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

Continuous irradiation of the thermodynamically stable (<i>Z</i>,<i>Z</i>)-cyclobisazobenzene does not lead to accumulation of a (<i>Z</i>,<i>E</i>) or (<i>E</i>,<i>E</i>) isomer as one might expect. Our combined experimental and computational investigation reveals that <i>Z</i> → <i>E</i> 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 (<i>E</i>)-isomers.