In general, ps2 is a more efficient DESC agent than ps1. However, we found one exception: ps2 proved unable to induce the switching of azobispyrazole E-9. To understand this result, we resorted to quantum chemical simulations (see Supplementary Section 8.1) and found the T1-T2 energy gap for the E-9⋅ps2 heterodimer to be exceptionally high (1.03 eV; compared to 0.26 eV for E-1⋅ps1 in Fig. ). Moreover, relaxed scans analogous to those in Fig. revealed that Φ in 9 must twist by 38º-a prohibitively large distortion-for the energies of these two triplet states to cross (Supplementary Fig. ). These computational results not only rationalize the experimental findings but provide further (although indirect) support for the involvement of the TET mechanism in our system.