To determine the SF contribution to this maximum possible triplet formation at 0.05 mM, we used the SF model described by Eqs. ( ) and ( ), with k SF set to the lower bound of the fit to the 8 mM data (2.1 × 10 9 M -1 s -1 ), and k ISC set to 0. As with the 8 mM sample, the excited-state concentration was significantly smaller than 0.05 mM (<1%), so we treated [S 0 ] as constant. This model gave a maximum of 2.6 × 10 -10 M of T 1 formed after 8 ns for a 0.05 mM sample (0.14% of the initial S 1 concentration). This concentration corresponds to a maximum absorption of about 0.004 mOD, which is well below the possible threshold of our TA spectrometer (supplementary material Fig. ), and still leaves 3.7 × 10 -9 M of T 1 that could be due to ISC.