Quantum yield standards for the photosensitized production of singlet oxygen in water

The photosensitized production of singlet molecular oxygen, O₂(a¹Δ_g), is a ubiquitous phenomenon and a common tool in a range of scientific endeavors. Quantifying photosensitized yields of O₂(a¹Δ_g) is key to understanding and ultimately controlling O₂(a¹Δ_g)-mediated processes. Accurate photosensitized O₂(a¹Δ_g) quantum yields, ϕ_Δ, also facilitate a better understanding of the fundamental photophysics involved in the interaction between the excited state of a photosensitizing molecule M and ground state oxygen, O₂(X³Σ_g^-), which, in itself, has far-reaching ramifications. The most accurate way to quantify ϕ_Δ for a light-absorbing molecule M is to compare the amount of O₂(a¹Δ_g) produced by M to that produced by a calibrated and standardized photosensitizer. Despite the importance of photosensitized O₂(a¹Δ_g) production in aqueous systems, commonly available water-soluble sensitizer standards are few, and many of the molecules used as such standards often have limitations that adversely affect their accuracy. In this study, we examined the photophysics of sensitized O₂(a¹Δ_g) production for nine water-soluble molecules that, combined, cover an excitation range across the near UV and visible spectrum (300-700 nm). This included determining cross-referenced O₂(a¹Δ_g) quantum yields in H₂O and D₂O, for which pronounced H/D isotope effects are observed in values of ϕ_Δ. In this way, we provide calibrated standards (a) for future experiments, and (b) that facilitate revision of published ϕ_Δ values and, thereby, help resolve misleading discrepancies.