Effects of substituent position on aminobenzoate relaxation pathways in solution

The negative effects of ultraviolet radiation (UVR) on human skin have led to the widespread use of sunscreens, <i>i.e.</i> skincare products containing UV filters to absorb, reflect or otherwise block UVR. The mechanisms by which UV filters dissipate energy following photoexcitation, <i>i.e.</i> their photodynamics, can crucially determine a molecule's performance as a sunscreen UV filter. In this work, we evaluate the effects of substituent position on the in-solution relaxation pathways of two derivates of methyl anthranilate (an <i>ortho</i> compound that is a precursor to the UV filter meradimate), <i>meta</i>- and <i>para</i>-methyl anthranilate, <i>m</i>-MA and <i>p</i>-MA, respectively. The photodynamics of <i>m</i>-MA were found to be sensitive to solvent polarity: its emission spectra show larger Stokes shifts with increasing polarity, and both the fluorescence quantum yield and lifetimes for <i>m</i>-MA increase in polar solvents. While the Stokes shifts for <i>p</i>-MA are much milder and more independent of solvent environment than those of <i>m</i>-MA, we find its fluorescence quantum yields to be sensitive not only to solvent polarity but to the hydrogen bonding character of the solvent. In both cases (<i>m</i>- and <i>p</i>-MA) we have found common computational methods to be insufficient to appropriately model the observed spectroscopic data, likely due to an inability to account for explicit solvent interactions, a known challenge in computational chemistry. Therefore, apart from providing insight into the photodynamics of anthranilate derivatives, the work presented here also provides a case study that may be of use to theoretical chemists looking to improve and develop explicit solvent computational methods.