Design, synthesis and characterization of a linear hydrogen bonded homologous series exhibiting reentrant smectic C ordering

Design, synthesis and characterization of seven linear hydrogen bonded liquid crystal complexes derived from mesogenic p–n-decyloxy benzoic acid and p–n-alkyl benzoic acids designated as 10OØn (where n varied from ethyl to octyl) are reported. FTIR studies confirm the hydrogen bond formation in all these complexes. The phase transition temperatures and their corresponding enthalpy values are experimentally deduced from Differential Scanning Calorimetry (DSC) studies. POM and DSC data are further utilized for the construction of 10OØn phase diagram. Two Odd–even effects have been evinced, one in enthalpy values and the other in corresponding transition temperatures across the isotropic to nematic phase transition. An interesting result is the observation of re-entrant smectic ordering, designated as smectic CR in three higher ordered mesogens. A new smectic ordering, smectic X, has been observed which is sandwiched between traditional smectic C and re-entrant smectic CR. Magnitudes of optical tilt angle in smectic C, smectic X and smectic CR are experimentally found to attain saturation with decrement of temperature in the corresponding phase. The occurrence of smectic X and smectic CR are discussed with relation to the molecular chemical structure. The optical filtering action in smectic C and re-entrant smectic CR phases have been analyzed.

Graphical abstractA novel series of hydrogen bonded monocomponent complexes have been isolated. Optical tilt angle and the filtering action in sequential smectic phases are discussed .Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Odd–even effect is distinct at isotropic to nematic transition temperatures. ► Even in enthalpy values of isotropic to nematic odd–even effect is noticed. ► Re-entrant phenomenon in smectic ordering is characterized. ► Temperature dependence of optical tilt angle is elucidated in all smectic phases. ► Filtering action of all smectic orderings are compared.