Under the optimised conditions (50 mol% DABCO, 5 mol% Cs2CO3; Supplementary Section 5.4), we determined an internal quantum yield of Φ = 0.136. While a value of Φ > 1 would strongly indicate a chain-propagation mechanism, where more than one equivalent of product is generated per photon absorbed, a value of Φ < 1 does not rule out a chain-propagation process, as it accounts for the entire reaction, including nonproductive pathways such as the rapid BET within the initial EDA complex. Low quantum yields are characteristic of many EDA activation pathways due to these nonproductive processes, particularly those using amine donors to activate electron-deficient RAEs, such as tert-chloro-RAEs (Φ = 0.02). For comparison, we observed an initiation quantum yield of Φ = 0.008. Using established methods, the chain lengths of this autocatalytic reaction can be estimated by dividing the observed quantum yield by the initiation quantum yield, the estimated chain length of this autocatalytic radical chain reaction was calculated to be 17.0 (Φestimated = Φmeasured / Φinitiation; Supplementary Section 5.4) . This supports the presence of a highly efficient autocatalytic chain process, consistent with the experimental and computational findings.