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Ayla Kruse

Lichtinduzierte Prozesse von Photosensibilisatoren

Universität Rostock, September 2023

https://doi.org/10.18453/rosdok_id00004653

Abstract: An efficient photosensitizer (PS), e.g. in photocatalytic water splitting, requires a broad absorption spectrum from the UV to the NIR and a long lifetime of the photocatalytically relevant charge-transfer states is essential. To quantify the usability of molecules as PS, ultrafast absorption spectroscopy on time scales from 100 fs to a few ns is a powerful tool. In this work, the lifetimes and decay mechanisms of the photocatalytically relevant excited states of a series of octahedral iron complexes with different ligand design were investigated to identify efficient iron-based PS. We found that specific modifications of the ligands by our cooperation partners extended the lifetime of the photocatalytically active 3MLCT (metal-to-ligand charge-transfer) state in Fe(II) complexes from 9 ps to 35 ps. For comparison to Fe(II) complexes, Ru(II) complexes with the same ligand design were studied showing long lifetimes of charge transfer (CT) states in the nanosecond range qualifying them for the application as PS. However, ruthenium is a rare and expensive metal. That is why iron complexes represent a more sustainable alternative. Measurements on Fe(III) complexes, started in this work, also show long lifetimes of CT states and emission. This raises the expectation for a successful application of iron complexes in photocatalysis. Another development in photocatalysis is the coupling of the PS to the catalyst within one molecule as a dyad. After optical excitation of zinc(II)-gold(III) porphyrin dyads, an ultrafast electron transfer from the zinc to the gold porphyrin takes place featuring a gold(II) center. Here, the electron is “trapped” which decelerates the back electron transfer and leads to a long-lived photocatalytically active charge-shifted state with a lifetime of 3 ns. This characteristic qualifies the dyads as good candidates for photocatalytic applications.

doctoral thesis   free access