Through-space interactions in charge-transfer reactions

Considering the importance of electron-transfer reactions in chemistry and nature, especially regarding light-to-energy conversion by (dye-sensitized) solar-cells and the
production of solar-fuels, a fundamental understanding of their mechanisms is necessary for the design of efficient systems. This thesis has its focus on the through-space
interaction of donor-acceptor pairs, which was investigated in two fundamentally different ways – photoinduced electron-transfer and organic mixed-valency.
A short perspective is outlined in Chapter I and brief overviews over the relevant aspects of electron-transfer reactions and mixed-valence systems are presented in Chapters II and IV, respectively.
In Chapter III, the synthesis and spectroscopic measurements of a right-angled and linear series of homologous complexes, bearing a [Ru(bpy)3]2+ photosensitizer and a triarylamine
electron-donor connected by fluorene bridges with different lengths, are described.
Their behavior upon photoexcitaion, with and without an external quencher, was investigated in terms of intramolecular electron-transfer. Electron-transfer rate
constants were determined and an unexpectedly weak distance-dependence was observed for the right-angled series. Molecular mechanics calculations indicate that this
is caused by the flexibility of the fluorene bridge leading to small through-space donoracceptor
separations. The very weak distance-dependence suggests a mainly throughspace pathway for the electron-transfer. For the linear complexes, electron-transfer was
too fast to be detected with the employed method.
In Chapter V, the synthesis of compounds with a "triple-decker" geometry, based on
phenothiazine and carbazole as redox-active moieties, is presented. Analysis of electrochemical
and spectroscopic data, obtained for the compounds in their singly-oxidized
mixed-valence state, revealedweak interaction for the phenothiazine-based compounds.
The through-space charge-transfer pathway may be explained by an n-pi-n- or pi-pi-interaction. The carbazole-based mixed-valence compounds suffered from inconclusive
data that were obtained from the spectroscopic measurements.