Redox-Gated Tristable Molecular Brakes of Geared Rotation

5 years ago

Redox-Gated Tristable Molecular Brakes of Geared Rotation

Chun-Wei Chiu, Chen-Yi Kao, Ito Chao, Chetti Prabhakar, Jye-Shane Yang, Ting Tseng, Hsiu-Feng Lu

p-Bis(arylcarbonyl)pentiptycenes 2 (aryl = 4-(trifluoromethyl)phenyl) and 3 (aryl = mesityl) have been prepared and investigated as redox-gated molecular rotors. For 2, rotations about the pentiptycene–carbonyl bond (the α rotation) and about the aryl–carbonyl bond (the β rotation) are independent, and the rotation barriers are 11.3 and 9.5 kcal mol^–1, respectively, at 298 K. In contrast, the α and β rotations in 3 are correlated (geared) in a 2-fold cogwheel pathway between the aryl and the pentiptycene groups with a much lower rotation barrier of 6.5 kcal mol^–1 at 298 K in spite of the bulkier aryl groups. Electrochemical reduction of the neutral forms led first to radical anions (2^•– and 3^•–) and then to a bis(radical anion) for 2^2– but a dianion for 3^2–. The redox operations switch the independent α and β rotations in 2 into a geared rotation in both 2^•– and 2^2– and result in a slow–fast–stop rotation mode for 2–2^•––2^2–. The two redox states 3^•– and 3^2– retain the geared α and β rotations and follow a fast–slow–stop mode for 3–3^•––3^2–. Both molecular systems mimic tristable molecular brakes and display 8–9 orders of magnitude difference in rotation rate through the redox switching.

Publisher URL: http://dx.doi.org/10.1021/acs.joc.7b00729

DOI: 10.1021/acs.joc.7b00729