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 Z → E 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.