0000000000056129
AUTHOR
Harald Romstedt
High-spin → low-spin relaxation in the two-step spincrossover compound [Fe(pic)3]Cl2EtOH (pic = 2-picolylamine)
Abstract The spin-crossover compound [Fe(pic) 3 ]Cl 2 EtOH (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 unperturb…
Intersystem Crossing and Light-Induced Bistability in Iron(?) Spin-Crossover Compounds
Abstract The dynamics of the high-spin→low-spin intersystem crossing process in iron(?) spin-crossover compounds are strongly influenced by cooperative effects of elastic origin which are due to the large difference in volume between high-spin and low-spin complexes. The deviation from first order kinetics is attributed to a build-up of an internal pressure as the relaxation proceeds, leading to a characteristic self-acceleration. The elastic interactions may lead to a light-induced bistability for systems which otherwise remain in the high-spin state down to cryogenic temperatures.
Modelling of two step high spin⇌low spin transitions using the cluster variation method
Abstract A thermodynamic description of high spin (HS)⇌low spin (LS) transition curves beyond the Bragg–Williams approximation is given using the Kikuchi cluster variation method (CVM). Transition curves of unusual behaviour (i.e. two step transition) are reproduced by short range interaction energies which are present in addition to long range elastic interaction between the spin changing molecules. The correlations in the distribution of the spin changing centers can be expressed analytically. They give rise to a reduced mixing entropy which was found experimentally in compounds with two step transitions.
Cooperative phenomena and light-induced bistability in iron(II) spin-crossover compounds
In iron(II) spin-crossover compounds, the transition from the 1A1 low-spin state at low temperatures to the 5T2 high-spin state at elevated temperatures is accompanied by a large increase in metal-ligand bond lengths. The resulting elastic interactions may be pictured as an internal pressure which is proportional to the concentration of the low-spin species. Because pressure stabilises the low-spin state relative to the high-spin state this results in a positive feedback. Thermal transition curves in neat iron(II) spin-crossover compounds are thus invariable much steeper than in diluted mixed crystals, and the high-spin→low-spin relaxation following the light-induced population of the high-…