Ultrafast Site-Selective Excitation Pathways Governing Short-Lived Photoinduced Spin States in a Cyanide-Bridged Fe–Cr Spin-Crossover System

We study the ultrafast light-induced excited spin state trapping (LIESST) process in a cyanide-bridged Fe–Cr spin-crossover material CsFe^II[Cr^III(CN)₆]·1.3H₂O. This material consists of an original Fe^II–NC–Cr^III units, responsible for the thermal spin transition (ST) of the low ligand field Fe ST site from a low-spin (LS) Fe^II_LS to a high-spin (HS) Fe^II_HS states.

Although electronic structures suggest that a metastable HS state can be reached by photoexcitation, no LIESST to this HS state has yet been observed using conventional techniques. In addition, part of the units exhibits a thermally driven cyanide flip toward the Fe^II_LS–CN–Cr^III sites, where Fe is always LS by the high ligand field. By combining optical and IR spectroscopic studies, we investigated whether light excitation is ineffective in inducing the LIESST or whether it causes the cyanide flip.