Towards photoinduced charge accumulation in purely molecular D⁶ metal complexes with oligo-triarylamine donors

One major problem humanity faces these days is the use of fossil fuels and even more pressing, the consequences of their combustion. Therefore, multiple research groups of various scientific backgrounds have made it their business to find a way to produce energy from sunlight and water. In order to successfully produce these so-called solar fuels, one has to go beyond charge transfer on a single-electron level. Most fuel-forming reactions, such as H2 production or CO2 reduction, require multiple redox equivalents.
The main focus of this thesis was on the creation of a simple, purely molecular (nanoparticle-free) donor-photosensitizer-acceptor (D-P-A) assembly capable of twofold photoinduced charge accumulation without the use of sacrificial agents.
Anthraquinone (AQ) was chosen as the two-electron acceptor, on which charge built-up can be mitigated by proton-coupled or metal ion-coupled electron transfer. Oligo-triarylamine (OTA) displays excellent ability to donate two electrons. In combination with d6 transition metal complexes, AQ and OTA present themselves as viable building blocks for D-P-A compounds for the study of photodriven charge accumulation.
Initial photophysical investigation of rhenium containing triads gave charge separation on a single-electron level. In an AQ-Re-OTA triad, the charge-separated state lifetime could be significantly increased in presence of a strong Bronsted acid due to PCET. A long lifetime of the first charge-separated state is an important prerequisite for a second, charge-accumulating step to occur.
Advancing the systems by incorporation of ruthenium polypyridyl photosensitizers allowed for higher excitation densities. AQ-Ru-OTA exhibited a lifetime in the microsecond range for the first charge-separated state already without acids. In presence of Sc3+, the lifetime was even increased beyond the detection limit of a nanosecond transient absorption setup. In the AQ-Ru-OTA triad, a ruthenium-OTA dyad, and in an AQ-Ru-OTA-Ru-AQ pentad, spectral evidence for OTA2+ was detected in presence of non-sacrificial additives, such as tetracyanobenzene or diphenylthiourea.
Furthermore, push-pull systems comprised of OTA and a boron mesityl acceptor were studied with regard to their emission behavior and the suitability of OTA for use in charge transfer emitters. This lead to guidelines for the design of new charge transfer fluorophores.