The neutral state of the polymer showed no affinity for binding O2 (Figure ). For the reduced states, the reduced complex (NDI-T2 + O2) -and the case where isolated O2 bears the entire negative charge are energetically identical and favorable relative to the case where the NDI-T2 alone bears the charge (Figure ). Inspection of the partial charge distribution for the reduced complex (Figure ) shows that the extra -1 e -charge is mainly localized on the O2 molecule, leaving the NDI-T2 monomer almost neutral, irrespective of the O2 location in the complex (Figure ). These results suggest that the lowest energy configuration is achieved when the added negative charge resides on O2 and that the charged O2 species (superoxide) is not stabilized by interaction with NDI-T2. Thus although there is electron transfer between the reduced polymer and O2 to form superoxide, it appears this process is chemical in nature and follows an outer-sphere process.