THERMAL-INDUCED AND LIGHT-INDUCED SPIN TRANSITION IN [FE(MTZ)6](BF4)2 - 1ST SUCCESSFUL FORMATION OF A METASTABLE LOW-SPIN STATE BY IRRADIATION WITH LIGHT AT LOW-TEMPERATURES

The iron(II) spin-crossover complex [Fe(mtz)6](BF4)2(mtz = 1 -methyl-1H-tetrazole) has two different iron(II) lattice sites A and B. The 57Fe Mössbauer spectra of the compound consist of only one quadrupole doublet above ~ 160 K, which is typical for iron(II) in the high-spin (HS) state. This indicates that the two sites A and B are indistinguishable above this temperature. Below ~160 K two different doublets are observed, both arising from iron(II) in the HS state. Several reasons for this phenomenon are discussed. Only the complex molecules, or rather the metal ions, in lattice site A undergo thermal spin transition to the low-spin (LS) state of iron(II), whereas those in lattice site B remain in the HS state down to 4.2 K. By irradiation with light of a Xe arc lamp (λ ~ 350-650 nm) at 20 K the well-known LIESST (light-induced excited spin state trapping) effect is observed for the lattice site A ions. A metastable HS state is formed, which has a practically infinitely long lifetime below 40 K. Irradiation of [Fe(mtz)6](BF4)2with red light (λ > 700 nm) at 20 K causes a light-induced HS → LS conversion at lattice site B. The populated LS state is also metastable with a practically infinitely long lifetime below 50 K. This is the first example for a successful light-induced formation of a metastable LS state in an iron(II) HS complex. © 1990, American Chemical Society. All rights reserved.