Photoinduced electron transfer in ruthenium-Bis (p-anisyl) amine dyads equipped with boronmesityl or tetramethoxybenzene bridging units

Abstract

Investigation of natural processes is one of the main topics for scientists. Designing molecular systems that mimic natural processes has become important to understand the reaction pathways. Multiple artificial systems were synthesized to study intramolecular electron transfer which is triggered by pho- toexcitation. These molecules are usually comprised of an electron acceptor moiety, a photosensitizer, and/or an electron donor moiety. The donor and acceptor moiety are linked by a bridge. The molecular dyads investigated in this thesis consist of a tris(bipyridine)ruthenium(II) part which acts as the pho- tosensitizer and electron acceptor. This part is linked by different bridging units to the electron donor bis(p-anisyl)amine. Two different bridging units are presented in this thesis. Chapter 1 starts with a general introduction to photosynthesis and photoinduced electron transfer. In the third part of this chapter the reasons for choosing tris(bipyridine)ruthenium(II) as photosensitizer are discussed. Chapter 2 focuses on donor-bridge-acceptor systems with an electron-poor benzene as bridging unit. A dimesitylboron substituent is attached to the central benzene ring of the bridge. The resulting three-coordinate organoboron compound is known as sensor for small anions like CN- or F- . The ability to be bound by anions might have an influence on the charge transfer rate which we wanted to investigate. We assumed that upon fluoride attachment the energy barrier of the bridge increases. Therefore, the photoinduced electron transfer might come to a stop or there might be a change from a hopping to a tunneling mechanism. The figure below shows the two synthesized donor-bridge-acceptor systems. To increase the probability for charge transfer the bridge of dyad 2 was shortened and the photosensitizer was modified to increase its electron-withdrawing character. Chapter 3 focuses on dyads with an electron rich benzene molecule in the bridge. Therefore, 1,2,4,5-tetramethoxybenzene is placed in the middle of the bridging unit. 1,2,4,5-tetramethoxybenzene is has a low oxidation potential. Our goal was to investigate the charge transfer rates and the charge transfer mechanism in these dyads. Due to the low oxidation potential of 1,2,4,5-tetramethoxybenzene it might act as a hopping station whereas in unsubstituted benzene bridging units the charge transfer through the whole bridge usually occurs via a tunneling mechanism. Hence, we synthesized four molecules with different spacers and three different donor-acceptor distances. Similar dyads with an unsubstituted benzene instead of tetramethoxybenzene served as reference molecules.