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

N.D.N Rodrigues,N.C Cole-Filipiak,M.D Horbury,M Staniforth,T.N.V Karsili,Y Peperstraete,V.G Stavros

doi:10.1016/j.jphotochem.2017.11.042

Abstract

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.

Highlights

The damaging effects of excessive exposure to ultraviolet (UV) radiation to living organisms are well documented in the literature [1,2,3]
Assuming dislocation of the intramolecularly bound hydrogen atom is as relevant in solution as we proposed in the discussion of our results in vacuum, and considering that hydrogen motion facilitates intersystem crossing (ISC), it would not be surprising that a polar protic solvent would affect the observed behaviour of methyl anthranilate (MA) and menthyl anthranilate (MenA)
The similarity of the results for MA and MenA in vacuum are in accordance with observations for other sunscreen molecules, which have similar photodynamics to those of their precursors [29]

Summary

Introduction

The damaging effects of excessive exposure to ultraviolet (UV) radiation to living organisms are well documented in the literature [1,2,3] Such damages include erythema [4], a result of excessive skin irradiation with UV-A (400–315 nm) and/or UV-B (315– 280 nm) radiation – both of which are directly absorbed by several chromophores in human skin (e.g. melanins, acids and kynurenines). While the human skin has its own natural photoprotection mechanisms (provided by melanin pigments), these are often insufficient for continued and excessive sun exposure. Photoprotection products, such as sunscreen lotions, are required in order to provide enhanced protection against UV-induced damage. There is, an obvious urgency for more effective sunscreens

Methods

Discussion

Conclusion