title: Cyanocarbanion-based spin-crossover materials: photocrystallographic and photomagnetic studies of a new iron(II) neutral chain.
abstract: A new iron(II) chain of formula [Fe(abpt)(2)(tcpd)] [1; (tcpd)(2-) = [C(10)N(6)](2-) = (C[C(CN)(2)](3))(2-) = 2-dicyanomethylene-1,1,3,3-tetracyanopropanediide anion, abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole] has been synthesized and characterized by IR spectroscopy, detailed variable-temperature single-crystal X-ray diffraction, magnetic and photomagnetic measurements. The crystal structure determination of 1 reveals a one-dimensional structural architecture in which the (tcpd)(2-) cyanocarbanion acts as a μ(2)-bridging ligand and the two abpt molecules act as chelating ligands. Detailed X-ray diffraction studies as a function of the temperature (293-10 K) showed a strong modification of the iron coordination sphere, whose characteristics are in agreement with the presence of a spin-crossover transition from high spin (HS) to low spin (LS) in 1. The average Fe-N distances at room temperature, at 10 K following a flash cooling, and at 10 K after subsequent HS-to-LS relaxation are in the range expected for 100%, 50%, and 25% fractions of HS Fe(II), respectively. These observations are consistent with the presence of ca. 25% residual HS species at low temperatures, as derived from the magnetic data. The signature of a photoinduced metastable HS state in 1 has been detected by performing coupled photomagnetic and photocrystallographic analyses. The limiting T(LIESST) value associated with the light-induced excited-spin-state trapping effect was derived as 35 K, in good agreement with the thermal dependence of the unit cell volume upon irradiation. Kinetic studies governing the photoinduced HS/LS process have been recorded at different temperatures; a reverse-LIESST effect has been evidenced at 10 K as a reduction of the residual HS fraction by irradiating the sample at 830 nm.
authors.affiliation: Université Européenne de Bretagne, UMR CNRS 6521, 6 Avenue V. Le Gorgeu, C.S. 93837, 29238 Brest, France.
authors.forename: Gaelle
authors.initials: G
authors.lastname: Dupouy
authors.forename: Smail
authors.initials: S
authors.lastname: Triki
authors.forename: Mathieu
authors.initials: M
authors.lastname: Marchivie
authors.forename: Nathalie
authors.initials: N
authors.lastname: Cosquer
authors.forename: Carlos J
authors.initials: CJ
authors.lastname: Gómez-García
authors.forename: Sébastien
authors.initials: S
authors.lastname: Pillet
authors.forename: El-Eulmi
authors.initials: el-E
authors.lastname: Bendeif
authors.forename: Claude
authors.initials: C
authors.lastname: Lecomte
authors.forename: Saket
authors.initials: S
authors.lastname: Asthana
authors.forename: Jean-François
authors.initials: JF
authors.lastname: Létard