Hexyl diethylaminohydroxybenzoylbenzoate (DHHB, Uvinul A Plus) is a photostable UV-A absorber. The photophysical properties of DHHB have been studied by obtaining the transient absorption, total emission, phosphorescence and electron paramagnetic resonance spectra. DHHB exhibits an intense phosphorescence in a hydrogen-bonding solvent (e.g., ethanol) at 77 K, whereas it is weakly phosphorescent in a non-hydrogen-bonding solvent (e.g., 3-methylpentane). The triplet-triplet absorption and EPR spectra for the lowest excited triplet state of DHHB were observed in ethanol, while they were not observed in 3-methylpentane. These results are explained by the proposal that in the benzophenone derivatives possessing an intramolecular hydrogen bond, intramolecular proton transfer is an efficient mechanism of the very fast radiationless decay from the excited singlet state. The energy level of the lowest excited triplet state of DHHB is higher than those of the most widely used UV-B absorbers, octyl methoxycinnamate (OMC) and octocrylene (OCR). DHHB may act as a triplet energy donor for OMC and OCR in the mixtures of UV-A and UV-B absorbers. The bimolecular rate constant for the quenching of singlet oxygen by DHHB was determined by measuring the near-IR phosphorescence of singlet oxygen. The photophysical properties of diethylaminohydroxybenzoylbenzoic acid (DHBA) have been studied for comparison. It is a closely related building block to assist in interpreting the observed data.

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.