Progress of multi functional properties of organic-inorganic hybrid system, A[Fe^IIFe^IIIX₃] (A = (n-C_nH_2n+1)₄N, spiropyran; X = C₂O₂S₂, C₂OS₃, C₂O₃S)

In the case of mixed-valence systems whose spin states are situated in the spin crossover region, new types of conjugated phenomena coupled with spin and charge are expected. From this viewpoint, we have investigated the multifunctional properties coupled with spin, charge and photon for the organic-inorganic hybrid system, A[Fe^IIFe^IIIX₃](A = (n-C_nH_2n+1)₄N, spiropyran; X = dto(C₂O₂S₂), tto(C₂OS₃), mto(C₂O₃S)). A[Fe^IIFe^III(dto)₃] and A[Fe^IIFe^III(tto)₃] undergo the ferromagnetic phase transitions, while A[Fe^IIFe^III(mto)₃] undergoes a ferrimagnetic transition. In (n-C_nH_2n+1)₄N [Fe^IIFe^III(dto)₃](n = 3,4), a new type of phase transition called charge transfer phase transition (CTPT) takes place around 120 K, where the thermally induced charge transfer between Fe^II and Fe^III occurs reversibly. At the CTPT, the iron valence state dynamically fluctuated with a frequency of about 0.1 MHz, which was confirmed by means of muon spin relaxation. The charge transfer phase transition and the ferromagnetic transition for (n-C_nH_2n+1)₄N[Fe^IIFe^III(dto)₃] remarkably depend on the size of intercalated cation. In the case of (SP)[Fe^IIFe^III(dto)₃](SP = spiropyran), the photoinduced isomerization of SP under UV irradiation induces the charge transfer phase transition in the [Fe^IIFe^III(dto)₃] layer and the remarkable change of the ferromagnetic transition temperature. In the case of (n-C_nH_2n+1)₄N[Fe^IIFe^III(mto)₃](mto = C₂O₃S), a rapid spin equilibrium between the high-spin state (S = 5/2) and the low-spin state (S = 1/2) at the Fe^IIIO₃S₃ site takes place in a wide temperature range, which induces the valence fluctuation of the FeS₃O₃ and FeO₆ sites through the ferromagnetic coupling between the low spin state (S = 1/2) of the Fe^IIIS3O3 site and the high spin state (S = 2) of the Fe^IIO₆ site.