Effects of substitution and terminal groups for liquid-crystallinity enhanced luminescence of disubstituted polyacetylenes carrying chromophoric terphenyl pendants

Liquid-crystalline and light-emitting poly(2-alkyne)s containing terphenyl cores with hexamethyleneoxy spacers, and cyano or n-propoxy tails −“[CH3C=C(CH2)6O-terphenyl-R]”n−, where R=CN, CH3PA6CN, R=OCH2CH2CH3, CH3PA6OPr, were synthesized. The effects of the substitution and terminal groups on the properties, especially the mesomorphic and optical properties of the polymers, were investigated. The disubstituted acetylene monomers (CH3A6CN, CH3A6OPr) were prepared through multistep reaction routes and were polymerized by WCl6-Ph4Sn in good yields (up to 82%). All the monomers and CH3PA6CN exhibited the enantiotropic SmA phase with a monolayer arrangement at elevated temperatures, whereas CH3PA6OPr formed a bilayer SmAd packing arrangement. Upon excitation at 330 nm, strong UV and blue emission peaks at 362 and 411 nm were observed in CH3PA6OPr and CH3PA6CN, respectively. The luminescent properties of CH3PA6CN and CH3PA6OPr have been improved by introducing the methyl substituted group, and the quantum yield of the polymer with cyano tail CH3PA6CN (Φ = 74%) was found to be higher than that of CH3PA6OPr (Φ = 60%). Compared to polyacetylene parents, both CH3PA6OPr and CH3PA6CN showed a narrower energy gap. This demonstrated that the electrical conductivities of polyacetylenes could be enhanced by attaching appropriate pendants to the conjugated polyene backbones.