Photocatalytic CO2 reduction by multinuclear metal complexes - A journey towards high-nuclearity supramolecular photocatalysts

The fossil fuels have been, until now, the most exploited energy source. The growth of world population, together with the increasing demand of energy, needs, economic and industrial developments have caused the increasing in the global energy consumption. The extensive fossil fuels consumption causes an increase in the atmospheric CO2, which induces several problems such as global warming and greenhouse effect. But there are a better life and a better world beyond the horizon: we can reach them having the research focused on the green and sustainable energy sources. There are various kinds of energy sources suitable to this aim, but the solar energy is the most promising. Solar light is renewable and environmentally clean, inexpensive, available everywhere and abundant. Moreover, the use of CO2 to obtain energy-rich compounds, like CO and HCOOH, using solar light, has been scientifically demonstrated. Increasing the field of research on this alternative energy source can gives us the possibility that reaching this horizon could not be just a hope. The aim of this PhD thesis is the synthesis and the study of new multinuclear metal-based supramolecular systems that functionally integrate all the principal components needed for the photocatalytic reduction of CO2. In particular, several multi-metallic multicomponent species have been prepared and their photophysical, redox and photocatalytic properties have been investigated. Most of the multinuclear species are made of Re(I) catalytic units coupled to Ru(II) photosensitizers. Light energy is absorbed by the Ru(II) polypyridine components that undergo to a series of electron transfer processes to the Re(I) catalysts, in which electrons are collected and used for CO2 reduction. The turnover numbers of the photocatalytic reduction of CO2 to CO is extremely high, reaching the outstanding values of 6038 in some cases, which are among the highest values reported in literature at the present. I performed part of the photocatalysis experiments at the Tokyo Institute of Technology (Japan), by taking advantage of a Italy-Japan collaborative research project. To complement the work on integrated systems for CO2 photocatalysis, an investigation on the Photoelectroreduction of CO2 was performed at the Universitè de Paris, where I performed the syntheses of suitable catalysts for the CO2 reduction, the assembly of multilayer electrodes and the electrochemical analysis on these electrodes.