Efficient red light–driven singlet oxygen photocatalysis with an osmium-based Coulombic dyad

Abstract

Photoactive osmium complexes are widely used sensitizers for the generation of singlet oxygen because they can be excited directly into their triplet states with low-energy red light. However, their short-lived excited states reduce quenching efficiencies and reaction quantum yields significantly. To elongate the excited state lifetime, osmium complexes have been linked to organic chromophores to form molecular dyads. This approach, although effective, is time- and resource-consuming, hampering larger-scale applications. Here, we demonstrate a straightforward approach by directly mixing a readily available cationic osmium complex and an anionic perylene derivative in solution. Strong Coulombic interactions facilitate rapid energy transfer (∼100 ps) from the excited osmium complex to the perylene derivative, mimicking a dyad-like system. Detailed spectroscopic investigations revealed an increased singlet oxygen formation rate by over one order of magnitude at sub-millimolar perylene concentrations, attributed to i) the three orders of magnitude longer lifetime of the perylene triplet state produced via intra-ion-pair energy transfer and ii) an inherently high singlet oxygen quantum yield of that key species. The novel catalyst system enables highly productive photooxygenations in water and in a MeOH/H2O 10:1 mixture, highlighting the broad applicability and versatility of the Coulombic dyad approach for photocatalytic synthesis and wastewater treatment.