Iron(II) polypyridyl chromophores are among one of the most promising earth-abundant alternatives to ruthenium-based complexes in the realms of photo-redox catalysis and solar energy harvesting. The biggest hurdle to their implementation is the sub-150 fs metal-to-ligand charge transfer (MLCT) excited-state lifetime which limits their implementation in diffusion-limited processes. The main way in which researchers have approached this problem is by attempting to invert the ligand-field (LF) and MLCT manifolds by increasing the donor ability of the ligands employed. To better understand the destabilizing nature of the ligands employed, a series of Co(III) complexes were synthesized as an isoelectronic stand-in for Fe(II) to measure the LF transitions of various polypyridyl ligands as well as the first carbene ligand coordinated to Fe(II) to determine the relevant ligand-field parameters and assess the splitting observed. These results indicate that polypyridyl complexes do not impose a strong enough LF to destabilize them above the MLCT. To that end, we synthesized a series of simple bis pyridinium-based polypyridyl ligands with increased sigma-donor ability compared to the widely used carbene systems, achieving a MLCT lifetime of 18 ps, a 2-fold increase from the first reported tetra-carbene system. A final thrust was to better understand the unique MLCT manifold of [Fe(dcpp)2]2+ (where dcpp is 2,6-di(2-carboxypyridyl)pyridine) using symmetric π-substitutions. With this method, we were able to synthetically deconvolute the MLCT spectrum and selectively enhance different transitions based upon the substitution pattern. The computational insights gleaned can now guide new promising ligands in this family of complexes.
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