Molecular Design And Characterization Of Liquid Crystalline Conjugated Polymers

Combining liquid crystallinity and luminescence into the polymer, liquid crystalline conjugated polymers (LCCP) are currently drawing interest from the viewpoint of multifunctional electrical and optical materials. The introduction of orientable mesogenic moieties onto a conjugated polymer backbone might increase the conjugation in the main chain and the spontaneous orientation and externally forced alignment of the mesogens enable us to control electrical and optical properties and to obtain the materials with excellent properties. The terphenyl is not only a chromophore but also a mesogenic core. Thus, in this paper, terphenyl mesogenic group was introduced onto different conjugated main chain, such as polyacetylene, polythiophene and poly(p-phenylene), to obtain novel LCCP containing terphenyl mesogenic pendant. The effects of the structural variation on their properties, especially their mesomorphism and photoluminescence behaviors, were studied.Novel acetylene monomers containing cyanoterphenyl groups, namely, 4-[(4'-cyano-4-terphenylyl)oxy]-1-butyl-1-butyne A(bu)TPhCN and 3-[(4'-cyano-4-terphenylyl)oxy]-1-phenyl-1-proyne A(Ph)TPhCN were synthesized. A(bu)TPhCN was polymerized with WCl₆/PhSn₄ catalyst successfully to give the liquid crystalline conjugated disubstituted polyacetylene containing cyanoterphenyl mesogenic pendant PA(bu)TPhCN. Polymerization of monomer A(Ph)TPhCN with phenyl substitute did not obtain any product. The results indicate that the stereoeffect of the bulky cyanoterphenyl group and phenyl seems to inhibit the reaction. The structures and properties of the disubstituted polyacetylene and monomers were characterized and evaluated with nuclear magnetic resonance (¹H NMR), infrared spectroscopy (FT-IR), X-ray diffraction (XRD), differential scanning calorimetry (DSC), polarized optical microscopy (POM), ultraviolet spectroscopy (UV), and photoluminescence (PL). The monomers show enantiotropic smectic phases in the heating and cooling processes, while the polymer PA(bu)TPhCN exhibits a nematic phase, due the coupling between the mesogen and main chain. The existence of the chromophoric cyanoterphenyl core endows the monomers with strong UV absorption and high photoluminescence, and the polymer PA(bu)TPhCN can emit a strong light of 411 nm.Novel disubstituted propiolates [C₅H₁₁C≡CCOO-terphenyl-R; R=CN, A(COO)TPhCN; R=OCH₃, A(COO)TPhOMe] bearing chromophoric terphenylene mesogenic groups with different terminal groups were synthesized by Suzuki coupling and esterification, where the terphenyl groups were connected to the C≡C through ester linkage directly. The polymerization and properties of the monomers were investigated. Using transition-metal catalysts such as the classical MoCl₅- and WCl₆-based metathesis catalysts, the polymerization of the monomers were carried out in a series of different solution, however, did not obtain any products. It suggests that the WCl₆- and MoCl₅- based catalysts are poisoned by the polar groups, on the other hand, the bulk terphenyl groups and the long alkyl chain around the C≡C bond might inhibit the reaction. The monomer with CN terminal group displays monotropic nematicity, whereas the one with OCH₃ terminal group exhibits enantiotropic nematicity and smecticity (SmA_d) with a bilayer arrangement. Ultraviolet spectroscopy and photoluminescence measurements also show that the terphenyl groups endow disubstituted propiolates with strong UV light absorption and high photoluminescence. The polymers with donor-acceptor pairs luminescent more strongly than those without such push-pull pairs.Series of polythiophenes containing terphenyl mesogenic side chain at the 3rd position with different terminal groups and spacer length {-[Thiopheneyl-CH₂COO-terphenyl-R]_n-; R=CN, PT(0)TPhCN; R=OCH₃, PT(0)TPhOMe; -[Thiopheneyl-CH₂COO-(CH₂)₆-O-terphenyl-CN]_n-, PT(6)TPhCN} were synthesized. The structures and properties of the monomers and polymers were characterized and evaluated. All of the polymers are stable, losing little of their weights when heated to≥300℃. The polymers show good solubility and can be dissolved in common solvents. The monomers all exhibit enantiotropic mesogen phases, while the polymer PT(0)TPhOMe and PT(0)TPhCN with short spacers could not exhibit liquid crystallinty at elevated temperature. With the space length increasing, the PT(6)TPhCN shows enantiotropic SmA_d phase, due to the longer spacer decreases the coupling between mesogen and main chain and favors the better packing arrangements. The existence of the chromophoric terphenyl core endows the polymers with high photoluminescence. Furthermore, push-pull pairs and the longer spacer favor the enabling light emission.Novel liquid crystalline poly(p-phenylene)s bearing cyanoterphenyl mesogenic pendants with varying spacer lengths and terminal groups{-[C₆H₃-COO-terphenyl-CN]_n-, PP(0)TPhCN; -[C₆H₃-COO-(CH₂)₆-O-terphenyl-R]_n-; R=CN, PP(6)TPhCN; R=OCH₃, PP(6)TPhOMe} were designed and synthesized, and the effects of structural variations on the liquid crystallinity behaviors and the optical properties of the polymers were studied. All the monomers and polymers show enantiotropic smectic phases, even including the polymer PP(0)TPhCN with short spacers. Photoexcitation of their solutions induces strong blue light emission. The PL of PP(6)TPhCN is much stronger and red-shifted to the visible spectral region, even extending to 600 nm, than that of PP(0)TPhCN, because the longer spacer may have better segregated the backbone and hence enhances the stronger emission in the photoluminescence. Push-pull pairs also exert influence on the photoluminescence. Another interesting and outstanding property also could be found in this type of polymers. Due to the stric crowding, without introducing any chiral center, terphenyl mesogen pendants orientating around the main chain forces the main chain showing spiral conformation along the main chain in the long region and a short spacer is favorable.Furthermore, we have attempted to synthesis a novel Mesogen-jacketed liquid crystalline conjugated polymer—polythiophene with directly attached bis(4-methoxyphenyl)terephthalate mesogen as pendant at the centrobaric position using FeCl₃ as the catalyst, CHCl₃ as the solvent. The structures and properties of the monomer and polymer were characterized and evaluated. The result indicates that the polymer has the expected structure, and the polymer is a random polymer with high thermal stability. Upon photoexcitation, the polymer can emit strong yellow light at 550nm. Different from the Mesogen-jacketed liquid crystalline polymers with flexible main chain, due to the strong interaction between the mesogen side chain and backbone, the polymer with rigid backone could not show any optical anisotropy, although its monomer possesses nematic phase.