where ∆𝐴 6𝑀𝑃 (𝜆 4 ) is the intensity of the triplet absorption of 6MP at 𝜆 4 , 𝜀 6𝑀𝑃 * (𝜆 4 ) is the triplettriplet extinction coefficient of 6MP at 𝜆 4 , and 𝛷 𝑇 (𝑅), 𝜀 𝑅 * (𝜆 3 ), ∆𝐴 𝑅 (𝜆 3 ) are the triplet quantum yield, the triplet-triplet extinction coefficient at 𝜆 3 , and the intensity of the triplet absorption at 𝜆 3 for a reference compound of known triplet yield, respectively. In this case, we used 6thioguanosine as the reference compound, which has a reported triplet yield of 0.8 ± 0.2 in aqueous buffer solution. 𝜆 3 and 𝜆 4 can be different wavelengths as long as they are wellseparated from the ground state bleaching signal.