A new design concept for organic, strongly oxidizing photocatalysts is described based upon dicationic acridinium/carbene hybrids. A highly modular synthesis of such hybrids is presented, and the dications are utilized as novel, tailor-made photoredox catalysts in the direct oxidative C-N coupling. Under optimized conditions, benzene and even electron-deficient arenes can be oxidized and coupled with a range of N-heterocycles in high to excellent yields with a single low-energy photon per catalytic turnover, while commonly used acridinium photocatalysts are not able to perform the challenging oxidation step. In contrast to traditional photocatalysts, the hybrid photocatalysts reported here feature a reversible two-electron redox system with regular or inverted redox potentials for the two-electron transfer. The different oxidation states could be isolated and structurally characterized supported by NMR, EPR, and X-ray analysis. Mechanistic experiments employing time-resolved emission and transient absorption spectroscopy unambiguously reveal the outstanding excited-state potential of our best-performing catalyst (+2.5 V vs SCE), and they provide evidence for mechanistic key steps and intermediates.

A Rational Way to Control the Triplet State Wave Function Confinement of Organic Chromophores: Effect of the Connection Sites and Spin Density Distribution-Guided Molecular Structure Design Principles in Bodipy Dimers

To study the intersystem crossing (ISC) and the spatial confinement of the triplet excited states of organic chromophores, we prepared a series of Bodipy dimers. We found that the connection position of the two units has a significant effect on the absorption and fluorescence. Singlet oxygen quantum yields of 3.8-12.4% were observed for the dimers, which are independent of solvent polarity. Nanosecond transient absorption spectra indicate the population of long-lived triplet excited states with lifetimes (τT) of 45-454 μs. Pulsed laser-excited time-resolved electron paramagnetic resonance (TREPR) spectra show that the T1 triplet states are essentially delocalized, which is different from the case for the previously reported Bodipy dimers. The TREPR spectra of the triplet states imply that the delocalization over the whole dimer essentially depends on the electron density of the carbon atoms at the connection sites. This property may become a universal rule for controlling the T1 state confinement in multichromophore organic molecules.

Efficient Spin-Orbit Charge-Transfer Intersystem Crossing and Slow Intramolecular Triplet-Triplet Energy Transfer in Bodipy-Perylenebisimide Compact Dyads and Triads

Bodipy (BDP)-perylenebisimide (PBI) donor-acceptor dyads/triad were prepared to study the spin-orbit charge-transfer intersystem crossing (SOCT-ISC). For BDP-PBI-3, in which BDP was attached at the imide position of PBI, higher singlet oxygen quantum yield (ΦΔ =85 %) was observed than the bay-substituted derivative BDP-PBI-1 (ΦΔ =30 %). Femtosecond transient absorption spectra indicate slow Förster resonance energy transfer (FRET; 40.4 ps) and charge separation (CS; 1.55 ns) in BDP-PBI-3, while for BDP-PBI-1, CS takes 2.8 ps. For triad BDP-PBI-2, ultrafast FRET (149 fs) and CS (4.7 ps) process were observed, the subsequent charge recombination (CR) takes 5.8 ns and long-lived 3 PBI* (179.8 μs) state is populated. Nanosecond transient absorption spectra of BDP-PBI-3 show that the CR gives upper triplet excited state (3 BDP*) and subsequently, via a slow intramolecular triplet energy transfer (14.5 μs), the 3 PBI* state is finally populated, indicating that upper triplet state is involved in SOCT-ISC. Time-resolved electron paramagnetic resonance spectroscopy revealed that both radical pair ISC (RP ISC) and SOCT-ISC contribute to the ISC. A rare electron spin polarization of (e, e, e, e, e, e) was observed for the triplet state formed via the RP ISC mechanism, due to the S-T+1 /T0 states mixing.

Electron-Poor Acridones and Acridiniums as Super Photooxidants in Molecular Photoelectrochemistry by Unusual Mechanisms

Electron-deficient acridones and in situ generated acridinium salts are reported as potent, closed-shell photooxidants that undergo surprising mechanisms. When bridging acyclic triarylamine catalysts with a carbonyl group (acridones), this completely diverts their behavior away from open-shell, radical cationic, 'beyond diffusion' photocatalysis to closed-shell, neutral, diffusion-controlled photocatalysis. Brønsted acid activation of acridones dramatically increases excited state oxidation power (by +0.8 V). Upon reduction of protonated acridones, they transform to electron-deficient acridinium salts as even more potent photooxidants (*E1/2 =+2.56-3.05 V vs SCE). These oxidize even electron-deficient arenes where conventional acridinium salt photooxidants have thusfar been limited to electron-rich arenes. Surprisingly, upon photoexcitation these electron-deficient acridinium salts appear to undergo two electron reductive quenching to form acridinide anions, spectroscopically-detected as their protonated forms. This new behaviour is partly enabled by a catalyst preassembly with the arene, and contrasts to conventional SET reductive quenching of acridinium salts. Critically, this study illustrates how redox active chromophoric molecules initially considered photocatalysts can transform during the reaction to catalytically active species with completely different redox and spectroscopic properties.

Pictet-Spengler synthesis of twisted quinoline-fused BODIPYs as heavy-atom-free photosensitizers

Singly and doubly quinoline-fused BODIPYs were effectively synthesized through a reaction sequence consisting of the reduction of nitrophenyl-substituted BODIPYs and subsequent Pictet-Spengler cyclization. The combination of the BODIPY core and fused quinoline rings imposed significantly twisted conformations in the quinoline-fused BODIPYs (around 20.0° deviation from coplanarity obtained from X-ray crystal structure analysis). These twisted BODIPYs showed significantly reduced LUMO, redshifted absorption/emission bands, high molar extinction coefficients and satisfactory reactive oxygen species generation efficiency up to 0.56, indicating potential use as heavy-atom-free photosensitizers.

Photochemical Sonogashira coupling reactions: beyond traditional palladium-copper catalysis

Sonogashira coupling is one of the Nobel reactions discovered in 1975 to form a C-C bond using palladium and copper as co-catalysts. Incorporating alkyne functionalities either in macro or micro molecules by using this Sonogashira reaction has already proven its viability and relevance in the sphere of synthetic chemistry. While aiming for sustainable chemistry, in recent years, visible light photoredox catalysts have appeared as an advanced tool in this regard. In this review, we aim to portray a comprehensive summary of modern visible light photo redox catalyzed Sonogashira reaction, which will leave space for the readers to rethink alternative strategies to conduct the Sonogashira reaction. This review briefly describes the implementation of various metal-based nanomaterial photocatalysts, developing either copper or palladium-free photocatalytic methods, and organo-photolytic and bioinspired photocatalysts for the Sonogashira coupling reactions. Besides, this review also gives a concise overview of the mechanistic aspects and highlights selective examples for substrate scope.

Fluoroalkyl Sulfoximines for Versatile Photocatalytic Radical Fluoroalkylations

Fluoroalkyl sulfoximines, which serve as electron-accepting fluoroalkyl radical sources, are easy-to-handle, solid, and bench-stable chemicals. Fluoroalkyl radicals can be generated from sulfoximine reagents using strong one-electron injectors, such as a highly reducing photoredox catalyst in the excited state. Our group has developed photocatalytic radical di- and mono-fluoromethylation and α-monofluoroalkylation of olefins with the corresponding fluoroalkyl sulfoximines. In this personal account, appropriate combinations of fluoroalkyl sulfoximines and photoredox catalysts, leading to successful radical fluoroalkylation, have been discussed.

Thieno[3,4-d]pyrimidin-4(3H)-thione: an effective, oxygenation independent, heavy-atom-free photosensitizer for cancer cells

All-organic, heavy-atom-free photosensitizers based on thionation of nucleobases are receiving increased attention because they are easy to make, noncytotoxic, work both in the presence and absence of molecular oxygen, and can be readily incorporated into DNA and RNA. In this contribution, the DNA and RNA fluorescent probe, thieno[3,4-d]pyrimidin-4(1H)-one, has been thionated to develop thieno[3,4-d]pyrimidin-4(3H)-thione, which is nonfluorescent and absorbs near-visible radiation with about 60% higher efficiency. Steady-state absorption and emission spectra are combined with transient absorption spectroscopy and CASPT2 calculations to delineate the electronic relaxation mechanisms of both pyrimidine derivatives in aqueous and acetonitrile solutions. It is demonstrated that thieno[3,4-d]pyrimidin-4(3H)-thione efficiently populates the long-lived and reactive triplet state generating singlet oxygen with a quantum yield of about 80% independent of solvent. It is further shown that thieno[3,4-d]pyrimidin-4(3H)-thione exhibits high photodynamic efficacy against monolayer melanoma cells and cervical cancer cells both under normoxic and hypoxic conditions. Our combined spectroscopic, computational, and in vitro data demonstrate the excellent potential of thieno[3,4-d]pyrimidin-4(1H)-thione as a heavy-atom-free PDT agent and paves the way for further development of photosensitizers based on the thionation of thieno[3,4-d]pyrimidine derivatives. Collectively, the experimental and computational results demonstrate that thieno[3,4-d]pyrimidine-4(3H)-thione stands out as the most promising thiobase photosensitizer developed to this date.

Triplet quenching pathway control with molecular dyads enables the identification of a highly oxidizing annihilator class

Metal complex - arene dyads typically act as more potent triplet energy donors compared to their parent metal complexes, which is frequently exploited for increasing the efficiencies of energy transfer applications. Using unexplored dicationic phosphonium-bridged ladder stilbenes (P-X2+) as quenchers, we exclusively observed photoinduced electron transfer photochemistry with commercial organic photosensitizers and photoactive metal complexes. In contrast, the corresponding pyrene dyads of the tested ruthenium complexes with the very same metal complex units efficiently sensitize the P-X2+ triplets. The long-lived and comparatively redox-inert pyrene donor triplet in the dyads thus provides an efficient access to acceptor triplet states that are otherwise very tricky to obtain. This dyad-enabled control over the quenching pathway allowed us to explore the P-X2+ photochemistry in detail using laser flash photolysis. The P-X2+ triplet undergoes annihilation producing the corresponding excited singlet, which is an extremely strong oxidant (+2.3 V vs. NHE) as demonstrated by halide quenching experiments. This behavior was observed for three P2+ derivatives allowing us to add a novel basic structure to the very limited number of annihilators for sensitized triplet-triplet annihilation in neat water.

Origin of intersystem crossing in highly distorted organic molecules: a case study with red light-absorbing N,N,O,O-boron-chelated Bodipys

To explore the relationship between the twisted π-conjugation framework of aromatic chromophores and the efficacy of intersystem crossing (ISC), we have studied a N,N,O,O-boron-chelated Bodipy derivative possessing a severely distorted molecular structure. Surprisingly, this chromophore is highly fluorescent, showing inefficient ISC (singlet oxygen quantum yield, Φ_Δ = 12%). These features differ from those of helical aromatic hydrocarbons, where the twisted framework promotes ISC. We attribute the inefficient ISC to a large singlet-triplet energy gap (ΔE_S1/T1 = 0.61 eV). This postulate is tested by critical examination of a distorted Bodipy having an anthryl unit at the meso-position, for which Φ_Δ is increased to 40%. The improved ISC yield is rationalized by the presence of a T₂ state, localized on the anthryl unit, with energy close to that of the S₁ state. The electron spin polarization phase pattern of the triplet state is (e, e, e, a, a, a), with the T_z sublevel of the T₁ state overpopulated. The small zero-field splitting D parameter (-1470 MHz) indicates that the electron spin density is delocalized over the twisted framework. It is concluded that twisting of π-conjugation framework does not necessarily induce ISC, but S₁/T_n energy matching may be a generic feature for increasing ISC for a new-generation of heavy atom-free triplet photosensitizers.

PCET-Mediated Ring-Opening Alkenylation of Cycloalkanols via Dual Photoredox and Cobalt Catalysis

The construction of molecular skeletons and modification of molecules using widely available and easily prepared alcohols as radical precursors for coupling reactions are significant and challenging subjects. We herein report a straightforward strategy for the dehydrogenative ring-opening alkenylation of cycloalkanols with alkenes by combining a proton-coupled electron transfer strategy and a dual photoredox and cobalt catalysis system. With this approach, a series of distally unsaturated ketones were obtained in 17-83% yields with high E selectivity.

Modern Photocatalytic Strategies in Natural Product Synthesis

Modern photocatalysis has proven its generality for the development and functionalization of native functionalities. To date, the field has found broad applications in diverse research areas, including the total synthesis of natural products. This contribution covers recent reports of total syntheses involving as a key step a photocatalytic reaction. Among the selected examples, the photocatalytic processes proceed in a highly chemo-, regio-, and stereoselective manner, thereby allowing the rapid access to structurally complex architectures under light-driven conditions.

Silver Halide-Based Nanomaterials in Biomedical Applications and Biosensing Diagnostics

In recent years, silver halide (AgX, X = Cl, Br, I)-based photocatalytic materials have received increasing research attention owing to their excellent visible-light-driven photocatalytic performance for applications in organic pollutant degradation, HER, OER, and biomedical engineering. Ag as a noble metal has a surface plasma effect and can form Schottky junctions with AgX, which significantly promotes electron transport and increases photocatalytic efficiency. Therefore, Ag/AgX can reduce the recombination rate of electrons and holes more than pure AgX, leading to using AgX as a photocatalytic material in biomedical applications. The use of AgX-based materials in photocatalytic fields can be classified into three categories: AgX (Ag/AgX), AgX composites, and supported AgX materials. In this review, we introduce recent developments made in biomedical applications and biosensing diagnostics of AgX (Ag/AgX) photocatalytic materials. In addition, this review also discusses the photocatalytic mechanism and applications of AgX (Ag/AgX) and supported AgX materials.

Recent Advances in the Heterogeneous Photocatalytic Hydroxylation of Benzene to Phenol

Phenol is an important chemical material that is widely used in industry. Currently, phenol is dominantly produced by the well−known three−step cumene process, which suffers from severe drawbacks. Therefore, developing a green, sustainable, and economical strategy for the production of phenol directly from benzene is urgently needed. In recent years, the photocatalytic hydroxylation of benzene to phenol, which is economically feasible and could be performed under mild conditions, has attracted more attention, and development of highly efficient photocatalyst would be a key issue in this field. In this review, we systematically introduce the recent achievements of photocatalytic hydroxylation of benzene to phenol from 2015 to mid−2022, and various heterogeneous photocatalysts are comprehensively reviewed, including semiconductors, polyoxometalates (POMs), graphitic carbon nitride (g−C3N4), metal–organic frameworks (MOFs), carbon materials, and some other types of photocatalysts. Much effort is focused on the physical and chemical approaches for modification of these photocatalysts. The challenges and future promising directions for further enhancing the catalytic performances in photocatalytic hydroxylation of benzene are discussed in the end.

Quantum effects in photosensitization: the case of singlet oxygen generation by thiothymines

Photosensitization is the indirect electronic excitation of a molecule with the aid of a photosensitizer and is a bimolecular nonradiative energy transfer. In this study, we have attempted to elucidate its mechanism, and we do this by calculating rate constants of photosensitization of oxygen by thiothymines (2-thiothymine, 4-thiothymine and 2,4 dithiothymine). The rate constants have been calculated using two approaches: (a) a classical limit of Fermi's Golden Rule (FGR), and (b) a time-dependent variant of FGR, where the treatment is purely quantum mechanical. The former approach has previously been developed for bimolecular systems and has been applied to the photosensitization reactions studied here. The latter approach, however, has so far only been used for unimolecular reactions, and in this work, we describe how it can be adapted for bimolecular reactions. Experimentally, all three thiothymines are known to have significant singlet oxygen yields, which are indicative of similar rates. Rate constants calculated using the time-dependent variant of FGR are similar across all three thiothymines. While the classical approximation gives reasonable rate constants for 2-thiothymine, it severely underestimates them for 4-thiothymine and 2,4 dithiothymine, by several orders of magnitude. This work indicates the importance of quantum effects in driving photosensitization.

Upcycling to Sustainably Reuse Plastics

Plastics are now indispensable in daily lives. However, the pollution from plastics is also increasingly becoming a serious environmental issue. Recent years have seen more sustainable approaches and technologies, commonly known as upcycling, to transform plastics into value-added materials and chemical feedstocks. In this review, the latest research on upcycling is presented, with a greater focus on the use of renewable energy as well as the more selective methods to repurpose synthetic polymers. First, thermal upcycling approaches are briefly introduced, including the redeployment of plastics for construction uses, 3D printing precursors, and lightweight materials. Then, some of the latest novel strategies to deconstruct condensation polymers to monomers for repolymerization or introduce vulnerable linkers to make the plastics more degradable are discussed. Subsequently, the review will explore the breakthroughs in plastics upcycling by heterogeneous and homogeneous photocatalysis, as well as electrocatalysis, which transform plastics into more versatile fine chemicals and materials while simultaneously mitigating global climate change. In addition, some of the biotechnological advances in the discovery and engineering of microbes that can decompose plastics are also presented. Finally, the current challenges and outlook for future plastics upcycling are discussed to stimulate global cooperation in this field.

Visible-Light-Induced Organophotocatalytic Difunctionallization: Open-Air Hydroxysulfurization of Aryl Alkenes with Aryl Thiols

Herein, we report a regioselective visible-light-induced organophotoredox catalytic difunctionalization method to prepare β-hydroxysulfides using aryl alkenes and aryl thiols as substrates. The reaction provides a wide substrate scope of aryl alkenes (from simple styrene to complex bioactive compounds) and aryl thiols (from diverse heteroaromatic thiols to nonheteroaromatic thiols) (total 45 examples, up to 88% yield). Based on the combined experimental and computational studies, we demonstrate that in situ generated hydroperoxyl radicals from O₂ in air react with benzylic radicals, which restrains the reaction between benzylic radicals and the acidic form of thiols in a classical thiol-ene radical reaction. We show that difunctionalization is possible due to the choice of bases, diluted substrate concentrations, increment in catalyst loading, and selection of suitable aryl thiols under aerobic conditions. Considering the biological importance of heteroaromatic thiols and the lack of methods to install them, our approach offers a platform to derive various β-hydroxysulfides that contain aromatic elements.

Photoinduced Arylation of Acridinium Salts: Tunable Photoredox Catalysts for C-O Bond Cleavage

A photoinduced arylation of N-substituted acridinium salts has been developed and has exhibited a high functional group tolerance (e.g., halogen, nitrile, ketone, ester, and nitro). A broad range of well-decorated C9-arylated acridinium-based catalysts with fine-tuned photophysical and photochemical properties, namely, excited-state lifetimes and redox potentials have been synthetized in a one-step procedure. These functionalized acridinium salts were later evaluated in the photoredox-catalyzed fragmentation of 1,2-diol derivatives (lignin models). Among them, 2-bromophenyl substituted N-methyl acridinium has outperformed all photoredox catalysts, including commercial Fukuzumi's catalyst, for the selective C^βO-Ar bond cleavage of diol monoarylethers to afford 1,2-diols in good yields.

Synthetic Molecular Photoelectrochemistry: New Frontiers in Synthetic Applications, Mechanistic Insights and Scalability

Synthetic photoelectrochemistry (PEC) is receiving increasing attention as a new frontier for the generation and handling of reactive intermediates. PEC permits selective single-electron transfer (SET) reactions in a much greener way and broadens the redox window of possible transformations. Herein, the most recent contributions are reviewed, demonstrating exciting new opportunities, namely, the combination of PEC with other reactivity paradigms (hydrogen-atom transfer, radical polar crossover, energy transfer sensitization), scalability up to multigram scale, novel selectivities in SET super-oxidations/reductions and the importance of precomplexation to temporally enable excited radical ion catalysis.

Acridinium salts as photoredox organocatalysts for photomediated cationic RAFT and DT polymerizations of vinyl ethers

A series of acridinium salts with high excited-state oxidative power are employed as photoredox organocatalysts for photomediated cationic RAFT and DT polymerizations under visible light.

Organic dyes supported on silicon-based materials: synthesis and applications as photocatalysts

The most important advance in photocatalysis in the last decade has been the synthesis and application of organic compounds to promote this process.

Visible light-driven copper(ii) catalyzed aerobic oxidative cleavage of carbon–carbon bonds: a combined experimental and theoretical study

By switching on visible blue light, aerobic oxidation of various substrates, such as α-substituted, β-substituted and α-halo styrenes, was first realized with a copper(ii) catalyst.

Visible light-promoted aerobic oxidative cleavage and cyclization of olefins to access 3-hydroxy-isoindolinones

An environmentally friendly synthetic approach is described from 2-vinylbenzamide to 3-hydroxy-isoindolinones through visible light-promoted transformations via iron/disulfide catalysis and molecular oxygen oxidation.

Light-mediated aerobic oxidation of C(sp3)–H bonds by a Ce(iv) hexachloride complex

A photochemical C(sp3)–H oxygenation of arene and alkane substrates (including methane) catalyzed by [NEt4]2[CeIVCl6] under mild conditions (1 atm, 25 °C) is described.

Charge Transfer, Intersystem Crossing, and Electron Spin Dynamics in a Compact Perylenemonoimide-Phenoxazine Electron Donor-Acceptor Dyad

With phenoxazine (PXZ) as the electron donor and perylene-3,4-dicarboximide (PMI) as the electron acceptor, we prepared a compact, orthogonal electron donor-acceptor dyad (PMI-PXZ) to study the spin-orbit charge transfer-induced intersystem crossing (SOCT-ISC). A weak charge transfer (CT) absorption band, due to S₀ → ¹CT transition, was observed (ε = 2840 M^-1 cm^-1 at 554 nm, FWHM: 2850 cm^-1), which is different from that of the previously reported analogue dyad with phenothiazine as the electron donor (PMI-PTZ), for which no CT absorption band was observed. A long-lived triplet state was observed (lifetime τ_T = 182 μs) with nanosecond transient absorption spectroscopy, and the singlet oxygen quantum yield (Φ_Δ = 76%) is higher than that of the previously reported analogue dyad PMI-PTZ (Φ_Δ = 57%). Ultrafast charge separation (ca. 0.14 ps) and slow charge recombination (1.4 ns) were observed with femtosecond transient absorption spectroscopy. With time-resolved electron paramagnetic resonance spectroscopy (TREPR), we confirmed the SOCT-ISC mechanism, and the electron spin polarization phase pattern of the triplet-state TREPR spectrum is (e, e, a, e, a, a), which is dramatically different from that of PMI-PTZ (a, e, a, e, a, e), indicating that the triplet-state TREPR spectrum of a specific chromophore in the electron donor-acceptor dyads is not only dependent on the geometry of the dyads but also dependent on the structure of the electron donor (or acceptor). Even one-atom variation in the donor structure may cause significant influence on the electron spin selectivity of the ISC of the electron donor-acceptor dyads.

Harnessing selective PET and EnT catalysis by chlorophyll to synthesize N-alkylated quinoline-2(1H)-ones, isoquinoline-1(2H)-ones and 1,2,4-trioxanes

Photocatalytic syntheses of quinoline-2(1H)-ones, isoquinoline-1(2H)-ones and 1,2,4-trioxanes were achieved by selective photo-induced electron transfer (PET) and energy transfer (EnT), respectively, by chlorophyll under visible light irradiation. Quinoline-2(1H)-ones, isoquinoline-1(2H)-ones and 1,2,4-trioxanes are biologically potent scaffolds and their syntheses following mild reaction protocols are highly sought after. This work showcases the divergent photocatalytic roles of chlorophyll viz., electron transfer in the case of quinolines or isoquinolines and energy transfer with allyl alcohols as substrates, affording their aerobic oxidation under green reaction conditions. The mechanistic investigations affirm that the catalytic cycle follows the electron-transfer pathway in carrying out the oxidation of N-alkyl(iso)quinolinium salts. Furthermore, the method provides an environmentally benign, simple reaction strategy for organic transformations of (N)-heterocycles.

Strategic Construction of Sulfur-Bridged BODIPY Dimers and Oligomers as Heavy-Atom-Free Photosensitizers

An efficient strategy for building sulfur-bridged oligo-BODIPYs based on the S_NAr reaction is described. These oligo-BODIPYs showed broadband and strong visible-near-infrared (NIR) light absorption, strong intramolecular exciton coupling, and efficient intersystem crossing (ISC). Generation of ¹O₂ as well as O₂^•- under irradiation was found to give high reactive oxygen species generation efficiencies for those oligomers.

Ni-catalyzed non-activated C-S bond cleavage at ambient temperature for the synthesis of sulfur-containing polycyclic compounds

A Ni-catalyzed intramolecular reaction of diarylthioether-tethered 1,8-diynes gave sulfur-containing tetracyclic compounds at ambient temperature. The transformation was initiated by non-activated sp² C-S bond cleavage along with consecutive alkyne insertions. A double intramolecular reaction of a tetrayne and an intermolecular reaction of monoynes were also available, and the corresponding polycyclic compounds were obtained. Moreover, the obtained cycloadduct showed photo-catalytic activity in benzylic oxidation.