Towards an artificial leaf – development of immobilizable photocatalysts

Abstract

Modern society is confronted with two major challenges: energy supply and prevention of environmental pollution. The development of a novel energetic scenario based on the direct conversion of solar energy to storable solar fuels is one promising means of providing for long-term, ecologically friendly energy supply. Based on the use of two renewable and unarguably the largest exploitable resources, water and sunlight, the light-driven water splitting indeed generates the environmentally benign oxygen gas and hydrogen, a carbon-free fuel with the highest energy output relative to molecular weight. Inspired by natural photosynthesis and the architecture of dye-sensitized solar cells (DSSCs), dye-sensitized photoelectrochemical cells (DSPECs) are now being designed and tested as auspicious approaches to generate hydrogen via solar-driven water splitting. In these devices, metal oxide semiconductors such as NiO are functionalized with molecular sensitizers in association with hydrogen evolving catalysts (HECs). Especially, intramolecular photocatalysts are interesting candidates within this context, since they comprise all necessary modules in a single photochemical molecular device (PMD). The photocenter and catalytic unit are chemically connected by a bridging ligand. This intramolecular approach and modular composition allows one-step integration into DSPECs and at the same time specific analyses and separate tuning of the different subunits. The main focus of this thesis was the optimization of an immobilizable PMD for visible light-induced hydrogen production based on a rutheniumpolypyridyl-platinum sensitizer-catalyst dyad. The PMD is equipped with potent phosphonate anchoring groups at its peripheral bipyridine (bpy) ligands to guarantee efficient binding on semiconducting metal oxides. Its general eligibility as hydrogen evolving photocatalyst was examined by homogenous catalysis experiments in the master thesis of the PhD candidate. In a first step of this work the hydrolysis and immobilization was optimized towards a facile processability of the PMD. Preliminary measurements in DSPEC setup lead to visible light-induced photocurrents, suggesting that hydrogen production in such a system is possible. To optimize the photocatalyst with regard to long charge-separated lifetimes and improved photon absorption characteristics a modular synthesis concept for elongated bipyridines was established. The distance between anchoring group and bpy core was increased by introduction of phenylene and triazole building blocks, generated by modern transition metal catalyzed coupling reactions. The distance adjustability, which was monitored by X-ray crystallography, should lead to improved cell efficiency, as it will allow tuning of charge carrier recombination and dye aggregation. Application of the new bpy derivatives in ruthenium(II)- and rhenium(I)-complexes revealed enhanced performance of the corresponding chromophores with regard to their solar light-harvesting ability. Resonance Raman experiments supported by theoretical calculations were conducted to further explore the effects of triazole and phenylene substitution. Additional investigation of the luminescence properties of this series of complexes lead to a surprising observation. Contrary to conventional rutheniumpolypyridyl-complexes the triazole containing compounds showed a solvent dependent light switch effect. Certain water content in solution quenches the luminescence of these complexes. The light-switch behavior could clearly be correlated to the presence and number of triazole subunits. Theoretical studies helped finding possible explanations concerning the origin of the observed light-switch effect. Metal complexes incorporating triazole subunits have a wide scope of applications since they can connect important function-owning molecular building blocks and are easy to generate via CLICK chemistry. The presented findings are highly relevant because they suggest a major restriction concerning the utilization of such complexes in DSPECs with aqueous electrolytes. Opening additional deactivation pathways by triazole introduction might reduce charge separated lifetimes and therewith diminish catalytic turnover. The combined research efforts of this thesis will further promote the targeted design of so-called artificial leafs for solar fuel production.