The search for new iron(II) chromophores as substitutes for ruthenium(II) dyes is an area of current interest due to the low cost and high abundance of iron. However, a major challenge with most iron(II) complexes is the decay of the initially-excited metal-to-ligand charge transfer (MLCT) states into the low-lying metal centered (MC) states (3MC and 5MC) via intersystem crossing. Here, we investigate the impact of aryl isocyanide ligands substituted with electron withdrawing and donating groups on the ligand field strength as well as ground and excited state electronic structure of iron(II)-polypyridines by means of density functional theory (DFT) and time-dependent DFT (TD-DFT). In this study, we systematically replace the bipyridine ligands of the parent complex [Fe(bpy)3]2+ (bpy = 2-2'-bipyridine) with aryl isocaynide ligands. Two series of Fe(II)-polypyridine complexes were investigated of the general form [Fe(bpy)2(L)2]2+and [Fe(bpy)(L)4]2+, where L = aryl isocyanide. Our results show that [Fe(bpy)(L)4]2+ complexes with four aryl isocyanide ligands display a significant change in the spin-state energetics (1A &lt; 3MC &lt; 5MC) compared to the [Fe(bpy)3]2+ parent complex (1A &lt; 5MC &lt; 3MC). Along with the destabilization of MC states by the aryl isocyanide ligands, MLCT states are also destabilized, resulting in the hypsochromic shift of the MLCT bands in the UV-Vis absorption spectrum relative to the [Fe(bpy)3]2+.

Tuning the ligand field strength of iron(II) polypyridine complexes with aryl isocyanide ligands

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