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This review focuses on how short X-ray pulses from synchrotrons and XFELs can be used to track light-induced structural changes in molecular complexes and proteins via the pump–probe method. The upgrade of the European Synchrotron Radiation Facility to a diffraction-limited storage ring, based on the seven-bend achromat lattice, and how it might boost future pump–probe experiments are described. We discuss some of the first X-ray experiments to achieve 100 ps time resolution, including the dissociation and in-cage recombination of diatomic molecules, as probed by wide-angle X-ray scattering, and the 3D filming of ligand transport in myoglobin, as probed by Laue diffraction. Finally, the use of femtosecond XFEL pulses to investigate primary chemical reactions, bond breakage and bond formation, isomerisation and electron transfer are discussed.

Cooperativity is essential for the proper functioning of numerous proteins by allosteric interactions. Hemoglobin from Scapharca inaequivalvis (HbI) is a homodimeric protein that can serve as a minimal unit for studying cooperativity. We investigated the structural changes in HbI after carbon monoxide dissociation using time-resolved resonance Raman spectroscopy and observed structural rearrangements in the Fe–proximal histidine bond, the position of the heme in the pocket, and the hydrogen bonds between heme and interfacial water upon ligand dissociation. Some of the spectral changes were different from those observed for human adult hemoglobin due to differences in subunit assembly and quaternary changes. The structural rearrangements were similar for the singly and doubly dissociated species but occurred at different rates. The rates of the observed rearrangements indicated that they occurred synchronously with subunit rotation and are influenced by intersubunit coupling, which underlies the positive cooperativity of HbI. [ABSTRACT FROM AUTHOR]

Time-resolved serial femtosecond crystallography (TR-SFX) has emerged as a crucial tool for studying the structural dynamics of proteins. In principle, TR-SFX has the potential to be a powerful tool not only for studying proteins but also for investigating chemical reactions. However, non-protein systems generally face challenges in indexing due to sparse Bragg spots and encounter difficulties in effectively exciting target molecules. Nevertheless, successful TR-SFX studies on chemical systems have been recently reported in a few instances, boding well for the application of TR-SFX to study chemical reactions in the future. In this context, we review the static SFX and TR-SFX studies conducted on chemical systems reported to date and suggest prospects for future research directions. [ABSTRACT FROM AUTHOR]

Metal‐semiconductor interfaces are crucial components of optoelectronic and electrical devices, the performance of which hinges on intricate dynamics involving charge transport and mechanical interaction at the interface. Nevertheless, structural changes upon photoexcitation and subsequent carrier transportation at the interface, which crucially impact hot carrier stability and lifetime, remain elusive. To address this long‐standing problem, they investigated the electron dynamics and resulting structural changes at the Au/TiO2 interface using ultrafast electron diffraction (UED). The analysis of the UED data reveals that interlayer electron transfer from metal to semiconductor generates a strong coupling between the two layers, offering a new way for ultrafast heat transfer through the interface and leading to a coherent structural vibration that plays a critical role in propagating mechanical stress. These findings provide insights into the relationship between electron transfer and interfacial mechanical and thermal properties. [ABSTRACT FROM AUTHOR]

We herein report a fundamental mechanistic investigation into photochemical metal‐nitrenoid generation and inner‐sphere transposition reactivity using organometallic photoprecursors. By designing Cp*Ir(hydroxamate)(Ar) complexes, we induced photo‐initiated ligand activation, allowing us to explore the amidative σ(Ir–aryl) migration reactivity. A combination of experimental mechanistic studies, femtosecond transient absorption spectroscopy, and density functional theory (DFT) calculations revealed that the metal‐to‐ligand charge transfer enables the σ(N−O) cleavage, followed by Ir‐acylnitrenoid generation. The final inner‐sphere σ(Ir–aryl) group migration results in a net amidative group transposition. [ABSTRACT FROM AUTHOR]

Despite extensive research on utilizing plasmonic hot carriers to advance photovoltaics and photocatalysts, achieving high hot‐carrier flux remains challenging due to their rapid relaxation. Recent studies have shown that combining plasmonic metals with perovskites improves hot‐electron flow, due to the slow hot‐electron relaxation in perovskites. Additionally, perovskites offer the advantage of facile bandgap tuning through composition changes. Herein, the influence of tuning the perovskite bandgap on the lifetime and flow of hot electrons in a perovskite/plasmonic Au/TiO2 nanodiode is explored. The findings reveal that perovskites with wider bandgaps exhibit improved hot‐electron lifetime and flow, attributed to the modified hot‐electron energy favoring a slower energy loss rate, as verified by ultrafast transient absorption spectroscopic analysis. It is believed that the results successfully demonstrate the integration of engineered hot‐carrier physics into device functions, providing valuable guidance for the design of optimized hot‐carrier‐based devices in the future. [ABSTRACT FROM AUTHOR]

The structure of molecules, particularly the dynamic changes in structure, plays an essential role in understanding physical and chemical phenomena. Time-resolved (TR) scattering techniques serve as crucial experimental tools for studying structural dynamics, offering direct sensitivity to molecular structures through scattering signals. Over the past decade, the advent of x-ray free-electron lasers (XFELs) and mega-electron-volt ultrafast electron diffraction (MeV-UED) facilities has ushered TR scattering experiments into a new era, garnering significant attention. In this review, we delve into the basic principles of TR scattering experiments, especially focusing on those that employ x-rays and electrons. We highlight the variations in experimental conditions when employing x-rays vs electrons and discuss their complementarity. Additionally, cutting-edge XFELs and MeV-UED facilities for TR x-ray and electron scattering experiments and the experiments performed at those facilities are reviewed. As new facilities are constructed and existing ones undergo upgrades, the landscape for TR x-ray and electron scattering experiments is poised for further expansion. Through this review, we aim to facilitate the effective utilization of these emerging opportunities, assisting researchers in delving deeper into the intricate dynamics of molecular structures. [ABSTRACT FROM AUTHOR]

Three-dimensional graphene architectures with periodic nanopores--reminiscent of zeolite frameworks--are of topical interest because of the possibility of combining the characteristics of graphene with a three-dimensional porous structure (1-6). Lately, the synthesis of such carbons has been approached by using zeolites as templates and small hydrocarbon molecules that can enter the narrow pore apertures (7-15). However, pyrolytic carbonization of the hydrocarbons (a necessary step in generating pure carbon) requires high temperatures and results in non-selective carbon deposition outside the pores. Here, we demonstrate that lanthanum ions embedded in zeolite pores can lower the temperature required for the carbonization of ethylene or acetylene. In this way, a graphene-like carbon structure can be selectively formed inside the zeolite template, without carbon being deposited at the external surfaces. X-ray diffraction data from zeolite single crystals after carbonization indicate that electron densities corresponding to carbon atoms are generated along the walls of the zeolite pores. After the zeolite template is removed, the carbon framework exhibits an electrical conductivity that is two orders of magnitude higher than that of amorphous mesoporous carbon. Lanthanum catalysis allows a carbon framework to form in zeolite pores with diameters of less than 1 nanometre; as such, microporous carbon nanostructures can be reproduced with various topologies corresponding to different zeolite pore sizes and shapes. We demonstrate carbon synthesis for large-pore zeolites (FAU, EMT and beta), a one-dimensional medium-pore zeolite (LTL), and even small-pore zeolites (MFI and LTA). The catalytic effect is a common feature of lanthanum, yttrium and calcium, which are all carbide-forming metal elements. We also show that the synthesis can be readily scaled up, which will be important for practical applications such as the production of lithium-ion batteries and zeolite-like catalyst supports., Zeolites are a family of microporous crystalline aluminosilicate materials, which fall into more than 200 structural types (16). Each structural type is distinguished by its unique pore structure--for example, in [...]

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 (1-3), 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).sub.2.sup.-]].sub.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 (4-6), 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 ~500 femtoseconds. We also determined the three-dimensional structures of reaction intermediates with sub-angstrom 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 (7) and advanced analysis of time-resolved solution scattering data., The functional efficiencies of photoactive molecules are governed by long-lived electronic excited states that are directly involved in functional transitions of the molecules. In fact, early stages of their photoinduced [...]

In this work, we present a detailed investigation on the recombination dynamics of mercury halides HgX2 (X = I, Br) in acetonitrile solution after UV-induced photodissociation. The study is performed by combining time-resolved wide-angle x-ray scattering (TRWAXS) and optical transient absorption spectroscopy. Up to 68% of the UV (266 nm) photodissociated HgX and X radicals that escape the solvent cage surrounding parent HgX2 recombine within a nanosecond after photodissociation. In contrast to classical primary geminate recombination, occurring on much faster time scales, we interpret the sub-nanosecond recombination channel as secondary geminate recombination (SGR), also referred to as diffusion-limited geminate recombination. The family of triatomic mercury halides therefore represents an important class of molecules to study chemical mechanisms of solvent-dependent SGR by TRWAXS. The methodology described here allows for direct mapping of the time-dependent inter-radical distance distribution function, a critical parameter for the assessment of the SGR dynamics in solution phase and solvation in general. [ABSTRACT FROM AUTHOR]

Diiodomethane, CH2I2, in a polar solvent undergoes a unique photoinduced reaction whereby I2− and I3− are produced from its photodissociation, unlike for other iodine-containing haloalkanes. While previous studies proposed that homolysis, heterolysis, or solvolysis of iso-CH2I–I, which is a major intermediate of the photodissociation, can account for the formation of I2− and I3−, there has been no consensus on its mechanism and no clue for the reason why those negative ionic species are not observed in the photodissociation of other iodine-containing chemicals in the same polar solvent, for example, CHI3, C2H4I2, C2F4I2, I3−, and I2. Here, using time-resolved X-ray liquidography, we revisit the photodissociation mechanism of CH2I2 in methanol and determine the structures of all transient species and photoproducts involved in its photodissociation and reveal that I2− and I3− are formed via heterolysis of iso-CH2I–I in the photodissociation of CH2I2 in methanol. In addition, we demonstrate that the high polarity of iso-CH2I–I is responsible for the unique photochemistry of CH2I2. [ABSTRACT FROM AUTHOR]