For TIPS-Pn, we determined the noise level in the TA data to be 0.019 mOD from the level of fluctuations around a smoothed spectrum obtained by applying a low-pass filter (supplementary material Sec. S5.1). We consider a change in absorption in the 8 ns time window less than three times this level (i.e. three standard deviations) to be unresolvable, according to the IUPAC recommendation for the limit of detection. That is, the maximum amount of triplet absorption that could be present in the 0.05 mM TIPS-Pn 8 ns spectrum is 0.057 mOD. Any signal higher than this level should be observable, and as Fig. (b) shows, we do not observe any T 1 absorption in the TA spectrum over time (at 504 nm where the T 1 ESA maximum is expected to occur, we in fact observe a slight decrease relative to the GSB, rather than the increase that would result from T 1 formation). Using the lower bound of the triplet basis spectrum at the peak maximum (70640 M -1 cm -1 at 504 nm), this maximum possible T 1 absorption corresponds to a T 1 concentration of 4.0 × 10 -9 M, or 2.1% of the initial S 1 concentration.