In fluid solutions at room temperature, the high-purity dibenzofuran (DBF) and dibenzothiophene (DBT) exhibit phosphorescence and delayed fluorescence from monomers as well as intermolecular triplet excimers. The delayed emissions assigned to the excimer are red shifted with respect to the corresponding emissions from the monomers. All four delayed emissions exhibit excitation spectra that are identical with the excitation spectrum of the prompt fluorescence of the corresponding monomer. Consistent with an emission originating from the bimolecular annihilation of the triplet excimers, the delayed excimer fluorescence is spectrally very similar to the normal excimer fluorescence of the compounds, and it decays with a lifetime which is one-half the decay time of the excimer phosphorescence. At longer delay times, where the triplet excimer−monomer equilibration is expected to occur, the ratio of the intensity of the excimer phosphorescence to that of the monomer phosphorescence becomes independent of time. Concomitantly, the lifetimes of the two emissions become identical to each other, as indicated by the very similar lifetimes of the delayed excimer fluorescence and the delayed monomer fluorescence. These results confirm the formation of intermolecular triplet excimers in fluid solutions of these compounds.

Effects of processing variables on the structural gradients developed in the injection molding of thermotropic wholly aromatic copolyester parts were investigated using optical macro and microscopy combined with image analysis techniques at various sections of the molded LCP parts.The studies indicate the existence of a multi-layer structure from skin to core of the parts. These include: a skin layer, a transition zone composed of a sub-skin layer, a shear layer and a core region. The effects of processing conditions including injection speed, mold temperature, and mold size on the structural features were evaluated. An interesting buckling phenomenon resembling features observed in jetting at core regions near the end of the cavity after the divergence section was observed for the samples molded at the lowest injection speed and mold temperature.Digital image analysis of the images of the sectioned parts revealed that there are more layers than one observes macroscopically. In certain instances as many as 15 layers were detected. Orientation behavior and the positional light reflection intensity appear to correlate with each other particularly away from the skin regions. This may be the manifestation of high visible light absorption anisotropy exhibited by these aromatic polymers.