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(a) XANES spectrum of the LS state of a 25 mM aqueous solution of Fe II bpy 3 2 (circles) and simulated MXAN spectrum (solid line) using an Fe-N bond distance of 2.0 A ˚. The inset shows details of the edge region and of the fit. The dots represent the XANES spectrum of the HS state retrieved via Eq. (1). The spectrum is plotted with respect to E 0 (7122.5 eV). (b) Experimental difference XANES recorded 50 ps after laser excitation (circles), and the fit results obtained by the FMS theory (solid line) with an Fe-N bond distance of 2.0 A ˚ and a chemical shift of ÿ2:5 eV (see text for details).
Source publication
Structural changes of the iron(II)-tris-bipyridine ([Fe(II)(bpy)(3)](2+)) complex induced by ultrashort pulse excitation and population of its short-lived (< or =0.6 ns) quintet high spin state have been detected by picosecond x-ray absorption spectroscopy. The structural relaxation from the high spin to the low spin state was followed over the ent...
Citations
... With our time resolution of 500 ps we exclusively probe the long-lived quintet state. This meta-stable state leads to an N-cage opening of 0.2 Å, which is reproduced by theory [69] and experiments [70]. The opening causes the observed energy shift of 0.3 eV between the ground and the excited state and an intensity change in the main absorption line. ...
With time-resolved soft X-ray spectroscopy, the element-specific dynamics of the electronic structure of disordered systems can be investigated. Measurements of species in dilute liquid solutions are particularly challenging and require high photon flux combined with low experimental noise. This mostly limited these experiments to large-scale facilities, especially for energies above the water window (533 eV). Based on a laser-produced plasma source, our system enables, to the best of our knowledge, the first static and transient experiments in the liquid phase to be performed in the laboratory for energies up to 1400 eV and with a time resolution of 500 ps. We benchmark the system with static investigations of ${[{\rm Ni}{({\rm CN})_4}]^{2 -}}$ [ N i ( C N ) 4 ] 2 − and transient experiments on the widely used model complex ${[{\rm Fe}{({\rm bpy})_3}]^{2 +}}$ [ F e ( b p y ) 3 ] 2 + , both in an aqueous solution. The introduced self-referencing concept ensures that the measurements are photon noise limited. Our results form the basis for further liquid-phase experiments investigating the dynamics in diluted solutions.
... This electron trap site behavior was revealed by Fe K-edge X-ray transient absorption (XTA) spectroscopy, which allows element-specific observation of laserinduced electronic structure changes. XTA has become an important investigative tool for probing the photophysics of molecular [23][24][25][26][27][28][29][30][31][32][33] and solid-state 34-37 metal-based systems, including other Fe-containing MOFs. 38,39 XTA spectra can reveal transient electronic and geometric structure changes of the absorbing element of interest upon optical laser excitation of the material, while time scan measurements collected at fixed X-ray probe energies provide associated kinetic data. ...
This study explores the nature, dynamics, and reactivity of the photo-induced charge separated excited state in a Fe3+-doped titanium-based metal organic framework (MOF), xFeMIL125-NH2, as a function of iron concentration. The MOF is synthesized with doping levels x = 0.5, 1 and 2 Fe node sites per octameric Ti-oxo cluster and characterized by powder x-ray diffraction, UV-vis diffuse reflectance, atomic absorption, and steady state Fe K-edge X-ray absorption spectroscopy. For each doping level, time-resolved X-ray transient absorption spectroscopy studies confirm the electron trap site role of the Fe sites in the excited state. Time scan data reveal multiexponential decay kinetics for the charge recombination processes which extend into the microsecond range for all three concentrations. A series of dye photodegradation studies, based on the oxidative decomposition of Rhodamine B, demonstrates the reactivity of the charge separated excited state and the photocatalytic capacity of these MOF materials compared to traditional heterometal-doped semiconductor photocatalysts.
... Fig. 7 tests the three corrections on a simple Real systems, like a molecule in solution, will most often be comprised of many different elements. This section will use the transition metal complex [Fe(bpy) 3 ] 2+ as an example, as it has been the center for many experimental studies using x-ray techniques [76,86,87], including scattering [18,21,25], as well as computational investigations [22,24,77]. The complex is shown in the lower right corner of fig. 8. From the geometry of the complex, it is evident that the simple spherical approximation of the excluded volume will be much less accurate than previously. ...
Structural studies using x-ray scattering methods for investigating molecules in solution are shifting focus towards describing the role and effects of the surrounding solvent. However, forward models based on molecular dynamics (MD) simulations to simulate structure factors and x-ray scattering from interatomic distributions such as radial distribution functions (RDFs) face limitations imposed by simulations, particularly at low values of the scattering vector q. In this work, we show how the value of the structure factor at q = 0 calculated from RDFs sampled from finite MD simulations is effectively dependent on the size of the simulation cell. To eliminate this error, we derive a new scheme to renormalize the sampled RDFs based on a model of the excluded volume of the particle-pairs they were sampled from, to emulate sampling from an infinite system. We compare this new correction method to two previous RDF- correction methods, developed for Kirkwood-Buff theory applications. We present a quantitative test to assess the reliability of the simulated low-q scattering signal, and show that our RDF-correction successfully recovers the correct q = 0 limit for neat water. We investigate the effect of MD-sampling time on the RDF-corrections, before advancing to a molecular example system, comprised of a transition metal complex solvated in a series of water cells with varying densities. We show that our correction recovers the correct q = 0 behaviour for all densities. Furthermore, we employ a simple continuum scattering model to dissect the total scattering signal from the solvent-solvent structural correlations in a solute-solvent model system to find two distinct contributions: a non-local density-contribution from the finite, fixed cell size in NVT simulations, and a local contribution from the solvent shell. We show how the second contribution can be approximated without also including the finite-size contribution. Finally, we provide a ’best-practices’-checklist for experimentalists planning to incorporate explicit solvation MD simulations in future work, offering guidance for improving the accuracy and reliability of structural studies using x-ray scattering methods in solution.
... Real systems, like a molecule in solution, will most often be comprised of many different elements. This section will use the transition metal complex [Fe(bpy) 3 ] 2+ as an example, as it has been the center for many experimental studies using x-ray techniques [75,85,86], including scattering [18,21,25], as well as computational investigations [22,24,76]. The complex is shown in the lower 10 right corner of figure 8. From the geometry of the complex, it is evident that the simple spherical approximation of the excluded volume will be much less accurate than previously. ...
Structural studies using x-ray scattering methods to investigate molecules in solution are shifting focus towards including the role and effects of the surrounding solvent. However, forward models based on molecular dynamics (MD) simulations to simulate structure factors and x-ray scattering from interatomic distributions such as radial distribution functions (RDFs) face limitations imposed by simulations, particularly at low values of the scattering vector q. In this work, we show how the value of the structure factor at q = 0 calculated from RDFs sampled from finite MD simulations is entirely dependent on the size of the simulation cell. We derive a new, simple method to correct the RDFs to eliminate this error, and compare it to two other RDF- correction methods developed for Kirkwood-Buff theory applications. We present a quantitative test to assess the reliability of the simulated low-q scattering signal, and show that our RDF- correction successfully recovers the correct q = 0 limit for neat water. We investigate the effect of MD-sampling time on the RDF-corrections, before advancing to a more realistic example system comprised of a solvated transition metal complex, sampled in a series of water cells with increasing density. We show that our correction recovers the correct q = 0 behaviour for increasing density. Furthermore, we employ a simple continuum scattering model to dissect the total scattering signal from the solvent-solvent structural correlations in a solute-solvent model system to find two distinct contributions: a non-local ’density’-contribution and a local contribution from the solvent shell. We show how the second contribution can be approximated without also including the density-contribution. Finally, we provide a ’best-practices’-checklist for experimentalists planning to incorporate explicit solvation MD simulations in future work, offering guidance for improving the accuracy and reliability of structural studies using x-ray scattering methods in solution.
... 32 The initially excited state relaxes to a high spin quintet on a sub 100 fs time scale, featuring significantly (∼10%) increased bond lengths. [32][33][34][35][36] Representative XTA spectra obtained at time delays near the laser are shown in Fig. 5(b). The ultrafast relaxation from the singlet state makes the high-spin quintet the only significant species in these spectra, as reflected by the four isosbestic points at 7.131, 7.155, 7.180, and 7.216 keV. ...
... The weak oscillations in the XTA spectrum occurring below 7.12 keV result from the Fe 1s → 3d quadrupole transitions, which reflect changes in the electronic structure and orbital occupancy between the quintet excited state and low spin ground state. [33][34][35][36][37] Large photoinduced changes are observed with maxima at ∼7.124 and ∼7.164 keV and minima at ∼7.144 and ∼7.202 keV. These features result from changes in the 1s → 4p dipole transitions and are attributed to a combination of electronic and structural effects in the excited state. ...
... These features result from changes in the 1s → 4p dipole transitions and are attributed to a combination of electronic and structural effects in the excited state. [33][34][35][36][37] The spectral dynamics shown in Figs. 5(a) and 5(b) illustrate the highly self-consistent XTA data obtained with AXMP. ...
Laser pump X-ray Transient Absorption (XTA) spectroscopy offers unique insights into photochemical and photophysical phenomena. X-ray Multiprobe data acquisition (XMP DAQ) is a technique that acquires XTA spectra at thousands of pump-probe time delays in a single measurement, producing highly self-consistent XTA spectral dynamics. In this work, we report two new XTA data acquisition techniques that leverage the high performance of XMP DAQ in combination with High Repetition Rate (HRR) laser excitation: HRR-XMP and Asynchronous X-ray Multiprobe (AXMP). HRR-XMP uses a laser repetition rate up to 200 times higher than previous implementations of XMP DAQ and proportionally increases the data collection efficiency at each time delay. This allows HRR-XMP to acquire more high-quality XTA data in less time. AXMP uses a frequency mismatch between the laser and x-ray pulses to acquire XTA data at a flexibly defined set of pump-probe time delays with a spacing down to a few picoseconds. AXMP introduces a novel pump-probe synchronization concept that acquires data in clusters of time delays. The temporally inhomogeneous distribution of acquired data improves the attainable signal statistics at early times, making the AXMP synchronization concept useful for measuring sub-nanosecond dynamics with photon-starved techniques like XTA. In this paper, we demonstrate HRR-XMP and AXMP by measuring the laser-induced spectral dynamics of dilute aqueous solutions of Fe(CN) 6 ⁴⁻ and [Fe II (bpy) 3 ] ²⁺ (bpy: 2,2′-bipyridine), respectively.
... Improved time resolution achieved with an X-ray free electron laser (XFEL) source TR-XANES allowed one to investigate nuclear wavepacket oscillations in solution and extract the information of the Fe-N breathing vibrational mode (Fig. 46). 7,137,138 The valence electronic structure of transition metal complexes in solution can also been studied with X-ray spectroscopy in the soft X-ray range with femtosecond time resolution at XFEL sources. Time-resolved RIXS (TR-RIXS) at the Fe L 3 edge has been performed to study the photoinduced ligand exchange dynamics of Fe(CO) 5 , concluding that the photoinduced removal of CO generates a 16-electron Fe(CO) 4 species in an excited singlet state, that either converts to the triplet ground state or combines with a solvent molecule to regenerate a penta-coordinated low spin singlet state Fe species (Fig. 47). ...
This Chapter briefly introduces modern and emerging methods to study ultrafast processes in coordination compounds, which are key to any photon-driven application from artificial photosynthesis to information storage. Ultrafast optical methods [namely electronic transient absorption, fluorescence upconversion, time-resolved infrared and resonance Raman], structural methods [X-ray spectroscopies and scattering], and ultrafast multidimensional methods are discussed. The relevance of each method to photochemistry of coordination compounds is illustrated on specific examples. The importance of using multiple methods—e.g., combining electronic, vibrational, and X-ray spectroscopies—to resolve the complex and entangled electronic, structural and spin dynamics happening on the short, fs–ps timescales, is emphasized. Finally, the approaches to direct and control photochemical dynamics are discussed.
... The pre-edge feature at 7.115 keV is associated with the quadrupolar 1s → 3d transition, which becomes partially allowed in the D 3 symmetry of the molecular complex. The edge feature at 7.125 keV is attributed to a dipolar 1s → 4p transition 31,33 . The 7.131 keV feature represents the onset of the multiple scattering region typical of the above-edge XANES. ...
Chirality is a structural property of molecules lacking mirror symmetry that has strong implications in diverse fields, ranging from life sciences to materials science. Chirality-sensitive spectroscopic methods, such as circular dichroism, exhibit weak signal contributions on an achiral background. Helical dichroism, which is based on the orbital angular momentum (OAM) of light, offers a new approach to probe molecular chirality, but it has never been demonstrated on disordered samples. Furthermore, in the optical domain the challenge lies in the need to transfer the OAM of the photon to an electron that is localized on an ångström-size orbital. Here we overcome this challenge using hard X-rays with spiral Fresnel zone plates, which can induce an OAM. We present the helical dichroism spectra of a disordered powder sample of enantiopure salts of the molecular complex of [Fe(4,4′-diMebpy)3]2+ at the iron K edge (7.1 keV) with OAM-carrying beams. The asymmetry ratios for the helical dichroism spectra are within one to five percent for OAM beams with topological charges of one and three. These results open a new window into the studies of molecular chirality and its interaction with the OAM of light. Linearly polarized orbital angular momentum-carrying hard X-ray beams are induced using spiral Fresnel zone plates. By sending the hard X-ray beams to disordered enantiopure molecular complexes, the helicity-dependent and chiral-sensitive signal is obtained.
... Previous X-raybased experimental studies on Fe-based complexes reported that the dynamics of MLCT and MC states heavily involves the Fe−ligand bonds as main reaction coordinates. 62 Thus, Figure 3 displays the evolution of the Fe−N bonds to the bipyridine ligand (Fe−N bpy ), the Fe−C bonds to the axial cyanides (Fe−CN ax ), and those to the equatorial cyanides (Fe−CN eq ). Blue or red indicates that the current active state is of predominant MLCT or MC character, respectively. ...
The ultrafast dynamical response of solute-solvent interactions plays a key role in transition metal complexes, where charge transfer states are ubiquitous. Nonetheless, there exist very few excited-state simulations of transition metal complexes in solution. Here, we carry out a nonadiabatic dynamics study of the iron complex [Fe(CN)4(bpy)]2- (bpy = 2,2'-bipyridine) in explicit aqueous solution. Implicit solvation models were found inadequate for reproducing the strong solvatochromism in the absorption spectra. Instead, direct solute-solvent interactions, in the form of hydrogen bonds, are responsible for the large observed solvatochromic shift and the general dynamical behavior of the complex in water. The simulations reveal an overall intersystem crossing time scale of 0.21 ± 0.01 ps and a strong reliance of this process on nuclear motion. A charge transfer character analysis shows a branched decay mechanism from the initially excited singlet metal-to-ligand charge transfer (1MLCT) states to triplet states of 3MLCT and metal-centered (3MC) character. We also find that solvent reorganization after excitation is ultrafast, on the order of 50 fs around the cyanides and slower around the bpy ligand. In contrast, the nuclear vibrational dynamics, in the form of Fe-ligand bond changes, takes place on slightly longer time scales. We demonstrate that the surprisingly fast solvent reorganizing should be observable in time-resolved X-ray solution scattering experiments, as simulated signals show strong contributions from the solute-solvent scattering cross term. Altogether, the simulations paint a comprehensive picture of the coupled and concurrent electronic, nuclear, and solvent dynamics and interactions in the first hundreds of femtoseconds after excitation.
... The tris-chelate iron(II)tris(bipyridine) ([Fe(bpy) 3 ] 2+ ) has emerged as a prototypical model complex, whose light-induced SCO mechanism serves as a reference for assessing the spin-switching dynamics of other Fe II complexes 4 . To this end, ultrafast spectroscopies in the optical [5][6][7][8] and X-ray domains [9][10][11][12][13] have determined that the initially excited singlet metal-to-ligand charge-transfer (MLCT) state is deactivated in <50 fs with near unity efficiency via an essentially barrierless passage through the triplet MLCT and metal-centred states to the lowest-energy metal-centred 5 T 2 state. The latter high-spin (HS; spin multiplicity S = 2) state then converts back to the low-spin (LS; S = 0) 1 A 1 ground state on the subnanosecond scale in aqueous solution. ...
Iron-based spin-crossover complexes hold tremendous promise as multifunctional switches in molecular devices. However, real-world technological applications require the excited high-spin state to be kinetically stable—a feature that has been achieved only at cryogenic temperatures. Here we demonstrate high-spin-state trapping by controlling the chiral configuration of the prototypical iron(II)tris(4,4′-dimethyl-2,2′-bipyridine) in solution, associated for stereocontrol with the enantiopure Δ- or Λ-enantiomer of tris(3,4,5,6-tetrachlorobenzene-1,2-diolato-κ2O1,O2)phosphorus(V) (P(O2C6Cl4)3– or TRISPHAT) anions. We characterize the high-spin-state relaxation using broadband ultrafast circular dichroism spectroscopy in the deep ultraviolet in combination with transient absorption and anisotropy measurements. We find that the high-spin-state decay is accompanied by ultrafast changes of its optical activity, reflecting the coupling to a symmetry-breaking torsional twisting mode, contrary to the commonly assumed picture. The diastereoselective ion pairing suppresses the vibrational population of the identified reaction coordinate, thereby achieving a fourfold increase of the high-spin-state lifetime. More generally, our results motivate the synthetic control of the torsional modes of iron(II) complexes as a complementary route to manipulate their spin-crossover dynamics. Despite much research, the high-spin-state relaxation mechanism of Fe(II) spin-crossover complexes is unresolved. Using ultrafast circular dichroism spectroscopy it has now been revealed that the spin relaxation is driven by a torsional twisting mode, which breaks the chiral symmetry of a prototypical Fe(II) compound. Stereocontrolling the configuration of the complex can thus be used to slow down the spin relaxation.
... The edge feature at 7.125 keV is attributed to a dipolar 1s→4p transition. 31,33 The 7.131 keV feature represents the onset of the multiple scattering region typical of the above-edge XANES. The XANES spectra have similar amplitudes (~1 ± 0.2 × 10 5 counts). ...
Chirality is a structural property of molecules lacking mirror symmetry that has strong implications in diverse fields, ranging from life to materials sciences. Established spectroscopic methods that are sensitive to chirality, such as circular dichroism (CD), exhibit weak signal contributions on an achiral background. Helical dichroism (HD), which is based on the orbital angular momentum (OAM) of light, offers a new approach to probe molecular chirality, but it has never been demonstrated on disordered samples. Furthermore, in the optical domain the challenge lies in the need to transfer the OAM of the photon to an electron that is localized on an {\AA}-size orbital. Here, we overcome this challenge using hard X-rays with spiral Fresnel zone, which can induce an OAM. We present the first HD spectra of a disordered powder sample of enantiopure molecular complexes of [Fe(4,4'-diMebpy)3]2+ at the iron K-edge (7.1 keV) with OAM-carrying beams. The HD spectra exhibit the expected inversions of signs switching from a left to a right helical wave front or from an enantiomer to the other. The asymmetry ratios for the HD spectra are within one to five percent for OAM beams with topological charges of one and three. These results open a new window into the studies of molecular chirality and its interaction with the orbital angular momentum of light.
