Orientation Effect And Modulation Of Terminal Groups, Spacers And Substituents For Liquid Crystalline Polyacetylenes Containing Terphenyl Pendants

Liquid crystalline conjugated polymers have been extensively studied for their intriguing applications and stimulate technological innovations in the development of novel electronic and photonic devices such as liquid crystal displays (LCD), light-emitting diodes (LED), photovoltaic cells, film transistors and plastic lasers. Polyacetylene is an archetypal conjugated polymer. Attachment of liquid-crystalline groups onto the polyacetylene backbone, the polymer is not only soluble in organic solvents, but also easily aligned by spontaneous orientation of the liquid-crystalline group.The terphenyl is not only a light-emitting chromophore but also a mesogenic core. Thus, if combining optically active terphenyl with electronically active polyacetylenes at the molecular level, the spontaneous orientation and chromophoric property of the terphenyl mesogens might likely lead to materials with novel, outstanding and interesting properties.Liquid-crystalline, monosubstituted acetylenes containing terphenyl pendant group with varying spacer lengths [HC≡C(CH2)nO-terphenyl-CN, AnCN, n=1,6] and its polymers PAnCN were synthesized, what's more, the effects of the structural variation on their properties, especially their mesomorphism, UV and photoluminescence behaviors, were studied. The monomers are prepared by simple Suzuki coupling and etherification reactions. High yield (>90%) and high trans-content (>95%) of polyacetylenes were obtained by polymerization using [Rh(nbd)Cl]2 catalyst. The polymer with long spacers (hexamethylene), that is, PA6CN, formed a nematic mesophase when heated and cooled, however the polymer with short spacers (methylene), that is, PA1CN, could not exhibit liquid crystallinity at elevated temperatures. Upon photoexcitation, PA1CN emits moderate ultraviolet emission (λmax～311nm) but PA6CN gives a very strong emission (λmax～308 nm), indicating that the emission intensity increases with the spacer length due to orientation of main chain by alignment of mesogenic pendant. Short spacer between polyacetylene and mesogens hindered alignment of the main chain, consequently lower fluorescence emission, and loss mesophase of the polymer was induced by short spacer due to disturbance of rigid main chain.Novel liquid-crystalline, monosubstituted acetylene containing terphenyl pendants with different spacer lengths and tails [HC≡C(CH2)mO-terphenyl-OPr, AmOPr, m=6,8], [HC≡C(CH2)60-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. 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 was observed in PA6CN(Φ=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.Poly(1-alkyne)s bearing terphenyl liquid crystalline pendant and containing octoxy spacer-{[HC=C(CH2)gO-terphenyl-CN]}n-(PA8CN) was synthesized, and the effects of structural variations on the optical properties, especially liquid crystallinity behaviors of the polymer and monomer were investigated. The monomer was prepared in high yields (92.3%) by coupling and etherification reactions and successfully polymerized by Rh catalyst and Et3N cocatalyst in high yields with trans-rich (86.27%) stereoregular structure. Compared to the monomer showing mixed monolayer structure, its corresponding polymer PA8CN forms homogeneous monolayer arrangement. Upon photoexcitation, strong blue emissions peaking was exhibited at 413 nm in PA8CN (ΦF=68% when excited at 330 nm in THF solution). Compared with its solid film when photoexcitated at 330 nm at room temperature, the UV light-emitting band of PA8CN solid film quenched rapidly with liquid nitrogen from their liquid crystalline state at 236℃was red-shift 10nm with higher intensity, it may be ascribed to that the effectual conjugation and the order degree of the molecular in the liquid crystalline state of the polymer were enhanced, the energy loss caused by the thermal vibration was decreased, thus resulting in the observed hyperchromic effect.Furthermore, liquid-crystalline and light-emitting poly(2-alkyne)s containing terphenyl cores with hexamethyleneoxy spacers, and cyano or n-propoxy tails-{[CH3C=C(CH2)6O-terphenyl-R]}n-where R=CN, CH3PA6CN, R=OCH2CH2CH3, CH3PA6OPr, were synthesized and the effects of the substituents and terminal groups on the properties, especially the mesomorphic and optical properties of the polymers, were investigated. The disubstituted acetylene monomers (CH3A6CN, CH3A6OPr) were prepared through multistep reaction routes and were polymerized by WCl6-Ph4Sn in good yields (up to 82%). All the monomers and CH3PA6CN exhibited enantiotropic SmA phase with a monolayer arrangement at elevated temperatures, whereas CH3PA6OPr formed a bilayer SmAd packing arrangement. Upon excitation at 330nm, strong UV and blue emission peaks at 362 and 411 nm were observed in CH3PA6OPr and CH3PA6CN, respectively. The luminescent properties of CH3PA6CN and CH3PA6OPr have been improved by introducing the methyl substituted group, and the quantum yield of the polymer with cyano tail CH3PA6CN (Φ=74%) was found to be higher than that of CH3PA6OPr (Φ=60%). Compared to polyacetylene parent, both CH3PA6OPr and CH3PA6CN showed a narrower energy gap.