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[FeIIFeIIII(dto)3] (A = (n-CnH2n+1)4N, spiropyran; dto = C2O2S2).
In (n-C2H2n+1)4N[FeIIFeIII(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 FeII and FeIII 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-C2H2n+1)4N[FeIIFeIII(dto)3] significantly depend on the size of (n-CnH2n+1)4N+. In
the case of (SP-Me)[FeIIFeIII(dto)3] (SP = spiropyran), the photo-isomerization of SP-Me by UV light irradiation induces
the CTPT on the two-dimensional [FeIIFeIII(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.
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