Liquid crystallinity and enhanced photoluminescence of terphenyl-containing poly(1-alkynes) with tuning spacers and tails

Novel liquid-crystalline, monosubstituted acetylene containing terphenyl pendant group with different spacer lengths and tails [HC≡C(CH2)mO-terphenyl-OPr, AmOPr, m = 6.8], [HC≡C(CH2)6O-terphenyl-CN, A6CN] and their corresponding polymers PAmOPr and PA6CN were synthesized. Also, the effects of the structural variation on their properties, especially on their mesomorphism and optical behaviors, were studied. The monomers are prepared by simple Suzuki coupling reactions. High yields and high trans content (>85%) of polyacetylenes were obtained by polymerization using [Rh(nbd)Cl]2 catalyst. The monomers A6CN and A6OPr exhibit SmA and SmAd + SmA mesogenic phase, respectively, whereas their corresponding polymer PA6CN shows nematic phase and PA6OPr possesses SmA. It indicates that flexible alkoxy tail favoring the better ordering of the mesogenic groups. None of the monomers shows any peaks at wavelengths longer than 350 nm, the polyene backbone absorptions of the PA6CN are very weak. The absorptions of the PA6OPr backbone are, however, observed at wavelengths longer than 360 nm and extend up to ∼570 nm, probably owing to the more order smectic phase allowing backbone to be more coplanar with a better conjugation. Upon excitation, strong blue emissions peaking at 416 were observed in PA6CN (ΦF = 64% when excited at 332 nm in solution). The emission efficiency was dramatically affected by the polymer structures. PAmOPr, whose quantum yields are found to increase with the spacer lengths. No significant wavelengths shifts in the emission maxima are observed when the polymers are fabricated into thin films, suggesting that the segregation of the backbone effectively decreases the strong interchain interaction. All the polymers were CD active, which may due to the steric crowding, terphenyl mesogen pendants orientating around the main chain forces the main chain showing spiral conformation along the main chain in the long region.