Synthesis of polymerizable liquid crystalline monomers and their side chain liquid crystalline polymers bearing azo-ester linked benzothiazole mesogen

A series of azo-ester linked benzothiazole mesogen containing new polymerizable liquid crystalline (LC) methacrylate monomers M1-M4 having different terminal substituents (H, CH3, OCH3, and OC2H5) on the benzothiazole moiety and their side chain liquid crystalline polymers (SCLCPs) P1-P4 were synthesized and characterized. The chemical structures, thermal stability, and LC phase behaviors of monomers and polymers were examined by the usage of different experimental techniques. All the synthesized monomers and polymers exhibited excellent thermal stability. Monomer M1 (without terminal substitution) displayed both nematic and smectic phases while monomers M2-M4 (CH3, OCH3, and OC2H5 substituted) showed only nematic phase. On the other hand, all the prepared polymers exhibited only nematic LC phase. Polymers P1-P4 showed two strong absorption bands in the range of 261-262 and 376-413 nm whereas polymers P1-P4 exhibited strong fluorescence emission in the range of 522-524 nm. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of the polymers were found to be -4.87 to -4.80 eV and -2.36 to -2.14 eV, respectively. The synthesized polymers could be potential candidate as fluorescent materials in the polymer light-emitting diode applications.