Syntheses and Mesomorphic and Luminescent Properties of Disubstituted Polyacetylenes Bearing Biphenyl Pendants

Liquid-crystalline and light-emitting poly(2-alkyne)s and poly(1-phenyl-1-alkyne)s containing biphenyl (Biph) cores, alkyl spacers, and alkoxy tails (-{RC=C[(CH 2 ) m -OCO-Biph-OC 7 H 15 ]} n -, where R, m = CH 3, 4 [P 1 (4)]; CH 3, 9 [P 1 (9)]; C 6 H 5, 4 [P 2 (4)]; and C 6 H 5, 9 [P 2 (9)]}, were synthesized and the effects of the spacer length and backbone structure on the properties of the polymers were investigated. The disubstituted acetylene monomers [ 1 ( m ) and 2 ( m ) ( m = 4, 9)] were prepared by esterification of biphenylcarboxylic acid ( 6 ) with ω-alkynyl alcohols [ 4 ( m ) or 5 ( m )]. Polymerizations of 1 ( m ) and 2 ( m ) were effected by WCl 6 −, MoCl 5 −, and NbCl 5 −Ph 4 Sn and the reactions catalyzed by WCl 6 −Ph 4 Sn under optimal conditions produce polymers P 1 ( m ) and P 2 ( m ) with high molecular weights ( M w up to 3.1 × 10 5 ) in good yields (up to 81%). The structures and properties of the disubstituted polyacetylenes were characterized and evaluated by nuclear magnetic resonance (NMR), thermogravimetry (TGA), differential scanning calorimetry (DSC), polarized optical microscope (POM), X-ray diffraction (XRD), ultraviolet spectroscopy (UV), and photoluminescence (PL) analyses. The polymers are resistant to thermolysis ( T d ≥ 360 °C) and all but P 2 (4) exhibit liquid crystallinity at elevated temperatures. The polymers with long methylene spacers, that is, P 1 (9) and P 2 (9), form a smectic A (SmA) mesophase with a monolayer arrangement. Upon excitation, strong UV and blue emissions peaking at 369 and 460 nm are observed in P 1 ( m ) and P 2 ( m ), respectively, whose quantum yields are found to increase with the spacer lengths.