Wireless Electrochemical Gel Actuators.

Soft actuators generate motion in response to external stimuli and are indispensable for soft robots, particularly future miniature robots with complex structure and motion. Similarly to conventional hard robots, electricity is suitable for the stimulation. However, previous electrochemical soft actuators require a tethered connection to a power supply, limiting their size, structure, and motion. Here, wireless electrochemical soft actuators composed of hydrogels and driven by bipolar electrochemistry are reported. Viologen, which dimerizes by one-electron reduction and dissociates by one-electron oxidation, is incorporated in the side chains of the gel networks and works as a reversible cross-link. Wireless and reversible electrochemical actuation of the hydrogels, i.e., muscle-like shrinking and swelling, is demonstrated at microscopic and even macroscopic scales.

High-Throughput Electrosynthesis of Gradient Polypyrrole Film Using a Single-Electrode Electrochemical System.

As an effective approach for materials synthesis, bipolar electrochemistry has been earning a renewed interest nowadays thanks to its unique features compared to conventional electrochemistry. Indeed, the wireless mode of electrode reactions and the generation of a gradient potential distribution above the bipolar electrode are among the most appealing qualities of bipolar electrochemistry. In particular, the gradient potential distribution is a highly attractive characteristic for the fabrication of surfaces with gradients in their chemical properties or molecular functionalities. Herein, we report the high-throughput electrosynthesis of gradient polypyrrole films by means of a new electrochemical cell design named the single-electrode electrochemical system (SEES). SEESs are made by attaching an inert plastic board with holes onto an indium tin oxide electrode, constructing multiple microelectrochemical cells on the same electrode. This type of arrangement enables parallel electrochemical reactions to be carried out simultaneously and controlled in a contactless manner by a single electrode. Several experimental conditions for polypyrrole film growth were extensively investigated. Furthermore, the gradient property of the polymer films was evaluated by thickness determination, surface morphology analysis, and contact angle measurements. The use of SEES has been demonstrated as a convenient and cost-effective strategy for high-throughput electrosynthesis and electroanalytical applications and has opened up a new door for gradient film preparation via a rapid condition screening process.

AC-Bipolar Electropolymerization of 3,4-Ethylenedioxythiophene in Ionic Liquids.

Recently, alternating current (AC)-bipolar electropolymerization of 3,4-ethylenedioxythiophene (EDOT) has been reported to produce poly(3,4-ethylenedioxythiophene) (PEDOT) fibers from the terminals of bipolar electrodes in acetonitrile solution (MeCN) containing low concentrations of supporting salts in a template-free manner. Here, we extend such methodology in ionic liquid (IL) media. Three kinds of ILs, diethylmethyl(2-methoxyethyl)ammonium tetrafluoroborate ([DEME][BF4]), 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]), and diethylmethyl(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide ([DEME][TFSI]), with different electric field transmission efficiencies and diffusion coefficients were employed as solvents for the AC-bipolar electropolymerization of EDOT. A variety of PEDOT morphologies were obtained in these three ILs, showing a relationship with the physicochemical properties of the ILs. We successfully confirmed the growth of PEDOT fibers in ILs and systematically discussed the factors that influenced their growth.

Synthesis, Structures, and Aggregation-Induced Emission Properties of Distyrylbenzene Derivatives Containing Fluorine Groups.

Distyrylbenzene derivatives with substituents on the vinylene moieties have been studied due to interest in their optoelectronic properties. In this study, we focused on distyrylbenzene derivatives with monofluoroolefin structures, expecting intermolecular H-F interactions in the solid state. UV-vis and fluorescence spectra of the obtained compounds were measured and compared with those of unsubstituted distyrylbenzene. The crystal structures of each compound were determined by single crystal X-ray diffraction and Hirshfeld surface analysis to understand the intermolecular contacts.

Site-Selective Synthesis and Concurrent Immobilization of Imine-Based Covalent Organic Frameworks on Electrodes Using an Electrogenerated Acid.

Imine-based covalent organic frameworks (COFs) are crystalline porous materials with prospective uses in various devices. However, general bulk synthetic methods usually produce COFs as powders that are insoluble in most of the common organic solvents, arising challenges for the subsequent molding and fixing of these materials on substrates. Here, we report a novel synthetic methodology that utilizes an electrogenerated acid (EGA), which is produced at an electrode surface by electrochemical oxidation of a suitable precursor, acting as an effective Brønsted acid catalyst for imine bond formation from the corresponding amine and aldehyde monomers. Simultaneously, it provides the corresponding COF film deposited on the electrode surface. The COF structures obtained with this method exhibited high crystallinities and porosities, and the film thickness could be controlled. Furthermore, such process was applied for the synthesis of various imine-based COFs, including a three-dimensional (3D) COF structure.

Cathodic generation of reactive (phenylthio)difluoromethyl species and its reactions: mechanistic aspects and synthetic applications.

The cathodic reduction of bromodifluoromethyl phenyl sulfide (1) using o-phthalonitrile as a mediator generated the (phenylthio)difluoromethyl radical, which reacted with α-methylstyrene and 1,1-diphenylethylene to provide the corresponding adducts in moderate and high yields, respectively. In contrast, chemical reduction of 1 with SmI2 resulted in much lower product yields. The detailed reaction mechanism was clarified based on the cathodic reduction of 1 in the presence of deuterated acetonitrile, CD3CN.

Mapping the Distribution of Potential Gradient in Bipolar Electrochemical Systems through Luminol Electrochemiluminescence Imaging.

Bipolar electrochemistry has been regarded as a powerful and sustainable electrochemical process for the synthesis of novel functional materials. The appealing features of this electrochemical technology, such as the wireless nature of the bipolar electrode (BPE) and the possibility to drive simultaneously electrochemical reactions on multiple BPEs placed in the same electrochemical cell, together with the possibility to change the shape and positioning of the driving electrodes, give significant freedom to design reaction systems. Nevertheless, the cell geometry dramatically affects the distribution and intensity of the potential gradient generated on the BPE surface and its monitoring is hampered due to the wireless nature of the BPE. In the present study, we propose the use of electrochemiluminescence (ECL) as an electrochemical imaging technique to map the distribution of potential gradient in bipolar electrochemical cells with different geometries. The proposed approach exploits the strong ECL emission of luminol/hydrogen peroxide (H2O2) system generated at the anodic pole of the BPE, when the total applied voltage (Etot) is strong enough to trigger the electrochemical reaction. Since luminol ECL emission is rather intense and relatively stable, the evolution of the potential distribution as a function of Etot can be monitored using a digital camera, allowing the elucidation of the potential distribution profile in every bipolar configuration. The suggested approach represents a valuable and reliable method to map the potential gradient in bipolar electrochemical systems and can be readily employed in every type of bipolar configuration.

Recent Advances in Electrochemical Systems for Selective Fluorination of Organic Compounds.

Organofluorine compounds are key materials applied in daily life because of their versatile utility as functional materials, pharmaceuticals, and agrochemicals. Development of the selective fluorination of organic molecules under safe conditions is therefore one of the most important subjects in modern synthetic organofluorine chemistry. Thus, various electrophilic fluorination reagents such as XeF2, (PhSO2)2NF (NFSI), Et2NSF3 (DAST), (MeOCH2CH2)2NSF3 (Deoxofluor), 1-chloromethyl-4-fluoro-1,4-diazoniabicyclo-[2.2.2]octane bis(tetrafluoroborate) (Selectfluor), N-fluoropyridinium salts, and 4-tert-butyl-2,6-dimethylphenylsulfur trifluoride (Fluolead) have been developed for chemical fluorination to date and the development of new fluorinating reagents is still ongoing. Electrochemical synthesis has recently attracted much attention from the perspective of green sustainable chemistry because no hazardous reagents are required and scale-up is generally easy. Although electrochemical perfluorination of organic compounds using a nickel anode in anhydrous HF has been well-established to manufacture perfluoro-functional materials, electrochemical partial fluorination (selective electrochemical fluorination) has been underdeveloped due to the low nucleophilicity of fluoride ions and anode passivation, which interferes with electrolysis. Selective electrochemical fluorination can be commonly achieved in aprotic solvents containing fluoride ions to provide mostly mono- and difluorinated products. Electrolysis is conducted at constant potentials slightly higher than the first oxidation potential of a substrate. Constant current electrolysis is also effective for selective fluorination in many cases. Choice of the combination of a supporting fluoride salt and an electrolytic solvent is most important to accomplish efficient selective fluorination. In this Account, we focus on our recent work on the electrochemical mono- and difluorination of various organic compounds and their synthetic application. We first briefly explain our research background of electrochemical fluorination. Main factors such as the effects of fluoride salts as supporting electrolytes, electrolytic solvents, and anode materials on the selectivity and efficiency of fluorination are discussed. Next, effects of PEG oligomer additives enhancing the nucleophilicity of fluoride ions and organic solvent-free systems using poly(HF) salt ionic liquids as well as recyclable mediatory systems for electrochemical fluorination are described. The desulfurizative monofluorination of xanthate and gem-difluorination of benzothioate and dithioacetals are briefly mentioned. Regioselective anodic fluorination of various heterocyclic compounds having a phenylthio group as electroauxiliary and heterocycles containing sulfur and other heteroatoms are also described. In addition, a boryl group is shown to be a good leaving group for anodic fluorination. Moreover, electrochemically α,α-difluorinated phenylsulfides and phenylselenides are illustrated to be useful for photochemical C-H difluoromethylation of aromatic and heteroaromatic compounds. Finally, this Account also highlights highly diastereoselective fluorination of aliphatic heterocyclic and open-chain compounds, as well as new electrolytic fluorination methods using inorganic fluoride salts such as KF and CsF.

Electricity-Driven Post-Functionalization of Conducting Polymers.

Electrochemical doping of conducting polymers (CPs) generates polarons (radical ionic species) and bipolarons (ionic species) in their backbone via multi-electron transfer between an electrode and the CP. In the electrochemical polymer reaction (ePR), these generated ionic species are regarded as reactive intermediates for further transformation of the chemical structures of CPs. This electrochemical post-functionalization can easily be used to control the degree of reactions by turning a power supply on/off, as well as tuning the applied electrode potential, which leads to fine-tuning of the various properties of the CPs, such as the HOMO/LUMO level and PL properties. This Account summarizes recent developments in the electrochemical post-functionalization of CPs. In particular, we focus on reaction design for the ePR, with respect to the preparation and structure of the precursor polymers, applicable functional groups, efficient reaction conditions, and electrolytic methodologies.

Alkali Metal Fluorides in Fluorinated Alcohols: Fundamental Properties and Applications to Electrochemical Fluorination.

Fundamental properties of alkali metal fluorides (MF, M = Cs, K) dissolved in 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) or in 3,3,3-trifluoroethanol (TFE) are investigated, including solubility, conductivity, and viscosity. Solid-state structures of single crystals obtained from CsF/HFIP and CsF/TFE are described for the first time, giving insights into the multiple interactions between fluorinated alcohols and CsF. Applications in electrochemical fluorination reactions are successfully demonstrated.

Fabrication of Gradient and Patterned Organic Thin Films by Bipolar Electrolytic Micelle Disruption Using Redox-Active Surfactants.

Bipolar electrochemistry could be regarded as a powerful approach for selective surface modification due to the beneficial feature that a wirelessly controllable potential distribution on bipolar electrodes (BPEs). Herein we report a bipolar electrolytic micelle disruption (BEMD) system for the preparation of shaped organic films. A U-shaped bipolar electrolytic system with a sigmoidal potential gradient on the BPE gave gradient-thin films including various interesting organic compounds, such as a polymerizable monomer, an organic pigment and aggregation induced emission (AIE) molecules. The gradient feature was characterized by UV-Vis absorption, thickness measurements and surface morphology analysis. Corresponding patterned films were also fabricated using a cylindrical bipolar electrolytic setup that enables site-selective application of the potential on the BPE. Such a facile BEMD approach will open a long-term perspective with respect to organic film preparation.

Bipolar Electrochemistry: A Powerful Tool for Electrifying Functional Material Synthesis.

Electrosynthesis is a powerful method for the synthesis of organic, inorganic, and polymeric materials based on electron-transfer-driven reactions at the substrate/electrode interface. The use of electricity for synthetic reactions without the need for hazardous chemical oxidants and reductants is recognized as a green and sustainable method. Other advantages include control of the reaction selectivity by tuning the electrode potentials. A different mode for driving electrochemical reactions has recently been proposed, in which bipolar electrodes (BPEs) are available as wireless electrodes that undergo anodic and cathodic reactions simultaneously. Bipolar electrochemistry is an old technology that has recently garnered renewed attention because of the interesting features of BPEs: (i) the wireless nature of a BPE is useful for sensors and material synthesis; (ii) the gradient potential distribution on BPEs is a powerful tool for the preparation of gradient surfaces and materials; and (iii) electrophoresis is available for effective electrolysis. In addition to these unique features, a BPE system only requires a small amount of supporting electrolyte in principle, whereas a large amount of electrolyte is necessary in conventional electrochemistry. Hence, bipolar electrochemistry is an inherently green and sustainable chemical process for the synthesis of materials. In this Account, recent progress in bipolar electrochemistry for the electrosynthesis of functional materials is summarized. The wireless nature of BPEs was utilized for symmetry breaking to produce anisotropic materials based on the site-selective modification of conductive objects by electrodeposition and electropolymerization. Potential gradients on a BPE interface have been successfully used as controllable templates to form molecular or polymeric gradient materials, which are potentially applicable for high throughput analytical equipment or as biomimetic materials. The electric field necessary to drive BPEs is also potentially useful to induce the directed migration of charged species. The synergetic effects of electrophoresis and electrolysis were also successfully demonstrated to obtain various functional materials. These features of bipolar electrochemistry and the various combinations of techniques have the potential to change the methodologies of material synthesis. Furthermore, the fundamental principle of bipolar electrochemistry infers that very small amounts of supporting electrolyte are necessary for an electrode system, which is expected to lead new methods of sustainable organic electrosynthesis.

Te-Li Exchange Reaction of Tellurophene-Containing π-Conjugated Polymer as Potential Synthetic Tool for Functional π-Conjugated Polymers.

On the basis of the facts that tellurophene-containing π-conjugated polymers are obtainable from organotitanium polymers and that the tellurium atoms in the tellurophene derivatives can be transformed into lithium atoms, the synthesis of reactive lithiated polymer precursor and its transformations into some functionalized π-conjugated polymers are described. A regioregular organometallic polymer having 1,4-dilithio-1,3-butadiene and 9,9-dioctylfluorene-2,7-diyl units is generated by the reaction of a tellurophene-containing polymer having the number-average molecular weight (Mn ) and molecular weight distribution (Mw /Mn ) of 5890 and 1.9, respectively, with n-butyllithium (2.4 equiv.) at -78 °C to -60 °C for 3 h. The lithiated polymer thus prepared is subjected to reactions with electrophiles to produce functionalized π-conjugated polymers. For example, a π-conjugated polymer possessing 1,4-bis(tri-n-butylstannyl)-1,3-butadiene-1,4-diyl unit is obtained in 67% yield by the reaction with tri-n-butyltin chloride (2.4 equiv.) at -60 °C to ambient temperature for 12 h in tetrahydrofuran, whose Mn and Mw /Mn are estimated as 7320 and 2.5, respectively, by size exclusion chromatography. The absorption maximum and onset of the obtained polymer are observed at 380 and 465 nm, respectively, in the UV-vis spectrum, from which the optical band gap of the polymer is estimated as 2.67 eV.

Synthesis of Stannole-Containing π-Conjugated Polymers by Post-Element Transformation of Organotitanium Polymer.

The synthesis of stannole-2,5-diyl-containing π-conjugated polymers by the post-element transformation of a regioregular organotitanium polymer is described. For example, a 1,1-diphenylstannole-containing polymer is obtained in 83% yield by the reaction of a regioregular organotitanium polymer, which is prepared from 1,4-bis(2-ethylhexyloxy)-2,5-diethynylbenzene and a low-valent titanium complex with diphenyltin dichloride at -50 °C to ambient temperature. The number-average molecular weight and molecular weight distribution (Mn and Mw /Mn ) of the stannole-containing polymer are estimated as 4800 and 1.8, respectively. The obtained polymer is found to have the extended π-conjugated backbone and relatively low-lying lowest unoccupied molecular orbital (LUMO) energy level (-3.12 eV), which is supported by its UV-vis absorption spectrum and cyclic voltammetric (CV) analysis. In addition, the stannole-containing polymer is found to be applicable to a chemosensor for fluoride anion where the color and photoluminescence intensity of the polymer solution exhibits a distinct change in the presence of a fluoride anion.

Synthesis of Poly(3,4-ethylenedioxythiophene)-Platinum and Poly(3,4-ethylenedioxythiophene)-Poly(styrenesulfonate) Hybrid Fibers by Alternating Current Bipolar Electropolymerization.

Alternating current (ac) bipolar electropolymerization of 3,4-ethylenedioxythiophene (EDOT) was performed in the presence of hexachloroplatinate ([PtCl6]2-) or poly(styrenesulfonate) (PSS). We demonstrated that both [PtCl6]2- and PSS were successfully incorporated into electrogenerated poly(3,4-ethylenedioxythiophene) (PEDOT) as dopants to offer hybrid fibers composed of (i) PEDOT and platinum nanoparticles (PtNPs) (PEDOT-Pt hybrid fibers) and (ii) PEDOT and PSS (PEDOT-PSS hybrid fibers), respectively, in one step, grown from the very edges of Au wires used as bipolar electrodes (BPEs).

The selective electrochemical fluorination of S-alkyl benzothioate and its derivatives.

We herein report that the regioselective anodic fluorination of S-alkyl benzothioate and its derivatives in various aprotic solvents using Et3N·nHF (n = 3-5) and Et4NF·nHF (n = 3-5) as supporting electrolyte and a fluorine source successfully provided the corresponding α-fluorinated products in moderate yields. Dichloromethane containing Et4NF·4HF was found to be the most suitable combination as electrolytic solvent and supporting salt as well as fluorine source for the anodic fluorination. The electrochemical fluorination of cyclic benzothioates such as benzothiophenone was also achieved.

Arsole-Containing π-Conjugated Polymer by the Post-Element-Transformation Technique.

A synthetic method to obtain an arsole-containing π-conjugated polymer by the post-transformation of the organotitanium polymer titanacyclopentadiene-2,5-diyl unit with an arsenic-containing building block is described. The UV/Vis absorption maximum and onset of the polymer were observed at 517 nm and 612 nm, respectively. The polymer exhibits orange photoluminescence with an emission maximum (Emax ) of 600 nm and the quantum yield (Φ) of 0.05. The polymer proved to exhibit a quasi-reversible redox behavior in its cyclic voltammetric (CV) analysis. The energy levels of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) were estimated to be -5.43 and -3.24 eV, respectively, from the onsets for oxidation and reduction signals in the CV analysis. Further chemical modification of the arsole unit in the π-conjugated polymer by complexation of gold(I) chloride occurred smoothly resulting in the bathochromic shift of the UV/Vis absorption and lowering of the LUMO energy level.

Parallel Synthesis of photoluminescent π-conjugated polymers by polymer reactions of an organotitanium polymer with a titanacyclopentadiene unit.

A regioregular organometallic polymer with titanacyclopentadiene unit, obtained by the reaction of a 2,7-diethynylfluorene derivative and a low-valent titanium complex, is subjected to the reaction with three kinds of electrophiles (i.e., sulfur monochloride, hydrochloric acid, and dichlorophenylphosphine) to give π-conjugated polymers possessing both fluorene and building blocks originated from the transformation of the titanacycles in the main chain. For example, a phosphole-containing polymer whose number-average molecular weight is estimated as 5000 is obtained in 50% yield. The obtained thiophene, butadiene, and phosphole-containing polymers exhibit efficient photoluminescence (PL) with emission colors of blue, green, and yellow, respectively. For example, the phosphole-containing polymer exhibits yellow PL with an emission maximum (Emax ) of 533 nm and a quantum yield (Φ) of 0.37.