Dynamics of Discotic Fluoroalkylated Triphenylene Molecules Studied by Proton NMR Relaxometry

The Larmor frequency and temperature dependence of the
proton nuclear magnetic resonance (NMR) spin–lattice relaxation
time was measured in the isotropic and columnar phases of both chain-end
fluorinated triphenylene disk-like and fully hydrogenated molecules.
In the columnar phase, the results are interpreted in terms of the
collective motions, due to the deformations of the columns, and individual
molecular translational self-diffusion displacements and rotations/reorientacions.
In the isotropic phase, local molecular motions and order fluctuations
as a pretransitional effect were considered. The activation energies
of the molecular motions of the partially fluorinated molecule were
found to be higher than those corresponding to the hydrocarbon homologue.
Our findings show a clear difference in the relaxation dispersion
between the two liquid crystals homologues. In particular it is observed
that the columnar undulations have a much stronger contribution to
the relaxation rate in the low frequency regime in the case of the
fully hydrogenated triphenylene. The effect of fluorination of the
pheripheral chain enhances the columnar mesophase’s stability.