SUPRAMOLECULAR SYSTEMS FOR ARTIFICIAL PHOTOSYNTESIS

Artificial photosynthesis, i.e., conversion of solar energy into fuels, represents one of the most promising research fields which could potentially provide clean and renewable sources of energy for a sustainable development of the future generations. Among several possibilities, water splitting into molecular hydrogen and oxygen is one of the most challenging and appealing reaction schemes. Taking inspiration from nature several functional units are required to this aim: (i) an antenna system, responsible for light harvesting, (ii) a charge separating system, where the absorbed energy is converted into an electrochemical potential (electron/hole pair), and (iii) appropriate catalyst units, capable of stepwise storing the photogenerated electrons and holes in order to drive multi-electron transfer processes at low activation energy. In the present thesis several points regarding both the photoinduced oxidation and reduction of water as well as charge separation are studied. In more detail, three different classes of water oxidation catalysts are examined, namely tetrametallic polyoxometalate, tetracobalt cubanes, and single-site cobalt salophen complexes, within light-activated catalytic cycles involving tris(bipyridine) ruthenium and persulfate as photosensitizer and sacrificial electron acceptor, respectively. Particular attention is paid to the evaluation of the interactions between the catalyst and the sensitizer and to the kinetics of both photochemical and thermal electron transfer steps. Concerning water reduction, the following systems are investigated: a self-assembling reductant/sensitizer/catalyst triad based on an aluminum pyridylporphyrin central unit, a cobaloxime catalyst, and an ascorbate electron donor, a cationic cobalt porphyrin catalyst in the presence of tris(bipyridine) ruthenium as sensitizer and ascorbic acid as electron donor, and a PAMAM dendrimer decorated with ruthenium polypyridine dyes at the periphery and inside of which platinum nanoparticles have been grown. Beside the optimization of the photocatalytic performance, detailed insights into the photoinduced hydrogen evolving mechanism are carefully provided. Finally, a triad system for photoinduced charge separation, based on a naphthalene bisimide electron acceptor, a zinc porphyrin electron donor, and a ferrocene secondary electron donor, connected via 1,2,3- triazole bridges, is also described. Detailed photophysical investigation of the system will show an unusual behavior with respect to photoinduced electron transfer. Results obtained from a side-project in the field of molecular electronics will be also discussed, where photoinduced electron transfer processes are used to different aims.