Photoinduced electron and proton transfer with ruthenium complexes and organic donors and acceptors

Proton-coupled electron transfer (PCET) plays a crucial role in photosynthesis and catalytic conversions such as water oxidation and carbon dioxide reduction. It can formally be split into proton and electron transfer. In photosynthesis, light is used as principal energy resource, which is also desirable for artificial conversions. Therefore, PCET and individual proton and electron transfers from the long-living triplet excited state of ruthenium polyimine complexes were investigated in this thesis.
In the first project, photoinduced PCET from the excited state of [Ru(bpy)2(pyimH)]2+ (bpy = 2,2’-bipyridine, pyimH = (2-pyridyl)imidazole) was investigated with electrochemical and time-resolved spectroscopic techniques. Depending on the pH, simple ET or PCET to a suitable organic substrate is favored. Simple excited state ET is facilitated significantly upon deprotonation of the ruthenium photosensitizer. The reducing power of this type of complex was further tuned by electron donating and electron withdrawing substituents on the bpy-spectator-ligands. Formal hydrogen atom donation is facilitated by approximately 50 kcal mol 1 in the photochemically generated 3MLCT, making these complexes strong formal hydrogen atom donors, even when compared to metal hydride complexes.
In the second project, a molecular triad was investigated which is inspired by photosystem II. This triad combines long-range photoinduced charge transfer with two PCETs. The investigated donor-photosensitizer-acceptor assembly is based on a phenol as combined electron and proton donor, a [Ru(bpy)3]2+-type photosensitizer and a 4,4’-bipyridinium proton and electron acceptor. The photochemically generated radicals are separated by 20 Å. They are formed via two PCETs which mimics enzymatic long-range charge transfer more closely than any previously reported molecular model system.
In the third project, the influence of a hydrogen bonded carboxylate on the luminescent excited state of acidic [Ru(bpy)2(biimH2)]2+ (biimH2 = 2,2’-biimidazole) type complexes was examined. Luminescence of monocrystalline samples was characterized by DFT calculations and monitored in the solid state at variable temperature and pressure. A pressure-induced red-shift in luminescence was observed in complexes with electron donating tert-butyl substituents on the bpy ligands whereas the more acidic complex with CF3-substituents showed only small pressure dependent luminescence. The origin of the difference in luminescence is either due to pressure induced proton transfer or secondary coordination sphere interactions via the hydrogen bonds.