Excited state distortions reverse the decay lifetime trend in Fe(ii) phenanthroline complexes

The correlation between increased ligand field strength and longer decay lifetimes in iron(ii) complexes is well established. In this work, we observe an unusual inverse relationship between lifetime and ligand field strength, where a stronger ligand field leads to a shorter lifetime. Two phenanthroline-iron(ii) complexes containing imidazol-2-ylidene (FephenImi) and benzimidazol-2-ylidene (FephenBzi) ligands are reported. Transient absorption experiments in acetonitrile reveal longer ³MLCT and ³MC lifetimes for FephenImi of 9 and 195 ps, respectively, compared to 2 and 53 ps, respectively, for FephenBzi. The ³MLCT lifetime is extended in dichloromethane for FephenImi to 27 ps. The differences in the excited-state dynamics are rationalized using DFT surface analysis. A closer-to-perfect octahedral geometry of FephenImi in the excited state increases the energy of the ³MC state and leads to a kinetic barrier between the triplet states. In contrast, the smaller axial angles in FephenBzi facilitate the ³MLCT → ³MC deactivation pathway despite the higher ligand field strength. These results provide a new perspective on geometric distortions in the excited states that may have an impact on the conversion between triplet states and increase the decay lifetime in iron(ii) complexes.