A Pentiptycene‐Derived Molecular Brake: Photochemical E→Z and Electrochemical Z→E Switching of an Enone Module

The synthesis and brakelike performance of a new molecular system ( 1 ) consisting of a pentiptycene rotor and a 2‐methyleneindanone brake are reported. The rotation kinetics of the rotor was probed by both variable‐temperature 1 H and 13 C NMR spectroscopy and DFT calculations, and the switching between the brake‐on and brake‐off states was conducted by a combination of photochemical and electrochemical isomerization. Because of the greater steric hindrance between the rotor and the brake units in the Z form (( Z )‐ 1 ) than in the E form (( E )‐ 1 ), rotation of the rotor is slowed down 500‐fold at room temperature (298 K) on going from ( E )‐ 1 to ( Z )‐ 1 , corresponding to the brake‐off and brake‐on states, respectively. The ( E )‐ 1 →( Z )‐ 1 photoisomerization in acetonitrile is efficient and reaches an ( E )‐ 1 /( Z )‐ 1 ratio of 11:89 in the photostationary state upon excitation at 290 nm, attributable to a much larger isomerization quantum efficiency for ( E )‐ 1 versus ( Z )‐ 1 . An efficient ( Z )‐ 1 →( E )‐ 1 isomerization (96 %) was also achieved by electrochemical treatment through the radical anionic intermediates. Consequently, the reversibility of the E–Z switching of 1 is as high as 85 %. The repeated E – Z switching of 1 with alternating photochemical and electrochemical treatments is also demonstrated.