Exploiting potential inversion for photoinduced charge-accumulation in molecular systems

In view of the steadily growing global population and concomitant increasing energy demand, the use of alternative energy sources becomes more and more important. Although the energy demand of the next few decades could in principle be met from fossil fuels, their supply progressively decreases. Moreover, the use of fossil fuels should be decreased because their combustion has devastating consequences for the environment. Aside from threatening human heath, the release of CO2 and other greenhouse gases contributes to global warming. For these reasons, the conversion of sunlight into useful and environmentally friendly energy has received significant attention in recent years and many research groups investigate the formation of so called solar fuels, especially from water. A major challenge of the light-driven water oxidation, or CO2 reduction, is the transfer and accumulation of multiple redox equivalents, which are required for successful conversion.
The focus of this thesis is the development of a purely molecular system, in which multiple charges can be accumulated without the use of sacrificial agents. Dibenzo[c,e][1,2]dithiin was used as a central two-electron acceptor, the reduction of which with potential inversion should facilitate electron-accumulation. This acceptor was covalently linked to two ruthenium trisbipyridyl photosensitizers and the electron-accumulation investigated. Furthermore, this triad was extended to a pentad by attaching triarylamine groups to the bipyridine ligands of the sensitizers, which act as internal electron donors. Upon irradiation with visible light, two electrons were successfully accumulated in the triad in the presence of a sacrificial electron donor, whereas the twofold charge-separated state of the pentad is formed without the use of external reductants. The lifetime of the charge-accumulated pentad was determined to be ∼ 100 ns. Investigations of the pentad in the presence of acid revealed that upon protonation of the thiolate groups, a stable photoproduct is formed, which does not undergo charge-recombination to the ground state. Furthermore, it was shown that both the triad and pentad can be used as multi-electron photoredox catalysts. Twofold reduction of the substrate disulfide occurs via thiol-disulfide interchange with the catalyst, and can be performed catalytically in the presence of a sacrificial agent.