Tracking the Light-induced Excited-State Dynamics and Structural Configurations of an Extraordinarily Long-lived Metastable State at Room Temperature.

Time-resolved X-ray (Tr-XAS) and optical transient absorption (OTA) spectroscopy in the pico-microsecond time scale 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 ground state.  Kinetic analysis of the macrocyclic complex reveals an unprecedented long-lived decay lifetime of ~42.6 µs. Comparative studies with a non-macrocyclic counterpart illustrate a significantly shortened ~568-fold decay lifetime of ~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 ~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.