Paradoxes in Thermal Stability of the Liquid-Crystal Phase of Phenylpropargyl Phenyl Ethers: I. Synthesis and Mesogenic Properties of Phenylpropargyl Phenyl Ethers

The ethers PhCCCH₂OC₆H₃RR' were prepared in 51-83% yields by reactions of phenylpropargyl tosylate in alkaline solution with appropriate p- and o-substituted phenols. Below are given R, R', and the ranges of existence of the nematic mesophase (°C): p-I, H, 104-137; p-Cl, H, 65-117; p-F, H, 33-53; p-OMe, H, 79-120; H, H, 44-115; p-(Me)₃C, H, 74-82; p-COOH, H; p-NO₂, H, 77-96; o-NO₂, H, 83-139; o-CHO, H, 75-115; p-Br, o-NO₂, 95-132; p-Cl, o-NO₂, 71-123; p-F, o-NO₂, 79-120; o-Cl, H, none; p-COOPr, H, none; and p-COOPh, H, 116-134. In contrast to the traditional views, the presence of the o-nitro group enhances, rather than distorts, the thermal stability of the mesophase. The stability increases in parallel with the -R effect of the o-substituent.