Fe N-Heterocyclic Carbene Complexes as Promising Photosensitizers

Abstract

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 (Fe(II)L6) 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 ((3)MLCT) 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 (3)MLCT 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 Ru(II)L6 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 Fe(II)L6 complexes with significantly extended (3)MLCT lifetimes. Already some of the first homoleptic bis(tridentate) complexes incorporating (CNHC^Npyridine^CNHC)-type ligands gratifyingly resulted in extension of the (3)MLCT 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 (3)MC instead of the (5)MC state likely dictates the deactivation of the (3)MLCT state, a behavior distinct from traditional Fe(II)L6 complexes but rather resembling Ru analogues. A heteroleptic Fe(II) NHC complex featuring mesoionic bis(1,2,3-triazol-5-ylidene) (btz) ligands also delivered a 100-fold elongation of the (3)MLCT lifetime relative to its parental [Fe(bpy)3](2+) (bpy = 2,2'-bipyridine) complex. Again, a Ru-like deactivation mechanism of the (3)MLCT state was indicated by QC studies. With a COOH-functionalized homoleptic complex, a record (3)MLCT 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 (3)MLCT lifetime indeed benefits efficient lig...

NHC-Based Iron Sensitizers for DSSCs

“…Their amplitude decays with the time constant τ2, which is the 3 MLCT lifetime (Figure 4b). For C1, we confirm the 3 MLCT lifetime of 9.0 ± 1.0 ps [24] and find τ2 = 16.5 ± 1.0 ps [28]. The two COOH groups on the central pyridine moiety almost double the excited-state lifetime.…”

“…Details are given in ref. [28] and in its Supporting information. A typical set ∆A (differential absorbance) data is displayed for C2 in Figure 4a, which shows these spectra for different delay times from −0.2 to 64 ps.…”

“…There is no indication for the occurrence of the quintuplet 5 T2 state, since the latter is characterized by a 20 nm narrow ESA at 355 nm for Fe(II)-polypyridine complexes [28]. In a recent The excited-state lifetimes of the complexes were studied by femtosecond transient absorption spectroscopy (TAS) with ≈80 fs time resolution and broadband white-light probing in the 300-700 nm range, in order to access the full spectra of photoinduced ground state bleach (GSB) and excited-state absorption (ESA).…”

“…There is no indication for the occurrence of the quintuplet 5 T 2 state, since the latter is characterized by a 20 nm narrow ESA at 355 nm for Fe(II)-polypyridine complexes [28]. In a recent work on [Fe(2,2 -bipyridine) 2 (CN) 2 ] complexes, the relaxation of 3 MLCT into metal-centered triplet states 3 MC with several-picosecond lifetimes was revealed [32,33].…”

“…Recently, as shown by Wärnmark and coworkers, N-heterocyclic carbenes (NHC) have been proven to promote notable improvements of 3 MLCT lifetimes, thanks to strong σ-donating effects inducing the destabilization of MC states [25][26][27][28]. While no DSSCs have been built from the newly designed complexes, this work has fostered the revival of research towards the development of iron-based sensitizers for DSSCs, which has been almost asleep since the pioneering works by Ferrere using the Fe(dcbpy) 2 (CN) 2 dye [14].…”