The third-order nonlinear optical susceptibility ͑3͒ ͑Ϫ3;,,͒ of thin films of poly͑p-phenylenevinylene͒ ͑PPV͒ and several corresponding oligomers ͑OPV-n͒ has been investigated by third-harmonic generation using variable laser wavelengths from 900 to 1520 nm. The oligomers show a single three-photon resonance of ͑3͒ ͑Ϫ3;,,͒ which is closely related to the linear absorption spectrum. We can identify, however, two maxima in the ͑3͒ spectrum of PPV. They are assigned to three-photon resonances with the maximum of the exciton absorption and with the threshold of the continuum of states, which can be located at 3.2Ϯ0.1 eV. This corresponds to an exciton binding energy of 0.7Ϯ0.1 eV. We observe a general scaling behavior for PPV, OPV-n, and other one-dimensional conjugated -electron systems in their neutral form. Their ͑3͒ values, evaluated at comparable resonant or low-resonant conditions, follow an empirical scaling relationship(3) /␣ max ϳ max x , where ␣ max and max denote the absorption coefficient and wavelength of the low-energy absorption maximum. We obtain an exponent xϭ10Ϯ1 which is much larger than expected from an earlier theory. Possible reasons for the difference between theory and experimental results are discussed.
We investigate the low-temperature phase of a semiflexible liquid crystalline polyester that was previously thought to display only a nematic mesophase. The polymer is the condensation product of 1,lO-decanediyldiosybis(benzoy1 chloride) and ethoxy-substituted hydroquinone and has a chemical structure that is strictly periodic. Polymers of two different molecular weights were prepared. For these viscous materials, standard techniques for phase identification are problematic; however, rheological studies easily reveal a nematic-smectic transition and pinpoint the transition temperature. According to X-ray diffraction on oriented fibers, both samples exhibit a smectic A mesophase, in which the layer repeat is equal to one-third the molecular repeat. TEM morphological investigation confirms the rheological and X-ray studies and reveals parabolic focal-conic textures, characteristic of smectics. The utility of rheology as a simple indication of a transition to a smectic phase is emphasized.
The first anthracene-containing conjugated polymer, poly(l,4-anthrylenevinylene) (PAV), was synthesized. Its processability was accomplished by using the precursor polymer route. Consideration of the mechanism responsible for the polymerization reaction led to selection of the adequate monomer to achieve polymerizability. UV spectra of PAV films showed a band-gap energy 0.3 eV smaller compared to that of poly(phenylenevinylene) (PPV) as a result of the minimization of the energy difference between aromatic and quinoid resonance structures by the anthracene unit. Model compound 13 was synthesized to exclude structural defects of PAV as well as to clarify the role of the steric and electronic effects on the conformation of the main chain. The optical properties of PAV thin films drastically changed upon reaction with a dienophile in the solid state.
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