Short-lived and Nonphosphorescent Triplet state of Mexoryl SX, a UV-A Sunscreen

Abstract

Mexoryl SX (terephthalylidene-3,3′-dicamphor-10,10′-disulfonic acid, Ecamsule) is a water-soluble UV-A absorber. The near-IR phosphorescence spectrum of singlet oxygen generated by photosensitization with Mexoryl SX was not observed in air-saturated water. On the other hand, the time-resolved near-IR phosphorescence spectrum was observed in oxygen-saturated phosphate buffer (pH 7.4). The quantum yield of the singlet oxygen generation (Φ_Δ) was determined to be 0.0021 ± 0.0005. The ability of Mexoryl SX as a photosensitizer is quite low. The question arises as to the quite low Φ_Δ value. No phosphorescence was detectable from Mexoryl SX in ethanol at 77 K. We elucidated the nature of the lowest excited triplet (T₁) state of Mexoryl SX using a time-resolved EPR technique, because this technique is powerful for the study of short-lived and nonphosphorescent T₁ molecules. The strong time-resolved EPR signals were observed. This fact shows that a considerable proportion of the lowest excited singlet (S₁) state molecules undergoes intersystem crossing (ISC) to the T₁ state and the deactivation process of the T₁ state is mainly radiationless. The observed zero-filed splitting parameters, T₁ lifetime, and S₁ → T₁ ISC anisotropy suggest that the T₁ state can be regarded as a ³nπ^*–³ππ^* mixing state in character and the two unpaired electrons in the T₁ state do not localize on (4-methylbenzylidene)camphor, a closely related component. Although the shorter T₁ lifetime (47 ns) prevents T₁ state quenching by ground-state oxygen, the ³nπ^* character may contribute something to the low Φ_Δ value.