The Effect of 2- and 3-Lateral Substituents on the Acid Side of 4,4′-Disubstituted Phenylbenzoates and Phenylthiobenzoates on Mesomorphic Properties

Abstract

A variety of esters/thioesters of the type where A or B=F, Cl, Me; Z=O, S and Y=C₁₀, OC₁₀, COC₉, CO₂C₉ and OCOC₉ for Z=O and C₅, C₁₀, OC₇, OC₁₀ for Z=S and R=C₈H₁₇, C₁₀H₂, C₁₂H₂₅ were

synthesized and their mesomorphic properties determined by hot-stage polarizing microscopy. These properties were compared with those for the parent compounds having no lateral substituents. Melting temperatures decreased in most, but not all, of the esters but decreased in all the thioesters studied whereas clearing temperatures always decreased. The amount of temperature depression varied considerably for both transitions. Melting temperature depressions in the esters were similar for the 2- and 3-fluorine isomers but were larger in the 2- than the 3-isomers with larger lateral substituents. Clearing temperature depressions were usually more for the 2-F ester than for the 3-isomer but not always. This difference was less in the thioesters. Often clearing temperature depressions were greater than melting temperature depressions giving narrower mesophase ranges.

Nematic and smectic C phases were found more often in the lateral substituted esters than in the parent compounds whereas the smectic A phase occurred less frequently. Thioesters showed a better suppression of smectics and a predominance of nematics than the esters. More smectic phases with larger phase ranges occurred in the 2- than the 3-F esters. As lateral substituent size increased the reverse became true.

A few lateral substituted thioesters containing branched chains (A or B=F, Me), Y=CH₂CH(Me)Pr or (CH₂)₂CH(Me)Et were also synthesized, mesomorphic properties determined and these properties compared to those for the parent thioesters. Lateral substituents lowered transition temperatures more than the branched chain did but unlike the branched chain, the clearing temperatures decreased more than the melting temperatures giving narrower mesophase ranges and fewer mesophases.