Computational Estimate of the Photophysical Capabilities of Four Series of Organometallic Iron(II) Complexes

In this study, we examine a large range of organometallic iron­(II) complexes with the aim of computationally identifying the most promising ones in terms of photophysical properties. These complexes combine polypyridine, bis­(phosphine), and carbon-bound ligands. Density functional theory has allowed us to establish a comparative Jablonski diagram displaying the lowest singlet, triplet, and quintet states. All of the proposed FeN<sub>5</sub>C or FeN<sub>3</sub>P<sub>2</sub>C complexes unfavorably possess a lowest triplet state of metal-centered (MC) nature. Among the FeN<sub>4</sub>C<sub>2</sub> and FeN<sub>2</sub>P<sub>2</sub>C<sub>2</sub> series, the carbene complexes display the least favorable excited-state distribution, also having a low-lying <sup>3</sup>MC state. Validating our design strategy, we are now able to propose seven iron­(II) complexes displaying a lowest excited state of triplet metal-to-ligand charge-transfer nature.