Janus all-cis-1,2,3,4,5,6-Hexafluorocyclohexane: A Molecular Motif for Aggregation-Induced Enhanced Polarization.

Recently synthesized all-cis-1,2,3,4,5,6-hexafluorocyclohexane is the least stable among all possible configurational isomers of 1,2,3,4,5,6-hexafluorocyclohexane. This molecule has a remarkably large dipole moment (6.2 D) as well as high facial polarization. Solid-state, dispersion-corrected DFT (DFT-D3) calculations are performed on the crystalline phase of all-cis-1,2,3,4,5,6- hexafluorocyclohexane, which reveal that dispersion interactions play a crucial role in its stabilization. A number of thermodynamically favorable orientations of dimers, trimers and tetramers are demonstrated for this molecule. Parallel-stacked aggregates, from dimers to higher-order aggregates, which are absent in the crystal, are found to be thermodynamically most favorable due to the presence of strong short-range C-H⋅⋅⋅F-C intermolecular hydrogen-bonding networks. Because of their cooperative nature, binding energies, dipole moments, and polarizations per molecule increase from monomer to tetramer, whereas the HOMO-LUMO gaps follow the opposite trend. Based on the DFT-D3 calculations, it is proposed that this parallel-stacked arrangement can be further extended to prepare stable a 1D crystal such that a large dipole moment and macroscopic polarizations can arise, which might be useful in designing electronic and nonlinear optical devices. Because the molecule has conformational flexibility, the potential energy surface is investigated for ring flipping and the effects of fluorine substitution are studied by comparing the barrier with respect to cyclohexane and all-cis-1,2,3-trifluorocyclohexane.