Generation of Stable Radical Cations (Polarons) and Dications (Bipolarons) on Model Thiophene Oligomers
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GENERATION OF STABLE RADICAL CATIONS (POLARONS) AND DICATIONS (BIPOLARONS) ON MODEL THIOPHENE OLIGONERS Denis FICHOU and Gilles HOROWITZ Laboratoire des Xat~riaux MolOculaires - C.N.R.S. - 2, rue Henry Dunant 94320 - Thiais, France ABSTRACT Oxidative doping of individual thiophene oligomers nT (n = 3 to 6) with FeCl3 in dilute CH2 Cl2 solution generates the corresponding radical cations (polarons) nT÷", as evidenced by in-situ visible/near infrared absorption and electron spin resonance spectroscopies. The nT÷" species in solution progressively precipitate as conducting (W3ooK = 10--10-2 S/cm) and strongly paramagnetic radical cation salts (g = 2.003. Hpp = 4-8 G). The conductivity of the 6T÷" salt is weakly temperature dependent, with an activation energy of 32 meV above 100 K and a tendency to level off at lower temperatures. In the case of the sexithienyl radical 6T÷", further doping results in the quantitative conversion into the dication 6T** (bipolaron). INTRODUCTION It is now well-admitted that the doping process of conjugated polymers leads to the formation of charged localized defects called polarons and bipolarons. In the case of polythiophene (PT). a nondegenerate ground state polymer. experimental studies have shown that at low doping levels (z 0.5 mol%) singly charged hole polarons are the dominant species while at higher doping levels (> 5 mol %) they recombine to form the more stable hole bipolarons [13. Numerous theoretical approaches have also been proposed to account for these observations, as that carried out by Bertho and Jouanin in the framework of the Huckel approximation (22. Recently, Br~das et al. presented VEE (Valence effective hamiltonian) band-structure calculations on doped PT as a function of the doping level using model oligomers such as the hexamer (6T) for the geometry optimization of the systems (33. Nevertheless, even if long thiophene oligomers are good qualitative approximations for the corresponding neutral and doted PT geometries, they remain hypothetic systems which may behave quite differently as synthesized molecules. In sharp contrast with the polymer, they have a finite structure and do not possess any structural disorder (side chains, conjugation interruptions) susceptible to influence their electronic properties. We have recently synthesized long oligothiophenes (nT, with n up to 10) (4) and studied their optical (5] and electrical [6) properties in the form of thin solid films as a function of the chain length. These compounds represent a compromise between small molecules and macromolecules. They remain very slightly soluble in organic solvents up to the hexamer and can then be studied in dilute solutions (= 10-6 mole. l-). In order to check whether or not real thiophene oligomers behave similarly to PT and could thus be considered as model systems for PT on both the experimental and theoretical levels, we have performed FeCl 3 doping experiments on 3T, 4T, 5T and 6T in dilute CH2 Cl2 solutions. We report in this article on the successive generation of the correspond
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