Martin Meedom Nielsen

Solvation plays a key role in many photochemical and biological processes. Yet, direct atomic-scale observation of solvent reorganization around photoexcited molecules containing only light atoms has so far remained elusive. Here, we use time-resolved X-ray scattering at an X-ray free-electron laser to track with a ~ 120 fs time resolution the photoinduced ultrafast rearrangement of polar acetonitrile solvent molecules around an organic hemithioindigo chromophore. The experiments reveal that the solvation shell reorganizes with a ~ 0.3 ps time constant driven by photoinduced charge transfer in the chromophore, and then reequilibrates over ~ 3 ps as the excited state population returns to the ground state. These results demonstrate the viability of using ultrashort X-ray pulses to directly visualize the motion of light atoms in solution, paving the way for atomic-scale understanding and control of photoinduced processes in biological and synthetic photoactive organic molecules.

With the start of the user program at the European XFEL in 2017, and more recently with the LCLS-II upgrade, the X-ray repetition rate at X-ray free-electron lasers (XFELs) has been pushed into the kilo- and megahertz regimes. These high X-ray repetition rates provide an increase in the integrated flux at these facilities by orders of magnitude, potentially facilitating measurements that were previously infeasible due to signal-to-noise constraints. However, the high repetition rates lead to new challenges for sample delivery and a shorter time for the sample to recover between X-ray pulses. For solution-phase techniques, the X-ray–sample interactions will strongly perturb or even vaporize the sample jet. Although the sample can be replenished, up to X-ray repetition rates of ∼100 kHz, by flowing the jet at high speeds, this does not completely mitigate the jet perturbations. In this work, we present a characterization of the jet perturbations induced by the high X-ray repetition rates at the European XFEL. We show how these can introduce background signals in time-resolved X-ray solution scattering data measured at the Femtosecond X-ray Experiments (FXE) instrument. We show that it is possible to mitigate these experimental artifacts by employing an alternating excitation scheme combined with careful background subtraction and that implementing this approach in the experimental design outperforms more simple background subtraction schemes. The methodology, the observations and analysis results are discussed in relation to the evolving landscape of XFEL sources.

Solvation plays a key role in photoinduced charge transfer, a fundamental process in many photochemical and biological systems. However, direct experimental observation of the atomic-scale solvent reorganization around an organic molecule following electronic excitation has so far remained elusive. We report time-resolved X-ray scattering measurements of a photoexcited organic hemithioindigo chromophore in the polar solvent acetonitrile, a system containing only light atoms. Exploiting an ~120 fs time resolution and the high X-ray photon flux offered by an X-ray free-electron laser, the experiments capture the ultrafast dipolar rearrangement of the solvent molecules surrounding the solute driven by photoinduced charge transfer. The transient solvation shell structure is found to be formed with a ~0.5 ps time constant and to reequilibrate over ~3 ps as the excited state population returns to the ground state. Transient absorption spectroscopy measurements and molecular dynamics simulations support the interpretation of the structural dynamics. These results demonstrate the viability of using ultrashort X-ray pulses to directly visualize the motion of light atoms in solution, paving the way for atomic-scale understanding and control of photoinduced processes in biological and synthetic photoactive organic molecules.

The energetics and dynamics of ion assembly in solution has broad influence in nanomaterials and inorganic synthesis. To investigate the fundamental processes involved, we present a time-resolved x-ray solution scattering (TR-XSS) study of the trinuclear silver and thallium complexes of the diplatinum ion PtPOP [Pt2(H2P2O5)44−] in aqueous solution. These complexes, their structural properties, and their electronic structure are not well understood and afford a unique opportunity to study the metal–metal bond formation that influences molecular and material assembly in solution. We present model-independent analysis of the observed dynamics as well as an analysis incorporating time-resolved structural refinements of key bond lengths with <100 fs time resolution. We find that upon photoexcitation, the Pt atoms contract ∼0.25 Å toward the center of both the Ag- and the Tl-PtPOP complexes, as previously observed for the PtPOP anion. For the AgPtPOP system, an ultrafast Ag-Pt bond expansion of ∼0.2 Å is observed, whereas in contrast, the TlPtPOP system exhibits a Tl-Pt bond contraction of ∼0.3 Å upon photoexcitation. For both complexes, the change in electronic state leads to coherent (“wave-packet”) oscillations along the metal–Pt coordinates. Based on these structural dynamics, we propose an electronic structure model that describes the metal–metal bonding behavior in both the ground and excited state for both complexes.

[ F e ( t e r p y ) 2 ] 2 + (terpy = 2,2′:6′,2″-terpyridine) is a transition metal complex where the spin state is photoswitchable and where the properties of the metal-centered quintet excited state (5MC) can be tuned by substituting different electron withdrawing or electron donating groups on the 4′ position of the terpyridine. To better understand the physics determining the photoswitching performance, a deeper insight into the positions of the relevant potential energy surfaces and the molecular structure of the 5MC state is needed. We present a structural investigation based on Time Resolved x-ray Solution Scattering (TR-XSS) by which we determine the average dFe–N bond-length elongation following population of the 5MC state as well as the lifetime of this state in a series of seven modified [Fe(terpy)2]2+ systems in aqueous solution following photo-excitation. The analysis of the TR-XSS data is supported by Density Functional Theory (DFT) and Molecular Dynamics calculations. The quintet state lifetime is determined to vary by more than a factor of 10 (from 1.5 to 16 ns) based on the electron withdrawing/donating properties of the substituting group. Both the DFT calculations and the structural analysis of the experimental data show that the main photo-induced change in metal–ligand bond lengths ΔdFe–N is ∼0.2 Å for all systems.

Dark field x-ray microscopy (DXFM) can visualize microstructural distortions in bulk crystals. Using the femtosecond x-ray pulses generated by x-ray free-electron lasers (XFELs), DFXM can achieve sub-μm spatial resolution and <100 fs time resolution simultaneously. In this paper, we demonstrate ultrafast DFXM measurements at the European XFEL to visualize an optically driven longitudinal strain wave propagating through a diamond single crystal. We also present two DFXM scanning modalities that are new to the XFEL sources: spatial 3D and 2D axial-strain scans with sub-μm spatial resolution. With this progress in XFEL-based DFXM, we discuss new opportunities to study multi-timescale spatiotemporal dynamics of microstructures.

[Fe(terpy)2]2+ (terpy = 2,2':6',2''-terpyridine) is a transition metal complex where the spin state is photoswitchable and where the properties of the metal-centered quintet excited state (5MC) can be tuned by substituting different Electron Withdrawing (EW) or Electron Donating (ED) groups on the 4' position of the terpyridine. To better understand the physics determining the photoswitching performance, a deeper insight into the positions of the relevant potential energy surfaces and the molecular structure of the 5MC state is needed. We present a structural investigation based on Time Resolved X-ray Solution Scattering (TR-XSS) by which we determine the average d(Fe-N) bond-length elongation following population of the 5MC state as well as the lifetime of this state in a series of seven modified [Fe(terpy)2]2+ systems in aqueous solution following photo-excitation. The analysis of the TR-XSS data is supported by Density Functional Theory (DFT) and Molecular Dynamics (MD) calculations. The quintet state lifetime is determined to vary by more than a factor of ten (from 1.5 to 16 nanoseconds) based on the electron withdrawing/donating properties of the substituting group. Both the DFT calculations and the structural analysis of the experimental data show that the main photo-induced change in metal-ligand bond lengths Δd(Fe-N) is ~0.2 Å for all systems.

This work investigates and describes the structural dynamics taking place following charge-transfer-to-solvent photo-abstraction of electrons from I⁻ and Br⁻ ions in aqueous solution following single- and 2-photon excitation at 202 nm and 400 nm, respectively. A Time-Resolved X-ray Solution Scattering (TR-XSS) approach with direct sensitivity to the structure of the surrounding solvent as the water molecules adopt a new equilibrium configuration following the electron-abstraction process is utilized to investigate the structural dynamics of solvent shell expansion and restructuring in real-time. The structural sensitivity of the scattering data enables a quantitative evaluation of competing models for the interaction between the nascent neutral species and surrounding water molecules. Taking the I⁰–O distance as the reaction coordinate, we find that the structural reorganization is delayed by 0.1 ps with respect to the photoexcitation and completes on a time scale of 0.5–1 ps. On longer time scales we determine from the evolution of the TR-XSS difference signal that I⁰: e⁻ recombination takes place on two distinct time scales of ∼20 ps and 100 s of picoseconds. These dynamics are well captured by a simple model of diffusive evolution of the initial photo-abstracted electron population where the charge-transfer-to-solvent process gives rise to a broad distribution of electron ejection distances, a significant fraction of which are in the close vicinity of the nascent halogen atoms and recombine on short time scales.

Dimeric complexes composed of d⁸ square planar metal centers and rigid bridging ligands provide model systems to understand the interplay between attractive dispersion forces and steric strain in order to assist the development of reliable methods to model metal dimer complexes more broadly. [Ir₂ (dimen)₄]²⁺ (dimen = para-diisocyanomenthane) presents a unique case study for such phenomena, as distortions of the optimal structure of a ligand with limited conformational flexibility counteract the attractive dispersive forces from the metal and ligand to yield a complex with two ground state deformational isomers. Here, we use ultrafast X-ray solution scattering (XSS) and optical transient absorption spectroscopy (OTAS) to reveal the nature of the equilibrium distribution and the exchange rate between the deformational isomers. The two ground state isomers have spectrally distinct electronic excitations that enable the selective excitation of one isomer or the other using a femtosecond duration pulse of visible light. We then track the dynamics of the nonequilibrium depletion of the electronic ground state population─often termed the ground state hole─with ultrafast XSS and OTAS, revealing a restoration of the ground state equilibrium in 2.3 ps. This combined experimental and theoretical study provides a critical test of various density functional approximations in the description of bridged d⁸-d⁸ metal complexes. The results show that density functional theory calculations can reproduce the primary experimental observations if dispersion interactions are added, and a hybrid functional, which includes exact exchange, is used.