Novel photocatalysts based on ruthenium complexes with NHC (N-heterocyclic carbene)-type bridging ligands have been prepared and structurally and photophysically characterised. The identity of the NHC-unit of the bridging ligand was established unambiguously by means of X-ray structural analysis of a heterodinuclear ruthenium-silver complex. The photophysical data indicate ultrafast intersystem crossing into an emissive and a non-emissive triplet excited state after excitation of the ruthenium centre. Exceptionally high luminescence quantum yields of up to 39% and long lifetimes of up to 2 μs are some of the triplet excited state characteristics. Preliminary studies into the visible light driven photocatalytic hydrogen formation show no induction phase and constant turnover frequencies that are independent on the concentration of the photocatalyst. In conclusion this supports the notion of a stable assembly under photocatalytic conditions.

We report on the synthesis of hybrid molecules consisting of a porphyrin or corrole chromophore axially coordinated to a [CoIII(dmgH)2(Cl)]±0 (dmg = dimethylglyoxime) unit via a pyridine group as potential hydrogen forming entities in H2O/THF medium. Photophysical, electrochemical, and pulse radiolysis studies on the hybrids and/or their separate components show that selective excitation of the porphyrin or corrole chromophore in its first singlet excited state leads to fast charge separation due to chromophore to cobalt electron transfer. However, this charge separation is followed by even faster charge recombination thereby preventing the accumulation of a reduced cobalt species which would lead to hydrogen production. It is important, nevertheless, that addition of a sacrificial electron donor slows the charge recombination down. In light of the latter it comes as hardly surprising that the photocatalysis experiments in the presence of a sacrificial electron donor (i.e., triethylamine) show modest rates of hydrogen production.

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A combined experimental and theoretical study shows that the photooxidative activity of two isostructural metal oxide clusters depends on their internal templates. To this end, two halide-templated bismuth vanadium oxide clusters [X(Bi(dmso)3 )2 V12 O33 ](-) (X=Cl(-) , Br(-) ) are reported and fully characterized. The two clusters show similar absorption features and illustrate that bismuth incorporation results in increased visible-light absorption. Significantly higher photooxidative activity is observed for the bromide-templated cluster compared with the chloride-templated one. Detailed photophysical assays and complementary DFT calculations suggest that the more efficient triplet excited state formation in the Br(-) -containing cluster is the decisive step in the photocatalysis and is due to the heavy-atom effect of the bromide. This concept can therefore open new pathways towards the optimization of photocatalytic activity in metal oxide clusters.