Our results demonstrate that due to the increased ligand field strength caused by NHC ligands, upon transition from the ground state to the 5 MC state, the metal to ligand bonds extend by unusually large values: Dashed lines separate the three main contributions to the temperature rise as discussed in the main text. In both a and b the spheres represent Fe and its first coordination shell, rods represent the rest of the molecular structure. Note that our refinement procedure allows for an experimental determination of the global molecular structure as a function of two bond distances rather than the refinement of these two distances alone. A fraction of the initial solvent temperature increase could not be attributed to the relaxation of the excited molecules as discussed further below. Box 12 4, Lund , Sweden. After thermal relaxation of the excited species we observe a temperature rise of 0.
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The difference signal for the bulk solvent temperature increase was measured according to the standard procedure using azobenzene molecules. This combined progress in experimental and theoretical capabilities will benefit not only the search for perspective earth-abundant photosensitizers, but also the advancement in the understanding of SCO compounds for future opto-magnetic materials.
The cage term was simulated with a purely classical MD, which accounts for solvation effects wn-400ls from the structural changes in the molecule, but it neglects the electronic charge redistribution in the molecule from the GS to 5 MC. A fraction of the initial solvent temperature increase could not be attributed to the relaxation of the excited molecules as discussed further below.
A 31 ps component was tentatively assigned to vibrational cooling of this MC state. Recent years have seen the development of new iron-centered Wn-040ps carbene NHC complexes for solar energy applications.
Laursen cGemma E. Overall, the presented results show that the transition in photophysical properties between weak LFS and strong LFS Fe—NHC compounds manifest itself not only in the destabilization of the MC states, but also in the significantly wn–400ps structural dynamics of these systems. The minor discrepancy between the data and the fit in the range 1. The key structural parameters defining the structure of the molecule are highlighted with colors.
This finding indicates that, at least for NHC systems, the short MC state lifetime is not indicative of its 3 MC character, as suggested for expanded cage and push—pull compounds in recent spectroscopic works 54,55 and observed in weak LFS Ru II based compounds.
After ps the temperature grows due to the release of wn-040ps 5 MC state energy following its decay to the GS.
Garage E. Kasper AG
These results imply that the transition in the photochemical properties from typical Fe complexes to novel NHC compounds is manifested not only in the destabilization of the MC states, but also in structural distortion of these states. Together with energy destabilization, this structural distortion of the scavenger MC states shortens their lifetime, and make them less accessible from the MLCT manifold, which in turn results in increased MLCT lifetimes. The experimentally determined values for the bond lengths and the energy levels are summarized in Table 1.
Understanding the nature of the observed temperature offset requires better time resolution which is beyond the scope of the present study. The interpretation of the TRWAXS data was conducted according to the established framework, 69 where the experimental curves are fitted with theoretical signals from three contributions: Harlang bcLisa A. The transient optical absorption spectroscopy experiments previously conducted on this complex, 49 indicated that upon excitation to the MLCT manifold, the system decays into a vibrationally hot MC state in fs.
The sharp temperature rise appears around time zero and is partly due to fast relaxation from the MLCT to 5 MC perhaps through a short lived 3 MC intermediate 50 and the associated redistribution of most of the photon energy into the solvent.
In both a and b the spheres represent Fe and its first coordination shell, rods represent the rest of the molecular structure. After thermal relaxation of the excited species we observe a temperature rise of 0. Improvements in the cage modelling will also impact the accuracy of the energetic parameters since both solvation and solvent contributions are present primarily in the low- q region of the difference signal. Our results demonstrate that due to the increased ligand field strength caused by NHC ligands, upon transition from the ground state to the 5 MC state, the metal to ligand bonds extend by unusually large values: Using information about the hydrodynamic state of the solvent, we also determine, for the first time, the energy of the 5 MC state as 0.
This increased lifetime is ascribed to strong ligand field splitting provided by the NHC ligands that raises the energy levels of the metal centered MC states and therefore reduces the deactivation efficiency of MLCT states. It confirms the computational prediction that NHC compounds exhibit excited MC states where the minima are pushed further away from the GS along the main reaction coordinate metal-to-ligand bond distance.
The hydrogens are omitted for clarity.
Garage E. Kasper AG –
Note that our refinement procedure allows for an experimental determination of the global molecular structure as a function of two bond distances rather than the refinement of these two distances alone. More generally this work illustrates how TRWAXS on a photosensitive molecule in solution provides information not only on the transient structures, but also on the energies of short-lived excited species, which is traditionally a difficult task since most experiments are either tailored towards structure or energy sensitivity.
Such information cannot be readily retrieved for MC states with optical methods, such as absorption or fluorescence spectroscopy due to the formally dipole-forbidden nature of the optical transitions between the GS and MC states which drastically reduces the interaction cross-section and, more importantly, because the relaxed MC states are geometrically inaccessible from vertical light excitations Fig.
To map the overall atomic positions characterized by the selected structural parameters, we have implemented a procedure based on the interpolation between the structures produced by varying the values of R ax and R eq and performing constrained DFT for each R axR eq point.
By varying these parameters and from those determine the resulting changes to the rest of the structure, one can refine the overall molecular structure within the framework of the proposed structural model.
Multidimensional photoemission spectroscopy—the space-charge limit
Moreover, TRWAXS can uniquely provide information about excited state energies by direct assessment of the amount of energy disposed into the solvent during excited state relaxation. Previous work on Fe II SCO complexes have established that the structural changes induced by the transition from the GS to 5 MC are well described by parameterizing the global structure through parameters that define the shape of the first Fe coordination shell, including metal—ligand bond lengths, ligand wn-400p angles and ligand rocking angles, as the reaction coordinates.
For all time points we find a very good agreement between the experimental data and the model fit. Further improvements of the experimental setup in terms of stability and signal-to-noise should help in increasing the precision of the extracted energies.