Redox States of Long Oligothiophenes: Two Polarons on a Single Chain

Can two isolated radical cations (polarons) exist simultaneously on a π-conjugated chain or do they coalesce into a single, doubly charged spinless (bipolaron) deformation? This long-standing question, fundamental to charge transport in conducting polymers, is now resolved by an investigation of the redox states of long oligothiophenes (below) in solution. In contrast to short oligothiophenes, for long oligothiophene dications two individual polarons prevail because of the decrease in Coulomb repulsion energy obtained by moving the two charges apart.