Journal
CRYSTAL GROWTH & DESIGN
Volume 11, Issue 6, Pages 2654-2659Publisher
AMER CHEMICAL SOC
DOI: 10.1021/cg200373g
Keywords
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Funding
- National Science Foundation [DMR0605688, CHE0551938, DMR0937243]
- National Science Foundation IGERT MCTP [DGE0114443]
- Direct For Mathematical & Physical Scien [1101934] Funding Source: National Science Foundation
- Division Of Materials Research [1101934] Funding Source: National Science Foundation
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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-terphenyOmethyl) 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 C-13 NMR with cross-polarization with magic angle spinning (C-13 CPMAS NMR) and by quadrupolar echo H-2 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.
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