title: Pentiptycene-derived light-driven molecular brakes: substituent effects of the brake component.
abstract: 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.
authors.affiliation: Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan.
authors.forename: Wei-Ting
authors.initials: WT
authors.lastname: Sun
authors.forename: Yau-Ting
authors.initials: YT
authors.lastname: Huang
authors.forename: Guan-Jhih
authors.initials: GJ
authors.lastname: Huang
authors.forename: Hsiu-Feng
authors.initials: HF
authors.lastname: Lu
authors.forename: Ito
authors.initials: I
authors.lastname: Chao
authors.forename: Shou-Ling
authors.initials: SL
authors.lastname: Huang
authors.forename: Shing-Jong
authors.initials: SJ
authors.lastname: Huang
authors.forename: Ying-Chih
authors.initials: YC
authors.lastname: Lin
authors.forename: Jinn-Hsuan
authors.initials: JH
authors.lastname: Ho
authors.forename: Jye-Shane
authors.initials: JS
authors.lastname: Yang