Liquid Crystalline Properties of Dimesogenic Compounds Consisting of Two Different Azobenzene Types of Mesogens Interlinked Through Central Polymethylene Spacers

A series of new dimesogenic compounds, S(+)-2-methylbutyl 4-[-(4-butylphenylazophenoxy)polymethyleneoxy] phenylazobenzoates, were synthesized and characterized for their liquid crystallinity and thermal transitions. The compounds are consisting of two different azobenzene mesogens interlinked through the central oxypolymethyleneoxy (oxytetramethyleneoxy through oxyoctamethyleneoxy) spacers. One of the two azobenzene mesogenic units carries the S(+)-2-methylbutyl ester tail group and the other the n-butyl tail. In other words, one of the mesogenic units is chiral and the other is not. Only the compounds of evennumbered polymethylene spacers form mesophases enantiotropically, whereas those of odd numbered spacers are monotropic. The compounds of penta- and octamethylene spacers exhibit only the cholesteric phase, while the rest show the SA phase in addition to the cholesteric phase. A racemic mixture containing hexamethylene spacers forms the SA and nematic phases enantiotropically. This series reveals a typical odd-even effect in the melting (Tm) and isotropization temperatures (Tj) and also in other themodynamic parameters for phase transitions. Differential scanning calorimetry, wide- and small-angel X-ray diffractometry and opitical microscopy are the principle methods utilized in the characterization of the present series of compounds.