Mimicking the electron transfer chain in photosystem II with a molecular triad thermodynamically capable of water oxidation

In the photosynthetic photosystem II, electrons are transferred from the manganese-containing oxygen evolving complex (OEC) to the oxidized primary electron-donor chlorophyll P680•+ by a proton-coupled electron transfer process involving a tyrosine-histidine pair. Proton transfer from the tyrosine phenolic group to a histidine nitrogen positions the redox potential of the tyrosine between those of P680•+ and the OEC. We report the synthesis and time-resolved spectroscopic study of a molecular triad that models this electron transfer. The triad consists of a high-potential porphyrin bearing two pentafluorophenyl groups (PF10), a tetracyanoporphyrin electron acceptor (TCNP), and a benzimidazole-phenol secondary electron-donor (Bi-PhOH). Excitation of PF10 in benzonitrile is followed by singlet energy transfer to TCNP (τ = 41 ps), whose excited state decays by photoinduced electron transfer (τ = 830 ps) to yield Bi-PhOH-PF 10•+-TCNP•-. A second electron transfer reaction follows (τ < 12 ps), giving a final state postulated as BiH+-PhO•-PF10-TCNP•-, in which the phenolic proton now resides on benzimidazole. This final state decays with a time constant of 3.8 μs. The triad thus functionally mimics the electron transfers involving the tyrosine-histidine pair in PSII. The final charge-separated state is thermodynamically capable of water oxidation, and its long lifetime suggests the possibility of coupling systems such as this system to water oxidation catalysts for use in artificial photosynthetic fuel production.Fil: Megiatto, Jackson D.. Arizona State University; Estados UnidosFil: Antoniuk Pablant, Antaeres. Arizona State University; Estados UnidosFil: Sherman, Benjamin D.. Arizona State University; Estados UnidosFil: Kodis, Gerdenis. Arizona State University; Estados UnidosFil: Gervaldo, Miguel Andres. Universidad Nacional de Río Cuarto; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba; ArgentinaFil: Moore, Thomas A.. Arizona State University; Estados UnidosFil: Moore, Ana L.. Arizona State University; Estados UnidosFil: Gust, Devens. Arizona State University; Estados Unido

Development of the B-Stark motional Stark effect diagnostic for measurements of the internal magnetic field in the DIII-D tokamak

A new diagnostic, B-Stark, has been developed at the DIII- D tokamak for measurements of the magnitude and direction of the internal magnetic field. The B-Stark system is a version of a motional Stark effect (MSE) diagnostic based on the Stark split D/[alpha] emission from injected neutral beams. This diagnostic uses the spacing of the Stark lines to measure the magnitude of the magnetic field, and the intensities of the [pi]₃ and [sigma]₁ lines to measure the magnetic pitch angle. These lines originate from the same upper level, and are therefore not dependent on the n=3 level populations. The measurement of the magnetic pitch angle requires a specific viewing geometry with respect to the neutral beams, which is provided by the B-Stark diagnostic installation. The B-Stark technique may have advantages over MSE polarimetry diagnostics in future devices with high densities and temperatures, such as ITER. Under these conditions coatings on the plasma facing mirrors are expected, which can cause changes in the polarization state of the reflected light. The B-Stark technique is insensitive to the polarization direction, and can calibrate for polarization dependent transmission by using an in-situ beam-into-gas calibration. This dissertation describes the development and characterization of the B-Stark diagnostic. The hardware design and spectral fitting techniques are discussed in detail. Calibration procedures are described including the in-situ determination of the beam emission line profiles, viewing geometry and properties of the collection optics. The performance of the system is evaluated over the range of plasma conditions accessible at DIII-D. Measurements of the magnetic field have been made with toroidal fields in the range 1.2 - 2.1Tesla, plasma currents in the range 0.5 - 2.0MA, densities between 1.7 - 9.0 x 10¹⁹m⁻³, and neutral beam voltages between 50 - 81keV. These results are compared to values found from plasma equilibrium reconstructions (EFIT) and the MSE polarimetry system on DIII-D. The B-Stark system has been shown to provide measurements with a random errors as low as 0.2-0.3° in the magnetic pitch angle and 0.001-0.002T in [B]. Finally, proposed future improvements for the B-Stark diagnostic are presente

Photoinduced Electron and Energy Transfer in a Molecular Triad Featuring a Fullerene Redox Mediator

In order to investigate the possibility of a fullerene acting as an electron and/or singlet energy relay between a donor chromophore and an acceptor, a triad consisting of a fullerene (C₆₀) covalently linked to both a porphyrin energy and electron donor (P) and a β-tetracyanoporphyrin energy and electron acceptor (CyP) was synthesized. Steady state and time-resolved spectroscopic investigations show that the porphyrin first excited singlet state donates singlet excitation and an electron to the fullerene and also donates singlet excitation to the CyP. All three processes differ in rate constant by factors of ≤1.3, and all are much faster than the decay of ¹P–C₆₀–CyP by unichromophoric processes. The fullerene excited state accepts an electron from P and donates singlet excitation energy to CyP. The P^•+–C₆₀^•––CyP charge-separated state transfers an electron to CyP to produce a final P^•+–C₆₀–CyP^•– state. The same state is formed from P–C₆₀–¹CyP. Overall, the final charge-separated state is formed with a quantum yield of 85% in benzonitrile, and has a lifetime of 350 ps. Rate constants for formation and quantum yields of all intermediate states were estimated from results for the triad and several model compounds. Interestingly, the intermediate P^•+–C₆₀^•––CyP charge-separated state has a lifetime of 660 ps. It is longer lived than the final state in spite of stronger coupling of the radical ions. This is ascribed to the fact that recombination lies far into the inverted region of the Marcus rate constant vs thermodynamic driving force relationship

Carotenoids as electron or excited-state energy donors in artificial photosynthesis: an ultrafast investigation of a carotenoporphyrin and a carotenofullerene dyad

Photophysical investigations of molecular donor-acceptor systems have helped elucidate many details of natural photosynthesis and revealed design principles for artificial photosynthetic systems. To obtain insights into the factors that govern the partition between excited-state energy transfer (EET) and electron transfer (ET) processes among carotenoids and tetrapyrroles and fullerenes, we have designed artificial photosynthetic dyads that are thermodynamically poised to favor ET over EET processes. The dyads were studied using transient absorption spectroscopy with ∼100 femtosecond time resolution. For dyad 1, a carotenoporphyrin, excitation to the carotenoid

Spectroscopic Analysis of a Biomimetic Model of Tyr(z) Function in PSII

Using natural photosynthesis as a model, bio-inspired constructs for fuel generation from sunlight are being developed. Here we report the synthesis and time-resolved spectroscopic analysis of a molecular triad in which a porphyrin electron donor is covalently linked to both a cyanoporphyrin electron acceptor and a benzimidazole-phenol model for the TyrZ-D1His190 pair of PSII. A dual-laser setup enabled us to record the ultrafast kinetics and long-living species in a single experiment. From this data, the photophysical relaxation pathways were elucidated for the triad and reference compounds. For the triad, quenching of the cyanoporphyrin singlet excited state lifetime was interpreted as photoinduced electron transfer from the porphyrin to the excited cyanoporphyrin. In contrast to a previous study of a related molecule, we were unable to observe subsequent formation of a long-lived charge separated state involving the benzimidazole-phenol moiety. The lack of detection of a long-lived charge separated state is attributed to a change in energetic landscape for charge separation/recombination due to small differences in structure and solvation of the new triad

Spectroscopic Analysis of a Biomimetic Model of Tyr_Z Function in PSII

Using natural photosynthesis as a model, bio-inspired constructs for fuel generation from sunlight are being developed. Here we report the synthesis and time-resolved spectroscopic analysis of a molecular triad in which a porphyrin electron donor is covalently linked to both a cyanoporphyrin electron acceptor and a benzimidazole–phenol model for the Tyr_Z-D₁His190 pair of PSII. A dual-laser setup enabled us to record the ultrafast kinetics and long-living species in a single experiment. From this data, the photophysical relaxation pathways were elucidated for the triad and reference compounds. For the triad, quenching of the cyanoporphyrin singlet excited state lifetime was interpreted as photoinduced electron transfer from the porphyrin to the excited cyanoporphyrin. In contrast to a previous study of a related molecule, we were unable to observe subsequent formation of a long-lived charge separated state involving the benzimidazole–phenol moiety. The lack of detection of a long-lived charge separated state is attributed to a change in energetic landscape for charge separation/recombination due to small differences in structure and solvation of the new triad