Observing light-induced reactions in liquids, where much of chemistry happens, and on the timescales during which these reactions occur, has been a long-standing objective. Knowledge of the mechanistic details at the molecular scale can inform rational design of new molecules for applications such as light harvesting and energy conversion and illuminate new means of controlling reactions. We have been exploring how time-resolved x-ray spectroscopy can be used to understand fundamental mechanisms underlying ligand exchange, a reaction common to coordination complexes in solution. Our recent focus has been on the aquation reaction of ferrous hexacyanide in water, where absorption of UV light initiates an exchange of a CN- ligand with a water molecule from the solvent. Using the MHz repetition-rate laser-pump/x-ray-probe capabilities our group has built at the Advanced Photon Source we have investigated the reaction using both absorption spectroscopy at the Fe K-edge and nonresonant Fe 1s emission spectroscopy. Pre-edge resonances sensitive to geometry revealed a 20-ps lived pentacoordinated intermediate species. QM/MM molecular dynamics simulations explain the rather long time for aquation as being due to fluctuations in the geometry of the pentacoordinated complex that leave very short time intervals when there is room for a water molecule to bond. The K alpha, K beta, and valence-to-core emission spectra yield complementary information beyond geometry for the species involved. The measured K beta line confirms the triplet spin state of the pentacoordinated intermediate, and also reveals a subtle spectral difference for the low-spin aquated product compared to the ground state complex. This work was supported by the US Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division.

Organic dyes present a promising alternative to the more expensive ruthenium and iridium complexes commonly used in photoredox reactions. Ideally, such a framework should offer facile tunability of the excited state redox potential while maintaining sufficiently long excited state lifetimes for intermolecular charge transfer. Here we have augmented the coumarin derivative 4-methylesculetin with dipicolylamine to form a tetrahedral binding pocket that can then coordinate different divalent first-row transition metals, allowing us to tune the excited state redox potential by simply adding a salt. Using X-ray transient absorption spectroscopy, we have observed photoinduced reductive shifts in the K-edge spectra of the corresponding complexes of Mn²⁺ through Zn²⁺ that are consistent with the varying degrees of intramolecular charge transfer to the metals predicted by density functional theory calculations. We have also combined these X-ray measurements with optical transient absorption spectroscopy to characterize the relaxation dynamics of these complexes on timescales ranging from 100s of femtoseconds to 10s of microseconds. As expected, the Zn²⁺ complex exhibits little to no charge transfer character, and any electronic or nuclear rearrangement at the metal site fully relax within the temporal resolution of our measurement. On the other hand, all other metal complexes exhibit long-lived charge transfer states that persist for 100s of nanoseconds. Notably, the relaxation dynamics of the Co²⁺ complex include an additional time component of approximately 10 nanoseconds that is entirely absent in the other metal complexes.

Using transient absorption (TA) spectroscopy with a barrage of spectral probes, we investigate the charge recombination dynamics following intervalence charge transfer (IVCT) excitation of a heterobimetallic Fe(II)Co(III) molecule. Femtosecond optical probes in the near-infrared and visible regions reveal long-lived excited states following IVCT excitation. In order to determine the electronic structure at each metal center in these excited states, we turn to element-specific extreme ultraviolet (XUV) and X-ray techniques. Femtosecond XUV TA at the Fe and Co M2,3 edges demonstrates that a local, Fe-centered, triplet (d-d) state is formed within 1 picosecond of IVCT excitation. This observation uncovers a unique IVCT relaxation mechanism whereby ultrafast intersystem crossing and back electron transfer populates a local, triplet excited state instead of the singlet ground state. The triplet state then evolves to a long-lived excited state, which we examined with hard and soft X-ray probes from synchrotron sources. Picosecond soft X-ray TA at the Fe and Co L3 edges shows that this state is also an Fe-centered (d-d) state. Hard X-ray TA at the Fe and Co K-edges reveal large bond length elongations of the Fe coordination sphere that are consistent with a high-spin quintet state. Altogether, these experiments highlight the importance of local, Fe-centered triplet and quintet (d-d) states on the IVCT relaxation dynamics of this heterobimetallic complex. By employing a diverse range of spectral probes, we were able to track charge and spin at each metal center over a broad range of timescales, which enabled the elucidation of this unique IVCT relaxation mechanism.

Electronic excited states enables novel non-equilibrium pathways to chemical transformations. The complexity of potential outcomes for light-driven reactions has made harnessing electronic excited states for chemistry challenging. One approach to addressing this challenge is advancing the tools we have for capturing the complex dynamics of electronic excited states. I will discuss how advances in ultrafast x-ray sources and experimental methods provide a pathway to capturing the non-equilibrium trajectories of electronic excited states with precision, particularly for transition metal complexes. Of particular importance for 3d metal compounds is characterizing the population dynamics of charge-transfer (CT) and metal-centered (MC) electronic excited states and understanding how the inner coordination sphere structural dynamics mediate the interaction between these states. I will focus on the value of using simultaneous X-ray emission spectroscopy (XES) and X-ray solution scattering (XSS) studies to disentangle the electronic and structural dynamics.

Recent spectro-electrochemical studies have shown that the Ru-Ru dimer, [Ru(tpy)(bpy)(μ-CN)Ru(bpy)₂(CH₃CN)]³⁺(tpy = terpyridine, bpy = bipyridine), exhibits Class III mixed valency (complete delocalization between metal centers) upon one electron oxidation. Photochemically, this could be achieved by metal-to-ligand-charge transfer (MLCT) excitation where one Ru center is oxidized while one of the polypyridine ligands is reduced. Characterizing the extent of electron delocalization on the excited states of donor-bridge-acceptor complexes remains an important target in the field of mixed-valence chemistry. Transient X-ray spectroscopies are well suited to investigate electron delocalization processes as they offer element and orbital specific probes of local electronic and atomic structure. Here, we utilized solution phase, femtosecond, Ru L₃-edge resonant inelastic X-ray scattering, performed at the Alvra Prime end station of the Swiss X-ray Free Electron Laser, to probe valence electron and hole motions during the ultrafast delocalization process following MLCT excitation. A complementary study using transient Infrared spectroscopy was done to determine role of the CN bridge in the delocalization process. Additionally, TD-DFT calculations of spectroscopic observables were performed to help interpret and describe the X-ray and infrared experiments.

One of the challenging problems in the field of mixed valence complexes is determining the extent of electronic delocalization between metal centers in the ground and excited states. In this study, we applied femtosecond transient Fe K-edge X-ray absorption spectroscopy measurement on a tri-metallic, mixed valence complex, [L₄Ru^II(NC-Fe^III(CN)₅)₂]⁴⁻ (L=pyridine and 4-methoxypyridine) , in methanol. The experiment was performed at X-ray Pump Probe end station of Linac Coherent Light Source. We observed 0.3 eV and 1.5 eV red shifts for the Fe 1s to 3d e_g and ligand π* transition, confirming the delocalized nature of the excited metal-to-metal charge transfer state. The appearance of a spectral feature at 7107.5 eV in the excited state was first reported, suggesting that a short-lived transition was enabled unique to the trimer system. Complementary TD-DFT calculations were performed to visualize the transition orbitals and interpret experimental results.

Photo-assisted catalysis using Ni complexes is an emerging field for cross-coupling reactions in organic synthesis. however, the mechanism by which light enables and enhances reactivity is poorly understood. Optical techniques can lend insight into the ground- and excited-states of the photocatalysts but lack the specificity to interrogate electronic and geometric structural changes at specific atoms. Through static and transient L- and K-edge X-ray absorption spectroscopy of a prototypical bipyridine-based Ni(II) photocatalyst, we are able to determine that the ground-state of complex features an unexpected mixed-spin character. We are also able to unambiguously determine that the long-lived (~5 ns) excited state is a tetrahedral metal-centered triplet state. The mixed-spin character of the ground state can have profound impact on the excited-state properties and reactivity that are not well understood. In addition, these findings can drive future synthetic design of improved photocatalysts via judicious tuning of the electronic and geometric properties of the ligands.

We present a sub-picosecond resolved investigation of the structural solvent reorganization and geminate recombination dynamics of photogenerated halogen atoms in aqueous solutions. Nascent iodine radicals were generated via 400 nm 2-photon ionization of the parent iodide, while nascent bromine and chlorine atoms were generated via 1-photon ionization with 200 nm light. Time-resolved X-ray Absorption Near Edge Structure Spectroscopy around the L₁-edge of the photogenerated nascent iodine atoms (I⁰) delivered kinetic traces in agreement with a purely diffusion-driven geminate iodine-electron recombination model without the need of a long-lived (I⁰:e⁻) contact pair [1]. Nonequilibrium classical MD simulations indicate a delayed (ca. 100 fs) response of the caging H₂O solvent shell supported by the structural analysis of the X-ray Solution Scattering data. In-house QM/MM simulations [2] indicate a decreasing cage orientation time in the homologue series from Cl to I, which may be due to a more rigid H bond network around the smaller halogen atom. We will compare the femtosecond photodetachment results on iodide, measured at the LCLS X-ray Free Electron Laser (XFEL), with those on aqueous bromide ions, measured at SACLA XFEL, together with laser-only spectroscopy measurements on the nascent solvated electrons. [1] P. Vester, K. Kubicek et al., J. Chem. Phys. 157, 224201 (2022); [2] M. Reidelbach et al., J. Phys. Chem. B 127, 1399–1413 (2023)

Pump-probe spectroscopy has been utilized for decades to take ultrafast “snapshots” of molecular systems as they traverse their excited state surfaces. Recently, linearly polarized pump and probe pulses have been manipulated to separate “magic angle” X-ray spectra from isotropic solutions into two components: signal from the transition dipole moment direction and signal from the two orthogonal directions. This polarized ultrafast spectroscopy provides more electronic and/or structural information compared to using magic angle. In the study presented here, polarized X-ray absorption near-edge structure (XANES) at the Cobalt K-edge and UV-visible transient absorption (TA) spectroscopy were used to monitor the evolution of the excited-state structure of n-propylcobalamin following visible excitation. Finite difference method near edge structure (FDMNES) is used in conjunction with polarized XANES experimental data to correlate changes in spectral features to changes in the structure of n-propylcobalamin on a subpicosecond timescale. These measurements provide a cobalt-centered movie of the excited molecule as it evolves to the local excited-state minimum.

Adenosylcobalamin (AdoCbl) is a highly light-sensitive member of the cobalamin family. It features a central cobalt coordinated to a corrin ring, a 5’-deoxyadenosyl upper ligand, and a dimethylbenzimidazole lower ligand. The photochemistry of AdoCbl is important due to its central role as the chromophore in the CarH gene regulatory photoreceptor found in various bacterial species. Time-resolved X-ray absorption near-edge structure (XANES) examines the excited states of AdoCbl through the lens of atomic structural change from the central cobalt’s perspective. The use of polarization selection deconvolves the changes into orthogonal molecule-defined directions, yielding more specific structural information instrumental in a thorough analysis with higher confidence. Extensive time-resolved XANES data have been collected on AdoCbl in both water and ethylene glycol solvent. To assign specific spectral features to structural changes in the AdoCbl, the XANES spectra are simulated and compared with the experimental difference spectra using the finite difference method near-edge structure (FDMNES) method. FDMNES allows for systematic investigation of the structural manipulations required to reproduce the experimental difference spectra. These simulations help to quantitatively uncover the sequential structural evolution of AdoCbl on an ultrafast timescale as a function of solvent, allowing for a better understanding of the fundamental relationship between this cofactor and its environment as well as the relationship between XANES spectral features and molecular structure in general.

r0pt Figure Understanding the elementary steps following ionization in aqueous systems provides a framework for radiation-matter interactions in chemistry and biology. However, a microscopic understanding of the reaction mechanisms in the relevant physico-chemical time regime is missing as typical techniques, EPR, and UV spectroscopies, lack either time resolution or spectral clarity. A powerful two-color sub-femtosecond time-resolved X-ray pump/X-ray probe scheme developed at LCLS provides a qualitatively new window to systematically understand the electronic and nuclear dynamics following outer-, inner-valence, and core ionization in aqueous systems. Recently we investigated radiation-induced reactions in liquid water by X-ray transient absorption in a sheet jet using sub-fs XLEAP pulses on the ChemRIXS beamline at LCLS. The x-ray pump (255 eV to 275 eV) produces outer- and inner-valence holes, and the probe (510-550 eV) covers the valence hole through oxygen K-edge absorption. Starting from 0.6 fs delay absorption spectra were measured to capture ultrafast processes, e.g. Auger decay, intermolecular Coulombic decay (ICD), electron transfer mediated decay (ETMD), and proton transfer.