STOICHIOMETRIC CONTROL OF THE SUCCESSIVE GENERATION OF THE RADICAL CATION AND DICATION OF EXTENDED ALPHA-CONJUGATED OLIGOTHIOPHENES - A QUANTITATIVE MODEL FOR DOPED POLYTHIOPHENE

The stoichiometric oxidation of extended alpha-conjugated oligothiophenes (C4H2S)nH2 (nT, with n = 3 to 6) by FeCl3 in dilute CH2Cl2 solutions allows the control of the oxidation state of the charged species successively generated. The respective nT.+ radical cations (polarons) are quantitatively produced in the first oxidation step and characterized by Vis-NIR absorption and ESR spectroscopies. The 6T.+ radical cation salt precipitates as a stable, conducting (sigma-300K = 0.5 S cm-1) and paramagnetic (g = 2.003, DELTA-H(PP) = 4 G) black powder, with FeCl4- as the counter anion. In the second step of the FeCl3 oxidation process, 6T.+ is quantitatively interconverted into the 6T++ dication (bipolaron), as revealed by the two isosbestic points at 1.51 eV (823 nm) and 1.09 eV (1135 nm) on the optical spectrum. A four-level molecular orbital (MO) diagram of the singly and doubly charged hexamer is proposed on the basis of the well-resolved optical spectra and transition assignment. We compare these quantitative results obtained on finite and isolated systems with the current polaron-bipolaron model of doped polythiophene.