Lifetimes of photoexcited charge transfer (CT) states in transition metal chromophores are influenced by low-lying ligand field (LF) excited states, especially for 3d metal complexes. To manipulate interactions between LF and CT states, it is important to be able to control LF excited state energies using tunable synthetic variables. In this report, we use Fe 2p3d L3-edge resonant inelastic X-ray scattering (RIXS) to measure LF excited state energies of three homoleptic iron chromophores coordinated by strong-field N-heterocyclic carbenes (NHCs). We investigate the effect of oxidation state and ligand scaffold on LF energies and covalency parameters. A cyclometalated bis(NHC) ligand affords both high LF excited state energies (and thus high 10 Dq) as well as high metal-ligand covalency compared to other iron complexes with very strong-field ligands. However, for the set of complexes investigated, we do not observe meaningful correlation between the LF excited state energies and the CT excited state lifetimes. These results illustrate that targeting long-lived CT excited states necessitates control of multiple molecular excited state properties, with destabilization of the LF excited state energies proving necessary, but insufficient, to control the CT excited state lifetime in Fe carbene complexes.

An Fe( ii ) complex bearing tris(carbene)borate ligands with imidazol-2-ylidene donors shows marked destabilization of metal centered states by the strongly σ-donating ligands. Yet, its high-energy 3 MLCT state is rapidly deactivated via a 3 MC state.

Two hydrogen-bonding monomers containing tetraethylene glycol (TEG) chains have been synthesized and characterized. The monomers are based on a bicyclic scaffold appended with either 4H-bonding benzyl-substituted ureidopyrimidinone motifs or 2H-bonding unsubstituted pyrrole-fused ureidopyrimidinone motifs, with the TEG chains introduced with the aim of developing amphiphilic monomers soluble in nonpolar and polar organic solvents in order to extend the utility of H-bonding in supramolecular chemistry. In CDCl3, both monomers formed cyclic tetramers. The monomer containing 2H-bonding motifs was found to form significantly less-stable aggregates than its previously reported alkylated analogue, most likely due to interference from the TEG chains. Despite the weaker aggregation, the 2H-bonded tetramers were able to stack into tubular polymers through orthogonal H-bonding in less polar solvent (toluene) or when a suitable guest (C70) was introduced. Comparison of the TEGylated monomers with previously reported alkylated analogues showed that the introduction of TEG chains resulted in increased solubility in a wide range of solvents. By using one of the TEGylated monomers, nonpolar C60 could be solubilized in polar solvent acetonitrile by forming an inclusion complex. This complex was used as a homogeneous catalyst for photochemical oxidation of sulfides to sulfoxides in acetonitrile.

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.

A diastereospecific synthesis of vicinally substituted 2-oxazolidinones from α,β-unsaturated lactams using m-chloroperoxybenzoic acid is reported. Several highly substituted 2-oxazolidinones were obtained in 19–46% yields in a one-pot reaction with complete control over the relative stereochemistry. The proposed reaction sequence consists of a Baeyer–Villiger oxidation, an epoxidation, and a concerted rearrangement. Experimental results and density functional theory calculations indicate that a CH2COOEt substituent at position 4 of the lactam is necessary for the diastereospecific rearrangement to take place.

In this work, four dyes-consisting of heteroleptic iron N-heterocyclic carbene (NHC) complexes-were investigated in dye-sensitized solar cell (DSC) applications to explore their behavior and interactions with the overall cell. Three of the four dyes contained amino-based electron donor moieties, while one was without this type of structure, and instead had only a hydrogen atom. Compared to earlier investigations, the characteristics of the DSC devices were improved by the use of a UV-curing glue to seal the cells, by lowering the concentration of sensitization baths, and by altering the composition of the TiO2 substrates. This resulted in devices with power conversion efficiencies of up to 1.3%. Optimizations previously made to the electrolyte for DSCs incorporating iron-NHC devices were however found to be detrimental for amino-substituted complexes, giving worse results for all measured characteristics of these dyes. There was furthermore a significant hysteresis effect for devices made from each dye, hitherto not reported for iron-NHC complexes as dyes, where the scan direction strongly influenced the open circuit voltage and thereby also the efficiency.

An Fe(III)-NHC complex has been employed for the green light driven catalysis of base-promoted homolytic aromatic substitution (BHAS) reactions. Tributylamine was used as a sacrificial electron donor, together with potassium carbonate as base in dimethyl sulfoxide as solvent. In contrast to previously studied photocatalysts, the excited Fe(III)-NHC complex is not reducing the arylhalide substrates. Instead, the latter are activated by α-aminoalkyl radicals formed upon reductive quenching of the photocatalyst by tributylamine. Avoiding strongly reducing photocatalysts as well as strong base, these mild reaction conditions allowed for the expansion of the substrate scope to accommodate also aldehyde and ester substituents. 100 % conversion was obtained after 48 h of irradiation. In this way a wide variety of cyclized products and their corresponding hydrodehalogenated products were obtained as isolated and pure compounds, in the vast majority of cases.

FeIII complexes based on the [FeIII(ImP)2]+ motif (ImP = bis(2,6-bis(3-methylimidazol-2-ylidene-1-yl)phenylene)), where the ligand contains both carbene and cyclometalated moieties, are a promising class of photoactive materials made from this abundant metal. In this work, it is shown that bromo or furanyl substituents attached to the cyclometalating moiety of the ImP ligands stabilize the 2LMCT excited state to very different extent resulting in opposing effects on the 2LMCT lifetime. For [FeIII(ImPBr)2]+, the lifetime (255 ps) of its moderately stabilized 2LMCT state (1.85 eV) is slightly increased compared to the parent complex (1.90 eV, 240 ps) pointing to an increased barrier for deactivation via the 4MC state and enabling applications as photoredox catalyst. In contrast, the 2LMCT energy of [FeIII(ImPFur)2]+ is lowered substantially to a value of 1.63 eV due to the extended π-system of the ligands and the reduced energy gap favors internal conversion directly to the ground state resulting in a considerably reduced 2LMCT lifetime of 59 ps. These findings have general implications for design of ligand modifications aiming at extended LMCT lifetimes and/or modified ground and excited state potentials.

Fe(II) N-heterocyclic carbene (NHC) complexes have emerged over the last decade as a promising class of light-harvesting complexes for a variety of photochemical applications relying on the presence of high-energy excited states of mainly charge-transfer character with excited state lifetimes of tens of picoseconds or longer. Recent spectroscopic investigations have significantly refined the understanding of some of the key prototype complexes of this kind and highlighted the subtle balance between population of triplet metal-to-ligand charge-transfer ((MLCT)-M-3) and triplet metal-centered ((MC)-M-3) states as a key issue to better understand and ultimately control the excited state dynamics in these complexes. To present a broader perspective on this issue, we here re-examine and discuss the excited state properties of a series of complexes with different side-groups on a common Fe NHC scaffold. Both the steady-state absorption spectrum and excited state dynamics are influenced by the side-group substitution, and the changes are rationalized based on shifting of the lowest metal-to-ligand charge-transfer (MLCT) state in energy based on the electron-withdrawing or electron-donating properties of the side-groups. Only electron-withdrawing substituents such as carboxylic acid groups ensured that the majority excited population stays in the (MLCT)-M-3 state for similar to 20 ps rather than rapidly converting into metal-centered (MC) states. In other complexes, the (MLCT)-M-3 state survived <300 fs after which the (MC)-M-3 state was populated for similar to 10 ps. The transient absorption results also show that the dynamics can be switched in a simple manner by deprotonating the carboxylic acid group, which renders some of the complexes pH-sensitive. For the here discussed complexes, the results from transient absorption measurements indicate that the (MLCT)-M-3 and (MC)-M-3 states were close enough in energy to enable the side-group to determine the photophysics. The emerging understanding of the (MLCT)-M-3-(MC)-M-3 balance, as well as the nature and properties of the (MC)-M-3 state in these complexes with intermediate ligand field strength is used to provide a broader fundamental perspective required to improve the ligand-design of Fe carbene complexes for issues such as to ensure a long-lived (MLCT)-M-3 state.

We here report the synthesis of the homoleptic iron(II) N-heterocyclic carbene (NHC) complex [Fe(miHpbmi)(2)](PF6)(4) (miHpbmi = 4-((3-methyl-1H-imidazolium-1-yl)pyridine-2,6-diyl)bis(3-methylimidazol-2-ylidene)) and its electrochemical and photophysical properties. The introduction of the pi-electron-withdrawing 3-methyl-1H-imidazol-3-ium-1-yl group into the NHC ligand framework resulted in stabilization of the metal-to-ligand charge transfer (MLCT) state and destabilization of the metal-centered (MC) states. This resulted in an improved excited-state lifetime of 16 ps compared to the 9 ps for the unsubstituted parent compound [Fe(pbmi)(2)](PF6)(2) (pbmi = (pyridine-2,6-diyl)bis(3-methylimidazol-2-ylidene)) as well as a stronger MLCT absorption band extending more toward the red spectral region. However, compared to the carboxylic acid derivative [Fe(cpbmi)(2)](PF6)(2) (cpbmi = 1,1 '-(4-carboxypyridine-2,6-diyl)bis(3-methylimidazol-2-ylidene)), the excited-state lifetime of [Fe(miHpbmi)(2)](PF6)(4) is the same, but both the extinction and the red shift are more pronounced for the former. Hence, this makes [Fe(miHpbmi)(2)](PF6)(4) a promising pH-insensitive analogue of [Fe(cpbmi)(2)](PF6)(2). Finally, the excited-state dynamics of the title compound [Fe(miHpbmi)(2)](PF6)(4) was investigated in solvents with different viscosities, however, showing very little dependency of the depopulation of the excited states on the properties of the solvent used.

Two iron complexes featuring the bidentate, nonconjugated N-heterocyclic carbene (NHC) 1,1 '-methylenebis(3-methylimidazol-2-ylidene) (mbmi) ligand, where the two NHC moieties are separated by a methylene bridge, have been synthesized to exploit the combined influence of geometric and electronic effects on the ground- and excited-state properties of homoleptic Fe-III-hexa-NHC [Fe(mbmi)(3)](PF6)(3) and heteroleptic Fe-II-tetra-NHC [Fe(mbmi)(2)(bpy)](PF6)(2) (bpy = 2,2 '-bipyridine) complexes. They are compared to the reported Fe-III-hexa-NHC [Fe(btz)(3)](PF6)(3) and Fe-II-tetra-NHC [Fe(btz)(2)(bpy)](PF6)(2) complexes containing the conjugated, bidentate mesoionic NHC ligand 3,3 '-dimethyl-1,1 '-bis(p-tolyl)-4,4 '-bis(1,2,3-triazol-5-ylidene) (btz). The observed geometries of [Fe(mbmi)(3)](PF6)(3) and [Fe(mbmi)(2)(bpy)](PF6)(2) are evaluated through L-Fe-L bond angles and ligand planarity and compared to those of [Fe(btz)(3)](PF6)(3) and [Fe(btz)(2)(bpy)](PF6)(2). The Fe-II/Fe-III redox couples of [Fe(mbmi)(3)](PF6)(3) (-0.38 V) and [Fe(mbmi)(2)(bpy)](PF6)(2) (-0.057 V, both vs Fc(+/0)) are less reducing than [Fe(btz)(3)](PF6)(3) and [Fe(btz)(2)(bpy)](PF6)(2). The two complexes show intense absorption bands in the visible region: [Fe(mbmi)(3)](PF6)(3) at 502 nm (ligand-to-metal charge transfer, (LMCT)-L-2) and [Fe(mbmi)(2)(bpy)](PF6)(2) at 410 and 616 nm (metal-to-ligand charge transfer, (MLCT)-M-3). Lifetimes of 57.3 ps ((LMCT)-L-2) for [Fe(mbmi)(3)](PF6)(3) and 7.6 ps ((MLCT)-M-3) for [Fe(mbmi)(2)(bpy)](PF6)(2) were probed and are somewhat shorter than those for [Fe(btz)(3)](PF6)(3) and [Fe(btz)(2)(bpy)](PF6)(2). [Fe(mbmi)(3)](PF6)(3) exhibits photoluminescence at 686 nm ((LMCT)-L-2) in acetonitrile at room temperature with a quantum yield of (1.2 +/- 0.1) x 10(-4), compared to (3 +/- 0.5) x 10(-4) for [Fe(btz)(3)](PF6)(3).

The α-functionalisation of N-containing compounds is an area of broad interest in synthetic chemistry due to their presence in biologically active substances among others. Visible light-induced generation of nucleophilic α-aminoalkyl radicals as reactive intermediates that can be trapped by electron-deficient alkenes presents an attractive and mild approach to achieve said functionalisation. In this work, [Fe(iii)(phtmeimb)2]PF6 (phtmeimb = phenyl(tris(3-methylimidazol-2-ylidene))borate), an N-heterocyclic carbene (NHC) complex based on Earth-abundant iron, was used as photoredox catalyst to efficiently drive the formation of α-aminoalkyl radicals from a range of different α-trimethylsilylamines and their subsequent addition to a number of electron-deficient alkenes under green light irradiation. Mechanistic investigations elucidated the different reaction steps of the complete photocatalytic cycle. In terms of yields and substrate scope, we show that [Fe(iii)(phtmeimb)2]PF6 can compete with noble metal photoredox catalysts, for instance outcompeting archetypal [Ru(bpy)3]Cl2 under comparable reaction conditions, illustrating that iron photocatalysts can efficiently facilitate photoredox reactions of synthetic value.

Iron-centered N-heterocyclic carbene compounds have attracted much attention in recent years due to their long-lived excited states with charge transfer (CT) character. Understanding the orbital interactions between the metal and ligand orbitals is of great importance for the rational tuning of the transition metal compound properties, e.g., for future photovoltaic and photocatalytic applications. Here, we investigate a series of iron-centered N-heterocyclic carbene complexes with +2, + 3, and +4 oxidation states of the central iron ion using iron L-edge and nitrogen K-edge X-ray absorption spectroscopy (XAS). The experimental Fe L-edge XAS data were simulated and interpreted through restricted-active space (RAS) and multiplet calculations. The experimental N K-edge XAS is simulated and compared with time-dependent density functional theory (TDDFT) calculations. Through the combination of the complementary Fe L-edge and N K-edge XAS, direct probing of the complex interplay of the metal and ligand character orbitals was possible. The σ-donating and π-accepting capabilities of different ligands are compared, evaluated, and discussed. The results show how X-ray spectroscopy, together with advanced modeling, can be a powerful tool for understanding the complex interplay of metal and ligand.

Quantum interference (QI) can strongly affect electric and thermoelectric properties of molecular junctions (MJs). So far, however, a limited number of experimental studies have explored the influence of QI on thermoelectric transport in MJs. To address this open point, we synthesized derivatives of meta-OPE3 with an electron-withdrawing nitro (-NO2) substituent or an electron-donating N,N-dimethyl amine (-NMe2) substituent, attached at two different positions of the central phenylene ring, and systematically studied the electrical conductance and thermopower of the corresponding gold-molecule-gold junctions. We show that (i) the electrical conductance of MJs depends weakly on the polarity of the substituents but strongly on the substitution position and (ii) MJs with the N,N-dimethyl amine group feature a higher thermopower than MJs with the nitro group. We also present calculations based on first principles, which explain these trends and show that the transport properties are highly sensitive to microscopic details in junctions, exhibiting destructive QI features.

Iron N-heterocyclic carbene (FeNHC) complexes with long-lived charge transfer states are emerging as a promising class of photoactive materials. We have synthesized [FeII(ImP)2] (ImP = bis(2,6-bis(3-methylimidazol-2-ylidene-1-yl)phenylene)) that combines carbene ligands with cyclometalation for additionally improved ligand field strength. The 9 ps lifetime of its 3MLCT (metal-to-ligand charge transfer) state however reveals no benefit from cyclometalation compared to Fe(ii) complexes with NHC/pyridine or pure NHC ligand sets. In acetonitrile solution, the Fe(ii) complex forms a photoproduct that features emission characteristics (450 nm, 5.1 ns) that were previously attributed to a higher (2MLCT) state of its Fe(iii) analogue [FeIII(ImP)2]+, which led to a claim of dual (MLCT and LMCT) emission. Revisiting the photophysics of [FeIII(ImP)2]+, we confirmed however that higher (2MLCT) states of [FeIII(ImP)2]+ are short-lived (<10 ps) and therefore, in contrast to the previous interpretation, cannot give rise to emission on the nanosecond timescale. Accordingly, pristine [FeIII(ImP)2]+ prepared by us only shows red emission from its lower 2LMCT state (740 nm, 240 ps). The long-lived, higher energy emission previously reported for [FeIII(ImP)2]+ is instead attributed to an impurity, most probably a photoproduct of the Fe(ii) precursor. The previously reported emission quenching on the nanosecond time scale hence does not support any excited state reactivity of [FeIII(ImP)2]+ itself.

Molecular junctions offer significant potential for enhancing thermoelectric power generation. Quantum interference effects and associated sharp features in electron transmission are expected to enable the tuning and enhancement of thermoelectric properties in molecular junctions. To systematically explore the effect of quantum interferences, we designed and synthesized two new classes of porphyrins, P1 and P2, with two methylthio anchoring groups in the 2,13- and 2,12-positions, respectively, and their Zn complexes, Zn-P1 and Zn-P2. Past theory suggests that P1 and Zn-P1 feature destructive quantum interference in single-molecule junctions with gold electrodes and may thus show high thermopower, while P2 and Zn-P2 do not. Our detailed experimental single-molecule break-junction studies of conductance and thermopower, the latter being the first ever performed on porphyrin molecular junctions, revealed that the electrical conductance of the P1 and Zn-P1 junctions is relatively close, and the same holds for P2 and Zn-P2, while there is a 6 times reduction in the electrical conductance between P1 and P2 type junctions. Further, we observed that the thermopower of P1 junctions is slightly larger than for P2 junctions, while Zn-P1 junctions show the largest thermopower and Zn-P2 junctions show the lowest. We relate the experimental results to quantum transport theory using first-principles approaches. While the conductance of P1 and Zn-P1 junctions is robustly predicted to be larger than those of P2 and Zn-P2, computed thermopowers depend sensitively on the level of theory and the single-molecule junction geometry. However, the predicted large difference in conductance and thermopower values between Zn-P1 and Zn-P2 derivatives, suggested in previous model calculations, is not supported by our experimental and theoretical findings.

Steady state and ultrafast spectroscopy on [FeIII(phtmeimb)2]PF6 (phtmeimb = phenyl(tris(3-methylimidazol-2-ylidene))borate) was performed over a broad range of temperatures. The intramolecular deactivation dynamics of the luminescent doublet ligand-to-metal charge-transfer (2LMCT) state was established based on Arrhenius analysis, indicating the direct deactivation of the 2LMCT state to the doublet ground state as a key limitation to the lifetime. In selected solvent environments photoinduced disproportionation generating short-lived Fe(iv) and Fe(ii) complex pairs that subsequently undergo bimolecular recombination was observed. The forward charge separation process is found to be temperature-independent with a rate of ∼1 ps-1. Subsequent charge recombination takes place in the inverted Marcus region with an effective barrier of 60 meV (483 cm-1). Overall, the photoinduced intermolecular charge separation efficiently outcompetes the intramolecular deactivation over a broad range of temperatures, highlighting the potential of [FeIII(phtmeimb)2]PF6 to perform photocatalytic bimolecular reactions.

Photoredox catalysis of organic reactions driven by iron has attracted substantial attention throughout recent years, due to potential environmental and economic benefits. In this Perspective, three major strategies were identified that have been employed to date to achieve reactivities comparable to the successful noble metal photoredox catalysis: (1) Direct replacement of a noble metal center by iron in archetypal polypyridyl complexes, resulting in a metal-centered photofunctional state. (2) In situ generation of photoactive complexes by substrate coordination where the reactions are driven via intramolecular electron transfer involving charge-transfer states, for example, through visible-light-induced homolysis. (3) Improving the excited-state lifetimes and redox potentials of the charge-transfer states of iron complexes through new ligand design. We seek to give an overview and evaluation of recent developments in this rapidly growing field and, at the same time, provide an outlook on the future of iron-based photoredox catalysis.