H-shaped dimeric LC compounds:: synthesis and thermotropic properties of α,ω-bis[2,5-bis(4-ethoxyphenoxycarbonyl)phenoxy]alkanes

A series of new dimeric liquid crystalline compounds were synthesized and their thermotropic properties studied by differential scanning calorimetry and observation of their melts by polarizing microscopy. These compounds consist of two bis(p-ethoxyphenoxy) terephthalate units interconnected through an oxypolymethyleneoxy spacer on the central terephthaloyl units resulting in a structure of 'H-shaped' dimeric twin compounds. The length of spacers was varied from oxytetramethyleneoxy (n = 4) to oxyoctamethyleneoxy (n = 8). The compound containing the oxydodecamethyleneoxy (n = 12) was also included in this study. With the exception only of the dodecamethylene compound, all of the present compounds are monotropically nematic. In contrast, the compound with the oxydodecamethyleneoxy spacer is enantiotropically nematic. It was found that the heats (Delta H-m) of melting of the monotropic compounds are exceptionally high, 70 similar to 100 kJ mol(-1). The Delta H-m value of the compound having the oxydodecamethyleneoxy spacer, however, is much less, about 17 kJ mol(-1). It is concluded that 'H-shaped' compounds possess less tendency to be thermotropic and also that they tend to vitrify when the spacer length exceeds 6.