Electron transfer optical bands of charge-ordered mixed valence compounds. Quasidynamical model

Valence tautomeric transformation in the [CrCo] compound: exploration of cooperative interactions

A crystal containing the heterometallic Cr-ligand-Co cluster with an unpaired electron on the ligand as a structural unit is examined. The developed model which describes the magnetic and polarizability characteristics of this crystal takes into account that the electron residing on the ligand can be transferred to the Co-ion, thus converting the diamagnetic ls-Co^III ion into the paramagnetic hs-Co^II one. Since this transformation is accompanied by electron density redistribution and elongation of the Co-N bond lengths, the vibronic interaction of the Co-ion with totally symmetric displacements of the nearest surroundings and cooperative dipole-dipole and electron-deformational interactions are accounted for as well. The exchange interactions between the Cr^III ion and the electron localized on the ligand as well as in the Cr^III-hs-Co^II pair are also included in consideration; the parameters of these interactions are estimated within the framework of the DFT method. Bistability in the magnetic and polarization characteristics is predicted for certain strengths of intra- and intercluster interactions in the crystal under study. Within the framework of the developed approach an explanation of the observed magnetic properties of the [Cr(SS-cth)(Co(RR-cth)(μ-dhbq))](PF₆)₂Cl crystal is given.

Modeling of Electron Transfer in a Linear Trinuclear Fe-Co-Fe Complex: Magnetic and Polarizability Properties

A microscopic approach to the problem of charge transfer-induced spin transitions in a system of interacting trinuclear Fe-Co-Fe clusters, which takes into account the switching of polarization during the transformation Fe_ls^II-Co_ls^III-Fe_ls^III→ Fe_ls^III-Co_hs^II-Fe_ls^III (ls-low spin, hs-high spin), has been developed. The cooperative electron-deformational and dipole-dipole interactions and the intracluster electron transfer have been proved to govern the observed significant changes in the magnetic and polarization characteristics of the examined system. The role of intra- and intercluster interactions in the spin transformation has been elucidated. On the basis of the developed approach, an explanation of the temperature dependence of the magnetic susceptibility and of the polar-nonpolar transformation in the {[FeTp(CN)₃]₂Co(Meim)₄}·6H₂O compound in response to thermal stimuli has been given.

Electric field control of the optical properties in magnetic mixed-valence molecules

In delocalized magnetic mixed-valence molecules the spin-crossover induced by an electric field may have a strong influence on their optical properties.

Influence of dehydration effects on the optical spectra of H4PVMo11O40 in the visible and near infrared range: intra- and intercenter optical transitions in the V-Mo cluster

UV-Vis-NIR diffuse reflectance spectra of heteropoly acids of the type of H4PVMo11O40.xH2O were recorded with improved spectroscopic equipment for different times on helium stream at room temperature. A broad asymmetric absorption band with a maximum at 740-780 nm in the visible range and a shoulder at 900-1000 nm in the near infrared range is identified. With increasing time on stream the intensity of this band grows appreciably: simultaneously, the water peaks at 1430 and 1925 nm decrease. Focusing on a single mixed-valence V4+-Mo6+-cluster, a model is suggested for the description of the experimental results. The model takes into account electron transfer, low symmetry crystal fields and vibronic interaction. The absorption band shape calculation is performed within the semiclassical limits. The band shape is shown to be determined both by the optical d-d transitions in V4+ and Mo5- and the intervalence V4+-Mo6+ optical transition. Appreciable changes in the band intensity are attributed to the increase in the electron transfer parameter due to water loss. Quite good agreement with the experiment was found for a reasonable set of transfer and vibronic parameters.