Singlet Oxygen Quantum Yields: Comparing Chemical Probe and Time-Resolved Phosphorescence.

Area of Science:

• Environmental Chemistry
• Photochemistry
• Analytical Chemistry

Background:

• Singlet oxygen (¹O₂) is a key reactive species in aquatic photochemistry, influencing pollutant transformation.
• The furfuryl alcohol (FFA) probe is commonly used to quantify ¹O₂ generation in aquatic environments.
• Direct measurement of ¹O₂ via its phosphorescence emission is an emerging, interest-generating alternative.

Purpose of the Study:

• To directly compare singlet oxygen (¹O₂) quantum yields (Φ_Δ) obtained using the FFA chemical probe and direct ¹O₂ phosphorescence methods.
• To evaluate the performance of these methods across diverse samples, including small molecule sensitizers and various forms of dissolved organic matter (DOM) and pyrogenic DOM (PyDOM).
• To investigate potential interference from FFA-mediated triplet excited state quenching processes.

Main Methods:

• Quantification of ¹O₂ quantum yields (Φ_Δ) using both the furfuryl alcohol (FFA) chemical probe and direct ¹O₂ phosphorescence spectroscopy.
• Application of methods to a range of samples: small molecule sensitizers, dissolved organic matter (DOM) isolates, and pyrogenic DOM (PyDOM).
• Laser spectroscopy was employed to explore FFA interactions with sensitizer triplet excited states.

Main Results:

• Significant discrepancies in measured ¹O₂ quantum yields (Φ_Δ) were observed between the FFA and phosphorescence methods for several small molecule sensitizers and DOM/PyDOM samples.
• The direct ¹O₂ phosphorescence method reported 1.4 to 3.8 times higher Φ_Δ values for DOM/PyDOM compared to the FFA method.
• FFA demonstrated effective quenching and/or reaction with triplet excited states of small molecule sensitizers, but minimal quenching was observed for DOM/PyDOM triplet states.

Conclusions:

• The furfuryl alcohol (FFA) probe may not accurately represent singlet oxygen (¹O₂) generation efficiencies in all aquatic systems due to potential interactions with sensitizer triplet states, particularly for dissolved organic matter (DOM).
• The direct ¹O₂ phosphorescence method appears to provide more reliable quantum yield measurements for DOM/PyDOM.
• This study highlights the importance of considering probe-sensitizer interactions in ¹O₂ quantification and supports the advancement of phosphorescence-based measurement techniques.