Two homologous series of highly anisotropic diimines containing five and six aromatic rings in the mesogenic core derived from 1,4-terephthaldehyde and 4-formylphenyl-4-formylbenzoate have been synthesized and characterized by spectroscopic methods. The liquid-crystalline phases and the phase-transition temperatures are investigated using a polarizing microscope attached to a hot stage and differential scanning calorimetry. Most of the compounds are thermally stable and exhibit smectic and nematic phases over a large thermal range. In the majority of the compounds studied, a SmC phase exists over a very broad temperature range. The importance of the alkyl chain length in competing with the size of the aromatic core in determining the phase variant and in the increase of thermal range of the mesomorphism in a homologous series is described.