The lowest excited triplet state of potassium 4-methoxysalicylate (4-MSK), a skin-whitening active ingredient, and its effects on riboflavin-photosensitized singlet oxygen generation

Potassium 4-methoxysalicylate (4-MSK) is a skin-whitening active ingredient used in skincare products. The lowest excited triplet (T₁) states of 4-MSK and its related molecules, sodium 4-aminosalicylate (4-ASNa) and 4-(trifluoromethyl)salicylic acid (4-FMSA), have been studied through measurements of phosphorescence and electron paramagnetic resonance in ethanol at 77 K. The energy levels, lifetimes and zero-field splitting (ZFS) parameters of the T₁ states of 4-MSK, 4-ASNa and 4-FMSA were determined. The observed ZFS parameters and T₁ lifetimes suggest that the T₁ states of these molecules are assigned to ³ππ^⁎ states. Riboflavin (RF) is a water-soluble vitamin B₂ and an efficient singlet oxygen photosensitizer. Time profiles of the near-IR phosphorescence of singlet oxygen generated by RF-photosensitization have been measured in the absence and presence of 4-MSK, 4-ASNa and 4-FMSA. These molecules suppress the RF-photosensitized singlet oxygen generation. The fluorescence and transient absorption measurements suggest that the observed suppression is due mainly to the quenching of the T₁ state of RF by 4-MSK, 4-ASNa and 4-FMSA. The T₁ energy levels of the quenching molecules are higher than that of RF. The triplet–triplet energy transfer from RF to the quenching molecules is energetically unlikely. The most probable quenching mechanism is the photoinduced electron transfer from the quenching molecules to the T₁ state of RF.