Energy-Storage Applications for a pH Gradient between Two Benzimidazole-Ligated Ruthenium Complexes That Engage in Proton-Coupled Electron-Transfer Reactions in Solution

The judicious selection of pairs of benzimidazole-ligated ruthenium complexes allowed the construction of a rechargeable proton-coupled electron-transfer (PCET)-type redox battery. A series of ruthenium­(II) and -(III) complexes were synthesized that contain substituted benzimidazoles that engage in PCET reactions. The formation of intramolecular Ru–C cyclometalation bonds stabilized the resulting ruthenium­(III) complexes, in which p<i>K</i>_a values of the imino N–H protons on the benzimidazoles are usually lower than those for the corresponding ruthenium­(II) complexes. As a proof-of-concept study for a solution redox battery based on such PCET reactions, the charging/discharging cycles of several pairs of ruthenium complexes were examined by chronopotentiometry in an H-type device with half-cells separated by a Nafion membrane in unbuffered CH₃CN/H₂O (1/1, v/v) containing 0.1 M NaCl. During the charging/discharging cycles, the pH value of the solution gradually changed accompanied by a change of the open-circuit potential (OCP). The changes for the OCP and pH value of the solution in the anodic and cathodic half-cells were in good agreement with the predicted values from the Pourbaix diagrams for the pairs of ruthenium complexes used. Accordingly, the careful selection of pairs of ruthenium complexes with a sufficient potential gradient and a suitably large p<i>K</i>_a difference is crucial: the charge generated between the two ruthenium complexes changes the OCP and the pH difference between the two cells in an unbuffered solution, given that the PCET reactions occur at both electrodes and that discharging leads to the original state. Because the electric energy is stored as a pH gradient between the half-cells, new possibilities for PCET-type rocking-chair redox batteries arise

Electronic Band Structure of Exfoliated Titanium- and/or Niobium-Based Oxide Nanosheets Probed by Electrochemical and Photoelectrochemical Measurements

Exfoliated two-dimensional (2D) unilamellar nanosheets of Ca₂Nb₃O₁₀^–, TiNbO₅^–, Ti₂NbO₇^–, and Ti₅NbO₁₄^3– were deposited layer-by-layer to produce multilayer films on indium–tin–oxide (ITO)-coated glass electrodes, and their electrochemical and photoelectrochemical properties were explored. The layer-by-layer assembly process via sequential adsorption with counter polycations was monitored by UV–visible absorption spectra and X-ray diffraction measurements, which confirmed the successful growth of films, where nanosheets and polycations are alternately stacked at a separation of 1.6–2.4 nm. Exposure to UV light totally removed polycations, producing inorganic films. Cyclic voltammetry on Ti and/or Nb oxide nanosheet electrodes thus fabricated showed reduction/oxidation (Ti³⁺/Ti⁴⁺ and Nb⁴⁺/Nb⁵⁺) peaks associated with insertion/extraction of Li⁺ ions into/from intersheet galleries of the films. The extent of the redox reaction is found to be governed by the cation density in the nanosheet gallery. Anodic photocurrents of the oxide nanosheet electrodes were observed under UV light irradiation. These action spectra showed close resemblance to optical absorption profiles of the colloidal nanosheets, indicating that the photocurrent was generated from the nanosheets. Their analysis indicates that the nanosheets of Ca₂Nb₃O₁₀^–, TiNbO₅^–, Ti₂NbO₇^–, and Ti₅NbO₁₄^3– are all indirect transition-type wide-gap semiconductors with bandgap energies of 3.44, 3.68, 3.64, and 3.53 eV, respectively. These values are larger than those for corresponding parent layered oxide compounds before delamination, suggesting confinement effects into 2D nanosheet structure. Furthermore, the value was invariable for the films with a different number of nanosheet layers, indicating that quantized nanosheets were electronically isolated with each other. In addition, photocurrent generation was measured as a function of applied electrode potential, and the flatband potential was estimated from the photocurrent onset values as −1.12, −1.33, −1.30, and −1.29 V vs Ag/Ag⁺, for Ca₂Nb₃O₁₀^–, TiNbO₅^–, Ti₂NbO₇^–, and Ti₅NbO₁₄^3– nanosheets, respectively, providing a diagram of electronic band structure for the nanosheets

Proton-Rocking-Chair-Type Redox Capacitors Based on Indium Tin Oxide Electrodes with Multilayer Films Containing Ru Complexes

A rechargeable proton-rocking-chair-type redox capacitor was fabricated using scalable layer-by-layer-(LbL)-assembled films composed of two dinuclear Ru complexes that exhibit proton-coupled electron-transfer (PCET) reactions with different Ru­(II/III) redox potentials (<b>RuNH–OH</b> and <b>RuCH–OH</b>). <b>RuNH–OH</b> and <b>RuCH–OH</b> contain different coordination environments that involve two phosphonate linker ligands at both ends and bridging 2,6,2′,6′-tetrakis­(benzimidazol-2-yl)-4,4′-bipyridine or 1,3,1′,3′-tetrakis­(benzimidazol-2-yl)-5,5′-biphenyl ligands, respectively. The molecular units were assembled onto indium tin oxide (ITO) electrodes by complexation between the phosphonate groups and zirconium­(IV) ions. The LbL growing process of these multilayer films was monitored by electrochemical or UV–vis spectroscopic measurements. The thus obtained LbL films on the ITO electrodes showed PCET reactions at different potentials, depending on the bridging ligands. The introduction of cyclometalated Ru–C bonds in the bridging ligand of <b>RuCH–OH</b> led to the stabilization of the ruthenium­(III) oxidation state, and therefore, <b>RuCH–OH</b> exhibited lower p<i>K</i>_a values for the imino N–H protons in the bridging benzimidazole groups compared to those of the corresponding <b>RuNH–OH</b> complex. The proton movements that accompany the redox reaction in the Ru multilayer films on the ITO electrode were confirmed using a pH indicator probe. For the performance test of a proton-rocking-chair-type redox capacitor, a two-electrode system composed of <b>RuNH–OH</b> and <b>RuCH–OH</b> multilayer films on ITO electrodes was examined in an aqueous solution of NaClO₄. Under galvanostatic conditions, stable, reversible, and repeatable charging/discharging processes occurred. The capacitance increased with an increasing number of LbL layers. For comparison, a similar redox capacitor composed of two <b>RuNMe–OH</b> and <b>RuCMe–OH</b> analogues, in which all four imino N–H protons on the benzimidazole moieties are protected by N–Me groups, was constructed and examined. In these complexes, the capacitance decreased by 77% compared to the PCET-type capacitor composed of a cell containing <b>RuNH–OH</b> and <b>RuCH–OH</b>; this result strongly suggests that the proton movement plays a more important role for the charge storage than the anion movement. In such LbL films composed of Ru complexes that exhibit PCET-type redox reactions, the capacitance is drastically improved with an increasing number of layers and using protons as charge carriers

Controlling the Adsorption of Ruthenium Complexes on Carbon Surfaces through Noncovalent Bonding with Pyrene Anchors: An Electrochemical Study

Surface modifications of carbon nanomaterials, such as graphene or carbon nanotubes, through noncovalent π–π interactions between π-conjugated carbon surfaces and pyrene anchors have received much attention on account of the applications of these materials in organic electronic and sensor devices. Despite the rapidly expanding use of pyrene anchors, little is known about the number of pyrene groups required in order to achieve a stable attachment of molecules on nanocarbon surfaces. So far, systematic studies on such surface modifications through adsorption isotherms and desorption behavior of molecules still remain scarce. In this study, we have investigated the effect of the number of pyrene anchors in redox-active Ru complexes on their adsorption on carbon nanomaterials through noncovalent π–π interactions. The Ru­(II/III) couple was used as a redox marker in order to determine the surface coverage on nanocarbon surfaces such as highly oriented pyrolytic graphite (HOPG), single-walled carbon nanotubes (SWCNTs), and multiwalled carbon nanotubes (MWCNTs). The amount of surface coverage as well as the kinetic stability of the Ru complexes was thereby observed to be directly proportional to the number of pyrene groups present in the ligands. The desorption rate from HOPG electrode increased in the order <b>Ru-1</b> with eight pyrene groups (<i>k</i> = 2.0 × 10^–5 s^–1) < <b>Ru-2</b> with four pyrenes (4.1 × 10^–5 s^–1) < <b>Ru-3</b> with two pyrenes (6.8 × 10^–5 s^–1) ≪ <b>Ru-4</b> with one pyrene (4.1 × 10^–3 s^–1). Furthermore, the electrochemical polymerization of the Ru complex with four pyrene groups proceeded more efficiently compared to complexes with one or two pyrene groups. As a consequence, compounds having more than two and/or optimally four pyrene groups revealed a stable adsorption on the nanocarbon surfaces. The heterogeneous electron transfer rate between the Ru complex, <b>Ru-2</b>, and the carbon nanomaterials increased in the order SWCNTs (<i>k</i>_ET = 1.3 s^–1) < MWCNTs (ϕ = 5–9 nm) (<i>k</i>_ET = 4.0 s^–1) < MWCNTs (ϕ = 110–170 nm) (<i>k</i>_ET = 14.9 s^–1) < HOPG (<i>k</i>_ET = 110 s^–1)

Long-Range Electron Transport of Ruthenium-Centered Multilayer Films <i>via</i> a Stepping-Stone Mechanism

We studied electron transport of Ru complex multilayer films, whose structure resembles redox-active complex films known in the literature to have long-range electron transport abilities. Hydrogen bond formation in terms of pH control was used to induce spontaneous growth of a Ru complex multilayer. We made a cross-check between electrochemical measurements and <i>I–V</i> measurements using PEDOT:PSS to eliminate the risk of pinhole contributions to the mechanism and have found small β values of 0.012–0.021 Å^–1. Our Ru complex layers exhibit long-range electron transport but with low conductance. On the basis of the results of our theoretical–experimental collaboration, we propose a modified tunneling mechanism named the “stepping-stone mechanism”, where the alignment of site potentials forms a narrow band around <i>E</i>_F, making resonant tunneling possible. Our observations may support Tuccito <i>et al</i>.’s proposed mechanism

Wisely Designed Phthalocyanine Derivative for Convenient Molecular Fabrication on a Substrate

An axial-substituted silicon phthalocyanine derivative, SiPc­(OR)₂ (R = C₄H₉), that is soluble in organic solvent is conveniently synthesized. This silicon phthalocyanine derivative reacts with a hydroxyl group on a substrate and then with another phthalocyanine derivative under mild conditions. The accumulation number of the phthalocyanine molecules on the substrates is easily controlled by the immersion time. On the basis of AFM (atomic force microscopy) images, the surface of the phthalocyanine-modified glass substrate has uneven structures on the nanometer scale. ITO electrodes modified with the composition of the phthalocyanine derivative and PCBM show stable cathodic photocurrent generation upon light irradiation

Simultaneous Formation and Spatial Patterning of ZnO on ITO Surfaces by Local Laser-Induced Generation of Microbubbles in Aqueous Solutions of [Zn(NH₃)₄]²⁺

We demonstrate the simultaneous formation and spatial patterning of ZnO nanocrystals on an indium–tin oxide (ITO) surface upon local heating using a laser (1064 nm) and subsequent formation of microbubbles. Laser irradiation of an ITO surface in aqueous [Zn­(NH₃)₄]²⁺ solution (1.0 × 10^–2 M at pH 12.0) under an optical microscope produced ZnO nanocrystals, the presence of which was confirmed by X-ray diffraction analysis and Raman microspectroscopy. Scanning the focused laser beam over the ITO surface generated a spatial ZnO pattern (height: ∼60 nm, width: ∼1 μm) in the absence of a template or mask. The Marangoni convection generated in the vicinity of the microbubbles resulted in a rapid concentration/accumulation of [Zn­(NH₃)₄]²⁺ around the microbubbles, which led to the formation of ZnO at the solid–bubble–solution three-phase contact line around the bubbles and thus afforded ZnO nanocrystals on the ITO surface upon local heating with a laser

Observation of an Orientation Change in Highly Oriented Layer-by-Layer Films of a Ruthenium Complex upon Oxidation Reaction

Layer-by-layer films composed of redox-active ruthenium dimer and Zr­(IV) ions were fabricated on an indium tin oxide electrode. The fabricating behavior was monitored by cyclic voltammetry and UV–vis absorption spectral measurements. The orientation of the film was also monitored by grazing-incidence small-angle and wide-angle X-ray scattering (GISAXS) measurements, and it has been clarified that this film has a crystalline structure. The peaks obtained by GISAXS were changed upon oxidation reaction, which indicates that a change in the orientation of the ruthenium dimer occurred in the film

Glycine Crystallization in Solution by CW Laser-Induced Microbubble on Gold Thin Film Surface

We have developed a novel laser-induced crystallization method utilizing local heat-induced bubble/water interface. Continuous laser beam of 1064 nm is focused on a gold nanoparticles thin film surface covered with glycine supersaturated aqueous solution. Light absorption of the film due to localized plasmon resonance caused local heating at the focal position and produced a single thermal vapor microbubble, which generated thermal gradient followed by convection flow around the bubble and eventually induced glycine crystallization and growth. The crystallization mechanism is discussed by considering gathering and accumulating molecules around the bubble/water interface assisted by convection flow and temperature jump

Tuning of Redox Potentials by Introducing a Cyclometalated Bond to Bis-tridentate Ruthenium(II) Complexes Bearing Bis(<i>N</i>-methylbenzimidazolyl)benzene or -pyridine Ligands

A series of asymmetrical bis-tridentate cyclometalated complexes including [Ru­(Mebib)­(Mebip)]⁺, [Ru­(Mebip)­(dpb)]⁺, [Ru­(Mebip)­(Medpb)]⁺, and [Ru­(Mebib)­(tpy)]⁺ and two bis-tridentate noncyclometalated complexes [Ru­(Mebip)₂]²⁺ and [Ru­(Mebip)­(tpy)]²⁺ were prepared and characterized, where Mebib is bis­(<i>N</i>-methylbenzimidazolyl)­benzene, Mebip is bis­(<i>N</i>-methylbenzimidazolyl)­pyridine, dpb is 1,3-di-2-pyridylbenzene, Medpb is 4,6-dimethyl-1,3-di-2-pyridylbenzene, and tpy is 2,2′:6′,2″-terpyridine. The solid-state structure of [Ru­(Mebip)­(Medpb)]⁺ is studied by X-ray crystallographic analysis. The electrochemical and spectroscopic properties of these ruthenium complexes were studied and compared with those of known complexes [Ru­(tpy)­(dpb)]⁺ and [Ru­(tpy)₂]²⁺. The change of the supporting ligands and coordination environment allows progressive modulation of the metal-associated redox potentials (Ru^II/III) from +0.26 to +1.32 V vs Ag/AgCl. The introduction of a ruthenium cyclometalated bond in these complexes results in a significant negative potential shift. The Ru^II/III potentials of these complexes were analyzed on the basis of Lever’s electrochemical parameters (<i>E</i>_L). Density functional theory (DFT) and time-dependent DFT calculations were carried out to elucidate the electronic structures and spectroscopic spectra of complexes with Mebib or Mebip ligands