Polypyridine based chromophores serve a key role in photo-induced electron transfer processes, with those based on iron(II) representing a promising earth-abundant alternative to the typical ruthenium-based complexes. However, the ultrafast deactivation of the metal-to-ligand charge transfer (MLCT) excited state in iron(II)-based complexes limits their ability to facilitate electron transfer. Studies have shown that the long-lived 5T2 excited state can induce electron transfer, yet there remain challenges in utilizing this excited state. To better understand how to manipulate the excited state dynamics of iron(II) polypyridyl chromophores, it is important to gain a fundamental understanding of excited state evolution in these complexes. A technique that is useful in this regard is variable temperature transient absorption (VT-TA) spectroscopy. By measuring the excited state lifetime as a function of temperature, both transition state and Marcus theory can be used to provide key information, such as the energy barrier or electronic coupling between states during a transition. Using VT-TA spectroscopy, the 5T2 excited state relaxation of two series of iron(II) polypyridines, one using substituted terpyridines and another using substituted bypiridines, was studied. VT-TA analysis on the terpyridine-based series provides experimental evidence that the relaxation from the 5T2 excited state occurs in the Marcus normal region. This provides important context on how changing the energy gap between the 5T2 excited state and 1A1 ground state will impact the lifetime of the 5T2 state. VT-TA analysis on the bipyridine-based complexes displays how substituent placement can be used to control electronic structure through leveraging resonance and induction effects. Furthermore, different substituent positions impact reorganization energy in different ways, which impacts the energy barrier for 5T2 relaxation. Lastly, ultrafast spectroscopy was used to study the vibrational coherence of several iron(II)-based polypyridine complexes during the early time post excitation. This reveals the vibrational motions that facilitate MLCT deactivation in these complexes. The information could be used to guide ligand design that will lengthen the MLCT lifetime.
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