Studies of Charge Separation in Molecular and Molecular-Inorganic Materials Assemblies for Solar Energy Conversion

Abstract: Conversion of solar energy and its storage in the form of chemical bonds is a current scientific and technological challenge. Different approaches to achieve light-harvesting followed by efficient charge separation are the focus on this thesis. This includes purely molecular approaches inspired by natural photosynthesis, and hybrid molecular-inorganic material (hybrid) approaches where the functions of both, molecules and solid materials are combined to obtain desired long-lived charge separation. The long-term goal is to create an assembly that is able to efficiently produce solar fuel. The study of excited state reactions such as photoinduced electron transfer is crucial for the understanding of solar energy conversion systems. In this thesis, electron-transfer processes in one molecular and one hybrid assembly were studied. In the molecular approach, a series of [Ru(bpy)3]2+-type photosensitizers have been covalently attached to a dinuclear Mn2-ligand that has previously shown photocatalytic water oxidation activity in bimolecular reactions. However, upon integrating photosensitizer and catalyst into one structure, water oxidation activity was shut off. Detailed investigation of the photophysical properties revealed unusually short-lived and strongly pH-dependent excited state decay patterns. The major contribution to the observed short lifetimes was presumably an electron-transfer quenching process originating from the ligand connecting the ruthenium and manganese centers. The hybrid assembly consisted of dye-sensitized mesoporous TiO2 nanoparticles, using the organic dye D35. The efficiency of this type of systems depends, to a great extent, on the kinetics of interfacial electron-transfer processes. Here, photoinduced back electron transfer, an efficiency limiting process, was investigated in the presence of ionic liquid (IL) electrolytes. The aim was to be able to reduce the rate of back electron transfer by taking advantage of the structural properties of the ionic liquids. It was hypothesized that cations accumulated at the TiO2 surface, could temporarily interfere with the recombination of the dye with the electrons in the conduction band of TiO2. Transient absorption measurements show that the kinetics of back electron transfer decreased in the presence of 1-butyl- and 1-hexyl-3-methylimidazolium hexafluorophsphate ILs (14000 and 7000 s-1 respectively) compared to that in organic solvent based electrolytes (24000 in CH3CN and 41000 s-1 in CH3CN/LiClO4). In conclusion, the bulkier the cations, the longer the lifetime of the charge separated state.