Theoretical studies on magnetic interactions in Prussian blue analogs and active controls of spin states by external fields.

The symmetry rules for the d-orbital interactions are considered for simple prediction of the sign of effective exchange integrals between transition metal ions in Prussian blue analogs. Ab initio UHF and DFT calculations were carried out for binuclear transition-metal cyanides in order to elucidate variations of the through-bond exchange integrals with combinations of transition metal ions having different d-electron configurations. It was shown that the sign of effective exchange integrals predicted from the symmetry rules and the ab initio calculations are consistent with the experiments but their magnitudes calculated for tetracentric systems MCNM' (M=Cr(III), M'=V(II), Mn(II), etc.) by the use of triple zeta basis sets are larger than the observed values for Prussian blue analogs. The calculated results clearly indicated that the direct exchange interactions between M and M' are negligible, whereas the through-bond superexchange and spin polarization interactions are crucial for the high-T-c ferri (or ferro) magnetism in Prussian blue analogs. The modifications of electronic states in antiferromagnetic of paramagnetic precursors are also discussed in relation to our previous theoretical proposals for molecular spinics and recent experiments to obtain the switching type molecular magnets controlled by photochemical, electrochemical and other techniques.