Nov 21, 2014

Highly Selective Solar-Driven Methanol from CO2 by a Photocatalyst/Biocatalyst Integrated System

Publication Date (Web): November 18, 2014 (Communication)
DOI: 10.1021/ja509650r
 
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The successful development of a photocatalyst/biocatalyst integrated system that carries out selective methanol production from CO2 is reported herein. The fine-tuned system was derived from a judicious combination of graphene-based visible light active photocatalyst (CCG-IP) and sequentially coupled enzymes. The covalent attachment of isatin-porphyrin (IP) chromophore to chemically converted graphene (CCG) afforded newly developed CCG-IP photocatalyst for this research endeavor. The current work represents a new benchmark for carrying out highly selective methanol formation from CO2 in an environmentally benign manner.
 

Nov 20, 2014

Levitation of molten uranium dioxide allowed structural determination of the solid and melt at high temperature. [Also see Perspective by Navrotsky]
Uranium dioxide (UO2) is the major nuclear fuel component of fission power reactors. A key concern during severe accidents is the melting and leakage of radioactive UO2 as it corrodes through its zirconium cladding and steel containment. Yet, the very high temperatures (>3140 kelvin) and chemical reactivity of molten UO2 have prevented structural studies. In this work, we combine laser heating, sample levitation, and synchrotron x-rays to obtain pair distribution function measurements of hot solid and molten UO2. The hot solid shows a substantial increase in oxygen disorder around the lambda transition (2670 K) but negligible U-O coordination change. On melting, the average U-O coordination drops from 8 to 6.7 ± 0.5. Molecular dynamics models refined to this structure predict higher U-U mobility than 8-coordinated melts.

Taking the measure of molten uranium oxide

  • Alexandra Navrotsky
Science 21 November 2014: 916-917.
Levitated droplets of uranium oxide reveal a complex structure below and above the melting point [Also see Report by Skinner et al.]

Thermochromic and Photoresponsive Cyanometalate Fe/Co Squares: Toward Control of the Electron Transfer Temperature

Publication Date (Web): October 23, 2014 (Article)
DOI: 10.1021/ja508280n


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Two structurally related and photoresponsive cyanide-bridged Fe/Co square complexes, {Fe2Co2}, are reported: {[(TpMe)Fe(CN)3]2[Co(bpy)2]2[(TpMe)Fe(CN)3]2}·12H2O (2) and {[(TpMe)Fe(CN)3]2[Co(bpy)2]2[BPh4]2}·6MeCN (3), where TpMe and bpy are hydridotris(3-methylpyrazol-1-yl)borate and 2,2′-bipyridine, respectively. Through electrochemical and spectroscopic studies, the TpMe ligand appears to be a moderate σ donor in comparison to others in the [NEt4][(TpR)FeIII(CN)3] series [where TpR = Tp, hydridotris(pyrazol-1-yl)borate; TpMe = hydridotris(3-methylpyrazol-1-yl)borate; pzTp = tetrakis(pyrazol-1-yl)borate; Tp* = hydridotris(3,5-dimethylpyrazol-1-yl)borate; Tp*Me = hydridotris(3,4,5-trimethylpyrazol-1-yl)borate]. The spectroscopic, structural, and magnetic data of the {Fe2Co2} squares indicate that thermally-induced intramolecular electron transfer reversibly converts {FeIILS(μ-CN)CoIIILS} pairs into {FeIIILS(μ-CN)CoIIHS} units near ca. 230 and 244 K (T1/2) for 2 and 3, respectively (LS: low spin; HS: high spin). These experimental results show that 2 and 3 display light-induced {FeIIILS(μ-CN)CoIIHS} metastable states that relax to thermodynamic {FeIILS(μ-CN)CoIIILS} ones at ca. 90 K. Ancillary TpR ligand donor strength appears to be the dominant factor for tuning electron transfer properties in these {Fe2Co2} complexes.
Publication Date (Web): November 7, 2014 (Article)
DOI: 10.1021/ja510719k
 
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For the first time quantitative measurements of the hydroperoxyl radical (HO2) in a jet-stirred reactor were performed thanks to a new experimental setup involving fast sampling and near-infrared cavity ring-down spectroscopy at low pressure. The experiments were performed at atmospheric pressure and over a range of temperatures (550–900 K) with n-butane, the simplest hydrocarbon fuel exhibiting cool flame oxidation chemistry which represents a key process for the auto-ignition in internal combustion engines. The same technique was also used to measure H2O2, H2O, CH2O, and C2H4 under the same conditions. This new setup brings new scientific horizons for characterizing complex reactive systems at elevated temperatures. Measuring HO2 formation from hydrocarbon oxidation is extremely important in determining the propensity of a fuel to follow chain-termination pathways from R + O2 compared to chain branching (leading to OH), helping to constrain and better validate detailed chemical kinetics models.
Publication Date (Web): November 14, 2014 (Communication)
DOI: 10.1021/ja510290t
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Converting CO2 into valuable compounds using sunlight as the energy input and an earth-abundant metal complex as the catalyst is an exciting challenge related to contemporary energy issues as well as to climate change. By using an inexpensive organic photosensitizer under visible-light excitation (λ > 400 nm) and a substituted iron(0) tetraphenylporphyrin as a homogeneous catalyst, we have been able to generate carbon monoxide from CO2 selectively with high turnover numbers. Sustained catalytic activity over a long time period (t > 50 h) did not lead to catalyst or sensitizer deactivation. A catalytic mechanism is proposed.
 

Nov 14, 2014

The Catalytic Mechanism of Diarylamine Radical-Trapping Antioxidants

Publication Date (Web): October 30, 2014 (Article)
DOI: 10.1021/ja509391u
 
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Diarylamine radical-trapping antioxidants are important industrial additives, finding widespread use in petroleum-derived products. They are uniquely effective at elevated temperatures due to their ability to trap multiple radicals per molecule of diarylamine. Herein we report the results of computational and experimental studies designed to elucidate the mechanism of this remarkable activity. We find that the key step in the proposed catalytic cycle–decomposition of the alkoxyamine derived from capture of a substrate-derived alkyl radical with a diarylamine-derived nitroxide–proceeds by different mechanisms depending on the structure of both the substrate and the diarylamine. N,N-Diarylalkoxyamines derived from saturated substrates and diphenylamines decompose by N–O homolysis followed by disproportionation. Alternatively, those derived from unsaturated substrates and diphenylamines, or saturated substrates and N-phenyl-β-naphthylamine, decompose by an unprecedented concerted retro-carbonyl-ene reaction. The alkoxyamines that decompose by the concerted process inhibit hexadecane autoxidations at 160 °C to the same extent as the corresponding diarylamine, whereas those alkoxyamines that decompose by the N–O homolysis/disproportionation pathway are much less effective. This suggests that the competing cage escape of the alkoxyl radicals following N–O homolysis leads to significantly less effective regeneration of diarylamines and implies that the catalytic efficiency of diarylamine antioxidants is substrate-dependent. The results presented here have significant implications in the future design of antioxidant additives: diarylamines designed to yield intermediate alkoxyamines that undergo the retro-carbonyl-ene reaction are likely to be much more effective than existing compounds and will display catalytic radical-trapping activities at lower temperatures due to lower barriers to regeneration.

Nov 13, 2014

A network of sodium and magnesium ions helps direct double deprotonation of aryl rings.
The regioselectivity of deprotonation reactions between arene substrates and basic metalating agents is usually governed by the electronic and/or coordinative characteristics of a directing group attached to the benzene ring. Generally, the reaction takes place in the ortho position, adjacent to the substituent. Here, we introduce a protocol by which the metalating agent, a disodium-monomagnesium alkyl-amide, forms a template that extends regioselectivity to more distant arene sites. Depending on the nature of the directing group, ortho-meta′ or meta-meta′ dimetalation is observed, in the latter case breaking the dogma of ortho metalation. This concept is elaborated through the characterization of both organometallic intermediates and electrophilically quenched products. 

Nov 10, 2014

Photodissociation Dynamics of Phenol: Multistate Trajectory Simulations including Tunneling

Publication Date (Web): October 27, 2014 (Article)
DOI: 10.1021/ja509016a
 
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We report multistate trajectory simulations, including coherence, decoherence, and multidimensional tunneling, of phenol photodissociation dynamics. The calculations are based on full-dimensional anchor-points reactive potential surfaces and state couplings fit to electronic structure calculations including dynamical correlation with an augmented correlation-consistent polarized valence double-ζ basis set. The calculations successfully reproduce the experimentally observed bimodal character of the total kinetic energy release spectra and confirm the interpretation of the most recent experiments that the photodissociation process is dominated by tunneling. Analysis of the trajectories uncovers an unexpected dissociation pathway for one quantum excitation of the O–H stretching mode of the S1 state, namely, tunneling in a coherent mixture of states starting in a smaller ROH (∼0.9–1.0 Å) region than has previously been invoked. The simulations also show that most trajectories do not pass close to the S1–S2 conical intersection (they have a minimum gap greater than 0.6 eV), they provide statistics on the out-of-plane angles at the locations of the minimum energy adiabatic gap, and they reveal information about which vibrational modes are most highly activated in the products.
 

Kinetics of Hydrogen Atom Abstraction from Substrate by an Active Site Thiyl Radical in Ribonucleotide Reductase

Publication Date (Web): October 29, 2014 (Article)
DOI: 10.1021/ja507313w
 
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Ribonucleotide reductases (RNRs) catalyze the conversion of nucleotides to deoxynucleotides in all organisms. Active E. coli class Ia RNR is an α2β2 complex that undergoes reversible, long-range proton-coupled electron transfer (PCET) over a pathway of redox active amino acids (β-Y122 → [β-W48] → β-Y356 → α-Y731 → α-Y730 → α-C439) that spans ∼35 Å. To unmask PCET kinetics from rate-limiting conformational changes, we prepared a photochemical RNR containing a [ReI] photooxidant site-specifically incorporated at position 355 ([Re]-β2), adjacent to PCET pathway residue Y356 in β. [Re]-β2 was further modified by replacing Y356 with 2,3,5-trifluorotyrosine to enable photochemical generation and spectroscopic observation of chemically competent tyrosyl radical(s). Using transient absorption spectroscopy, we compare the kinetics of Y· decay in the presence of substrate and wt-α2, Y731F-α2 ,or C439S-α2, as well as with 3′-[2H]-substrate and wt-α2. We find that only in the presence of wt-α2 and the unlabeled substrate do we observe an enhanced rate of radical decay indicative of forward radical propagation. This observation reveals that cleavage of the 3′-C–H bond of substrate by the transiently formed C439· thiyl radical is rate-limiting in forward PCET through α and has allowed calculation of a lower bound for the rate constant associated with this step of (1.4 ± 0.4) × 104 s–1. Prompting radical propagation with light has enabled observation of PCET events heretofore inaccessible, revealing active site chemistry at the heart of RNR catalysis.
 

Nov 7, 2014

A dye molecule channels the energy of two visible photons into aryl substitution reactions.
Biological photosynthesis uses the energy of several visible light photons for the challenging oxidation of water, whereas chemical photocatalysis typically involves only single-photon excitation. Perylene bisimide is reduced by visible light photoinduced electron transfer (PET) to its stable and colored radical anion. We report here that subsequent excitation of the radical anion accumulates sufficient energy for the reduction of stable aryl chlorides giving aryl radicals, which were trapped by hydrogen atom donors or used in carbon-carbon bond formation. This consecutive PET (conPET) overcomes the current energetic limitation of visible light photoredox catalysis and allows the photocatalytic conversion of less reactive chemical bonds in organic synthesis.
Shape-tunable metal nanoparticles form by replicating the hollow space inside designed DNA nanostructures.
We report a general strategy for designing and synthesizing inorganic nanostructures with arbitrarily prescribed three-dimensional shapes. Computationally designed DNA strands self-assemble into a stiff “nanomold” that contains a user-specified three-dimensional cavity and encloses a nucleating gold “seed.” Under mild conditions, this seed grows into a larger cast structure that fills and thus replicates the cavity. We synthesized a variety of nanoparticles with 3-nanometer resolution: three distinct silver cuboids with three independently tunable dimensions, silver and gold nanoparticles with diverse cross sections, and composite structures with homo- and heterogeneous components. The designer equilateral silver triangular and spherical nanoparticles exhibited plasmonic properties consistent with electromagnetism-based simulations. Our framework is generalizable to more complex geometries and diverse inorganic materials, offering a range of applications in biosensing, photonics, and nanoelectronics.
Discrepancies in the reported structures of the difficult-to-isolate solid form of a common acid have been resolved
A cerium oxide support rendered palladium-rhodium nanoparticles more reactive and harder to reduce under reaction conditions.
Catalysts used for heterogeneous processes are usually composed of metal nanoparticles dispersed over a high–surface-area support. In recent years, near-ambient pressure techniques have allowed catalyst characterization under operating conditions, overcoming the pressure gap effect. However, the use of model systems may not truly represent the changes that occur in real catalysts (the so-called material gap effect). Supports can play an important role in the catalytic process by providing new active sites and may strongly affect both the physical and chemical properties of metal nanoparticles. We used near-ambient pressure x-ray photoelectron spectroscopy to show that the surface rearrangement of bimetallic (rhodium-palladium) nanoparticles under working conditions for ethanol steam reforming with real catalysts is strongly influenced by the presence of a reducible ceria support. 

Controlling the Intercalation Chemistry to Design High-Performance Dual-Salt Hybrid Rechargeable Batteries

Publication Date (Web): November 3, 2014 (Communication)
DOI: 10.1021/ja508463z
 
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We have conducted extensive theoretical and experimental investigations to unravel the origin of the electrochemical properties of hybrid Mg2+/Li+ rechargeable batteries at the atomistic and macroscopic levels. By revealing the thermodynamics of Mg2+ and Li+ co-insertion into the Mo6S8 cathode host using density functional theory calculations, we show that there is a threshold Li+ activity for the pristine Mo6S8 cathode to prefer lithiation instead of magnesiation. By precisely controlling the insertion chemistry using a dual-salt electrolyte, we have enabled ultrafast discharge of our battery by achieving 93.6% capacity retention at 20 C and 87.5% at 30 C, respectively, at room temperature.
 

Sn Cation Valency Dependence in Cation Exchange Reactions Involving Cu2-xSe Nanocrystals

Publication Date (Web): October 23, 2014 (Article)
DOI: 10.1021/ja508161c


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We studied cation exchange reactions in colloidal Cu2-xSe nanocrystals (NCs) involving the replacement of Cu+ cations with either Sn2+ or Sn4+ cations. This is a model system in several aspects: first, the +2 and +4 oxidation states for tin are relatively stable; in addition, the phase of the Cu2-xSe NCs remains cubic regardless of the degree of copper deficiency (that is, “x”) in the NC lattice. Also, Sn4+ ions are comparable in size to the Cu+ ions, while Sn2+ ones are much larger. We show here that the valency of the entering Sn ions dictates the structure and composition not only of the final products but also of the intermediate steps of the exchange. When Sn4+ cations are used, alloyed Cu2–4ySnySe NCs (with y ≤ 0.33) are formed as intermediates, with almost no distortion of the anion framework, apart from a small contraction. In this exchange reaction the final stoichiometry of the NCs cannot go beyond Cu0.66Sn0.33Se (that is Cu2SnSe3), as any further replacement of Cu+ cations with Sn4+ cations would require a drastic reorganization of the anion framework, which is not possible at the reaction conditions of the experiments. When instead Sn2+ cations are employed, SnSe NCs are formed, mostly in the orthorhombic phase, with significant, albeit not drastic, distortion of the anion framework. Intermediate steps in this exchange reaction are represented by Janus-type Cu2-xSe/SnSe heterostructures, with no Cu–Sn–Se alloys.
Publication Date (Web): October 16, 2014 (Article)
DOI: 10.1021/ja5079514


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Nickel-containing superoxide dismutase (NiSOD) is a mononuclear cysteinate-ligated nickel metalloenzyme that catalyzes the disproportionation of superoxide into dioxygen and hydrogen peroxide by cycling between NiII and NiIII oxidation states. All of the ligating residues to nickel are found within the first six residues from the N-terminus, which has prompted several research groups to generate NiSOD metallopeptide-based mimics derived from the first several residues of the NiSOD sequence. To assess the viability of using these metallopeptide-based mimics (NiSOD maquettes) to probe the mechanism of SOD catalysis facilitated by NiSOD, we computationally explored the initial step of the O2 reduction mechanism catalyzed by the NiSOD maquette {NiII(SODm1)} (SODm1 = HCDLP CGVYD PA). Herein we use spectroscopic (S K-edge X-ray absorption spectroscopy, electronic absorption spectroscopy, and circular dichroism spectroscopy) and computational techniques to derive the detailed active-site structure of {NiII(SODm1)}. These studies suggest that the {NiII(SODm1)} active-site possesses a NiII-S(H+)-Cys(6) moiety and at least one associated water molecule contained in a hydrogen-bonding interaction to the coordinated Cys(2) and Cys(6) sulfur atoms. A computationally derived mechanism for O2 reduction using the formulated active-site structure of {NiII(SODm1)} suggests that O2 reduction takes place through an apparent initial outersphere hydrogen atom transfer (HAT) from the NiII-S(H+)-Cys(6) moiety to the O2 molecule. It is proposed that the water molecule aids in driving the reaction forward by lowering the NiII-S(H+)-Cys(6) pKa. Such a mechanism is not possible in NiSOD itself for structural reasons. These results therefore strongly suggest that maquettes derived from the primary sequence of NiSOD are mechanistically distinct from NiSOD itself despite the similarities in the structure and physical properties of the metalloenzyme vs the NiSOD metallopeptide-based models.

Mechanistic Contrasts between Manganese and Rhenium Bipyridine Electrocatalysts for the Reduction of Carbon Dioxide

Publication Date (Web): October 17, 2014 (Article)
DOI: 10.1021/ja508192y


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[Re(bpy)(CO)3] is a well-established homogeneous electrocatalyst for the reduction of CO2 to CO. Recently, substitution of the more abundant transition metal Mn for Re yielded a similarly active electrocatalyst, [Mn(bpy)(CO)3]. Compared to the Re catalyst, this Mn catalyst operates at a lower applied reduction potential but requires the presence of a weak acid in the solution for catalytic activity. In this study, we employ quantum chemistry combined with continuum solvation and microkinetics to examine the mechanism of CO2 reduction by each catalyst. We use cyclic voltammetry experiments to determine the turnover frequencies of the Mn catalyst with phenol as the added weak acid. The computed turnover frequencies for both catalysts agree to within one order of magnitude of the experimental ones. The different operating potentials for these catalysts indicate that different reduction pathways may be favored during catalysis. We model two different pathways for both catalysts and find that, at their respective operating potentials, the Mn catalyst indeed is predicted to take a different reaction route than the Re catalyst. The Mn catalyst can access both catalytic pathways, depending on the applied potential, while the Re catalyst does not show this flexibility. Our microkinetics analysis predicts which intermediates should be observable during catalysis. These intermediates for the two catalyzed reactions have qualitatively different electronic configurations, depending on the applied potential. The observable intermediate at higher applied potentials possesses an unpaired electron and therefore should be EPR-active; however, the observable intermediate at lower applied potentials, accessible only for the Mn catalyst, is diamagnetic and therefore should be EPR-silent. The differences between both catalysts are rationalized on the basis of their electronic structure and different ligand binding affinities.

Neat and Complete: Thiolate-Ligand Exchange on a Silver Molecular Nanoparticle

Publication Date (Web): October 27, 2014 (Communication)
DOI: 10.1021/ja508860b


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Atomically precise thiolate-protected noble metal molecular nanoparticles are a promising class of model nanomaterials for catalysis, optoelectronics, and the bottom-up assembly of true molecular crystals. However, these applications have not fully materialized due to a lack of ligand exchange strategies that add functionality, but preserve the properties of these remarkable particles. Here we present a method for the rapid (<30 s) and complete thiolate-for-thiolate exchange of the highly sought after silver molecular nanoparticle [Ag44(SR)30]−4. Only by using this method were we able to preserve the precise nature of the particles and simultaneously replace the native ligands with ligands containing a variety of functional groups. Crucially, as a result of our method we were able to process the particles into smooth thin films, paving the way for their integration into solution-processed devices.

Gold–Copper Nanoalloys Supported on TiO2 as Photocatalysts for CO2 Reduction by Water

Publication Date (Web): October 20, 2014 (Article)
DOI: 10.1021/ja506433k
 
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Commercial P25 modified by Au–Cu alloy nanoparticles as thin film exhibits, for CO2 reduction by water under sun simulated light, a rate of methane production above 2000 μmol (g of photocatalyst)−1 h–1. Although evolution of hydrogen is observed and O2 and ethane detected, the selectivity of conduction band electrons for methane formation is almost complete, about 97%. This photocatalytic behavior is completely different from that measured for Au/P25 (hydrogen evolution) and Cu/P25 (lower activity, but similar methane selectivity). Characterization by TEM, XPS, and UV–vis spectroscopy shows that Au and Cu are alloyed in the nanoparticles. FT-IR spectroscopy and chemical analysis have allowed one to detect on the photocatalyst surface the presence of CO2•–, Cu–CO, and elemental C. Accordingly, a mechanism in which the role of Au is to respond under visible light and Cu binds to CO and directs the reduction pathway is proposed.
 

Reduction of CO2 to Methanol Catalyzed by a Biomimetic Organo-Hydride Produced from Pyridine

Publication Date (Web): October 17, 2014 (Article)
DOI: 10.1021/ja510131a
 
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We use quantum chemical calculations to elucidate a viable mechanism for pyridine-catalyzed reduction of CO2 to methanol involving homogeneous catalytic steps. The first phase of the catalytic cycle involves generation of the key catalytic agent, 1,2-dihydropyridine (PyH2). First, pyridine (Py) undergoes a H+ transfer (PT) to form pyridinium (PyH+), followed by an e transfer (ET) to produce pyridinium radical (PyH0). Examples of systems to effect this ET to populate PyH+’s LUMO (E0calc ∼ −1.3 V vs SCE) to form the solution phase PyH0 via highly reducing electrons include the photoelectrochemical p-GaP system (ECBM ∼ −1.5 V vs SCE at pH 5) and the photochemical [Ru(phen)3]2+/ascorbate system. We predict that PyH0 undergoes further PT–ET steps to form the key closed-shell, dearomatized (PyH2) species (with the PT capable of being assisted by a negatively biased cathode). Our proposed sequential PT–ET–PT–ET mechanism for transforming Py into PyH2 is analogous to that described in the formation of related dihydropyridines. Because it is driven by its proclivity to regain aromaticity, PyH2 is a potent recyclable organo-hydride donor that mimics important aspects of the role of NADPH in the formation of C–H bonds in the photosynthetic CO2 reduction process. In particular, in the second phase of the catalytic cycle, which involves three separate reduction steps, we predict that the PyH2/Py redox couple is kinetically and thermodynamically competent in catalytically effecting hydride and proton transfers (the latter often mediated by a proton relay chain) to CO2 and its two succeeding intermediates, namely, formic acid and formaldehyde, to ultimately form CH3OH. The hydride and proton transfers for the first of these reduction steps, the homogeneous reduction of CO2, are sequential in nature (in which the formate to formic acid protonation can be assisted by a negatively biased cathode). In contrast, these transfers are coupled in each of the two subsequent homogeneous hydride and proton transfer steps to reduce formic acid and formaldehyde.
 

Driving Force Dependent, Photoinduced Electron Transfer at Degenerately Doped, Optically Transparent Semiconductor Nanoparticle Interfaces

Publication Date (Web): October 20, 2014 (Communication)
DOI: 10.1021/ja508862h
 
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Photoinduced, interfacial electron injection and back electron transfer between surface-bound [RuII(bpy)2(4,4′-(PO3H2)2-bpy)]2+ and degenerately doped In2O3:Sn nanoparticles, present in mesoporous thin films (nanoITO), have been studied as a function of applied external bias. Due to the metallic behavior of the nanoITO films, application of an external bias was used to vary the Fermi level in the oxide and, with it, the driving force for electron transfer (ΔGo′). By controlling the external bias, ΔGo′ was varied from 0 to −1.8 eV for electron injection and from −0.3 to −1.3 eV for back electron transfer. Analysis of the back electron-transfer data, obtained from transient absorption measurements, using Marcus–Gerischer theory gave an experimental estimate of λ = 0.56 eV for the reorganization energy of the surface-bound RuIII/II couple in acetonitrile with 0.1 M LiClO4 electrolyte.
 

Photoelectrochemical Hole Injection Revealed in Polyoxotitanate Nanocrystals Functionalized with Organic Adsorbates

Publication Date (Web): October 22, 2014 (Article)
DOI: 10.1021/ja509270f
 
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We find that crystallographically resolved Ti17O24(OPri)20 nanoparticles, functionalized by covalent attachment of 4-nitrophenyl-acetylacetonate or coumarin 343 adsorbates, exhibit hole injection into surface states when photoexcited with visible light (λ = 400–680 nm). Our findings are supported by photoelectrochemical measurements, EPR spectroscopy, and quantum dynamics simulations of interfacial charge transfer. The underlying mechanism is consistent with measurements of photocathodic currents generated with visible light for thin layers of functionalized polyoxotitanate nanocrystals deposited on FTO working electrodes. The reported experimental and theoretical analysis demonstrates for the first time the feasibility of p-type sensitization of TiO2 solely based on covalent binding of organic adsorbates.
 

Oct 29, 2014

Light-Induced Proton-Coupled Electron Transfer Inside a Nanocage

Publication Date (Web): October 21, 2014 (Communication)
DOI: 10.1021/ja509761a
 
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Triggering proton-coupled electron-transfer (PCET) reactions with light in a nanoconfined host environment would bring about temporal control on the reactive pathways via kinetic stabilization of intermediates. Using a water-soluble octahedral Pd6L4 molecular cage as a host, we show that optical pumping of host–guest charge transfer (CT) states lead to generation of kinetically stable phenoxyl radical of the incarcerated 4-hydroxy-diphenylamine (1-OH). Femtosecond broadband transient absorption studies reveal that CT excitation initiates the proton movement from the 1-OH radical cation to a solvent water molecule in ∼890 fs, faster than the time scale for bulk solvation. Our work illustrates that optical host–guest CT excitations can drive solvent-coupled ultrafast PCET reactions inside nanocages and if optimally tuned should provide a novel paradigm for visible-light photocatalysis.
 

Selective Electrocatalytic Oxidation of a Re–Methyl Complex to Methanol by a Surface-Bound RuII Polypyridyl Catalyst

Publication Date (Web): October 17, 2014 (Communication)
DOI: 10.1021/ja507979c
 
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The complex [Ru(Mebimpy)(4,4′-((HO)2OPCH2)2bpy)(OH2)]2+ surface bound to tin-doped indium oxide mesoporous nanoparticle film electrodes (nanoITO-RuII(OH2)2+) is an electrocatalyst for the selective oxidation of methylrhenium trioxide (MTO) to methanol in acidic aqueous solution. Oxidative activation of the catalyst to nanoITO-RuIV(OH)3+ induces oxidation of MTO. The reaction is first order in MTO with rate saturation observed at [MTO] > 12 mM with a limiting rate constant of k = 25 s–1. Methanol is formed selectively in 87% Faradaic yield in controlled potential electrolyses at 1.3 V vs NHE. At higher potentials, oxidation of MTO by nanoITO-RuV(O)3+ leads to multiple electrolysis products. The results of an electrochemical kinetics study point to a mechanism in which surface oxidation to nanoITO-RuIV(OH)3+ is followed by direct insertion into the rhenium–methyl bond of MTO with a detectable intermediate.