Shunsuke Nozawa

Direct observation of bond formation in solution with femtosecond X-ray scattering.

The making and breaking of atomic bonds are essential processes in chemical reactions. Although the ultrafast dynamics of bond breaking have been studied intensively using time-resolved techniques, it is very difficult to study the structural dynamics of bond making, mainly because of its bimolecular nature. It is especially difficult to initiate and follow diffusion-limited bond formation in solution with ultrahigh time resolution. Here we use femtosecond time-resolved X-ray solution scattering to visualize the formation of a gold trimer complex, [Au(CN)2-]3 in real time without the limitation imposed by slow diffusion. This photoexcited gold trimer, which has weakly bound gold atoms in the ground state, undergoes a sequence of structural changes, and our experiments probe the dynamics of individual reaction steps, including covalent bond formation, the bent-to-linear transition, bond contraction and tetramer formation with a time resolution of ∼500xa0femtoseconds. We also determined the three-dimensional structures of reaction intermediates with sub-ångström spatial resolution. This work demonstrates that it is possible to track in detail and in real time the structural changes that occur during a chemical reaction in solution using X-ray free-electron lasers and advanced analysis of time-resolved solution scattering data.

Crystal Melting by Light: X-ray Crystal Structure Analysis of an Azo Crystal Showing Photoinduced Crystal-Melt Transition

Trans-cis photoisomerization in an azo compound containing azobenzene chromophores and long alkyl chains leads to a photoinduced crystal-melt transition (PCMT). X-ray structure analysis of this crystal clarifies the characteristic coexistence of the structurally ordered chromophores through their π···π interactions and disordered alkyl chains around room temperature. These structural features reveal that the PCMT starts near the surface of the crystal and propagates into the depth, sacrificing the π···π interactions. A temporal change of the powder X-ray diffraction pattern under light irradiation and a two-component phase diagram allow qualitative analysis of the PCMT and the following reconstructive crystallization of the cis isomer as a function of product accumulation. This is the first structural characterization of a compound showing the PCMT, overcoming the low periodicity that makes X-ray crystal structure analysis difficult.

Solar-driven Z-scheme water splitting using tantalum/nitrogen co-doped rutile titania nanorod as an oxygen evolution photocatalyst

A visible-light-driven water-splitting system that involves two-step photoexcitation (Z-scheme) was constructed using rutile TiO2 nanorod doped with Ta and N (TiO2:Ta/N) as an O2 evolution photocatalyst. The Ta-doped TiO2 nanorods, prepared by a solvothermal synthesis, underwent nitridation to possess visible-light absorption under mild conditions, even at 623 K under an ammonia flow. The TiO2:Ta/N powders modified with a RuO2 cocatalyst were active under visible light up to 540 nm for water oxidation for producing O2 in the presence of reversible electron acceptors (IO3− or Fe3+), while TiO2:N exhibited negligible activity. The results of time-resolved infrared absorption spectroscopy indicated that co-doping Ta with N into TiO2 prolonged the lifetime of photogenerated free electrons, leading to high photocatalytic activity. Simultaneous H2 and O2 evolution via water splitting was achieved using a combination of RuO2-modified TiO2:Ta/N, Ru-loaded SrTiO3:Rh and an Fe3+/Fe2+ redox couple under visible-light irradiation (λ > 420 nm) and under AM 1.5G simulated sunlight.

Cobalt Oxide Nanoclusters on Rutile Titania as Bifunctional Units for Water Oxidation Catalysis and Visible Light Absorption: Understanding the Structure–Activity Relationship

The structure of cobalt oxide (CoOx) nanoparticles dispersed on rutile TiO2 (R-TiO2) was characterized by X-ray diffraction, UV-vis-NIR diffuse reflectance spectroscopy, high-resolution transmission electron microscopy, X-ray absorption fine-structure spectroscopy, and X-ray photoelectron spectroscopy. The CoOx nanoparticles were loaded onto R-TiO2 by an impregnation method from an aqueous solution containing Co(NO3)2·6H2O followed by heating in air. Modification of the R-TiO2 with 2.0 wt % Co followed by heating at 423 K for 1 h resulted in the highest photocatalytic activity with good reproducibility. Structural analyses revealed that the activity of this photocatalyst depended strongly on the generation of Co3O4 nanoclusters with an optimal distribution. These nanoclusters are thought to interact with the R-TiO2 surface, resulting in visible light absorption and active sites for water oxidation.

A time-resolved powder diffraction study of in-situ photodimerization kinetics of 9-methylanthracene using a CCD area detector and parametric Rietveld refinement

The [4π + 4π] photodimerization process of the 9-substituted anthracene derivative crystalline 9-methylanthracene (9-MA) was investigated using time-resolved X-ray powder diffraction. The study was carried out in-situ using a CCD area detector. Sequential and parametric Rietveld refinement was applied for quantitative phase analysis. The results of traditional sequential Rietveld refinement showed that the evolution of the dimerization process can be described using the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model. The parameters of the JMAK equation were obtained successfully by parametric Rietveld refinement and suggest that the reaction follows heterogeneous nucleation and one-dimensional growth with a decreasing nucleation rate.

A Ruthenium(III)–Oxyl Complex Bearing Strong Radical Character

Proton-coupled electron-transfer oxidation of a RuII -OH2 complex, having an N-heterocyclic carbene ligand, gives a RuIII -O. species, which has an electronically equivalent structure of the RuIV =O species, in an acidic aqueous solution. The RuIII -O. complex was characterized by spectroscopic methods and DFT calculations. The oxidation state of the Ru center was shown to be close to +3; the Ru-O bond showed a lower-energy Raman scattering at 732u2005cm-1 and the Ru-O bond length was estimated to be 1.77(1)u2005Å. The RuIII -O. complex exhibits high reactivity in substrate oxidation under catalytic conditions; particularly, benzaldehyde and the derivatives are oxidized to the corresponding benzoic acid through C-H abstraction from the formyl group by the RuIII -O. complex bearing a strong radical character as the active species.

Rotational dephasing of a gold complex probed by anisotropic femtosecond x-ray solution scattering using an x-ray free-electron laser

The orientational dynamics of a gold trimer complex in a solution are investigated by using anisotropic femtosecond x-ray solution scattering measured by an x-ray free-electron laser. A linearly polarized laser pulse preferentially excites molecules with transition dipoles oriented parallel to the laser polarization, leading to the transient alignment of excited molecules. Such photoselectively aligned molecules give rise to an anisotropic scattering pattern that has different profiles in parallel and perpendicular directions with respect to laser polarization. Anisotropic x-ray scattering patterns obtained from the transiently aligned molecules contain information on the molecular orientation. By monitoring the time evolution of the anisotropic scattering pattern, we probe the rotational dephasing dynamics of [Au(CN)2 −]3 in a solution. We found that rotational dephasing of [Au(CN)2 −]3 occurs with a time constant of 13 ± 4 ps. By contrast, time-resolved scattering data on FeCl3 in a water solution, which does not accompany any structural change and gives only the contributions of solvent heating, lacks any anisotropy in the scattering signal.

Global reaction pathways in the photodissociation of I3 (-) ions in solution at 267 and 400 nm studied by picosecond X-ray liquidography.

The mechanism of a photochemical reaction involves the formation and dissociation of various short-lived species on ultrafast timescales and therefore its characterization requires detailed structural information on the transient species. By making use of a structurally sensitive X-ray probe, time-resolved X-ray liquidography (TRXL) can directly elucidate the structures of reacting molecules in the solution phase and thus determine the comprehensive reaction mechanism with high accuracy. In this work, by performing TRXL measurements at two different wavelengths (400 and 267 nm), the reaction mechanism of I3 (-) photolysis, which changes subtly depending on the excitation wavelength, is elucidated. Upon 400 nm photoexcitation, the I3 (-) ion dissociates into I2 (-) and I. By contrast, upon 267 nm photoexcitation, the I3 (-) ion undergoes both two-body dissociation (I2 (-) +I) and three-body dissociation (I(-) +2I) with 7:3 molar ratio. At both excitation wavelengths, all the transient species ultimately disappear in 80 ns by recombining to form the I3 (-) ion nongeminately. In addition to the reaction dynamics of solute species, the results reveal the transient structure of the solute/solvent cage and the changes in solvent density and temperature as a function of time.

Tracking ligand-migration pathways of carbonmonoxy myoglobin in crystals at cryogenic temperatures.

In order to explore the ligand-migration dynamics in myoglobin induced by photodissociation, cryogenic X-ray crystallographic investigations of carbonmonoxy myoglobin crystals illuminated by continuous wave and pulsed lasers at 1-15 kHz repetition rate have been carried out. Here it is shown that this novel method, extended pulsed-laser pumping of carbonmonoxy myoglobin, promotes ligand migration in the protein matrix by crossing the glass transition temperature repeatedly, and enables the visualization of the migration pathway of the photodissociated ligands in native Mb at cryogenic temperatures. It has revealed that the migration of the CO molecule into each cavity induces structural changes of the amino-acid residues around the cavity which result in the expansion of the cavity. The sequential motion of the ligand and the cavity suggests a self-opening mechanism of the ligand-migration channel arising by induced fit.

Current status of 50-picosecond resolved x-ray diffraction at Photon Factory Advanced Ring (PF-AR)

Ultra-fast time-resolved x-ray crystallography using electron-accelerator-based x-ray sources has been becoming a general and powerful tool to explore structural dynamics of crystalline state in material and biological sciences. Photon Factory Advanced Ring (PF-AR) is a full-time single-bunch synchrotron radiation source operated for such time-resolved x-ray studies using pulsed x-rays. Here we report instrumentation, feasibility, and applications of time-resolved x-ray diffraction at PF-AR. The time-resolved x-ray diffraction equipment consists of an x-ray pulse selector, an x-ray diffractometer, a femtosecond laser system and their timing modules which provide synchronized x-ray pulses with 50-picosecond duration (rms) and 150-femtosecond laser pulses for laser-pump-x-ray-probe experiment. The current status of the beam lines NW2 and NW14, and their applications are described.