To further support the assignment of the third lifetime to the decay of the T 1 state, nanosecond-tomicrosecond transient absorption spectroscopy was conducted. The results are presented in Figures S8 and S9 in PBS and ACN, respectively and Table . From approximately 3 ns to 6 µs, the positive transient species with maxima at 425 nm and 360 nm exhibits a monotonic decay back to the baseline following excitation at 390 nm. As shown in Figures and, and summarized in Table , dTAT1 exhibits a long-lived triplet decay lifetime. In ambient air, the triplet decay lifetimes were measured as 0.54 ± 0.02 µs in PBS and 0.125 ± 0.010 µs in ACN. Under oxygen-saturated conditions, these lifetimes decreased to 0.20 ± 0.01 µs in PBS and 0.023 ± 0.002 µs in ACN, while they are significantly longer lifetimes under N 2saturated conditions, reaching 0.90 ± 0.03 µs in PBS and 1.00 ± 0.05 µs in ACN. From the lifetimes, bimolecular triplet quenching rate constants by O 2 (k q ) were determined for dTAT1. The k q values were found to be (4.7 ± 0.2) × 10 9 M -1 s -1 in ACN and (3.0 ± 0.1) × 10 9 M -1 s -1 in PBS, as illustrated in Figure and Table . The noticeable quenching of the T 1 (ππ*) state by O 2 , suggests that dTAT1 should be able to efficiently generate ROS upon excitation with UVA to near-visible radiation.