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首页> 外文期刊>Chemistry: A European journal >Understanding the Two-Step Spin-Transition Phenomenon in Iron(II) 1D Chain Materials
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Understanding the Two-Step Spin-Transition Phenomenon in Iron(II) 1D Chain Materials

机译：了解铁（II）1D链材料中的两步自旋转变现象

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Three analogous one dimensional (1D) polymeric iron(II) spin crossover (SCO) materials containing the new ligand 4,6-bis(2',2"-pyridyl)pyrazine (bdpp) have been comprehensively characterised magnetically (thermal and light-induced) and structurally. Within this series are two polymorphs of the formula [Fe(NCS)(2)(bdpp)], 1 and 2a, which differ magnetically in that phase 1. undergoes a full two-step SCO (T-1/2(1) = 135K and T-1/2(2) = 90 K) whereas phase 2a remains high spin (HS) over all temperatures. The central distinction between these two materials lies in the presence of intermolecular pi-pi interactions generated by the crystal packing in 1, which are absent in 2a. The isostructural selenocyanate analogue of 2a, [Fe(NCSe)(2)(bdpp)], 2b, undergoes a full two-step SCO (T-1/2(1) = 200K and T-1/2(2) = 125 K). Structural analyses of 1 and 2b at a range of temperatures provide deep insight into their two-step SCO nature. Structural analysis of 1 at 25 K (1(LS-LS)), 123 K (1(LS-HS)) and 250 K (1(HS-HS)) reveals two distinct iron(II) centres at each temperature, with ordered, alternating HS and LS (low spin) sites at the intermediate plateau (1P) temperatures. In contrast, structural analysis of 2b at 90 K (2b(LS)), 150 K (2b(LS/HS)) and 250 K (2b(HS)) reveals one unique iron(II) centre at each temperature with an "averaged" LS/HS character at the IP temperature. Weak planes of diffuse scattering in the single-crystal X-ray diffraction patterns were observed for this phase at 90 and 150 K, indicating that ID long range ordering of alternating HS/LS iron(II) centres occurs along the ID coordination chains, but that there is no correlation between chains. The lack of observable diffuse scattering at 250 K suggests that the onset of the 1D structural ordering in the chain direction corresponds to the first step of the SCO and that this structural transition is electronically driven. The photomagnetic properties of both 1 and 2b have been investigated and show approximate to 62 and 53% photo-excitation of a HS metastable state at low temperatures and T(LIESST) values of 55 and 49 K, respectively. Relaxation studies on the HS fraction in 2b fitted well to a stretched exponential model with kinetic parameters indicative of weak cooperativity.
机译：三种具有新型配体4,6-双（2'，2“-吡啶基）吡嗪（bdpp）的相似的一维（1D）聚合铁（II）自旋交联（SCO）材料已通过磁学方法（热和光在这个系列中，分子式为[Fe（NCS）（2）（bdpp）]，1和2a的两个多晶型物，在相1中磁性不同，经历了完整的两步SCO（T-1 / 2（1）= 135K和T-1 / 2（2）= 90 K），而2a相在所有温度下均保持高自旋（HS），这两种材料之间的主要区别在于存在分子间pi-pi相互作用由1中的晶体堆积产生，而2a中没有。2a的同结构硒氰酸酯类似物[Fe（NCSe）（2）（bdpp）]，2b经历完整的两步SCO（T-1 / 2（ 1）= 200K，T-1 / 2（2）= 125 K）。在一定温度范围内1和2b的结构分析提供了对其两步SCO性质的深入了解。在25 K（1（ LS-LS）），123 K（1（LS-HS））和250 K（1（HS-HS））在每个温度下都显示出两个不同的铁（II）中心，在中间高原（1P）温度下有序，交替的HS和LS（低自旋）位点。相反，对2b在90 K（2b（LS）），150 K（2b（LS / HS））和250 K（2b（HS））处的结构分析显示，每个温度下一个唯一的铁（II）中心带有一个“ IP温度下的平均LS / HS字符。在90和150 K时，在该相中观察到了单晶X射线衍射图的弱散射平面，表明交替的HS / LS铁（II）中心的ID长程有序沿着ID配位链发生，但是链之间没有关联。在250 K处缺乏可观察到的漫散射，这表明一维结构顺序在链方向上的出现与SCO的第一步相对应，并且这种结构转变是电子驱动的。已经研究了1b和2b的光磁特性，分别显示了在低温和T（LIESST）值分别为55和49 K时，HS亚稳态的光激发分别约为62％和53％。 2b中HS分数的弛豫研究很好地拟合了具有动态参数指示弱协同性的拉伸指数模型。

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