Electronic absorption of ferricenium derivatives: an unexpected twist

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Our results revealed that the introduction of either electron-donating or electron-withdrawing group(s) on the cyclopentadienyl rings leads to a substantial red shift of the lowest energy ligand-to-metal charge transfer (LMCT) band, generally known as 2E2g → 2E1u, relative to that of the parent ferricenium complex. The observed stabilization of the lowest unoccupied molecular orbitals (LUMO), i.e. e2g orbitals, in the electron-deficient system, was ascribed to the δ back donation from the iron atom into the antibonding molecular orbitals of the ligand rings. A series of DFT calculations reproduce the experimental trend highlighting the shift in energies of the molecular orbitals involved in the charge transfer transition. The direct relationship between the location of the LMCT band and reduction potential for specific class of substituents is also presented which allows for more informed structural modifications in the development of these complexes.