When the fs-TAS experiment was repeated for a 1:2 mixture of (ps1)2⊂C and (E-9)2⊂C (i.e., a pair with a high tendency to form a heterodimer), the bleach at 483 nm was significantly less pronounced (Fig. , inset), indicating a TET to E-9 (see step iii in Fig. ). Importantly, the TET and the subsequent formation of Z-9 occur in the ns time regime, i.e., much faster than the lifetime of the ps1 triplet state, which explains why DESC does not require exclusion of oxygen. In fact, we found the process to be equally efficient in strictly deoxygenated vs. thoroughly oxygenated water (Supplementary Fig. ).