Synthesis, optical properties, crystal structures and phase behaviour of symmetric, conjugated ethynylarene-based rigid rods with terminal carboxylate groups

Sonogashira cross-coupling reactions of 4-ethynylmethylbenzoate ( 1) with aryl halides gave 1,2-bis(4-carbomethoxyphenyl) ethyne ( 2), 1,4-bis(4-carbomethoxyphenylethynyl) benzene ( 3), 1,4-bis(4-carbomethoxyphenylethynyl) tetrafluorobenzene ( 4) and 9,10-bis(4-carbomethoxyphenylethynyl) anthracene ( 5), protected precursors to conjugated rigid rod bis(carboxylato) ligands. The compound 1,4-bis(4-carbomethoxyphenyl) butadiyne ( 6) was prepared by the oxidative homo-coupling of 1. The hydrolysis of 2 and 3 by NaOH in methanol gave the sodium salts of the respective carboxylate anions and subsequent treatment with HCl gave the corresponding free carboxylic acids. Compounds 2 - 6 were characterised by absorption, emission, IR, NMR and mass spectrometry and examined for liquid crystal phase behaviour by differential thermal analysis (DTA) and transmitted polarised light microscopy (TPLM). Compound 4 exhibited a nematic phase stable from 233.6 degreesC until decomposition at 320 degreesC. The single-crystal structures of 2, 5 and 6 were determined by X-ray diffraction at 110 - 160 K.