Ferromagnetism and its photo-induced effect in 2D iron mixed-valence complex coupled with photochromic spiropyran

Iron mixed-valence complex, (n-C3H7)4N[FeIIFeIII(dto)3] (dto = C2O2S2), shows a spin-entropy driven phase transition called charge transfer phase transition around 120 K and a ferromagnetic transition at 6.5 K. These phase transitions remarkably depend on the hexagonal ring size in two-dimensional honeycomb network structure of [FeIIFeIII(dto)3]∞. In order to control the magnetic properties and the electronic state in the dto-bridged iron mixed-valence system by means of photo-irradiation, we have synthesized a photo-sensitive organic–inorganic hybrid system, (SP)[FeIIFeIII(dto)3] (SP = spiropyran), and investigated the photo-induced effect on the magnetic properties by the measurements of magnetic susceptibility, ESR, IR, UV–vis and 57Fe Mössbauer spectra. Upon UV irradiation at 337 nm, a broad absorption band between 500 nm and 600 nm appears and continuously increases with the photo-irradiation time, which implies that the UV irradiation changes the structure of spiropyran from closed form to open one in solid state. The photochromism in spiropyran changes the ferromagnetic transition temperature and the coercive force.