2D sp2 Carbon‐Conjugated Porphyrin Covalent Organic Framework for Cooperative Photocatalysis with TEMPO

2D covalent organic frameworks (COFs) are receiving ongoing attention in semiconductor photocatalysis. Herein, we present a photocatalytic selective chemical transformation by combining sp² carbon‐conjugated porphyrin‐based covalent organic framework (Por‐sp²c‐COF) photocatalysis with TEMPO catalysis illuminated by 623 nm red light‐emitting diodes (LEDs). Highly selective conversion of amines into imines was swiftly afforded in minutes. Specifically, the π‐conjugation of porphyrin linker leads to extensive absorption of red light; the sp² ?C=C? double bonds linkage ensures the stability of Por‐sp²c‐COF under high concentrations of amine. Most importantly, we found that crystalline framework of Por‐sp²c‐COF is pivotal for cooperative photocatalysis with (2,2,6,6‐tetramethylpiperidin‐1‐yl)oxyl (TEMPO). This work foreshadows that the outstanding hallmarks of COFs, particularly crystallinity, could be exploited to address energy and environmental challenges by cooperative photocatalysis.     

Visible‐Light Photocatalysis: Does It Make a Difference in Organic Synthesis?

Visible‐light photocatalysis has evolved over the last decade into a widely used method in organic synthesis. Photocatalytic variants have been reported for many important transformations, such as cross‐coupling reactions, α‐amino functionalizations, cycloadditions, ATRA reactions, or fluorinations. To help chemists select photocatalytic methods for their synthesis, we compare in this Review classical and photocatalytic procedures for selected classes of reactions and highlight their advantages and limitations. In many cases, the photocatalytic reactions proceed under milder reaction conditions, typically at room temperature, and stoichiometric reagents are replaced by simple oxidants or reductants, such as air, oxygen, or amines. Does visible‐light photocatalysis make a difference in organic synthesis? The prospect of shuttling electrons back and forth to substrates and intermediates or to selectively transfer energy through a visible‐light‐absorbing photocatalyst holds the promise to improve current procedures in radical chemistry and to open up new avenues by accessing reactive species hitherto unknown, especially by merging photocatalysis with organo‐ or metal catalysis.     

Photocatalytic single electron transfer reactions on TiO2 semiconductor

The use of inorganic semiconductor particles such as titanium dioxide(TiO_2) has received relatively less attention in organic chemistry, although semiconductor particles have been widely used as a single electron transfer photocatalyst in waterpurification, air-cleaning, and self-cleaning. In recent years, the photocatalysis on semiconductor particles has become an active area of research even in organic chemistry, since the heterogeneous semiconductor photocatalysis leads to the unique redox organic reactions. In an early stage, the semiconductor photocatalysis was applied to the oxidation of organic molecules.Semiconductor particles have also the potential to induce the reductive chemical transformations in the absence of oxygen(O_2),by using the suitable sacrificial hole scavenger. In this review, we summarize the representative examples of the reductive and oxidative organic reactions using semiconductor particles and the recent applications to the stereoselective reactions.     

Electrophotocatalytic Decarboxylative C−H Functionalization of Heteroarenes

Decarboxylative C?H functionalization reactions are highly attractive methods for forging carbon–carbon bonds considering their inherent step‐ and atom‐economical features and the pervasiveness of carboxylic acids and C?H bonds. An ideal approach to achieve these dehydrogenative transformations is through hydrogen evolution without using any chemical oxidants. However, effective couplings by decarboxylative carbon–carbon bond formation with proton reduction remain an unsolved challenge. Herein, we report an electrophotocatalytic approach that merges organic electrochemistry with photocatalysis to achieve the efficient direct decarboxylative C?H alkylation and carbamoylation of heteroaromatic compounds through hydrogen evolution. This electrophotocatalytic method, which combines the high efficiency and selectivity of photocatalysis in promoting decarboxylation with the superiority of electrochemistry in effecting proton reduction, enables the efficient coupling of a wide range of heteroaromatic bases with a variety of carboxylic acids and oxamic acids. Advantageously, this method is scalable to decagram amounts, and applicable to the late‐stage functionalization of drug molecules.     

Ketones as Molecular Co‐catalysts for Boosting Exciton‐Based Photocatalytic Molecular Oxygen Activation

Excitonic processes in semiconductors open up the possibility for pursuing photocatalytic organic synthesis. However, the insufficient spin relaxation and robust nonradiative decays in semiconductors place restrictions on both quantum yield and selectivity of these reactions. Herein, by taking polymeric carbon nitride (PCN)/acetone as a prototypical system, we propose that extrinsic aliphatic ketones can serve as molecular co‐catalysts for promoting spin‐flip transition and suppressing non‐radiative energy losses. Spectroscopic investigations indicate that hot excitons in PCN can be transferred to ketones, while triplet excitons in ketones can be transferred to PCN. As such, the PCN/ketone systems exhibit considerable triplet‐exciton accumulation and extended visible‐light response, leading to excellent performance in exciton‐based photocatalysis, such as singlet oxygen generation. This work provides a fundamental understanding of energy harvesting in semiconductor/molecule systems, and paves the way for optimizing exciton‐based photocatalysis via molecular co‐catalyst design.     

The Polarization Effect in Surface‐Plasmon‐Induced Photocatalysis on Au/TiO2 Nanoparticles

Controlling the interaction of polarization light with an asymmetric nanostructure such as a metal/semiconductor heterostructure provides opportunities for tuning surface plasmon excitation and near‐field spatial distribution. However, light polarization effects on interfacial charge transport and the photocatalysis of plasmonic metal/semiconductor photocatalysts are unclear. Herein, we reveal the polarization dependence of plasmonic charge separation and spatial distribution in Au/TiO₂ nanoparticles under 45° incident light illumination at the single‐particle level using a combination of photon‐irradiated Kelvin probe force microscopy (KPFM) and electromagnetic field simulation. We quantitatively uncover the relationship between the local charge density and polarization angle by investigating the polarization‐dependent surface photovoltage (SPV). The plasmon‐induced photocatalytic activity is enhanced when the polarization direction is perpendicular to the Au/TiO₂ interface.     

金属-有机框架衍生的多功能光催化剂

在光催化过程中,光催化剂被太阳能激发产生光生电子和空穴,来实现环境净化或能量转换,是应对全球变暖和能源短缺的有效途径之一.然而,光催化技术面临的主要瓶颈问题是光生载流子的低分离效率和高反应能垒.而催化剂本身的特性对这一点起到了决定性的作用.因此,催化剂的合理设计和改性是提高光催化效率的关键.金属有机框架(MOFs)是一...     

结合(光)电化学方法研究光催化降解污染物反应

半导体光催化在环境保护和未来新能源开发等领域中具有重要的作用和意义. 由于参与光催化反应的主体是光生载流子,并涉及光生电子和空穴的界面转移与复合,(光)电化学方法是研究光催化反应微观动力学和机理的重要手段. 本文主要介绍本课题组应用这类研究方法在液相光催化去污方面所获得的部分研究结果,并对今后研究重点提出了某些看法.     

Plasmonic Nickel–TiO2 Heterostructures for Visible‐Light‐Driven Photochemical Reactions

Plasmon‐mediated carrier transfer (PMCT) at metal–semiconductor heterojunctions has been extensively exploited to drive photochemical reactions, offering intriguing opportunities for solar photocatalysis. However, to date, most studies have been conducted using noble metals. Inexpensive materials capable of generating and transferring hot carriers for photocatalysis via PMCT have been rarely explored. Here, we demonstrate that the plasmon excitation of nickel induces the transfer of both hot electrons and holes from Ni to TiO₂ in a rationally designed Ni–TiO₂ heterostructure. Furthermore, it is discovered that the transferred hot electrons either occupy oxygen vacancies (V_O) or produce Ti³⁺ on TiO₂, while the transferred hot holes are located on surface oxygens at TiO₂. Moreover, the transferred hot electrons are identified to play a primary role in driving the degradation of methylene blue (MB). Taken together, our results validate Ni as a promising low‐cost plasmonic material for prompting visible‐light photochemical reactions.     

微波场助光催化氧化及其应用

从半导体光催化机理、微波制备光催化剂和微波场助光催化反应等几个方面,简述微波在光催化领域中应用的研究现状和进展,探讨了微波-紫外光的耦合对提高光催化量子效率的作用.     

The Nature of Photocatalytic “Water Splitting” on Silicon Nanowires

Silicon should be an ideal semiconductor material if it can be proven usable for photocatalytic water splitting, given its high natural abundance. Thus it is imperative to explore the possibility of water splitting by running photocatalysis on a silicon surface and to decode the mechanism behind it. It is reported that hydrogen gas can indeed be produced from Si nanowires when illuminated in water, but the reactions are not a real water‐splitting process. Instead, the production of hydrogen gas on the Si nanowires occurs through the cleavage of Si? H bonds and the formation of Si? OH bonds, resulting in the low probability of generating oxygen. On the other hand, these two types of surface dangling bonds both extract photoexcited electrons, whose competition greatly impacts on carrier lifetime and reaction efficiency. Thus surface chemistry holds the key to achieving high efficiency in such a photocatalytic system.     

Semi‐heterogeneous Dual Nickel/Photocatalysis using Carbon Nitrides: Esterification of Carboxylic Acids with Aryl Halides

Cross‐coupling reactions mediated by dual nickel/photocatalysis are synthetically attractive but rely mainly on expensive, non‐recyclable noble‐metal complexes as photocatalysts. Heterogeneous semiconductors, which are commonly used for artificial photosynthesis and wastewater treatment, are a sustainable alternative. Graphitic carbon nitrides, a class of metal‐free polymers that can be easily prepared from bulk chemicals, are heterogeneous semiconductors with high potential for photocatalytic organic transformations. Here, we demonstrate that graphitic carbon nitrides in combination with nickel catalysis can induce selective C?O cross‐couplings of carboxylic acids with aryl halides, yielding the respective aryl esters in excellent yield and selectivity. The heterogeneous organic photocatalyst exhibits a broad substrate scope, is able to harvest green light, and can be recycled multiple times. In situ FTIR was used to track the reaction progress to study this transformation at different irradiation wavelengths and reaction scales.     

A Photowelding Strategy for Conductivity Restoration in Flexible Circuits

Light‐driven micropumps, which are based on electro‐osmosis with the electric field generated by photocatalytic reactions, are among most attractive research topics in chemical micromotors. Until now, research in this field has mainly been focused on the directional motion or collective behavior of microparticles, which lack practical applications. In this study, we have developed a photowelding strategy for repeated photoinduced conductivity recovery of cracked flexible circuits. We immersed the circuit in a suspension of conductive healing particles and applied photoillumination to the crack; photocatalysis of a predeposited pentacene (PEN) layer triggered electro‐osmotic effects to gather conductive particles at the crack, thus leading to conductivity recovery of the circuit. This photowelding strategy is a novel application of light‐driven micropumps and photocatalysis for conductivity restoration.     

Visible‐Light‐Induced Organic Photochemical Reactions through Energy‐Transfer Pathways

Visible‐light photocatalysis is a rapidly developing and powerful strategy to initiate organic transformations, as it closely adheres to the tenants of green and sustainable chemistry. Generally, most visible‐light‐induced photochemical reactions occur through single‐electron transfer (SET) pathways. Recently, visible‐light‐induced energy‐transfer (EnT) reactions have received considerable attentions from the synthetic community as this strategy provides a distinct reaction pathway, and remarkable achievements have been made in this field. In this Review, we highlight the most recent advances in visible‐light‐induced EnT reactions.     

Synthesis of Thiophene‐annulated Naphthalene Diimide‐based Small‐Molecular Acceptors via Two‐step C−H Activation

Thiophene‐annulated naphthalene diimide (NTI)‐based molecules have recently emerged as an important class of n‐type electronic materials. However, their synthesis has predominantly been achieved by Stille or Suzuki coupling reactions despite the presence of a potential C?H bond in NTI. Additionally, the synthesis of NTI or more generally mono‐functionalization of naphthalene diimide (NDI) starts with a cumbersome bromination that results in a low yield, is unselective, and requires tedious purification. We herein thus address these issues via a two‐step C?H activation: a rhodium‐catalyzed direct C?H iodinization is first presented for NDI, followed by establishing an efficient direct arylation protocol for NTI with high yield and robustness. Coupling of up to four NTI units on a benzene or pyrene core is demonstrated along with other aryl bromide substrates. All the herein reported NTI‐based small molecules showed n‐type semiconductor behavior under air.     

Amide‐Directed Tandem C?C/C?N Bond Formation through C?H Activation

The transformation of C? H bonds into other chemical bonds is of great significance in synthetic chemistry. C? H bond‐activation processes provide a straightforward and atom‐economic strategy for the construction of complex structures; as such, they have attracted widespread interest over the past decade. As a prevalent directing group in the field of C? H activation, the amide group not only offers excellent regiodirecting ability, but is also a potential C? N bond precursor. As a consequence, a variety of nitrogen‐containing heterocycles have been obtained by using these reactions. This Focus Review addresses the recent research into the amide‐directed tandem C? C/C? N bond‐formation process through C? H activation. The large body of research in this field over the past three years has established it as one of the most‐important topics in organic chemistry.     

Electrophotocatalysis with a Trisaminocyclopropenium Radical Dication

Visible‐light photocatalysis and electrocatalysis are two powerful strategies for the promotion of chemical reactions. Here, these two modalities are combined in an electrophotocatalytic oxidation platform. This chemistry employs a trisaminocyclopropenium (TAC) ion catalyst, which is electrochemically oxidized to form a cyclopropenium radical dication intermediate. The radical dication undergoes photoexcitation with visible light to produce an excited‐state species with oxidizing power (3.33 V vs. SCE) sufficient to oxidize benzene and halogenated benzenes via single‐electron transfer (SET), resulting in C?H/N?H coupling with azoles. A rationale for the strongly oxidizing behavior of the photoexcited species is provided, while the stability of the catalyst is rationalized by a particular conformation of the cis‐2,6‐dimethylpiperidine moieties.     

Highly Efficient Photocatalysts and Continuous‐Flow Photocatalytic Reactors for Degradation of Organic Pollutants in Wastewater

One of the most important applications for photocatalysis is engineered water treatment that photodegrades organic pollutants in wastewater at low cost. To overcome the low efficiency of batch degradation methods, continuous‐flow photocatalytic reactors have been proposed and have become the most promising method for mass water treatment. However, most commercial semiconductor photocatalysts are granular nanoparticles with low activity and a narrow active light wavelength band; this creates difficulties for direct use in continuous‐flow photocatalytic reactors. Therefore, a high‐performance photodegradation photocatalyst with proper morphology or structure is key for continuous photocatalytic degradation. Moreover, a well‐designed photocatalytic device is another important component for continuous‐flow photocatalysis and determines the efficiency of photocatalysis in practical water treatment. This review describes the basic design principles and synthesis of photocatalysts with excellent performance and special morphologies suitable for a filtering photocatalysis process. Certain promising continuous photodegradation reactors are also categorized and summarized. Additionally, selected scientific and technical problems that must be urgently solved are suggested.     

New insight into daylight photocatalysis of AgBr@Ag: synergistic effect between semiconductor photocatalysis and plasmonic photocatalysis

Noble metal nanoparticles (NPs) are often used as electron scavengers in conventional semiconductor photocatalysis to suppress electron-hole (e(-)-h(+) ) recombination and promote interfacial charge transfer, and thus enhance photocatalytic activity of semiconductors. In this contribution, it is demonstrated that noble metal NPs such as Ag NPs function as visible-light harvesting and electron-generating centers during the daylight photocatalysis of AgBr@Ag. Novel Ag plasmonic photocatalysis could cooperate with the conventional AgBr semiconductor photocatalysis to enhance the overall daylight activity of AgBr@Ag greatly because of an interesting synergistic effect. After a systematic investigation of the daylight photocatalysis mechanism of AgBr@Ag, the synergistic effect was attributed to surface plasmon resonance induced local electric field enhancement on Ag, which can accelerate the generation of e(-)-h(+) pairs in AgBr, so that more electrons are produced in the conduction band of AgBr under daylight irradiation. This study provides new insight into the photocatalytic mechanism of noble metal/semiconductor systems as well as the design and fabrication of novel plasmonic photocatalysts.     

Graphene‐Like Covalent Organic Framework with a Wide Band Gap Synthesized On Surface via Stepwise Reactions

Developing graphene‐like two‐dimensional materials naturally possessing a band gap has sparked enormous interest. Thanks to the inherent wide band gap and high mobility in the 2D plane, covalent organic frameworks containing triazine rings (t‐COFs) hold great promise in this regard, whilst the synthesis of single‐layer t‐COFs remains highly challenging. Herein, we present the fabrication of a well‐defined graphene‐like t‐COF on Au(111). Instead of single/multiple‐step single‐type reactions commonly applied for on‐surface synthesis, distinct stepwise on‐surface reactions, including alkynyl cyclotrimerization, C?O bond cleavage, and C?H bond activation, are triggered on demand, leading to product evolution in a controlled step‐by‐step manner. Aside from the precise control in sophisticated on‐surface synthesis, this work proposes a single‐atomic‐layer organic semiconductor with a wide band gap of 3.41 eV.