Oxoglaucine (OG) is an oxoaporphine alkaloid, which has been linked to plant defense mechanisms. It contains a phenalenone (PN)–like chromophore, which suggests a role as singlet oxygen (¹O₂) photosensitizer. Indeed, OG is able to photosensitize ¹O₂ with 100% efficiency in nonpolar environments, similar to PN. However, this efficiency decreases in polar and protic media, although ¹O₂ is formed in all environments ranging from benzene to water. OG is a rather inefficient ¹O₂ quencher (k_q = 8 × 10⁵ M⁻¹ s⁻¹) unlike the related alkaloids boldine and glaucine, for which an antioxidant role has been suggested. The results of this study contribute to the view that plant defense mediated by PN-like secondary metabolites may have a photochemical component.