Aniline tetramers: Comparison with aniline octamer and polyaniline

Abstract

Transport and magnetic properties of a charge transfer complex of a phenyl-end-capped aniline tetramer are studied. This oligomer is an analog of the emeraldine salt form of polyaniline. Magnetic susceptibility, four probe conductivity and thermopower of BF − 4-aniline tetramer single crystals are reported together with ESR susceptibility, linewidth and g-factor of the C1O − 4-aniline tetramer powder. Experimental results demonstrate that the “bipolaron” state is stabilized in the tetramer, in contrast to the polaron state being preferentially stabilized in the octamer and polymer (emeraldine salt) and in agreement with bipolaron stabilization in the protonated amorphous emeraldine and emeraldine salt in solution. Its conductivity is activated, in contrast to that of the polymer in which charging effects between metallic islands are important. Susceptibility, ESR and thermopower indicate some polarons are thermally excited amongst the bipolarons. The activation energy of polarons suggest that their origin is defect sites. The cause of the stability of the bipolaron with respect to the polaron state in the tetramer is suggested to be the reduced delocalization energy in this system.