Femtosecond Infrared Spectroscopy Resolving the Multiplicity of High-Spin Crossover States in Transition Metal Iron Complexes

Tuning the photophysical properties of iron-based transition-metal complexes is crucial for their employment as photosensitizers in solar energy conversion. For the optimization of these new complexes, a detailed understanding of the excited-state deactivation paths is necessary. Here, we report femtosecond transient mid-IR spectroscopy data on a recently developed octahedral ligand-field enhancing [Fe(dqp)(2)](2+) (<bold>C1</bold>) complex with dqp = 2,6-diquinolylpyridine and prototypical [Fe(bpy)(3)](2+) (<bold>C0</bold>). By combining mid-IR spectroscopy with quantum chemical DFT calculations, we propose a method for disentangling the (5)Q(1) and T-3(1) multiplicities of the long-lived metal-centered (MC) states, applicable to a variety of metal-organic iron complexes. Our results for <bold>C0</bold> align well with the established assignment toward the (5)Q(1), validating our approach. For <bold>C1</bold>, we find that deactivation of the initially excited metal-to-ligand charge-transfer state leads to a population of a long-lived MC (5)Q(1) state. Analysis of transient changes in the mid-IR shows an ultrafast sub 200 fs rearrangement of ligand geometry for both complexes, accompanying the MLCT -> MC deactivation. This confirms that the flexibility in the ligand sphere supports the stabilization of high spin states and plays a crucial role in the MLCT lifetime of metal-organic iron complexes.