Electronic and Structural Dynamics During the Switching of the Photomagnetic Complex [Fe(L222 N5 )(CN)2 ]

Abstract

The [Fe(L₂₂₂ N₅ )(CN)₂ ] compound, where L₂₂₂ N₅ refers to the macrocyclic Schiff-base ligand, 2,13-dimethyl-3,6,9,-12,18-pentaazabicyclo[12.3.1]octadeca-1(18),2,12,14,- 16-pentaene, is a photomagnetic Fe^II 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 its ultrafast spin-state photoswitching dynamics are studied, 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(L₂₂₂ N₅ )(CN)₂ ] occurs on the 100 fs timescale under different types of photoexcitation. In addition, coherent oscillations were observed, resulting from the structural dynamics accompanying LIESST, which were recently evidenced in more conventional octahedral Fe^II N₆ systems.

Keywords: LIESST; X-ray absorption spectroscopy; intersystem crossing; spin crossover; time-resolved spectroscopy.