The femtosecond m-edge approach to the photodynamics of iron(II) complexes

Polypyridyl complexes of ruthenium(II) and iridium(III) have long been used as photosensitizers for photocatalysis and photovoltaic energy production. Low-spin iron(II) complexes with polypyridyl ligands also have d6 ground-state electron configuration and are strong absorbers of visible light, and are consequently considered to hold promise as affordable alternatives to their more expensive ruthenium and iridium analogs. However, conventional iron complexes with polypyridyl ligands are unsuitable for use as photosensitizers because the excitation energy is dissipated in 200 fs by ultrafast population of the low-lying metal-centered quintet state. This process, which formally requires flipping the spins of two electrons, is conjectured to proceed through a metal-centered triplet intermediate state, but spectroscopic evidence of such an intermediate has remained elusive. X-ray absorption spectroscopy (XAS) has long been used to study the electronic structure of transition metal compounds. For 1st-row transition metals, XAS employing soft X-rays (L2,3-edge XAS) or extreme ultraviolet light (XUV, M2,3-edge XAS) is particularly sensitive to the metal-centered electronic structure. Innovations in ultrafast light sources in the soft X-ray and XUV energy ranges, particularly via high-harmonic generation (HHG), allow short-lived states to be examined with X-ray XAS. Despite of these favorable characteristics, ultrafast soft X-ray or XUV absorption spectroscopy has seldom been applied to study molecular coordination complexes of 1st-row transition metals due to the difficulties of sample preparation and spectroscopic interpretation. In this work, we build the theoretical and experimental framework to study molecular complexes of 1st-row transition metals by transient femtosecond M2,3-edge XAS, and use these capabilities to investigate the ultrafast photodynamics of low-spin iron(II) complexes. We first tackle the challenges associated with interpreting the L2,3-/M2,3-edge spectra of excited state species. The L2,3-/M2,3-edge spectra of transition metal species are conventionally modelled using charge transfer multiplet theory (CTM). The difficulty of handling excited states using existing CTM theory software is primarily due to the use in the software of spin-orbit coupled basis functions, a choice that is mathematically efficient but obscures chemically relevant information (e.g., the spin multiplicity of the system). Here, we show that by projecting the CTM theory eigenstates onto a spin-orbit decoupled basis, CTM theory eigenstates can be decomposed into pure-spin Russell-Saunders terms, allowing the spin-multiplicities and orbital symmetry to be directly read off. We then considered the issue of sample preparation. The characteristics of XUV light produced by the HHG light source require that the samples must be thin films of uniform, sub-micrometer thickness. We found that thermal evaporation and spin-coating can give thin film samples compatible with XUV spectroscopy, although these techniques do not always work for samples of interest. We show using five model complexes that M2,3-edge XAS is sensitive to the elemental identity, oxidation state, spin-state, and coordination geometry of the metal centers in a sample. Having established the framework of M2,3-edge XAS, we then turned to a study of photoinduced processes in iron(II) complexes. We performed femtosecond M2,3-edge XAS on a thin film of Fe(phen)3(SCN)2 deposited by thermal evaporation, where phen is 1,10-phenanthroline. The transient response following excitation into the metal-to-ligand charge transfer (MLCT) band shows that the sample undergoes the expected photoinduced conversion from its singlet ground state into its metastable quintet state. Transient response in the first 300 fs after excitation clearly shows the presence of an intermediate state, which was identified as a metal-centered 3T1g/3T2g state based on CTM theory simulations. The transient response at longer times also shows oscillations consistent with the evolution of a vibrational wave-packet on the quintet potential energy surface. To explore the effects of ligand changes on metal-centered photophysics, we then performed femtosecond M2,3-edge XAS on the iron(II) complex Fe[(4-CF3)2bpca]2 ((4-CF3)2bpca = N,N-bis(4-trifluoromethyl)pyridylcarbonyl amide), a sublimable molecule designed to model the behavior of Fe(terpy)2 2+ (terpy = 2,2’;6’,2’’-terpyridine). Compared to Fe(phen)3 2+ or Fe(bpy)3 2+ (bpy = 2,2’-bipyridine), Fe[(4-CF3)2bpca]2 and Fe(terpy)2 2+ have weaker ligand field strengths and undergo more extensive geometric reorganization upon low-spin to high-spin transition. The M2,3-edge transient response of Fe[(4-CF3)2bpca]2 upon irradiation into the MLCT band is consistent with photoinduced low-spin to high-spin conversion, but signs of a triplet intermediate state are no longer apparent. The frequency of the oscillation in the transient response is also considerably lower compared to what is observed in Fe(phen)3 2+. The results described in this work adds a hitherto missing piece to our knowledge of the photodynamics of iron(II) complexes such as Fe(bpy)3 2+ and Fe(phen)3 2+. The behavior of Fe[(4-CF3)2bpca]2 points to the possibility of steering the relaxation pathway through different states by judicious choice of ligands. The overall theoretical and experimental work described in this thesis goes a long way toward making time-resolved M2,3-edge XAS in the XUV energy an accessible and generally applicable tool for studying the metal-centered photodynamics of transition metal complexes.U of I OnlyAuthor requested U of Illinois access only (OA after 2yrs) in Vireo ETD syste