A novel type of optically active helical liquid crystalline polymers: Synthesis and characterization of poly(p-phenylene)s containing terphenyl mesogen with different terminal groups

Series of poly(p-phenylene)s (PPPs) containing terphenyl mesogenic pendants with cyano and methoxy terminal groups by flexible -COO(CH₂)₆Obridge [P(CN) and P(OCH₃)] are synthesized through Yamamoto polycondensation with Ni-based complex catalysts. The effects of the structural variation on their properties, especially their mesomorphism, ultraviolet-visible (UV), and photoluminescence behaviors, are studied. All of the polymers are stable, losing little of their weights when heated to >340 °C. The polymers show good solubility and can be dissolved in common solvents. P(CN) with cyano terminal group shows enantiotropic SmA_dphase with bilayer packing arrangement, while P(OCH₃) with methoxy terminal group readily forms nematic and SmA_dphase when heated and cooled. Photoexcitation of their solutions induces strong blue light emission. Compared with P(OCH₃), the light-emitting bands of polymer P(CN) is slightly redshifted to 428 nm and the emission intensity of P(CN) is much stronger, due to the existence of donor-acceptor pairs. More interestingly, both of the polymers exhibit obvious Cotton effect on the CD spectra, resulting from the predominant screw sense of the backbone. This indicates that the bulky mesogenic pendant orientating around the backbone will force the main chain with helical conformation in the long region due to steric crowdedness.