Improved understanding of processes relevant for artificial photosynthesis: studies on the distance dependence of electron transfer, charge-separated states and the photosynthetic Z-Scheme

Due to the growing world population and the enormous progress in technology over the last decades, there is an increasing demand on energy. To date, the worlds energy needs are mainly covered by the combustion of fossil fuels like natural gas, oil or coal. However, besides the limited availability in the future, combustion of fossil fuels is harmful to human health and the released gases contribute to global warming. An alternative and more sustainable energy source would be the sun since the solar energy that reaches the surface of the earth can already cover the worlds energy demands, but efficient solar energy conversion and storage is a challenge. Natural photosynthesis offers a blueprint for such processes. In this thesis, fundamental processes relevant for artificial photosynthesis are investigated to gain better understanding of and to improve future artificial systems.
Electron transfer over large distances is one of the major processes involved in photosynthesis. In the first project of this thesis (Chapter 3), the distance dependence of electron-transfer rates in donor-photosensitizer-acceptor triads, with either a low (ca. 1.2 eV in TAA-ph$_{n}$-Ru-ph$_{n}$-NDI) or a high (ca. 2.0 eV in TPA-ph$_{n}$-Ir-ph$_{n}$-AQ) driving force for thermal charge recombination, was studied. Symmetrical addition of phenyl spacers allowed the elongation of the donor-acceptor distances in the given triads (n =1,2). Earlier investigations in the WENGER group, on a comparable molecular triad with a driving force of ca. 1.6 eV, revealed an increase of the electron-transfer rate with increasing donor-acceptor distance. This was the first unambiguous experimental proof for a counterintuitive phenomenon that was predicted more than 20 years ago. To increase the understanding of this counterintuitive phenomenon, a systematic investigation regarding the influence of the driving force for thermal charge recombination was performed. The results of this project showed that highly exergonic electron-transfer reactions can exhibit fundamentally different distance dependences than the more commonly investigated weakly exergonic electron transfers. Elongation of the donor-acceptor distance in the set of triads with a low driving force resulted in a decrease of the electron-transfer rate. On the other hand, for the sets of triads with a high driving force, an increase of the transfer rate was observed upon elongation. These observations are in agreement with the MARCUS theory of electron transfer. For low driving forces of ca. 1.2 eV, electron transfer proceeds in an activationless manner in the shorter triad, whereas the electron-transfer step takes place in the normal regime of the MARCUS model in the longer compound. Thus, a decrease of the electron-transfer rate with increasing donor-acceptor distance results. At high driving forces of ca. 2.0 eV, thermal charge recombination in the shorter triad occurs in the inverted regime. With elongation of the system, activationless electron transfer can be observed. As a consequence, the electron-transfer rate for thermal charge recombination in the triads with a high driving force increases with elongation of the donor-acceptor distance.
Charge-separated states (CSSs) are key intermediates in natural photosynthesis. Therefore, the second project in this thesis (Chapter 4) investigated the highly energetic CSSs of the iridium-based triads introduced in Chapter 3 in more detail. In particular, quantitative determinations of the quantum yields for CSS formation were performed to gain more insights into the factors that govern the CSS formation efficiency. Additionally, two-pulse laser experiments revealed the fate of the CSSs after absorption of a second photon. The CSS formation quantum yield reached ca. 80% when the formation proceeded via an MLCT transition. Admixture of an intraligand charge transfer (CT) transition, which is exclusively possible in the shorter triad, decreased the quantum yield significantly and gave rise to an unusual wavelength-dependence of the CSS quantum yield. One key finding of the investigations in this project is that light-induced charge recombination shows opposite behavior compared to thermal charge recombination in terms of their distance dependences. Thermal charge recombination in the longer triad was significantly more efficient than in the shorter one. However, light-induced charge recombination is much more efficient in the shorter triad.
In nature, the Z-scheme can be seen as the ʽheartʼ of light-dependent natural photosynthesis and can be regarded as one of the most important processes in life. A detailed understanding of the Z-scheme is therefore highly desirable. However, molecular mimics of the Z-scheme are scarce. In the third project of this thesis (Chapter 5), a purely organic molecular mimic of the photosynthetic Z-scheme was designed. Like in nature, two photosystems are incorporated into the molecular design. Each photosystem is represented by a dyad (NMI-TPDB and PT-TAA) and the photosystems are linked with each other by a ${p}$-(di-${n}$-hexyl)phenyl (hxy) spacer to afford a tetrad. To estimate if the approach of the tetrad was promising enough to pursue, reference dyads (NMI-TPDB, TPDB-PT and PT-TAA) were developed. While the successful synthesis of PT-TAA is still pending, the applicability of NMI-TPDB as one photosystem and the introduced linker between the two dyads were verified. NMI-TPDB formed a CSS after excitation with visible light whereas the hxy spacer minimized the possibility of a CT transition between the TPDB and PT units. A weak CT absorption band could be observed, but thermal charge recombination after excitation with visible light occurred rapidly and no CSS formation was detectable. Based on these results, further investigations of this all-organic tetrad approach seem to be very promising.