Investigating the photodegradation of organic sunscreens in the gas phase and in solution: towards natural alternatives as means of photoprotection

UV filters found in commercial sunscreens are an important group of molecules which play a key role in human health by reducing cases of melanoma. The development of better performing sunscreens is an active area of research, particularly as some sunscreens are photounstable. It is therefore important to develop new methods to measure the photostability of sunscreens and understand their photodegradation products better.
The first gas-phase study of the common UVB filter, octyl-methoxycinnamate (OMC), using laser-interfaced mass spectrometry (LIMS) was performed to probe the gaseous absorption profile and associated UV breakdown products. OMC was studied in its protonated form and displayed an absorption spectrum with increased absorption in the UVA and UVC regions, compared to the neutral species which absorbs primarily in the UVB region. Photofragmentation of protonated OMC across the UV was found to occur primarily with production of the thermal fragments that are observed upon heating of the electronic ground state. This indicates that protonated OMC decays statistically, in line with ultrafast excited state decay. A minor, photochemical photofragment was also observed, indicating that a triplet state is also being accessed following UV excitation.
The solution-phase photoproducts of OMC and 2-ethylhexyl 4-(dimethylamino) benzoate (OD-PABA) were explored using novel UVA and UVB LED photolysis cells. For OMC, the main photofragments produced match the ones identified via LIMS, indicating that it is a powerful tool for photoproduct detection. The photoproducts observed match those previously reported for the two molecules, hence validating our new methodology that has a lower energy requirement than previous ones. However, a potential, general issue was identified in the identification of photoproducts using mass spectrometry, since the collision induced dissociation experiments performed as part of this work suggest that the identified “photoproducts” are in fact thermal fragments produced during electrospray.
The thermal fragmentation and photodegradation pattern in solution for vanillin, a lignin monomer, was also explored. The solution-phase UV absorption profile of OMC-vanillin and avobenzone-vanillin mixtures was also investigated as a step towards understanding the synergistic effect between lignin and chemical UV filters. An enhancement was observed in the photostability of both organic sunscreens in the presence of vanillin.