Létard Jean-François, Guionneau Philippe, Rabardel Louis, Howard Judith A. K., Goeta Andres E., Chasseau Daniel, Kahn Olivier
Laboratoire des Sciences Moléculaires, Institut de Chimie de la Matière Condensée de Bordeaux, UPR CNRS No. 9048, 33608 Pessac, France, and Durham Chemical Crystallography Group, Chemistry Department, Durham University, South Road, Durham DH1 3LE, U.K.
Inorg Chem. 1998 Aug 24;37(17):4432-4441. doi: 10.1021/ic980107b.
The new spin-crossover compound Fe(PM-BiA)(2)(NCS)(2) with PM-BiA = N-(2-pyridylmethylene)aminobiphenyl has been synthesized. The temperature dependence of chi(M)T (chi(M) = molar magnetic susceptibility and T = temperature) has revealed an exceptionally abrupt transition between low-spin (LS) (S = 0) and high-spin (HS) (S = 2) states with a well-reproducible hysteresis loop of 5 K (T(1/2) downward arrow = 168 K and T(1/2) upward arrow = 173 K). The crystal structure has been determined both at 298 K in the HS state and at 140 K in the LS state. The spin transition takes place without change of crystallographic space group (Pccn with Z = 4). The determination of the intermolecular contacts in the LS and HS forms has revealed a two-dimensional structural character. The enthalpy and entropy variations, DeltaH and DeltaS, associated with the spin transition have been deduced from heat capacity measurements. DeltaS (= 58 J K(-)(1) mol(-)(1)) is larger than for other spin transition bis(thiocyanato) iron(II) derivatives. At 10 K the well-known LIESST (light-induced excited spin state trapping) effect has been observed within the SQUID cavity, by irradiating a single crystal or a powder sample with a Kr(+) laser coupled to an optical fiber. The magnetic behavior recorded under light irradiation in the warming and cooling modes has revealed a light-induced thermal hysteresis (LITH) effect with 35 < T(1/2) < 77 K. The HS --> LS relaxation after LIESST has been found to deviate from first-order kinetics. The kinetics has been investigated between 10 and 78 K. A thermally activated relaxation behavior at elevated temperatures and a nearly temperature independent tunneling mechanism at low temperatures have been observed. The slow rate of tunneling from the metastable HS state toward the ground LS state may be explained by the unusually large change in Fe-N bond lengths between these two states.
合成了新型自旋交叉化合物Fe(PM - BiA)₂(NCS)₂，其中PM - BiA = N-(2-吡啶基亚甲基)氨基联苯。χ(M)T（χ(M) = 摩尔磁化率，T = 温度）的温度依赖性揭示了低自旋（LS）（S = 0）和高自旋（HS）（S = 2）状态之间异常陡峭的转变，具有可重现的5 K滞后回线（T(1/2)向下箭头 = 168 K，T(1/2)向上箭头 = 173 K）。已确定该化合物在298 K的HS状态和140 K的LS状态下的晶体结构。自旋转变发生时晶体学空间群不变（Pccn，Z = 4）。对LS和HS形式的分子间接触的测定揭示了二维结构特征。通过热容测量推导出了与自旋转变相关的焓变和熵变，ΔH和ΔS。ΔS（= 58 J K⁻¹ mol⁻¹）比其他自旋转变双(硫氰酸根)铁(II)衍生物的大。在10 K时，通过用耦合到光纤的Kr⁺激光照射单晶或粉末样品，在超导量子干涉仪腔内观察到了著名的光诱导激发自旋态捕获（LIESST）效应。在光照下升温及降温模式下记录的磁行为揭示了35 < T(1/2) < 77 K的光诱导热滞后（LITH）效应。发现LIESST后HS→LS弛豫偏离一级动力学。在10至78 K之间研究了动力学。观察到高温下的热激活弛豫行为和低温下几乎与温度无关的隧穿机制。从亚稳态HS态向基态LS态的缓慢隧穿速率可能由这两种状态之间Fe - N键长的异常大变化来解释。
