Multiple trajectories towards excited states – multicross

Experiment:
-Time-resolved X-ray spectroscopy
-Time-resolved diffraction

Theory:
-Quantum dynamics
-Electronic structure theory

One of the central questions around spin transition in molecular systems is whether the photoinduced charge transfer induces a spin transition or vice versa.We have shown, by using femtosecond XANES at X-FEL, that optical excitation in CoFe systems induces the spin transition and that this structural reorganization then induces charge transfer. This work was published in Nature Chemistry.
We investigated the case of cyanide-bridged MnFe PBA, showing existence of different optical excitation bands.This opens up different transformation paths on the potential energy surface, initiated by d-dM or ICT excitations.By combining theoretical calculations and ultrafast optical spectroscopy, we have shown that a d-dMn excitation of unpaired a spins first initiates the spin transition of the Mn (Jahn-Teller reorganization), which closes the t2g-eg gap of Mn and induces Mn to Fe charge transfer (STICT pathway). The direct Mn to Fe charge transfer excitation of b spins populates the non-bonding Mn t2g states. The spin transition on the Mn is then slower, as not coupled to the change of electronic state. This is the charge-transfer-induced spin transition (CTIST) path. This work was selected as a «hot paper« in Angewandte Chemie.
On the theory sie, we have created a complete theoretical model for understanding the electron- nuclear couplings in iron tris-bipyridine, simulating the forward and reversed light-induced excited spin-state trapping (LIESST). We have devised a new protocol for obtaining such couplings by performing a fitting of the spin- mixed potential energy surface using imaginary couplings in a diabatic basis instead of directly taking the spin-orbit matrix elements in the adiabatic basis as it is usually done. An article is in preparation for this protocol. We built a model of 9 vibrational dimensions sufficient to reproduce the main kinetic steps of the reaction 1MLCT > 3MLCT > 3MC > 5MC, consisting of the totally symmetric stretching representing the low-to-high spin reaction coordinate, 2 Jahn-Teller vibrations (axial-equatorial Fe-N distortions) that split the vacant 3d orbitals of Fe and 6 bipyridine distortions, that reproduce the relaxation of anionic bipyridine. Two articles are in preparation for the forward and reverse LIESST reactions.We also started computing the time resolved XANES spectrum up to the rising edge. In the literature, filtering strategies have been proposed to overcome this problem, which we are currently exploring. In terms of theory, we are more or less on time with the objectives of the project. We are finishing up the models/publications for Fe(bpy)3.

Commissioning of time-tool and experiments with sub-20 fs NOPA at SwissFEL. Further developments on theory side on systems experimentally studied at SwissFEL and in Rennes. New setup for liquid jet delivery of nano-crystals of PBA for ultrafast experiments.

3 publications :
-J. Mater. Chem. C 9 (2021) 6773
-Angew. Chem. Int. Ed. 60 (2021) 23267
-Nature Chemistry 13, 10-14 (2021)

9 conferences

Multicross aims at understanding transition metal photophysics to a new level of detail thanks to a joint experimental and theoretical research program spanning three laboratories and two countries. Ultrafast optical spectroscopy and X-ray techniques will be pushed to time resolutions approaching 10 fs. Quantum models will be used to solve time dependent Schrodinger equation to follow the photoinduced wavepacket motion and dispersion along different excited state trajectories that will be controlled by different pump laser pulses. Because the same physical model will be able to explain the different experimental findings, the outcomes will be little biased and the resulting representations can be used to clarify the mechanisms behind the unexpected properties of ultrafast intersystem crossing of transition metal compounds.

Indeed transition metals play often a central role in photoreceptors, catalysts and biological active sites. This is due to their capability of changing oxidation states (favouring charge transfers) and of being coordinated by different molecular geometries. Typical examples are organometallic systems. Organic ligands lift the degeneracy of 3d orbitals usually resulting in non bonding and antibonding levels. Such energy gap creates different electronic/structural configurations that can be stabilized by enthalpic (low spin, LS) or entropic (high spin, HS) contributions. Spin CrossOver (SCO) from LS to HS state is usually phototriggered by electronic excitation via a metal-to-ligand charge transfer (MLCT) band. Experiments have tried to disclose what immediately follows these excitations and have delivered unexpectedly high intersystem crossing rates. Such observation spurred a wealth of experimental and theoretical investigations all attempting to understand the sub picosecond (1 ps = 10-12 s) LS to HS mechanism and dynamics.

Today, while the ultrafast nature of SCO photophysics is undiscussed a detailed understand of the process is still debated. For example, the most studied SCO compound has been scrutinised with optical and X-ray spectroscopy, still yielding completely different switching mechanisms in terms of time scale (from sub-50-fs to nearly 200 fs) and visited intermediates electronic states. The physical picture is even less clear for non octahedral SCO systems that only very recently have been experimentally studied, or hetero-bimetallic compounds in which both charge transfer and spin transition characterize the the difference between low and high temperature phases.

Multicross will focus on those different systems in search of an underlying physical picture that could for example evidence the role of particular structural degrees of freedom in driving the system to the metastable HS state.