ON THE INFLUENCE OF SHORT-RANGE ORDER UPON THE PHYSICAL-PROPERTIES OFTRIPHENYLENE NEMATIC DISCOGENS

The dielectric and elastic properties of two novel triphenylene nemati c discogens are measured and compared with those of calamitic nematics and the known material H7OBT. The new compounds have nematic phases a t much lower temperatures than H7OBT. All three compounds have a small positive permittivity anisotropy, indicating the presence of a perman ent dipole contribution parallel to the director. Measurement of the m olecular moment dipole provides a value consistent with nearly-free ro tation of the substituent groups. The temperature dependence of epsilo nBAR and the concentration dependence of the dipolar strength DELTA sh ow evidence of antiparallel dipole correlations in the N(D) phase. The strong temperature dependence of \k33/DELTAchi\, the large value of k 11/k33 and the rise in k11/k33 as the temperature approaches the clear ing point are believed to be due to short range columnar order. Theore tical study suggests that dipole correlations favour local columnar or der, and we predict that antiparallel dipole correlations require that the molecules be tilted within the columns.