Pentiptycene‐Derived Light‐Driven Molecular Brakes: Substituent Effects of the Brake Component

Five pentiptycene‐derived stilbene systems ( 1 R ; R =H, OM, NO, Pr, and Bu) have been prepared and investigated as light‐driven molecular brakes that have different‐sized brake components ( 1 H < 1 OM < 1 NO < 1 Pr < 1 Bu ). At room temperature (298 K), rotation of the pentiptycene rotor is fast ( k rot =10 8 –10 9  s −1 ) with little interaction with the brake component in the trans form (( E )‐ 1 R ), which corresponds to the brake‐off state. When the brake is turned on by photoisomerization to the cis form (( Z )‐ 1 R ), the pentiptycene rotation can be arrested on the NMR spectroscopic timescale at temperatures that depend on the brake component. In the cases of ( Z )‐ 1 NO , ( Z )‐ 1 Pr , and ( Z )‐ 1 Bu , the rotation is nearly blocked ( k rot =2–6 s −1 ) at 298 K. It is also demonstrated that the rotation is slower in [D 6 ]DMSO than in CD 2 Cl 2 . A linear relationship between the free energies of the rotational barrier and the steric parameter A values is present only for ( Z )‐ 1 H , ( Z )‐ 1 OM , and ( Z )‐ 1 NO , and it levels off on going from ( Z )‐ 1 NO to ( Z )‐ 1 Pr and ( Z )‐ 1 Bu . DFT calculations provide insights into the substituent effects in the rotational ground and transition states. The molar reversibility of the E–Z photoswitching is up to 46 %, and both the E and Z isomers are stable under the irradiation conditions.