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 CsFeII[CrIII(CN)6]·1.3H2O. This material consists of an original FeII–NC–CrIII units, responsible for the thermal spin transition (ST) of the low ligand field Fe ST site from a low-spin (LS) FeIILS to a high-spin (HS) FeIIHS 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 FeIILS–CN–CrIII 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. Our results reveal that photoexcitation indeed generates the HS state, but its lifetime is limited to a few picoseconds by ultrafast structural relaxation. Excitation at 800 nm predominantly triggers a Fe-centered d–d transition at the ST site, whereas excitation at 530 nm induces metal-to-metal charge transfer (MM’CT) at both ST and flipped sites. For the ST site, both excitation pathways generate the HS state within less than 200 fs, which launches coherent torsional motion of the cyanide-bridged network, and this HS state decays to a LS state within 3.5 ps. In contrast, at the flipped site, 530 nm excitation selectively populates a transient MM’CT FeIIILS–CrII*, which decays without inducing spin state switching nor cyanide flipping. Overall, the photoinduced HS state rapidly decays, due to the counteracting pressure of the rigid cyanide-bridged lattice, preventing long-lived LIESST and cooperative switching.