The deactivation mechanism of photoexcited [Co(terpy) 2 ] 3+ (terpy = 2,2':6',2''-terpyridine) in acetonitrile is investigated using transient absorption spectroscopy and density functional theory calculations. Ultrafast measurements reveal the formation of a quintet metal-centred ( 5 MC) state, which decays with a time constant of 170 fs via 5 MC → 3 MC intersystem crossing (ISC). Subsequent vibrational cooling occurs on a 5.7 ps timescale, followed by a long-lived 3 MC excited state with a lifetime of 3.4 ns. In contrast to Fe(II) polypyridines, where early-time relaxation proceeds via 3 MC states despite similar accessibility of the quintet manifold in the Franck–Condon (FC) region, [Co(terpy) 2 ] 3+ exhibits direct population of the 5 MC state, thereby reversing the early-time relaxation pathway. Furthermore, the persistence of a nanosecond-lived 3 MC state is unexpected given the strong spin–orbit coupling of Co(III), which would typically enable efficient triplet–singlet relaxation on the picosecond timescale, as observed in Fe(II) analogues. Instead, relaxation is governed by a dynamical bottleneck arising from unfavorable 3 MC → ground-state energetics. These results demonstrate that ultrafast population pathways in 3d 6 complexes are not dictated solely by ligand-field strength or spin–orbit coupling, but by nonequilibrium dynamical factors that can invert established relaxation pathways.

The quintet-to-singlet back-switching process in [Fe(tpy)2]2+(tpy=2,2’:6’,2"-terpyridine), [Fe(dftpy)2]2+ (dftpy=6,6"-difluoro-2,2’:6’,2"-terpyridine) and [Fe(dctpy)2]2+(dctpy=6,6"-dichloro-2,2’:6’,2"-terpyridine), together with the photorelaxation dynamics of the [Fe(dftpy)2]2+ spin-crossover and the [Fe(dctpy)2]2+ high-spin complexes were investigated by full-dimensional trajectory surface hopping (TSH) dynamics. Following photoexcitation, the initial population of the 5Eg (5MC) state decays quickly via a branching mechanism of direct (5Eg → 5T2g) and 3MC-crossed (5Eg → 3MC→5T2g) processes, with the latter dominating the dynamics. The quintet-to-singlet photoconversion, the key step of the back-switching process, remains inefficient (<10%). This low efficiency arises because the singlet–triplet–quintet crossing region is readily accessible only during the initial sub-picosecond dynamics (<500 fs). Shifting the crossing region to improve its accessibility could substantially enhance the quintet-to-singlet conversion efficiency, suggesting that more extensive chemical modification could significantly optimize the back-switching process. Upon excitation to the quintet MLCT manifold, the population decays within <200 fs to the dark 5Eg (5MC) state (followed by the relaxation to the 5T2g (5MC) in less than 1 ps); only 1-2% of the initial population remains in the ligand-centered states in all complexes. In contrast, previous transient absorption spectroscopy experiments identified a long-lived 5/7MLCT state for [Fe(dftpy)2]2+ and [Fe(dctpy)2]2+, potentially because the analysis was restricted to picosecond timescales and probe wavelengths were used at which 5MC states are dark.

The effect of substitution in the axial 4' position by electron withdrawing (EW) and donating (ED) groups on the ground and excited state properties of the \feterpy complex is investigated in a systematic study. DFT calculations are used to determine the geometric and electronic structures of the substituted complexes, and these calculated results were compared to experimental results from stationary and time-resolved spectroscopy. The question how this interplay of theory and experiment can be used in the context of ligand engineering is also considered.

[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.

It has long been known that irradiation with visible light converts Fe(II) polypyridines from their low-spin (singlet) to high-spin (quintet) state, yet mechanistic interpretation of the photorelaxation remains controversial. Herein, we simulate the full singlet-triplet-quintet dynamics of the [Fe(terpy) 2 ] 2+ (terpy = 2,2’:6’,2”-terpyridine) complex in full dimension, in order to clarify the complex photodynamics. Importantly, we report a branching mechanism involving two sequential processes: a dominant 3 MLCT→ 3 MC( 3 T 2g )→ 3 MC( 3 T 1g )→ 5 MC, and a minor 3 MLCT→ 3 MC( 3 T 2g )→ 5 MC component. (MLCT = metal-to-ligand charge transfer, MC = metal-centered). While the direct 3 MLCT→ 5 MC mechanism is considered as a relevant alternative, we show that it could only be operative, and thus lead to competing pathways, in the absence of 3 MC states. The quintet state is populated on the sub-picosecond timescale involving non-exponential dynamics and coherent Fe-N breathing oscillations. The results are in agreement with the available time-resolved experimental data on Fe(II) polypyridines, and fully describe the photorelaxation dynamics.

The properties of transition metal complexes and their chemical dynamics can be effectively modified with ligand substitutions, and theory can be a great aid to such molecular engineering. In this paper we first theoretically explore how substitution with a Cl atom at different positions of the terpyridine ligand affects the electronic structure of the [Fe(terpy)2]2+ complex. We found that besides the substitution at position 4’, the next most promising candidate to cause substantial electronic effects is that where the side pyridine ring is substituted at position 5 (beta). Therefore, next we examine in detail the Fe(II) complexes of the 5- chloro and 5,5”-di-chloro derivatives of terpy, theoretically and experimentally, to reveal how these substitutions modify the ground state properties and the lifetime of the excited quintet state in such complexes. In addition, we extend the investigation to the complexes of the analogously substituted derivatives of 4’-SMe-terpy. The substitution at position(s) 5 (and 5”) with Cl lowers the energy of the quintet state and increases its lifetime; the results on the 4’-SMe substituted complexes show similar changes with these two substitutions, verifying that these effects are more or less additive. This study contributes to the enhancement of our molecular engineering toolset for modifying the potential energy landscape of similar complexes.

Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Twitter Facebook Reddit LinkedIn Tools Icon Tools Reprints and Permissions Cite Icon Cite Search Site Citation Peter Vester, Katharina Kubicek, Roberto Alonso-Mori, Tadesse Assefa, Elisa Biasin, Morten Christensen, Asmus O. Dohn, Tim B. van Driel, Andreas Galler, Wojciech Gawelda, Tobias C. B. Harlang, Niels E. Henriksen, Kasper S. Kjær, Thomas S. Kuhlman, Zoltán Németh, Zhangatay Nurekeyev, Mátyás Pápai, Jochen Rittman, György Vankó, Hasan Yavas, Diana B. Zederkof, Uwe Bergmann, Martin M. Nielsen, Klaus B. Møller, Kristoffer Haldrup, Christian Bressler; Publisher's Note: "Tracking structural solvent reorganization and recombination dynamics following e− photoabstraction from aqueous I− with femtosecond x-ray spectroscopy and scattering" [J. Chem. Phys. 157, 224201 (2022)]. J. Chem. Phys. 7 April 2023; 158 (13): 139903. https://doi.org/10.1063/5.0151289 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAIP Publishing PortfolioThe Journal of Chemical Physics Search Advanced Search |Citation Search

We previously studied the effect of 4' substitution in iron(II)-bis-terpyridine complexes, showing that the photoexcited high-spin quintet-state is stabilized by electron-donating substituents. In this paper we explore the effects of electron-donating ( X = NH2 , Cl ) and withdrawing ( X = NO2 ) substituents in the 5,5" positions on the stability and lifetime of the quintet-state. We used a simple densitiy-functional theory (DFT) based method that had been proven fairly accurate in the case of 4' substitution to estimate the energy barrier of the quintet-singlet transition and thereby predict the quintet state lifetime. We synthetized the complexes and used ultrafast transient optical absorption spectroscopy to experimentally determine the quintet lifetimes, in order to test the applicability of these quantum-chemistry based predictive methods for these side-ring substitution cases. UV-Visible spectra of the complexes have shown that the metal-to-li­gand charge transfer (MLCT) and ligand-localized transitions of these complexes change according to the previous observations. We have shown that in the 5,5" positions, electron withdrawing groups stabilize the quintet state, while donating groups destabilize it. This is in stark contrast to the effects previously observed for the 4' case, and indicates that unlike the latter case, the simple concept of inductive and mesomeric effects may not be adequate to describe the changes due to 5,5" substitution, warranting further study of the area.

Photochemical reactions in solution are governed by a complex interplay between transient intramolecular electronic and nuclear structural changes and accompanying solvent rearrangements. State-of-the-art time-resolved X-ray solution scattering has emerged in the last decade as a powerful technique to observe solute and solvent motions in real time. However, disentangling solute and solvent dynamics and how they mutually influence each other remains challenging. Here, we simultaneously measure femtosecond X-ray emission and scattering to track both the intramolecular and solvation structural dynamics following photoexcitation of a solvated copper photosensitizer. Quantitative analysis assisted by molecular dynamics simulations reveals a two-step ligand flattening strongly coupled to the solvent reorganization, which conventional optical methods could not discern. First, a ballistic flattening triggers coherent motions of surrounding acetonitrile molecules. In turn, the approach of acetonitrile molecules to the copper atom mediates the decay of intramolecular coherent vibrations and induces a further ligand flattening. These direct structural insights reveal that photoinduced solute and solvent motions can be intimately intertwined, explaining how the key initial steps of light harvesting are affected by the solvent on the atomic time and length scale. Ultimately, this work takes a step forward in understanding the microscopic mechanisms of the bidirectional influence between transient solvent reorganization and photoinduced solute structural dynamics.

The application of N-heterocyclic carbene (NHC) ligands represents a groundbreaking advance towards environment-friendly light-harvesting complexes, yet, even the excited-state dynamics of the first-discovered Fe-NHC photosensitiser [Fe(bmip)2]2+ (bmip = 2,6-bis(3-methyl-imidazole-1-ylidine)-pyridine) remain controversial. Using full-dimensional trajectory surface hopping (TSH) spin-vibronic dynamics simulations, we fully resolve and clarify the deactivation mechanism of [Fe(bmip)2]2+. In agreement with the most recent fs-resolved X-ray experiments, we find the excited-state population branches into triplet metal-to-ligand charge transfer (3MLCT) and metal-centered states (3MC) on early sub-ps timescales ~200 fs. The ultrafast 3MLCT quenching by 3MC is driven by multidimensional excited-state ligand motion; it is this deactivation process that ultimately weakens the photosensing efficiency of [Fe(bmip)2]2+. Crucially, the preservation of vibrational coherence along the covalent Fe-C bonds (~300 fs vibrational period) and the appearance of resulting coherent oscillations in various time-resolved experimental data unambiguously evidences the presence of the early sub-ps 3MC component.

X-ray spectroscopy is a demanded tool across multiple user communities. Here we report on a new station for X-ray emission spectroscopy at the Extreme Light Infrastructure Beamlines Facility. The instrument utilizes the von Hamos geometry and works with a number of different sample types, notably including liquid systems. We demonstrate a simple and reliable method for source position control using two cameras. This approach addresses energy calibration dependence on sample position, which is a characteristic source of measurement uncertainty for wavelength dispersive spectrometers in XES arrangement. We also present a straightforward procedure for energy calibration of liquid and powder samples to a thin film reference. The developed instrumentation enabled us to perform the first experimental determination of the Kα lines of liquidized K3Fe(CN)6 as well as powdered and liquidized FeNH4(SO4)2. Finally, we report on proof-of-principle use of a colliding jet liquid sample delivery system in an XES experiment.

The present work demonstrates the performance of a von Hámos high-energy-resolution X-ray spectrometer based on a non-conventional conical Si single-crystal analyzer. The analyzer is tested with different primary and secondary X-ray sources as well as a hard X-ray sensitive CCD camera. The spectrometer setup is also characterized with ray-tracing simulations. Both experimental and simulated results affirm that the conical spectrometer can efficiently detect and resolve the two pairs of two elements (Ni and Cu) Kα X-ray emission spectroscopy (XES) peaks simultaneously, requiring a less than 2 cm-wide array on a single position-sensitive detector. The possible applications of this simple yet broad-energy-spectrum crystal spectrometer range from quickly adapting it as another probe for complex experiments at synchrotron beamlines to analyzing X-ray emission from plasma generated by ultrashort laser pulses at modern laser facilities.

We present a sub-picosecond resolved investigation of the structural solvent reorganization and geminate recombination dynamics following 400 nm two-photon excitation and photodetachment of a valence p electron from the aqueous atomic solute, I-(aq). The measurements utilized time-resolved X-ray Absorption Near Edge Structure (TR-XANES) spectroscopy and X-ray Solution Scattering (TR-XSS) at the Linac Coherent Light Source x-ray free electron laser in a laser pump/x-ray probe experiment. The XANES measurements around the L1-edge of the generated nascent iodine atoms (I0) yield an average electron ejection distance from the iodine parent of 7.4 ± 1.5 Å with an excitation yield of about 1/3 of the 0.1M NaI aqueous solution. The kinetic traces of the XANES measurement are in agreement with a purely diffusion-driven geminate iodine-electron recombination model without the need for a long-lived (I0:e-) contact pair. Nonequilibrium classical molecular dynamics simulations indicate a delayed response of the caging H2O solvent shell and this is supported by the structural analysis of the XSS data: We identify a two-step process exhibiting a 0.1 ps delayed solvent shell reorganization time within the tight H-bond network and a 0.3 ps time constant for the mean iodine-oxygen distance changes. The results indicate that most of the reorganization can be explained classically by a transition from a hydrophilic cavity with a well-ordered first solvation shell (hydrogens pointing toward I-) to an expanded cavity around I0 with a more random orientation of the H2O molecules in a broadened first solvation shell.

Time-resolved X-ray absorption spectroscopy has been utilized to monitor the bimolecular electron transfer in a photocatalytic water splitting system. This has been possible by uniting the local probe and element specific character of X-ray transitions with insights from high-level ab initio calculations. The specific target has been a heteroleptic [IrIII (ppy)2 (bpy)]+ photosensitizer, in combination with triethylamine as a sacrificial reductant and Fe3(CO)12 as a water reduction catalyst. The relevant molecular transitions have been characterized via high-resolution Ir L-edge X-ray absorption spectroscopy on the picosecond time scale and restricted active space self-consistent field calculations. The presented methods and results will enhance our understanding of functionally relevant bimolecular electron transfer reactions and thus will pave the road to rational optimization of photocatalytic performance.

Substitution of terpyridine at the 4' position with electron withdrawing and donating groups is used to tune the quintet lifetime of its iron(ii) complex. DFT calculations suggest that the energy barrier between the quintet and singlet states can be altered significantly upon substitution, inducing a large variation of the lifetime of the photoexcited quintet state. This prediction was experimentally verified by transient optical absorption spectroscopy and good agreement with the trend expected from the calculations was found. This demonstrates that the potential energy landscape can indeed be rationally tailored by relevant modifications based on DFT predictions. This result should pave the way to advancing efficient theory-based ligand engineering of functional molecules to a wide range of applications.

This corrects the article DOI: 10.1103/PhysRevLett.117.013002.

Femtosecond electronic and nuclear dynamics in [Cu(dmp) 2 ] + complex upon 550 nm photoexcitation are studied with X-ray emission spectroscopy and X-ray solution scattering, revealing pseudo Jahn-Teller distortion (~410 fs) coupled with coherent vibrational motion.

Disentangling the dynamics of electrons and nuclei during nonadiabatic molecular transformations remains a considerable experimental challenge. Here we have investigated photoinduced electron transfer dynamics following a metal-to-ligand charge-transfer (MLCT) excitation of the [Fe(bmip)2]2+ photosensitizer, where bmip = 2,6-bis(3-methyl-imidazole-1- ylidine)-pyridine, with simultaneous femtosecond-resolution Fe K{\alpha} and K\b{eta} X-ray Emission Spectroscopy (XES) and Wide Angle X-ray Scattering (WAXS). This measurement clearly shows temporal oscillations in the XES and WAXS difference signals with the same 278 fs period oscillation. The oscillatory signal originates from an Fe-ligand stretching mode vibrational wavepacket on a triplet metal-centered (3MC) excited state surface. The vibrational wavepacket is created by 40% of the excited population that undergoes electron transfer from the non-equilibrium MLCT excited state to the 3MC excited state with a 110 fs time constant, while the other 60% relaxes to a 3MLCT excited state in parallel. The sensitivity of the K{\alpha} XES spectrum to molecular structure results from core-level vibronic coupling, due to a 0.7% average Fe-ligand bond length difference in the lowest energy geometry of the 1s and 2p core-ionized states. These results highlight the importance of vibronic effects in time-resolved XES experiments and demonstrate the role of metal-centered excited states in the electronic excited state relaxation dynamics of an Fe carbene photosensitizer.

This corrects the article DOI: 10.1103/PhysRevLett.117.013002.

AbstractIron N‐heterocyclic carbene (NHC) complexes have received a great deal of attention recently because of their growing potential as light sensitizers or photocatalysts. We present a sub‐ps X‐ray spectroscopy study of an FeIINHC complex that identifies and quantifies the states involved in the deactivation cascade after light absorption. Excited molecules relax back to the ground state along two pathways: After population of a hot 3MLCT state, from the initially excited 1MLCT state, 30 % of the molecules undergo ultrafast (150 fs) relaxation to the 3MC state, in competition with vibrational relaxation and cooling to the relaxed 3MLCT state. The relaxed 3MLCT state then decays much more slowly (7.6 ps) to the 3MC state. The 3MC state is rapidly (2.2 ps) deactivated to the ground state. The 5MC state is not involved in the deactivation pathway. The ultrafast partial deactivation of the 3MLCT state constitutes a loss channel from the point of view of photochemical efficiency and highlights the necessity to screen transition‐metal complexes for similar ultrafast decays to optimize photochemical performance.