Photoactive metallo-conjugates for the homogeneous conversion of solar-light energy

The work of the present PhD thesis is divided in six chapters. In the first chapter, a general introduction on natural and artificial photosynthesis is presented, highlighting the key aspects and selected of the examples of metal-mediated systems proposed in the literature as synthetic candidates for performing relevant photocatalytic processes, such as water oxidation, H2 evolution and CO2 reduction are briefly described. Specific aspects related to the present work are separately presented in the introductory sections of the following chapters. In the second chapter, the design and synthesis and characterization of a small library of dyads of the type zinc-monopyridylporphyrin-Re(I)-diimine-tricarbonyl complexes, together with a detailed electrochemical and photophysical investigation, are discussed. From the state-of-the-art results obtained for the dyad fac-[Re(CO)3(bipy)(Zn†¢4'MPyP)](CF3SO3), a proper modification of the single components has been envisaged in order to improve the efficiency of the photoinduced charge separation process. With this aim, three different dyads have been designed. In two of them, electron-donor tert-butyl (t-Bu) groups (either three, or six, respectively) have been introduced on the meso-phenyl moieties of the zinc-porphyrin, in order to make the it a better electron donor. In the third dyad electron-withdrawing ethyl ester groups have been introduced on the bipy ligand, in order to make the rhenium fragment a better electron acceptor. The photophysical investigation proves that the latter system promote the fast formation of a charge separated upon visible light irradiation. In the third chapter, the synthesis and characterization of a new triad is reported. Starting from the rhenium(I) precursor fac-[Re(CO)3(dmso-O)3](CF3SO3), we present a variation on the synthetic pathway shown in the first chapter, in which the zinc-porphyrin is first coordinated to the rhenium(I)(CO)3 fragment via an amido-bipyridyl linker unit, leaving the third apical position of the metal center free for the subsequent introduction of a pyridyl functionalized fullerene. The detailed NMR analysis, also in comparison with intermediate systems and appropriate model compounds, has elucidated several structural features of the adduct, and, in particular, highlighted the complexity deriving from the simultaneous presence of two stereogenic centers. A detailed photophysical investigation is also reported, that evidence the formation of the charge separated species, with the positive charge located on the zinc-porphyrin fragment and the negative charge located on the fullerene component. The fourth chapter is focused on the development of new photoactive triads obtained by a metalloporphyrin-mediated self-assembling strategy. An aluminium-monopyridylporphyrin platform has been successfully used to form, via based metal hard/soft discriminations, robust photoactive two-component and three-component systems, in one-pot reactions with quantitative yields. As complementary photoactive components, a ruthenium-porphyrin and either a free-base porphyrin or a fulleropyrrolidine, both suitably functionalized with a carboxylic acid group, have been employed. A complete characterization of the two-component and three-component systems is presented. The photophysical properties arising from the two- and three- component systems containing the three different types of porphyrins, investigated by stationary and nanosecond time-resolved emission techniques, are described. In the fifth chapter, the preliminary investigations on the potential use of liposomes as supramolecular scaffolds for the anchoring of Re(bipy)-based catalysts and metalloporphyrin chromophores, and their potential employment in the photoreduction of CO2 in water media has presented. Successful inclusion of a Re(bipy) complex, suitably functionalized with a pendant alkyl chain, alone or together with either a tetracationc or a tetraanionic zinc-porphyrin, on different types of liposomes is reported. The preliminary UV-vis surveys focused on the monitoring of the photogeneration of the One-Electron-Reduced species (OER), with either UV or visible light in buffer solution, and on the monitoring of the photostability of the systems, are described. The sixth, and final chapter contains the experimental section.