Simple Air-Stable [3]Radialene Anion Radicals as Environmentally Switchable Catholytes in Water.

The hexacyano[3]radialene radical anion (1) is an attractive catholyte material for use in redox flow battery (RFB) applications. The substitution of cyano groups with ester moieties enhances solubility while maintaining redox reversibility and favorable redox potentials. Here we show that these ester-functionalized, hexasubstituted [3]radialene radical anions dimerize reversibly in water. The dimerization mode is dependent on the substitution pattern and can be switched in solution. Stimuli-responsive behavior is achieved by exploiting an unprecedented tristate switching mechanism, wherein the radical can be toggled between the free radical, a π-dimer, and a σ-dimer-each with dramatically different optical, magnetic, and redox properties-by changing the solvent environment, temperature, or salinity. The symmetric, triester-tricyano[3]radialene (3) forms a solvent-responsive, σ-dimer in water that converts to the radical anion with the addition of organic solvents or to a π-dimer in brine solutions. Diester-tetracyano[3]radialene (2) exists primarily as a π-dimer in aqueous solutions and a radical anion in organic solvents. The dimerization behavior of both 2 and 3 is temperature dependent in methanol solutions. Dimerization equilibrium has a direct impact on catholyte stability during galvanostatic charge-discharge cycling in static H-cells. Specifically, conditions that favor the free radical anion or π-dimer exhibit significantly enhanced cycling profiles.

Design of an Electrochemical Cell for Continuous Wave EPR Measurements of Radical Ions.

The combination of continuous wave electron paramagnetic resonance (cw-EPR) with electrochemistry is highly attractive as it allows a clean in-situ generation and the subsequent spectroscopic characterisation of radical ions, which are important intermediates in many photocatalytic cycles as well as light-induced processes occurring in biological systems or optoelectronic devices. Although commercial setups for spectroelectrochemical EPR are available, they are often expensive and tailored to a particular spectroscopic setup.  Here we present a design for a low-cost electrochemical EPR cell that can be used in combination with any commercial cw-EPR instrumentation. The cell design is compared to existing setups and the performance of the cell is evaluated by comparison of EPR spectra obtained by chemical and electrochemical oxidation of a graphene fragment.

Bisphosphonium Benzene Diimides.

The incorporation of cationic groups onto electron-poor compounds is a viable strategy for achieving potent electron acceptors, as evidenced by reports of air-stable radical forms of large aromatic diimides such as naphthalene and perylene diimides. These ions have also been observed to exhibit anion-π interaction tendencies of interest in molecular recognition applications. The benefits of phosphonium incorporation, however, have not yet been extended to the smallest benzene diimides. Here, we report that dibrominated pyromellitic diimide and mellophanic diimide both readily undergo substitution reactions with phosphine sources to yield bisphosphonium compounds. In the single crystalline form, these dications display anion-π interactions and, in the case of mellophanic diimide, the stabilization of a bromide-water H-bonding ring pattern. The reaction of these dications with chemical reductants readily provides the singly and doubly reduced redox states, which were characterized by UV-vis spectroscopy and found to exhibit intense absorptions extending into the near-IR region. Taken together, this work demonstrates that phosphonium incorporation onto congested aromatic diimide scaffolds is synthetically viable and produces unusual electron-poor compounds.

Sequential C-F Bond Transformation of the Difluoromethylene Unit in Perfluoroalkyl Groups: A Combination of Fine-Tuned Phenothiazine Photoredox Catalyst and Lewis Acid.

A sequential process via photoredox catalysis and Lewis acid mediation for C-F bond transformation of the CF2 unit in perfluoroalkyl groups has been achieved to transform perfluoroalkylarenes into complex fluoroalkylated compounds. A phenothiazine-based photocatalyst promotes the defluoroaminoxylation of perfluoroalkylarenes with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) under visible light irradiation, affording the corresponding aminoxylated products. These products undergo a further defluorinative transformation with various organosilicon reagents mediated by AlCl3 to provide highly functionalized perfluoroalkyl alcohols. Our novel phenothiazine catalyst works efficiently in the defluoroaminoxylation. Transient absorption spectroscopy revealed that the catalyst regeneration step is crucial for the photocatalytic aminoxylation.

Photo-induced Ultrafast Charge Transfer and Air-Stable Radical Formation in Tetraphenylpyrene Derivatives.

Stable organic radicals generated by photo-excitation hold applications in molecular switching devices and information storage. It remains challenging to develop photo-generated radical materials with rapid response and air stability in the solid state. Here, we report a structure based on 1,3,6,8-tetraphenylpyrene derivative (Py-TTAc) displaying photo-induced radicals with air stability in the solid state. Photo-induced electron transfer, exposed to a 365 nm ultraviolet lamp for 1 minute, affords radicals in Py-TTAc powder as confirmed by electron paramagnetic resonance (EPR) spectroscopy. The maximum radical concentration reaches 2.21% after continuous irradiation for 1 hour and recurs more than 10 times without any chemical degradation. The mechanistic study according to the femtosecond transient absorption (fsTA) and X-ray technology suggests that the radicals are derived from photo-induced symmetry-breaking charge separation (SB-CS) and stabilized through non-covalent interactions. The photo-generated stable radical system is employed in anti-counterfeiting paper and optoelectronic device applications. This study will provide insights into the development of photoactive organic radical materials.

A Bimetallic Benzene-1,2,4,5-Tetrathiolate (btt) Molybdenocene Complex Cp2 Mo(btt)MoCp2 : Radical and Diradical States.

Benzene-1,2,4,5-tetrathiolate (btt) has been used as a bridging ligand to prepare a redox active (molybdenocene dithiolene)-based bimetallic complex Cp2 Mo(btt)MoCp2 , which exhibits four successive electron transfers up to the tetracation. Spectro-electrochemical investigations together with DFT and TD-DFT calculations evidence that the two electroactive MoS2 C2 metallacycles are electronically coupled in the monocationic as in the dicationic state. Two salts of the dication [Cp2 Mo(btt)MoCp2 ]2+ have been structurally characterized with PF6 - and HSO4 - counterions, showing different chair or boat conformations associated with variable folding angles of the two MoS2 C2 metallacycles along the S-S hinge. The bis-oxidized dicationic complex exhibits a diradical character, with both radicals essentially localized on the metallacycles and with antiferromagnetic coupling evidenced from magnetic susceptibility measurements.

Solvent Dependency of Catalyst-Substrate Aggregation Through π-π Stacking in Photoredox Catalysis.

Assemblies of photoredox catalysts and their target substrates prior to photoexcitation is a phenomenon naïvely overlooked by the majority of synthetic chemists, but can have profound influences on reactivity and selectivity in photocatalytic reactions. In this study, we determine the aggregation states of triarylamine radical cationic photocatalysts with various target arene substrates in different solvents by specifically parameterized polarizable molecular dynamics simulations. A π-stacking interaction previously implicated by more expensive, less-representative quantum calculations is confirmed. Critically, this study presents new insights on: i) the ability of solvents (MeCN vs DMF) to make or break a photocatalytic reaction by promoting (MeCN) or demoting (DMF) its catalyst-substrate assemblies, which is a determining factor for reactivity, ii) the average "lifetimes" of assemblies in solution from a dynamic simulation. We find that both in the ground state and the photoexcited state, the cationic radical assemblies remain intact for periods often higher than 60 ps, rendering them ideally suitable to undergo intra-assembly electron transfer reactions upon photoexcitation. Such aspects have not addressed by previous studies on synthetic photocatalytic reactions involving non-covalent assemblies.

Photoinduced Processes in Lysine-Tryptophan-Lysine Tripeptide with L and D Tryptophan.

Optical isomers of short peptide Lysine-Tryptophan-Lysine (Lys-{L/D-Trp}-Lys) and Lys-Trp-Lys with an acetate counter-ion were used to study photoinduced intramolecular and intermolecular processes of interest in photobiology. A comparison of L- and D-amino acid reactivity is also the focus of scientists' attention in various specialties because today, the presence of amyloid proteins with D-amino acids in the human brain is considered one of the leading causes of Alzheimer's disease. Since aggregated amyloids, mainly Aß42, are highly disordered peptides that cannot be studied with traditional NMR and X-ray techniques, it is trending to explore the reasons for differences between L- and D-amino acids using short peptides, as in our article. Using NMR, chemically induced dynamic nuclear polarization (CIDNP) and fluorescence techniques allowed us to detect the influence of tryptophan (Trp) optical configuration on the peptides fluorescence quantum yields, bimolecular quenching rates of Trp excited state, and the photocleavage products formation. Thus, compared with the D-analog, the L-isomer shows a greater Trp excited state quenching efficiency with the electron transfer (ET) mechanism. There are experimental confirmations of the hypothesis about photoinduced ET between Trp and the CONH peptide bond, as well as between Trp and another amide group.

On-Demand Control of Short-Wave Infrared Light Transparency Based on Stimuli-Responsive Association of Tetrathiafulvalene Radical Cations.

The light-transmissive properties of a solid-state tetrathiafulvalene radical cation-bis(trifluoromethanesulfonyl)imide, 1-C5 ⋅+ ⋅ NTf2 - , underwent instantaneous changes in the short-wave infrared (SWIR) region (1000-2500 nm) upon exposure to solvent vapor or the application of mechanostress at room temperature. The initial solid state of 1-C5 ⋅+ ⋅ NTf2 - exhibited strong absorption in the near-infrared (NIR; 700-1000 nm) and SWIR regions, whereas the absorption in the SWIR region was significantly diminished in the stimulated state induced by dichloromethane vapor. Upon cessation of vapor stimulation, the solid state spontaneously and promptly reverted to its original state, characterized by absorption bands in the NIR/SWIR region. Moreover, the SWIR absorption was absent upon the application of mechanical stress using a steel spatula. The reversal was fast and occurred within 10 s. These changes were visualized using a SWIR imaging camera under 1450-nm light irradiation. Experimental investigations demonstrated that the transparency to the SWIR light in the solid states was modulated through significant structural transformations of the associated radical cations, with transitions between columnar and isolated π-dimer structures under ambient and stimulated conditions, respectively.

Photoredox catalysis harvesting multiple photon or electrochemical energies.

Photoredox catalysis (PRC) is a cutting-edge frontier for single electron-transfer (SET) reactions, enabling the generation of reactive intermediates for both oxidative and reductive processes via photon activation of a catalyst. Although this represents a significant step towards chemoselective and, more generally, sustainable chemistry, its efficacy is limited by the energy of visible light photons. Nowadays, excellent alternative conditions are available to overcome these limitations, harvesting two different but correlated concepts: the use of multi-photon processes such as consecutive photoinduced electron transfer (conPET) and the combination of photo- and electrochemistry in synthetic photoelectrochemistry (PEC). Herein, we review the most recent contributions to these fields in both oxidative and reductive activations of organic functional groups. New opportunities for organic chemists are captured, such as selective reactions employing super-oxidants and super-reductants to engage unactivated chemical feedstocks, and scalability up to gram scales in continuous flow. This review provides comparisons between the two techniques (multi-photon photoredox catalysis and PEC) to help the reader to fully understand their similarities, differences and potential applications and to therefore choose which method is the most appropriate for a given reaction, scale and purpose of a project.

Carbon-Supported Potassium Hydride for Efficient Low-Temperature Desulfurization.

The industrial removal of organosulfur impurities from fossil fuels relies on transition-metal-based catalysts in harsh conditions (ca. 400 °C, up to 100 bar H2 ), yet desulfurization (DS) of refractory alkyl dibenzothiophenes (DBTs) remains challenging. Here, we report that carbon-supported potassium hydride (KH/C) enables efficient DS of DBTs in mild conditions, viz. >97 % conversion of DBTs is achieved at 165 °C in 3-6 h while the yields of respective biphenyls are 84-95 % by using only 15 % excess of KH per a C-S bond. In addition, KH/C allows to lower the concentration of 4,6-Me2 DBT in the mesitylene solution from 1000 ppm to <3 ppm (165 °C, 20 h) and provides deoxygenation, denitrogenation and catalytic aromatic hydrogenation reactions. DS of various sulfur heterocycles by using KH/C, a transition-metal-free material based on earth abundant elements, is viable at low temperature and has prospects for the further development towards decentralized removal of organosulfur species from fossil fuels.

Synthesis and Characterization of Stable Iron Pentacarbonyl Radical Cation Salts.

The open-shell iron pentacarbonyl cation [Fe(CO)5 ].+ was isolated by deelectronation, i.e., the single-electron oxidation of the parent neutral Fe(CO)5 using [phenazineF ].+ as the [Al(ORF )4 ]- and [F-{Al(ORF )3 }2 ]- salt (RF =C(CF3 )3 ; phenazineF =perfluoro-5,10-bis(perfluorophenyl)-5,10-dihydrophenazine). [Fe(CO)5 ].+ [Al(ORF )4 ]- was fully characterized (scXRD analysis, IR, NMR, EPR, 57 Fe spectroscopy, CV and SQUID magnetization study) and, apart from being a compound of fundamental interest, may serve as a precursor for low-valent iron coordination chemistry.

Photoredox Generation of Isothiouronyl Radical Cations: A New Platform in Covalent Radical Catalysis.

Thiyl radicals offer unique catalytic patterns for the direct covalent activation of alkenes. However, important limitations in terms of structural diversity and handling have hampered the routine use of thiyl radicals in covalent radical catalysis. Herein, we report a new class of cationic sulfur-centered radicals to achieve covalent radical catalysis. Their generation from highly modular thioureas by photoredox catalysis make their utilization very simple and reliable. The synthetic potential and the versatility of the catalytic system were finally evaluated in a (3+2)-radical cascade between vinylcyclopropanes and olefins.

Crystalline Germanium(I) and Tin(I) Centered Radical Anions.

An isostructural series of heavy Group 14 E(I) radical anions (Ge, Sn, Pb), stabilized by a bulky xanthene-based diamido ligand are reported. The radical anions were synthesised by the one-electron reduction of their corresponding E(II) precursor complexes with sodium naphthalenide in THF, yielding the radical anions as charge-separated sodium salts. The series of main group radicals have been comprehensively characterized by EPR spectroscopy, X-ray crystallography and DFT analysis, which reveal that in all cases, the spin density of the unpaired electron almost exclusively resides in a p-orbital of π symmetry located on the Group 14 center.

Synthesis, Structure and Electronic Properties of a Stable π-Type 3-Electron-2-Center-Bonded Species: A Silicon Analogue of a Bicyclo[1.1.0]butane Radical Anion.

σ-Type 3-electron-2-center (3e-2c) bonds have been extensively studied as one of the key bonding motifs in radical chemistry and some biological systems. "π-Type 3e-2c-bonded species" that contain a 3e-2c π-bond without an underlying σ-bond framework, however, have been unexplored so far both theoretically and experimentally. Herein, we report the synthesis of the first stable π-type 3e-2c-bonded species, a silicon analogue of a bicyclo[1.1.0]butane radical anion. This compound exhibits an extremely long bridgehead Si-Si bond (3.0638(8) Å) and a strong near-IR absorption at 922 nm in solution which arises from a HOMO→SOMO [π(Si-Si)→π*(Si-Si)] transition. DFT calculations revealed a π-type bonding interaction between the two bridgehead silicon atoms with an unpaired electron mainly delocalized across the corresponding π*-type orbital, which introduces a novel bonding motif for constructing π-electron systems.

Metalloradical Cations and Dications Based on Divinyldiphosphene and Divinyldiarsene Ligands.

Metalloradicals are key species in synthesis, catalysis, and bioinorganic chemistry. Herein, two iron radical cation complexes (3-E)GaCl4 [(3-E).+ = [{(IPr)C(Ph)E}2 Fe(CO)3 ].+ , E = P or As; IPr = C{(NDipp)CH}2 , Dipp = 2,6-iPr2 C6 H3 ] are reported as crystalline solids. Treatment of the divinyldipnictenes {(IPr)C(Ph)E}2 (1-E) with Fe2 (CO)9 affords [{(IPr)C(Ph)E}2 Fe(CO)3 ] (2-E), in which 1-E binds to the Fe atom in an allylic (η3 -EECvinyl ) fashion and functions as a 4e donor ligand. Complexes 2-E undergo 1e oxidation with GaCl3 to yield (3-E)GaCl4 . Spin density analysis revealed that the unpaired electron in (3-E).+ is mainly located on the Fe (52-64 %) and vinylic C (30-36 %) atoms. Further 1e oxidation of (3-E)GaCl4 leads to unprecedented η3 -EECvinyl to η3 -ECvinyl CPh coordination shuttling to form the dications (4-E)(GaCl4 )2 .

The Simplest Model for Doped Poly(3,4-ethylenedioxythiophene) (PEDOT): Single-crystalline EDOT Dimer Radical Cation Salts.

Although doped poly(3,4-ethylenedioxythiophene) (PEDOT) is extensively used in electronic devices, their molecular-weight distributions and inadequately defined structures have hindered the elucidation of their underlying conduction mechanism. In this study, we introduce the simplest discrete oligomer models: EDOT dimer radical cation salts. Single-crystal structural analyses revealed their one-dimensional (1D) columnar structures, in which the donors were uniformly stacked. Band calculations identified 1D metallic band structures with a strong intracolumnar orbital interaction (band width W≈1 eV), implying the origin of the high conductivity of doped PEDOT. Interestingly, the salts exhibited semiconducting behavior reminiscent of genuine Mott states as a result of electron-electron repulsion (U) dominant over W. This study realized basic models with tunable W and U to understand the conduction mechanism of doped PEDOT through structural modification in oligomers, including the conjugation length.

Highly Efficient Doping of Conjugated Polymers Using Multielectron Acceptor Salts.

Chemical doping is a vital tool for tuning electronic properties of conjugated polymers. Most single electron acceptors used for p-doping necessitate high dopant concentrations to achieve good electrical conductivity. However, high-molar doping ratios hamper doping efficiency. Here a new concept of using multielectron acceptor (MEA) salts as dopants for conjugated polymers is presented. Two novel MEA salts are synthesized and their doping efficiency towards two polymers differing in their dielectric properties are compared with two single electron acceptors such as NOPF6 and magic blue. Cutting-edge methods such as ultraviolet photoelectron spectroscopy/X-ray photoelectron spectroscopy (XPS), impedance spectroscopy, and density of states analysis in addition to UV-vis-NIR absorption, spectroelectrochemistry, and Raman spectroscopy methods are used to characterize the doped systems. The tetracation salt improves the conductivity by two orders of magnitude and quadruples the charge carrier concentration compared to single electron acceptors for the same molar ratio. The differences in charge carrier density and activation energy on doping are delineated. Further, a strong dependency of the carrier release on the polymer polarity is observed. High carrier densities at reduced dopant loadings and improved doping efficacies using MEA dopants offer a highly efficient doping strategy for conjugated polymers.

Hysteretic Control of Near-infrared Transparency Using a Liquescent Radical Cation.

A liquescent dihydrophenazine radical cation, 1.+ ⋅NTf2 - , showed drastic changes in near-infrared (near-IR) transparency and opaqueness through hysteretic phase transitions with no measurable degradation of the compound even under aerated conditions. During the heating and slow cooling process (0.5 K min-1 ), its electronic and magnetic properties were altered clearly and repeatedly changed between solid and liquid states. The liquid state was transparent to near-IR light (940 nm), but the solid state was opaque, despite both samples exhibiting a similar green color under room light. Additionally, the liquid state was changed to a glass state under a fast cooling process (2-10 K min-1 ). UV/Vis/near-IR and electron spin-resonance spectroscopy revealed that these drastic changes were attributable to the dynamic dissociation and association of a π-dimer structure for 1.+ accompanying with the solid-liquid phase transitions even under the neat conditions.

A Sterically Hindered Derivative of 2,1,3-Benzotelluradiazole: A Way to the First Structurally Characterised Monomeric Tellurium-Nitrogen Radical Anion.

Interaction of the tetradentate redox-active 6,6'-[1,2-phenylenebis(azanediyl)]bis(2,4-di-tert-butylphenol) (H4 L) with TeCl4 leads to neutral diamagnetic compound TeL (1) in high yield. The molecule of 1 has a nearly planar TeN2 O2 fragment, which suggests the formulation of 1 as TeII L2- , in agreement with the results of DFT calculations and QTAIM and NBO analyses. Reduction of 1 with one equivalent of [CoCp2 ] leads to quantitative formation of the paramagnetic salt [CoCp2 ]+ [1].- , which was characterised by single-crystal XRD. The solution EPR spectrum of [CoCp2 ]+ [1].- at room temperature features a quintet due to splitting on two equivalent 14 N nuclei. Below 150 K it turns into a broad singlet line with two weak satellites due to the splitting on the 125 Te nucleus. Two-component relativistic DFT calculations perfectly reproduce the a(14 N) HFI constants and A∥ (125 Te) value responsible for the low-temperature satellite splitting. Calculations predict that the additional electron in 1.- is localised mainly on L, while the spin density is delocalised over the whole molecule with significant localisation on the Te atom (≥30 %). All these data suggest that 1.- can be regarded as the first example of a structurally characterised monomeric tellurium-nitrogen radical anion.