ConspectusThe photophysics and photochemistry of transition metal complexes (TMCs) has long been a hot field of interdisciplinary research. Rich metal-based redox processes, together with a high variety in electronic configurations and excited-state dynamics, have rendered TMCs excellent candidates for interconversion between light, chemical, and electrical energies in intramolecular, supramolecular, and interfacial arrangements. In specific applications such as photocatalytic organic synthesis, photoelectrochemical cells, and light-driven supramolecular motors, light absorption by a TMC-based photosensitizer and subsequent excited-state energy or electron transfer constitute essential steps. In this context, TMCs based on rare and expensive metals, such as ruthenium and iridium, are frequently employed as photosensitizers, which is obviously not ideal for large-scale implementation. In the search for abundant and environmentally benign solutions, six-coordinate FeII complexes (FeIIL6) have been widely considered as highly desirable alternatives. However, not much success has been achieved due to the extremely short-lived triplet metal-to-ligand charge transfer (3MLCT) excited state that is deactivated by low-lying metal-centered (MC) states on a 100 fs time scale. A fundamental strategy to design useful Fe-based photosensitizers is thus to destabilize the MC states relative to the 3MLCT state by increasing the ligand field strength, with special focus on making eg σ* orbitals on the Fe center energetically less accessible. Previous efforts to directly transplant successful strategies from RuIIL6 complexes unfortunately met with limited success in this regard, despite their close chemical kinship. In this Account, we summarize recent promising results from our and other groups in utilizing strongly σ-donating N-heterocyclic carbene (NHC) ligands to make strong-field FeIIL6 complexes with significantly extended 3MLCT lifetimes. Already some of the first homoleptic bis(tridentate) complexes incorporating (CNHC^Npyridine^CNHC)-type ligands gratifyingly resulted in extension of the 3MLCT lifetime by more than 2 orders of magnitude compared to the parental [Fe(tpy)2]2+ (tpy = 2,2′:6′,2″-terpyridine) complex. Quantum chemical (QC) studies also revealed that the 3MC instead of the 5MC state likely dictates the deactivation of the 3MLCT state, a behavior distinct from traditional FeIIL6 complexes but rather resembling Ru analogues. A heteroleptic FeII NHC complex featuring mesoionic bis(1,2,3-triazol-5-ylidene) (btz) ligands also delivered a 100-fold elongation of the 3MLCT lifetime relative to its parental [Fe(bpy)3]2+ (bpy = 2,2′-bipyridine) complex. Again, a Ru-like deactivation mechanism of the 3MLCT state was indicated by QC studies. With a COOH-functionalized homoleptic complex, a record 3MLCT lifetime of 37 ps was recently observed on an Al2O3 nanofilm. As a proof of concept, it was further demonstrated that the significant improvement in the 3MLCT lifetime indeed benefits efficient light harvesting with FeII NHC complexes. For the first time, close-to-unity electron injection from the lowest-energy 3MLCT state to a TiO2 nanofilm was achieved by a stable FeII complex. This is in complete contrast to conventional FeIIL6-derived photosensitizers that could only make use of high-energy photons. These exciting results significantly broaden the understanding of the fundamental photophysics and photochemistry of d6 FeII complexes. They also open up new possibilities to develop solar energy-converting materials based on this abundant, inexpensive, and intrinsically nontoxic element.

Fe N-Heterocyclic Carbene Complexes as Promising Photosensitizers