Covalent Modification of Glassy Carbon Surfaces by Electrochemical Grafting of Aryl Iodides

The reduction of an aryl iodide is generally believed to involve a clean-cut two-electron reduction to produce an aryl anion and iodide. This is in contradiction to what is observed if a highly efficient grafting agent, such as an aryldiazonium salt, is employed. The difference in behavior is explained by the much more extreme potentials required for reducing an aryl iodide, which facilitates the further reduction of the aryl radical formed as an intermediate. However, in this study we disclose that electrografting of aryl iodides is indeed possible upon extended voltammetric cycling. This implies that even if the number of aryl radicals left unreduced at the electrode surface is exceedingly small, a functionalization of the surface may still be promoted. In fact, the grafting efficiency is found to increase during the grafting process, which may be explained by the inhibiting effect the growing film exerts on the competing reduction of the aryl radical. The slow buildup of the organic film results in a well-ordered structure as shown by the well-defined electrochemical response from a grafted film containing ferrocenyl­methyl groups. Hence, the reduction of aryl iodides allows a precisely controlled, albeit slow, growth of thin organic films

Surface-Attached Poly(glycidyl methacrylate) as a Versatile Platform for Creating Dual-Functional Polymer Brushes

Novel types of dual-functional surface-attached polymer brushes were developed by post-polymerization modification of poly­(glycidyl methacrylate) brushes on glassy carbon substrates. Azide and alcohol groups were initially introduced by epoxide ring-openings of the side chains. These polymer brushes represent an attractive chemical platform to deliberately introduce other molecular units at specific sites. In this work, ferrocene and nitrobenzene redox units were immobilized through the two groups to create redox polymers. In-depth analysis by infrared reflection–absorption spectroscopy and X-ray photoelectron spectroscopy revealed an almost quantitative conversion of the modification reactions. The electrochemical activity of the ferrocenyl part of this diode-like system was fully expressed with an electron transfer rate constant = 1.2 s^–1 and surface density = 0.19 nmol cm^–2 per nm section of the film, independent of its thickness. In contrast, for the nitrobenzene moieties diffusion of counterions (i.e., tetraalkylammonium) easily becomes the rate-controlling step, thereby leaving a substantial fraction of them electrochemically inactive

Redox Grafting of Diazotated Anthraquinone as a Means of Forming Thick Conducting Organic Films

Thick conductive layers containing anthraquinone moieties are covalently immobilized on gold using redox grafting of the diazonium salt of anthraquinone (i.e., 9,10-dioxo-9,10-dihydroanthracene-1-diazonium tetrafluoroborate). This grafting procedure is based on using consecutive voltammetric sweeping and through this exploiting fast electron transfer reactions that are mediated by the anthraquinone redox moieties in the film. The fast film growth, which is followed by infrared reflection absorption spectroscopy, atomic force microscopy, X-ray photoelectron spectroscopy, ellipsometry, and coverage calculation, results in a mushroom-like structure. In addition to varying the number of sweeps, layer thickness control can easily be exerted through appropriate choice of the switching potential and sweep rate. It is shown that the grafting of the diazonium salt is essentially a diffusion-controlled process but also that desorption of physisorbed material during the sweeping process is essentially for avoiding blocking of the film due to clogging of the electrolyte channels in the film. In general, sweep rates higher than 0.5 V s^–1 are required if thick, porous, and conducting films should be formed

Electron Transport through a Diazonium-Based Initiator Layer to Covalently Attached Polymer Brushes of Ferrocenylmethyl Methacrylate

A versatile method based on electrografting of aryldiazonium salts was used to introduce covalently attached initiators for atom transfer radical polymerization (ATRP) on glassy carbon surfaces. Polymer brushes of ferrocenylmethyl methacrylate were prepared from the surface-attached initiators, and these films were thoroughly analyzed using various techniques, including X-ray photoelectron spectroscopy (XPS), infrared reflection–absorption spectroscopy (IRRAS), ellipsometry, and electrochemistry. Of particular interest was the electrochemical characterization of the electron transfer through the diazonium-based initiator layer to the redox centers in the polymer brush films. It was found that the apparent rate constant of electron transfer decreases exponentially with the dry-state thickness of this layer. To investigate the electron transfer in the brushes themselves, scanning electrochemical microscopy (SECM) was applied, thereby allowing the effect from the initiator layer to be excluded. The unusual transition feature of the approach curves recorded suggests that an initial fast charge transfer to the outermost-situated ferrocenyl groups is followed by a slower electron transport involving the neighboring redox units

On Electrogenerated Acid-Facilitated Electrografting of Aryltriazenes to Create Well-Defined Aryl-Tethered Films

The mechanism of electrogenerated acid-facilitated electrografting (EGAFE) of the aryltriazene, 4-(3,3-dimethyltriaz-1-enyl)­benzyl-1-ferrocene carboxylate, was studied in detail using electrochemical quartz crystal microbalance (EQCM) and cyclic voltammetry. The measurements support the previously suggested mechanism that electrochemical oxidation of the EGA agent (i.e., <i>N,N′</i>–diphenylhydrazine) occurs on the forward oxidative sweep to generate protons, which in turn protonate the aryltriazene to form the corresponding aryldiazonium salt close to the electrode surface. On the reverse sweep, the electrochemical reduction of the aryldiazonium salt takes place, resulting in the electrografting of aryl groups. The EGAFE-generated film consists of a densely packed layer of ferrocenyl groups with nearly ideal electrochemical properties. The uncharged grafted film contains no solvent and electrolyte, but counterions and solvent can easily enter and be accommodated in the film upon charging. It is shown that all ferrocene moieties present in the multilayered film are electrochemically active, suggesting that the carbon skeleton possesses a sufficiently high flexibility to allow the occurrence of fast electron transfers between the randomly located redox stations. In comparison, EQCM measurements on aryldiazonium-grafted films reveal that they have a substantially smaller electrolyte uptake during charging and that they contain only 50% electroactive ferrocenyl groups relative to weight. Hence, half of these films consist of entrapped supporting electrolyte/solvent and/or simply electrochemically inactive material due to solvent inaccessibility

Utilizing Glycerol as an Ex Situ CO-Source in Pd-Catalyzed Alkoxycarbonylation of Styrenes

We report on an efficient Ir-catalyzed decarbonylation of glycerol, which could be coupled to an ensuing Pd-catalyzed alkoxycarbonylation of styrenes. The formation of hydrogen could be avoided by employing 1,4-benzoquinone (BQ) as an external oxidant. A wide variety of styrenes underwent the esterification in good yields and high regioselectivity. Applying catalytic amounts of hexafluoroisopropanol provided access to alcohols other than methanol, which this transformation is often limited to. Finally, we demonstrate the suitability of this methodology for the preparation of three well-known nonsteroidal anti-inflammatory drugs (NSAIDs)

Elucidation of the Mechanism of Redox Grafting of Diazotated Anthraquinone

Redox grafting of aryldiazonium salts containing redox units may be used to form exceptionally thick covalently attached conducting films, even in the micrometers range, in a controlled manner on glassy carbon and gold substrates. With the objective to investigate the mechanism of this process in detail, 1-anthraquinone (AQ) redox units were immobilized on these substrates by electroreduction of 9,10-dioxo-9,10-dihydroanthracene-1-diazonium tetrafluoroborate. Electrochemical quartz crystal microbalance was employed to follow the grafting process during a cyclic voltammetric sweep by recording the frequency change. The redox grafting is shown to have two mass gain regions/phases: an irreversible one due to the addition of AQ units to the substrate/film and a reversible one due to the association of cations from the supporting electrolyte with the AQ radical anions formed during the sweeping process. Scanning electrochemical microscopy was used to study the relationship between the conductivity of the film and the charging level of the AQ redox units in the grafted film. For that purpose, approach curves were recorded at a platinum ultramicroelectrode for AQ-containing films on gold and glassy carbon surfaces using the ferro/ferricyanide redox system as redox probe. It is concluded that the film growth has its origin in electron transfer processes occurring through the layer mediated by the redox moieties embedded in the organic film

Efficient Graphene Production by Combined Bipolar Electrochemical Intercalation and High-Shear Exfoliation

In this study, we demonstrate that bipolar electrochemistry is a viable strategy for “wireless” electrochemical intercalation of graphite flakes and further large-scale production of high-quality graphene suspensions. Expansion of the graphite layers leads to a dramatic 20-fold increase in the yield of high-shear exfoliation. Large graphite flakes, which do not produce graphene upon high shear if left untreated, are exfoliated in a yield of 16.0 ± 0.2%. Successful graphene production was confirmed by Raman spectroscopy and scanning transmission electron microscopy, showing that the graphene flakes are 0.4–1.5 μm in size with the majority of flakes consisting of 4–6 graphene layers. Moreover, a low intensity of the D peak relative to the G peak as expressed by the <i>I</i>_D/<i>I</i>_G ratio in Raman spectroscopy along with high-resolution transmission electron microscopy images reveals that the graphene sheets are essentially undamaged by the electrochemical intercalation. Some impurities reside on the graphene after the electrochemical treatment, presumably because of oxidative polymerization of the solvent, as suggested by electron energy loss spectroscopy and X-ray photoelectron spectroscopy. In general, the bipolar electrochemical exfoliation method provides a pathway for intercalation on a wider range of graphite substrates and enhances the efficiency of the exfoliation. This method could potentially be combined with simultaneous electrochemical functionalization to provide graphene specifically designed for a given composite on a larger scale

Efficient Fluoride-Catalyzed Conversion of CO₂ to CO at Room Temperature

A protocol for the efficient and selective reduction of carbon dioxide to carbon monoxide has been developed. Remarkably, this oxygen abstraction step can be performed with only the presence of catalytic cesium fluoride and a stoichiometric amount of a disilane in DMSO at room temperature. Rapid reduction of CO₂ to CO could be achieved in only 2 h, which was observed by pressure measurements. To quantify the amount of CO produced, the reduction was coupled to an aminocarbonylation reaction using the two-chamber system, COware. The reduction was not limited to a specific disilane, since (Ph₂MeSi)₂ as well as (PhMe₂Si)₂ and (Me₃Si)₃SiH exhibited similar reactivity. Moreover, at a slightly elevated temperature, other fluoride salts were able to efficiently catalyze the CO₂ to CO reduction. Employing a nonhygroscopic fluoride source, KHF₂, omitted the need for an inert atmosphere. Substituting the disilane with silylborane, (pinacolato)­BSiMe₂Ph, maintained the high activity of the system, whereas the structurally related bis­(pinacolato)­diboron could not be activated with this fluoride methodology. Furthermore, this chemistry could be adapted to ¹³C-isotope labeling of six pharmaceutically relevant compounds starting from Ba¹³CO₃ in a newly developed three-chamber system

NHC-CDI Ligands Boost Multicarbon Production in Electrocatalytic CO₂ Reduction by Increasing Accumulated Charged Intermediates and Promoting *CO Dimerization on Cu

Copper-based materials exhibit significant potential as catalysts for electrochemical CO2 reduction, owing to their capacity to generate multicarbon hydrocarbons. The molecular functionalization of Cu electrodes represents a simple yet powerful strategy for improving the intrinsic activity of these materials by favoring specific reaction pathways through the creation of tailored microenvironments around the surface active sites. However, despite its success, comprehensive mechanistic insights derived from experimental techniques are often limited, leaving the active role of surface modifiers inconclusive. In this work, we show that N-heterocyclic carbene-carbodiimide-functionalized Cu catalysts display a remarkable activity for multicarbon product formation, surpassing bare Cu electrodes by more than an order of magnitude. These hybrid catalysts operate efficiently using an electrolyzer equipped with a gas diffusion electrode, achieving a multicarbon product selectivity of 58% with a partial current density of ca. −80 mA cm–2. We found that the activity for multicarbon product formation is closely linked to the surface charge that accumulates during electrocatalysis, stemming from surface intermediate buildup. Through X-ray photoelectron spectroscopy, we elucidated the role of the molecular additives in altering the electronic structure of the Cu electrodes, promoting the stabilization of surface CO. Additionally, in situ Raman measurements established the identity of the reaction intermediates that accumulate during electrocatalysis, indicating preferential CO binding on Cu step sites, known for facilitating C–C coupling. This study underscores the significant potential of molecular surface modifications in developing efficient electrocatalysts for CO2 reduction, highlighting surface charge as a pivotal descriptor of multicarbon product activity