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.