Fe N-Heterocyclic Carbene Complexes as Promising Photosensitizers

The 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 Fe-II complexes ((FeL6)-L-II) 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((MLCT)-M-3) 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 e(g) sigma* orbitals on the Fe center energetically less accessible. Previous efforts to directly transplant successful strategies from (RuL6)-L-II 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 sigma-donating N-heterocyclic carbene (NHC) ligands to make strong-field (FeL6)-L-II complexes with significantly extended 3MLCT lifetimes. Already some of the first homoleptic bis(tridentate) complexes incorporating (CNHCNpyridineCNHC)-N-boolean AND-C-boolean AND)-type ligands gratifyingly resulted in extension of the (MLCT)-M-3 lifetime by more than 2 orders of magnitude compared to the parental Fe(tpy)(2) (tpy = 2,2':6',2 ''-terpyridine) complex. Quantum chemical (QC) studies also revealed that the (MC)-M-3 instead of the (MC)-M-5 state likely dictates the deactivation of the (MLCT)-M-3 state, a behavior distinct from traditional (FeL6)-L-II complexes but rather resembling Ru analogues. A heteroleptic Fe-II NHC complex featuring mesoionic bis(1,2,3-triazol-S-ylidene) (btz) ligands also delivered a 100-fold elongation of the (MLCT)-M-3 lifetime relative to its parental Fe(bpy)(3) (bpy = 2,2'-bipyridine) complex. Again, a Ru-like deactivation mechanism of the (MLCT)-M-3 state was indicated by QC studies. With a COOH functionalized homoleptic complex, a record (MLCT)-M-3 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 (MLCT)-M-3 lifetime indeed benefits efficient light harvesting with Fe-II NHC complexes. For the first time, close-to-unity electron injection from the lowest-energy (MLCT)-M-3 state to a TiO2 nanofilm was achieved by a stable Fell complex. This is in complete contrast to conventional (FeL6)-L-II-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 d(6) Fe-II complexes. They also open up new possibilities to develop solar energy-converting materials based on this abundant, inexpensive, and intrinsically nontoxic element.