The Role of Ligand- Field States in the Ultrafast Photophysical Cycle of the Prototypical Iron( II) Spin- Crossover Compound [Fe(ptz)6]( BF4)

Light-induced excited spin-state trapping (LIESST) in iron(II) spin-crossover compounds, that is, the light-induced population of the high-spin (S=2) state below the thermal transition temperature, was discovered thirty years ago. For irradiation into metal-ligand charge transfer (MLCT) bands of the low-spin (S=0) species the acknowledged sequence takes the system from the initially excited (MLCT)-M-1 to the high-spin state via the (MLCT)-M-3 state within ca. 150fs, thereby bypassing low-lying ligand-field (LF) states. Nevertheless, these play a role, as borne out by the observation of LIESST and reverse-LIESST on irradiation directly into the LF bands for systems with only high-energy MLCT states. Herein we elucidate the ultrafast reverse-LIESST pathway by identifying the lowest energy S=1 LF state as an intermediate state with a lifetime of 39ps for the light-induced high-spin to low-spin conversion on irradiation into the spin-allowed LF transition of the high-spin species in the NIR.