Non-conventional three-armed star-shaped mesogens based on 1,3,5-trisubstituted benzene with azobenzene moieties at the periphery: synthesis, and mesomorphic behaviour

A series of new symmetrical trimeric star-shaped mesogens in which the molecular architecture composed of three 4-[(4-substituted-phenyl)diazenyl]phenoxy-6-bromohexane connected as the pheripheral units to 1,3,5-positions of the benzene core group have been synthesised and fully characterised by physical measurements and spectroscopic techniques (carbon, hydrogen and nitrogen microanalysis, Fourier transform-infrared spectroscopy and nuclear magnetic resonance: CHN, FT-IR and NMR). At one end of the para-position of the azobenzene fragment consists of different terminal substituents X=F, Cl, Br, I, C2H5 and OC2H5. The thermal behaviour and mesomorphic properties of the star-shaped compounds were investigated by means of differential scanning calorimetry, polarizing optical microscopy (POM) and X-ray diffraction analyses. Moreover, the liquid-crystalline properties of all the intermediates 4-[(4-substituted-phenyl)diazenyl]phenoxy-6-bromohexane were also evaluated. Whilst all the star-shaped compounds exhibit nematic and smectic A phases, the mesogens with chloro- and bromo-terminal substituents were found to show soft crystalline phase. The smectic phase and the possible molecular arrangement of the star-shaped molecule within the anisotropic region were further substantiated by the XRD. In addition to a low-angle sharp peak (reflecting the layered structure), a diffuse wide-angle peak indicating the liquid-like order within the smectic layer was recorded. A comparative study between the properties of the present azo mesogens and the earlier reported star-shaped mesogens from Schiff bases is also discussed.