New molecular devices: In search of a molecular ratchet

The triptycene-substituted [3]- and [4]helicenes 1 and 2 were examined as possible molecular versions of mechanical ratchets, where the triptycene serves as the ratchet wheel and the helicenes as pawl and spring. The syntheses of 1 and 2b are described. H-1 NMR was employed to examine rotation around the triptycene/helicene single bond; at 20 degrees C rotation is frozen for both 1 and 2b, but the NMR of 1 revealed a plane of symmetry, indicating that 1 cannot function as a unidirectional ratchet. In contrast, NMR revealed that, like a ratchet, triptycyl[4]helicene 2b lacks the symmetry of 1 and has a barrier to rotation of 24.5 kcal/mol, but spin polarization transfer NMR experiments indicated the triptycene in 2b nonetheless rotates equally in both directions. That outcome is rationalized from the standpoint of thermodynamics.