Photoexcitation of Fe3 O Nodes in MOF Drives Water Oxidation at pH=1 When Ru Catalyst Is Present.

Artificial photosynthesis strives to convert the energy of sunlight into sustainable, eco-friendly solar fuels. However, systems with light-driven water oxidation reaction (WOR) at pH=1 are rare. Broadly used [Ru(bpy)3 ]2+ (bpy=2,2'-bipyridine) photosensitizer has a fixed +1.23 V potential which is insufficient to drive most water oxidation catalysts (WOCs) in acid, while Fe2 O3 , featuring the highly oxidizing holes, is not stable at low pH. Here, the key examples of Fe-based metal-organic framework (MOF) water oxidation photoelectrocatalysts active at pH=1 are presented. Fe-MIL-126 and Fe MOF-dcbpy structures were formed with 4,4'-biphenyl dicarboxylate (bpdc), 2,2'-bipyridine-5,5'-dicarboxylate (dcbpy) linkers and their mixtures. Presence of dcbpy linkers allows integration of metal-based catalysts via coordination to 2,2'-bipyridine fragments. Fe-based MOFs were doped with Ru-based precursors to achieve highly active MOFs bearing [Ru(bpy)(dcbpy)(H2 O)2 ]2+ WOC. Materials were analyzed with X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infra-red (FTIR) spectroscopy, resonance Raman, X-ray absorption spectroscopy, fs optical pump-probe, electron paramagnetic resonance (EPR), diffuse reflectance and electric conductivity measurements and were modeled by band structure calculations. It is shown that under reaction conditions, FeIII and RuIII oxidation states are present, indicating rate-limiting electron transfer in MOF. Fe3 O nodes emerge as photosensitizers able to drive prolonged O2 evolution in acid. Further developments are possible via MOF's linker modification for enhanced light absorption, electrical conductivity, reduced MOF solubility in acid, Ru-WOC modification for faster WOC catalysis, or Ru-WOC substitution to 3d metal-based systems. The findings give further insight for development of light-driven water splitting systems based on Earth-abundant metals.

Combined Spectroscopic and Computational Investigation on the Oxidation of exo-Tetrahydrodicyclopentadiene (JP-10; C10H16) Doped with Titanium-Aluminum-Boron Reactive Metal Nanopowder.

We report the results on the combustion of single, levitated droplets of exo-tetrahydrodicyclopentadiene (JP-10) doped with titanium-aluminum-boron (Ti-Al-B) reactive metal nanopowders (RMNPs) in an oxygen (60%)-argon (40%) atmosphere by exploiting an ultrasonic levitator with droplets ignited by a carbon dioxide laser. Ultraviolet-visible (UV-vis) emission spectroscopy revealed the presence of gas-phase aluminum (Al) and titanium (Ti) atoms. These atoms can be oxidized in the gas phase by molecular oxygen to form spectroscopically detected aluminum monoxide (AlO) and titanium monoxide (TiO) transients. Analysis of the optical ignition videos supports that the nanoparticles are ignited before JP-10. The detection of boron monoxide (BO) further proposes an active surface chemistry through the oxidation of the RMNPs and the release of at least BO into the gas phase. The oxidation of gas-phase BO by molecular oxygen to boron dioxide (BO2) plus atomic oxygen might operate in the gas phase, although the involvement of surface oxidation processes of RMNPs to BO2 cannot be discounted. The UV-vis emission spectra also revealed the key reactive intermediates (OH, CH, C2, and HCO) of the oxidation of JP-10. Electronic structure calculations reveal that the presence of reactive radicals has a profound impact on the oxidation of JP-10. Although titanium monoxide (TiO) reacts to produce titanium dioxide (TiO2), it does not engage in an active JP-10 chemistry as all abstraction pathways are endoergic by more than 217 kJ mol-1. This is similar for atomic aluminum and titanium, whose hydrogen abstraction reactions from JP-10 were revealed to be endoergic by at least 77 kJ mol-1. Therefore, aluminum and titanium react preferentially with molecular oxygen to produce their monoxides. However, the formation of BO, AlO, and BO2 supplies a pool of highly reactive radicals, which can abstract hydrogen from JP-10 via transition states ranging from only 1 to 5 kJ mol-1 above the separated reactants, forming JP-10 radicals along with the hydrogen abstraction products (boron hydride oxide, aluminum monohydroxide, and metaboric acid) in the overall exoergic reactions. These abstraction barriers are well below the barriers of abstractions for ground-state atomic oxygen and molecular oxygen. In this sense, gas-phase BO, AlO, and BO2 catalyze the oxidation of gas-phase JP-10 via hydrogen abstraction, forming highly reactive JP-10 radicals. Overall, the addition of RMNPs to JP-10 not only provides a higher energy density fuel but is also expected to lead to shorter ignition delays compared to pure JP-10 due to the highly reactive pool of radicals (BO, AlO, and BO2) formed in the initial stage of the oxidation process.

Early Binding of Substrate Oxygen Is Responsible for a Spectroscopically Distinct S2 State in Photosystem II.

The biological generation of oxygen by the oxygen-evolving complex (OEC) in photosystem II (PS II) is one of nature's most important reactions. The OEC is a Mn4Ca cluster that has multiple Mn-O-Mn and Mn-O-Ca bridges and binds four water molecules. Previously, binding of an additional oxygen was detected in the S2 to S3 transition. Here we demonstrate that early binding of the substrate oxygen to the five-coordinate Mn1 center in the S2 state is likely responsible for the S2 high-spin EPR signal. Substrate binding in the Mn1-OH form explains the prevalence of the high-spin S2 state at higher pH and its low-temperature conversion into the S3 state. The given interpretation was confirmed by X-ray absorption spectroscopic measurements, DFT, and broken symmetry DFT calculations of structures and magnetic properties. Structural, electronic, and spectroscopic properties of the high-spin S2 state model are provided and compared with the available S3 state models. New interpretation of the high-spin S2 state opens opportunity for analysis of factors controlling the oxygen substrate binding in PS II.

Model of the Oxygen Evolving Complex Which Is Highly Predisposed to O-O Bond Formation.

Light-driven water oxidation is a fundamental reaction in the biosphere. The Mn4Ca cluster of photosystem II cycles through five redox states termed S0-S4, after which oxygen is evolved. Critically, the timing of O-O bond formation within the Kok cycle remains unknown. By combining recent crystallographic, spectroscopic, and DFT results, we demonstrate an atomistic S3 state model with the possibility of a low barrier to O-O bond formation prior to the final oxidation step. Furthermore, the associated one electron oxidized S4 state does not provide more advantages in terms of spin alignment or the energy of O-O bond formation. We propose that a high energy peroxide isoform of the S3 state can preferentially be oxidized by Tyr zox in the course of final electron transfer leading to O2 evolution. Such a mechanism may explain the peculiar kinetic behavior of O2 evolution as well as serve as an evolutionary adaptation to avoid release of the harmful peroxides.

Rapid evolution of the Photosystem II electronic structure during water splitting.

Photosynthetic water oxidation is a fundamental process that sustains the biosphere. A Mn4Ca cluster embedded in the photosystem II protein environment is responsible for the production of atmospheric oxygen. Here, time-resolved x-ray emission spectroscopy (XES) was used to observe the process of oxygen formation in real time. These experiments reveal that the oxygen evolution step, initiated by three sequential laser flashes, is accompanied by rapid (within 50 µs) changes to the Mn Kß XES spectrum. However, no oxidation of the Mn4Ca core above the all MnIV state was detected to precede O-O bond formation, and the observed changes were therefore assigned to O-O bond formation dynamics. We propose that O-O bond formation occurs prior to the transfer of the final (4th) electron from the Mn4Ca cluster to the oxidized tyrosine YZ residue. This model resolves the kinetic limitations associated with O-O bond formation, and suggests an evolutionary adaptation to avoid releasing of harmful peroxide species.

Uncovering the Role of Oxygen Atom Transfer in Ru-Based Catalytic Water Oxidation.

The realization of artificial photosynthesis carries the promise of cheap and abundant energy, however, significant advances in the rational design of water oxidation catalysts are required. Detailed information on the structure of the catalyst under reaction conditions and mechanisms of O-O bond formation should be obtained. Here, we used a combination of electron paramagnetic resonance (EPR), stopped flow freeze quench on a millisecond-second time scale, X-ray absorption (XAS), resonance Raman (RR) spectroscopy, and density functional theory (DFT) to follow the dynamics of the Ru-based single site catalyst, [RuII(NPM)(4-pic)2(H2O)]2+ (NPM = 4-t-butyl-2,6-di(1',8'-naphthyrid-2'-yl)pyridine, pic = 4-picoline), under the water oxidation conditions. We report a unique EPR signal with g-tensor, gx = 2.30, gy = 2.18, and gz = 1.83 which allowed us to observe fast dynamics of oxygen atom transfer from the RuIV═O oxo species to the uncoordinated nitrogen of the NPM ligand. In few seconds, the NPM ligand modification results in [RuIII(NPM-NO)(4-pic)2(H2O)]3+ and [RuIII(NPM-NO,NO)(4-pic)2]3+ complexes. A proposed [RuV(NPM)(4-pic)2═O]3+ intermediate was not detected under the tested conditions. We demonstrate that while the proximal base might be beneficial in O-O bond formation via nucleophilic water attack on an oxo species as shown by DFT, the noncoordinating nitrogen is impractical as a base in water oxidation catalysts due to its facile conversion to the N-O group. This study opens new horizons for understanding the real structure of Ru catalysts under water oxidation conditions and points toward the need to further investigate the role of the N-O ligand in promoting water oxidation catalysis.

Density perturbation theory.

Despite the fundamental importance of electron density in density functional theory, perturbations are still usually dealt with using Hartree-Fock-like orbital equations known as coupled-perturbed Kohn-Sham (CPKS). As an alternative, we develop a perturbation theory that solves for the perturbed density directly, removing the need for CPKS. This replaces CPKS with a true Hohenberg-Kohn density perturbation theory. In CPKS, the perturbed density is found in the basis of products of occupied and virtual orbitals, which becomes ever more over-complete as the size of the orbital basis set increases. In our method, the perturbation to the density is expanded in terms of a series of density basis functions and found directly. It is possible to solve for the density in such a way that it makes the total energy stationary even if the density basis is incomplete.

Hydrogen-bonded intermediates and transition states during spontaneous and acid-catalyzed hydrolysis of the carcinogen (+)-anti-BPDE.

Understanding mechanisms of (+)-anti-BPDE detoxification is crucial for combating its mutagenic and potent carcinogenic action. However, energetic-structural correlations of reaction intermediates and transition states during detoxification via hydrolysis are poorly understood. To gain mechanistic insight we have computationally characterized intermediate and transition species associated with spontaneous and general-acid catalyzed hydrolysis of (+)-anti-BPDE. We studied the role of cacodylic acid as a proton donor in the rate limiting step. The computed activation energy (ΔG‡) is in agreement with the experimental value for hydrolysis in a sodium cacodylate buffer. Both types of, spontaneous and acid catalyzed, BPDE hydrolysis can proceed through low-entropy hydrogen bonded intermediates prior to formation of transition states whose energies determine reaction activation barriers and rates.