Study on spin configuration in photoresponsive iron mixed-valence complexes by Mössbauer spectroscopy

We have investigated magnetic properties in a series of photoresponsive dithiooxalato (dto)-bridged iron mixed-valence complexes, (SP-R)[Fe^IIFe^III(dto)₃] (SP-R = R-substituted pyridospiropyran cation; R = Me, Et, and Pr; abbreviated as 1 ^Me, 1 ^Et, and 1 ^Pr, respectively). As for our previous reports, 1 ^Me and 1 ^Et show two-step succeeding ferromagnetic transitions at T _C = 25 & 8 K and 22 & 5 K, respectively. However, 1 ^Et has no hysteresis in the magnetic susceptibility, while 1 ^Me undergoes the charge transfer phase transition with thermal hysteresis around 75 K. To elucidate the two-step transitions of them, we measured ⁵⁷Fe Mössbauer spectra of 1 ^Et. The spectra of Fe^II (S = 2) and Fe^III (S = 1/2) in the HTP were observed in the magnetically ordered state as well as the paramagnetic state, and revealed that only HTP exists in a temperature range up to 5 K. The result is consistent with that of 1 ^Pr, where one ferromagnetic phase transition occurs at T _C = 10 K. ⁵⁷Fe Mössbauer spectroscopy is useful to clarify the origin of the succeeding magnetic transition for these systems.