Photophysics of Iodine-Substituted Oxyluciferins: Observation of Simultaneous High Fluorescence Quantum Yields and Singlet Oxygen Sensitization

quantum yield (SOQY). Specifically, in the phenolate form, diiodo-oxyluciferin has FLQY = 0.42 and SOQY = 0.41, as compared to FLQY = 0.33 and SOQY <0.02 for unsubstituted oxyluciferin. Electronic structure calculations reveal that deprotonation reorders the low-lying singlet manifold, leading to a marked difference in the effective singlet-state charge localization for phenol and phenolate forms. Consequently, in the phenol form, a large difference in total orbital angular momentum between singlet and triplet states enhances ISC. Iodination predominantly affects the triplet state energetics and suppresses a competing nonradiative decay channel that likely involves internal conversion. Our results show how new functionalities can be engineered into the luciferin chromophore, unlocking the possibility of using this compound to sensitize reactive oxygen species for biomedical applications.