ELECTROCHEMISTRY OF END-CAPPED OLIGOTHIENYLS - NEW INSIGHTS INTO THE POLYMERIZATION MECHANISM AND THE CHARGE STORAGE, CONDUCTION AND CAPACITIVE PROPERTIES OF POLYTHIOPHENE

The kinetics of anodic coupling to dimers of thiophene oligomers (n = 3-5), methyl protected at one alpha-terminal position, is second order in oligomer concentration and evidences high activation enthalpies an d negative activation entropies. Activation free energies are linearly related to the inverse of the oligomer length n (the dimerization rat e decreases as n is increased). Thin films of methyl end-capped thioph ene oligomers (n = 4, 6, 8 and 10) display reversible oxidations from a single one-electron step (tetramer) to a single two-electron step (o ctamer and decamer) through two separate one-electron steps (hexamer). ESR indicates strong magnetic dimerization for the one-electron-oxidi zed hexamer. The close resemblance of the electrochemical and ESR beha viour of the hexamer with that of polythiophene suggests that oxidatio n of the latter occurs via hexameric spin-dimerized polarons. The cond uctive and capacitive proper-ties of the end-capped oligomers (n = 6, 8 and 10) were investigated by in situ conductivity and chronopotentio metry. While conductivity of octamer and decamer is displayed at the t wo-electron (bipolaron) state, the hexamer, insulating at this state, is conducting at the mixed-valence polaron-bipolaron state; capacitive responses are evidenced at the bipolaron state for the octamer and de camer only. The difference of conductive and capacitive behaviour betw een the hexamer and the higher oligomers is explained by charge locali zation in hexameric segments.