Low-Spin→High-Spin Relaxation Dynamics in the Highly Diluted Spin-Crossover System [FexZn1−x(bbtr)3](ClO4)2

Whereas the neat polymeric iron(II) compound [Fe(bbtr)3](ClO4)2, bbtr = 1,4-di(1,2,3-triazol-1-yl)butane, shows a quantitative spin transition triggered by a crystallographic phase transition centered at 107 K with a 13 K wide hysteresis, the iron(II) complexes in the diluted mixed crystals [FexZn1−x(bbtr)3](ClO4)2, x = 0.02 and 0.1, stay predominantly in the 5T2 high-spin state down to cryogenic temperatures. However, the 1A1 low-spin state can be populated as metastable state via irradiation into the spin-allowed 5T2→5E ligand-field transition of the high-spin species in the near-infrared. The quantum efficiency of the light-induced conversion is approximately 10% at low temperatures and decreases rapidly above 160 K. The lifetime of the light-induced low-spin state decreases from 15 days at 40 K to 30 ns at 220 K, that is, by 14 orders of magnitude. In the high-temperature regime the activation energy for the low-spin→high-spin relaxation is 1840(20) cm−1