Electrochimica Acta 45 (2000) 2273 – 2278 Polythiophene S,S dioxides: an investigation on electrochemical doping Catia Arbizzani a , Marina Mastragostino b, *, Francesca Soavi a a Department of Chemistry ‘G. Ciamician’, Uniersity of Bologna, ia Selmi 2, I -40126 Bologna, Italy b Department of Physical Chemistry, Uniersity of Palermo, iale delle Scienze, Parco d’Orleans II, I -90128 Palermo, Italy Received 5 October 1999; received in revised form 4 January 2000 Abstract A new strategy for functionalizing oligothiophenes is the transformation of the thienyl sulphurs into the corresponding S,S dioxides, with the effect of lowering the LUMO energy without significantly affecting the HOMO one. From a quinquethiophene S,S dioxide derivative, a polymer (pQTDO) which can be reversibly n-doped at not very negative potentials still maintaining the property of being p-doped at moderate potential values was electrosyn- thesized. There is, however, a great difference in the ability to store charge of the polymer’s p- and n-doped forms: a great amount of injected negative charge irreversibly modifies the structure of pQTDO. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Conducting polymer; Electrochemical doping; n-Doping; Polythiophene S,S dioxide; Thiophene derivative www.elsevier.nl/locate/electacta 1. Introduction Conjugated polymers have acquired a growing im- portance in many areas of modern chemistry and in condensed matter physics. The new applications envi- sioned for electronically conducting polymers (ECPs) in the last few years have given rise to considerable re- search efforts in basic and applied materials science to develop new materials for electronics, optics and energy conversion. The main synthetic strategies for the design of new ECPs have focused on the modulation of such factors which affect the HOMO and LUMO energy as the bond length alternation, the mean deviation from planarity, the aromatic resonance energy of the ring, the inductive or mesomeric electronic effects of substi- tution and the intermolecular interchain coupling in the solid state [1–4]. Owing to the feasible applications of ECPs in ad- vanced symmetric electrochemical devices, there is great interest in achieving stable n-doping at moderate poten- tial. Particularly, the shifting of LUMO without signifi- cantly affecting the HOMO energy produces polymers that can be easily reduced and oxidized. Among the various molecules studied during the past two decades, thiophene has acquired great importance because of its electronic properties and structural versatility; these characteristics make thiophene suitable for a great vari- ety of functionalizations and, consequently, the family of the polymers based on thiophene derivatives one of the most numerous. It has already been demonstrated by theoretical cal- culations [5] that S-oxide and S,S dioxide substitution at sulphur in polythiophene induce considerable modifi- cations in the electronic structures, particularly in the frontier orbital patterns and, hence, also in the band gap values. A new strategy for functionalizing oligoth- iophenes has recently been reported, consisting of the chemical transformation of the thienyl sulphurs into the * Corresponding author. Fax: +39-051-2099456. E-mail address: mastrag@ciam.unibo.it (M. Mastragostino) 0013-4686/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII:S0013-4686(00)00320-0