rate constant, k T 1 , was fixed to 1/9 µs -1 , using the decay time of TIPS-Pn in toluene observed by Schaberle et al.. These authors suggested that the T 1 lifetime may be much longer in a lowoxygen environment, but 9 µs is still substantially longer than the timescales here, so this value had no effect on the fit. Finally, the initial S 1 concentration, [S 1 ](0), and the bimolecular singlet fission rate constant, k SF , were fit to the S 1 and T 1 kinetic data, respectively. The fitted concentrations are shown in Fig. (a), where the shaded gray region represents the range of possible kinetics that fit the triplet concentration. These rate constants are summarized in Table . ) and ( ) for TIPS-Pn and TIPS-Tn. The S 1 lifetimes, τ S 1 , were determined from the rate constant of a single exponential fit to the 0.05 mM GSB TA kinetics, minus the ISC and SF rate constants. The T 1 lifetimes, τ T 1 were taken from literature. The ISC rate constant, k ISC , was either fixed to zero or determined as in Sec. III C, and the SF rate constant, k SF was fit to the 8 mM TA data. The rise in triplet concentration of TIPS-Pn can be reproduced well by this SF model (Fig. ).