Fused state 13C-NMR study on liquid crystalline polyacetylene derivatives — magnetically forced alignment and orientational behavior

Magnetically forced alignments and orientational behaviors of a series of side-chain liquid crystalline polymers, poly(monosubstituted acetylenes), as well as a liquid crystalline monomer, have been investigated through the fused-state 13C-NMR measurements with proton dipolar decoupling. Upon a phase transition from isotropic to nematic or smectic phase, aromatic carbon signals notably shifted to the lower magnetic field, whereas aliphatic carbon signals slightly shifted to the higher magnetic field. This indicates that in the nematic or smectic phase, the side-chain liquid crystal group is preferentially aligned along the direction of magnetic field. Analyses of chemical shift tensors were also carried out to evaluate an order parameter and shielding anisotropy in liquid crystalline phase. We demonstrated that the liquid crystalline conjugated polymers are uniaxially aligned due to the magnetically forced alignment of the liquid crystalline side-chain, giving rise to a mono-domain structure.