Synthesis and Helical Conformation of Novel Optically Active Liquid Crystalline Poly(<i>p</i>-phenylene)s Containing Cyanoterphenyl Mesogen as Pendant

Novel liquid crystalline poly(p-phenylene)s (PPP) bearing cyanoterphenyl mesogenic pendants with varying spacer lengths {-[C₆H ₃-COO-terphenyl-CN]_n-, P(0); -[C₆H ₃-COO-(CH₂)₆- O-terphenyl-CN]_n-, P(6)} were designed and synthesized, and the effects of structural variations on the liquid crystallinity behaviors and the optical properties of the polymers are studied. P(0) shows anSmAphase with faint birefringence, while P(6) readily forms the enantiotropic SmAd phase with a colorful fan texture, due to the longer flexible spacer favoring the better packing arrangements. Photoexcitation of their solutions induces strong blue light emission. The photoluminescence of P(6) in THF solution is much stronger and redshifted to the visible spectral region, even extending to 600 nm, than that of P(0). The longer spacer may have better segregated the bulky chromophoric pendants, which effectively hampers the excitons from traveling to the quenching sites of the backbone and hence enhances the stronger emission in the photoluminescence. No significant 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. Another interesting and outstanding property also could be found in this type of polymers. Due to the stric crowding, cyanoterphenyl mesogen pendants orientating around the main chain forces the main chain showing spiral conformation along the main chain in the long region; moreover, a short spacer is favorable.