Control of Charge Transfer Phase Transition and Ferromagnetism by Photoisomerization of Spiropyran for an Organic-Inorganic Hybrid System, (SP)[FeIIFeIII(dto)3] (SP = spiropyran, dto = C2O2S2)

Iron mixed-valence complex, (n-C3H7)(4)N[(FeFeIII)-Fe-II(dto)(3)](dto = C2O2S2), shows a spin entropy-driven phase transition called charge transfer phase transition in [(FeFeIII)-Fe-II(dto)(3)](infinity)(-) around 120 K and a ferromagnetic transition at 7 K. These phase transitions remarkably depend on the hexagonal ring size in the two-dimensional honeycomb network structure of [(FeFeIII)-Fe-II(dto)(3)](infinity)(-). In order to control the magnetic properties and the electronic state in the dto-bridged iron mixed-valence system by means of photoirradiation, we have synthesized a photosensitive organic-inorganic hybrid system, (SP)[(FeFeIII)-Fe-II(dto)(3)](SP = spiropyran), and investigated the photoinduced effect on the magnetic properties. Upon UV irradiation at 350 nm, a broad absorption band between 500 and 600 nm appears and continuously increases with the photoirradiation time, which implies that the UV irradiation changes the structure of spiropyran from the closed form to the open one in solid state. The photochromism in spiropyran changes the ferromagnetic transition temperature from 5 to 22 K and the coercive force from 1400 to 6000 Oe at 2 K. In this process, the concerted phenomenon coupled with the charge transfer phase transition in [(FeFeIII)-Fe-II(dto)(3)](infinity)(-) and the photoisomerization of spiropyran is realized.