Redox-Gated Tristable Molecular Brakes of Geared Rotation

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 22– but a dianion for 32–. The redox operations switch the independent α and β rotations in 2 into a geared rotation in both 2•– and 22– and result in a slow–fast–stop rotation mode for 2–2•––22–. The two redox states 3•– and 32– retain the geared α and β rotations and follow a fast–slow–stop mode for 3–3•––32–. Both molecular systems mimic tristable molecular brakes and display 8–9 orders of magnitude difference in rotation rate through the redox switching.