Interchain interactions raised the photo-induced [LS] → [HS*] transition temperature to 78 K in a cyanide-bridged [Fe III2CoII] chain†

Bistable mixed valence compounds have thermodynamically accessible phases at certain temperatures, and the electron transfer switches the electronic configurations by applying external stimuli like heat and light. Thermally induced phase transition temperatures range widely, while the photo-induced state conversions need irradiation at very low temperatures, such as below 30 K, and the photo-induced metastable state relaxes rapidly at low temperatures. We prepared new mixed-valence compounds of [Fe(bipy)(CN)₄]₂[CoL₂] (L = 4-[(1E)-2-phenyldiazenyl]pyridine for 1-papy and 4-(2-phenylethynyl)pyridine for 1-pepy) in which cyanide-bridged squared cores form corner-shared chains with substantial interchain π–π contacts. Mössbauser spectra revealed that 1-papy and 1-pepy are in the high-spin (HS) state [(Fe^III_LS)₂Co^II_HS] at 300 K and the low-spin (LS) state [Fe^II_LSFe^III_LSCo^III_LS] at 78 K, confirming the occurrence of the electron transfer coupled spin transition (ETCST). Magnetic susceptibility measurements suggested their T_c values of 231 and 260 K, respectively. Photoirradiation (808 nm) for 1-papy and 1-pepy at 10 K induced the state conversion from the [LS] to the [HS*] state, and the metastable [HS]* state relaxed to the thermodynamically stable [LS] states at temperatures (T_relax) of 130 and 90 K, respectively. Furthermore, the [LS] states in 1-papy and 1-pepy were fully converted to the [HS*] states by light irradiation at 78 and 50 K, respectively. The X-ray structural analyses showed characteristic coordination bond lengths for the metal ions in each electronic state before and after light irradiation, but shortened intrachain π_L⋯π_L contact distances, from 3.726(4) to 3.688(4) Å, were observed for 1-papy upon the state conversion from the [LS] to the [HS*] state, despite the swollen cell volumes from 2479 to 2566 ų, respectively. Photomagneto and structural studies suggest that the intermolecular interactions increase the light-induced state conversion and relaxation temperatures.