Fluorescence Control of Boron Enaminoketonate Using a Rotaxane Shuttle

The effect of rotaxane shuttling on the fluorescence properties of a fluorophore was investigated by exploiting fluorophore-tethered [2]rotaxanes. A fluorescent boron enaminoketonate (BEK) moiety was introduced in a rotaxane via transformation of an isoxazole unit generated as a result of an end-capping reaction using a nitrile <i>N</i>-oxide. The rotaxane exhibited a red shift of the fluorescence maximum along with a remarkable enhancement of the fluorescence quantum yield through wheel translation to the fluorophore

Intercrystal Self-Assembly for the Design of High-Quality Nickel Molybdate Nanocrystals

Nanowire of nickel molybdate hydrate, being recognized as an emerging supercapacitor material, was synthesized from the intercrystal self-assembly process (commonly referred to as oriented aggregation or attachment). The detailed lattice image of a NiMoO₄·0.75H₂O nanowire and the intermediate nanostructure before reaching the interplanar binding were successfully captured by means of high-resolution transmission and scanning electron microscopies. NiMoO₄·0.75H₂O possessed highly crystalline surface and internal nanostructures

Monoclinic Ag₂Mo₂O₇ Nanowire: A New Ag–Mo–O Nanophotocatalyst Material

We report a template-free facile technique that allows for the first ever synthesis of a <i>monoclinic</i> Ag₂Mo₂O₇ nanowire (m-Ag₂Mo₂O₇-NW), using a commercially available MoO₃ particle. The nanowire possessed high crystallinity and structural homogeneity and strongly suggested that the nanowire was grown through an oriented aggregation mechanism in contrast to the case of a typical solution-phase method. The corresponding bulky counterpart showed no photoresponse; however, a complete structural transformation toward a nanowire triggered activity for O₂ evolution in the presence of Ag⁺ as an electron acceptor under visible-light irradiation

New Series of Dinuclear Ruthenium(II) Complexes Synthesized Using Photoisomerization for Efficient Water Oxidation Catalysis

A new series of <i>proximal,proximal</i>-[Ru₂(tpy)₂(L)­XY]^<i>n</i>+ (<i>p</i>,<i>p</i>-<b>Ru</b>_<b>2</b><b>XY</b>, tpy = 2,2′:6′,2″-terpyridine, L = 5-phenyl-2,8-di­(2-pyridyl)-1,9,10-anthyridine, X and Y = other coordination sites) were synthesized using photoisomerization of a mononuclear complex. The <i>p</i>,<i>p</i><b>-Ru</b>_<b>2</b><b>XY</b> complexes undergo unusual reversible bridge-exchange reactions to generate <i>p</i>,<i>p</i>-<b>Ru</b>_<b>2</b><b>(μ-Cl)</b>, <i>p</i>,<i>p</i>-<b>Ru</b>_<b>2</b><b>(μ-OH)</b>, and <i>p</i>,<i>p</i>-<b>Ru</b>_<b>2</b><b>(OH)­(OH</b>_<b>2</b><b>)</b> with μ-Cl, μ-OH, as well as hydroxo and aquo ligands at X and Y sites of <i>p</i>,<i>p</i>-<b>Ru</b>_<b>2</b><b>XY</b>, respectively. The geometric and electronic structures of these complexes were characterized based on UV–vis and ¹H NMR spectra, X-ray crystallography, and density functional theory (DFT) calculations. ¹H NMR data showed <i>C</i>₂ symmetry of <i>p</i>,<i>p</i>-<b>Ru</b>_<b>2</b><b>(OH)­(OH</b>_<b>2</b>) with the distorted L chelate and nonequivalence of two tpy ligands, in contrast to the <i>C</i>_2<i>v</i> symmetry of <i>p</i>,<i>p</i>-<b>Ru</b>_<b>2</b><b>(μ-Cl)</b> and <i>p</i>,<i>p</i>-<b>Ru</b>_<b>2</b><b>(μ-OH)</b>. However, irrespective of the lower symmetry, <i>p</i>,<i>p</i>-<b>Ru</b>_<b>2</b><b>(OH)­(OH</b>_<b>2</b><b>)</b> is predominantly formed in neutral and weakly basic conditions due to the specially stabilized core structure by multiple hydrogen-bond interactions among aquo, hydroxo, and backbone L ligands. The electrochemical data suggested that <i>p</i>,<i>p</i>-<b>Ru</b>_<b>2</b><b>(OH)­(OH</b>_<b>2</b><b>)</b> (Ru^II–OH:Ru^II–OH₂) is oxidized to the Ru^III–OH:Ru^III–OH state at 0.64 V vs saturated calomel electrode (SCE) and further to Ru^IVO:Ru^IVOH at 0.79 V by successive 1-proton-coupled 2-electron processes at pH 7.0. The cyclic voltammogram data exhibited that the <i>p</i>,<i>p</i>-<b>Ru</b>_<b>2</b><b>(OH)­(OH</b>_<b>2</b><b>)</b> complex works more efficiently for electrocatalytic water oxidation, compared with a similar mononuclear complex <i>distal-</i>[Ru­(tpy)­(L)­OH₂]²⁺ (<i>d-</i><b>RuOH</b>_<b>2</b>) and <i>p</i>,<i>p</i>-<b>Ru</b>_<b>2</b><b>(μ-Cl)</b> and <i>p</i>,<i>p</i>-<b>Ru</b>_<b>2</b><b>(μ-OH)</b>, showing that the <i>p</i>,<i>p</i>-<b>Ru</b>_<b>2</b> core structure with aquo and hydroxo ligands is important for efficient electrocatalytic water oxidation. Bulk electrolysis of the <i>p</i>,<i>p</i>-<b>Ru</b>_<b>2</b><b>(OH)­(OH</b>_<b>2</b><b>)</b> solution corroborated the electrocatalytic cycle involving the Ru^III–OH:Ru^III–OH state species as a resting state. The mechanistic insight into O–O bond formation for O₂ production was provided by the isotope effect on electrocatalytic water oxidation by <i>p,p</i>-<b>Ru</b>_<b>2</b><b>(OH)­(OH</b>_<b>2</b><b>)</b> and <i>d-</i><b>RuOH</b>_<b>2</b> in H₂O and D₂O media

Light Energy Accumulation from Pyrene Derivative to Tris(bipyridine)ruthenium on Clay Surface

A novel type of energy donor–acceptor system on a clay surface has been prepared. The energy transfer between an energy-donating cationic pyrene derivative (An-Py²⁺) and an energy-accepting tris­(bipyridine)ruthenium complex (Ru²⁺) on the clay surface was investigated using absorption, emission, and lifetime measurements. An obvious energy transfer was observed, and one Ru²⁺ molecule quenched the emission from five molecules of An-Py²⁺ with an emission quenching efficiency of 85% on the clay surface. This suggests that the light energies absorbed by five of the An-Py²⁺ molecules were accumulated in the one Ru²⁺ molecule. Near-quantitative emission quenching was observed for stoichiometric amounts of An-Py²⁺ and Ru²⁺. The apparent quenching rate constant is approximately 10¹⁷ L mol^–1 s^–1, and thus the quenching rate constant is 10⁷–10⁸ times higher than the diffusion rate constant in a homogeneous solution

Remarkable Stimulation of Emission Quenching on a Clay Surface

Tetra-cationic pyrene derivative (Py⁴⁺) and tris­(bipyridine)­ruthenium­(II) (Ru²⁺) were hybridized onto the surface of a synthesized clay. We observed the remarkable stimulation of excited Py⁴⁺ emission quenching on the clay surface, with a very large apparent quenching rate constant (<i>k</i>_q = 7.4 ± 0.7 × 10¹⁵ L mol^–1 s^–1)

Hybridization between Periodic Mesoporous Organosilica and a Ru(II) Polypyridyl Complex with Phosphonic Acid Anchor Groups

A new method for the hybridization of a ruthenium­(II) polypyridyl complex ([Ru­(bpy)₂((CH₂PO₃H₂)₂-bpy)]²⁺ (<b>RuP</b>_<b>2</b>^<b>2+</b>: bpy =2,2′-bipyridine; (CH₂PO₃H₂)₂-bpy =2,2′-bipyridine-4,4′di­(metylphosphonic acid)) with biphenylene-bearing periodic mesoporous organosilica <b>(Bp–PMO</b> made from 4,4′bis­(triethoxysilyl)­biphenyl [(C₂H₅O)₃Si-(C₆H₄)₂-Si­(OC₂H₅)₃]) was developed. Efficient and secure fixation of the ruthenium­(II) complex with methylphosphonic acid groups (<b>RuP</b>_<b>2</b>^<b>2+</b>) in the mesopores of <b>Bp–PMO</b> occurred. This method introduced up to 660 μmol of <b>RuP</b>_<b>2</b>^<b>2+</b> in 1 g of <b>Bp–PMO</b>. Two modes of adsorption of <b>RuP</b>_<b>2</b>^<b>2+</b> in the mesopores of <b>Bp–PMO</b> were observed: one is caused by the chemical interaction between the methylphosphonic acid groups of <b>RuP</b>_<b>2</b>^<b>2+</b> and the silicate moieties of <b>Bp–PMO</b> and the other is attributed to aggregation of the <b>RuP</b>_<b>2</b>^<b>2+</b> complexes. In the case of the former mode, adsorbed <b>RuP</b>_<b>2</b>^<b>2+</b> (up to 80–100 μmol g^–1) did not detach from <b>Bp–PMO</b> after washing with acetonitrile, dimethylformamide, or even water. Emission from the excited biphenylene (Bp) units was quantitatively quenched by the adsorbed <b>RuP</b>_<b>2</b>^<b>2+</b> molecules in cases where more than 60 μmol g^–1 of <b>RuP</b>_<b>2</b>^<b>2+</b> was adsorbed, and emission from <b>RuP</b>_<b>2</b>^<b>2+</b> was observed. Quantitative emission measurements indicated that emission from approximately 100 Bp units can be completely quenched by only one <b>RuP</b>_<b>2</b>^<b>2+</b> molecule in the mesopore, and photons absorbed by approximately 400 Bp units are potentially accumulated in one <b>RuP</b>_<b>2</b>^<b>2+</b> molecule

Superior Inorganic Ion Cofactors of Tetraborate Species Attaining Highly Efficient Heterogeneous Electrocatalysis for Water Oxidation on Cobalt Oxyhydroxide Nanoparticles

A heterogeneous catalyst incorporating an inorganic ion cofactor for electrochemical water oxidation was exploited using a CoO­(OH) nanoparticle layer-deposited electrode. The significant catalytic current for water oxidation was generated in a Na₂B₄O₇ solution at pH 9.4 when applying 0.94 V versus Ag/AgCl in contrast to no catalytic current generation in the K₂SO₄ solution at the same pH. HB₄O₇^– and B₄O₇^2– ions were indicated to act as key cofactors for the induced catalytic activity of the CoO­(OH) layer. The Na₂B₄O₇ concentration dependence of the catalytic current was analyzed based on a Michaelis–Menten-type kinetics to provide an affinity constant of cofactors to the active sites, <i>K</i>_m = 28 ± 3.6 mM, and the maximum catalytic current density, <i>I</i>_max = 2.3 ± 0.13 mA cm^–2. The <i>I</i>_max value of HB₄O₇^– and B₄O₇^2– ions was 1.4 times higher than that (1.3 mA cm^–2) for the previously reported case of CO₃^2– ions. This could be explained by the shorter-range proton transfer from the active site to the proton-accepting cofactor because of the larger size and more flexible conformation of HB₄O₇^– and B₄O₇^2– ions compared with that of CO₃^2– ions

Intercalation of a Surfactant with a Long Polyfluoroalkyl Chain into a Clay Mineral: Unique Orientation of Polyfluoroalkyl Groups in Clay Layers

Eight novel polyfluorinated surfactants (C_<i>n</i>F_2<i>n</i>+1CONH­(CH₂)₂ N⁺(CH₃)₂C₁₆H₃₃ Br^–; abbreviated as C<i>n</i>F–S, where <i>n</i> = 1, 2, 3, 4, 5, 6, 8, 10) were synthesized and their intercalation into cation-exchangeable clay was investigated. All of the polyfluorinated surfactants intercalated in amounts exceeding the cation exchange capacity (CEC) of the clay. The C4F–S and C5F–S surfactants exhibited intercalation up to 480% of the CEC as a saturated adsorption limit. On the basis of X-ray analysis, C<i>n</i>F–S surfactants intercalated between clay nanosheets to form a bilayer structure in which the surfactant molecules tilt at an angle of ∼60° with respect to the clay surface. The saturated adsorption limits and layer distances differed between surfactants with short (<i>n</i> = 1, 2) and long (<i>n</i> = 3–10) perfluoroalkyl chains. For long-chain surfactants, their saturated adsorption limits were independent of the perfluoroalkyl chain length and the layer distances systematically increased with increasing perfluoroalkyl chain length. These results suggest that the microscopic orientation differed between the short and long chains. X-ray analysis showed that the long-chain surfactants orient the perfluoroalkyl chains at a tilt of 41 ± 5° with respect to the clay layer. This value was in good agreement with polarized IR measurements of 42 ± 2° for this angle

Mechanistic Insight into Reversible Core Structural Changes of Dinuclear μ‑Hydroxoruthenium(II) Complexes with a 2,8-Di-2-pyridyl-1,9,10-anthyridine Backbone Prior to Water Oxidation Catalysis

<i>proximal,proximal</i>-(<i>p</i>,<i>p</i>)-[Ru^II₂(tpy)₂LXY]^<i>n</i>+ (tpy = 2,2′;6′,2″-terpyridine, L = 5-phenyl-2,8-di-2-pyridyl-1,9,10-anthyridine, and X and Y = other coordination sites) yields the structurally and functionally unusual Ru^II(μ-OH)­Ru^II core, which is capable of catalyzing water oxidation with key water insertion to the core (<i>Inorg. Chem.</i> <b>2015</b>, <i>54</i>, 7627). Herein, we studied a sequence of bridging-ligand substitution among <i>p</i>,<i>p</i>-[Ru₂(tpy)₂L­(μ-Cl)]³⁺ (<b>Ru</b>_<b>2</b><b>(μ-Cl)</b>), <i>p</i>,<i>p</i>-[Ru₂(tpy)₂L­(μ-OH)]³⁺ (<b>Ru</b>_<b>2</b><b>(μ-OH)</b>), <i>p</i>,<i>p</i>-[Ru₂(tpy)₂L­(OH)­(OH₂)]³⁺ (<b>Ru</b>_<b>2</b><b>(OH)­(OH</b>_<b>2</b><b>)</b>), and <i>p</i>,<i>p</i>-[Ru₂(tpy)₂L­(OH)₂]²⁺ (<b>Ru</b>_<b>2</b><b>(OH)</b>_<b>2</b>) in aqueous solution. <b>Ru</b>_<b>2</b><b>(μ-Cl)</b> converted slowly (10^–4 s^–1) to <b>Ru</b>_<b>2</b><b>(μ-OH)</b>, and further <b>Ru</b>_<b>2</b><b>(μ-OH)</b> converted very slowly (10^–6 s^–1) to <b>Ru</b>_<b>2</b><b>(OH)­(OH</b>_<b>2</b><b>)</b> by the insertion of water to reach equilibrium at pH 8.5–12.3. On the basis of density functional theory (DFT) calculations, <b>Ru</b>_<b>2</b><b>(OH)­(OH</b>_<b>2</b><b>)</b> was predicted to be thermodynamically stable by 13.3 kJ mol^–1 in water compared to <b>Ru</b>_<b>2</b><b>(μ-OH)</b> because of the specially stabilized core structure by multiple hydrogen-bonding interactions involving aquo, hydroxo, and L backbone ligands. The observed rate from <b>Ru</b>_<b>2</b><b>(μ-OH)</b> to <b>Ru</b>_<b>2</b><b>(OH)</b>_<b>2</b> by the insertion of an OH^– ion increased linearly with an increase in the OH^– concentration from 10 to 100 mM. The water insertion to the core is very slow (∼10^–6 s^–1) in aqueous solution at pH 8.5–12.3, whereas the insertion of OH^– ions is accelerated (10^–5–10^–4 s^–1) above pH 13.4 by 2 orders of magnitude. The kinetic data including activation parameters suggest that the associative mechanism for the insertion of water to the Ru^II(μ-OH)­Ru^II core of <b>Ru</b>_<b>2</b><b>(μ-OH)</b> at pH 8.5–12.3 alters the interchange mechanism for the insertion of an OH^– ion to the core above pH 13.4 because of relatively stronger nucleophilic attack of OH^– ions. The hypothesized <i>p</i>,<i>p</i>-[Ru₂(tpy)₂L­(μ-OH₂)]⁴⁺ and <i>p</i>,<i>p</i>-[Ru₂(tpy)₂L­(OH₂)₂]⁴⁺ formed by protonation from <b>Ru</b>_<b>2</b><b>(μ-OH)</b> and <b>Ru</b>_<b>2</b><b>(OH)­(OH</b>_<b>2</b><b>)</b> were predicted to be unstable by 71.3 and 112.4 kJ mol^–1 compared to <b>Ru</b>_<b>2</b><b>(μ-OH)</b> and <b>Ru</b>_<b>2</b><b>(OH)­(OH</b>_<b>2</b><b>)</b>, respectively. The reverse reactions of <b>Ru</b>_<b>2</b><b>(μ-OH)</b>, <b>Ru</b>_<b>2</b><b>(OH)­(OH</b>_<b>2</b><b>)</b>, and <b>Ru</b>_<b>2</b><b>(OH)</b>_<b>2</b> to <b>Ru</b>_<b>2</b><b>(μ-Cl)</b> below pH 5 could be caused by lowering the core charge by protonation of the μ-OH^– or OH^– ligand