Organic Photocatalysis Research Articles

Enhancing the Photocatalytic Performance of CsPbBr3 Nanocrystals through Ferrocene-Assisted Exciton Dissociation and Halide Vacancies.

Excited-state interactions at the interfaces of nanocrystals play a crucial role in determining photocatalytic efficiency. CsPbBr3 nanocrystals (CPB NCs), celebrated for their exceptional photophysical properties, have been explored for organic photocatalysis. However, their intrinsic limitations, such as charge carrier recombination and stability issues, hinder their full potential. Strategies to enhance exciton dissociation, such as complexing CPB NCs with charge-shuttling molecules, have shown promise but remain underexplored for fully realizing their potential in improving the photocatalytic performance. We coupled ferrocene carboxylic acid (FcA) with CPB to extract the photogenerated holes, leveraging them to oxidize (1,2-dibromoethyl)benzene to phenacyl bromide. Optimization using pristine CPB NCs achieved a production rate of 5 μmol gcat-1 h-1, which increased to 13.1 μmol gcat-1 h-1 upon FcA incorporation, marking a 2.5-fold enhancement. Mechanistic investigations revealed the simultaneous involvement of electrons and holes, with oxygen acting as a reactant contributing to the oxygenated product. Halide vacancies were identified as critical adsorption sites for the substrate, with post-synthetic treatments enhancing these vacancies, resulting in over a 2-fold increase in the reaction rate. This work not only establishes an effective approach for phenacyl bromide synthesis but also highlights the potential of leveraging dissociated charge carriers to enhance photocatalysis using CPB NCs.

Photocatalytic Mechanisms of Organic Thermally Activated Delayed Fluorescence Compounds.

Reverse intersystem crossing (RISC) has become possible by minimizing the energy gap between the first excited singlet (S1) and triplet state (T1), which facilitates the thermally activated delayed fluorescence (TADF). Due to the small singlet-triplet energy gap, the S1 and T1 states exhibit comparable redox reactivity, leading organic TADF compounds to be potent photocatalysts. Here, we report such TADF compounds with multiple donor units designed as an efficient photocatalyst for the direct C(sp3)-H carbamoylation of saturated aza-heterocycles. The results obtained by photophysical investigations and chemical calculations confirm that both the S1 and T1 states are involved in the photocatalysis cycle, with the fast spin-flip from the S1 to triplet states being a crucial factor in the enhancement of catalytic performance. The findings will be beneficial for the design of novel, efficient organic photocatalysis with TADF characteristics and aid in the development of organic photocatalysis.

Catalytic Construction of C(sp3)−Ge Bonds: Recent Advances and Future Perspectives

AbstractGermanium (Ge), a congener of carbon, possesses unique properties that hold extensive potential for applications across multiple domains. Recent years have seen significant progress in the development of carbon‐germanium bond formation strategies, particularly those for more challenging C(sp3)−Ge bonds. This review systematically summarizes the recent advances in C(sp3)−Ge bond forming methodologies, with particular emphasis on (1) the versatility of transition‐metals, including iron, nickel, copper, rhodium and palladium, as catalysts in broadening reaction scope and controlling selectivity; (2) the powerfulness of organic photocatalysis in achieving mild and selective bond formation, and (3) the sustainability of catalytic electrosynthesis in facilitating chemical oxidant‐/reductant‐ free conversions. Additionally, examples of (4) non‐catalytic strategies are also discussed. The representative scopes, as well as mechanistic proposals, of these protocols are highlighted. Through an overview on the current state of research, this review aims to offer insights into the catalytic construction of C(sp3)−Ge bonds, and provide perspectives on future research directions to address the current challenges.

Organic Two-Photon-Absorbing Photosensitizers Can Overcome Competing Light Absorption in Organic Photocatalysis.

Conventional organic photocatalysis typically relies on ultraviolet and short-wavelength visible photons as the energy source. However, this approach often suffers from competing light absorption by reactants, products, intermediates, and co-catalysts, leading to reduced quantum efficiency and side reactions. To address this issue, we developed novel organic two-photon-absorbing (TPA) photosensitizers capable of functioning under deep red and near-infrared light irradiation. Three model reactions including cyclization, Sonogashira Csp2-Csp cross-coupling, and Csp2-N cross-coupling reactions were selected to compare the performance of the new photosensitizers under both blue (427 nm) and deep red (660 nm) light irradiation. The obtained results unambiguously prove that for reactions involving blue light-absorbing reactants, products, and/or co-catalysts, deep red light source resulted in better performance than blue light when utilizing our TPA photosensitizers. This work highlights the potential of our metal-free TPA photosensitizers as a sustainable and effective solution to mitigate the competing light absorption issue in photocatalysis, not only expanding the scope of organic photocatalysts but also reducing reliance on expensive Ru/Ir/Os-based photosensitizers.

Modulating Decarboxylative Oxidation Photocatalysis by Ligand Engineering of Atomically Precise Copper Nanoclusters.

Copper nanoclusters (Cu NCs) characterized by their well-defined electronic and optical properties are an ideal platform for organic photocatalysis and exploring atomic-level behaviors. However, their potential as greener, efficient catalysts for challenging reactions like decarboxylative oxygenation under mild conditions remains unexplored. Herein, we present Cu13(Nap)3(PPh3)7H10 (hereafter Cu13Nap), protected by 1-naphthalene thiolate (Nap), which performs well in decarboxylative oxidation (90% yield) under photochemical conditions. In comparison, the isostructural Cu13(DCBT)3(PPh3)7H10 (hereafter Cu13DCBT), stabilized by 2,4-dichlorobenzenethiolate (DCBT), yields only 28%, and other previously reported Cu NCs (Cu28, Cu29, Cu45, Cu57, and Cu61) yield in the range of 6-18%. The introduction of naphthalene thiolate to the surface of Cu13 NCs influences their electronic structure and charge transfer in the ligand shell, enhancing visible light absorption and catalytic performance. Density functional theory (DFT) and experimental evidence suggest that the reaction proceeds primarily through an energy transfer mechanism. The energy transfer pathway is uncommon in the context of previous reports for decarboxylative oxidation reactions. Our findings suggest that strategically manipulating ligands holds significant potential for creating composite active sites on atomically precise copper NCs, resulting in enhanced catalytic efficacy and selectivity across various challenging reactions.

A Hydrogen-Bonded Organic Framework Based on Triphenylamine for Photocatalytic Silane Hydroxylation.

Employing hydrogen-bonded organic frameworks (HOFs) as mild photocatalysts for organic conversions is still considerably challenging. In this work, we synthesized a hydrogen-bonded organic framework (HOF-16) and achieved the photocatalytic oxidation of silanes to generate silanols. Considering the constraints imposed by the framework structure, a significant improvement in the efficacy of singlet oxygen (1O2) generation is observed. HOF-16 exhibits remarkable photocatalytic performance when it comes to silane hydroxylation, displaying high efficiency, low catalyst loading, and good recyclability. This research highlights the immense potential of HOFs in the realm of organic photocatalysis.

A supramolecular photosensitizer based on triphenylamine and pyrazine with aggregation-induced emission properties for high-efficiency photooxidation reactions

Recently, there has been a great interest in the study of photocatalysts (PCs) and photosensitizers (PSs) in the field of organic photocatalysis. In the present study, a pure organic thermally activated delayed fluorescence (TADF) molecule 4,4′-(12-(pyridin-4-yl)dibenzo[f,h]pyrido[2,3-b]quinoxaline-3,6-diyl)bis(N,N-diphenylaniline) (DPQ-TPA) was designed and synthesized, which not only have excellent TADF property and small energy splitting (ΔEST), but also can self-assembly in water to form cross-linked nanoparticles with exceptional aggregation-induced emission (AIE) characteristics. DPQ-TPA exhibits excellent remarkable selectivity and notably enhances the production capacity of reactive oxygen species (ROS), particularly 1O2, which was employed as a highly effective photocatalyst in the photooxidation reaction of phosphine and hydroazobenzenes under blue light irradiation with high yields up to 94% and 91%, respectively. This work expands the potential application of (donor-acceptor) D-A type AIE-TADF molecules in photocatalytic organic transformations through supramolecular self-assembly.

Supramolecular confinement effect enabling light-harvesting system for photocatalytic α-oxyamination reaction

The supramolecular Förster resonance energy transfer (FRET) is seen as a promising approach for organic photocatalysis using dyes as catalysts, because it combines the high efficiency of energy transfer with the dynamic responsiveness based on non-covalent interactions. Here we propose a supramolecular FRET photocatalysis strategy for α-oxyamination reaction based on supramolecular confinement effect. The well-designed benzothiadiazole-based cationic monomer as energy donor and the dyes of Nile Red as acceptor are doped into the amphiphilic surfactants of sodium dodecyl sulfate (SDS). Benefitting from the supramolecular confinement space provided by SDS in aqueous environment, the FRET process between the monomer and Nile Red is effectively achieved (exciton migration rate: 3.99 × 1014 L mol‒1 s‒1). On this basis, the supramolecular FRET system is used as an efficient energy source to catalyze α-oxyamination reactions between a series of 1,3-dicarbonyl compounds and 2,2,6,6-tetramethylpiperidine-1-oxyl under white LED light, showing a yield as high as 94% and a turnover frequency value of 3.92 h‒1. This photocatalytic result shows a great enhancement compared to that of Nile Red alone.

Photoinduced Metal-Free Atom Transfer Radical Polymerization for the Modification of Cellulose with Poly(N-isopropylacrylamide) to Create Thermo-Responsive Injectable Hydrogels.

Photoinduced metal-free ATRP has been successfully applied to fabricate thermo-responsive cellulose graft copolymer (PNIPAM-g-Cell) using 2-bromoisobuturyl bromide-modified cellulose as the macroinitiator. The polymerization of N-isopropylacrylamide (NIPAM) from cellulose was efficiently activated and deactivated with UV irradiation in the presence of an organic-based photo-redox catalyst. Both FTIR and 13C NMR analysis confirmed the structural similarity between the obtained PNIPAM-g-Cell and that synthesized via traditional ATRP methods. When the concentration of the PNIPAM-g-Cell is over 5% in water, it forms an injectable thermos-responsive hydrogel composed of micelles at 37 °C. Since organic photocatalysis is a metal-free ATRP, it overcomes the challenge of transition-metal catalysts remaining in polymer products, making this cellulose-based graft copolymer suitable for biomedical applications. In vitro release studies demonstrated that the hydrogel can continuously release DOX for up to 10 days, and its cytotoxicity indicates that it is highly biocompatible. Based on these findings, this cellulose-based injectable, thermo-responsive drug-loaded hydrogel is suitable for intelligent drug delivery systems.

Integrating Photoactive Ligands into Crystalline Ultrathin 2D Metal-Organic Framework Nanosheets for Efficient Photoinduced Energy Transfer.

3D metal-organic frameworks (MOFs) have gained attention as heterogeneous photocatalysts due to their porosity and unique host-guest interactions. Despite their potential, MOFs face challenges, such as inefficient mass transport and limited light penetration in photoinduced energy transfer processes. Recent advancements in organic photocatalysis have uncovered a variety of photoactive cores, while their heterogenization remains an underexplored area with great potential to build MOFs. This gap is bridged by incorporating photoactive cores into 2D MOF nanosheets, a process that merges the realms of small-molecule photochemistry and MOF chemistry. This approach results in recyclable heterogeneous photocatalysts that exhibit an improved mass transfer efficiency. This research demonstrates a bottom-up synthetic method for embedding photoactive cores into 2D MOF nanosheets, successfully producing variants such as PCN-641-NS, PCN-643-NS, and PCN-644-NS. The synthetic conditions were systematically studied to optimize the crystallinity and morphology of these 2D MOF nanosheets. Enhanced host-guest interactions in these 2D structures were confirmed through various techniques, particularly solid-state NMR studies. Additionally, the efficiency of photoinduced energy transfer in these nanosheets was evidenced through photoborylation reactions and the generation of reactive oxygen species (ROS).

Understanding charge carrier dynamics in organic photocatalysts for hydrogen evolution

This review surveyed the charge behavior in organic photocatalysis, and elucidated a correlation between molecular structure, charge dynamics and photocatalytic performance.

Titanium in photocatalytic organic transformations: current applications and future developments.

Titanium, as an important transition metal, has garnered extensive attention in both industry and academia due to its excellent mechanical properties, corrosion resistance, and unique reactivity in organic synthesis. In the field of organic photocatalysis, titanium-based compounds such as titanium dioxide (TiO2), titanocenes (Cp2TiCl2, CpTiCl3), titanium tetrachloride (TiCl4), tetrakis(isopropoxy)titanium (Ti(OiPr)4), and chiral titanium complexes have demonstrated distinct reactivity and selectivity. This review focuses on the roles of these titanium compounds in photocatalytic organic reactions, and highlights the reaction pathways such as photo-induced single-electron transfer (SET) and ligand-to-metal charge transfer (LMCT). By systematically surveying the latest advancements in titanium-involved organic photocatalysis, this review aims to provide references for further research and technological innovation within this fast-developing field.

3 + 2] radical sulfuration of alkenes by organic photocatalysis

A new model involving radicals for formation of C–S bonds enables the direct [3 + 2] radical sulfuration reaction of alkenes using CS2. This study has provided a new method for constructing complex sulfur-containing molecules.

Visible light-mediated direct C8–H arylation of quinolines and C2–H arylation of quinoline-N-oxides and pyridines under organic photocatalysis

In this study, we have successfully developed a regioselective C–H arylation by using natural photocatalyst chlorophyll.

Tetrabutylammonium decatungstate (TBADT), a compelling and trailblazing catalyst for visible-light-induced organic photocatalysis.

Tetrabutylammonium decatungstate (TBADT) has recently emerged as an intriguing photocatalyst under visible-light or near-visible-light irradiation in a wide range of organic reactions that were previously not conceivable. Given its ability to absorb visible light and excellent effectiveness in activating unactivated chemical bonds, it is a promising addition to traditional photocatalysts. This review covers some of the contemporary developments in visible-light or near-visible-light photocatalysis reactions enabled by the TBADT catalyst to 2023, with the contents organized by reaction type.

Optical and electrical properties of thin films of MnS/metal/MnS for photocatalysis and gas sensing applications

Thin films of manganese sulfide (MnS) with interlayers of silver and copper were prepared using the thermal evaporation technique for applications in photocatalysis and gas sensing. The structure, morphology, optical, and electrical properties of the deposited films were described by X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), UV/VIS/NIR spectrophotometer, and two-probe technique. Various parameters such as crystallite size, micro-deformation, dislocation density, optical energy gap, refractive index, extinction coefficient, Urbach energy, dielectric constants, dispersion parameters, activation energy, mobility and, Seebeck coefficient were determined. According to the experimental results, XRD analysis revealed that the MnS powder has a crystalline structure with a cubic phase. While the pure MnS thin films exhibited an amorphous structure, the addition of metals (Ag and Cu) as interlayers resulted in a distinct crystallinity. The pure MnS films had a direct optical energy gap of 3.17 eV, which changed to an indirect transition with energy gaps of 2.43 eV and 2.50 eV when Ag and Cu interlayers were added, respectively. The refractive index values and dispersion of the films depended on the type of interlayer, with all films exhibiting normal dispersion. The temperature dependence of the sheet resistance indicates a semiconducting behavior of the samples, and the addition of the interlayer metal reduced the sheet resistance. Overall, the results suggest that thin films of MnS and MnS/metal/MnS hold promise for organic photocatalysis and gas sensor applications, respectively.

Graphitic Carbon Nitride Meets Molecular Oxygen: New Sustainable Photocatalytic Ways for the Oxidation of Organic Molecules

In recent years, organic chemists have taken a resolute step toward green photocatalytic synthesis. In this regard, the oxidation of organic compounds with molecular oxygen is one of the most important classes of transformations, as it increases molecular complexity while avoiding the use of toxic and harmful oxidants. To this aim, the development of new and efficient photocatalysts capable of driving valuable oxidative reactions in a sustainable manner is highly desirable. These novel photocatalytic systems need to be metal-free, easy-to-prepare, and potentially recyclable. Carbon nitride (CN) fulfils all these requirements because of its outstanding physicochemical properties, thus emerging as a promising heterogeneous photocatalytic platform. The growing popularity of this material is also substantiated by its fast and facile preparation from readily available and inexpensive molecular precursors. This Review aims at highlighting the recent advances in synthesis of carbon nitride-based materials and their applications in organic photocatalysis for the oxidation of organic molecules in presence of molecular oxygen. Lastly, forward-looking opportunities within this intriguing research field are mentioned.

Sustainable Organic Photocatalysis for Site-Selective Hydrazocoupling of Electron-Rich Arenes.

An efficient photocatalytic para- and ortho-selective amination and aminative dearomatization of phenols, naphthols, and anilines with azodicarboxylates was developed using riboflavin tetraacetate (RFTA) as an organic photocatalyst. The site selectivity was controlled using tetrabutylammonium bromide (TBAB), which also acts as a phase transfer catalyst. The reaction conditions are simple and mild, giving high regioselectivity with good to excellent yields. A broad substrate scope and nice functional group tolerance with scalability and post-functionalization make this protocol both useful and regioselective.

Phenols as Novel Photocatalytic Platforms for Organic Synthesis

AbstractIn recent years, organic chemists have devoted a great deal of effort towards the implementation of novel green photocatalytic synthetic protocols. To this end, the development of new effective, non‐toxic, inexpensive photocatalysts, which are capable of driving value‐added chemical transformations, is highly desirable. Interestingly, phenols fulfill all these requirements due to their outstanding physicochemical features, therefore emerging as promising metal‐free photocatalytic platforms for organic synthesis. This Perspective aims at highlighting the most recent applications of phenols in organic photocatalysis. More specifically, phenolate anions, formed upon deprotonation of phenols, are photo‐active organic intermediates that may absorb light within the visible region. Thus, when in the excited states, these anions may be used as reductants to generate reactive open shell species from suitable precursors under mild operative conditions. Alternatively, phenolate anions and suitable radical precursors can form electron donor‐acceptor (EDA) complexes. Specifically, the photochemical activity of these molecular aggregates can be used to initiate organic radical reactions. Lastly, forward‐looking opportunities within this research field have been discussed.

Effect of a redox-mediating ligand shell on photocatalysis by CdS quantum dots.

Semiconductor quantum dots (QDs) are efficient organic photoredox catalysts due to their high extinction coefficients and easily tunable band edge potentials. Despite the majority of the surface being covered by ligands, our understanding of the effect of the ligand shell on organic photocatalysis is limited to steric effects. We hypothesize that we can increase the activity of QD photocatalysts by designing a ligand shell with targeted electronic properties, namely, redox-mediating ligands. Herein, we functionalize our QDs with hole-mediating ferrocene (Fc) derivative ligands and perform a reaction where the slow step is hole transfer from QD to substrate. Surprisingly, we find that a hole-shuttling Fc inhibits catalysis, but confers much greater stability to the catalyst by preventing a build-up of destructive holes. We also find that dynamically bound Fc ligands can promote catalysis by surface exchange and creation of a more permeable ligand shell. Finally, we find that trapping the electron on a ligand dramatically increases the rate of reaction. These results have major implications for understanding the rate-limiting processes for charge transfer from QDs and the role of the ligand shell in modulating it.