The synthesis and properties of a class of high molecular weight, filmand fiber-forming, rodlike, aromatic polyamides containing all-para-linked substituted-biphenylene and/or -stilbene repeat units are described. Substituents are placed on the aromatic rings (or double bond of the stilbene moiety) so as to force them into a noncoplanar conformation. The effect of this noncoplanarity is to reduce the crystallinity (to <10%) of even highly oriented films, enhance solubility, and reduce or completely eliminate absorption of visible radiation. Another consequence of this noncoplanar conformation is to confer a substantially cylindrical electron density distribution about the long axis of the polymer backbone which in many examples enhances the repeat unit anisotropy and may contribute to the unusually high birefringence.
Reduced dimensionality and quantum confinement in conjugated organic and polymer structures enhance the effects of electron correlation on virtual electronic excitation processes and nonlinear-optical responses. A microscopic many-electron description of the third-order susceptibilities Yijkl(-W4; w1, 2, c3) of conjugated structures is reviewed for one-dimensional chains and extended to two-dimensional conjugated cyclic structures. Electron correlation effects in effectively reduced dimensions result in highly correlated 7-electron virtual excitations that lead to large, ultrafast nonresonant nonlinear-optical responses. The increase of dimensionality from linear to cyclic chains is found to reduce the nonresonant isotropic third-order susceptibility yg. Resonant experimental studies of saturable absorption and optical bistability in ultrathin films of quasi-two-dimensional naphthalocyanine oligomers are also presented. In the saturable-absorption studies, the resonant nonlinear refractive index n was measured to be 1 X 10-4 cm2/kW in the wavelength range of operating laser diodes. Based on this result, electronic absorptive optical bistability is observed on a nanosecond time scale in a nonlinear Fabry-Perot interferometer employing the saturably absorbing naphthalocyanine film as the nonlinear-optical medium.
The solubility of a series of para‐linked, substituted phenyl, biphenyl, terphenyl, and quaterphenyl polyamides is reported. Most of these polymers are soluble in amide solvents without lithium chloride, and several are soluble in ethers and/or ketones. The results indicate that solubility in this class of polymers is affected by the position, polarizability and size of the substituents, enantiomeric purity, and the number of non‐coplanar biphenyl rings per repeat unit.
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