A series of symmetrical chiral, liquid crystal dimeric molecules possessing ester-linked, biphenyl-naphthyl cores with varied spacer\r\nlengths and terminal vinyl groups have been synthesized. (S)-(+)-2-(6-Methoxy-2-naphthyl)propionic acid was used as the synthetic\r\nprecursor to achieve the target dimers. The synthesized symmetrical chiral dimers were characterized by 1H NMR spectroscopy, and\r\ntheir liquid crystalline behavior was confirmed by DSC and HOPM studies. Structural effects on the mesomorphic and\r\nphysicochemical properties were investigated in terms of variation of chiral chain length. The synthesized dimeric compounds\r\nexhibited SmX*, SmC*, SmA*, N*, BPI*, and BPII* mesophase sequences. An odd-even effect was observed in which the dimers\r\nwith an even number of spacers generated SmX*, SmC*, SmA*, N*, BPI*, and BPII* phases whereas dimers with an odd spacers\r\nexhibited SmX*, SmA*, N*, BPI*, and BPII* phases, and the duration of the mesophase decreased with increasing spacer length.\r\nThe differences in transitional properties for odd- and even-numbered dimers could be attributed to the difference in conformation of\r\nthe molecules of the liquid crystalline dimers. When the number of methylene units in the bridging chain (spacer) is odd, then the\r\nspacer will be in its cis conformation, which causes the two aromatic units to be tilted in the same direction with respect to eachother\r\n(Fig. 1A). In the even membered dimers since the aromatic units are tilted in the opposite directions, a bent structure is produced\r\nwhich when packed with other dimers together will lead to a zig-zag layer-like structure. In general, the trans conformation (Fig.\r\n1B) always leads to layer regularity and promotes the crystal-crystal polymorphism. This kind of layer formation may assist the\r\nformation of SmC* phase. However, the cis conformation destroys the regularity and as well as the polymorphism in the liquid\r\ncrystal melt. The synthesized vinyl substituted liquid crystalline dimers are particularly useful in understanding liquid crystal\r\npolymorphism and act as model compounds for liquid crystal polymers.
