Dynamical Spin Crossover Phenomenon in Organic-Inorganic Composite Complex and Its Development to Molecular System

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In the case of mixed-valence complexes whose spin states are situated in the spin-crossover region, it is expected that a cooperative phenomenon coupled with spin and charge takes place between neighboring metal ions in order to minimize the free energy in the whole system. From this viewpoint, we have investigated the charge transfer phase transition, which is a conjugated phenomenon coupled with spin and charge, (n-C_nH_<2n+1>)_4N[Fe^<II>Fe^<III> (dto)_3] (n=3-6,dto=C_2O_2S_2). The charge transfer phase transition takes place for n = 3 and 4, while it does not take place between 300 K and 4 K for n=5 and 6. In order to reveal the dynamics of the charge transfer phase transition, we have investigated muon spin relaxation (μSR) for n - 3 and 5. In the temperature range of the charge transfer phase transition for n = 3, an anomalous enhancement of the dynamic muon-spin depolarization rate with thermal hysteresis loop was observed at around 80 K in the temperature dependence of the depol … More arization rate. The origin of this anomaly has been successfully assigned to the dynamic process of electron-transfer between Fe^<II> and Fe^<III> and its frequency has been estimated from LF-μSR to be about the order of 0.1 MHz. This result is the first observation of dynamical electron transfer process by μSR technique for such mixed valence complexes.Moreover, 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)[Fe^<II>Fe^<III>(dto)_3] (SP = spiropyran), and investigated the photo-induced effect on the magnetic properties by the measurements of magnetic susceptibility, UV-vis and ^<57>Fe Mossbauer spectra. The UV irradiation changes the structure of spiropyran from the closed form to the open one in (SP)[Fe^<II>Fe^<III> (dto)_3], which induces the charge transfer phase transition in the two-dimensional honeycomb network structure of [Fe^<II>Fe^<III>(dto)_3]., and changes the ferromagnetic transition temperature. Less