STRUCTURAL, MAGNETIC, AND PHOTOMAGNETIC STUDIES OF A MONONUCLEAR IRON(II) DERIVATIVE EXHIBITING AN EXCEPTIONALLY ABRUPT SPIN TRANSITION - LIGHT-INDUCED THERMAL HYSTERESIS PHENOMENON

The new spin-crossover compound Fe(PM-BiA)(2)(NCS)(2) with PM-BiA = N- (2-pyridylmethylene)aminobiphenyl has been synthesized. The temperatur e 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 (PIS) (S = 2) states with a well- reproducible hysteresis loop of 5 K (T(1/2)down arrow = 168 K and T(1/ 2)up arrow = 173 K). The crystal structure has been determined both at 298 K in the I-IS state and at 140 K in the LS state. The spin transi tion takes place without change of crystallographic space group (Pccn with Z = 4). The determination of the intermolecular contacts in the L S and HS forms has revealed a two-dimensional structural character. Th e enthalpy and entropy variations, Delta H and Delta S, associated wit h the spin transition have been deduced from heat capacity measurement s. Delta S(= 58 J K-1 mol(-1)) is larger than for other spin transitio n 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 s ample with a Kr+ laser coupled to an optical fiber. The magnetic behav ior 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 t o deviate from first-order kinetics. The kinetics has been investigate d between 10 and 78 K. A thermally activated relaxation behavior at el evated temperatures and a nearly temperature independent tunneling mec hanism at low temperatures have been observed. The slow rate of tunnel ing from the metastable HS state toward the ground LS state may be exp lained by the unusually large change in Fe-N bond lengths between thes e two states.