Optimized Octahedral Geometry and Hidden Rigidity of a <i>mer</i> -Tridentate N-Heterocyclic Carbene Ligand Enhance Luminescence of an Fe(III) Complex

Fe(III) N-heterocyclic carbene (NHC) complexes are emerging photoactive first-row transition metal complexes. While optimizing ligand bite angles is a widely employed strategy to extend photoactive excited states, the lack of tridentate NHC ligands prevents exploration of better coordination geometries in the first-row transition metal complexes. Here, we demonstrate that a newly designed methylene-bridged mer -tridentate NHC ligand provides an improved coordination bite angle. Continuous Shape Measure (CShM) analysis reveals that the Fe(III) complex exhibits a significantly improved octahedral geometry (0.35) compared to existing phenylene-bridged mer -tridentate ligands (3.15). This optimized geometry results in a prolonged 2 LMCT lifetime (τ = 670 ps), which is longer compared to the Fe(III) complexes with reported bidentate or mer -tridentate ligands. The Fe(III) complex retains a better octahedral geometry even in the distorted quartet metal-centered ( 4 MC) state, indicating enhanced structural rigidity of the coordination environment. With this structural rigidity, the Fe(III) complex achieved an enhanced photoluminescence quantum yield (Φ = 0.6%). The methylene bridges seem inherently flexible; however, we revealed that the geometric constraints of the mer -tridentate arrangement and methylene moieties provide counterintuitive hidden structural rigidity. This work establishes the new mer -tridentate NHC ligand scaffold as a promising ligand platform for developing photoactive first-row transition metal complexes.