Abstract
A new series of teraryl 2-(4-alkoxybiphen-4′-yl)-5-methylpyridines (nO-PPPyMe, n = 3–8) nematic liquid crystal compounds, bearing a biphenylene core and a picoline terminus, were synthesized using a short two-step reaction, and overall yields between 34% and 38% were obtained. Spectral analysis results were in accordance with the expected structures. The thermotropic behavior of the teraryl liquid crystal compounds was investigated through polarized optical microscopy and differential scanning calorimetry. All compounds exhibited a solely enantiotropic nematic phase at the medium–high temperature range of 162.4–234.2 °C. Furthermore, the results for the nO-PPPyMe series were analyzed relative to three other compound series, mO-PPPyCN (m = 2–8), iO-PPQMe (i = 3–8) and xO-PPyPMe (x = 1–10). Consequently, the effect of pyridine on the mesophase of teraryl liquid crystals was demonstrated.
Highlights
In liquid crystals (LCs), when the structure of a molecule is changed, many molecular parameters are affected, which may differentially influence the properties of the LCs
We previously reported a novel method for synthesizing pyridine-containing LC compounds [20,21,22,23,24,25]
A new homologous series of nO-PPPyMe (n = 3–8) was obtained first through the regioselective addition of Grignard reagents to activated 1-acyl-3-picolinium salts to preferentially form 1,2-dihydro-5-picolines, which were subsequently aromatized through a mild oxidation reaction (Scheme 1)
Summary
In liquid crystals (LCs), when the structure of a molecule is changed, many molecular parameters are affected, which may differentially influence the properties of the LCs. Synthesis of pyridine-containing LCs generally involves a cyclisation reaction using an enamine to react with a vinyl ketone to obtain a dihydropyran derivative, which was used to react with hydroxylamine hydrochloride to form pyridine [6] Another method in preparing 2,5-disubstituted pyridines is using the reaction of acetophenone with ethyl formate in the presence of sodium, followed by cyclisation with cyanoacetamide, substitution of oxygen by chlorine, and reductive elimination of chlorine [8,9,10]. Et al [19] modified this method to prepare a new class of mesogenic materials that exhibit the smectic C phase These methods are valuable in preparing heterocyclic LC molecules, most methods have their limitations, requiring a relatively expensive catalyst, and occasionally requiring many low-yields synthetic steps. The targeted molecules contain an alkoxy tail, a biphenyl moiety, and a picoline terminus
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