Investigations on liquid crystalline partially fluorinated ‘one-chain’ and ‘two-chain’ benzoic acids

Abstract

The mesomorphic properties of perfluoroalkyl alkyloxy-substituted benzoic acids with one (K1S) and two (K2S) partially fluorinated alkyl chains are presented. These classes of compound were investigated by means of differential scanning calorimetry, polarizing optical microscopy, X-ray diffraction of non-oriented samples and, in one case, temperature-dependent IR spectroscopy. Because of dimerization via H-bonding, the presence of carboxy groups leads to an enhanced thermodynamic mesophase stability with respect to the corresponding alkyl esters. The ‘one-chain’ acids K1S form SmC phases, while the ‘two-chain’ acids K2S exhibit hexagonal columnar phases (Col_h). In the case of one-chain compounds, fluorination leads to the suppression of nematic phases. Furthermore, elongation of the fluorinated chain segments causes a reduction of the mesophase range, due to an increased crystallization tendency of these chains. In the case of two-chain acids K2S, an increased thermodynamic stability of the columnar phases can be observed on increasing the length of the perfluoroalkyl segments. Here the thermal behaviour of the compounds is also sensitive to the non-fluorinated alkyl chain length.