PHOTOINDUCED PROCESSES IN SUPRAMOLECULAR SYSTRMS FOR SOLAR ENERGY CONVERSION

Artificial photosynthesis, defined as the conversion of solar energy into fuels, could provide a solution to the problem of the intermittent avalaibility of sunlight, one of the key issues to overcome in order to implement widespread use of solar energy. Among the possible applications of artificial photosynthesis, particularly interesting are photochemical water splitting, since it represents a possible way to solar hydrogen generation, and the photocatalytic reduction of CO2 to CO, HCOOH, CH3OH or CH4, important industrial intermediates or fuels. In a biomimetic approach, a generic artificial photosynthetic device consists of an antenna system, a charge-separating reaction center, an oxidation catalyst and a reduction catalyst. Antenna systems absorb visible photons, thus converting them to electronic excitation energy which is then conveyed via energy transfer processes to the reaction center where it drives electron transfer processes leading to charge separation. The photogenerated electrons and holes provide to the catalysts the oxidizing and reducing equivalents necessary to drive redox reactions on a substrate. In this work, a number of systems proposed as possible components of an artificial photosynthetic device or as models for the investigation of related key processes are studied. The systems described here are designed organizing molecular components in spatially defined architectures, following the principles of supramolecular chemistry. In particular, Chapter 3 describes a triad for charge-separation obtained by selfassembling of a Ru-porphyrin electron donor, an Al-porphyrin as the photoexcitable chromophore and a naphtalenebisimide electron acceptor. The intrinsically asymmetric nature of triad systems required the development of assembling strategies based on molecular recognition between the subunits, implemented exploiting highly selective metalligand interactions. Photoinduced charge-separation was demostrated by a detailed photophysical and electrochemical characterization. In Chapter 4, a Sn-porphyrin component and a Ru-porphyrin component are combined in a series of tri-, penta- and heptanuclear supramolecular arrays. A number of photoinduced intercomponent electron-transfer processes, leading to a common chargeseparated state, could be identified by use of time-resolved UV-Vis absorption and emission spectroscopy and their kinetics rationalized in terms of standard electron-transfer theory. Chapter 5 describes a model complex of the [FeFe] hydrogenase enzyme active site, tested as a reduction catalyst for photochemical hydrogen production. High turnover numbers were obtained in a photocatalytic cycle using Ru(bpy)3 2+ as photosensitizer and ascobic acid as sacrificial electron donor. In Chapter 6, a kinetic study on [Ru4(μ-O)4(-OH)2(H2O)4(γ-SiW10O36)2]10-, a recently proposed Ruthenium Polyoxometalate catalyst for the oxygen-evolving side of water splitting, is presented. Hole transfer from photogenerated oxidants to the catalyst has been investigated by means of nanosecond laser flash photolysis, both in solution and at a sensitized TiO2 surface. The very fast rates observed open the possibility to include [Ru4(μ- O)4(-OH)2(H2O)4(γ-SiW10O36)2]10- in photochemical water splitting devices. Chapter 7 describes the photophysical investigation of supramolecular systems containing a Zn-porphyrin chromophore and the [fac-Re(CO)3(bpy)]+ fragment as components. Polypyridine-Re(I)-tricarbonyl complexes are known to catalyze CO2 reduction, opening the possibility to exploit them for photocatalysis. The results obtained provided guidelines for the realization of new adducts to be tested in photocatalytic cycles. A side-project in the field of molecular electronics is also reported in Chapter 8. Photoinduced electron transfer is demonstrated here as the working principle of a prototype photochromic switch for non-destructive read-out optical memory systems. The system proposed is composed of a diarylethene as photochromic unit and a perylene bisimide as fluorophore.