Pentiptycene‐Derived Light‐Driven Molecular Brakes: Substituent Effects of the Brake Component |
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Authors: | Wei‐Ting Sun Yau‐Ting Huang Guan‐Jhih Huang Dr. Hsiu‐Feng Lu Prof. Ito Chao Shou‐Ling Huang Dr. Shing‐Jong Huang Prof. Ying‐Chih Lin Dr. Jinn‐Hsuan Ho Prof. Jye‐Shane Yang |
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Affiliation: | 1. Department of Chemistry, National Taiwan University, Taipei 10617 (Taiwan), Fax: (+886)?223636359;2. Institute of Chemistry, Academia Sinica, Taipei 11529 (Taiwan), Fax: (+886)?227831237;3. Current address: Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10607 (Taiwan) |
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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 (krot=108–109 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 (krot=2–6 s?1) at 298 K. It is also demonstrated that the rotation is slower in [D6]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)‐ 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. |
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Keywords: | conformation analysis fused‐ring systems isomerization photochromism substituent effects |
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