Beyond Classical Ruthenium(II) Polypyridyl Complexes Photosensitizers as Building Blocks For Linear Donor-Photosensitizer-Acceptor Assemblies

Abstract: This thesis describes ruthenium(II) polypyridyl-type complexes tailored for artificial photosynthesis. Inspired by Nature, the primary events in photosystem II are mimicked by donor-photosensitizer-acceptor (D-P-A) assemblies. The photosensitizer plays a key role in such processes, and the combination of structural and photophysical properties is essential to control the electron transfer steps. In the first part, the general requirements for photosensitizers are discussed.The second part deals with [Ru(bpy)3]2+-benzoquinone (Q) dyads (bpy is 2,2´-bipyridine) based on an asymmetric 5,5´-bisamide substituted bpy. Rapid electron-transfer from the excited state is observed to generate the RuIII-Q- charge separated states but preliminary results show no effect of the directionality of the amide link.In the main part, a strategy to overcome the photophysical limitations of RuII bistridentate complexes (e.g. [Ru(tpy)2]2+, tpy is 2,2´:6´,2´´-terpyridine) is explored. The prototypical [Ru(dqp)2]2+ complex (dqp is 2,6-di(quinolin-8-yl)pyridine) is synthesized which displays a 3000 ns excited state lifetime at room temperature, reversible redox chemistry and high photostability. The synthesis of 4-substituted dqp is achieved via SUZUKI coupling using 8-quinoline boronic acid or ring-formation of the central pyridine. A markedly rich Ru coordination chemistry was observed, e.g. facial and meridional isomers of [Ru(dqp)2]2+. Using a chloride-free [Ru(dqp-R)(MeCN)3]2+ intermediate allows the synthesis of heteroleptic meridional [Ru(dqp-R)(dqp-R’)]2+ (R,R’ = -H, -CO2Et, -NH2, -OMe, -Br, -PhBr, …) in high yields. The meridional complexes show long-lived luminescence (450 - 5500 ns) and reversible redox chemistry. The photochemical reactivity has been investigated in typical electron-transfer reactions, e.g. in a supramolecular P-A dyad and in a multimolecular approach using biomimetic components (Mn and Fe complexes).The dqp ligand is further used to synthesize FeII, RhIII, cyclometallating RuII complexes and an aza-analogue of [Ru(dqp)2]2+ and is discussed in the final part. These complexes were prepared with the aim to further tune the redox properties while maintaining good photophysical properties.