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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.