Tuning Iridium Photocatalysts and Light Irradiation for Enhanced CO₂ Reduction

Efficient photocatalytic conversion of carbon dioxide into valuable reduction products is a priority goal for artificial photosynthesis. Iridium­(III) photocatalysts with a combined 2-phenylpyridine (ppy) and 2,2′:6′,2″-terpyridine (tpy) ligand set have been shown to selectively reduce CO₂ to CO. Here, terpyridine modifications have been investigated that yield a turnover number (TON) of up to 265, a quantum yield of 0.10, and a photocatalyst lifespan of over 10 days. The key to success is the combined effect of adding aromatic substituents to the tpy ligand 4′-position and optimizing lighting conditions. Insights into the photocatalyst fate are provided by kinetics analysis and spectroelectrochemistry, which point out the critical role of the reductively quenched catalyst and its evolution to a spent “green” state via a dark deactivation pathway. The stereoelectronic effect of adding a 9-anthryl substituent together with the use of low-energy blue light proves instrumental in the management of excited and reduced species, dictating the overall performance of the molecular photocatalyst

Photoinduced Water Oxidation by a Tetraruthenium Polyoxometalate Catalyst: Ion-pairing and Primary Processes with Ru(bpy)₃²⁺ Photosensitizer

The tetraruthenium polyoxometalate [Ru₄(μ-O)₄(μ-OH)₂(H₂O)₄(γ-SiW₁₀O₃₆)₂]^10‑ (<b>1</b>) behaves as a very efficient water oxidation catalyst in photocatalytic cycles using Ru­(bpy)₃²⁺ as sensitizer and persulfate as sacrificial oxidant. Two interrelated issues relevant to this behavior have been examined in detail: (i) the effects of ion pairing between the polyanionic catalyst and the cationic Ru­(bpy)₃²⁺ sensitizer, and (ii) the kinetics of hole transfer from the oxidized sensitizer to the catalyst. Complementary charge interactions in aqueous solution leads to an efficient static quenching of the Ru­(bpy)₃²⁺ excited state. The quenching takes place in ion-paired species with an average <b>1</b>:Ru­(bpy)₃²⁺ stoichiometry of 1:4. It occurs by very fast (ca. 2 ps) electron transfer from the excited photosensitizer to the catalyst followed by fast (15–150 ps) charge recombination (reversible oxidative quenching mechanism). This process competes appreciably with the primary photoreaction of the excited sensitizer with the sacrificial oxidant, even in high ionic strength media. The Ru­(bpy)₃³⁺ generated by photoreaction of the excited sensitizer with the sacrificial oxidant undergoes primary bimolecular hole scavenging by <b>1</b> at a remarkably high rate (3.6 ± 0.1 × 10⁹ M^–1 s^–1), emphasizing the kinetic advantages of this molecular species over, e.g., colloidal oxide particles as water oxidation catalysts. The kinetics of the subsequent steps and final oxygen evolution process involved in the full photocatalytic cycle are not known in detail. An indirect indication that all these processes are relatively fast, however, is provided by the flash photolysis experiments, where a single molecule of <b>1</b> is shown to undergo, in 40 ms, ca. 45 turnovers in Ru­(bpy)₃³⁺ reduction. With the assumption that one molecule of oxygen released after four hole-scavenging events, this translates into a very high average turnover frequency (280 s^–1) for oxygen production

Surface Immobilization of a Tetra-Ruthenium Substituted Polyoxometalate Water Oxidation Catalyst Through the Employment of Conducting Polypyrrole and the Layer-by-Layer (LBL) Technique

A tetra Ru-substituted polyoxometalate Na₁₀[{Ru₄O₄(OH)₂(H₂O)₄}­(γ-SiW₁₀O₃₆)₂] (Ru₄POM) has been successfully immobilised onto glassy carbon electrodes and indium tin oxide (ITO) coated glass slides through the employment of a conducting polypyrrole matrix and the layer-by-layer (LBL) technique. The resulting Ru₄POM doped polypyrrole films showed stable redox behavior associated with the Ru centres within the Ru₄POM, whereas, the POM’s tungsten-oxo redox centres were not accessible. The films showed pH dependent redox behavior within the pH range 2–5 whilst exhibiting excellent stability towards redox cycling. The layer-by-layer assembly was constructed onto poly­(diallyldimethylammonium chloride) (PDDA) modified carbon electrodes by alternate depositions of Ru₄POM and a Ru­(II) metallodendrimer. The resulting Ru₄POM assemblies showed stable redox behavior for the redox processes associated with Ru₄POM in the pH range 2–5. The charge transfer resistance of the LBL films was calculated through AC-Impedance. Surface characterization of both the polymer and LBL Ru₄POM films was carried out using atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Initial investigations into the ability of the Ru₄POM LBL films to electrocatalytically oxidise water at pH 7 have also been conducted

Photocatalytic Water Oxidation: Tuning Light-Induced Electron Transfer by Molecular Co₄O₄ Cores

Isostructural cubane-shaped catalysts [Co^III₄(μ-O)₄(μ-CH₃COO)₄(<i>p</i>-NC₅H₄X)₄], <b>1-X</b> (X = H, Me, <i>t</i>-Bu, OMe, Br, COOMe, CN), enable water oxidation under dark and illuminated conditions, where the primary step of photoinduced electron transfer obeys to Hammett linear free energy relationship behavior. Ligand design and catalyst optimization are instrumental for sustained O₂ productivity with quantum efficiency up to 80% at λ > 400 nm, thus opening a new perspective for in vitro molecular photosynthesi

Data_Sheet_1_Selective Targeting of Proteins by Hybrid Polyoxometalates: Interaction Between a Bis-Biotinylated Hybrid Conjugate and Avidin.docx

<p>The Keggin-type polyoxometalate [γ-SiW₁₀O₃₆]⁸⁻ was covalently modified to obtain a bis-biotinylated conjugate able to bind avidin. Spectroscopic studies such as UV-vis, fluorimetry, circular dichroism, coupled to surface plasmon resonance technique were used to highlight the unique interplay of supramolecular interactions between the homotetrameric protein and the bis-functionalized polyanion. In particular, the dual recognition mechanism of the avidin encompasses (i) a complementary electrostatic association between the anionic surface of the polyoxotungstate and each positively charged avidin subunit and (ii) specific host-guest interactions between each biotinylated arm and a corresponding pocket on the tetramer subunits. The assembly exhibits peroxidase-like reactivity and it was used in aqueous solution for L-methionine methyl ester oxidation by H₂O₂. The recognition phenomenon was then exploited for the preparation of layer-by-layer films, whose structural evolution was monitored in situ by ATR-FTIR spectroscopy. Finally, cell tracking studies were performed by exploiting the specific interactions with a labeled streptavidin.</p

Highly Sensitive Membrane-Based Pressure Sensors (MePS) for Real-Time Monitoring of Catalytic Reactions

Functional, flexible, and integrated lab-on-chips, based on elastic membranes, are capable of fine response to external stimuli, so to pave the way for many applications as multiplexed sensors for a wide range of chemical, physical and biomedical processes. Here, we report on the use of elastic thin membranes (TMs), integrated with a reaction chamber, to fabricate a membrane-based pressure sensor (MePS) for reaction monitoring. In particular, the TM becomes the key-element in the design of a highly sensitive MePS capable to monitor gaseous species production in dynamic and temporally fast processes with high resolution and reproducibility. Indeed, we demonstrate the use of a functional MePS integrating a 2 μm thick polydimethylsiloxane TM by monitoring the dioxygen evolution resulting from catalytic hydrogen peroxide dismutation. The operation of the membrane, explained using a diffusion-dominated model, is demonstrated on two similar catalytic systems with catalase-like activity, assembled into polyelectrolyte multilayers capsules. The MePS, tested in a range between 2 and 50 Pa, allows detecting a dioxygen variation of the μmol L^–1 s^–1 order. Due to their structural features, flexibility of integration, and biocompatibility, the MePSs are amenable of future development within advanced lab-on-chips

Knitting the Catalytic Pattern of Artificial Photosynthesis to a Hybrid Graphene Nanotexture

The artificial leaf project calls for new materials enabling multielectron catalysis with minimal overpotential, high turnover frequency, and long-term stability. Is graphene a better material than carbon nanotubes to enhance water oxidation catalysis for energy applications? Here we show that functionalized graphene with a tailored distribution of polycationic, quaternized, ammonium pendants provides an sp² carbon nanoplatform to anchor a totally inorganic tetraruthenate catalyst, mimicking the oxygen evolving center of natural PSII. The resulting hybrid material displays oxygen evolution at overpotential as low as 300 mV at neutral pH with negligible loss of performance after 4 h testing. This multilayer electroactive asset enhances the turnover frequency by 1 order of magnitude with respect to the isolated catalyst, and provides a definite up-grade of the carbon nanotube material, with a similar surface functionalization. Our innovation is based on a noninvasive, synthetic protocol for graphene functionalization that goes beyond the ill-defined oxidation–reduction methods, allowing a definite control of the surface properties