Extending Photoinduced Charge Separation. Molecular-semiconductor assemblies for solar energy conversion

Abstract: The conversion of solar energy into chemical energy by harvesting visible-light with synthetic molecules presents several scientific and technological challenges. This thesis is dedicated to the investigation of approaches to long-lived charge separation, one of the crucial aspects for the photochemical generation of solar fuels. Charge separation was characterized in molecular and molecular-semiconductor hybrid assemblies by using optical spectroscopic techniques. The assemblies studied, were designed with the purpose of either solar fuel generation, or for mere mechanistic understanding and proof of principle studies on relevant aspects for solar-to-chemical energy conversion.The photophysical characterization of a Ru-Mn supramolecular complex for photo-chemical water oxidation, and related Ru(II)-complexes, revealed the reasons behind the difficulty of obtaining a long-lived charge separation state. This prevented the photo-chemical water oxidation, exemplifying the limitations of supramolecular approaches to solar fuels. This work justifies the need to explore other alternatives for the creation of a stable material where all the basic functions of natural photosynthesis can be imitated in a simplified way.A viable option is the construction of nanoarchitectures that incorporate light-harvesting units and catalysts on a semiconductor surface. As demonstrated in this thesis, the modification of the individual components of these assemblies, e.g. macroscopic structure of the dye-sensitized semiconductor and electrolyte, can be used to extend not only the lifetime, but also the distance of charge separation. One of the most remarkable findings of this thesis, is that the lifetime of charge separation in dye-sensitized semicon-ductors can be extended by several orders of magnitude, by implementing a photoanode design consisting of a repetitive patterns of SnO2 and TiO2 µm-thick layers. The main feature of this design is the possibility of trapping electrons at dye-free areas on the film, where they reside for longer times before recombining with dye molecules. In addition, such materials can be used for visible-light generation of catalytically active sites on the surface through electron transfer from the photosensitizer. This process, being facilitated by the conduction band of the semiconductor.The work summarized in this thesis is intended to encourage the development of dye-sensitized semiconductors to expand the possibilities of their application in solar fuel technology.