STRUCTURAL EFFECT ON THE REDOX THERMODYNAMICS OF POLY(THIOPHENES)

The redox thermodynamics of poly(thiophene) (PT), poly(3-methylthiophene) (PMeT), and poly(3-nonylthiophene) (PNT) have been analyzed by using the Nernst plots (E vs log [O]/[R] constructed from the in situ absorbance measurements performed at various doping levels. The apparent standard potential E°′ and the initial slope of the Nernst plots of the oxidation process decrease in the order PT > PMeT > PNT. Concurrently, the redox process becomes progressively more complex with an increasing deviation from linearity above E°′ and the appearance of two distinct oxidation stages for PMeT and PNT. Whereas hysteresis is evident for PT and PMeT, the redox process appears fully reversible in the case of PNT. Although the slope corresponding to initial step of the charging process decreases from PT to PNT, it remains of much larger magnitude than expected for a simple one-electron redox couple. These super-Nernstian behaviors are interpreted by (i) a decrease of the interactions among charged sites due to modifications in the populations of polarons and bipolarons caused by the structurally induced extension of the conjugation and by (ii) the overpotential related to the contribution of the mechanical strain associated with the insertion of the charge-compensating anion in the polymer matrix. © 1990 American Chemical Society.