Photophysics and Photochemistry of Iron Carbene Complexes

Sammanfattning: Nature captures sunlight via light-absorbing molecules.Similarly, photosensitisers are used in applications of solar cells and artificial photosynthesis to absorb sunlight, and transfer the excited electron.Successful photosensitisers have in the past been based on a Ru polipyridyl scaffold, despite Ru being one of the scarcest elements in Earth's crust.This thesis work aims to replace Ru polipyridyl complexes by Fe carbene complexes, that by clever ligand design have approached suitable photosensitiser properties.One crucial property that is not yet competitive for Fe carbene photosensitisers is how long they stay in the excited state, i.e. their lifetime. This is controlled by the deactivation pathways of the molecule, dictated by the excited state landscape. Several Fe carbene photosensitisers were in this thesis investigated by spectroscopic and computational methods, to understand their deactivation pathways. For the Fe(II) carbene complexes investigated, small changes in the ligand structure influenced both what excited state (charge-transfer or metal-centred) that was mainly populated and the lifetime of the state.For the Fe(III) carbene complexes investigated, there was instead one dominating charge-transfer excited state that was rather unaffected by changes to the ligand. Furthermore, for the Fe(II) complexes metal-centred states played a large role in the deactivation pathway but for the Fe(III) complexes this was not the case.Also, one Co(III) carbene complex was investigated which displays remarkable long lifetime and emission from a metal-centred state.As a first step towards application, the electron-transfer properties of some of the photosensitisers were investigated.Fe(II) complexes with a push-pull design were able to transfer electrons to TiO2 in a solar cell configuration.The solar cell performance was however limited by an ultrafast recombination reaction, that brought a majority of the transferred electrons back to the photosensitiser.The Fe(III) complexes investigated had long enough lifetime to participate in electron transfer with other molecules in solution, if the concentration was high.Furthermore, at very high concentrations of the photosensitiser a light-induced charge-disproportionation reaction outcompeted all other deactivation pathways. In a heterogeneous catalysis configuration, this reaction could generate long-lived Fe(IV) species with the correct additives.The thesis work thus provide fundamental insights to the early implementations of Fe carbene photosensitisers in applications, by resolving key electron-transfer processes on the ultrafast timescale.