Dipolar rotor-rotor interactions in a difluorobenzene molecular rotor crystal

In this paper, we characterize the rotational dynamics and observe rotor-rotor interactions within a crystalline, three-dimensional array of dipolar molecular rotors. The rotating portion of each rotor molecule consists of a dipolar fluorine-substituted phenyl group. The phenyl rotors are connected by acetylene linkages to bulky triphenyl methyl groups which are held rigid in the crystal lattice. These custom synthesized rotor molecules allow control over the molecular spacing in the lattice, the dipole strength, and the rotational hindrance, thus permitting formation of systems with rapid thermal rotation and strong dipole-dipole interactions, which is of interest for studying new phases and collective phenomena. Dielectric and H-2 NMR spectroscopy measurements are used to map the rotational potential, and to explore the influence of rotor-rotor interactions. Interactions due to dipole-dipole effects are studied using a Monte Carlo simulation, while contributions from steric interactions between rotors are investigated using molecular mechanics methods. Both contributions are needed explain the dielectric spectroscopy results.