Cyril Bachmann

Clean Donor Oxidation Enhances the H2 Evolution Activity of a Carbon Quantum Dot–Molecular Catalyst Photosystem

Carbon quantum dots (CQDs) are new-generation light absorbers for photocatalytic H2 evolution in aqueous solution, but the performance of CQD-molecular catalyst systems is currently limited by the decomposition of the molecular component. Clean oxidation of the electron donor by donor recycling prevents the formation of destructive radical species and non-innocent oxidation products. This approach allowed a CQD-molecular nickel bis(diphosphine) photocatalyst system to reach a benchmark lifetime of more than 5 days and a record turnover number of 1094±61 molH2  (molNi )(-1) for a defined synthetic molecular nickel catalyst in purely aqueous solution under AM1.5G solar irradiation.

Quinones as Reversible Electron Relays in Artificial Photosynthesis

We explore the potential of various hydroquinone/quinone redox couples as electron relays in a homogenous water reduction system between a Re-based photosensitizer and a sacrificial electron donor [tris-(2-carboxyethyl)-phosphine, TCEP]. By using transient IR spectroscopy, flash photolysis as well as stopped-flow techniques covering timescales from picoseconds to 100 ms, we determine quenching rates and cage escape yields, the kinetics of the follow-up chemistry of the semiquinone, the recombination rates, as well as the re-reduction rates by TCEP. The overall quantum yield of hydrogen production is low, and we show that the limiting factors are the small cage escape yields and, more importantly, the slow regeneration rate by TCEP in comparison to the undesired charge recombination with the reduced water reduction catalyst.

Light-Induced H2 Evolution with a Macrocyclic Cobalt Diketo-Pyrphyrin as a Proton-Reducing Catalyst

Cobalt complexes are well-known catalysts for photocatalytic proton reduction in water. Macrocyclic tetrapyridyl ligands (pyrphyrins) and their CoII complexes emerged in this context as a highly efficient class of H2 evolution catalysts. On the basis of this framework, a new macrocyclic CoII complex consisting of two keto-bridged bipyridyl units (Co diketo-pyrphyrin) is presented. The complex is synthesized along a convenient route, is well soluble in water, and shows high activity as a water reduction catalyst (WRC). In an aqueous system containing [Ru(bpy)3]Cl2 as a photosensitizer and NaAscO as a sacrificial electron donor, turnover numbers (TONs) of 2500 H2/Co were achieved. Catalysis is terminated by a limited electron supply and decomposition of the photosensitizer but not of the WRC, highlighting the distinct stability of Co diketo-pyrphyrin.