The finding that E-9 and ps1 form the heterodimer in a near-quantitative yield allowed us to determine the quantum yield (QY) of DESC for this pair. Here, we note that ps1 within (E-9⋅ps1)⊂C is highly emissive, but its fluorescence with (ps1)2⊂C is largely quenched -therefore, exposing (E-9⋅ps1)⊂C to a 515 nm pulsed laser led to a gradual decrease of emission (Fig. ). When the experiment was repeated in the presence of an extra 2 and 4 eq of (E-9)2⊂C, however, we observed lag periods of ~8 and ~18 min, respectively. The stable fluorescence-despite the ongoing E→Z isomerization of 9-indicates that the concentration (E-9⋅ps1)⊂C remains steady, which confirms the rapid exchange kinetics in our system: as soon as the isomerized Z-9 is expelled from the cage, it is replaced by another copy of E-9 (if available). By assessing the mean number of absorbed photons required to convert the excess of E-9, we found that each successful E→Z isomerization event requires 17 photons on average (see Supplementary Section 6 for derivation), which corresponds to a QY of ~6% -a remarkably high value, provided the number of steps separating the excitation of ps1 from the formation of Z-9 (Fig. ).