Increased Water Reduction Efficiency of Polyelectrolyte‐Bound Trimetallic [Ru,Rh,Ru] Photocatalysts in Air‐Saturated Aqueous Solutions

The groundbreaking use of polyelectrolytes to increase the efficiency of supramolecular photocatalysts in solar H₂ production schemes under aqueous aerobic conditions is reported. Supramolecular photocatalysts of the architecture [{(TL)₂Ru(BL)}₂RhX₂]⁵⁺ (BL=bridging ligand, TL=terminal ligand, X=halide) demonstrate high efficiencies in deoxygenated organic solvents but do not function in air‐saturated aqueous solution because of the quenching of the metal‐to‐ligand charge‐transfer (MLCT) excited state under these conditions. The new photocatalytic system incorporates poly(4‐styrenesulfonate) (PSS) into aqueous solutions containing [{(bpy)₂Ru(dpp)}₂RhCl₂]⁵⁺ (bpy=2,2′‐bipyridine, dpp=2,3‐bis(2‐pyridyl)pyrazine). PSS has a profound impact on the photocatalyst efficiency, increasing H₂ production over three times that of deoxygenated aqueous solutions alone. H₂ photocatalysis proceeds even under aerobic conditions for PSS‐containing solutions, an exciting consequence for solar hydrogen‐production research.     

Visualizing zeptomole (electro)catalysis at single nanoparticles within an ensemble

The relationship between the structural properties, such as the size and the shape, of a catalytic nanoparticle and its reactivity is a key concept in (electro)catalysis. Current understanding of this relationship is mainly derived from studies involving large ensembles of nanoparticles (NPs). However, the results necessarily reflect the average catalytic behavior of an ensemble, even though the properties of individual particles may vary widely. Here, we demonstrate a novel approach using scanning electrochemical cell microscopy (SECCM) to locate and map the reactivity of individual NPs within an electrocatalytic ensemble, consisting of platinum NPs supported on a single carbon nanotube. Significantly, our studies show that subtle variations in the morphology of NPs lead to dramatic changes in (potential-dependent) reactivity, which has important implications for the design and assessment of NP catalysts. The instrumental approach described is general and opens up new avenues of research in functional imaging, nanoscale electron transfer, and catalysis.     

Thermally driven ionic aggregation in poly(ethylene oxide)-based sulfonate ionomers

A series of sulfonate polyester ionomers with well-defined poly(ethylene oxide) spacer lengths between phthalates and alkali metal cations as counterions are designed for improved ionic conductivity. Ion conduction in these chemically complex materials is dominated by the polymer mobility and the state of ionic aggregation. While the aggregation decreases dramatically at room temperature as the cation size increases from Li to Na to Cs, the extents of ionic aggregation of these ionomers are comparable at elevated temperatures. Both the Na and Cs ionomers exhibit thermally reversible transformation upon heating from 25 to 120 °C as isolated ion pairs aggregate. This seemingly counterintuitive aggregation of ions on heating is driven by the fact that the dielectric constant of all polar liquids decreases on heating, enhancing Coulomb interactions between ions.     

Hydrolysis of serine-containing peptides at neutral pH promoted by [MoO4]2- oxyanion

Hydrolysis of the dipeptides glycylserine (GlySer), leucylserine (LeuSer), histidylserine (HisSer), glycylalanine (GlyAla), and serylglycine (SerGly) was examined in oxomolybdate solutions by means of (1)H, (13)C, and (95)Mo NMR spectroscopy. In the presence of a mixture of oxomolybdates, the hydrolysis of the peptide bond in GlySer proceeded under neutral pD conditions (pD = 7.0, 60 °C) with a rate constant of k(obs) = 5.9 × 10(-6) s(-1). NMR spectra did not show evidence of the formation of paramagnetic species, excluding the possibility of Mo(VI) reduction to Mo(V), indicating that the cleavage of the peptide bond is purely hydrolytic. The pD dependence of k(obs) exhibits a bell-shaped profile, with the fastest cleavage observed at pD 7.0. Comparison of the rate profile with the concentration profile of oxomolybdate species implicated monomolybdate MoO(4)(2-) as the kinetically active complex. Kinetics experiments at pD 7.0 using a fixed amount of GlySer and increasing amounts of MoO(4)(2-) allowed for calculation of the catalytic rate constant (k(2) = 9.25 × 10(-6) s(-1)) and the formation constant for the GlySer-MoO(4)(2-) complex (K(f) = 15.25 M(-1)). The origin of the hydrolytic activity of molybdate is most likely a combination of the polarization of amide oxygen in GlySer due to the binding to molybdate, followed by the intramolecular attack of the Ser hydroxyl group.     

A unique three-dimensional coordination cluster based on a silver carbene complex

In an attempt to perform a simple anion-exchange reaction on a pincer-carbene-ligated nickel complex using AgNO(3), we instead obtained an unexpected three-dimensional (3D) Ag(7) cluster containing a [Ag(6)] core in a twisted-bowtie geometry. The reverse-transmetalation reaction by which the carbene is transferred from nickel to silver is virtually unprecedented. The CNC pincer-carbene ligands exhibit unusual bridging modes of ligand bonding for all three donor atoms. Another unique feature is that the final structure exhibits a 3D structure brought about by the connection of two-dimensional layers of the [Ag(6)] core via a seventh Ag ion.     

Experimental results of a high Q quasioptical reflection cavity

A quasi-optical reflection cavity intended to be used as the resonant load of a submillimeter wavelength oscillator has been designed, constructed, and tested. The resonator consists of three elements in cascade: a high Q, high finesse folded Fabry-Perot resonator, a lower Q, low finesse Fabry-Perot resonator, and a Littrow mounted diffraction grating. The resonator exhibits a Q of 6600 and 2.5 dB of loss at 63 GHz. The design can be scaled to 1 THz, where waveguide structures become impractical, to serve as the resonator for high frequency quantum well resonant tunneling device oscillators.