Photophysics of the sunscreen ingredient menthyl anthranilate and its precursor methyl anthranilate: A bottom-up approach to photoprotection

The ultrafast excited state dynamics of the sunscreen ingredient menthyl anthranilate (MenA) and its precursor methyl anthranilate (MA) were studied in vacuum (using time-resolved ion yield spectroscopy) and in solution (using transient electronic absorption spectroscopy). MenA and MA both show long-lived dynamics, with the observation of a kinetic isotope effect suggesting that hydrogen motion acts as the rate determining process in the overall decay. Complementary computational studies exploring the intuitive decay pathways of MA revealed a bound S1 state with a shallow ‘up-hill’ gradient with respect to proton transfer. From these results, it is suggested that photoexcited population is trapped in this excited state from which luminescence occurs as a prominent decay pathway. This work has shown that the photophysics of MA and MenA – and hence their photoprotection capabilities – are not drastically influenced by aliphatic structure or solvent environment alone. A bottom-up approach, such as the one described herein, is essential to understand the combination of factors that afford optimum photoprotection and to develop a new generation of tailor made, efficacious sunscreens.