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Fast and Ferrous!

Electronic and structural dynamics during the switching of the photomagnetic complex [Fe(L222N5)(CN)2].

 
The [Fe(L222N5)(CN)2] compound is a photomagnetic FeII (ferrous) based coordination compound, which undergoes light-induced excited spin-state trapping (LIESST). The low spin state is hexacoordinated and the high spin state heptacoordinated. This system also serves as complex for the design of trinuclear or one-dimensional compounds made of other types of bricks with diverse coordinated metals. Here we study its ultrafast spin-state photoswitching dynamics, by combining femtosecond optical spectroscopy and femtosecond X-ray absorption measurements at the XPP station of the X-ray free-electron laser LCLS. DFT and TD-DFT calculations are used to interpret experimental findings. These studies performed in the solution phase show that LIESST in [Fe(L222N5)(CN)2] occurs on the 100 fs timescale under different types of photoexcitation. In addition, we observe coherent oscillations resulting from the structural dynamics accompanying LIESST, which were recently evidenced in more conventional octahedral FeIIN6 systems.

S. Zerdane, E. Collet,* X. Dong, S. F. Matar, H. F. Wang, C. Desplanches, G. Chastanet, M. Chollet, J. M. Glownia, H.T. Lemke, M. Lorenc, M. Cammarata,*
Electronic and structural dynamics during the switching of the photomagnetic complex [Fe(L222N5)(CN)2]
      Chemistry A European Journal doi 10.1002/chem.201704746 (2017)

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Chemical Reviews: M. Chergui and E. Collet

Photoinduced structural dynamics of molecular systems mapped by time-resolved x-ray methods.

We review the tremendous advances in ultrafast X-ray science, over the past 15 years, making the best use of new ultrashort x-ray sources including table-top or large-scale facilities. Different complementary x-ray based techniques, including spectroscopy, scattering and diffraction, are presented. The broad and expanding spectrum of these techniques in the ultrafast time domain, is delivering new insight into the dynamics of molecular systems, of solutions, of solids and of Biosystems. Probing the time evolution of the electronic and structural degrees of freedom of these systems on the timescales of femtosecond to picoseconds delivers new insight into our understanding of dynamical matter.

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Chemical Science:

Comparison of structural dynamics and coherence of d-d and MLCT light-induced spin state trapping

S. Zerdane, L. Wilbraham, M. Cammarata,* O. Iasco, E. Rivière, M.-L. Boillot, I. Ciofini, and E. Collet*

Light Induced Excited Spin State Trapping (LIESST) in FeII spin-crossover systems is a process switching molecules from low (LS, S=0) to high spin (HS, S=2) states. The direct LS-to-HS conversion being forbidden by selection rules, LIESST involves intermediate states such as 1,3MLCT or 1,3T. The intersystem crossing sequence results in a HS state, structurally trapped by metal-ligand bond elongation through the coherent activation and damping of molecular breathing. The ultrafast dynamics of this process was so far investigated in FeN6 ligand field systems, under MLCT excitation. Here we study LIESST in an FeIIN4O2 spin-crossover material of lower symmetry, which allows quite intense and low-energy shifted d-d bands. By combining ab-initio DFT and TD-DFT calculations and fs optical absorption measurements we demonstrate that, compared to MLCT, d-d excitation induces faster LS-to-HS switching and that the shorter intermediates enhance coherent structural dynamics.

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Nature Communication:

     Coherent structural trapping through wave packet dispersion during photoinduced spin state switching

H. T. Lemke*, K. S. Kjær, R. Hartsock, T. B. van Driel, M. Chollet, J. M. Glownia, S. Song, D.  Zhu, E. Pace, S. F. Matar, M. M Nielsen, M. Benfatto, K. J. Gaffney, E. Collet, Marco Cammarata*

The description of ultrafast nonadiabaticchemical dynamics during molecular photo-transformations remains challenging because electronic and nuclear structures impact each other and cannot be treated independently. Here we gain new experimental insights, beyond the Born-Oppenheimer approximation, into the light-induced spin-state trapping (LIESST) dynamics of the prototypical [Fe(bpy)3]2+ compound by time-resolved XANES spectroscopy at sub-30-femtosecond resolution and high signal to noise ratio. The electronic decay from the initial optically excited electronic state towards the high spin state is distinguished from the structural trapping dynamics, which launches a coherent oscillating wave packet (265 fs period) clearly identified as molecular breathing. Throughout the structural trapping, the dispersion of the wave packet along the reaction coordinate reveals details of intramolecular vibronic coupling before a slower vibrational energy dissipation to the solution environment. These findings revisiting the LIESST process illustrate how modern time-resolved XANES provides key information to unravel dynamic details of photo-functional molecules.