Investigating the radical properties of oxidized carbon materials under photo-irradiation: behavior of carbon radicals and their application in catalytic reactions

Oxidized carbon materials have abundant surface functional groups and customizable properties, making them an excellent platform for generating radicals. Unlike reactive oxygen species such as hydroxide or superoxide radicals that have been reported previously, oxidized carbon also produces stable carbon radicals under photo-irradiation. This has been confirmed through electron spin resonance. Among the various oxidized carbon materials synthesized, graphene oxide shows the largest number of carbon radicals when exposed to blue LED light. The light absorption capacity, high surface area, and unique structural characteristics of oxidized carbon materials offer a unique function for radical-mediated oxidative reactions.

Chemistry of zipping reactions in mesoporous carbon consisting of minimally stacked graphene layers

The structural evolution of highly mesoporous templated carbons is examined from temperatures of 1173 to 2873 K to elucidate the optimal conditions for facilitating graphene-zipping reactions whilst minimizing graphene stacking processes. Mesoporous carbons comprising a few-layer graphene wall display excellent thermal stability up to 2073 K coupled with a nanoporous structure and three-dimensional framework. Nevertheless, advanced temperature-programmed desorption (TPD), X-ray diffraction, and Raman spectroscopy show graphene-zipping reactions occur at temperatures between 1173 and 1873 K. TPD analysis estimates zipping reactions lead to a 1100 fold increase in the average graphene-domain, affording the structure a superior chemical stability, electrochemical stability, and electrical conductivity, while increasing the bulk modulus of the framework. At above 2073 K, the carbon framework shows a loss of porosity due to the development of graphene-stacking structures. Thus, a temperature range between 1873 and 2073 K is preferable to balance the developed graphene domain size and high porosity. Utilizing a neutron pair distribution function and soft X-ray emission spectra, we prove that these highly mesoporous carbons already consist of a well-developed sp2-carbon network, and the property evolution is governed by the changes in the edge sites and stacked structures.

Generation of aryl radicals by redox processes. Recent progress in the arylation methodology

Arylation methods based on the generation and use of aryl radicals have been a rapidly growing field of research in recent years and currently represent a powerful strategy for carbon – carbon and carbon – heteroatom bond formation. The progress in this field is related to advances in the methods for generation of aryl radicals. The currently used aryl radical precursors include aryl halides, aryldiazonium and diaryliodonium salts, arylcarboxylic acids and their derivatives, arylboronic acids, arylhydrazines, organosulfur(II, VI) compounds and some other compounds. Aryl radicals are generated under mild conditions by single electron reduction or oxidation of precursors induced by conventional reagents, visible light or electric current. A crucial role in the development of the radical arylation methodology belongs to photoredox processes either catalyzed by transition metal complexes or organic dyes or proceeding without catalysts. Unlike the conventional transition metal-catalyzed arylation methods, radical arylation reactions proceed very often at room temperature and have high functional group tolerance. Without claiming to be exhaustive, this review covers the most important advances of the current decade in the generation and synthetic applications of (het)aryl radicals. Examples of reactions are given and mechanistic insights are highlighted.

The bibliography includes 341 references.

Unveiling the Mechanism of Polymer Grafting on Graphene for Functional Composites: The Behavior of Radicals

Polymer-graphene composites have attracted significant attention; however, their formation mechanisms are a focus of debate. This work tries to clarify how grafting occurs on graphene by electron spin resonance techniques. As a result, two pathways are found. One passes through the radicals formed by cleaving CO bonds on graphene are transferred to monomers, then grafting and polymerization proceed. Another mechanism passes through the oxy-radicals, which react with monomers in solution and finally react with carbon radicals on graphene. Based on the mechanism, various types of polymer-graphene composites are prepared, and applied to electrical conductive sheets, basic catalysts, and acidic catalysts.

Graphene-based carbocatalysts for carbon-carbon bond formation

Organic transformations are usually catalyzed by metal-based catalysts. In contrast, metal-free catalysts have attracted considerable attention from the viewpoint of sustainability and safety. Among the studies in metal-free catalysis, graphene-based materials have been introduced in the reactions that are usually catalyzed by transition metal catalysts. This review covers the literature (up to the beginning of April 2020) on the use of graphene and its derivatives as carbocatalysts for C-C bond-forming reactions, which are one of the fundamental reactions in organic syntheses. Besides, mechanistic studies are included for the rational understanding of the catalysis. Graphene has significant potential in the field of metal-free catalysis because of the fine-tunable potential of the structure, high stability and durability, and no metal contamination, making it a next-generation candidate material in catalysis.

Redox reactions of small organic molecules using ball milling and piezoelectric materials

Over the past decade, photoredox catalysis has harnessed light energy to accelerate bond-forming reactions. We postulated that a complementary method for the redox-activation of small organic molecules in response to applied mechanical energy could be developed through the piezoelectric effect. Here, we report that agitation of piezoelectric materials via ball milling reduces aryl diazonium salts. This mechanoredox system can be applied to arylation and borylation reactions under mechanochemical conditions.

Selective Hydrogenation by Carbocatalyst: The Role of Radicals

The selective hydrogenation of the nitro moiety is a difficult task in the presence of other reducible functional groups such as alkenes or alkynes. We show that the carbon-based (metal-free) catalyst can be used to selectively reduce substituted nitro groups using H₂ as a reducing agent, providing a great potential to replace noble-metal catalysts and contributing to simple and greener strategies for organic synthesis.

Biaryl synthesis with arenediazonium salts: cross-coupling, CH-arylation and annulation reactions

The rich legacy of arenediazonium salts in the synthesis of unsymmetrical biaryls, built around the seminal works of Pschorr, Gomberg and Bachmann more than a century ago, continues to make important contributions at various evolutionary stages of modern biaryl synthesis. Based on in-depth mechanistic analysis and design of novel pathways and reaction conditions, the scope of biaryl synthesis with arenediazonium salts has enormously expanded in recent years through applications of transition metal/photoredox-catalysed cross-coupling, thermal/photosensitized radical chain CH-arylation of (hetero)arenes and arylative radical annulation reactions with alkynes. These recent developments have provided facile synthetic access to a wide variety of unsymmetrical biaryls of pharmaceutical, agrochemical and optoelectronic importance with green scale-up options and created opportunities for late-stage modification of peptides, nucleosides, carbon nanotubes and electrodes, the details of which are captured in this review.

Graphene oxide-phenalenyl composite: transition metal-free recyclable and catalytic C-H functionalization

An efficient route towards a heterogeneous transition metal-free catalytic C-H functionalization using a covalently linked graphene oxide-phenalenyl conjugate is described herein (28 examples, which include a core of some biologically relevant biaryl and hetero-biaryls). It is an environmentally benign, economical and heterogeneous platform, whose catalytic activity can easily be regenerated through a simple washing-drying technique and the catalytic activity can be retained even after 10 cycles.

A photochemical approach for a fast and self-limited covalent modification of surface supported graphene with photoactive dyes

Herein, we report a simple method for a covalent modification of surface supported graphene with photoactive dyes. Graphene was fabricated on cubic-SiC/Si(001) wafers due to their low cost and suitability for mass-production of continuous graphene fit for electronic applications on millimetre scale. Functionalisation of the graphene surface was carried out in solution via white light induced photochemical generation of phenazine radicals from phenazine diazonium salt. The resulting covalently bonded phenazine-graphene hybrid structure was characterised by scanning tunnelling microscopy (STM) and spectroscopy (STS), Raman spectroscopy and density functional theory (DFT) calculations. It was found that phenazine molecules form an overlayer, which exhibit a short range order with a rectangular unit cell on the graphene surface. DFT calculations based on STM results reveal that molecules are standing up in the overlayer with the maximum coverage of 0.25 molecules per graphene unit cell. Raman spectroscopy and STM results show that the growth is limited to one monolayer of standing molecules. STS reveals that the phenazine-graphene hybrid structure has a band gap of 0.8 eV.

Integrating reduced graphene oxide with microwave-subcritical water for cellulose depolymerization

Subcritical water compensates for the loss of functionalities in reduced graphene oxide to facilitate high depolymerization rate of cellulose under microwave.

White light induced covalent modification of graphene using a phenazine dye

A photochemical method to covalently modify graphene with a dye was developed. The hybrid material has a band-gap of 1.95 eV and emits light at 591 nm.