X-ray study of the light-induced metastable state of a spin-crossover compound

Iron(II) complexes exhibiting thermal spin crossover map be converted from the (1)A(1) low-spin (LS) state to the T-5(2) high-spin (HS) state by irrad iation with green light (light-induced excited spin-state trapping, LIESST) . The lifetime of the metastable LIESST state may be sufficiently long for X-ray diffraction study. The lattice parameters of a single crystal of [Fe( ptz)(6)](BF4)(2) (ptz = propyltetrazole) were measured between 300 and 10 K , while the crystal changed from the HS to the LS state near 135 K. Using t he green light (514 nm) of an argon-ion laser, the crystal was quantitative ly converted to the metastable LIESST state at 10 K; its lattice parameters were measured up to 50 K, at which point the LIESST state begins to decay on a minute timescale. The change of the lattice parameters can be interpre ted by a superposition of a normal temperature dependence, for which the is ostructural zinc compound served as a reference, and an almost temperature- independent part which is proportional to the fraction of molecules in the HS state. (C) 2000 International Union of Crystallography Printed in Great Britain - all rights reserved.