ИСТИНА

Copper octacyanomolybdate molecular systems present reversible photomagnetic transitions. Before irradiation, the molecular entities behave as non-coupled CuII paramagnetic ions with a central diamagnetic MoIV ion. At low temperature, light irradiation at 406 nm is associated with a sharp variation of the magnetic susceptibility. The proposed mechanism to explain the photomagnetic transition was first based on a photoinduced charge transfer from MoIV(S=0) - CuII(S=1/2) pair to the formation of MoV(S=1/2) - CuI(S=0) pair with strong ferromagnetic coupling between MoV and the other CuII neighbors. Through an extensive set of SQUID magnetic measurements, x-ray absorption spectroscopy (XAS), and x-ray magnetic circular dichroism (XMCD), we have determined that the nature of the metastable photomagnetic phase is not connected to a charge transfer but to a spin cross-over from a low spin MoIV(S=0) ion to a high spin MoIV(S=1) ion. This finding has been firmly determined on the cyano-bridged 3D network Cs2Cu7[Mo(CN)8]4 (sample 1) [1] and on molecules such as [Mo(CN)6 (CN-CuL′2)2], with L′ being N ,N′,dimethyl ethylene diamine (sample 2) and [Mo(CN)2 (CN-CuL)6]8+, L being tris(2-amino)ethylamine (sample 3) [2]. For the three samples, the XMCD signal at the Mo L2,3 edges is null before light irradiation as expected for diamagnetic MoIV(S=0) ions. After light irradiation, a large XMCD signal appears, which directly reflects the formation of spin density on the Mo ion. After warming up to room temperature, the paramagnetic Mo ion disappears, indicating the reversibility of the photoinduced transition. The application of orbital, spin and isotropic sum rules shows that in the photomagnetic metastable state at T=10K and B=6T -Mo ions are not oxidized and remain MoIV ions, -the orbital magnetic moment is equal to 0.13 ± 0.05 μB per Mo ion, -the spin magnetic moment is equal to 1.22 ± 0.1 μB per Mo ion. These results demonstrate that the photomagnetic transition is induced by the photoconversion of low spin MoIV(S=0) ions into high spin MoIV(S=1) ions. The magnetic and spectroscopic measurements fully support the low spin to high spin photomagnetic conversion, whereas there is no spectroscopic evidence for the charge transfer mechanism from Mo(IV)-Cu(II) to Mo(V)-Cu(I). From the joint perspective of density functional theory (DFT) and ligand field multiplet theory (LFM), we performed the simulation of the XMCD spectra at the Mo L2,3 edges. This provides a theoretical justification for the Mo spin cross over process. [1] S. Brossard et al. J. Am. Chem. Soc., 2012, 134, 222–228 [2] M.-A. Arrio et al. J. Phys. Chem. C, 2010, 114, 593–600 [3] O. Bunau et al. J. Phys. Chem. A, 2012, 116, 8678−8683