Extended π-Conjugated Ligands Tune Excited-State Energies of Iron(II) Polypyridine Dyes

Over the past decade, iron(II) polypyridines have gained a lot of attention as potential chromophores and sensitizers due to the low cost and high abundance of iron. Unfortunately, most iron(II) polypyridines are poor chromophores since their initially excited, photoactive metal-to-ligand charge transfer (MLCT) states quickly decay into non-photoactive metal-centered (MC) states. Many strategies to increase their lifetime have been pursued, built mainly around increasing the ligand field strength of these complexes and thus destabilizing the MC states. In this work, we aim to design a new class of Fe(II) complexes by stabilizing the energies of their MLCT states. To this end, we employ density functional theory (DFT) and time-dependent DFT to investigate a series of Fe(II) complexes, [Fe(L/X)_2,4(N^N)]^2+/2- where L/X represents either cyanide, isocyanide, or bipyridine ligands and N̂N stands for bidentate-extended π-conjugated ligands derived from the bipyridine. The L/X ligands tune the energetics of the Fe-based t_2g molecular orbitals, while the amount of π-conjugation on the N^N ligand impacts the energies of its π and π* orbitals, thus tuning the energetics of the MLCT and the ligand-centered (LC) states. Overall, our results suggest that the use of N^N ligands with the extended π-conjugation is a viable strategy to tune the relative energies of MLCT, LC, and MC states.