The Connection between NHC Ligand Count and Photophysical Properties in Fe(II) Photosensitizers: An Experimental Study

Four homo- and heteroleptic complexes bearing both polypyridyl units and N-heterocyclic carbene (NHC) donor functions are studied as potential noble metal-free photosensitizers. The complexes [Fe^II(L1)­(terpy)]­[PF₆]₂, [Fe^II(L2)₂]­[PF₆]₂, [Fe^II(L1)­(L3)]­[PF₆]₂, and [Fe^II(L3)₂]­[PF₆]₂ (terpy = 2,2′:6′,2″ terpyridine, L1 = 2,6-bis­[3-(2,6-diisopropylphenyl)­imidazol-2-ylidene]­pyridine, L2 = 2,6-bis­[3-isopropylimidazol-2-ylidene]­pyridine, L3 = 1-(2,2′-bipyridyl)-3-methylimidazol-2-ylidene) contain tridentate ligands of the C^N^C and N^N^C type, respectively, resulting in a Fe-NHC number between two and four. Thorough ground state characterization by single crystal diffraction, electrochemistry, valence-to-core X-ray emission spectroscopy (VtC-XES), and high energy resolution fluorescence detected X-ray absorption near edge structure (HERFD-XANES) in combination with ab initio calculations show a correlation between the geometric and electronic structure of these new compounds and the number of the NHC donor functions. These results serve as a basis for the investigation of the excited states by ultrafast transient absorption spectroscopy, where the lifetime of the ³MLCT states is found to increase with the NHC donor count. The results demonstrate for the first time the close interplay between the number of NHC functionalities in Fe­(II) complexes and their photochemical properties, as revealed in a comparison of the activity as photosensitizers in photocatalytic proton reduction