Abstract

In mixed-valence assembled metal complexes whose spin states are situated in the spin-crossover region, multifunctional properties induced by the synergetic effect between spin and charge are expected. Based on this viewpoint, we have developed a ferromagnetic mixed-valence system, A[Fe^IIFe^IIII(dto)₃] (A = (n-C_nH_2n+1)₄N, spiropyran; dto = C₂O₂S₂). In (n-C₂H_2n+1)₄N[Fe^IIFe^III(dto)₃], we observed a charge transfer phase transition (CTPT) at 122.4 K and 142.8 K for n = 3 and 4, respectively, where a thermally induced charge transfer occurs reversibly between the Fe^II and Fe^III sites. In contrast, the CTPT does not occur for n = 5 and 6 at ambient pressure. From the analysis of heat capacity, the CTPT is regarded as a spin-entropy driven phase transition to minimize the Gibbs energy in the whole system. The appearance of CTPT and the Curie temperature for (n-C₂H_2n+1)₄N[Fe^IIFe^III(dto)₃] significantly depend on the size of (n-C_nH_2n+1)₄N⁺. In the case of (SP-Me)[Fe^IIFe^III(dto)₃] (SP = spiropyran), the photo-isomerization of SP-Me by UV light irradiation induces the CTPT on the two-dimensional [Fe^IIFe^III(dto)₃] layer and the remarkable change of Curie temperature.

Keywords: Calorimetry, charge transfer interaction, charge transfer phase transition, dithiooxalate ligand, ferromagnetism, heat capacity, Gibbs energy, mixed-valence, molecular magnetism, photo-induced phase transition, pressure effect, spin entropy.

Graphical Abstract