Syntheses and mesomorphic and luminescent properties of disubstituted polyacetylenes bearing biphenyl pendants

Liquid-crystalline and light-emitting poly(2-alkyne)s and poly(-phenyl-1-alkyne)s containing biphenyl (Biph) cores, alkyl spacers, and alkoxy tails (-{RC=C[(CH ₂) _m-OCO-Biph-OC ₇H ₁₅]} _n-, where R, m = CH ₃, 4 [P1(4)]; CH ₃, 9 [P1(9)]; C ₆H ₅, 4 [P2(4)]; and ₆H ₅, 9 [P2(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 ₄Sn and the reactions catalyzed by WCl ₆-Pl ₄Sn under optimal conditions produce polymers P1(m) and P2(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 P2(4) exhibit liquid crystallinity at elevated temperatures. The polymers with long methylene spacers, that is, P1(9) and P2(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 P1(m) and P2(m), respectively, whose quantum yields are found to increase with the spacer lengths.