title: Spin-transition in [Fe(II)(L5)2][ClO4]2 [L5 = 2-[3-(2'-pyridyl)pyrazol-1-ylmethyl](1-methylimidazole)]: a further example of coexistence of features typical for disorder and cooperativity.
abstract: An iron(II) complex [Fe(L5)2][ClO4]2 of a new nonplanar tridentate ligand [L5 = 2-[3-(2'-pyridyl)pyrazol-1-ylmethyl](1-methylimidazole)] has been synthesized and its spin-transition properties have been investigated. X-ray structural analysis of the solvated compound [Fe(L5)2][ClO4]2.CH3CN at 100 K and at 298 K revealed a mononuclear Fe(II)N6 pseudooctahedron with the tridentate ligand in the meridional coordination mode. Magnetic susceptibility measurements revealed LS (1A1) <--> HS (5T2) transition starting above room temperature and remained incomplete even up to 350 K, however providing evidence for the hysteretic behaviour of the transition. Photo-magnetic measurements using a 532 nm laser revealed incomplete light-induced excited spin-state trapping (LIESST). Relaxation curves in the dark exhibited a stretched exponential shape, typical for broad distributions of relaxation times. The complete photo-magnetic data were modelled using a non-cooperative master equation including a tunnelling process with distributed parameters and accounting for the bulk absorption of light. We also measured the diffuse reflectance properties, with detailed investigation of the absorption spectra and discussed the obtained features with respect to structural properties, including the suggested presence of disorder.
authors.affiliation: Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur, 208 016, India.
authors.forename: Vibha
authors.initials: V
authors.lastname: Mishra
authors.forename: Haritosh
authors.initials: H
authors.lastname: Mishra
authors.forename: Rabindranath
authors.initials: R
authors.lastname: Mukherjee
authors.forename: Epiphane
authors.initials: E
authors.lastname: Codjovi
authors.forename: Jorge
authors.initials: J
authors.lastname: Linarès
authors.forename: Jean-François
authors.initials: JF
authors.lastname: Létard
authors.forename: Cédric
authors.initials: C
authors.lastname: Desplanches
authors.forename: Chérif
authors.initials: C
authors.lastname: Baldé
authors.forename: Cristian
authors.initials: C
authors.lastname: Enachescu
authors.forename: François
authors.initials: F
authors.lastname: Varret