Engineering Crystal Packing and Internal Dynamics in Molecular Gyroscopes by Refining their Components. Fast Exchange of a Phenylene Rotator by 2H NMR

Using quadrupolar echo H-2 NMR, we have determined that a relatively simple change on the periphery of the triptycene stators of molecular gyroscopes may have a profound effect oil the packing arrangements, packing coefficients, and rotary dynamics of the central phenylene rotators. The previously reported crystal structure of 1,4-bis-[2-(9-triptycyl)-ethynyl]benzene (1) is characterized by the inclusion of meta-xylene and a very tightly interdigitated packing arrangement that effectively prevents the rotary motion of the phenylene rotator. Structural modifications to prevent this interdigitation led to the design and synthesis of 1,4-bis[2-(2,3,6,7,12,13-hexamethyl-10-propyl-9-triptycyl)ethynyl]benzene (2), which had been shown to crystallize in the desired manner but with the inclusion of bromobenzene. Using crystals of 2 with a H-2-labeled phenylene rotator, we determined by quadrupolar echo H-2 NMR line shape analysis that rotation occurs by a 180 degrees site exchange (2-fold flip) with frequencies in the MHz regime at low temperatures (150-183 K). From the temperature dependence of the rotational exchange frequency, we determined a barrier of 4.4 kcal/mol, which is only 1.4 kcal/mol higher than the internal barrier for ethane in the gas phase. Additional spectral narrowing observed at higher temperature was analyzed in terms of a model that considers larger amplitude excursions between the 180 degrees jumps.