THIOPHENE OLIGOMERS AS POLYTHIOPHENE MODELS .1. ANODIC COUPLING OF THIOPHENE OLIGOMERS TO DIMERS - A KINETIC INVESTIGATION

The kinetics of anodic coupling to dimers of thiophene oligomers (n = 3-5), methyl protected at one alpha-terminal position, has been invest igated in 1,2-dichloroethane by cyclic voltammetry and chronoamperomet ry. The process, second-order in oligomer concentration, is characteri zed by relatively high activation enthalpies (25/70 kJ mol-1) and nega tive activation entropies (-100/-4 J K-1 mol-1). Though activation fre e energies are linearly related to the inverse of the oligomer length n (the dimerization rate decreases as n is increased), the activation enthalpy and entropy for the pentameric radical cation are peculiarly high, reflecting strong electron delocalization and ring coplanarizati on.