Electron accumulation role of bridging ligands in photocatalytic donor-acceptor assemblies

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Abstract
The possible mechanistic roles of bridging ligands in chromophore-catalyst assemblies are examined to assess if the bridge can play an active role in light-driven electron storage towards multi-redox photocatalysis, as opposed to its traditional role facilitating electron transfer (ET) from chromophore-to-catalyst. In a dyad linking a Ru polypyridyl chromophore and Rh proton reduction catalyst, [Ru-Rh]3+, the central azaacene motif in the tetrapyridophenazine (tpphz) bridging ligand is shown here to accumulate electrons, rather than supporting ET to the catalyst. Ultrafast time-resolved X-ray absorption spectroscopies probing the chromophore (Ru L3-edge) and catalyst (Rh L3- and Cl K-edges) atoms demonstrate that photoexcitation induces ET away from the chromophore, but not to the catalyst motif. Ab initio calculations show that the spectral features are consistent with electron accumulation on tpphz. An alternative photocatalytic mechanism is identified, in which tpphz stores electrons and the multi-electron Rh catalyst becomes activated by a disproportionation mechanism. The results indicate a broader role for bridging ligands in photocatalytic assemblies to be considered in their future design.
Funding Information
  • National Science Foundation Graduate Research Fellowship Program (DGE-2234667)