Holli

Organic electron donors are of importance for a number of applications. However, the factors that are essential for a directed design of compounds with desired reduction power are not clear. Here, we analyze these factors in detail. The intrinsic reduction power, which neglects the environment, has to be separated from extrinsic (e.g., solvent) effects. This power could be quantified by the gas-phase ionization energy. The experimentally obtained redox potentials in solution and the calculated ionization energies in a solvent (modeled with the conductor-like screening model (COSMO)) include both intrinsic and extrinsic factors. An increase in the conjugated π-system of organic electron donors leads to an increase in the intrinsic reduction power, but also decreases the solvent stabilization. Hence, intrinsic and extrinsic effects compete against each other; generally the extrinsic effects dominate. We suggest a simple relationship between the redox potential in solution and the gas-phase ionization energy and the volume of an organic electron donor. We finally arrive at formulas that allow for an estimate of the (gas-phase) ionization energy of an electron donor or the (gas-phase) electron affinity of an electron acceptor from the measured redox potentials in solution. The formulas could be used for neutral organic molecules with no or only small static dipole moment and relatively uniform charge distribution after oxidation/reduction.

Size matters: A relationship between the gas-phase ionization energy and the experienced redox potential in solution is derived. The results highlight the importance of molecular volume in determining the reduction potential in solution (see figure).

Spermbots are biocompatible hybrid machines that consist of microtubes which are propelled by single spermatozoa and have promising features for powering nano and microdevices. This article presents three approaches on how to improve the performance of such spermbots. First, 20 μm microtubes produce faster spermbots compared to the previously reported 50 μm long microtubes. Furthermore, biofunctionalization by microcontact printing and surface chemistry of biomolecules on the inner tube surface improve the coupling efficiency between sperm cell and microtube, and the addition of caffeine results in a speed boost of the sperm-driven micromotor.

Improved spermbot performance is demonstrated by biofunctionalization of the inner tube surface, shorter tube design, and caffeine addition. Firstly, spermbot velocity is improved by the use of shorter microtubes; secondly, better coupling efficiency is achieved by binding of fibronectin inside the microtube; and finally, caffeine addition gives a temporary speed boost to the spermbot.

In the Z-scheme of photosynthesis, two photosynthetic proteins, photosystem II and photosystem I, excite electrons step-wise using the light energy. On page 2369, K. T. Nam and team report a hybrid Z-scheme using photosystem I and a BiVO₄ semiconductor. In this study, step-wise charge separation in photosystem I and BiVO₄ enables the production of hydrogen from only water under visible light, for the first time.

The cover picture shows four different pathways for methanol synthesis over Cu-based catalysts depending on the feed composition and the presence of the Zn-promoter. In their Full Paper, F. Studt et al. present a DFT-based microkinetic model and consistent experimental data that explain the Janus-faced character of Cu in CO_x hydrogenation allowing only one path, CO or CO₂ hydrogenation, to be effective at a time. The industrial Zn-promoter accelerates CO₂ conversion over Cu/ZnO, but can simultaneously act as a poison for CO hydrogenation. On the other hand, CO₂ in the feed was found to poison unpromoted catalysts such as Cu/MgO, which was highly efficient for CO hydrogenation. The article highlighted by this cover can be found on p. 1105 ff. of Issue 7, 2015.

Setting sail towards new horizons is the iron(II)-catalyzed asymmetric transfer hydrogenation of CO bonds. With the robust, highly active N₂P₂ macrocyclic catalysts described by A. Mezzetti and co-workers in their Communication on page 5171 ff., a wide scope of substrates can be hydrogenated, obtaining high yields and enantioselectivities by literally paying the iron price.

A near-stoichiometric amount of O₂ was evolved as observed in the visible-light irradiation of an aqueous buffer (pH 8) containing [Ru^II(2,2′-bipyridine)₃] as a photosensitizer, Na₂S₂O₈ as a sacrificial electron acceptor, and a heteropolynuclear cyanide complex as a water-oxidation catalyst. The heteropolynuclear cyanide complexes exhibited higher catalytic activity than a polynuclear cyanide complex containing only Co^III or Pt^IV ions as C-bound metal ions. The origin of the synergistic effect between Co and Pt ions is discussed in relation to electronic and local atomic structures of the complexes.

A platinum assist: A near-stoichiometric amount of O₂ was evolved as observed in the visible-light irradiation of an aqueous buffer (pH 8) containing [Ru^II(2,2′-bipyridine)₃] as a photosensitizer, Na₂S₂O₈ as a sacrificial electron acceptor, and a heteropolynuclear cyanide complex as a water-oxidation catalyst. The synergistic effect between the Co and Pt ions was confirmed to facilitate the water-oxidation catalysis by the heteropolynuclear complex.

A Ni⁰-NCN pincer complex featuring a six-membered N-heterocyclic carbene (NHC) central platform and amidine pendant arms was synthesized by deprotonation of its Ni^II precursor. It retained chloride in the square-planar coordination sphere of nickel and was expected to be highly susceptible to oxidative addition reactions. The Ni⁰ complex rapidly activated ammonia at room temperature, in a ligand-assisted process where the carbene carbon atom played the unprecedented role of proton acceptor. For the first time, the coordinated (ammine) and activated (amido) species were observed together in solution, in a solvent-dependent equilibrium. A structural analysis of the Ni complexes provided insight into the highly unusual, non-innocent behavior of the NHC ligand.

Fast and reversible ligand-assisted ammonia activation on a nickel-NCN pincer complex featuring a non-innocent N-heterocyclic carbene produced a solvent-dependent equilibrium mixture of ammine and amide complexes (see scheme, Dipp=2,6-diisopropylphenyl). These species were present in solution in comparable proportions, and EXSY NMR experiments as well as exchange with ND₃ further supported the equilibrium.

Nearly stoichiometric amounts of O₂ are evolved on visible-light irradiation of an aqueous buffer (pH 8) containing [Ru^II(2,2′-bipyridine)₃], Na₂S₂O₈, and a heteropolynuclear cyanide complex containing Co and Pt ions as a photosensitizer, a sacrificial electron acceptor, and a water oxidation catalyst, respectively, as reported by Y. Yamada, S. Fukuzumi, et al. in their Communication (DOI: 10.1002/anie.201501116). A synergistic effect between the Co and Pt ions was confirmed for the water oxidation catalysis by the heteropolynuclear complex.

A micromotor-based strategy for energy generation, utilizing the conversion of liquid-phase hydrogen to usable hydrogen gas (H₂), is described. The new motion-based H₂-generation concept relies on the movement of Pt-black/Ti Janus microparticle motors in a solution of sodium borohydride (NaBH₄) fuel. This is the first report of using NaBH₄ for powering micromotors. The autonomous motion of these catalytic micromotors, as well as their bubble generation, leads to enhanced mixing and transport of NaBH₄ towards the Pt-black catalytic surface (compared to static microparticles or films), and hence to a substantially faster rate of H₂ production. The practical utility of these micromotors is illustrated by powering a hydrogen–oxygen fuel cell car by an on-board motion-based hydrogen and oxygen generation. The new micromotor approach paves the way for the development of efficient on-site energy generation for powering external devices or meeting growing demands on the energy grid.

Fuel metal jacket: Micromotors give enhanced energy generation by the movement of Pt-black/Ti Janus microparticles in liquid-phase chemical fuel. The autonomous motion of these micromotors leads to enhanced mixing and transport of NaBH₄ fuel compared to static microparticles or films, and hence to a substantially faster hydrogen-generation rate. The practical utility is illustrated by powering a hydrogen–oxygen fuel cell car.

The hole-driving oxidation of titanium-coordinated water molecules on the surface of TiO₂ is both thermodynamically and kinetically unfavorable. By avoiding the direct coordinative adsorption of water molecules to the surface Ti sites, the water can be activated to realize its oxidation. When TiO₂ surface is covered by the H-bonding acceptor F, the first-layer water adsorption mode is switched from Ti coordination to a dual H-bonding adsorption on adjacent surface F sites. Detailed in situ IR spectroscopy and isotope-labeling studies reveal that the adsorbed water molecules by dual H-bonding can be oxidized to O₂ even in the absence of any electron scavengers. Combined with theoretical calculations, it is proposed that the formation of the dual H-bonding structure can not only enable the hole transfer to the water molecules thermodynamically, but also facilitate kinetically the cleavage of OH bonds by proton-coupled electron transfer process during water oxidation.

Switching the water adsorption mode on TiO₂ to dual H-bonding by the presence of fluorine atoms at the surface not only thermodynamically enables the hole transfer to the water molecules, but also facilitates the proton-coupled electron transfer during water oxidation. This phenomenon is established by IR spectroscopy studies and calculations.

Reported are two highly efficient metal-free perylene dyes featuring N-annulated thienobenzoperylene (NTBP) and N-annulated thienocyclopentaperylene (NTCP), which are coplanar polycyclic aromatic hydrocarbons. Without the use of any coadsorbate, the metal-free organic dye derived from the NTCP segment was used for a dye-sensitized solar cell which attained a power conversion efficiency of 12 % under an irradiance of 100 mW cm⁻², simulated air mass global (AM1.5G) sunlight.

Power trip: A perylene dye derived from N-annulated thienocyclopentaperylene, which is characterized by a low-energy gap and a high electron injection yield, was synthesized for dye-sensitized solar cells. A high power conversion efficiency of 12 %, at an irradiance of the AM1.5G sunlight, was achieved. This efficiency is the highest achieved thus far by using just a metal-free organic dye.