Synthesis and Solid-State Rotational Dynamics of Molecular Gyroscopes with a Robust and Low Density Structure Built with a Phenylene Rotator and a Tri(meta-terphenyl)methyl Stator

Recent studies suggest that the rotational dynamics in crystals of molecular gyroscopes become more favorable (i.e., faster) when the packing coefficient of the corresponding lattice is decreased by increasing the steric bulk of the stator, as expected for structures with high protuberances or deep cavities. In an effort to explore the effects of increased stator size on the solid-state dynamics of these crystalline models for molecular machines, molecular gyroscope 4 with an “exploded” bis(tri(meta-terphenyl)methyl) stator was synthesized. Single crystal X-ray diffraction analysis revealed a packing structure with two crystallographically distinct gyroscope molecules and four ethyl acetate molecules per unit cell. Although a relatively low packing coefficient of 0.68 was determined for the corresponding packing motif, we noticed that rotators at the two sites have significantly different environments. The solid state rotational dynamics of the two central phenylenes in an ethyl acetate clathrate of 4 were explored by variable-temperature 13C NMR with cross-polarization with magic angle spinning (13C CPMAS NMR) and by quadrupolar echo 2H NMR measurements with isotopically labeled samples. It was found that the increased stator size does indeed allow for more free-volume and faster rotational dynamics as compared to molecular gyroscopes with smaller or more globular stators. However, the two crystallographic sites experience different rotational dynamics, suggesting that the average density available from the packing coefficient is a very crude indicator of solid-state dynamics.