Molecular design of correlated internal rotation

Disrotatory coupling of the internal rotational degrees of freedom in double rotor molecules has been studied for bis(9-triptycyl)methanes (Tp₂CH₂) and bis-(9-triptycyl) ethers (Tp₂O). These molecules undergo rapid internal rotation in a strictly gear-meshed fashion, giving rise to new stereoisomerism in the 2, 2′- and 3,3′-dichloro derivatives. The racemic and meso isomers which arise due to different phase relationship between the substituted benzene rings were separated by HPLC. The high energy barrier to the gear-slipping process was obtained as 32 - 33 and 42 - 43 kcal mol^?1 for Tp₂CH₂ and Tp₂O, respectively, by the isomerization study. The low energy barrier to the gear-meshing process was determined to be ca. 4.5 kcal mol^?1 by analyzing the exciplex fluorescence dynamics for a Tp₂O carrying the naphthalene chromophore on one Tp unit and the tert-amino group on the other. The molecular structure of Tp₂O has been determined by an X-ray analysis; the molecule adopts a bevel gear-shaped structure with the C_s point group in which the CO bond lengths average 1.412 Å and the ∠COC angle is widened to 135.8°. An attempt at reproducing the observed structural features by means of an empirical force-field calculation is described. A possible extension of the concept and further results on correlated internal rotation and phase isomers are discussed.