Substituent position effects on sunscreen photodynamics: A closer look at methyl anthranilate

Towards the development of a bottom-up rationale for sunscreen design, the effects of substituent position on the ultrafast photodynamics of the sunscreen precursor methyl anthranilate (MA, an ortho compound) were evaluated by studying para- and meta-MA in vacuum. Time-resolved ion yield (TR-IY) measurements reveal a long-lived S-1 excited state (>> 1.2 ns) for para-MA, proposed to be the result of a weakly fluorescent, bound excited state. In the case of meta-MA, TR-IY transients reveal a much faster (similar to 2 ns) excited state relaxation, possibly due to multiple low-lying S-1/S-0 conical intersections of prefulvenic character. While meta-MA may not be an ideal sunscreen ingredient due to a low ultraviolet absorbance, its comparatively efficient relaxation mechanism may constitute an alternative to common sunscreen relaxation pathways. Thus, our results should prompt further studies of prefulvenic relaxation pathways in potential sunscreen agents.