Synthesis and thermal behaviour of non-linear optical materials. 1: Liquid crystals containing long conjugate and high polar terminal groups

Abstract

A number of non-symmetrical liquid crystalline compounds with a long conjugate length (phenyltolane) and three different polar terminal groups (-NO₂, -CN, -CF₃) for non-linear optical applications were synthesized. These compounds are liquid crystalline and expected to exhibit a large first molecular hyperpolarizability (β). They all exhibit crystal E (CrE), smectic A (S_A), and nematic (N) phases except the compounds containing a trifluoromethyl terminal group which have CrE, S_B and S_A phases. The layer thickness and the intermolecular spacing were investigated using powder X-ray diffraction. A bilayer structure is present in the S_A phase, which disappears on going into the CrE phase. The layer thickness of the S_A phase increases as the temperature decreases. Moreover, this family of phenyltolane chromophores exhibits excellent thermal stability as shown by differential scanning calorimetry and thermogravimetric analysis. Their thermal degradation temperatures are approximately 300° C. UV-VIS absorption characteristics of all compounds show short cut-off wavelength (λ _cut-off<460nm). More polar terminal groups (polarity: -NO₂> -CN > -CF₃) cause a red shift of the absorption peaks (λ _cut-off 460 > 390 > 370 nm). Such properties make these compounds potentially useful in non-linear optical applications.