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The effect on mesomorphic properties of a triple bond in a terminal chain of 4,4′‐disubstituted phenyl benzoates and phenyl thiobenzoates

The effect of a triple bond in a terminal chain on the mesomorphic properties of rod‐like mesogens was studied by replacing a straight alkyl chain with a terminal 1,2‐alkynyl chain in some esters and thioesters of the type: or by incorporating the triple bond into an alkyl ester chain in esters and thioesters of the types: The triple bond should provide increased rigidity to the triple bond region but increased flexibility at the adjacent single bond. These should have opposite effects on the mesomorphic properties. The first compounds were synthesized by esterification of the appropriate alkynylbenzoic acids with various phenols or thiols using the carbodiimide method. Detailed syntheses are provided for the alkynylbenzoic acids. Preparation of the alkynyl ester compounds was by acylation of the precursor hydroxyl esters or thioesters using either the acid chloride or carbodiimide method. The synthesis of the phenolic or thiolic ester intermediates, usually using a protection group, is described. Mesomorphic properties of these new esters and thioesters were determined using hot stage polarizing microscopy and DSC and compared with those for the known parent compounds. Fewer mesophases having shorter temperature ranges and lower transition temperatures were usually observed. Some of the intermediate phenols or benzyl ethers were also studied. Long chain benzyloxy esters showed a nematic phase as did some of the hydroxy thioesters but no mesophases were observed in the hydroxy esters.