Infrared spectral studies of triphenylene mesogens possessing terminal functional groups in the peripheral chains for hydrogen-bond interaction

Temperature dependence. of infrared spectra for three homologues of the triphenylene mesogens with two terminal carboxylic acids was measured. These mesogens show Col(h) mesophase. These results revealed the existence of some hydrogen bonding interactions in the isotropic, mesomorphic as well as solid phases. The increase of the dimeric C=O (ca. 1712cm(-1)) and the decrease of the monomeric C=O (ca. 1740 cm(-1)) were observed at the isotropic to Col(h) phase transition on cooling. The absorption intensity of the dimeric C=O stretching band in Col(h) phase is larger than that in the isotoropic phase. It was indicated that for the explanation of Col(h) thermal stability for these homologues molecular rotational fluctuation around the columnar axis in Col(h) is significantly suppressed by the hydrogen bonding interaction and the suppression depends on the spacer length in their homologues, considering that the thermal stability of Col(h) phase is related to both the rotational fluctuation and the instability of hexagonal packing of molecules and these could be operated by hydrogen bond interaction.