Quantum Yield Standards for the Photosensitized Production of Singlet Oxygen in Water

Abstract

The photosensitized production of singlet molecular oxygen, O2(a1Δg), is a ubiquitous phenomenon and a common tool in a range of scientific endeavors. Quantifying photosensitized yields of O2(a1Δg) is key to understanding and ultimately controlling O2(a1Δg)-mediated processes. Accurate photosensitized O2(a1Δg) quantum yields, ϕΔ, 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(X3Σg-), which, in itself, has far-reaching ramifications. The most accurate way to quantify ϕΔ for a light-absorbing molecule M is to compare the amount of O2(a1Δg) produced by M to that produced by a calibrated and standardized photosensitizer. Despite the importance of photosensitized O2(a1Δg) 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(a1Δg) 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(a1Δg) quantum yields in H2O and D2O, for which pronounced H/D isotope effects are observed in values of ϕΔ. In this way, we provide calibrated standards (a) for future experiments, and (b) that facilitate revision of published ϕΔ values and, thereby, help resolve misleading discrepancies.