Photocatalyst System Research Articles

Photocatalytic degradation of tetracycline using surface defective black TiO2–ZnO heterojunction photocatalyst under visible light

Fabrication of heterojunction and surface defective engineering, through the formation of oxygen vacancies, are among the various photocatalytic enhancement techniques. A combination of these techniques has the prospect of enhancing photocatalytic activities through improved light absorption capabilities and charge separation process of the photocatalysts. In this study, a heterojunction of black titanium oxide-zinc oxide (BTiO2–ZnO) nanocomposite was synthesized using the conventional sol-gel approach, coupled with aluminum foil-assisted NaBH4 reduction. The structure, morphology, surface properties, and optical characteristics of the synthesized material were studied using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), UV–vis absorption spectra, scanning electron microscope (SEM), Energy-dispersive X-ray spectroscopy (EDS), and transmission electron microscope (TEM). The XRD confirmed the successful formation of BTiO2–ZnO heterostructure, while SEM revealed the structural morphology as pseudo-spherical with slight agglomeration. BTiO2–ZnO was found to be more efficient than BTiO2 and BZnO for the removal of tetracycline with degradation efficiencies of 63, 58, and 56 % respectively. The effects of process parameters such as the amount of photocatalyst, pollutant's concentration, and the initial solution pH on photocatalytic degradation study were systematically explored. The results confirm that the formation of the heterostructure from BTiO2 and BZnO could offer a facile route to improving the catalytic degradation of tetracycline. Therefore, this study offers a novel perspective on the design of efficient metal oxide photocatalyst systems that rely on the integration of defect engineering and heterojunction for the removal of organic contaminants.

Creation of robust oxygen vacancies in 2D ultrathin BiOBr nanosheets by irradiation through photocatalytic memory effect for enhanced CO2 reduction

An interesting photocatalytic memory effect was found in 2D ultrathin BiOBr nanosheets, which endowed them the capability to trap photogenerated electrons on Bi3+ and Br- in this material system for an extended period of time and resulted in the creation of stable oxygen vacancies (Ov) by just visible light irradiation due to the charge balance requirement. Thus, a broader visible light absorption, more negative conduction band minimum, stronger piezoelectric response, better charge carrier separation and transfer, and enhanced CO2 adsorption and activation were occurred in these BiOBr nanosheets with prior visible light irradiation, all of which were beneficial for their enhanced CO2 reduction performances in photocatalysis, piezocatalysis, and photopiezocatalysis, respectively, than their counterparts without it. This work demonstrated a novel approach to create oxygen vacancies by light irradiation through the photocatalytic memory effect, which could be readily applied to similar photocatalyst systems to boost their performances and generate novel functionalities.

Facile synthesis of a GO-g-C3N4/BaTiO3 ternary nanocomposites for visible-light-driven photocatalytic degradation of rhodamine B

Photogenerated charge carriers can undergo rapid recombination in conventional photocatalyst systems, reducing their photocatalytic efficiency. To address this bottleneck, a g-C3N4/BaTiO3 (CNB) heterojunction composite was decorated with different mass ratios of graphene oxide (GO) to form a novel visible-light responsive ternary GO-g-C3N4/BaTiO3 (GOCNB) nanocomposite using a facile fabrication method. The GOCNB photocatalyst exhibited significantly higher light absorption and greater charge transfer than CNB, g-C3N4, or BaTiO3. The photodegradation performance of GOCNB was optimized with a 2% mass loading of GO, and it achieved a degradation rate constant of 14.9 × 10−3 min−1 for rhodamine B with an efficiency of 94% within 180 min. The rate constant was 8-fold and 6-fold higher than that of bare BaTiO3 and CNB, respectively. The stronger photocatalytic activity was attributed to the synergistic effect of GO, g-C3N4, and BaTiO3, with g-C3N4 and BaTiO3 promoting charge transfer within a wider visible light range and GO promoting electron mobility and the photocatalyst's adsorption capacity. In particular, the proposed system maintained the spatial separation of photogenerated electron–hole pairs, which is vital for high photocatalytic activity. This study provides new insights into semiconductor-based photocatalytic systems and suggests a route for more environmentally sustainable technologies.

Effects of Surface‐Immobilization on Photobleaching of Xanthene Dye Photocatalysts

AbstractWhile organic photocatalysts provide increasingly versatile chemical pathways under mild conditions, their long‐term stability remains understudied. Here, the photobleaching behavior of xanthene dye photocatalysts is investigated. Rose Bengal, Eosin Y, and fluorescein are studied when in solution, when grafted to glass beads, and when incorporated into polymer brushes that are tethered to glass beads. This provides a comparison between xanthene's stability as a homogeneous and as a heterogeneous photocatalyst. Photobleaching is investigated using UV–vis, diffuse reflectance UV–vis (DR UV–vis), and fluorescence microscopy. Xanthene dyes as homogeneous photocatalysts exhibit the highest photostability, while the grafted systems appeared to fade more rapidly. Notably, heterogenization appears to have different effects based on the photocatalyst system, and further altering the photocatalyst environment with reagents may improve stability.

Ternary-role of NiCo-MOFs for boosting the photocatalytic H2-evolution and long term stability of CdS

CdS is a promising candidate for visible-light-driven photocatalytic H2-evolution, but its practical application is still limited by self-photo-corrosion and noble co-catalysts needing. Herein, CdS/NiCo-MOFs (CM) heterojunction was synthesized by an in-situ hydrothermal method, where NiCo-MOFs (BMOFs) acted as platform to inhibit the aggregation of CdS nanoparticles. Furthermore, BMOFs provided dual-reaction sites to improve the photocatalytic H2-evolution speed and suppress the photo-corrosion of CdS by taking its Ni-linker as catalytic active sites for water splitting and Co-linker as hole trapping sites to suppress the self-oxidation between S2− and holes. Therefore, the optimal CM-20% exhibited a remarkable photocatalytic H2-evolution rate of 1.810 mmol g−1 h−1 under sunlight irradiation, which is 4.55 times higher than that of CdS. Moreover, CM-20% maintained this high photocatalytic H2-evolution rate for 100 h reaction, which for CdS is just 50 h. This research provides insight into designing high-efficiency and photo-stable sulfide-based photocatalyst systems for H2-evolution.

3D In2S3/C/Fe3C nanofibers for Z-scheme photocatalytic CO2 conversion to acetate

Constructing a Z-scheme heterostructure is of great significance to achieve efficient photocatalytic CO2 conversion without sacrificial reagents. However, the fabrication of a well-suited Z-scheme remains a challenge. In this work, we constructed a Z-scheme system with a suitable band structure by in-situ hydrothermal growth of In2S3 nanosheets on electrospun Fe3C/Carbon fibers with 3D structure. The Z-scheme electron transport path is verified by the calculation of the energy band structure calculation and the method of photodeposition, indicating that In2S3 and Fe3C are reduction reaction sites and oxidation reaction sites respectively. Carbon fibers serve as both the skeleton of the 3D structure and the electron mediator from Fe3C to In2S3. Moreover, the DFT calculation demonstrates that the introduction of Fe3C can reduce the energy barrier of *CO and *COH coupling on In2S3, and weaken the bonding of In-S, thereby enhancing the product selectivity towards acetate. Owing to the efficient charge transfer of the Z-scheme system, the photocorrosion in In2S3 is also greatly reduced, showing a relatively stable chemical composition after several hours of reaction. Compared with In2S3 and Fe3C/C, In2S3-C/Fe3C composites showed a significantly increased acetate evolution rate of 11.33 μmol/h/g without any sacrificial reagents. This work provides important insights into the design and research of the photocatalyst system that combines a monolithic 3D structure and a Z-scheme charge flow for efficient CO2 conversion.

Directional surface hydroxylation on 6H-SiC induces surface electron polarization and proton activation promoting photocatalytic water splitting for hydrogen

Photocatalytic water splitting for hydrogen production is a promising solution to converse solar energy to green fuel. A high-performance photocatalyst system always requires fast surface reaction rate and efficient charge separation. Herein, a novel strategy that photo-assisted silanizing-hydrolyzing method is used to trigger in situ surface hydroxylation selectively on Si atoms of micro-sized 6H-SiC particles. Due to this functionalization, the results show that the silanizing-hydrolyzing process occurs at the Si sites while the C is passivated, leading the Si atoms with electron deficiency property by XPS characterization. It also drives the SiC surface from native hydrophobicity to hydrophilicity, which is benefit to proton absorption and desorption of generated H2 by FT-IR and contact angle (CA) result. Furthermore, the photo-electron-chemical characterization demonstrate the charge migration and separation of SiC are promoted. The DFT calculation further indicates these surface properties changes are mainly attribute to the surface electron polarization through the hydroxylation, inducing the Si-O spatial charge separation. More importantly, the positive Si atoms exhibit an appropriate proton absorption energy, resulting in easily H2 desorption from SiC surface. The hydrogen evolution is significantly enhanced by 33 times compared to pristine SiC without any cocatalysts introduced. Our method provides an efficient route to design an environmental-friendly photocatalytic material SiC and construct highly efficient solar energy conversion systems, which can simultaneously facilitate carrier generation, transportation and surface reduction reaction rate.

Single‐Atom Co‐Ultrafine RuOx Clusters Codecorated TiO2 Nanosheets Promote Photocatalytic Hydrogen Evolution: Modulating Charge Migration, H+ Adsorption, and H2 Desorption of Active Sites

AbstractRu as a cocatalyst has attracted wide attention as a substitute for Pt precious metal. However, the strong interaction between Ru and H atoms reduces the efficiency of hydrogen (H2) desorption and slows the overall hydrogen evolution reaction (HER) efficiency. How to further improve the efficiency of Ru‐based cocatalyst is challenging. Herein, single‐atom Co‐ultrafine RuOx clusters codecorated TiO2 nanosheets for photocatalytic HER are synthesized using a simple hydrothermal–calcination method. Experiments and theoretical calculations demonstrate that Co atoms can serve as a specific electronic medium, promoting electron enrichment at RuOx, thereby enhancing the adsorption of H+. Moreover, the electronic state of the interaction between adjacent Co atoms and RuOx is conducive to the desorption of H2. As a result, the proposed photocatalyst system gives a classy photocatalytic HER performance. The hydrogen production rate at stationary point reaches up to 20.20 mmol g−1 h−1, and the apparent quantum yield at 365 nm is 86.5%. It is worth noting that the hydrogen production rate in seawater is as high as 9.83 mmol g−1 h−1. This work offers precise guidance to design catalysts for efficient photocatalytic H2 production from the in‐depth understanding of the electronic coupling effect of coupling active sites.

In2O3/NIO/MOS2 Composite as a Novel Photocatalytic towards Imatinib and 5-Fluorouracil Degradation

Photocatalysts with high efficiency in water and wastewater treatment have gained increasing attention in recent years. This study synthesized an In2O3/NiO/MoS2 composite using the hydrothermal method and characterized its crystal structure, particle size, morphology, elemental purity, and optical properties. This nanocomposite exhibits high photocatalytic activity under visible light radiation. It achieved efficiencies of 91.57% and 88.23% in decomposing Imatinib and 5-fluorouracil, respectively. The formation of heterogeneity between MoS2 and NiO enhances the photocatalytic activity, which facilitates the separation and transfer efficiency of photo-generated electron-hole pairs, increases the catalytic active sites, and inhibits the rate of electron-hole recombination. The photocatalytic mechanism shows that both O2− and H+ are reactive species for the degradation of the studied pollutant. The stability and reusability tests deposited that the In2O3/NiO/MoS2 composite photocatalyst has superior stability during four reuse cycles. The results of the study show that a unique photocatalyst system can provide a new perspective and create new opportunities for the design of efficient composite photocatalysts.

Efficient activation of peroxymonosulfate by Z-scheme NiCo2O4/BiVO4 heterojunctions for rapid degradation of tetracycline under visible light irradiation

The development of visible-light-assisted photocatalysts for the rapid reduction of transition metals is an effective approach for researchers to overcome the limited activation efficiency of peroxymonosulfate (PMS). Herein, Z-scheme NiCo2O4/BiVO4 heterojunction composite catalysts were designed to effectively activate PMS for the degradation of tetracycline (TC) under visible light irradiation. The structure, morphology, composition, and optical properties of the composite catalysts were investigated comprehensively and carefully, and the degradation performance of TC in the PMS/vis system was examined. 2-NiCo2O4/BiVO4 exhibited the highest degradation performance, and the effective removal of TC could be achieved within 10 min. The effects of different conditions on degradation experiments were clarified in detail. The optical behavior of the composite catalysts was analyzed in detail by surface photovoltage (SPV), transient photovoltage (TPV), UV–vis diffuse reflectance spectroscopy, and photoluminescence (PL) measurements. The experimental results showed that the constructed Z-scheme heterojunction achieved effective separation of photogenerated carriers, which enabled the rapid reduction of Ni3+ and Co3+, thus accelerating the activation of PMS and obtaining a large number of free radicals for pollutant removal. This Z-scheme NiCo2O4/BiVO4 heterojunction composite photocatalyst system provides new insights for the further development of photocatalytic synergistic PMS-based AOPs.

Mechanisms for Charge Selectivity on Semiconducting Photocatalyst Particles

Overall-water splitting on photocatalysts provides a direct-unassisted path in the conversion of solar energy to chemical energy. Optoelectronic properties (absorbing incident light with long excited state lifetimes) and carrier selective contacts (transporting and transferring the desired electronic carrier to the targeted redox active species) are two key parameters that govern photocatalyst behavior. However, the advantageous small geometry of the highest achieving photocatalysts makes measurement of their carrier selective contacts difficult. Additionally, several mechanisms that enable high energy-conversion efficiencies and large gradients in free energy manifest only at the nanoscale at individual nanoparticle sites. SrTiO3 has been extensively studied and shown to split water with high quantum yields when decorated with HER and OER catalysts. SrTiO3 is used, in this work, as a model system to study mechanisms for charge selectivity on overall-water splitting photocatalyst particles.Here, we use three sample geometries to measure local, facet, and catalyst-dependent, electrochemical driving forces and their synergistic enhancement of carrier selectivity, (single crystal planar (b), single crystal co-facet exposed planar (a), facetted nanoparticles (c) loaded with OER and HER catalysts [{Pt/CrO3, Rh/CrO3} and CoOOH respectively]). We use ex-situ conductive atomic force microscopy to individually interrogate the electrical properties of nano-scale catalyst/semiconductor junctions decorated on facet-engineered surfaces of SrTiO3. Supporting experiments of SECM and Operando AP-XPS will be used to measure the charge carrier selectivity when an electrolyte is present and connect the ex-situ characterization observations to operando conditions. In summary, these findings demonstrate the underlying mechanisms and the degree that they play, enabling directed research of photocatalyst systems under the two dominant engineering conditions, high optoelectronic properties and charge selective contacts. Figure 1

(Invited) Z-Scheme Water Splitting Using Dye-Sensitized Metal Oxide Nanosheets

Solar water splitting by a heterogeneous photocatalyst, producing H2 and O2, is a potential solution for realizing large-scale H2 production. While dye-sensitized metal oxides are good candidates as H2 evolution photocatalysts for solar-driven Z-scheme water splitting, their solar-to-hydrogen (STH) energy conversion efficiencies remains low because of uncontrolled charge recombination reactions. Here it is shown that modification of Ru dye-sensitized, Pt- intercalated HCa2Nb3O10 nanosheets (Ru/Pt/HCa2Nb3O10) with both amorphous Al2O3 and poly(styrenesulfonate) (PSS) improves the STH efficiency of Z-scheme overall water splitting by a factor of ~100, when the nanosheets are used in combination with a WO3-based O2 evolution photocatalyst and an I3 –/I– redox mediator, relative to an analogous system that uses unmodified Ru/Pt/HCa2Nb3O10. The Al2O3 and PSS modifiers, which have previously been shown to suppress back electron transfer reactions in a dye-sensitized H2 evolution photocatalyst, enabled operation of the Z-scheme system even at low intensity of simulated sunlight without a decrease in the STH values. By using the optimized photocatalyst, PSS/Ru/Al2O3/Pt/HCa2Nb3O10, a maximum STH of 0.12% and an apparent quantum yield of 4.1% at 420 nm were obtained, by far the highest among dye-sensitized water splitting systems and also comparable to conventional semiconductor-based suspended particulate photocatalyst systems.

B2O3‐Nanoparticles‐Decorated N‐Rich Reduced Graphene Oxide Composites for the Enhanced Visible‐Light‐Assisted Photocatalytic Degradation of Ciprofloxacin

The occurrence of pharmaceutical pollutants in the environment is a mounting concern due to their detrimental effects on living organisms, including humans. Ciprofloxacin (CIP) is a regularly used antibiotic detected in wastewater and surface water. Therefore, developing effective, sustainable strategies for its elimination is paramount. Photocatalysis is an efficient approach that has been widely recommended for the removal of CIP from water. However, improved photocatalyst performance, stability, and activation in abundant visible light are essential for better photocatalyst systems. In this study, a novel boron‐oxide‐decorated nitrogen‐rich reduced graphene oxide (B2O3/N‐rGO) nanocomposite is prepared and characterized for its photocatalytic performance toward CIP degradation. The B2O3/N‐rGO nanocomposites are in situ synthesized via wet chemical route, and the morphology and structural properties of nanocomposites are extensively characterized. In the results, it is shown that the B2O3/N‐rGO nanocomposite demonstrated significantly enhanced photocatalytic performance (98%, 3 h) compared to N‐rGO and pure B2O3 toward the degradation of CIP under visible‐light irradiation. The enhanced photocatalytic activity is attributed to the synergistic benefit of B2O3 and N‐rGO, which improves light absorption, charge separation efficiency, adsorption sites, and delayed electron–hole recombination. In summary, the synthesized B2O3/N‐rGO nanocomposite photocatalyst can potentially be applied for various environmental remediation and energy‐related applications.

Enhanced synergistic photocatalytic degradation of oxytetracycline antibiotic using novel Ag2MoO4/Co-zeolitic imidazolate framework (ZIF-67) Z-type heterojunction

Enabling efficient separation of photocarriers is a critical approach for the development of high-performance photocatalyst systems to efficiently remove environmental contaminants. Herein, a novel heterojunction of Co-zeolitic imidazolate framework (ZIF-67) assembled on Ag2MoO4 has been developed using simple solvothermal and precipitation routes. The structure, composition, morphological, and surface and optical features of asprepared Ag2MoO4/ZIF-67 photocatalyst were determined by XRD, EDX, XPS, FESEM, TEM, PL, EIS, UV–vis DRS and BET analytical methods. The activity of developed composite photocatalyst was evaluated by photodegradation of a pharmaceutical oxytetracycline (OTC) under simulated visible light, in which 98.2% of was photodegraded by Ag2MoO4/ZIF-67 within 75 min. The improved photocatalytic performance of Z-type Ag2MoO4/ZIF-67 heterojunction is due to the active electron-hole separation, charge transfer between Ag2MoO4 and ZIF-67, and therefore, degradation efficiency enhanced via increasing the production of radicals. Moreover, the active species trapping experiments approved that •O2– and h+ were the most critical species, and reinforced the suggested photocatalytic mechanism of Ag2MoO4/ZIF-67 Z-type system.

Recent progress in photocatalytic reactions involving the excitation of electron-primed catalysts

Open-shell species are attracting significant attention owing to their unique physicochemical properties and highly reactive characteristics. Over the past decade, photoredox catalysis (PRC) has emerged as a powerful strategy for radical reactions. Recently, photocatalysis involving the excitation of open-shell catalytic species generated from in situ photo- or electrochemical electron transfer has also attracted significant attention in synthetic organic chemistry. These systems can achieve redox potentials that are difficult to achieve in the ground state of the photocatalyst or by simple excitation of the photocatalyst. In this review article, we discuss recent advancements in highly reducing organic photocatalyst (OPC) systems involving the photoexcitation of electron-primed catalytic species, which can be engaged in photochemically (conPET: consecutive photoinduced electron transfer) or electrochemically (e-PRC: electrochemically mediated photoredox catalysis). We believe that expanding the redox windows of catalysts to activate inert substrates from the viewpoint of redox potential will improve rational reaction design, and the use of sophisticated OPC systems will be promising for achieving elusive molecular transformations.

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.

Self-introduction of carbon nitride quantum dots into carbon nitride planar structure for enhanced photocatalytic hydrogen production

The separation efficiency and migration rate of photogenerated carriers are important factors determining the activity of photocatalysts. In this work, CN/CNQDs were prepared by self-introducing carbon nitride quantum dots (CNQDs) into the planar structure of carbon nitride (CN) using a novel photo-triggered self-assembly strategy. Different from conventional interfacial modification strategies based on van der Waals forces or hydrogen bonding, CN/CNQDs atomic junctions formed chemical bonds between the two components, which have stronger interfacial interactions and facilitate efficient carrier migration between the two components. The continuous π-conjugated structure of CN/CNQDs also provided unique conditions for carrier migration. In addition, the presence of in-plane electric field (IEF) in CN/CNQDs acted as a direct driving force for the migration of electrons and holes in the opposite directions, which greatly facilitates the separation and transfer of photogenerated carriers in CN/CNQDs. This work provides an atomic-level strategy for the construction of heterojunction photocatalyst systems.

Recent Progress on Semiconductor Heterogeneous Photocatalysts in Clean Energy Production and Environmental Remediation

Semiconductor-based photocatalytic reactions are a practical class of advanced oxidation processes (AOPs) to address energy scarcity and environmental pollution. By utilizing solar energy as a clean, abundant, and renewable source, this process offers numerous advantages, including high efficiency, eco-friendliness, and low cost. In this review, we present several methods to construct various photocatalyst systems with excellent visible light absorption and efficient charge carrier separation ability through the optimization of materials design and reaction conditions. Then it introduces the fundamentals of photocatalysis in both clean energy generation and environmental remediation. In the other parts, we introduce various approaches to enhance photocatalytic activity by applying different strategies, including semiconductor structure modification (e.g., morphology regulation, co-catalysts decoration, doping, defect engineering, surface sensitization, heterojunction construction) and tuning and optimizing reaction conditions (such as photocatalyst concentration, initial contaminant concentration, pH, reaction temperature, light intensity, charge-carrier scavengers). Then, a comparative study on the photocatalytic performance of the various recently examined photocatalysts applied in both clean energy production and environmental remediation will be discussed. To realize these goals, different photocatalytic reactions including H2 production via water splitting, CO2 reduction to value-added products, dye, and drug photodegradation to lessen toxic chemicals, will be presented. Subsequently, we report dual-functional photocatalysis systems for simultaneous energy production and pollutant photodegradation for efficient reactions. Then, a brief discussion about the industrial and economical applications of photocatalysts is described. The report follows by introducing the application of artificial intelligence and machine learning in the design and selection of an innovative photocatalyst in energy and environmental issues. Finally, a summary and future research directions toward developing photocatalytic systems with significantly improved efficiency and stability will be provided.

Mechanism insights for efficient photocatalytic reforming of formic acid with tunable selectivity: Accelerated charges separation and spatially separated active sites

Photocatalytic reforming of formic acid (FA) is considered a promising energy conversion method for producing solar fuels and valuable chemical feedstocks to achieve the carbon-neutral goal. However, the application of FA photocatalytic reforming has been restricted by the low efficiency and relatively fixed selectivity due to limited knowledge about the reaction mechanism. Herein, we study the efficient CdS/W2N3 photocatalyst system for photoreforming of FA with tunable selectivity to realize the conversion of products from H2 to syngas under simulated sunlight. Widely tunable CO:H2 ratios between 0 and 2.18 along with a record-high apparent quantum yield (AQY) of 61.00% (H2) and 76.84% (syngas) at 420 nm have been demonstrated. Both theoretical investigation and experimental results show that intrinsic N vacancies and spatially separated active sites are vital factors in achieving tunable selectivity. This work provides a insightful information to construct and understand the photocatalytic system for efficiently reforming of FA with tunable selectivity.

Discovery and Development of the Enantioselective Minisci Reaction.

ConspectusThe class of reactions now known as Minisci reactions is broadly defined as the addition of nucleophilic carbon-based radicals to basic heteroarenes with subsequent rearomatization to form a new carbon-carbon bond. Since the pioneering work of Minisci in the 1960s and 1970s, these reactions are now widely used in medicinal chemistry due to the ubiquity of basic heterocycles in druglike molecules. One of the long-standing challenges of Minisci chemistry has been that of regioselectivity due to the mixtures of positional isomers commonly obtained on many substrates if there is a choice between similarly activated sites. At the outset of the work described herein, we hypothesized that it may be possible to tackle this using a catalytic strategy whereby a bifunctional Brønsted acid catalyst simultaneously activates the heteroarene and engages attractive non-covalent interactions with the incoming nucleophile, resulting in a proximal attack. Using chiral BINOL-derived phosphoric acids, we not only were able to achieve this goal of regiocontrol but also discovered that we could control the absolute stereochemistry at the new stereocenter formed when prochiral α-amino radicals were employed. At the time, this discovery was unprecedented in the context of Minisci reactions.This Account details the discovery of this protocol and the further development, expansion, and investigations into the mechanism that we have carried out since then, several in collaboration with other research groups. Collaborative efforts have involved an expansion of the scope to diazines guided by multivariate statistical analysis through the development of a predictive model (collaboration with Sigman). Also, a mechanistic study involving detailed DFT analysis (collaboration with Goodman and Ermanis) unveiled the selectivity-determining step as being the deprotonation of a key cationic radical intermediate by the associated chiral phosphate anion. We have additionally carried out a number of synthetic developments of the protocol such as removing the need to prefunctionalize the radical nucleophile; hydrogen-atom transfer can be used to enable a formal coupling of two C-H bonds to form a C-C bond while retaining high enantio- and regioselectivity. Most recently, we have been able to expand the protocol so that α-hydroxy radicals can be used: until this point, all examples had concerned α-amino radicals. Again, HAT was used to generate the α-hydroxy radicals, and DFT studies carried out in collaboration (Ermanis) provided mechanistic insights.Since our original report, there have appeared a number of exciting developments from other research groups whereby the protocol has been applied to new substrates or using different precursors to generate the requisite α-amino radical. There have also been several examples in which alternative photocatalyst systems have been used to reduce the redox-active esters in the original enantioselective Minisci protocol. While primarily an Account, these contributions from other research groups will be covered briefly for context toward the end of the article.