Nanosecond Metal-to-Ligand Charge Transfer State in an Fe(II) Chromophore: Lifetime Enhancement via Nested Potentials

Examples of Fe complexes with long-lived (≥1 ns) charge-transfer states are limited to pseudo-octahedral geometries with strong -donor chelates. Alternative strategies based on varying both coordination motifs and ligand donicity are highly desirable. Reported herein is an air-stable, tetragonal FeII complex, Fe(HMTI)(CN)2 (HMTI = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene) with a 1.25 ns metal-to-ligand charge transfer (MLCT) lifetime. The structure has been determined, and the photophysical properties examined in a variety of solvents. The HMTI ligand is highly pi-acidic due to low lying pi*(C=N), which enhances DeltaFe via stabilizing t2g orbitals. The inflexible geometry of the macrocy-cle results in short Fe-N bonds, and density functional theory calculations show that this rigidity results in an unusual set of nested potential energy surfaces. Moreover, the lifetime and energy of the MLCT state depends strongly on the solvent environment. This dependence is caused by modulation of the axial ligand-field strength by Lewis acid-base interactions between the solvent and the cyano ligands. This work represents the first example of a long-lived charge transfer state in an Fe(II) macrocyclic species.