Tetrakis(N-heterocyclic Carbene)-Diboron(0): Double Single-Electron-Transfer Reactivity.

The use of 1,3,4,5-tetramethylimidazol-2-ylidene (IMe) to coordinate with diatomic B2 species afforded a tetrakis(N-heterocyclic carbene)-diboron(0) [(IMe)2B-B(IMe)2] (2). The singly bonded B2 moiety therein possesses a valence electronic configuration 1σg21πu21πg*2 with four vacant molecular orbitals (1σu*, 2σg, 1πu', 1πg'*) coordinated with IMe. Its unprecedented electronic structure is analogous to the energetically unfavorable planar hydrazine with a D2h symmetry. The two highly reactive πg* antibonding electrons enable double single-electron-transfer (SET) reactivity in small-molecule activation. Compound 2 underwent a double SET reduction with CO2 to form two carbon dioxide radical anions CO2•-, which then reduced pyridine to yield a carboxylated pyridine reductive coupling dianion [O2CNC5(H)5-C5(H)5NCO2]2- and converted compound 2 to the tetrakis(N-heterocyclic carbene)-diborene dication [(IMe)2B═B(IMe)2]2+ (32+). This is a remarkable transition-metal-free SET reduction of CO2 without ultraviolet/visible (UV/vis) light conditions.

Non-Fullerene Electron Acceptors Based on Hybridisation of Corannulene and Thiophene-S,S-Dioxide Motifs.

Herein we show that hybridisation of buckybowl corannulene and thiophene-S,S-dioxide motifs is a general approach for the preparation of high electron affinity molecular materials. The devised synthesis is modular and relies on thienannulation of corannnulene-based phenylacetylene scaffolds. The final compounds are highly soluble in common organic solvents. These compounds also exhibit interesting optical properties such as absorption and emission in the blue/green regions of the electromagnetic spectrum. Importantly, a bis-S,S-dioxide derivative exhibits three reversible reductions similar in their strength to the prevalent fullerene-based electron acceptor phenyl-C61 -butyric acid methyl ester (PC61 BM).

Electrochemical and Spectroscopic Characterization of Oxidized Intermediate Forms of Vitamin E.

Vitamin E, a collection of lipophilic phenolic compounds based on chroman-6-ol, has a rich and fascinating oxidative chemistry involving a range of intermediate forms, some of which are proposed to be important in its biological functions. In this review, the available electrochemical and spectroscopic data on these oxidized intermediates are summarized, along with a discussion on how their lifetimes and chemical stability are either typical of similar phenolic and chroman-6-ol derived compounds, or atypical and unique to the specific oxidized isomeric form of vitamin E. The overall electrochemical oxidation mechanism for vitamin E can be summarized as involving the loss of two-electrons and one-proton, although the electron transfer and chemical steps can be controlled to progress along different pathways to prolong the lifetimes of discreet intermediates by modifying the experimental conditions (applied electrochemical potential, aqueous or non-aqueous solvent, and pH). Depending on the environment, the electrochemical reactions can involve single electron transfer (SET), proton-coupled electron transfer (PCET), as well as homogeneous disproportionation and comproportionation steps. The intermediate species produced via chemical or electrochemical oxidation include phenolates, phenol cation radicals, phenoxyl neutral radicals, dications, diamagnetic cations (phenoxeniums) and para-quinone methides. The cation radicals of all the tocopherols are atypically long-lived compared to the cation radicals of other phenols, due to their relatively weak acidity. The diamagnetic cation derived from α-tocopherol is exceptionally long-lived compared to the diamagnetic cations from the other ß-, γ- and δ-isomers of vitamin E and compared with other phenoxenium cations derived from phenolic compounds. In contrast, the lifetime of the phenoxyl radical derived from α-tocopherol, which is considered to be critical in biological reactions, is typical for what is expected for a compound with its structural features. Over longer times via hydrolysis reactions, hydroxy para-quinone hemiketals and quinones can be formed from the oxidized intermediates, which can themselves undergo reduction processes to form intermediate anion radicals and dianions. Methods for generating the oxidized intermediates by chemical, photochemical and electrochemical methods are discussed, along with a summary of how the final products vary depending on the method used for oxidation. Since the intermediates mainly only survive in solution, they are most often monitored using UV-vis spectroscopy, FTIR or Raman spectroscopies, and EPR spectroscopy, with the spectroscopic techniques sometimes combined with fast photoinitiated excitation and time-resolved spectroscopy for detection of short-lived species.

MXene Titanium Carbide-based Biosensor: Strong Dependence of Exfoliation Method on Performance.

Transition metal carbides, known as MXenes, are generated via the selective etching of "A" layers from their layered, ternary parent compounds, MAX phases, where M corresponds to early d-transition metal, A being a main group sp-element from either Group 13 or 14 and carbon or nitrogen being denoted by X. MXenes are being recognized as a new and uprising class of 2D materials with extraordinary physical and electrochemical properties. The huge specific surface area and outstanding electrical conductivity of MXenes, make them ideal candidates for sensing and energy applications. Herein, we demonstrated the successful incorporation of pristine MXene, Ti3C2 produced via HF etching and subsequent delamination with TBAOH, as a transducer platform toward the development of a second generation electrochemical glucose biosensor. Chronoamperometric studies demonstrate that the proposed biosensing system exhibits high selectivity and excellent electrocatalytic activity toward the detection of glucose, spanning over wide linear ranges of 50-27 750 µM and possess a low limit of detection of 23.0 µM. The findings reported in this study conceptually proves the probable applications of pristine MXenes toward the field of biosensors and pave ways for the future developments of highly selective and sensitive electrochemical biosensors for biomedical and food sampling applications.

Functional 2D Germanene Fluorescent Coating of Microrobots for Micromachines Multiplexing.

Micromachines are at the forefront of materials research as they are self-propelled, smart autonomous systems capable of acting as an intelligent matter. One of the obstacles the field faces is tracking individual micromachines carrying molecular cargo from the rest of the micromachines. Highly stable fluorescent markers based on chemically modified 2D germanene compounds are developed. Two different 2D germanene derivatives, 4-fluorophenylgermanane (2D-Ph-Ge) and methylgermanane (2D-Me-Ge), exhibit different fluorescence under UV light irradiation (excitation at 365 nm), which allows one particular micromotor to be easily distinguished in a mixture of micromotors. This offers a paradigm shift toward a new approach of multiplex detection of self-propelled micromachines. The utility is demonstrated on a drug delivery system, where micromachines carrying a drug are labeled with 2D-Ph-Ge with blue emission while bare micromachines are labeled by 2D-Me-Ge with red emission. This approach of functional fluorescent labeling will pave the way to multiple simultaneous functionalized micromachines identification in complex environments.

Corannulene-Based Electron Acceptors: Combining Modular and Practical Synthesis with Electron Affinity and Solubility.

It is shown in this work that high electron affinity can be combined with high solubility and practical accessibility in corannulene-based electron acceptors. The electron affinity originates from the presence of three different types of electron-withdrawing groups (imide, sulfone, and trifluoromethyl) on the aromatic scaffold. The imide substituent further hosts a long alkyl chain (C18 H37 ) to boast solubility in a wide range of organic solvents. The synthesis is modular and consists of three simple steps from a commonly available corannulene derivative with an overall isolated yield of 22-27 %.

Mechanochemical Synthesis of Corannulene-Based Curved Nanographenes.

It is shown that corannulene-based strained π-surfaces can be obtained through the use of mechanochemical Suzuki and Scholl reactions. Besides being solvent-free, the mechanochemical synthesis is high-yielding, fast, and scalable. Therefore, gram-scale preparation can be carried out in a facile and sustainable manner. The synthesized nanographene structure carries positive (bowl-like) and negative (saddle-like) Gaussian curvatures and adopts an overall quasi-monkey saddle-type of geometry. In terms of properties, the non-planar surface exhibits a high electron affinity that was measured by cyclic voltammetry, with electrolysis and in situ UV/vis spectroscopy experiments indicating that the one-electron reduced state displays a long lifetime in solution. Overall, these results indicate the future potential of mechanochemistry in accessing synthetically challenging and functional curved π-systems.

Photochemical Synthesis and Electronic Properties of Extended Corannulenes with Variable Fluorination Pattern.

The first family of extended and fluorinated corannulenes is prepared through a highly efficient and modular synthetic strategy. In this strategy, corannulene aldehyde could be combined with the fluorine-carrying phosphonium ylides to furnish stilbene-like vinylene precursors. A photochemically induced oxidative cyclization process of these precursors gives rise to the fluorinated and curved polycyclic aromatic hydrocarbons. A UV-vis absorption study shows that aromatic extension results in a bathochromic shift of about 12 nm. Fluorination further shifts the absorption spectrum to the red region, and a maximum shift of about 22 nm is detected for a compound carrying two trifluoromethyl groups. A cyclic and square-wave voltammetry investigation reveals that the extension of the corannulene scaffold increases the reduction potential by 0.11 V. Placement of fluorine or trifluoromethyl groups further enhances the electron affinities. In this regard, the presence of one trifluoromethyl group equals the effect of three aromatic fluorine atoms. Molecules with two trifluoromethyl groups, meanwhile, exhibit the highest reduction potentials of -1.93 and -1.83 V. These values are 0.37 and 0.46 V higher than those of the parental corannulene and demonstrate the utility of the present design concept by efficiently accessing effective electron acceptors based on the buckybowl motif.

A Crystalline Diazadiborinine Radical Cation and Its Boron-Centered Radical Reactivity.

One-electron oxidation of 1,4,2,5-diazadiborinine 1 has been studied. While the reaction of 1 a bearing phenyl groups on the B atoms with AgAl{OC(CF3 )3 }4 afforded a complex mixture, the same oxidation reaction with 1 b featuring bulky mesityl substituents on the B atoms rendered the corresponding cation radical 2 b as an isolable species. X-ray diffraction analysis, EPR spectroscopy, and DFT calculations of 2 b revealed the delocalization of the unpaired electron over the entire π-system of 2 b, as well as a large spin density (0.76 in total) on the two equivalent boron atoms. The chemical trapping reaction of 2 b with p-benzoquinone and triphenyltin hydride afforded the dicationic species 3 containing two newly formed B-O bonds and the monocationic product 2b-H containing a B-H bond, respectively, thus confirming the boron-centered radical reactivity of 2 b.

Direct Visible-Light-Excited Asymmetric Lewis Acid Catalysis of Intermolecular [2+2] Photocycloadditions.

A reaction design is reported in which a substrate-bound chiral Lewis acid complex absorbs visible light and generates an excited state that directly reacts with a cosubstrate in a highly stereocontrolled fashion. Specifically, a chiral rhodium complex catalyzes visible-light-activated intermolecular [2+2] cycloadditions, providing a wide range of cyclobutanes with up to >99% ee and up to >20:1 d.r. Noteworthy is the ability to create vicinal all-carbon-quaternary stereocenters including spiro centers in an intermolecular fashion.

The Dual Roles of Phenylenediamines: Using their Voltammetric Behavior to Measure the Hydrogen Donor and Acceptor Abilities of Alcohols in Acetonitrile.

Cyclic voltammetry experiments on 2,3,5,6-tetramethyl-1,4-phenylenediamine (P) in acetonitrile in the presence of varying concentrations of alcohols indicate that the oxidized forms of the compound (P.+ and P2+ ) interact with the alcohols through a hydrogen-bonding mechanism where P.+ and P2+ act as the hydrogen donor and the alcohols act as acceptors. However, the neutral form (P) largely acts as a hydrogen acceptor but only for strong hydrogen donors that do not undergo proton-transfer reactions with the phenylenediamine. These results were ascertained based on measuring the difference in potential of the two one-electron transfer reactions (ΔEPox(1, 2) =|EPox(1) -EPox(2) |) in the oxidative electrochemistry of P, which thereby allows a simple measure of relative hydrogen bonding strengths.

Using Voltammetry to Measure the Relative Hydrogen-Bonding Strengths of Pyridine and Its Derivatives in Acetonitrile.

The voltammetric behavior of 2,3,5,6-tetramethyl-1,4-phenylenediamine was found to be able to differentiate the hydrogen acceptor abilities of electroinactive pyridine compounds in acetonitrile. Weak and strong hydrogen acceptors were distinguished through the onset of a third oxidation process that came about at sub-stoichiometric amounts of strong hydrogen acceptors, but not in the presence of weak hydrogen acceptors. This additional oxidation reaction occurred at a potential between the two 1 e- -oxidation reactions that phenylenediamines typically undergo (i.e. EPox(1)

Asymmetric Catalysis with Organic Azides and Diazo Compounds Initiated by Photoinduced Electron Transfer.

Electron-acceptor-substituted aryl azides and α-diazo carboxylic esters are used as substrates for visible-light-activated asymmetric α-amination and α-alkylation, respectively, of 2-acyl imidazoles catalyzed by a chiral-at-metal rhodium-based Lewis acid in combination with a photoredox sensitizer. This novel proton- and redox-neutral method provides yields of up to 99% and excellent enantioselectivities of up to >99% ee with broad functional group compatibility. Mechanistic investigations suggest that an intermediate rhodium enolate complex acts as a reductive quencher to initiate a radical process with the aryl azides and α-diazo carboxylic esters serving as precursors for nitrogen and carbon-centered radicals, respectively. This is the first report on using aryl azides and α-diazo carboxylic esters as substrates for asymmetric catalysis under photoredox conditions. These reagents have the advantage that molecular nitrogen is the leaving group and sole byproduct in this reaction.

Ball-milled sulfur-doped graphene materials contain metallic impurities originating from ball-milling apparatus: their influence on the catalytic properties.

Graphene materials have found applications in a wide range of devices over the past decade. In order to meet the demand for graphene materials, various synthesis methods are constantly being improved or invented. Ball-milling of graphite to obtain graphene materials is one of the many versatile methods to easily obtain bulk quantities. In this work, we show that the graphene materials produced by ball-milling are spontaneously contaminated with metallic impurities originating from the grinding bowls and balls. Ball-milled sulfur-doped graphene materials obtained from two types of ball-milling apparatus, specifically made up of stainless steel and zirconium dioxide, were investigated. Zirconium dioxide-based ball-milled sulfur-doped graphene materials contain a drastically lower amount of metallic impurities than stainless steel-based ball-milled sulfur-doped graphene materials. The presence of metallic impurities is demonstrated by their catalytic effects toward the electrochemical catalysis of hydrazine and cumene hydroperoxide. The general impression toward ball-milling of graphite as a versatile method for the bulk production of 'metal-free' graphene materials without the need for post-processing and the selection of ball-milling tools should be cautioned. These findings would have wide-reaching implications for graphene research.

Extended Bis(benzothia)quinodimethanes and Their Dications: From Singlet Diradicaloids to Isoelectronic Structures of Long Acenes.

Extended bis(benzothia)quinodimethanes and their dications were synthesized as stable species. The neutral compounds mainly have a quinoidal structure in the ground state but show increased diradical character with extension of the central quinodimethane unit. The dications exhibit similar electronic absorption spectra, NMR spectra, NICS values, and diatropic ring currents to their aromatic all-carbon acene analogues and thus can be regarded as genuine isoelectronic structures of pentacene, hexacene, and heptacene, respectively. Our research gave some insights into the design and synthesis of stable longer acene analogues.

Theoretical Modelling and Facile Synthesis of a Highly Active Boron-Doped Palladium Catalyst for the Oxygen Reduction Reaction.

A highly active alternative to Pt electrocatalysts for the oxygen reduction reaction (ORR), which is the cathode-electrode reaction of fuel cells, is sought for higher fuel-cell performance. Our theoretical modelling reveals that B-doped Pd (Pd-B) weakens the absorption of ORR intermediates with nearly optimal binding energy by lowering the barrier associated with O2 dissociation, suggesting Pd-B should be highly active for ORR. In fact, Pd-B, facile synthesized by an electroless deposition process, exhibits 2.2 times and 8.8 times higher specific activity and 14 times and 35 times less costly than commercial pure Pd and Pt catalysts, respectively. Another computational result is that the surface core level of Pd is negatively shifted by B doping, as confirmed by XPS, and implies that filling the density of states related to the anti-bonding of oxygen to Pd surfaces with excess electrons from B doping, weakens the O bonding to Pd and boosts the catalytic activity.

N-heterocyclic carbene-catalyzed radical reactions for highly enantioselective ß-hydroxylation of enals.

An N-heterocyclic carbene-catalyzed ß-hydroxylation of enals is developed. The reaction goes through a pathway involving multiple radical intermediates, as supported by experimental observations. This oxidative single-electron-transfer reaction allows for highly enantioselective access to ß-hydroxyl esters that are widely found in natural products and bioactive molecules.

Human transport protein carrier for controlled photoactivation of antitumor prodrug and real-time intracellular tumor imaging.

Current anticancer chemotherapy often suffers from poor tumor selectivity and serious drug resistance. Proper vectors for targeted delivery and controlled drug release play crucial roles in improving the therapeutic selectivity to tumor areas and also overcoming the resistance of cancer cells. In this work, we developed a novel human serum albumin (HSA) protein-based nanocarrier system, which combines the photoactivatable Pt(IV) antitumor prodrug for realizing the controlled release and fluorescent light-up probe for evaluations of drug action and efficacy. The constructed Pt(IV)-probe@HSA platform can be locally activated by light irradiation to release the active Pt species, which results in enhanced cell death at both drug-sensitive A2780 and cisplatin-resistant A2780cis cell lines when compared to the free prodrug molecules. Simultaneously, the cytotoxicity caused by light controlled drug release would further lead to the cellular apoptosis and trigger the activation of caspases 3, one crucial protease enzyme in apoptotic process, which could cleave the recognition peptide moiety (DEVD) with a flanking fluorescent resonance energy transfer (FRET) pair containing near-infrared (NIR) fluorophore Cy5 and quencher Qsy21 on the HSA nanocarrier surface. The turn-on fluorescence in response to caspase-3 could be assessed by fluorescence microscopy and flow cytometry analysis. Our results supported the hypothesis that such a unique design may present a successful platform for multiple roles: (i) a biocompatible protein-based nanocarrier for drug delivery, (ii) the controlled drug release with strengthened therapeutic effects, (iii) real-time monitoring of antitumor drug efficacy at the earlier stage.

Interplay of hole transfer and host-guest interaction in a molecular dyad and triad: ensemble and single-molecule spectroscopy and sensing applications.

A new molecular dyad consisting of a Cy5 chromophore and ferrocene (Fc) and a triad consisting of Cy5, Fc, and ß-cyclodextrin (CD) are synthesized and their photophysical properties investigated at both the ensemble and single-molecule levels. Hole transfer efficiency from Cy5 to Fc in the dyad is reduced upon addition of CD. This is due to an increase in the Cy5-Fc separation (r) when the Fc is encapsulated in the macrocyclic host. On the other hand, the triad adopts either a Fc-CD inclusion complex conformation in which hole transfer quenching of the Cy5 by Fc is minimal or a quasi-static conformation with short r and rapid charge transfer. Single-molecule fluorescence measurements reveal that r is lengthened when the triad molecules are deposited on a glass substrate. By combining intramolecular charge transfer and competitive supramolecular interaction, the triad acts as an efficient chemical sensor to detect different bioactive analytes such as amantadine hydrochloride and sodium lithocholate in aqueous solution and synthetic urine.

Measuring the relative hydrogen-bonding strengths of alcohols in aprotic organic solvents.

Voltammetric experiments with 9,10-anthraquinone and 1,4-benzoquinone performed under controlled moisture conditions indicate that the hydrogen-bond strengths of alcohols in aprotic organic solvents can be differentiated by the electrochemical parameter ΔEp (red) =|Ep (red(1)) -Ep (red(2)) |, which is the potential separation between the two one-electron reduction processes. This electrochemical parameter is inversely related to the strength of the interactions and can be used to differentiate between primary, secondary, tertiary alcohols, and even diols, as it is sensitive to both their steric and electronic properties. The results are highly reproducible across two solvents with substantially different hydrogen-bonding properties (CH3 CN and CH2 Cl2 ) and are supported by density functional theory calculations. This indicates that the numerous solvent-alcohol interactions are less significant than the quinone-alcohol hydrogen-bonding interactions. The utility of ΔEp (red) was illustrated by comparisons between 1) 3,3,3-trifluoro-n-propanol and 1,3-difluoroisopropanol and 2) ethylene glycol and 2,2,2-trifluoroethanol.