On the influence of short range order upon the physical properties of triphenylene nematic discogens

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Abstract
The dielectric and elastic properties of two novel triphenylene nematic discogens are measured and compared with those of calamitic nematics and the known material H7OBT. The new compounds have nematic phases at much lower temperatures than H7OBT. All three compounds have a small positive permittivity anisotropy, indicating the presence of a permanent dipole contribution parallel to the director. Measurement of the molecular moment dipole provides a value consistent with nearly-free rotation of the substituent groups. The temperature dependence of ∊ and the concentration dependence of the dipolar strength Δ show evidence of antiparallel dipole correlations in the ND phase. The strong temperature dependence of |k 33/Δχ|, the large value of k11/k 33 and the rise in k 11/k 33 as the temperature approaches the clearing point are believed to be due to short range columnar order. Theoretical study suggests that dipole correlations favour local columnar order, and we predict that antiparallel dipole correlations require that the molecules be tilted within the columns.