Shaping D-π-A dyes for Dye-Sensitized Photocatalysis and new generation photovoltaics

The phenomenon of global warming, coupled with the progressive depletion of feedstocks, demands a rapid replacement of fossil fuels with renewable energy sources. Among them, solar energy appears very appealing since it is abundant, ubiquitous and practically inexhaustible. Furthermore, this kind of energy can be exploited in several different ways, since it can be converted both to electricity (photovoltaics) and fuels, overcoming the problem of storage and distribution. In this, H2 seems to be one of the best candidates since it is carbon-free, can be produced from water and has a high energy content. Contrary to traditional electrolysis, photocatalytic H2 production can be considered a promising and cheap alternative to produce H2 in a green and sustainable way. The main project explicated during this Ph.D. career focused on the Dye-Sensitized Photocatalysis (DSP), a relatively new method to convert sunlight into a fuel, such as H2. In this approach, the photoactive system is made with by Pt0/TiO2 nanoparticles decorated by colored dyes capable to extend the active spectral range to the Vis wavelengths, so to increase the efficiency of the catalytic process. The mechanism of such devices involves an electron flow from the photo-excited sensitizer to the Pt hydrogen evolution catalyst, while a molecule of sacrificial electron donor (SED) is required to regenerate the dye. The required optical and electrochemical properties of the dyes employed in DSP are very similar to those required for dye-sensitized solar cells (DSSC), as a result, the classes of compounds most commonly employed in DSP are those that have shown the highest efficiencies in DSC. Nevertheless, it’s still not clear how the dye structure affects the efficiency of catalysis in H2 production experiments. Thus, the aim of this research project has been to find a reliable guideline for the synthesis of D-π-A dyes with optimized structures for hydrogen production. The activity of such dyes in DSSC application has been also investigated. In order to prepare a wide-ranging family of D-π-A dyes optimized for H2 production, we decided to face the two most critical issues: hydrophilicity and bulkiness. In this sense, we explored sensitizers with increasing hydrophilicity in the donor moiety, to promote dye-SED interaction, and increasing bulkiness of the backbone, to shield the TiO2 surface and avoid dye leaching and negative recombination processes. We selected a class of compounds containing the 2,1,3-benzothiadiazole moiety, a scaffold that was already shown to have impressive power conversion efficiencies in DSSC, even if no systematic study on their structural optimization were carried out. Furthermore, to date, there are only a few reports concerning their use in DSP. Here, the preparation and application in photocatalytic H2 production of several D-π-A benzothiadiazole-based dyes will be reported. The design of the structures proposed has been optimized in order to tune the photophysical and chemical properties by a proper choice of terminal groups, π-spacers and side chains. All the new structures have been synthesized and fully characterized and used to build the corresponding DSSC, which showed good efficiencies. Photocatalytic tests were conducted by varying the SED nature, as well as the dyes concentration in the system and the type of TiO2 exploited, leading to very interesting results in terms of H2 production rate and TONs and highlighting an actual correlation between dyes architectures and catalytic efficiencies.