Synthesis and Properties of Polyacetylenes Containing Terphenyl Pendent Group with Different Spacers

Liquid-crystalline, monosubstituted acetylenes containing terphenyl pendent group with varying spacer lengths [HC C(CH2)(n) O-terphenyl-CN, AnCN, n = 1, 6] and its polymers PAnCN were synthesized. Also, the effects of the structural variation on their properties, especially their mesomorphism, UV and photoluminescence behaviors, were studied. The monomers are prepared by simple Suzuki coupling reactions. High yields and high trans-content of polyacetylenes were obtained by polymerization using [Rh(nbd)Cl](2) catalyst. The polymer with long spacers (hexamethylene), that is, PA6CN, formed a nematic mesophase when heated and cooled, however the polymer with short spacers (methylene), that is, PA1CN, could not exhibit liquid crystallinity at elevated temperatures. Upon photoexcitation by keep the constant photons of excitation, PA1CN emits ultraviolet emission (lambda(max) similar to 311 nm) but PA6CN gives a stronger emission (lambda(max) similar to 308 nm), indicating that the emission intensity increases with the spacer length due to orientation of main chain by alignment of mesogenic pendant. Short spacer between polyacetylene and mesogens hindered alignment of the main chain, consequently lower fluorescence emission, and loss mesophase of the polymer was induced by short spacer due to disturbance of rigid main chain.