Iron(II) polypyridyl chromophores represent an earth-abundant alternative to ruthenium-based complexes in photo-induced electron transfer applications, yet the sub-150 fs metal-to-ligand charge transfer (MLCT) excited-state lifetime endemic to low-spin Fe(II) polypyridyls has hampered their widespread use. One promising avenue towards achieving a longer-lived MLCT excited-state lifetime is through the exertion of kinetic control, made possible through the identification and subsequent disruption of the nuclear coordinate of excited-state deactivation. With this aim, a series of structurally similar iron(II) polypyridyl complexes spanning from low-spin to high-spin, including a spin crossover complex, were synthesized, which allowed for the determination of reorganization energy from the lowest-energy excited state (5T2) to the ground state (1A1) through a combination of variable temperature transient absorption and magnetic susceptibility measurements. In addition to experimentally determining the reorganization energy and electronic coupling constant associated with this conversion, we will deduce the kinetically competent degree of freedom associated with this transition through a convergence of analyses from semi-classical to fully quantum mechanical non-radiative decay theories. A ruthenium(II)-based analog of the spin crossover complex provided insight into the geometric distortions coupled to the deactivation of the MLCT excited states. Coupled together, these results offered new guidelines for ligand design, inspiring the synthesis of new iron(II) complexes with unique photophysical dynamics and establishing new roadmaps towards controlling excited-state dynamics in this class of compounds.
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