Oligomeric liquid crystals: From monomers to trimers

A homologous series of linear liquid crystal trimers, the 4,4'-bis[omega-(4-methoxyazobenzene-4'-yloxy)alkoxy]azobenzenes, has been synthesized and characterized. The transitional properties of the trimers are compared with those of the corresponding series of dimers, the alpha,omega-bis(4-methoxyazobenzene-4'-oxy)alkanes, and monomers, the 4-methoxy-4'-alkoxyazobenzenes. Characteristically pronounced odd-even effects were seen for the transitional properties of both dimers and trimers on varying the spacer lengths. The clearing temperatures of the trimers were higher than those of the corresponding dimers, but as the length of the flexible spacers was increased this difference became rather small. The ratios of T-NI, and DeltaS(NI)/R for monomer:dimer and dimer:trimer are discussed. These are very similar to reported values for similar materials, suggesting that there may be a rather general relationship between the transitional properties of liquid crystal oligomers as the number of mesogenic units is increased.