Chain length effect on intrachain electronic excitation and interchain coupling in poly- and oligo-anilines

The electronic absorption profiles of a polyaniline base (using NMP as a solvent) and of a polyaniline salt (dissolved in sulfuric and methanesulfonic acid) have been studied from 0.5 to 5.5 eV and compared with those of some constitutive bricks of the polyaniline repeated units: diphenylamine (BB), N,N'-diphenyl-1,4-phenylene-diamine (BBB) and -diimine (BQB) and the oxidized tetramer (BQBBa, a stands for a terminal amine group). Standard (similar to 100 Mn) and short (10-20 nm) polyaniline chain lengths are considered. Resonant Raman profiles were determined for exciting energies ranging from 1 to 3 eV. In the cationic chains, the electronic absorbance per cycle is increased with the chain length but the formation of NH2+ groups in strong acids destroys the conjugation, leading to isolated phenyl rings. On the other hand, for polymer base chains, constant behavior is observed with length greater than or equal to 16 nm and pernigraniline chain portions are evidenced from the analysis of the Resonant Raman profile. Although a semi-conduction amorphous-like behavior is deduced from Urbach's tail analysis for polyaniline base nanofilaments, a crystalline behavior isrecognized for HCl intercalated films. (C) 2000 Elsevier Science S.A. All rights reserved.