Photophysical properties of novel Porphyrin-Flavin Dyads

Photosynthesis belongs to the fundamentals of life on earth, therefore it is an important matter in natural sciences. The basic principle of photosynthesis is the transformation of solar light into chemical energy. The starting steps of photosynthesis are light-induced energy- and electron-transfer-steps with singular efficiency. One attempt to enlighten the molecular processes involved is to synthesize simpler model systems with similar properties. Important research goals are the dependencies of light-induced processes on distance and orientation of donor and acceptor. A second aim next to the clarification of the molecular conditions of photosynthesis is to create molecular light-driven machines. The most simple so-called biomimetic model system consists of an electron-donor connected to an electron-acceptor via a spacer-group. This simplest form is also referred to as dyad. Beyond dyads far more complicated compounds have been introduced consisting of several donors and/or acceptors, so-called triads, tetrads, pentads etc. Usually porphyrin serves as electron-donor. Next to chinones several other electron-acceptors are used, e.g. anthracene, pyromellitimide and fullerene. Artificial photosynthetic centers are often more stable and/or the excited states are easier to detect compared to the natural photosynthetic center. The photophysical characteristics of four dyads are reported in this work. The dyads consist of porphyrin (either free-base or zinc-metallated) and flavin, connected by different spacers. These dyads reveal photo-induced electron transfer from porphyrin to flavin and energy-transfer in the reversed direction with different efficiencies. The object of the study is the dependency of these processes on the structural features. The spacer of the dyads 1a-1c is an aromatic bridge which leads to well defined donor-acceptor distances. Because of this structure conjugation through the spacer is increased, whereas the absorption in the visible and near UV-area is not affected. Dyad 1b consists of zn-metallated porphyrin instead of its free-base form. The distance of flavin and porphyrin of dyad 1c is extended by one aromatic unit in comparison to dyads 1a and 1b. The spacer of dyad 2 consists of a norbornyl-group. The compound is so-called 'u-shaped', holding porphyrin and flavin closer and obliquely to each other within two conformations: boat and chair. Following methods will be used to study the photophysical behavior of the dyads: absorption and fluorescence spectroscopy, fluorescence lifetime determinations, cyclovoltammetric measurements, transient absorption spectroscopy and semi-empirical calculations. When flavin is excited, a virtually complete quenching of its fluorescence emission is detected for all dyads. Instead two processes compete: energy transfer to porphyrin and a polar quenching process. Both practical studies and semi-empirical calculations prove on the occupation of a short-lived charge-separated state which results from electron transfer of porphyrin to flavin. While the photophysical behaviour of the excited porphyrin-subunit of dyad 1a and 1c remains unchanged compared to the reference compound, the polar quenching process is also detected for the metallated dyad 1b and the 'u-shaped' dyad 2. The different efficiencies of electron- and energy-transfer can be explained by different structures of the dyads. (author)