Interest in the development of new iron(II)-based chromophores for applications in photo-induced electron transfer has been increasing in recent years. Newer, more strongly donating ligands have become the key to producing long-lived metal-to-ligand charge transfer (MLCT) states. Though we are gaining a more detailed understanding of the iron(II) d-π* excited-states, the effects of the ligand scaffold on ligand field (LF) excited-states are difficult to assess due to the fact that the d-d transitions are usually obscured by the MLCT absorptions. Cobalt(III) provides a facile means for measuring these transitions due to the fact that the LMCT transitions associated with this isoelectronic ion are shifted to higher energy, revealing the low energy and low intensity LF absorptions. We have carried out a study of the ligand field transitions of various substituted and unsubstituted cobalt(III) polypyridyl complexes in an effort to assess the Racah B, Racah C, and ligand field splitting (10Dq) parameters of ligands commonly employed as the basis for iron(II)-based chromophores. The data reveal that while substitutions on the polypyridyl ligands do modulate the energy of the CT state, the effect at the metal center is significantly attenuated from what has been assumed previously. From what has been gleaned from the cobalt(III) complexes, a spectrochemical series for polypyridyl ligands can be constructed and the relative differences in 10Dq can be used to infer how the LF manifold is modulated in the iron(II) analogues. With this information we will have a better understanding of the energetic positioning of the LF states and be able to link this with synthetic modifications in the hopes of extending the lifetime of the MLCT of iron(II)-based chromophores.

Development of a cobalt(III) polypyridyl spectrochemical series for application in understanding the ligand field electronic structure in iron(II)-based chromophores

2019 Spring National Meeting

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