This thesis is focussed on the development, optimisation and study of different molecular photocatalytic systems for the reduction of protons, in both organic solvents and pure water. Several polypyridine complexes of rhodium and cobalt have been studied by electrochemistry in organic solvents, in the presence of a source of protons, in order to evaluate their catalytic performance for the reduction of protons. They have been studied in the presence of [Ru(bpy)3]2+, which is used as a photosensitiser, and a sacrificial electron donor, either triethanolamine or sodiumascorbate, to construct a photocatalytic system for the reduction of protons to dihydrogen. The production of H2 has been quantified by gas chromatography, and the mechanism of formation of hydrogen has been studied by transient absorption spectroscopy. We have shown that the system [Ru(bpy)3]2+/[Rh(dmbpy)2Cl2]+/NaHA/H2A is amongst the most efficient yet described for the reduction of protons to hydrogen in pure water, with more than 1000 catalytic cycles per molecule of catalyst observed. Furthermore, the [Rh(dmbpy)2Cl2]+ catalyst and the [Ru(bpy)3]2+ photosensitiser have been joined by an alkyl bridge and the effect of this on the photocatalytic activity of the system has beenstudied. We have demonstrated that the non-conjugated covalent link allows not only maintenance ofthe redox properties of both sub-units, but also significantly increases the stability and the efficiency of the system.The final part of this thesis is devoted to the synthesis, characterisation and use of a macrocyclic cobalt complex as a catalyst for photocatalysis of the reduction of protons, in the presence of both [Ru(bpy)3]2+ and sodium ascorbate. This catalyst shows promising activity; its performance is better than that of [Rh(dmbpy)2Cl2]+ under the same catalytic conditions.
