Characterization of soluble oligomers produced by electrochemical oxidation of o-phenylenediamine by electrospray ionization sequential mass spectrometry

Soluble species generated during the electropolymerization of o-phenylenediamine (o-PD) on platinum electrodes in aqueous buffers at different pH values were investigated by electrospray ionization ion trap sequential mass spectrometry (ESI-ITMS(n)). The main protonated molecules (MH(+)) detected in the full scan ESI-MS spectra of the electrolytic solutions were isolated in the ion trap and sequentially fragmented (MS(n), with n up to 5) to obtain fragmentation patterns. The latter led to hypotheses as to the molecular structures of the soluble products of o-PD electropolymerization; it appeared that all of them are actually oligomeric species in different oxidation states. In particular, o-PD dimers, trimers and tetramers could be identified and three common structural features were found, namely: the presence of phenazine, 1,4-benzoquinonediimine, and secondary amine (acting as bridges between benzene rings) units. These findings are in agreement with those already reported for the surface structure of the polymeric films formed on the platinum electrodes during o-PD polymerization, thus suggesting that a close relationship exists between the soluble oligomers and the polymer itself.