On the molecular recognition and associations between electrically conducting polyaniline and solvents

The specific interactions between sulphonic acid protonated polyaniline (PANI) and solvents are here studied both by the semiempirical AM1 method and experimentally. Phenolic solvents are shown to have a relatively large interaction with the sulphonate anions of the counterions and with the amines in PANI. In addition, a properly functionalized counterion may form cyclic associations provided that there is a steric match between the molecules concerned. This concept is called molecular recognition and it is a novel concept in the context of PANI. For example, the carbonyl group in (±)-10-camphor sulphonic acid (CSA) can form a hydrogen bond to the hydroxyl group of m-cresol, whereby the phenyl ring becomes coplanar with one of the PANI rings thus enabling enhanced van der Waals interaction. This additional specific interaction agrees with our observed increased solubility with CSA doped PANI in m-cresol, compared to its solubility in dimethyl sulphoxide or chloroform, or to tosylene sulphonic acid doped PANI in m-cresol. The above cyclic associations are suggested for dilute solutions and for the amorphous domains of solid films. In the latter case, they provide a potential mechanism to yield planar conformation in the crystalline domains: during the evaporation of m-cresol, stacking to crystal structure may twist the rings due to the planar m-cresol molecules on top of PANI rings. This is in agreement with the observed high conductivity. The present results indicate that computational methods combined with the concept of molecular recognition may open new possibilities to tailor the rigidities and solubilities of macromolecules.