Photosensitizing Diiron Hydrogenase Mimics: Excited State Dynamics

Catalytically evolving hydrogen from a system that photosensitizes diiron hydrogenase could be a carbon neutral method for converting and storing energy. Two methods for producing a photocatalytic system are investigated; the covalently linking of photosensitizer and catalyst moieties to enhance electron transfer, or keeping them separate and relying on collisions to transfer the energy necessary to drive the reaction.
A new covalently linked dyad has been synthesized that has a Charge Separated State (CSS) as its excited state. The charge separation is between the platinum (II) photosensitizer (PS) and the diiron hydrogenase mimic catalyst moieties. A CSS state has been observed, quenching the emission of the PS moiety. The CSS has a lifetime of 247 ± 25 ps determined by picosecond time-resolved infrared spectroscopy. This is a similar lifetime to previously studied PS-hydrogenase dyads and is unlikely to be long enough to effectively initiate a hydrogen evolution reaction.
New photosensitizer complexes have been developed to drive photo-catalysed hydrogen evolution. These complexes feature a platinum (II) centre ligated by a phenyl-bipyridine cyclometalating ligand and a substituted phenyl acetylide ligand. Their ground and excited states have been probed using photophysical techniques, revealing emissive states with lifetimes of 63 – 703 ns and quantum yields of 0.002 – 0.27. This broad range of lifetimes and quantum yields was further investigated using picosecond time-resolved infrared spectroscopy to reveal that there is intramolecular quenching of emissive states by an equilibrium with a dark charge transfer excited state.
Investigation of how these new PS complexes in their excited state interact with [FeFe] complexes was undertaken by utilizing Stern Volmer quenching kinetics. These PS complexes were found to be quenched at close to the diffusion limited rate (1.4 – 2.6 x109 mol-1 dm3 s-1), indicating that there is energy transfer between the PS and catalyst complexes. A false Marcus inverted region was observed for these complexes and further investigation revealed new information on the nature of the excited state equilibria present in the PS complexes.