Interfacial Self-Assembly as Catalytic Platform for Multielectron Redox Reactions

In Nature, many vital biological processes take place across the interface formed between two dielectric media. For instance, photosynthesis and aerobic respiration involve a series of redox reactions occurring across energy-transducing membranes, in charge of converting the energy released in the exothermic multielectron redox reactions into an electrical potential difference across the membrane that finally drives the synthesis of ATP. Thus, the interface between two immiscible electrolyte solutions (ITIES) is considered a bio-inspired medium that provides electrochemical control of processes as ion transfer, assisted ion transfer and heterogeneous electron transfer. Moreover, the ITIES is recognized as a catalytic platform providing separation of the reactants and products in two different phases making feasible to perform reactions that are thermodynamically unfavorable in homogeneous systems. Self-assembled metalloporphyrins are fundamental units in the biochemical machineries that carry out vital processes in living organisms. For instance, in photosynthesis, magnesium complexes of porphyrins known as chlorophyll and bacteriochlorophylls self assemble to build the functional units that carry out the fundamental steps, light-harvesting and charge separation. Such supramolecular arrangement significantly enhances the catalytic properties of porphyrins. iiThis thesis represents an important contribution to the development of new biomimetic catalysts based on artificial supramolecular architectures able to resemble Nature’s way to address kinetically challenging multielectron reactions that are fundamental in the development of renewable energy devices. Particularly in the case of the four-electron reduction of oxygen, reaction that determines the efficiency of fuel cells and batteries, the development of selective precious metal-free catalysts would contribute to improve considerably the performance and stability of the devices at a relative low cost. The biomimetic catalysts developed in this thesis consist of self-assembled cobalt porphyrins adsorbed at ITIES, where the self assembled system intend to mimic the function of the multi metal complexes in charge of respiration in mitochondria, and the liquid/liquid interface mimics the function of an energy-transducing membrane by providing a media suitable for proton and electron transfer. The systems were found to be highly selective towards the four-electron reduction, opening new projects on interfacial self-assembled systems to catalyze other challenging multielectron reactions, as photo induced hydrogen evolution and water splitting. Considering that in parallel projects our group have successfully employed ITIES to drive important photo induced reactions as hydrogen evolution and water splitting, we have designed and built up a time resolved surface second harmonic generation (TR-SSHG) and sum frequency generation (SFG) setup in order to selectively characterize the behavior of photoactiv