Polarized X-ray Absorption Near-edge Structure Research Articles

Ultrafast XANES Monitors Femtosecond Sequential Structural Evolution in Photoexcited Coenzyme B12.

Polarized X-ray absorption near-edge structure (XANES) at the Co K-edge and broadband UV-vis transient absorption are used to monitor the sequential evolution of the excited-state structure of coenzyme B12 (adenosylcobalamin) over the first picosecond following excitation. The initial state is characterized by sub-100 fs sequential changes around the central cobalt. These are polarized first in the y-direction orthogonal to the transition dipole and 50 fs later in the x-direction along the transition dipole. Expansion of the axial bonds follows on a ca. 200 fs time scale as the molecule moves out of the Franck-Condon active region of the potential energy surface. On the same 200 fs time scale there are electronic changes that result in the loss of stimulated emission and the appearance of a strong absorption at 340 nm. These measurements provide a cobalt-centered movie of the excited molecule as it evolves to the local excited-state minimum.

Probing the Excited State of Methylcobalamin Using Polarized Time-Resolved X-ray Absorption Spectroscopy.

We use picosecond time-resolved polarized X-ray absorption near-edge structure (XANES) measurements to probe the structure of the long-lived photoexcited state of methylcobalamin (MeCbl) and the cob(II)alamin photoproduct formed following photoexcitation of adenosylcobalamin (AdoCbl, coenzyme B12). For MeCbl, we used 520 nm excitation and a time delay of 100 ps to avoid the formation of cob(II)alamin. We find only small spectral changes in the equatorial and axial directions, which we interpret as arising from small (<∼0.05 Å) changes in both the equatorial and axial distances. This confirms expectations based on prior UV-visible transient absorption measurements and theoretical simulations. We do not find evidence for the significant elongation of the Co-C bond reported by Subramanian [ J. Phys. Chem. Lett. 2018 , 9 , 1542 - 1546 ] following 400 nm excitation. For AdoCbl, we resolve the difference XANES contributions along three unique molecular axes by exciting with both 540 and 365 nm light, demonstrating that the spectral changes are predominantly polarized along the axial direction, consistent with the loss of axial ligation. These data suggest that the microsecond "recombination product" identified by Subramanian et al. is actually the cob(II)alamin photoproduct that is produced following bond homolysis of MeCbl with 400 nm excitation. Our results highlight the pronounced advantage of using polarization-selective transient X-ray absorption for isolating structural dynamics in systems undergoing atomic displacements that are strongly correlated to the exciting optical polarization.

Strain Engineering for Anion Arrangement in Perovskite Oxynitrides.

Mixed-anion perovskites such as oxynitrides, oxyfluorides, and oxyhydrides have flexibility in their anion arrangements, which potentially enables functional material design based on coordination chemistry. However, difficulty in the control of the anion arrangement has prevented the realization of this concept. In this study, we demonstrate strain engineering of the anion arrangement in epitaxial thin films of the Ca1-xSrxTaO2N perovskite oxynitrides. Under compressive epitaxial strain, the axial sites in TaO4N2 octahedra tend to be occupied by nitrogen rather than oxygen, which was revealed by N and O K-edge linearly polarized X-ray absorption near-edge structure (LP-XANES) and scanning transmission electron microscopy combined with electron energy loss spectroscopy. Furthermore, detailed analysis of the LP-XANES indicated that the high occupancy of nitrogen at the axial sites is due to the partial formation of a metastable trans-type anion configuration. These results are expected to serve as a guide for the material design of mixed-anion compounds based on their anion arrangements.

Nature of empty states in superconducting CaC6and related Li-Ca ternary graphite intercalation compounds using polarized x-ray absorption near-edge structure at the CaKedge

The angular dependence of the x-ray absorption near-edge structure (XANES) was recorded at the calcium $K$ edge in three graphite intercalated compounds: CaC${}_{6}$, Li${}_{3.1}$Ca${}_{2.1}$C${}_{6}$ (phase $\ensuremath{\beta}$), and Li${}_{0.4}$Ca${}_{2.7}$C${}_{6}$ (phase $\ensuremath{\alpha}$). The polarized XANES spectra provide experimental pictures of the Ca ${p}_{x,y}$ and ${p}_{z}$ empty states of these three materials. In the case of CaC${}_{6}$, first-principles calculations based on density functional theory were performed, including XANES modeling and projected density-of-state simulations. A good agreement is obtained between theoretical and experimental polarized spectra. This enables us to assign the main XANES peaks in terms of orbital hybridization. The thorough analysis of the CaC${}_{6}$ polarized XANES spectra is used to interpret the spectral differences observed between the three compounds. The absence of pre-edge for all these phases indicates that Ca atoms are in locally centrosymmetric sites. The presence of Li in the graphitic interlayer drastically reduces the anisotropy of the Ca $p$ empty states, as compared to CaC${}_{6}$. Structural and electronic information is extracted from the spectra of the $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ phases, leading to substantial results, in absence of fully determined crystallographic structures.

Comparative study on LB films and crystals of TTF derivatives with long alkyl chains: In-plane structure

Polarized X-ray absorption near-edge structure (XANES) was studied for iodine L I-edge on the single crystal of [EDT-TTF(SC 18) 2] 2I 3 which has identical physical properties to the Langmuir-Blodgett (LB) films. The polarization dependence of XANES spectra showed that I − 3 anions incline 10 ° from the conducting axis ( b axis). Incorporating the present result into the lateral packing model of EDT-TTF(SC 18) 2, we proposed a possible molecular arrangement of this system.

X-ray absorption and diffraction investigations of iodine-intercalated conducting Langmuir-Blodgett film

X-Ray absorption and diffraction have been used to study a Langmuir-Blodgett film of an iodine-intercalated charge transfer (CT) salt based on EDT-TTF(SC 18) 2. The polarized X-ray absorption near-edge structure (XANES) of I 3 − exhibits a strong angular dependence of the 2s–5p(σ ∗) resonance intensity. To obtain precise information about the I 3 − orientation, the observed polarization dependence is compared with the theoretical calculation. It is shown that the I 3 − molecular axes are distributed randomly in a plane parallel to the substrate. From the viewpoint of the I 3 − distribution, the effect of annealing is also investigated with polarized XANES spectroscopy and X-ray diffraction. It is confirmed that an annealing process is necessary to achieve a well-stabilized and highly conducting film of the present CT salt. In addition, a possible molecular assembly of this annealed purple phase is proposed via a simulation of the X-ray diffraction profile based on the XANES results.

ChemInform Abstract: Polarized X‐Ray Absorption Near‐Edge Structure of Highly Oxidized Chromium Porphyrins

Chemischer InformationsdienstVolume 17, Issue 44 Physical Organic Chemistry ChemInform Abstract: Polarized X-Ray Absorption Near-Edge Structure of Highly Oxidized Chromium Porphyrins J. E. PENNER-HAHN, J. E. PENNER-HAHNSearch for more papers by this authorM. BENFATTO, M. BENFATTOSearch for more papers by this authorB. HEDMAN, B. HEDMANSearch for more papers by this authorT. TAKAHASHI, T. TAKAHASHISearch for more papers by this authorS. DONIACH, S. DONIACHSearch for more papers by this authorJ. T. GROVES, J. T. GROVESSearch for more papers by this authorK. O. HODGSON, K. O. HODGSONSearch for more papers by this author J. E. PENNER-HAHN, J. E. PENNER-HAHNSearch for more papers by this authorM. BENFATTO, M. BENFATTOSearch for more papers by this authorB. HEDMAN, B. HEDMANSearch for more papers by this authorT. TAKAHASHI, T. TAKAHASHISearch for more papers by this authorS. DONIACH, S. DONIACHSearch for more papers by this authorJ. T. GROVES, J. T. GROVESSearch for more papers by this authorK. O. HODGSON, K. O. HODGSONSearch for more papers by this author First published: November 4, 1986 https://doi.org/10.1002/chin.198644038AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume17, Issue44November 4, 1986 RelatedInformation

Polarized x-ray absorption near-edge structure of highly oxidized chromium porphyrins

Polarized X-ray absorption near-edge spectra have been measured for Cr/sup IV/(TTP)O and Cr/sup V/(TTP)N (TTP is the dianion of 5,10,15,20-tetra-p-tolylporphyrin). These spectra, which are very similar for the two molecules, show an intense preedge absorption feature polarized perpendicular to the porphyrin plane. The absorption edges are relatively featureless when the polarization is parallel to the porphyrin plane. On the basis of multiple-scattered-wave X..cap alpha.. calculations, the intense preedge feature is interpreted as a bound ..-->.. bound transition with significant metal character in the excited state. This transition, which has been observed in many metal-oxo species, was not observed in the putative ferryl (Fe=o) intermediates of horseradish peroxidase. The reasons for this absence in Fe=O species and its relationship to the reactivity of metal-oxo porphyrin complexes are discussed in light of the present results.