Laser flash photolysis studies of the UVA sunscreen Mexoryl (R) SX

The results of a nanosecond laser hash photolysis investigation of the WA s unscreen Mexoryl* SX in various solvent environments and within a commercia l 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 fo llowing nanosecond 355 nn laser excitation reveals a short-lived species wi th a solvent-dependent absorption maximum around 470-500 nm and a solvent-d ependent lifetime of similar to 50-120 ns, This transient absorption is att ributed to the triplet state of Mexoryl(R) SX on the basis that it is quenc hed by molecular oxygen leading to the formation of singlet oxygen in aceto nitrile, The singlet oxygen quantum yield (Phi(Delta)), determined by compa rative time-resolved near-infrared luminescence measurements and extrapolat ed to the limit of complete triplet state quenching, is estimated as 0.09 /- 0.03 in acetonitrile. In aqueous solution the shorter triplet state life time combined with lower ambient oxygen concentrations precludes significan t triplet state quenching, For the commercial sunscreen formulation there w as no observable difference in the measured triplet lifetime between sample s exposed to oxygen or argon, suggesting that the singlet oxygen quantum yi eld in such environments is likely to be orders of magnitude lower than tha t measured in acetonitrile.