Published papers

Double-bond character of phosphate groups has important roles for biological reactions and was investigated using the P═O π* peaks in the oxygen K-edge X-ray absorption spectroscopy (XAS). The O K-edge XAS spectra of phosphates in the solid phase found that the double-bond character of phosphates is enhanced as the negative charge increases, where the weak P═O π* peak was observed in NaH2PO4, but distinct P═O π* peaks were obtained in Na2HPO4 and Na3PO4. Conversely, the P═O π* peaks were not observed in the XAS spectra of any phosphates in the liquid phase, indicating a substantial reduction in the double-bond character of the phosphate groups owing to the interaction of Na+ ions and the formation of hydration structures. The XAS spectra of diphosphates validated that change in the double-bond character of the phosphate groups is a general phenomenon also observed in high-energy phosphate compounds, which is closely related to the reactivity of adenosine triphosphate hydrolysis.

The electronic structures of high-purity solid chlorophyll a were investigated using nitrogen K-edge X-ray absorption spectroscopy. The C═N π* peaks of the chlorins exhibited characteristic split profiles, which changed progressively with successive scans owing to radiation-induced effects. Inner-shell quantum chemical calculations for structural models of chlorophyll a with different phytol chain lengths assigned the electronic structures of the C═N π* peaks and confirmed that the intensity changes of the C═N π* peaks are caused by cleaving the phytol chain. The influence of the vibration of the phytol chain to the C═N π* peaks of the chlorins was also discussed.

We present a framework for interpreting nitrogen K-edge x-ray absorption of ionic liquids and imidazole derivatives in water on a single absolute energy scale. Liquid transmission measurements provide reproducible spectra that can be compared across solutes without ad hoc shifting. Experimental line shapes fall into three patterns defined by the availability of low-lying π acceptors. Pyrrolidinium and ammonium-like centers show no discrete pre-edge. Imidazolium cations show a single pre-edge. Imidazole derivatives show a robust doublet. Real-space calculations with explicit inclusion of water molecules test how hydration controls these patterns. A radius scan in simulation identifies a practical cluster size that captures the first hydration shell while avoiding spurious long-range structure. Direct summation of simulated spectra based on 100 molecular dynamics snapshots on a common energy scale converts discrete transitions into a continuous liquid-phase profile. It merges the two ring nitrogen atoms of imidazolium into one pre-edge and preserves the imidazole doublet with improved intensity ratios. A minimal protocol that combines absorber-centered p DOS with targeted COOP maps separates σ and π channels and locates hydration-sensitive π windows without broader projections. Hydration increases low-energy π overlap while leaving the σ onset comparatively stable. The result is a transferable set of rules for assignment and for choosing simulation settings that match spectra for liquid phases.

Metal–ligand delocalization in metal complexes participating in various chemical processes was probed from the ligand side using nitrogen K-edge x-ray absorption spectroscopy (XAS). The electronic structures, spin multiplicities, and hydration structures of metal complexes in solutions with different metal ion centers, such as iron and cobalt protoporphyrin IX complexes, were investigated by N K-edge XAS. The influence of a phytol chain in chlorophyll a in the solid phase was studied by the XAS measurements with a radiation-induced effect which may cleave the phytol chain. Time-resolved N K-edge XAS measurements at the 100 ps timescale were used to examine the photorelaxation process of iron phenanthroline complexes in aqueous solution. Future directions of studies of various chemical and biological processes that involve metal complexes using N K-edge XAS measurements of ligands are discussed.

Simultaneous measurement methods for soft X-ray absorption spectroscopy (XAS) of solid–liquid interfaces and bulk liquids have been developed. The O K-edge XAS spectra of H2O/Au interfaces and bulk H2O were measured using a transmission-type liquid cell, in which the liquid layer was sandwiched between Au/Cr/Si3N4 and Si3N4 membranes. The XAS spectrum of bulk H2O was obtained using the transmission method by controlling the thickness of the liquid layer, whereas the XAS spectrum of the H2O/Au interface was obtained using the electron-yield method by measuring the drain currents from the Au surface after soft X-ray absorption. The XAS spectra of bulk H2O and the H2O/Au interfaces were differentiated to determine the appropriate measurement conditions for the solid–liquid interfaces interfaces.

A µ-oxo-bridged iron phthalocyanine dimer with eight peripheral tert-butyl substituents was investigated using soft X-ray absorption spectroscopy in its solid state. The O K-edge spectrum of the dimer showed a peak at 529.1 eV, corresponding to that of the Fe–µO–Fe core. The peak position is apparently at a lower energy than that of single-bonded oxygens, such as the oxygen of H2O (>535 eV), and is almost comparable to the peak of oxytitanium phthalocyanine, whose oxygen atom has a double-bond character (530.6 eV), despite the fact that the µO–Fe bonds of the µ-oxo-bridged dimer have a single-bond character. DFT calculations clarified that the peak of the µ-oxo dimer at 529.1 eV is assignable to the excitation to the antibonding orbital localized in the µO–Fe unit.

Carbon (C) K-edge X-ray absorption spectra for firefly luciferin were measured and assigned using time-dependent density functional theoretical calculations for luciferin anion and dianion to elucidate the effect of hydroxy-group deprotonation. It was found that the C K-edge spectra for luciferin had four characteristic peaks. The effect of deprotonation of the hydroxy group appears in the energy difference of the first and second peaks of these spectra. This energy difference is 1.0 eV at pH 7 and 2.3 eV at pH 10. The deprotonation of the hydroxy group can be distinguished based on the soft X-ray absorption spectra.

The cononsolvency mechanism of poly(N-isopropylacrylamide) (PNIPAM), dissolving in pure methanol (MeOH) and water (H2O) but being insoluble in MeOH-H2O mixtures, was investigated by O K-edge X-ray absorption spectroscopy (XAS). The cononsolvency emerges from the aggregation of PNIPAM with MeOH clusters, leading to the collapse of the hydrophobic hydration of PNIPAM.

Operando time-resolved soft x-ray absorption spectroscopy (TR-SXAS) is an effective method to reveal the photochemical processes of metal complexes in solutions. In this study, we have developed the TR-SXAS measurement system for observing various photochemical reactions in solutions by the combination of laser pump pulses with soft x-ray probe pulses from the synchrotron radiation. For the evaluation of the developed TR-SXAS system, we have measured nitrogen K-edge x-ray absorption spectroscopy (XAS) spectra of aqueous iron phenanthroline solutions during a photoinduced spin transition process. The decay process of the high spin state to the low spin state in the iron complex has been obtained from the ligand side by N K-edge XAS, and the time constant is close to that obtained from the central metal side by time-resolved Fe K-edge XAS in the previous studies.

A methodology for solution Al K-edge X-ray absorption spectroscopy (XAS) has been developed to investigate the coordination geometry and electronic structure of aluminium complexes in organic solvents. Using a controllable thin liquid flow cell under a helium atmosphere, Al K-edge XAS spectra of both penta- and hexacoordinate aluminium complexes were successfully obtained. Tris(acetylacetonato)aluminium (Al(acac)3) exhibited identical spectral features in various solvents, reflecting its rigid and symmetric octahedral structure. In contrast, an Al-salen complex displayed pronounced solvent-dependent spectral variations corresponding to coordination number changes induced by Lewis base coordination. In non-coordinating solvents such as 1,2-dichloroethane, the Al-salen complex retained a pentacoordinate structure, whereas coordination of strongly donating solvents such as DMSO led to the formation of a hexacoordinate species, accompanied by a high-energy shift in the absorption edge. Notably, these structural and electronic changes were detected by XAS, highlighting the superior sensitivity of Al K-edge XAS to local electronic environments. This study demonstrates the feasibility and utility of solution-phase Al K-edge XAS as a powerful spectroscopic probe for elucidating coordination and electronic structures of aluminium complexes.