Photoinduced formation and decay of charge-separated states in d⁶ metal complexes

Abstract

One of the major challenges mankind faces these days is the growing demand for energy and the associated consequences of combusting fossil fuels. Accordingly, numerous research groups of different scientific backgrounds focus their efforts towards the production of energy and fuels from sunlight and water. For efficient production of so-called solar fuels, a thorough understanding of fundamental characteristics of charge transfer reactions is crucial. In this context, the following thesis will focus on three key aspects of charge separation: (i) the mechanisms of electron transfer reactions that are coupled to concomitant proton transfer, (ii) the kinetics of electron transfer reactions over large distances, and (iii) charge accumulation in a molecular system without the need of sacrificial agents. In ruthenium(II) and rhenium(I) dyads with an attached phenol which acts as a combined electron and proton donor, the dependence of proton-coupled electron transfer (PCET) reactions on the electron donor-electron acceptor distance was investigated. These measurements yielded the first distance-decay constants (β) for bidirectional concerted PCET reactions. Additionally, a mechanistic change-over was observed for short donor-acceptor distances. When linking the phenol directly to the photosensitizer these dyads exhibit photoacid behavior. In contrast, by introducing one p-xylene bridging molecule, proton-coupled electron transfer was operative. In addition to these dyad studies the PCET chemistry of thiophenols was investigated in bimolecular reactions. This study demonstrated that thiophenols, dependent on their substituents, show the whole variety of PCET mechanisms ranging from concerted to stepwise processes. However, in comparison to phenols, the sulfur analogs tend towards a stepwise transfer of the electron and proton, due to easier oxidation combined with a higher acidity. In a second topic of my thesis, the kinetics of thermal back electron transfer of charge-separated states was investigated in linear triads of variable donor-acceptor distances. These triads are comprised of a ruthenium(II) photosensitizer, a triarylamine (TAA) electron donor, and anthraquinone (AQ) as an electron acceptor. The charge recombination kinetics as a function of spatial separation between donor and acceptor revealed a maximum in rate constants for large distances. This observation was attributed to an increase in the outer-sphere reorganization energy with increasing donor-acceptor distances. According to Marcus theory, this causes a change-over for electron transfer operating in the inverted region for short distances to the normal region at large separations, via an activationless ET for intermediate distances. This is the first unambiguous experimental evidence for rate maxima at large spatial separations, as predicted already 30 years ago by theoretical studies. With respect to the main objective of light-driven solar fuel production, the accumulation of multiple charges on a single molecule is pivotal. For the reduction of protons to hydrogen, or the oxidation of water, a single redox equivalent is not sufficient. Therefore, in a third topic of my thesis, the already mentioned donor-photosensitizer-acceptor assembly was extend to a pentad and investigated with respect to its ability to perform photoinduced charge accumulation. Hereby, a central AQ unit served as a potential two-electron acceptor. Upon excitation with light, the formation of twofold reduced anthraquinone was confirmed in transient IR and UV/Vis experiments. Furthermore, in presence of acid the signal of this charge-accumulated state persists even after several hundred microseconds. This is a clear improvement compared to charge-accumulating systems reported in the literature, which either rely on sacrificial agents or do not exhibit sufficiently long lifetimes for the charge-accumulated state.