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 watersoluble 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 (T 1 ) state of Mexoryl SX using a time-resolved EPR technique because this technique is powerful for the study of short-lived and non-phosphorescent T 1 molecules. The strong time-resolved EPR signals were observed. This fact shows that a considerable proportion of the lowest excited singlet (S 1 ) state molecules undergoes intersystem crossing (ISC) to the T 1 state and the deactivation process of the T 1 state is mainly radiationless. The observed zero-filed splitting parameters, T 1 lifetime and S 1 → T 1 ISC anisotropy suggest that the T 1 state can be regarded as a 3 nπ * -3 ππ * mixing state in character and the two unpaired electrons in the T 1 state do not localize on (4methylbenzylidene)camphor, a closely related component. Although the shorter T 1 lifetime (47 ns) prevents T 1 state quenching by ground-state oxygen, the 3 nπ * character may contribute something to the low Φ Δ value.