Photoexcited triplet states of UV-B absorbers: ethylhexyl triazone and diethylhexylbutamido triazoneElectronic supplementary information (ESI) available: Stern-Volmer plots of the phosphorescence lifetime of singlet oxygen in acetonitrile at room temperature. The emission, phosphorescence and EPR spectra of MePABA and PAMeB in ethanol at 77 K. See DOI: 10.1039/c4pp00373j

The excited states of UV-B absorbers, ethylhexyl triazone (EHT) and diethylhexylbutamido triazone (DBT), have been studied through measurements of UV absorption, fluorescence, phosphorescence, triplet-triplet absorption and electron paramagnetic resonance spectra in ethanol. The energy levels of the lowest excited singlet (S 1 ) and triplet (T 1 ) states and quantum yields of fluorescence and phosphorescence of EHT and DBT were determined. In ethanol at 77 K, the deactivation process of EHT and DBT is predominantly fluorescence, however, a significant portion of the S 1 molecules undergoes intersystem crossing to the T 1 state. The observed phosphorescence spectra, T 1 lifetimes and zero-field splitting parameters suggest that the T 1 state of EHT can be assigned to a locally excited 3 ππ* state within p -( N -methylamino)benzoic acid, while the T 1 state of DBT can be assigned to a locally excited 3 ππ* state within p -( N -methylamino)benzoic acid or p -amino- N -methylbenzamide. The quantum yields of singlet oxygen generation by EHT and DBT were determined by time-resolved near-IR phosphorescence measurements in ethanol at room temperature. EHT and DBT did not exhibit significantly antioxidative properties by quenching singlet oxygen, in contrast to the study by Lhiaubet-Vallet et al. The lowest excited triplet state of a UV-B absorber with extremely high molecular extinction, ethylhexyl triazone, can be assigned to a locally excited 3 ππ* state within a chromophore, p -( N -methylamino)benzoic acid.