Molecular systems involving a phosphonated Ru trisbipyridyl photosensitizer (Ru, pictured), a Ru bipyridyl carbonyl catalyst (RuC, pictured) and a covalently linked dyad (RuRu, pictured) were synthesized. RuRu was immobilized on Al2O3 where the formic acid (HCOOH) activity (turnover number (TONHCOOH) of 78) was significantly higher than the co-adsorbed monomers RuC and Ru under the same conditions (TONHCOOH) of 15). On the contrary, RuRu in solution (TONHCOOH of 337) performed better than the surface-bound equivalent, which is attributed to the faster diffusion of the free molecule leading to increased collisions with the required sacrificial reductant. Additionally, the effect of adsorption concentration of RuRu on Al2O3 was investigated and, perhaps counterintuitively, the performance decreased with increased loading. It was discovered that this is primarily the result of a loss of stability from formation of an inactive Ru(0) polymer via coupling of bipyridyl ligands — a consequence which is significantly amplified by the decreased separation of molecular units as concentration increases.
To suppress inactive polymer formation while taking advantage of enhanced light absorption, co-adsorption of RuRu and Ru on Al2O3 proved to be an effective strategy. Intermolecular transfer of additional photoexcited electrons from Ru augmented the internal performance of RuRu (TONHCOOH of 520) while spatially separating the RuRu units. Overall, this study shines light on the complex considerations underpinning the design of durable and active hybrid photocatalytic systems.
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