Photoinduced Magnetization in Copper Octacyanomolybdate

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Abstract
This article describes the studies of a photomagnetic cyanide-bridged Cu−Mo bimetallic assembly, CuII2[MoIV(CN)8]·8H2O (CuII, S = 1/2; MoIV, S = 0) (1), which has an intervalence transfer (IT) band from MoIV−CN−CuII to MoV−CN−CuI around 480 nm. Wide-angle X-ray scattering and X-ray spectroscopic studies provide precise information about the 3D connectivity and the local environment of the transition metal ions. Irradiating with blue light causes solid 1 to exhibit a spontaneous magnetization (Curie temperature = 25 K). The thermal reversibility is carefully studied and shows the long-time stability of the photoinduced state up to 100 K. Photoreversibility is also observed; i.e., the magnetization is induced by irradiation with light below 520 nm, while the magnetization is reduced by irradiation with light above 520 nm. The UV−vis absorption spectrum after irradiation shows a decrease of the IT band and the appearance of the reverse-IT band in the region of 600−900 nm (λmax = 710 nm). This UV−vis absorption spectrum is recovered to the original spectrum by irradiation with 658-, 785-, and 840-nm light. In this photomagnetic effect, the excitation of the IT band causes an electron transfer from MoIV to CuII, producing a ferromagnetic mixed-valence isomer of CuICuII[MoV(CN)8]·8H2O (CuI, S = 0; CuII, S = 1/2; MoV, S = 1/2) (1‘). 1‘ returns to 1 by irradiation of the reverse-IT band, which obeys the scheme for the potential energy surface in mixed-valence class II compounds.

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