Triazine-based Dendrimers as Liquid Crystals: Synthesis and Characterization.

Different generations of dendritic macromolecules based on triazine were synthesized by the divergent growth approach and functionalization with mesogenic units based on peripherally located dihydroxybiphenyl derivatives. Thermotropic liquid crystal dendrimers, containing poly(ethylene glycol) (PEG) as a core and triazine dendrons with peripherals mesogenic groups were synthesized. First generation (G1) was prepared by coupling of hydroxyl group of PEG with cyanuric chloride in dichloromethane at 0-5°C. Reaction of compound G1 with amino group of ethanolamine in dichloromethane resulted G1.5. Second generation of linear dendritic compounds (G2) was synthesized using coupling reaction of hydroxyl groups of compound G1.5 and cyanuric chloride in dichloromethane. Reaction of compound G2 with amino group of ethanolamine in water/dioxane resulted G2.5. The growth of dendrons on the PEG core and their structures were investigated using common spectroscopy methods. In the next step, compounds Cn with different alkyl tail groups were synthesized via nucleophilic displacement of bromine from 1-bromoalkane by potassium-4,4 -biphenoxide in DMF. The mesogenic compounds 4-bromo-(4-alkyloxybiphenyl- 4 -oxy) butane (CnC4Br) were synthesized by reaction with Cn and 1,4-dibromobutane undergoing SN2 substitution. Liquid crystalline dendrimers (LCD) were synthesized via coupling of hydroxyl group of G2.5 and bromine from mesogenic units, 4-bromo-(4-alkyloxy-biphenyl-4 -oxy) butane. The liquid crystal property of mesogenic compound has been studied in details with differential scanning calorimetry and optical polarization microscopy. The experiments showed that the thermal properties are influenced with the spacer length.