Influence of The Heavy Atom Effect on The Cage Escape Yields of Iron(III)-Photosensitizers with N-Heterocyclic Carbene Ligands

Sammanfattning: The recent emergence of earth-abundant iron(III)-photosensitizers with N-heterocyclic carbene ligands, exhibiting nanosecond ligand-to-metal charge-transfer states with suitable redox potentials to activate a broad range of substances, offers the possibility to convert sunlight into chemical substances, such as fuels or pharmaceuticals, in an environmentally friendly way. These complexes, however, often suffer from low yields of charge-separated products escaping the solvent cage, making such applications inherently inefficient. This behavior has been attributed to the fact that back electron transfer is a spin-allowed process in these systems, as both the charge separated as well as the ground state are doublet states. The back electron transfer may hence be slowed down by a heavy atom-induced spin-orbit coupling to a quartet charge-separated state. This hypothesis has recently been postulated to explain a thirty-fold increase in cage escape yield for the reductive excited state quenching of [FeIII(pthmeimb)2]+ (phtmeimb = phenyl [tris(3-methyl-imidaz-olin-2-ylidene)]borate) by substituted alkyl/aryl amines in dichloromethane compared to the same reactions in acetonitrile. The work conducted in this thesis aimed to shine light onto this spin-orbit coupling effect by either introducing heavy atoms in the quencher or the solvent and quantifying their influence on the cage escape yields for both reductive quenching using [FeIII(pthmeimb)2]+ and [FeIII(btz)3]3+ (btz = 4,4’-bis(1,2,3-triazol-5-ylidene), as well as for previously unaddressed oxidative quenching using [FeIII(pthmeimb)2]+ by use of steady-state as well as time-resolved spectroscopic experiments. The results in this thesis highlighted that heavy-atom induced spin-orbit coupling, if present, may only have a small influence on the cage escape yield and that this effect is unlikely to be the sole contributor of the previously observed increase in dichloromethane.