Exploring the light-induced dynamics in solvated metallogrid complexes with femtosecond pulses across the electromagnetic spectrum

Out-of-equilibrium dynamics of a grid-like Fe(ii) spin crossover dimer triggered by a two-photon excitation

The application of two-photon excitation (TPE) in the study of light-responsive materials holds immense potential due to its deeper penetration and reduced photodamage. Despite these benefits, TPE has been underutilised in the investigation of the photoinduced spin crossover (SCO) phenomenon. Here, we employ TPE to delve into the out-of-equilibrium dynamics of a SCO FeII dimer of the form [FeII(HL)2]2(BF4)4·2MeCN (HL = 3,5-bis{6-(2,2'-bipyridyl)}pyrazole). Optical transient absorption (OTA) spectroscopy in solution proves that the same dynamics take place under both one-photon excitation (OPE) and TPE. The results show the emergence of the photoinduced high spin state in less than 2 ps and with a lifetime of 147 ns. Time-resolved photocrystallography (TRXRD) reveals a single molecular reorganisation within the first 500 ps following TPE. Additionally, variable temperature single crystal X-ray diffraction (VTSCXRD) and magnetic susceptibility measurements confirm that the thermal transition is silenced by the solvent. While the results of the OTA and TRXRD utilising TPE are intriguing, the high pump fluencies required to excite enough metal centres to the high spin state may impair its practical application. Nonetheless, this study sheds light on the potential of TPE for the investigation of the out-of-equilibrium dynamics of SCO complexes.

Experimental capabilities for liquid jet samples at sub-MHz rates at the FXE Instrument at European XFEL

The Femtosecond X-ray Experiments (FXE) instrument at the European X-ray Free-Electron Laser (EuXFEL) provides an optimized platform for investigations of ultrafast physical, chemical and biological processes. It operates in the energy range 4.7-20 keV accommodating flexible and versatile environments for a wide range of samples using diverse ultrafast X-ray spectroscopic, scattering and diffraction techniques. FXE is particularly suitable for experiments taking advantage of the sub-MHz repetition rates provided by the EuXFEL. In this paper a dedicated setup for studies on ultrafast biological and chemical dynamics in solution phase at sub-MHz rates at FXE is presented. Particular emphasis on the different liquid jet sample delivery options and their performance is given. Our portfolio of high-speed jets compatible with sub-MHz experiments includes cylindrical jets, gas dynamic virtual nozzles and flat jets. The capability to perform multi-color X-ray emission spectroscopy (XES) experiments is illustrated by a set of measurements using the dispersive X-ray spectrometer in von Hamos geometry. Static XES data collected using a multi-crystal scanning Johann-type spectrometer are also presented. A few examples of experimental results on ultrafast time-resolved X-ray emission spectroscopy and wide-angle X-ray scattering at sub-MHz pulse repetition rates are given.

Structural dynamics of a thermally silent triiron(II) spin crossover defect grid complex

The structural evolution of spin crossover (SCO) complexes during their spin transition at equilibrium and out-of-equilibrium conditions needs to be understood to enable their successful utilisation in displays, actuators and memory components. In this study, diffraction techniques were employed to study the structural changes accompanying the temperature increase and the light irradiation of a defect [2 × 2] triiron(II) metallogrid of the form [FeII3LH2(HLH)2](BF4)4·4MeCN (FE3), LH = 3,5-bis{6-(2,2'-bipyridyl)}pyrazole. Although a multi-temperature crystallographic investigation on single crystals evidenced that the compound does not exhibit a thermal spin transition, the structural analysis of the defect grid suggests that the flexibility of the grid, provided by a metal-devoid vertex, leads to interesting characteristics that can be used for intermolecular cooperativity in related thermally responsive systems. Time-resolved photocrystallography results reveal that upon excitation with a ps laser pulse, the defect grid shows the first two steps of the out-of-equilibrium process, namely the photoinduced and elastic steps, occurring at the ps and ns time scales, respectively. Similar to a previously reported [2 × 2] tetrairon(II) metallogrid, FE3 exhibits a local distortion of the entire grid during the photoinduced step and a long-range distortion of the lattice during the elastic step. Although the lifetime of the pure photoinduced high spin (HS) state is longer in the tetranuclear grid than in the defect grid, suggesting that the global nuclearity plays a crucial role for the lifetime of the photoinduced species, the influence of the co-crystalising solvent on the lifetime of the photoinduced HS state remains unknown. This study sheds light on the out-of-equilibrium dynamics of a thermally silent defect triiron SCO metallogrid.

Short- vs. long-range elastic distortion: structural dynamics of a [2 × 2] tetrairon(II) spin crossover grid complex observed by time-resolved X-Ray crystallography

Spin crossover complexes (SCO) are among the most studied molecular switches due to their potential use in displays, sensors, actuators and memory components. A prerequisite to using these materials is the understanding of the structural changes following the spin transition at out-of-equilibrium conditions. So far, out-of-equilibrium studies in SCO solids have been focused on mononuclear complexes, though a growing number of oligonuclear SCO complexes showing cooperative effects are being reported. Here, we use time-resolved pink Laue crystallography to study the out-of-equilibrium dynamics of a [2 × 2] tetranuclear metallogrid of the form [FeII4LMe4](BF4)4·2MeCN ([LMe]- = 4-methyl-3,5-bis{6-(2,2'-bipyridyl)}pyrazolate). The out-of-equilibrium spin state switching induced by a ps laser pulse demonstrates that the metallogrid exhibits a multi-step response similar to that reported for mononuclear complexes. Contrary to the mononuclear complexes, the metallogrid shows two types of elastic distortions at different time scales. The first is a short-range distortion that propagates over the entire Fe4 grid complex during the ps time scale, and it is caused by the rearrangement of the coordination sphere of the photo-switching ion and the constant feedback between strongly linked metal ions. The second is a long-range distortion caused by the anisotropic expansion of the lattice during the ns time scale, observed in mononuclear materials. The structural analysis demonstrates that the long-range prevails over the short-range distortion, inducing the largest deformation of both the entire grid and the coordination sphere of each metal ion. The present study sheds light on the out-of -equilibrium dynamics of a non-cooperative oligonuclear complex.

Metal-to-metal communication during the spin state transition of a [2 × 2] Fe(II) metallogrid at equilibrium and out-of-equilibrium conditions

Spin crossover (SCO) complexes are prototypes of materials with bi- or multi-stability in the solid state. The structural evolution during their spin transition is a key feature to establish the foundations of how to utilize this type of material. So far, ultrafast time-resolved structural investigations of SCO solids have been focused on monometallic complexes, though an increasing number of oligometallic SCO complexes showing cooperativity effects are being reported. Here, we used single crystal X-ray crystallography and time-resolved pink Laue photocrystallography to study the molecular reorganisation during the thermal and photoinduced SCO of a [2 × 2] tetranuclear metallogrid of the form [FeII4LMe4](BF4)4·2MeCN ([LMe]- = 4-methyl-3,5-bis{6-(2,2'-bipyridyl)}pyrazolate). A multitemperature crystallographic investigation on single crystals reveals an effective communication between the metal centres during thermal SCO, observed by the simultaneous transformation of the coordination polyhedra of both crystallographic-symmetry independent metal atoms accompanying the SCO in only one of them. Time-resolved photocrystallography results reveal the different molecular responses between mononuclear and oligonuclear complexes, after light irradiation with a picosecond laser pulse. While mononuclear SCO complexes reorganise once during the first nanosecond after excitation, the tetranuclear metallogrid exhibits a multiple structural rearrangement in the same span of time. Such behaviour is attributed to the elastic communication between metal atoms, which allows the propagation of a short-range elastic distortion over the entire Fe4 grid complex. The present study sheds light on the importance of strong elastic coupling of metal atoms during the correlated spin transition of oligometallic complexes.

Ultrafast photoinduced dynamics in Prussian blue analogues

A review on ultrafast photoinduced processes in molecule-based magnets with an emphasis on Prussian blue analogues.

Spin-state dependence of the structural and vibrational properties of solvated iron(ii) polypyridyl complexes from AIMD simulations: III. [Fe(tpen)]Cl2 in acetonitrile

In order to achieve an in-depth understanding of the role played by the solvent in the photoinduced low-spin (LS) → high-spin (HS) transition in solvated Fe(ii) complexes, an accurate description of the solvated complexes in the two spin states is required. To this end, we are applying state-of-the-art ab initio molecular dynamics (AIMD) simulations to the study of the structural and vibrational properties of iron(ii) polypyridyl complexes. Two aqueous LS complexes were investigated in this framework, namely, [Fe(bpy)3]2+ (bpy = 2,2'-bipyridine) [Lawson Daku and Hauser, J. Phys. Chem. Lett., 2010, 1, 1830; Lawson Daku, Phys. Chem. Chem. Phys., 2018, 20, 6236] and [Fe(tpy)2]2+ (tpy = 2,2':6',2''-ter-pyridine) [Lawson Daku, Phys. Chem. Chem. Phys., 2019, 21, 650]. For aqueous [Fe(bpy)3]2+, combining the results of forefront wide-angle X-ray scattering experiments with those of the AIMD simulations allowed the visualization of the interlaced coordination and solvation spheres of the photoinduced HS state [Khakhulin et al., Phys. Chem. Chem. Phys., 2019, 21, 9277]. In this paper, we report the extension of our AIMD studies to the spin-crossover complex [Fe(tpen)]2+ (tpen = N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine) in acetonitrile (ACN). The determined LS and HS solution structures of the complex are in excellent agreement with the experimental results obtained by high-resolution transient X-ray absorption spectroscopy [Zhang et al., ACS Omega, 2019, 4, 6375]. The first solvation shell of [Fe(tpen)]2+ consists of ACN molecules located in the grooves defined by the chelating coordination motif of the tpen ligand. Upon the LS → HS change of states, the solvation number of the complex is found to increase from ≈9.2 to ≈11.9 and an inner solvation shell is formed. This inner solvation shell originates from the occupancy by about one ACN molecule of the internal cavity which results from the arrangement of the 4 pyridine rings of the tpen ligand, and which becomes accessible to the solvent molecules in the HS state only thanks to the structural changes undergone by the complex. The presence of this inner solvation shell for the solvated HS complex probably plays a key role in the spin-state dependent reactivity of [Fe(tpen)]2+ in liquid solutions.

A Review of Density Functional Models for the Description of Fe(II) Spin-Crossover Complexes

Spin-crossover (SCO) materials have for more than 30 years stood out for their vast application potential in memory, sensing and display devices. To reach magnetic multistability conditions, the high-spin (HS) and low-spin (LS) states have to be carefully balanced by ligand field stabilization and spin-pairing energies. Both effects could be effectively modelled by electronic structure theory, if the description would be accurate enough to describe these concurrent influences to within a few kJ/mol. Such a milestone would allow for the in silico-driven development of SCO complexes. However, so far, the ab initio simulation of such systems has been dominated by general gradient approximation density functional calculations. The latter can only provide the right answer for the wrong reasons, given that the LS states are grossly over-stabilized. In this contribution, we explore different venues for the parameterization of hybrid functionals. A fitting set is provided on the basis of explicitly correlated coupled cluster calculations, with single- and multi-dimensional fitting approaches being tested to selected classes of hybrid functionals (hybrid, range-separated, and local hybrid). Promising agreement to benchmark data is found for a rescaled PBE0 hybrid functional and a local version thereof, with a discussion of different atomic exchange factors.