Laser flash photolysis studies of the UVA sunscreen Mexoryl® SX

The results of a nanosecond laser hash photolysis investigation of the WA sunscreen Mexoryl* SX in various solvent environments and within a commercial sunscreen formulation are reported, To the best of our knowledge this is the first laser hash photolysis study of a commercial suncare formulation, In each of these environments kinetic UV-visible absorption measurements following nanosecond 355 nn laser excitation reveals a short-lived species with a solvent-dependent absorption maximum around 470-500 nm and a solvent-dependent lifetime of similar to 50-120 ns, This transient absorption is attributed to the triplet state of Mexoryl(R) SX on the basis that it is quenched by molecular oxygen leading to the formation of singlet oxygen in acetonitrile, The singlet oxygen quantum yield (Phi(Delta)), determined by comparative time-resolved near-infrared luminescence measurements and extrapolated to the limit of complete triplet state quenching, is estimated as 0.09 +/- 0.03 in acetonitrile. In aqueous solution the shorter triplet state lifetime combined with lower ambient oxygen concentrations precludes significant triplet state quenching, For the commercial sunscreen formulation there was no observable difference in the measured triplet lifetime between samples exposed to oxygen or argon, suggesting that the singlet oxygen quantum yield in such environments is likely to be orders of magnitude lower than that measured in acetonitrile.