Structural, Magnetic, and Photomagnetic Studies of a Mononuclear Iron(II) Derivative Exhibiting an Exceptionally Abrupt Spin Transition. Light-Induced Thermal Hysteresis Phenomenon

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Abstract
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 χMT (χM = 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 (T1/2↓ = 168 K and T1/2↑ = 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, ΔH and ΔS, associated with the spin transition have been deduced from heat capacity measurements. ΔS (= 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 < T1/2 < 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.

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