Optical and electron paramagnetic resonance studies of the excited triplet states of UV-B absorbers: 2-ethylhexyl salicylate and homomenthyl salicylate

Abstract

The energy levels and lifetimes of the lowest excited triplet (T₁) states of UV-B absorbers, 2-ethylhexyl salicylate (EHS) and homomenthyl salicylate (HMS), and their deprotonated anions (EHS⁻ and HMS⁻) were determined through measurements of phosphorescence and electron paramagnetic resonance (EPR) spectra in rigid solutions at 77 K. The observed T₁ energies of EHS and HMS are higher than those of butylmethoxydibenzoylmethane, the most widely used UV-A absorber, and octyl methoxycinnamate, the most widely used UV-B absorber. The T₁ states of EHS, HMS, EHS⁻ and HMS⁻ were assigned to almost pure ³ππ* state from the observed T₁ lifetimes and zero-field splitting parameters. EHS and HMS with an intramolecular hydrogen bond show a photoinduced phosphorescence enhancement in ethanol at 77 K. The EPR signals of the T₁ states of EHS and HMS also increase in intensity with UV-irradiation time (photoinduced EPR enhancement). The T₁ lifetimes of EHS and HMS at room temperature were determined through triplet–triplet absorption measurements in ethanol. The quantum yields of singlet oxygen production by EHS and HMS were determined by using time-resolved near-IR phosphorescence.