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.