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2009 BIOLOGY, CHEMISTRY AND SOFT MATTERConcerted spin crossover and symmetry breaking yield three thermally- andone light-induced crystallographic phases of a novel molecular materialSpin crossover (SC) complexes have been widely studiedover the last decades: the reversible low-spin(LS) ⇄ high-spin (HS) switching triggered by a changein temperature or by light irradiation, has attractedmuch interest for potential applications in informationstorage. A new SC material [Fe II H 2 L 2−Me ][PF 6 ] 2 ,1 where H 2 L 2−Me denotes bis[((2-methylimidazol-4-yl)methylidene)-3-aminopropyl] ethylene diamine, hasbeen synthesized.Figure 139 shows the variation of the χ M T product of 1,evidencing a two-step SC process. The INT ⇄ LS step centeredaround 97 K shows a thermal hysteresis loop of 6 Kwidth, characteristic of a first order transition. When 1 wasirradiated with green light, a quantitative light-induced excitedspin state trapping effect (LIESST) was clearly observed.When the irradiation was stopped, two steps relaxationto the fully LS state was observed upon increasing T.Figure 139: Temperature dependence of the χ M T product oncooling () then warming mode () and after irradiating the sampleat 10 K with a 532 nm laser light. ()at sweeping rate of 1 Kmin-1The crystal structures of 1 were determined by singlecrystalXRD. In the HS state (250 K, P22 1 2 1 ), the averageFe-N bond length 〈Fe-N〉 = 2.190 Å is typical of an HSFe II site with six N donors. The crystal structure is madeof hydrogen bonded cation layers and anion layers in the(a,b) plane (Figure 140a). In the INT phase, an orderingof spin-states occurs among the Fe II sites, as a cell doublingalong the crystalline axis. The space group decreasesto the non-isomorphic monoclinic subgroup P2 1 : there aretwo non equivalent cation sites (Figure 140b), one mainlyHS (〈Fe1-N〉 = 2.13(1) Å) and the other one is mainly LS(〈Fe2-N〉 = 2.04(1) Å). The ordering in the INT phase resultsin the LS-HS-HS-LS pattern. Below 97 K, in the LSphase (P2 1 2 1 2 1 ), the unit cell has changed to (4a,b,c) andincludes two independent LS cations per unit cell (〈Fe1-N〉= 2.012 Å = 〈Fe2-N〉) (Figure 140c). Because of the symmetrybreakings, a related distortion of each molecule occurs,accompanied with slight tilts as well as displacementsof the ions.A novel structural reorganization occurs upon generationof the PIHS state: the P22 1 2 1 space group is the same asin the HS phase but with different translation symmetry.The structure includes two independent complex cationsper unit cell (Figure 140d) with Fe-N bond lengths typicalof HS Fe II sites (〈Fe1-N〉 = 2.17(1) Å and 〈Fe2-N〉 =2.18(1) Å). This is the first-case of photoinduced SC involvingsymmetry breaking while previous report demonstratedisostructural reorganization. The occurrence of four differentphases with different symmetries is unique and shouldbe related to the strong intermolecular interactions and tothe specific packing of anion and cation layers.Figure 140: Projection of the crystal packing in the HS phase(a), for the INT phase (b), the LS state (c) and PIHS (d). [HS(blue) and LS (red) sites] Additional projections along the multipliedcrystals axis on the right show the motions of the ions.N. BréfuelL. Toupet, E. Collet (University of Rennes), H. Watanabe, K. Tanaka (University of Kyoto, Japan), J.-P. Tuchagues (Universityof Toulouse), N. Matsumoto (University of Kumamoto, Japan)97