Significant Variation of the Singlet–Quintet Intersystem Crossing Rate Constant in an Iron(II) High‐Spin Complex as a Function of Temperature

Relax! In the dilute mixed-crystal system [Zn1−xFex(bbtr)3](ClO4)2, x=2 % (bbtr=1,4-di(1,2,3-triazol-1-yl)butane), the iron(II) centers are predominantly in the high-spin state. The low-spin state can be populated as a metastable state by irradiation with near-IR light; the rate constant of the low-spin→high-spin relaxation spans 14 orders of magnitude between 40 and 220 K (see picture).