Methodology for determining singlet oxygen quantum yield in dimethyl sulfoxide

Background: Singlet oxygen (¹O₂) is a highly reactive oxygen species, playing an important role in a wide range of fields. The singlet oxygen quantum yield (Ф_Δ) is a fundamental parameter for evaluating the efficiency of ¹O₂ generation by dye molecules. Various methods have been developed for Ф_Δ measurement by exploiting the photoluminescence and chemical reactivity of ¹O₂. We recently proposed a method for determining Ф_Δ based on the reaction between Dimethyl Sulfoxide (DMSO) and ¹O₂. However, due to the limited solubility of the reaction product, we supposed that the DMSO–¹O₂ reaction would exhibit a termination point, which may affect the applicability of this method. Results: In this work, we found that upon ¹O₂ generation, the absorption spectrum of DMSO initially changes and then reaches a plateau, which is attributed to the occurrence of a reaction termination in DMSO–¹O₂. Moreover, the occurrence time of the saturation point is inversely proportional to the excitation power density. When using the DMSO absorption change method to determine Ф_Δ, the measurement must be conducted before this saturation point. Furthermore, due to the presence of the termination in DMSO–¹O₂ reaction, the chemical probe method can still be applied for the determination of Ф_Δ in DMSO under certain conditions. The relative degradation rate (k) of the probe exhibits an inflection point, with the slope k₂ after this point being greater than k₁ before it. This is because, prior to the inflection point, ¹O₂ generated by dye molecules react with both DMSO and chemical probe, and thus the degradation of probe reflects only a fraction of the total ¹O₂. Hence, for the chemical probe method, measurements should be taken after inflection point. Significance: This work provides a robust methodology for accurate determination of Ф_Δ in DMSO, highlighting the necessity of selecting appropriate time windows tailored to each measurement method.