Photoelectrochemical alcohol oxidation by mixed-linker metal-organic frameworks

Metal-organic frameworks (MOFs) provide a suitable platform for stable and efficient heterogeneous photoelectrochemical oxidation catalysis due to their highly ordered structure, large surface area, and synthetic tunability. Herein, a mixed-linker MOF comprising of a photosensitizer [Ru(dcbpy)(bpy)₂]²⁺ (bpy = 2,2'-bipyridine, dcbpy = 5,5'-dicarboxy-2,2'-bipyridine) and catalyst [Ru(tpy)(dcbpy)Cl]⁺ (tpy = 2,2':6',2''-terpyridine) that were incorporated into the UiO-67 framework and grown as thin films on a TiO₂-coated, fluorine-doped tin oxide (FTO) electrode (RuB-RuTB-UiO-67/TiO₂/FTO). When used as an electrode for the photoelectrochemical oxidation of benzyl alcohol, the mixed-linker MOF film showed a faradaic efficiency of 34%, corresponding to a 3-fold increase in efficiency relative to the RuB-UiO-67/TiO₂/FTO control. This increase in catalytic efficiency is ascribed to the activation of RuTB moieties via oxidation by photogenerated Ru^IIIB. Transient absorption spectroscopy revealed the delayed appearance of Ru^IIITB* or Ru^IIITB formation, occurring with a lifetime of 21 ns, due to energy and/or electron transfer. The recovery kinetics of the charge separated state was increased (283 μs) in comparison to single-component control experiments (105 μs for RuB-UiO-67/TiO₂/FTO and 7 μs for RuTB-UiO-67/TiO₂/FTO) indicating a cooperative effect that could be exploited in chromophore/catalyst MOF motifs.