Iron N-Heterocyclic Carbene Photoactive Complexes with Rigid Phenylethynyl Substituents as Ligand π-System Extensions

The design of iron complexes with long-lived charge transfer states suitable for applications as photosensitizers remains a formidable challenge. Here, we investigated the effect of an extended ligand π-system on the ground- and excited-state properties of iron(II) complexes with N-heterocyclic carbene (NHC) ligands. For this purpose, three iron complexes based on the established [Fe(II)(pbmi)₂]²⁺ motif (pbmi = (1,1'-(pyridine-2,6-diyl)bis(3-methylimidazole-2-ylidene))) have been modified with phenylethynyl moieties attached to the pyridine part of the ligand. In general, the introduction of the phenylethynyl units served to red shift the main absorption band, as well as to increase the extinction coefficient of the same, compared to the parent complex. The lowered MLCT energies are in line with the electrochemical data that revealed substantially easier reduction of the phenylethynyl-modified ligands, while the potentials of the Fe(III/II) couple are only moderately increased. Only minor modifications of the electronic effect intrinsic to the phenylethynyl moieties could be implemented with bromide and dimethylamino substituents on the phenylene units. As a result, all three complexes experience similar stabilization of their ³MLCT states, about 0.3 eV compared to the parent complex, and feature transient absorption data in line with ES dynamics that are dominated by a moderately long-lived (∼17 ps) ³MLCT state. These values exceed the ³MLCT lifetimes reported for the parent complex (up to 9 ps) and resemble the results for carboxylic acid and imidazolinium derivatives with comparable ³MLCT energies and lifetimes.