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Artificial photosynthesis utilises solar-light for clean fuel H2 production and is emerging as a potential solution for renewable energy generation. Photocatalytic systems that combine a light harvester and catalysts in one-pot reactor are promising strategies towards this direction. Yet, most of the reported systems function by consuming excess amount of expensive sacrificial reagents, preventing commercial development. In this thesis, carbon nitrides (CNx) have been selected as non-toxic, stable and low-cost photocatalysts. CNx are first introduced as efficient light harvesters, to couple alcohol oxidation with proton reduction, in the presence of a Ni-based molecular catalyst. This system operated in a single compartment while the oxidation and reduction products were collected in the solution and gaseous phases, respectively, demonstrating a closed redox system. In the presence of an organic substrate and absence of a proton reduction catalyst, photoexcited CNx was found to accumulate long-lived "trapped-electrons", which enables decoupling oxidation and reduction reactions temporarily and spatially. This allows solar H2 generation in the dark, following light exposure, replication light and dark cycle of natural photosynthesis in an artificial set-up. The stability of the designed system was found to be limited by the Ni-based molecular catalyst, and the spectroscopic studies revealed electron transfer from CNx to catalyst as the kinetic bottleneck. Graphene based conductive scaffolds were introduced to the CNx-Ni system, to accelerate the rate of electron transfer from CNx to the Ni catalyst. Time-resolved spectroscopic techniques revealed that introducing these conductive binders enabled better electronic communication between CNx and Ni, resulting in significantly enhanced photocatalytic activity. To improve the solar-light utilisation and the photocatalytic performance of bulk CNx, a straightforward ultra-sonication approach was introduced. This pre-treatment was found to break aggregates of bulk CNx, and the resulting activated CNx had significantly improved activity. The activated CNx showed record activities per gram of the material used, for H2 evolution with a molecular Ni catalyst. The use of abundant waste sources instead of organic substrates was investigated in the presence of activated CNx. The system demonstrated to photoreform purified and raw lignocellulose samples into H2 in the presence of various H2 evolution catalysts over a wide range of pH.

[FeFe]-Hydrogenases (H2ases) are metalloenzymes that can catalyze the reversible reduction of protons to molecular hydrogen as part of the metabolism of certain cyanobacteria and green algae. Due to the low availability of the enzyme, synthetic complexes that mimic the natural active site in structure, function and activity are highly sought after. In this thesis, a number of [FeFe]-H2ases active site model complexes were synthesized to answer open questions of the active site and to develop unprecedented bio-inspired proton reduction catalysts. The first part describes the synthesis and the protonation properties of a [Fe2(μ-adt)(CO)4(PMe3)2] (adt = azadithiolate) complex which contains two basic sites that are similar to those found in the enzyme active site. Unusual kinetic factors give rise to four discrete protonation states. The twofold protonated state is the first model complex that simultaneously carries a proton at the azadithiolate nitrogen and a bridging hydride at the Fe-Fe bond. In the second part, a model complex with an unprecedented amine ligand was synthesized and studied. In analogy to the enzyme active site, the labile amine ligand is expelled after electrochemical reduction. The third part describes a series of model complexes with electronically different aromatic dithiolate ligands. It is demonstrated in one case that the tuning of the ligand by electron-withdrawing substituents results in proton reduction catalysis at an overpotential that is lower than that required by the non-substituted parent compound. The design and the synthetic work towards a new ruthenium-diiron dyad for light-driven hydrogen production are presented in the fourth part. In the final part, differently isotope-labelled mixed valent Fe(I)-Fe(II) model complexes were synthesized, in particular the unprecedented 15N labelled analogue, with the aim to provide EPR-spectroscopic references that will allow the elucidation of the nature of the central atom in the dithiolate bridge of the [FeFe] hydrogenase active site.