Poly(2,5-dialkoxy-p-phenylenebutadiynylene)s—Novel rigid conjugated polymers with liquid crystalline property

Novel rigid conjugated poly(p-phenylenebutadiynylene)s having long alkoxy side chains, octyloxy (C8), decyloxy (C10), and dodecyloxy (C12) at 2,5-positions of a phenylene unit, are synthesized by oxidative polycondensation of corresponding 2,5-dialkoxy-1,4-diethynylbenzenes with Hay catalyst. The polymers are soluble in hot solvents such as CHCl3 and fusible upon heating. UV–Vis spectra of the polymers show an absorption maximum (λmax) around 420 nm due to the π-conjugation along the main chain. An intense fluorescence exhibiting the λmax around 475 nm is observed in all of the polymers. Cyclic voltammetry of the polymer shows two Li ion-participated redox reactions occurred at the dialkoxyphenylene moiety and in the conjugated polymer chain due to an n-doping process. Electrical conductivity is an order of 10−8 S cm−1 which increases to an order of 10−4 S cm−1 upon vapor phase doping by H2SO4. Thermochemical investigations by DSC and polarized optical microscopy suggest that the polymers have a nematic liquid crystalline phase. The molecular arrangements in the phase is evaluated by XRD and a molecular dynamics simulation. A nematic phase is shown to arise from a side-by-side accretion of the rigid rodlike main chains. A lamellar structure is suggested in the crystalline phase.