Liquid crystalline and light emitting polyacetylenes: Synthesis and properties of biphenyl-containing poly(1-alkynes) with different functional bridges and spacer lengths

Biphenyl-(Biph-) containing 1-alkynes (3 and 4) and their polymers (1 and 2) with varying bridge groups and spacer lengths were synthesized and the effects of the structural variation on their properties, especially their mesomorphism and photoluminescence behaviors, were studied. The acetylene monomers 3(3) [HC≡C(CH₂)₃O-Biph-OCO(CH₂)₁₀CH ₃] and 4(m) [HC≡C(CH₂)_m OCO-Biph-OCO(CH₂)_10-CH₃, m = 3,4] were prepared by sequential etherization and esterification reactions of 1-alkynes. While 3(3) exhibits enantiotropic crystal E and SmB mesophases, its structural cousin 4(3) displays only monotropic SmB phase. Enantiotropic SmA and SmB mesophases are, however, developed when the spacer length is increased to 4. Polymerizations of the monomers are affected by Mo-, W-, Rh-, and Fe-based catalysts, with the WCl₆-Ph₄Sn catalyst giving the best results (isolation yield up to 85% and M_w up to 59000). The polymers were characterized by IR, UV, NMR, TGA, DSC, POM, XRD, and PL analyses. Compared to 1(3), 2(3) shows a red-shifted absorption, a higher T_i, and a better packed interdigitated bilayer SmA_d structure, while the mesophase of 2(4) involves mionolayer-packing arrangements of the mesogens. Upon photoexcitation, 1(3) emits almost no light but 2(m) gives a strong ultraviolet emission (λ_max ∼ 350 nm), whose intensity increases with the spacer length.