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Iron complexes are emerging as favourable substitutes to noble metal complexes as photocatalysts due to iron being earth-abundant and inexpensive. Much of the recent progress has been enabled by the strong electron-donating character of N-heterocyclic carbene (NHC) ligands that strongly destabilizes metal-centred (MC) states of FeNHC complexes and thereby greatly extends the lifetimes of their charge transfer (CT) states that are otherwise rapidly deactivated via low-lying MC states.

The first part of this thesis successfully employed FeNHC complexes in different photoredox catalysis (PRC) reactions and an example of high-turnover catalytic hydrogen production. The latter was accomplished with the benchmark ferric bis-tridentate scorpionate complex [Fe^III(phtmeimb)²]⁺ (phtmeimb = phenyl(tris(3-methylimidazol-1-ylidene))borate) which has a ²LMCT state lifetime of two nanoseconds and excellent photostability. It was further employed in two PRC reactions that yielded synthetically-useful organic compounds, where fast and efficient reductive quenching of the ²LMCT state by various amine donors with cage escape yields between 2 and 22 % were observed. A tris-bidentate complex with favourable excited-state (ES) redox properties and lifetimes in both oxidation states, [Fe^II,III(btz)₃]^2+,3+ (btz = 3,3’-dimethyl-1,1’-bis(p-tolyl)-4,4’-bis(1,2,3-triazol-5-ylidene)), was employed in a two-photon PRC reaction utilizing both oxidative and reductive quenching steps, making the PRC reaction overall more efficient. 

The second part of this thesis describes the electrochemical and photophysical characterization of novel FeNHC complexes with three different motifs in view of their potential suitability as photocatalysts. (i) For a series of ferric bis-tridentate complexes with cyclometalating ligands, not only were their emissive ²LMCT states with lifetimes of hundreds of picoseconds approaching values previously obtained with the [Fe^III(phtmeimb)₂]⁺ motif, their electrochemical and ES properties were more tunable by substituent effects. (ii) For the (NHC)₄(bpy)₂ bis-tridentate complexes [Fe(btz)₂bpy]^2+,3+ (bpy = 2,2'-bipyridyl) and [Fe(btz)₂mbpy]^2+,3+ (mbpy = 4,4'-dimethyl-2,2'-bipyridyl), both ferrous and ferric analogues offered insufficient ES lifetimes on the order of ten ps. The ferrous mbpy variant featured however a more long-lived, presumably MC state that deserves further characterization, also in regard to its potential reactivity. (iii) For the ferrous analogue of [Fe^III(phtmeimb)₂]+, the strikingly-short picosecond ³MLCT state lifetime concludes that even the phtmeimb- ligand with superior σ-donating ability cannot sufficiently prevent the relatively high-energy ³MLCT state from rapid deactivation. 

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 N-heterocyclic carbene (FeNHC) complexes with long-lived charge transfer states are emerging as a promising class of photoactive materials. We have synthesized [Fe-II(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 (MLCT)-M-3 (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 ((MLCT)-M-2) state of its Fe(III) analogue [Fe-III(ImP)(2)](+), which led to a claim of dual (MLCT and LMCT) emission. Revisiting the photophysics of [Fe-III(ImP)(2)](+), we confirmed however that higher ((MLCT)-M-2) states of [Fe-III(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 [Fe-III(ImP)(2)](+) prepared by us only shows red emission from its lower (LMCT)-L-2 state (740 nm, 240 ps). The long-lived, higher energy emission previously reported for [Fe-III(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 [Fe-III(ImP)(2)](+) itself.

Symmetry-breaking charge separation in molecular materials has attracted increasing attention for optoelectronics based on single-material active layers. To this end, Fe(III) complexes with particularly electron-donating N-heterocyclic carbene ligands offer interesting properties with a (LMCT)-L-2 excited state capable of oxidizing or reducing the complex in its ground state. In this Communication, we show that the corresponding symmetry-breaking charge separation occurs in amorphous films of pristine [Fe(III)L-2]PF6 (L = [phenyl(tris(3-methylimidazol-2-ylidene))borate](-)). Excitation of the solid material with visible light leads to ultrafast electron transfer quenching of the (LMCT)-L-2 excited state, generating Fe(II) and Fe(IV) products with high efficiency. Sub-picosecond charge separation followed by recombination in about 1 ns could be monitored by transient absorption spectroscopy. Photoconductivity measurements of films deposited on microelectrode arrays demonstrated that photogenerated charge carriers can be collected at external contacts.

Herein we report the first high turnover photocatalytic hydrogen formation reaction based on an earth-abundant Fe-III-NHC photosensitiser. The reaction occurs via reductive quenching of the (LMCT)-L-2 excited state that can be directly excited with green light and employs either Pt-colloids or [Co(dmgH)(2)pyCl] as proton reduction catalysts and [HNEt3][BF4] and triethanolamine/triethylamine as proton and electron donors. The outstanding photostability of the Fe-III-NHC complex enables turnover numbers >1000 without degradation.

Fe(III) complexes with N-heterocyclic carbene (NHC) ligands belong to the rare examples of Earth-abundant transition metal complexes with long-lived luminescent charge-transfer excited states that enable applications as photosensitizers for charge separation reactions. We report three new hexa-NHC complexes of this class: [Fe(brphtmeimb)2]PF6 (brphtmeimb = [(4-bromophenyl)tris(3-methylimidazol-2-ylidene)borate]–, [Fe(meophtmeimb)2]PF6 (meophtmeimb = [(4-methoxyphenyl)tris(3-methylimidazol-2-ylidene)borate]–, and [Fe(coohphtmeimb)2]PF6 (coohphtmeimb = [(4-carboxyphenyl)tris(3-methylimidazol-2-ylidene)borate]–. These were derived from the parent complex [Fe(phtmeimb)2]PF6 (phtmeimb = [phenyltris(3-methylimidazol-2-ylidene)borate]– by modification with electron-withdrawing and electron-donating substituents, respectively, at the 4-phenyl position of the ligand framework. All three Fe(III) hexa-NHC complexes were characterized by NMR spectroscopy, high-resolution mass spectroscopy, elemental analysis, single crystal X-ray diffraction analysis, electrochemistry, Mößbauer spectroscopy, electronic spectroscopy, magnetic susceptibility measurements, and quantum chemical calculations. Their ligand-to-metal charge-transfer (2LMCT) excited states feature nanosecond lifetimes (1.6–1.7 ns) and sizable emission quantum yields (1.7–1.9%) through spin-allowed transition to the doublet ground state (2GS), completely in line with the parent complex [Fe(phtmeimb)2]PF6 (2.0 ns and 2.1%). The integrity of the favorable excited state characteristics upon substitution of the ligand framework demonstrates the robustness of the scorpionate motif that tolerates modifications in the 4-phenyl position for applications such as the attachment in molecular or hybrid assemblies.

Fe-N-heterocyclic carbene (NHC) complexes attract increasing attention as photosensitisers and photoredox catalysts. Such applications generally rely on sufficiently long excited state lifetimes and efficient bimolecular quenching, which leads to there being few examples of successful usage of Fe-NHC complexes to date. Here, we have employed [Fe(iii)(btz)(3)](3+) (btz = (3,3 '-dimethyl-1,1 '-bis(p-tolyl)-4,4 '-bis(1,2,3-triazol-5-ylidene))) in the addition of alkyl halides to alkenes and alkynes via visible light-mediated atom transfer radical addition (ATRA). Unlike other Fe-NHC complexes, [Fe(iii/ii)(btz)(3)](3+/2+) benefits from sizable charge transfer excited state lifetimes >= 0.1 ns in both oxidation states, and the Fe(iii) (LMCT)-L-2 and Fe(ii) (MLCT)-M-3 states are strong oxidants and reductants, respectively. The combined reactivity of both excited states enables efficient one-electron reduction of the alkyl halide substrate under green light irradiation. The two-photon mechanism proceeds via reductive quenching of the Fe(iii) (LMCT)-L-2 state by a sacrificial electron donor and subsequent excitation of the Fe(ii) product to its highly reducing (MLCT)-M-3 state. This route is shown to be more efficient than the alternative, where oxidative quenching of the less reducing Fe(iii) (LMCT)-L-2 state by the alkyl halide drives the reaction, in the absence of a sacrificial electron donor.

Dopant-free organic hole transport materials (HTMs) remain highly desirable for stable and efficient ni-p perovskite solar cells but are rarely applied in formamidinium lead-bromide (FAPbBr₃) underambient processing. Herein, we compare four dopant-free HTMs on FAPbBr₃- perovskite solar cells (FAPbBr₃-PSCs) according to their structure-property relationship. Among these, P3HT presents higher hole mobility, lower interface trap density, and lower nonradiative recombination, resulting in superior charge extraction and transport. The optimized device utilizing dopant-free P3HT shows a high open circuit voltage of 1.47 V and a champion power conversion efficiency (PCE) of 9.38% with greatly improved operational stability, making it among the highest performance in FAPbBr₃-PSCs based ondopant-free HTMs. Also, to further improve the stability of P3HT- FAPbBr₃ solar cells, the lower cost Carbon electrode was applied to replace the Au, and the resultant carbon-PSCs presented an impressive PCE of 8.9% with a high voltage of 1.44 V. It also keeps excellent stability that almost no degradation nearly one year.