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 (∼100 nm) 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 ≥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 Urbachʹs tail analysis for polyaniline base nanofilaments, a crystalline behavior is recognized for HCl intercalated films.