TOLANE OLIGOMERS - MODEL THERMOTROPIC LIQUID-CRYSTALS

The liquid crystalline phases of several rigid-rod, non-polar tolane o ligomers are characterized by differential scanning calorimetry and tr ansmitted polarized light microscopy. We determine that a stable nemat ic phase can be formed at ambient pressure if the molecular axial rati o (length-to-width ratio) is greater than 4.5. A smectic phase forms i n addition to the nematic phase if the axial ratio exceeds 6.1. Symmet rical fluorination of the terminal phenyl groups reveals that the liqu id crystalline phase behaviour of these rigid rods is highly sensitive to perturbations of the charge distribution along the molecules. Nema tic tolane oligomers can exhibit high strength disclinations (s = +/-3 /2 and +/-2) in their schlieren textures, and we discuss conditions th at promote the stability of these defects.