Excited states of menthyl anthranilate: a UV-A absorber

Abstract

The photophysical properties of menthyl anthranilate (MA), a UV-A absorber, have been studied through measurements of UV absorption, fluorescence, triplet–triplet absorption and time-resolved thermal lens in ethanol at room temperature and/or 77 K. The phosphorescence and time-resolved electron paramagnetic resonance spectra have also been observed in ethanol at 77 K. The energy levels of the lowest excited singlet (S₁) state and triplet (T₁) state were determined. The quantum yields of fluorescence, phosphorescence and S₁ → T₁ intersystem crossing (ISC) were also determined. From the data on the lifetime and quantum yield, the rate constants of the radiative decay, internal conversion and ISC of the excited states of MA were determined. The exceptionally high ISC quantum yield of MA shows that the deactivation processes of the S₁ state of MA are different from those of most UV absorbers such as cinnamate, salicylate, 2-hydroxybenzophenone, benzotriazole and dibenzoylmethane derivatives, where the internal conversion rates of the S₁ states are much faster than the ISC rates. The observed T₁ lifetime and zero-field splitting parameters suggest that the T₁ state of MA possesses almost pure ³ππ* character.