Twist and Return−Induced Ring Strain Triggers Quick Relaxation of a (Z)‑Stabilized Cyclobisazobenzene

Continuous irradiation of the thermodynamically stable (Z,Z)-cyclobisazobenzene does not lead to accumulation of a (Z,E) or (E,E) isomer as one might expect. Our combined experimental and computational investigation reveals that ZE photoisomerization still takes place on an ultrafast time scale but induces large ring strain in the macrocycle, which leads to a very fast thermal back-isomerization, preventing photostationary accumulation of (E)-isomers.