Octyl methoxycinnamate (OMC) is a common UVA and UVB filter molecule that is widely used in commercial sunscreens. Here, we used gas-phase laser photodissociation spectroscopy to characterise the intrinsic photostability and photodegradation products of OMC by studying the system in its protonated form, i.e., [OMC·H]+. The major photofragments observed were m/z 179, 161, and 133, corresponding to fragmentation on either side of the ether oxygen of the ester group (m/z 179 and 161) or the C–C bond adjacent to the ester carbonyl group. Additional measurements were obtained using higher-energy collisional dissociation mass spectrometry (HCD-MS) to identify fragments that resulted from the breakdown of the vibrationally hot electronic ground state. We found that the m/z 179 and 161 ions were the main fragments produced by this route. Notably, the m/z 133 ion was not observed through HCD-MS, revealing that this product ion is only produced through a photochemical route. Our results demonstrate that UV photoexcitation of OMC is able to access a dissociative excited-state surface that uniquely leads to the rupture of the C–C bond adjacent to the key ester carbonyl group.

Octyl methoxycinnamate (OMC) is a common UVA and UVB filter molecule that is widely used is commercial sunscreens. Here, we use gas-phase laser photodissociation spectroscopy to characterize the intrinsic photostability and photodegration products of OMC, by studying the system as its protonated form, i.e. [OMC·H]+. The major photofragments observed have, m/z 179, 161, and 133, corresponding to fragmentation on either side of the of the ether oxygen of the ester group (m/z 179, and 161) or the C-C bond adjacent to the ester carbonyl group. Additional measurements were obtained using higher-energy collisional dissociation mass spectrometry (HCD-MS), to identify fragments which result from breakdown of the vibrationally hot electronic ground state. We found that the m/z 179 and 161 ions are the main fragments produced by this route. Notably, the m/z 133 ion was not observed through HCD-MS, revealing that this product ion is only produced through a photochemical route. Our results demonstrate that UV photoexcitation of OMC is able to access a dissociative excited state surface that uniquely leads to rupture of the C-C bond adjacent to the key ester carbonyl group.