High-spin→low-spin relaxation in the two-step spin-crossover compound [Fe(pic)3]Cl2EtOH (pic equals 2-picolylamine)

The spin-crossover compound [Fe(pic)(3)]Cl2EtOH (pic = 2-picolylamine) shows an unusual two-step spin transition. This is thought to be caused by specific nearest-neighbour interactions and short-range correlations and requires a theoretical treatment of the elastic interactions between the spin-changing molecules beyond the mean-field approximation. Such short-range correlations also influence the high-spin --> low-spin relaxation following the light-induced population of the high-spin state at cryogenic temperatures, leading to characteristic deviations from the predictions of a mean-field treatment. These deviations are directly observable by comparison of the full and unperturbed relaxation curves with curves for which the short-range correlations were destroyed using an appropriate irradiation technique. Monte Carlo simulations including both nearest-neighbour and long-range interactions give a description of the observed relaxation curves which is consistent with the thermal spin equilibrium. (C) 1997 Elsevier Science Ltd. All rights reserved.