Observation of biphasic kinetics in light-induced spin-state crossover in an iron(II) complex in solution

Relaxation of the 1A1 [graphic omitted] 5T2 spin equilibrium in acetonitrile of the complex of FeII with the multidentate pyridyl macrocyclic ligand N,N′,N″-tris(2-pyridylmethyl)-1,4,7-triazacyclodecane (tp[10]aneN3) after perturbation by a pulsed laser provides the first example of biphasic kinetics for spin crossover in solution with a fast (τ < 30 ns) temperature-independent process coupled to a slower, temperature-dependent step; barrierless spin change is so far unique among spin equilibrium systems.