Oxidative coupling of small oligothiophenes and oligopyrroles in water in the presence of cyclodextrin. Pulse radiolysis investigations

The oxidation of small oligothiophenes and oligopyrroles (bipyrroles) has b een investigated by pulse radiolysis in aqueous solutions containing hydrox ypropyl-beta-cyclodextrin. Oxidation by the N-3(.), Cl-2(.-), or Br-2(.-) r adicals produces the corresponding radical cations, which are identical to those observed in organic solvent. Detailed kinetic studies with methylbipy rrole show that the homogeneous oxidation occurs after dissociation of the complex but also with the complex itself, contrarily to what is commonly de scribed for electrochemical reduction or oxidation. The produced cation rad ical is not associated with the cylodextrin and is rapidly expelled from th e host after its formation. On the contrary, if the cation radical is depro tonated to form the neutral radical, the neutral radical is associated with the cyclodextrin. Dissociation kinetics rate constants were found to be fa st for both cation radical and neutral radical.