Synthesis and properties of polyacetylenes with directly attached bis(4‐alkoxyphenyl)terephthalate mesogens as pendants

Abstract

Liquid‐crystalline, monosubstituted polyacetylenes containing lateral pendants of bis(4‐alkoxyphenyl)terephthalate with no flexible spacers and alkoxy tails {RO, where R is CH3 [P(1)] or C6H13 [P(6)]} were synthesized, and the effects of the backbone structure and alkoxy tails on the properties of the polymers were investigated. The polymerizations of acetylene monomers were carried out with chloronorbornadiene rhodium(I) dimer as a 1,2‐insertion catalyst in toluene. The structures and properties of the monosubstituted polyacetylenes were characterized and evaluated with nuclear magnetic resonance, infrared spectroscopy, thermogravimetry, differential scanning calorimetry, polarized optical microscopy, ultraviolet spectroscopy, and photoluminescence analyses. The molecular weights of the polymers were measured by gel permeation chromatography. The polymer with long tails (p‐hexyloxy), that is, P(6), formed a smectic mesophase upon heating above the melting temperature, but the other one with short tails (p‐methoxy), that is, P(1), could not exhibit liquid crystallinity at elevated temperatures. The steric effect of bulky, liquid‐crystalline mesogens and a direct connection with the main chain prevented the planar conformation of the polyene backbone and, therefore, led to the lower absorption and emission wavelength of the polymers. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2499–2509, 2006

Enhanced Photoluminescence, Mesomorphism and Conformation of Liquid‐Crystalline Conjugated Polymers with Terphenyl Mesogen Pendants

“…The monomer 21 with methoxy tail, in which the mesogen is directly linked to the triple bond, only displays a nematic phase, while its cousin polymer 19 with methylene spacer exhibits a SmB mesophase (Scheme 4). [20,36] Generally, a liquid-crystalline monomer would result in a polymer with at least one liquid-crystalline phase; however, some monomers such as monomer 8, 19, 20 and 21 show bright colorful texture when heating and cooling while their corresponding polymers are completely nonmesomorphic. Because ''coupled effects'' resulting from the mesogenic pendants close to backbone destroy the packing arrangements of the mesogens and demolish the stability of the mesophases.…”

“…[29,36,74,90,91,99] Compared with polymer 9, the emission intensities of its cousin polymers 11 and 12 are much stronger by keeping the constant photons of excitation. Moreover, obvious increase of absorbance of the polymers 9, 11 and 12, so called ''hyperchromic effects'', were observed with respect to their matching monomers.…”

“…This may be due to the steric effect of the bulky mesogens directly attached to the polyene backbone, which twisted the double bonds and reduced the effective conjugation length along the main chain. [36] Our group tried to measure their quantum yields, but no reliable results could be obtained (Table 2 no. 3).…”

“…Previous studies [17,36,70] on the mesomorphic properties of the type of LCCP with rigid backbone and short spacers, have revealed that the short spacers and bulky mesogen pendants prevented the polymers from showing any mesomorphism. So the case may also occur in polymer 30, because the bulk cyanoterphenyl pendant was linked to the backbone only through a short ester group.…”

“…[17,18] The third type are the side-chain liquidcrystalline conjugated polymers (SCLCCPs), where the mesogen units as the side-chain are hung to the rigid main-chain. [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] Particularly, SCLCCPs have been considered with growing interest by various research groups, [37][38][39][40][41][42][43][44][45][46] because their electrical and optical properties are expected to be controlled by the molecular orientation of LC side chain. Besides, SCLCCPs could be macroscopically aligned by an external perturbation, such as shear stress, electric or magnetic force field.…”