Exploration of Thermally Activated Delayed Fluorescence (TADF)-Based Photoredox Catalyst To Establish the Mechanisms of Action for Photodynamic Therapy.

The mechanisms of action (MoA) have been proposed to further reduce the O dependence of photodynamic therapy (PDT) significantly. However, the triplet states of traditional photosensitizers are relatively short and also are easily deactivated by the quenching of HO or O. This is not conducive for the electron transfer in the photocatalytic process and poses a great obstacle to establish the MoA. Therefore, we selected and synthesized a zirconium(IV) complex (Zr(PDP)) reported by Milsmann to address this issue. The specific symmetric and intact geometry endowed Zr(PDP) NPs with long-lived triplet excited state (τ = 350 μs), desired sensitized ability, and improved anti-interfering performance on O, which was matched with the requirements of photoredox catalyst significantly. The results showed that while PDT (I) and PDT (II) could be achieved simultaneously by leveraging Zr(PDP) NPs, it also could be served as a rare example of thermally activated delayed fluorescence (TADF)-based photoredox catalyst to implement the MoA of PDT. It involved the oxidation of NADH and the establishment of catalytic cycle collaborating by O and cytochrome c (cyt ) in normoxia and hypoxia, respectively. As a result, the oxygen-free PDT and tumor-growth inhibition was realized.