The cooperative spin-state transition of an iron(III) compound [FeIII(3-MeO-SalEen)2]PF6: thermal- vs. ultra-fast photo-switching
Résumé
The switching properties of the spin-transition solid [FeIII(3-MeO-SalEen)2]PF6 (H-3-MeO-SalEen resulting from the condensation of 3-methoxy-substituted salicylaldehyde and N-ethylethylenediamine), exhibiting a first-order transition, were investigated by using temperature and light as stimulation. The structural analysis reveals a first-order isostructural transition occurring with a 3 K width thermal hysteresis (T↓ = 162.5 K, T↑ = 165.5 K) coupled to the magnetic transition. The microscopic origin of this bistable behavior derives from strong coupling between pairs of complexes and the molecular packing. An extensive 3D network of intermolecular interactions, reinforced near the transition temperature, appears as a key feature for the cooperativity. The LMCT (ligand-to-metal charge transfer) transitions, detected by single-crystal transmission measurements, were selected for analyzing the photoswitching process of this cooperative solid. Only optical pump-probe experiments could detect photoinduced low-spin to high-spin conversion because of the too short lived excited state. The spin-state transformation induced by the femtosecond laser flash is very fast: electronic excitation corresponding to the low-spin LMCT transition relaxes toward the transient photoexcited high-spin state within 170 ± 50 fs.