Linac Coherent Light Source

Ultrafast control of functional materials by light is key for developing optoelectronic devices. Still, the mechanisms by which microscopic photoinduced excitations evolve into a macroscopic phase transformation remain poorly understood, partly due to the experimental challenges of isolating multiscale electronic and structural dynamics. Here we use femtosecond X-ray techniques to track the non-equilibrium dynamics leading to macroscopic transformation in a ferroelastic charge-transfer Prussian blue analogue. The experimental data evidence a sequence of phenomena. The initial electronic excitation leads to reverse Jahn-Teller distortion within 50 fs, causing intermetallic charge-transfer polarons within 200 fs. Polarons generate considerable lattice strain and trigger phase nucleation within 60 ps. This elastically driven cooperativity, launched by the photoinduced polarons, offers an efficient route to the generation and stabilization of photoinduced phases in many volume-changing materials.

ZUSAMMENFASSUNG Die Photopharmakologie entwickelt lichtresponsive Wirkstoffe zur gezielten Modulation von Proteinaktivitäten, die präzise und dynamische Untersuchungen ihrer Funktionen in Gesundheit und Krankheit ermöglichen. Adrenerge Rezeptoren sind hierfür besonders geeignete Ziele, da sie prototypische G‐Protein‐gekoppelte Rezeptoren mit hoher klinischer Relevanz für bronchiale und kardiovaskuläre Erkrankungen darstellen. In dieser Studie kombinierten wir die azobenzenbasierte Verbindung Photoazolol‐1 mit zeitaufgelöster serieller Kristallographie an Röntgen‐Freie‐Elektronen‐Lasern, um die molekularen Mechanismen aufzuklären, durch die photoschaltbare β‐Blocker die Aktivität des β 2 ‐Adrenozeptors (β 2 AR) modulieren. Zeitaufgelöste Strukturen des Rezeptors in Komplexen mit trans‐Photoazolol‐1 (vor der Photokonversion), einem gespannten Zwischenzustand im Nanosekundenbereich sowie vollständig photoisomerisiertem cis‐Photoazolol‐1 zeigen, wie die Isomerisierung der Azobenzeneinheit unterschiedliche konformationelle Veränderungen innerhalb der orthosterischen Ligandenbindetasche induziert. Innerhalb von Sekunden nimmt lichtangeregtes Photoazolol‐1 eine neue Bindepose ein, wodurch Wechselwirkungen mit der extrazellulären Schleife 2 verändert und die Positionen der Transmembranhelices 5, 6 und 7 verschoben werden. Funktionelle Assays des β 2 AR in zellulären Membranen belegen, dass Photoazolol‐1 als Wirksamkeits‐Photoschalter fungiert, der sich bei der Isomerisierung von einem inversen Agonisten zu einem neutralen Antagonisten wandelt, ohne die Bindetasche zu verlassen. Zusammengenommen weisen diese Ergebnisse auf einen molekularen Mechanismus der Aktivitätsmodulation durch Wirksamkeits‐Photoschalter hin und liefern einen konzeptionellen Rahmen für das Design von Liganden, die lichtgetriebene Übergänge innerhalb der Bindetasche zur räumlichen und zeitlichen Kontrolle der Rezeptorfunktion nutzen.

Protein-hydroxylation catalysed by Fe(II) and 2-oxoglutarate (2OG) dependent oxygenases is an important regulatory mechanism in human biology. Such oxygenases typically coordinate their Fe(II) cofactor via a conserved triad of an aspartate- or glutamate- and two histidine-residues. By contrast, aspartate/asparagine β-hydroxylase (AspH), which catalyses asparagine/aspartate-residue oxidation in epidermal growth factor-like domains (EGFDs), has only two histidine-residues (H679, H725), with a water occupying the site normally occupied by an aspartate- or glutamate-residue. We describe mechanistic studies with catalytically active AspH crystals. Turnover studies with single crystals under cryogenic conditions give (3 R)-hydroxylated EGFDs with the product alcohol coordinating Fe(II) trans to H725. Time-resolved serial crystallography of microcrystals using an acoustic droplet ejection system, coupled to X-ray emission analyses, demonstrate turnover within 1.5 s, giving a product complex in which Fe(II) is regenerated. Solution and crystallographic studies with the O2 surrogate nitric oxide imply O2 binds to Fe(II) trans to H725. The additional Fe-chelating water is maintained throughout AspH catalysis and is not directly involved in substrate hydroxylation, because O2 is the sole oxygen source in alcohol products, as shown by 18O labelling studies. The results reveal how AspH accommodates both aspartate- and asparagine-substrates and will assist in efforts targeting AspH for cancer treatment.

Recent advancements in photocatalysis, photovoltaics, and quantum information science take advantage of electron spin, and determining how spin multiplicity affects electron transfer is key to understanding these phenomena. In this study, we examine how metal spin state affects electronic communication in an organometallic mixed-valence dimer, ferrocenyl cobaltocenium ([FeIICp2CoIIICp2]+). This complex can be photoexcited from its low-spin singlet FeII ground state to form intermediate-spin triplet FeII and high-spin quintet FeII excited states. Using femtosecond optical transient absorption (OTA) spectroscopy with visible (vis), near-infrared (NIR), and short-wave IR (SWIR) probes, supported by time-dependent density functional theory (TD-DFT) calculations, we measure FeIICoIII→FeIIICoII intervalence charge transfer (IVCT) bands in each of the FeII spin states. Mulliken-Hush analysis of the excited-state IVCT bands was used to compute the electronic coupling between the metal centers in all three spin states, which increased as quintet < triplet < singlet. Meanwhile, the peak energy of the bands, and thus the ΔG of the IVCT transition, increased as triplet < quintet < singlet. Then, to directly probe the electronic structure at each metal center, we employed picosecond soft X-ray transient absorption (XTA) spectroscopy at the Fe and Co L3 edges. Our results show that the low-spin and high-spin states of [FeIICp2CoIIICp2]+ are valence-localized, while the intermediate-spin state is partially delocalized. The differences in charge delocalization are attributed to differences in orbital occupation and geometry that affect the free energy and superexchange coupling.

Abstract Three-dimensional velocity-map imaging and covariance analysis were used to identify over 50 fragmentation channels produced following the inner-shell ionization of selenophene at 120 eV. The majority resulted in two or three 'heavy' products containing selenium or carbon, many of which had identical mass-to-charge ratios but different chemical compositions due to the degree of hydrogenation and the selenium isotope involved. Covariance analysis was used to isolate these reaction channels and to provide estimates of their relative yields. In combination with prior similar studies on thiophene and furan, the current results indicate that the nature of the heteroatom significantly influences the charge redistribution and bond cleavage dynamics induced by the Auger-Meitner process, and demonstrate the sensitivity of inner-shell ionization dynamics to the molecular and electronic structures of heterocyclic systems.

Proton transfer is a fundamental process underlying chemical and biological phenomena, and its dynamics are significantly influenced by the surrounding environment. This paper studies the excited state intramolecular proton transfer (ESIPT) process, which is crucial to the photostability of hydroxyanthraquinone-based pigments through efficient energy dissipation, by investigating how crystalline packing influences photoinduced proton transfer dynamics in single crystals of dihydroxyanthraquinone (DHAQ) constitutional isomers. Comparing the proton transfer dynamics in crystalline and solution phases, we show substantial differences due to the crystalline environment, particularly in the 1,4- and 1,5-DHAQ isomers. These isomers show intermolecular hydrogen bonding within the crystal lattice, resulting in larger excitonic couplings, which significantly alters their reaction pathways compared to their behavior in solution. We show that 1,8-DHAQ, which does not form intermolecular hydrogen bonds in the crystal, shows minimal changes in dynamics between the phases. In contrast, in the case of 1,4-DHAQ, intermolecular interactions within the molecular crystal phase open up an ESIPT relaxation channel, which is not observed in the solution phase. These findings highlight the critical role of crystal packing in modulating proton transfer dynamics and offer insights into how molecular packing can be strategically manipulated to control and optimize reaction pathways in solid-state environments.

Abstract We report on the coherence properties and characteristics of the split-and-delay unit at the Materials Imaging and Dynamics instrument of the European XFEL under seeded-beam conditions. Our investigation focuses on the speckle contrast extracted from the scattering patterns from static scatterers and pulse splitting characteristics. Seeded-beam operation enabled a high throughput of the split-and-delay unit. We highlight the invaluable potential of the split-and-delay unit for experimental investigations for enhancing our understanding of ultrafast phenomena in molecular liquids, such as water and aqueous solutions.

In this work, we describe the instrumentation used to perform the first operando transmission X-ray microscopy (TXM) and simultaneous X-ray diffraction of laser melting simulating laser powder bed fusion on the XCS instrument at the Linac Coherent Light Source (LCLS) X-ray free-electron laser (XFEL). Our TXM with 40× magnification in the X-ray regime at 11 keV gave spatial resolutions down to 940 nm per line pair, with effective pixel sizes down to 206 nm, image integration times of <100 fs, and frame rates tunable between 2.1 and 119 ns for two probe frames (0.48 GHz to 8.4 MHz). Images were recorded on Zyla and Icarus (UXI) detectors to trade off between spatial resolution and time dynamics. A 1 kW CW IR laser was coupled into the interaction point to conduct pump-probe studies of laser melting and solidification dynamics. Our temporal and spatial resolution with attenuation-based contrast exceeds that currently possible with synchrotron-based high-speed radiography. This system was sensitive to feature velocities of 10-12000 m s-1 but we did not observe any motion in this range in the laser melting of Al6061 alloy. Shockwaves were not observed and hot cracking proceeded at velocities below the detection limits. Pore accumulation was observed between successive shots, indicating that bubble escape mechanisms were not active. With proper experimental design, the spatial resolution, contrast and field of view could be further improved or modified. The increased brightness and narrower bandwidth of the XFEL allowed for this imaging technique and it lays the groundwork for a wide range of operando techniques to study additive manufacturing.

Charge transport in materials has an impact on a wide range of devices based on semiconductor, battery, or superconductor technology. Charge transport in sliding charge density waves (CDW) differs from all others in that the atomic lattice is directly involved in the transport process. To obtain an overall picture of the structural changes associated to the collective transport, the large coherent x-ray beam generated by an x-ray free-electron laser (XFEL) source was used. The CDW phase can be retrieved over the entire sample from diffracted intensities using a genetic algorithm. For currents below threshold, increasing shear deformation is observed in the central part of the sample while longitudinal deformation appears above threshold when shear relaxes. Shear thus precedes longitudinal deformation, with relaxation of one leading to the appearance of the other. Moreover, strain accumulates on surface steps in the sliding regime, demonstrating the strong pinning character of these surface discontinuities. The sliding process of nanometric CDW involves macroscopic sample dimensions.