Direct Observation of Photochemical Free Radical Production from the Sunscreen 2-Phenylbenzimidazole-5-Sulfonic Acid via Laser-Interfaced Mass Spectrometry

The common sunscreen molecule 2-phenylbenzimidazole-5-sulfonic acid (PBSA) is studied in its gas-phase deprotonated form ([PBSA-H](-)) for the first time as an important step in achieving a better understanding of its behavior as a photosensitizer. UV laser-interfaced mass spectrometry is employed, revealing that [PBSA-H](-) photofragments into three ionic products (m/z 208, 193, and 80) with distinctive wavelength-dependent production profiles. Both the m/z 208 and 80 channels produce associated neutral free radical species. Collision-induced dissociation is performed on [PBSA-H](-), showing that its hot ground-state dissociates only into m/z 193 (statistical fragment). Therefore, the m/z 208 and 80 fragments which are produced strongly through the UVA/UVB are characterized as non-statistical photofragments associated with non-ergodic excited-state decay. Our observation of non-statistical photofragments reveal that [PBSA-H](-) is not behaving as a model sunscreen molecule. Further, our results indicate that the T-1 state, associated with photosensitization, decays with direct free radical production.