Rigid-Rod Polyesters with Flexible Side Chains Based on 1,4-Dialkyl Esters of Pyromellitic Acid and 4,4‘-Biphenol. 7. Fluorescence Studies on Crystalline and Liquid Crystalline Layered Phases over a Wide Range of Temperatures

Abstract

Rigid-rod polyesters with flexible side chains denoted as B-Cn form two types of layered crystals and two types of layered mesophases. These layered phases are characterized by a layered structure, in which the aromatic main chains are paced into a layer with the side chains occupying the space between the layers. The detailed aggregated structures for these two crystalline phases were studied by fluorescence spectroscopy, from which it was found that the ground-state charge-transfer complexes are formed between the biphenyl and pyromellitic moieties in the adjacent main chains and that these two moieties have an alternating lateral alignment inside a layer. Two layered crystals, K1 and K2, show charge-transfer fluorescence (475 and 505 nm) and its excitation spectra (400 and 425 nm) in different wavelengths, respectively. This difference in wavelengths is related to the difference in lateral packing distances (4.6 Å for K1 and 3.45 Å for K2 crystals) and suggested to be due to the distance between the electron-donating and -accepting units in the adjacent chains. In the isotropic phase, the charge-transfer bands were also observed. This result means that the isotropic state in this polymeric system can be defined by the state that a layered segregation is maintained, but the molecular ordering and the interlayer correlation are lost. It is well-known that the driving force of the adoption for layered structures is a segregation of aliphatic and aromatic domains. We have demonstrated here, moreover, that a charge-transfer interaction between the electron-donating and -accepting units in the neighboring main chains contributes to the organization of phase structures and spatial arrangements.