Photoinduced Magnetization on Mo Ion in Copper Octacyanomolybdate: An X-ray Magnetic Circular Dichroism Investigation

Recently synthesized copper octacyanomolybdate molecules present interesting photomagnetic properties. Before irradiation, the molecules behave as noncoupled Cu II paramagnetic ions (with central diamagnetic Mo IV ion), whereas after irradiation (406 nm) they behave as superparamagnetic molecules with Cu II ions coupled to the molybdenum ion. The proposed mechanism to explain these photomagnetic properties is based on the photoinduced charge transfer from Mo IV ( S = 0) to Cu II ( S = 1/2) leading to the formation of Mo V ( S = 1/2) and Cu I ( S = 0) with strong ferromagnetic coupling between Mo V and the other Cu II spin carriers. This paper presents X-ray magnetic circular dichroism (XMCD) measurements at the molybdenum L 2,3 edges. Two bimetallic molecules have been investigated: [Mo(CN) 2 (CN-CuL) 6 ] 8+, L being tris(2-amino)ethylamine and [Mo(CN) 6 (CN-CuL′ 2 ) 2 ], with L′ being N, N ′,dimethyl ethylene diamine (labeled MoCu 2 -Meen). In both cases, before irradiation the XMCD signal is null as expected for diamagnetic Mo IV ( S = 0). After irradiation, an XMCD signal appears, which directly demonstrates the formation of spin density on the Mo ion. After reaching room temperature, the photoinduced spectral features disappear, indicating the reversibility of the photoinduced process. In the case of MoCu 2 -Meen, the XMCD experiments allow the observation of an X-ray-induced metastable state made of high-spin Mo IV ion ( S = 1). The application of the sum rule for isotropic spectra shows that there is no variation of the molybdenum oxidation state during the X-ray-induced magnetic process. From the Mo L 2,3 XMCD signal of the X-ray-photoexcited MoCu 2 -Meen, we obtain an orbital magnetic moment equal to 0.13 μ B and a spin magnetic moment equal to 1.22 μ B at T = 10 K and H = 6 T. These results demonstrate that the Mo ion in the X-ray-photoinduced excited state of the MoCu 2 -Meen complex is high-spin Mo IV ( S = 1).