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All the starting materials required for the synthesis were produced from sigma-Aldrich Company and used as received without further purification. Most the intermediate compounds in this study were prepared according to the procedures in our references [20] and [21] with a few simple modifications. The structures of all intermediates and more of final products were characterized by spectroscopic methods. FT-IR spectra of solid sample in ATR technique were recorded on Shimadzu 8400S, 1 H NMR spectra were obtained with Bruker spectrophotometer model ultrashield at 300 MHz using tetramethylsilane (TMS) as interval standard and CDCl3, DMSO-d6 as solvents. Elemental analysis for carbon, hydrogen and nitrogen were conducted using a Perkin-Elmer model 2400 instrument. Mass spectra of some compounds were recorded on Shimadzu QP1000EX using (SCI) mode. Investigation the liquid crystals properties of the final compounds were carried out by polarized optical microscopy (POM) model PW-BK 5000 PR equipped with a hot-stage system of HS-400 (KER 3100-08S). Phase transition temperatures were recorded by differential scanning calorimetry (DSC) model Linseis instrument STA PT-1000, Ramp rate: 5 °C/ min. on heating and cooling, temperature and heat flow calibrated with standard indium with heating and cooling rates of 5 °C/min -1 . Synthesis Methyl p-toluate [A]This compound was prepared by classical esterification method described by Chiang et al. [22].

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Two series of ten ester derivatives with four aromatic rings linked together by three different linkages (azo/ester/imine), 4-((4-substituted)diazenyl)phenyl 4-((4alkoxybenzylidene)amino)benzoate (IV n X=OCH 3 and V n X=Br) were synthesized. These derivatives have an alkoxy chain in the para position of the phenyl ring with different lengths (n = 3, 4, 6, 7, and 12) on one terminal side while the other side was substituted by methoxy and bromo groups linked to the pposition of phenyl. The structures of all synthesized derivatives were proven by nuclear magnetic resonance ( 1 H-NMR, 13 C-NMR), elemental analysis (C,H,N), and Fourier-transform infrared spectroscopy (FT-IR). The Mesophase textures of the final esters were observed by polarized optical microscopy (POM) and differential scanning calorimetry (DSC). Only the nematic texture was observed in the methoxy derivatives while nematic and smectic A textures were shown in the bromo mesogens. All the homologous showed mesogenic behavior with wide thermal stability. From the results, it was found that the liquid crystalline behaviors of these derivatives are dependent on the substituted group (-X) and the length of the alkoxy chain. The mesogenic behavior of these compounds was compared with other mesogens that have similar structures.

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