H-shaped mesogenic dimers containing polar -NO₂/-Cl terminus

Two homologous series of H-shaped symmetrical dimers were synthesised and their thermotropic properties studied by differential scanning calorimetry and on the hot-stage of a polarising microscope. These compounds consist of two mesogenic units of azoester interconnected through flexible spacers (n = 4) resulting in the structure of ‘H-shaped’ dimeric compounds. The difference between the two series is in the structure of the terminal substituents (-NO₂ for series I and -Cl for series II) attached on the azoester mesogens at one terminus. All these compounds were found to be smectogenic. The effect of different terminal substituents on mesomorphism is discussed. The trans–azobenzene groups of the H-shaped dimeric compounds display a high-intensity π–π* transition at about 370 nm and a low-intensity π–π* transition at around 460 nm; therefore, photochromism can be achieved by the introduction of the azo linkage to these H-shaped dimeric compounds.