From light-induced processes in solution toward solid/liquid and liquid/liquid interfaces: the use of peri-xanthenoxanthene derivatives as photoactive compounds

The efficient exploitation of solar energy for chemical bond formation in homogeneous solutions is often hampered by the fast rate of charge recombination within the solvent cage. Heterogeneous biphasic systems can segregate the charge carriers in different chemical environments, allowing the charge transfer to occur only across the interface between the phases. In this way, increased charge separation yield and catalyst recyclability can be obtained. Nevertheless, the use of chemical interfaces to foster photoinduced transformations has not been extensively explored. Aiming to explore the advantages that biphasic systems can provide compared to homogeneous solutions, this work focuses on using derivatives of peri-xanthenoxanthene (PXX), an oxygen-doped analogue of anthanthrene already employed in homogeneous photocatalysis, for photoinduced processes in heterogeneous systems. In particular, the interface between solid/liquid and liquid/liquid immiscible solutions have been investigated. In chapter 2, spectroscopic, electrochemical and computational analyses on PXX derivatives are presented. Specifically, the effect of the elongation of the aromatic surface and the replacement of the oxygen with sulphur atoms was investigated. In chapter 3, novel PXX derivatives were loaded on polymeric surfaces and the functionalised materials employed in the photodebromination of p-bromoacetophenone, both dispersed in bulk solution and within a microfluidic reactor. Aiming to develop a biphasic liquid system to foster photoinduced electron transfer (PET) between a fluorinated and organic liquid phase, in chapter 4 the attempts to obtain selective orthogonal solubility of the electron donors and acceptors by functionalisation with alkylic or fluorinated pendants are reported. This project was the first to extensively investigate the use of PXX derivatives in heterogeneous photoreactions and characterise the photophysical properties of the functionalised materials. The studies revealed a rich band gap tunability of up to 1.3 eV for PXX derivatives in solution. In addition, the loading on polymeric scaffolds not only improved the reaction yield but product conversion up to 30% after three cycles of catalyst recycling was obtained. Consequently, biphasic systems can be considered a promising way to foster the applicability of organic photocatalysis in industrial processes