Femtosecond Infrared Spectroscopy Resolving the Multiplicity of High-Spin Crossover States in Transition Metal Iron Complexes
Clark Zahn, Mariachiara Pastore, J. Luis Pérez Lustres, Philippe C. Gros, Stefan Haacke, Karsten Heyne
Abstract
High Resolution Image Download MS PowerPoint Slide 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+ ( C1 ) complex with dqp = 2,6-diquinolylpyridine and prototypical [Fe(bpy) 3 ] 2+ ( C0 ). By combining mid-IR spectroscopy with quantum chemical DFT calculations, we propose a method for disentangling the 5 Q 1 and 3 T 1 multiplicities of the long-lived metal-centered (MC) states, applicable to a variety of metal–organic iron complexes. Our results for C0 align well with the established assignment toward the 5 Q 1, validating our approach. For C1, 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.