Dielectric studies on strongly polar discotic liquid crystals

We report dielectric investigations on the fourth and fifth homologues of the series 1-nitro-2,3,6,7,10,11-hexaalkoxytriphenylene. A hexagonally ordered columnar mesophase (D_ho) is exhibited by both the compounds but the fourth homologue shows in addition, a more ordered mesophase at lower temperatures. The dielectric constants observed are high compared to the values observed in other discotic materials. In the frequency range studied (1 Hz-1 MHz) one relaxation mode is observed in the D_ho phase which can perhaps be correlated with a glass transition. A strong reduction in the magnitude of the relaxation frequency and an associated increase in the dielectric strength of the mode are observed at the transition from D_ho to the lower temperature phase. The data in the two-phase region of this first-order transition are also presented.