Photoinduced magnetization in copper octacyanomolybdate

This article describes the studies of a photomagnetic cyanide-bridged Cu-Mo bimetallic assembly, Cu-2(II)[Mo-IV(CN)(8)]center dot 8H(2)O (Cu-II, S = 1/2; Mo-IV, S = 0) (1), which has an intervalence transfer (IT) band from Mo-IV-CN-Cu-II to Mo-V-CN-Cu-I 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 (lambda(max) = 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 Mo-IV to Cu-II, producing a ferromagnetic mixed-valence isomer of (CuCuII)-Cu-I[Mo-V(CN)(8)]center dot 8H(2)O (Cu-I, S = 0; CUII, S = 1/2; Mo-V, 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.