THIOPHENE OLIGOMERS AS POLYTHIOPHENE MODELS .2. ELECTROCHEMISTRY AND INSITU ESR OF END-CAPPED OLIGOTHIENYLS IN THE SOLID-STATE - EVIDENCE FOR PI-DIMERIZATION OF HEXAMERIC POLARONS IN POLYTHIOPHENE

Thin films of thiophene oligomers (n = 4, 6, 8, and 10) methyl-protect ed toward anodic coupling at the terminal alpha positions were investi gated by cyclic voltammetry and in situ ESR in acetonitrile. Reversibl e oxidation goes from a single one-electron step (tetramer) to a singl e two-electron step (octamer and decamer) through two separate one-ele ctron steps (hexamer). ESR indicates strong magnetic dimerization for the one-electron-oxidized hexamer. The close resemblance of the electr ochemical and ESR behavior of the hexamer with that of polythiophene s uggests that oxidation of the latter occurs via hexameric spin-dimeriz ed polarons. The hexameric model is discussed taking account of the ol igomeric nature of polythiophene.