Ultrafast charge transfer processes in solution

Charge transfer reactions in solution are of fundamental importance in many areas of chemistry and biology. As commonly accepted, electron transfer reaction is regarded as the simplest chemical reaction because of the absence of bond-breaking and bondforming processes. It plays a key role in biocatalysis and in photosynthesis. On the other hand, neutralisation reactions between Brønsted acids and bases involve proton transfer which is important for other processes as well: for instance the function of enzymes, the transport mechanism in biological membranes and the aforementioned photosynthesis. Research on charge transfer reactions began in the nineteenth century. In the 1950s, the development of concepts started, in order to describe the mechanism on a microscopic level. Based on transition state theory Rudolph A. Marcus proposed a theory which allows a description of electron transfer reactions in such a way as the rate constant could be related to the free energy of reaction. With some modification this theory can be applied to proton transfer reactions as well. If a charge is transferred from one molecule to another the charge transfer reaction is referred to as intermolecular or bimolecular. The research on the dynamics of fast bimolecular charge transfer reactions in solution is challenging because of the variable distance between the reactants. Consequently, the reaction rate is influenced by their diffusion. A model, which can be traced back to Marian von Smoluchowski, allows to disentangle the intrinsic reaction within an encounter complex from the preceding diffusion of the reactants resulting in the formation of the encounter complex. Considering different encounter complexes/ reactive complexes the application of this model to bimolecular proton transfer reactions yields a reasonable description. In contrast, bimolecular electron transfer reactions can only be described insufficiently. Different explanations are invoked in the literature: first, the electron can be transferred from donor to acceptor over long distances. This leads to a multitude of reactive complexes, within which the reaction proceeds with different characteristic reaction rates. Second, in case excited state products are involved, the free energy of reaction can be smaller than expected. As a result, the obvious relation is shifted relatively to the true relation. Both explanations could neither be proved nor disproved in an experimental way. Models which are applied to describe reaction dynamics of charge transfer reactions such as the aforementioned Marcus theory assume reactants to be of spherical symmetry. The Smoluchowski model does this as well: the encounter complex is solely characterised by the complex radius. In light of the molecular structure of typical reactants in photoinduced bimolecular charge transfer reactions, which is far from isotropic, such a simplification is inappropriate. An ideal mutual orientation of the reactants leads to a strong interaction and consequently to a fast reaction. This specific mutual orientation depends on the nature of the reaction. A multitude of reactive complexes with different geometries exist in solution. The motivation of this thesis is to gain an insight into the geometry of molecular reactive complexes and to connect them with the reaction pathways and the corresponding dynamics. With the appearance of pulsed laser technology, it became possible to follow chemical reactions in real time with ultrafast time resolution. Time-resolved polarisationsensitive infrared spectroscopy became an important experimental method for studies on ultrafast structural changes because of its chemical specificity and of the possible local nature of vibrational modes. Electron transfer reaction between two neutral molecules leads to charge separation whereas the back reaction is specified as charge recombination. In the 1990s, a