Lifetime of Triplet Photosensitizers in Aerosol Using Time-Resolved Photoelectric Activity

Abstract

Time-resolved photoelectric activity of aerosol (TPEAA) experiments measure the kinetics of photochemically initiated reactions near the surface of aerosol particles. In the demonstration presented here, TPEAA experiments monitor the evolution of excited triplet states, which are key intermediates in the photochemical formation of secondary organic aerosol (SOA). One example is the decay of the triplet excited state of imidazole-2-carboxaldehyde (IC) in aqueous NaCl aerosol particles. The decay of the triplet IC shows two distinct timescales, indicative of two populations of IC in the particles: aqueous and pure IC. The lifetime in the pure-IC phase is less than 20 ns, and the lifetime associated with aqueous IC is limited by reaction with chloride ions (k = 5.6 × 105 M–1 s–1). The results demonstrate how phase separation in aerosol particles can control reaction pathways and kinetics. In another example, TPEAA monitors the decay of the triplet anthraquinone-2-sulfonate (AQ2S) anion in aqueous NaCl aerosol. The AQ2S triplet reacts with water (k′ = 8.7 × 106 s–1) to produce adduct species. In this case, tuning the wavelength of the photoemission laser permits the preferential observation of the triplet or a combination of the AQ2S triplet and the water adduct. These results bridge a gap between aerosol phase measurements, which are normally steady-state and bulk-phase transient absorption measurements, which typically probe homogeneous phases.