Tracking the Light-Induced Excited-State Dynamics and Structural Configurations of an Extraordinarily Long-Lived Metastable State at Room Temperature

Chemistry - a European journal 26(47), 10801 - 10810 (2020). doi:10.1002/chem.202001393
Time���resolved X���ray (Tr���XAS) and optical transient absorption (OTA) spectroscopy on the pico���microsecond timescale coupled with density functional theory calculations are applied to study the light���induced spin crossover processes of a Fe���based macrocyclic complex in solution. Tr���XAS analysis after light illumination shows the formation of a seven���coordinated high���spin quintet metastable state, which relaxes to a six���coordinated high���spin configuration before decaying to the ground state. Kinetic analysis of the macrocyclic complex reveals an unprecedented long���lived decay lifetime of approximately 42.6 ��s. Comparative studies with a non���macrocyclic counterpart illustrate a significantly shortened approximately 568���fold decay lifetime of about 75 ns, and highlight the importance of the ligand arrangement in stabilizing the reactivity of the excited state. Lastly, OTA analysis shows the seven���coordinated high���spin state to be formed within approximately 6.2 ps. These findings provide a complete understanding of the spin crossover reaction and relaxation pathways of the macrocyclic complex, and reveal the importance of a flexible coordination environment for their rational design.
Published by Wiley-VCH, Weinheim