CONJUGATED AROMATIC POLY(AZOMETHINES) .1. CHARACTERIZATION OF STRUCTURE, ELECTRONIC-SPECTRA, AND PROCESSING OF THIN-FILMS FROM SOLUBLE COMPLEXES

A series of conjugated aromatic poly(azomethines), poly(1,4-phenylenemethylidynenitrilo-1,4-phenylenenitrilomethylidyne) (PPI), poly(2-methyl-1,4-phenylenemethylidynenitrilo-1,4-phenylenenitrilomethylidyne) (PMPI), poly(1,4-phenylenemethylidynenitrilo-2,5-dimethoxy-1,4-phenylenenenitrilo-methylidyne) (PMOPI), poly(1,4-phenylenemethylidynenitrilo-2,5-dihydroxy-1,4-phenylenenitrilo-methylidyne) (PHOPI), and copolymer PPI/PMPI, have been prepared and solubilized in organic solvents by reversible complexation with either gallium chloride or di-m-cresyl phosphate (DCP) and characterized. The molecular structure of the aromatic polyazomethines was characterized by H-1 NMR spectra of the polymers in GaCl3/deuterated nitromethane. The conjugated aromatic poly(azomethines) were processed into optical-quality thin films from their soluble complexes in organic solvents and characterized. The solid-state electronic absorption spectra showed that the lambda-max for pi-pi* transition varies from 405 nm for PPI to 497 nm for PHOPI. The corresponding solid-state bandgap was in the range 2.07-2.50 eV. The relatively low bandgap of PHOPI (2.07 eV) compared to PMOPI (2.34 eV) and PPI (2.50 eV) is due to its intramolecular hydrogen-bonding-mediated coplanar structure. Intramolecular hydrogen bonding in PHOPI was further evidenced by FTIR spectra and the results of solubility and complexation studies. The solid-state electronic absorption spectra of complexes of these conjugated aromatic poly(azomethines) were found to exhibit greater electronic delocalization and smaller bandgaps (1.85-2.15 eV) than the pure polymers. Origin of this remarkable red shift of electronic spectra was attributed to complexation-induced change in polymer conformation. The preparation of soluble complexes of conjugated aromatic poly(azomethines) and their processing to optical-quality thin films has now opened up investigation of the solid-state properties of this class of liquid-crystalline conjugated polymers and high-temperature fiber-forming materials.