Quercetin•Zirconium Metal Complexes: Insights from Femto to Millisecond Photobehavior

Metal complexation modulates the antioxidant properties of many flavonoids, including quercetin (QRT). However, competition between the antioxidant and autoxidation processes limits their longevity. This necessitates a deeper understanding of the fate of QRT phenoxy radicals following antioxidant activity. Here, we report on the formation of QRT•Zr complexes and their photobehaviors, which account for the photochemical reactivity of the generated radicals. In methanol, N, N-dimethylformamide, and water, the UV-visible absorption spectra of the 1:1 and 1:2 complexes exhibit a significant bathochromic shift (50-100 nm) and high global binding constants (1 - 14 × 10⁹ M^-2). While the formation of charge-transfer states through ligand-to-metal charge transfer in the ground state prevents the excited-state intramolecular proton transfer that typically occurs in QRT, it leads to the generation of excited metal-bound ligand radicals. We present details on the dynamics and further relaxation from femto to millisecond regimes of the excited complexes and generated species. The lifetimes of the formed radicals are considerably shorter than those of the free QRT radicals due to rapid back electron transfer, regenerating the neutral complexes. These findings will facilitate the design of other systems for antioxidant activity, photocatalysis, and various applications in chemistry, biochemistry, and biomedicine.