Ultrafast Dynamics of Iron(II)-Based Complexes in Solution and Semiconductor-Chromophore Assemblie

The desire to capitalize on the photochemical and photophysical properties of transition metal complexes is the driving force for a number of applications, such as solar energy conversion and photocatalysis. Work in these areas traditionally employ ruthenium-based chromophores that are successful due to the presence of long-lived metal-to-ligand charge transfer (MLCT) states. Unfortunately, ruthenium is a rare metal, prompting the search for a suitable replacement that is earth abundant. Iron represents one such option that also happens to be isoelectronic with ruthenium. However, as a first-row transition metal, the splitting between the ligand field (LF) states is smaller, such that the lowest energy excited state of iron polypyridyl complexes is a metal-centered state rather than MLCT. The fact that the MLCT manifold in Fe(II) polypyridyls undergoes rapid conversion to the lowest energy LF state on the order of 100 fs is also challenging. Despite this, solar cells featuring Fe(II) sensitizers are able to produce a photocurrent, thus, inspiring research into the fundamentals of Fe(II) dye-semiconductor assemblies.;Before studying the electron transfer processes associated with Fe(II) complexes in devices for solar energy conversion, it is important to understand the excited state dynamics inherent to the chromophores themselves, wherein an interesting discovery was made about the influence of solvent for certain relaxation processes along the way. While the solvent environment is known to shape the dynamics associated with a charge transfer excited state (as it involves a redistribution of charge across the molecule), that is not the case for a LF excited state, which entails an electronic rearrangement on the metal center. Work shown here demonstrates that a solvent dependence can indeed exist for LF transitions, with a particular focus on the ground state recovery process in tris(2,2'-bipyridine)iron(II), [Fe(bpy)3]2+. Possible reasons behind what could be driving this solvent dependence, such as ion pairing and changes in solvation energy, are considered and assessed via ultrafast time-resolved absorption spectroscopic and computational methods. Work has been done in an effort to understand the physical origin behind this solvent dependence and get a better idea of how and why outer-sphere variables seem to affect these inner-sphere processes.;Although the discovery that the solvent environment modulates LF state lifetimes in iron(II) polypyridyl complexes is noteworthy, MLCT states are more relevant for the purposes of solar energy conversion. And so, other studies of mine have sought to clarify the results of dye-sensitized solar cells incorporating iron polypyridyl sensitizers, starting with cis-bis(cyano)bis(4,4'-dicarboxylic acid-2,2'-bipyridine)iron(II), hereafter referred to as F2CA. Interestingly, the initial report on this dye revealed a band-selective behavior for interfacial electron transfer (IET) where the absorbed photon-to-current conversion efficiency of F2CA on titanium dioxide (TiO2) did not match its absorption spectrum, highlighting a discrepancy between the injection efficiency associated with the two absorption bands. To investigate this, ultrafast time-resolved spectroscopic measurements were carried out on F2CA-TiO2 assemblies as a function of excitation wavelength and also incorporated studies where the additives in the electrolyte of the solar cell were adjusted to either raise or lower the conduction band edge in an effort to study its impact on the injection yield. Synthetic variants of F2CA, which differ by the number of polypyridyl ligands to the number of cyano groups and incorporate either the carboxylate or hydroxamate linker, were also studied in order to identify how these changes affect the IET dynamics. Altogether, these results help to identify which components should be optimized for more efficient Fe(II) dye-sensitized solar cells.