Catalytic and photocatalytic processes in sustainable small molecule activation

Small-molecule activation is a fundamental process for several organic transformations, where abundant, cheap and readily accessible molecules such as O2, N2, CO2, H2 and H2O2 are involved. Such molecules are usually kinetically inert and the research of appropriate catalysts or photocatalysts to activate them is widely active. Moreover, several studies concerning the development of new active molecules to construct artificial photosynthetic devices are in progress. In this framework, in our research group, few years ago a new class of quinoid compounds called KuQuinones (KuQ) has been synthesized. These compounds are characterized by a pentacyclic, planar and fully conjugated skeleton, which is responsible of their intense absorption in the visible region of the spectrum. Thanks to their quinoid structure, which derives from the condensation of two hydroxy-naphtoquinone units, KuQuinones can accept multiple electrons and their reduction potential for the generation of the radical anion (KuQ•-) is very low if compared with the same process in similar compounds. Such favourable properties were studied quite in detail, and Chapter 2 of this thesis is devoted to the application of KuQuinones as sensitizers in photoelectrochemical devices. In particular, among this class of devices our interest has been focused on the application of KuQuinones as photocatalyst in the light driven water oxidation process, which is considered a valid approach to achieve renewable energy production. To explore this field of application, KuQuinones have been firstly immobilized on an electrode surface, such as Indium-Tin Oxide (ITO). Afterwards, the photoinduced electron transfer process has been studied, using a simple model system where a mono-electronic electron donor molecule, such as triethanolamine or alternatively ascorbic acid, was used. In this context, good results have been achieved using an amphiphilic KuQuinone derivative, which has been deposited on ITO through Langmuir-Blodgett technique, obtaining organized and well packed mono- and multilayers on the electrode surface. Such modified electrodes showed good efficiencies in the photoinduced electron transfer process, in the presence of TEOA. In particular, upon irradiation of the electrode, one electron is promoted to the excited state, where KuQuinone is a strong oxidant and it can take an electron from the sacrificial electron donor species, thus forming the radical anion KuQ•-. Afterwards, a further electron transfer occurs toward the ITO surface, to close the circuit. These promising results prompted us to explore the use of KuQuinones as photocatalysts in the water splitting process. In principle, in the photoelectrochemical cell, once excited, KuQuinones can oxidize water through a multiple electrons process. In fact, KuQuinones can accept two electrons and for this reason H2O2 is the expected reaction product, coming from the 2-electrons oxidation of water. As a matter of fact, preliminary results showed that in our experimental conditions a significant photocurrent signal was detected, indicating that water oxidation occurred. Additionally, thanks to the promising results obtained in such photoelectrochemical devices, carboxylic acid-functionalized KuQuinones have been synthesized and used as sensitizers in ptype Dye Sensitized Solar Cells, using Nickel-Oxide as supporting electrode. Results highlighted that KuQuinones-sensitized cells showed better performance than Ery B, a conventional benchmark dye. Interestingly, despite the lack of electronic conjugation between the anchoring group and the light-absorbing unit, the photoinduced charge transfer occurred through space and not through the conjugated linker, as predictable. Finally, in order to better understand KuQuinones spectroscopic behaviour, a detailed investigation of possible equilibria in different solvents has been performed. Chapter 3 of the thesis will be focused on the development of photoelectrochemical devices for the oxygen activation. In this chapter it is demonstrated that photocatalysis resulted an efficient tool to activate dioxygen using a porphyrin sensitizer. In fact, in our research group over the last decade, the appealing properties of meso-tetraferrocenylporphyrins have been widely investigated. Such porphyrins are characterized by interesting electronic and electrochemical properties, hence, their application as photocatalyst for dioxygen reduction has been already studied. However, devices showed low efficiencies and the synthesis of the appropriate porphyrin derivative in some cases proceeded through laborious procedures. Therefore, Chapter 3 summarize the optimization of the photoelectrochemical cell in order to evaluate the best compromise among the feasibility of the synthetic procedure, the stability of the porphyrin layer on ITO and the optimal photoelectrochemical response toward the generation of the superoxide anion, which is one of the most reactive oxygen species. Finally, together with dioxygen, also hydrogen peroxide constitutes an efficient and sustainable oxidant. As for molecular oxygen, hydrogen peroxide is kinetically inert toward oxidation reactions so it must be activated. Chapter 4 will concern the application of a sustainable vanadium- based system for the synthesis of biologically active compounds. Specifically, reaction between H2O2 and commercially available vanadium catalyst precursors, leads to the generation of peroxido-V derivatives, which are very effective species for the oxidation of several organic and inorganic substrates. Among them, V-peroxocomplexes have been used to oxidize bromide ion to effective brominating species, thus mimicking the mechanisms of action of the Vdependent bromoperoxidase enzyme. This system allowed us to synthesize important brominated thymol derivatives. Among other properties, 4-bromothymol resulted a very effective antibacterial agent against important bacterial strains, which are responsible of dangerous infections for humans. These results open the opportunity to introduce 4-bromothymol as active ingredient in several industrial products in substitution of existing antibacterial agents.