Exploiting molecular symmetry reduction to enrich liquid crystal phase diversity

Abstract

The strategic tuning of liquid crystalline phase behaviour by adjusting molecular symmetry was investigated. A family of sixteen symmetrical and unsymmetrical 2,6-di(4'-n-alkoxybenzoyloxy) naphthalene derivatives were prepared and their liquid crystal properties examined by differential scanning calorimetry, polarised optical microscopy, and x-ray diffraction. All mesogens formed nematic phases, with longer-chain analogues also exhibiting smectic C phases at lower temperatures. Melting temperatures of the compounds strongly depend on molecular symmetry, whereas clearing transitions are relatively insensitive to this effect. A detailed analysis indicates that the clearing point can be predicted based on the nature of the terminal alkyl chains, with only a secondary effect from molecular symmetry. Moreover, low symmetry molecules showed a greater tendency to form smectic C phases, which was ascribed to the selective depression of the melting point versus the SmC-N transition. This demonstrates that molecular symmetry-breaking is a valuable tool both for tuning liquid crystalline phase range and for increasing a material's polymorphism.

Strategies for promoting discotic nematic phases

“…This effect leads to a broadening of the LC temperature range, 30,31 and promotes enantiotropic over monotropic phases. 32 In the present case, the mixed-tail derivative dDBP-OHx(6,10) melts almost 50 °C lower than dDBP-OHx(6,6) , whereas its clearing temperature is depressed by less than 40 °C.…”

“…broadening of the LC temperature range, 30,31 and promotes enantiotropic over monotropic phases. 32 In the present case, the mixed-tail derivative dDBP-OHx(6,10) melts almost 50 1C lower than dDBP-OHx (6,6), whereas its clearing temperature is depressed by less than 40 1C. Monomeric and dimeric dibenzophenazine mesogens almost exclusively organize into columnar liquid crystalline phases.…”

“…Even lower melting points were observed for dDBP(6,10) and dDBP(10,6) (186 and 188 °C, respectively), consistent with previous observations that show a depression in the melting point of mixed tailed systems. 30–32…”

“…Even lower melting points were observed for dDBP(6,10) and dDBP(10,6) (186 and 188 1C, respectively), consistent with previous observations that show a depression in the melting point of mixed tailed systems. [30][31][32] The lack of liquid crystallinity for members of the dDBP(m,n) series is in marked contrast to the behaviour of their noncovalent analogues A(m)-Pyr(n), all of which form columnar and/or nematic phases (Fig. 1).…”

Induced Nematic Phase of New Synthesized Laterally Fluorinated Azo/Ester Derivatives

“…These compounds were found to exhibit N mesophases, and the position of F-atom in the molecular structure center was found to influence the physical and geometrical parameters of the molecules. In other studies [ 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 ], the investigation was directed toward measuring the impact of incorporation of lateral moiety on their mesomorphic properties in the terminal or central benzene rings.…”