Photoexcited triplet state and singlet oxygen generation of quinine, an antimalarial drug.

Energy transfer from the lowest excited triplet (T) state of quinine (QN) to ground-state molecular oxygen produces singlet oxygen. In aqueous solutions, a neutral form QN, a singly protonated cation QNH and doubly protonated cation QNH are present according to their pK values. To the best of our knowledge, the pH dependence of QN-photosensitized singlet oxygen generation has not been reported. In the present study, the quantum yields of photosensitized singlet oxygen generation (Φ) by QN, QNH and QNH have been determined through the measurements of time-resolved near-IR phosphorescence. Φ decreases in the following order: Φ (QNH) > Φ (QNH) > Φ (QN). The nature of the T states of QN, QNH and QNH has been studied through the measurements of transient absorption, phosphorescence and EPR by changing the pH of the medium. This is the first report of EPR for the T state of QN. The photoexcited T state of 6-methoxyquinoline (6-MeOQL), a closely related component, has been studied for comparison. The observed zero-field splitting parameters, phosphorescence spectra and triplet lifetimes suggest that the nature of the T state of QN can be regarded as a locally excited ππstate within 6-MeOQL. The two unpaired electrons localize mainly on 6-MeOQL. The nature of the T state of QN scarcely changes when the quinuclidine nitrogen site is protonated. Applying the Förster cycle to the T states of QN and its protonated cations, it was found that QNH becomes more basic when excited to its T state.