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

Five pentiptycene-derived stilbene systems (1R: R = H, OM, NO, Pr, and Bu) have been prepared and investigated as light-driven molecular brakes that have different-sized brake components (1H < 1OM < 1NO < 1Pr < 1Bu). 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)-1R), which corresponds to the brake-off state. When the brake is turned on by photoisomerization to the cis form ((Z)-1R), the pentiptycene rotation can be arrested on the NMR spectroscopic time-scale at temperatures that depend on the brake component. In the cases of (Z)-1NO, (Z)-1Pr, and (Z)-1Bu, 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 CD2Cl2. A linear relationship between the free energies of the rotational barrier and the steric parameter A values is present only for (Z)-1H, (Z)-1OM, and (Z)-1NO, and it levels off on going from (Z)-1NO to (Z)-1Pr and (Z)-1Bu. 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.