Articles Poly(thieno[3,4-b]thiophene)s from Three Symmetrical Thieno[3,4-b]thiophene Dimers Byoungchul Lee, Mustafa S. Yavuz, and Gregory A. Sotzing* Department of Chemistry and the Polymer Program, Institute of Materials Science, UniVersity of Connecticut, 97 N. EagleVille Rd., Storrs, Connecticut 06269 ReceiVed December 14, 2005; ReVised Manuscript ReceiVed February 15, 2006 ABSTRACT: Herein we report the synthesis of symmetrical bis(thieno[3,4-b]thiophene)s and their electrochemical polymerization. The 2,2′-bis(T34bT) has an oxidation peak at 0.73 V for electropolymerization whereas both 4,4′-bisT34bT and 6,6′-bisT34bT have peak oxidation potentials for polymerization at 0.49 and 0.53 V (0.44 V vs NHE), respectively. For comparison, thieno[3,4-b]thiophene (T34bT) polymerizes with an oxidation peak at 0.9 V. Conjugated T34bT polymers prepared from T34bT, 4,4′-bisT34bT, and 6,6′-bisT34bT exhibit similar redox behavior showing oxidation and reduction peaks located at ca. 0.1 V and ca. -0.3 V, respectively, and optical band gaps of ca. 0.9 eV (1377 nm), whereas the conjugated polymer from 2,2′-bisT34bT has redox peaks centered at 0.5 and 0.4 V. Like PT34bT prepared from T34bT, both PT34bTs prepared from 4,4′-bisT34bT and 6,6′-bisT34bT are pale blue to colorless in their oxidized states and sky blue in their neutral forms. PT34bT prepared from 2,2′-bisT34bT is brown in the oxidized state. The conductivities of the PT34bTs from both 4,4′- and 6,6-bisT34bTs were found to be ca. 2 × 10 -5 S/cm in the undoped state, increasing to 0.2 S/cm after iodine doping. The conductivity of PT34bT from 2,2′-bisT34bT was 2 × 10 -5 and 0.007 S/cm in the neutral and oxidized forms, respectively. Introduction One of the significant goals in the field of conjugated and conductive polymers is the preparation of optically transparent conductors. Numerous potential applications for such materials exist for optically transparent conductive polymers exhibiting <10 -4 to very high conductivities. Low conductivities would have applicability in areas such as charge dissipation films, 1 as ion storage layers in dual polymer electrochromic devices, 2 and, with a proper work function, as hole injection layers for light- emitting diodes (LEDs). 3 Optically transparent conductive polymers exhibiting high conductivities of >200 S/cm have the potential for indium-doped tin oxide (ITO) replacement in numerous device applications. All plastic electrochromic 4 and photovoltaic devices 5 have been fabricated from poly(3,4- ethylenedioxy)thiophene (PEDOT)-poly(styrenesulfonate) (PSS), a conducting polymer having a conductivity of ∼200 S/cm when processed under the proper conditions. When a conjugated polymer undergoes oxidation, typically the spectral absorbances shift to a lower energy. Hence, most studies focusing on the preparation of optically transparent conductive polymers have been focused on the synthesis of low- band-gap conjugated polymers. However, recently there was a report of a high-band-gap polymer exhibiting high optical transparency in the oxidized conductive state. 6 Among several strategies to tune the band gap of conjugated polymers, 7 polymerization of fused heterocyclic rings has long been known to yield polymers with very low band gaps. This is attributed to the stabilization effect of the fused ring on the quinoidal form of the main-chain polymer. On the basis of this principle, poly- (isothianaphtene) (PITN) is reported to exhibit a band gap of 1.0-1.2 eV. 8 The oxidative chemical polymerization of 2-de- rivatized thieno[3,4-b]thiophenes has been reported to yield linear polymers with low band gaps ranging from 0.85 to 0.92 eV. 9,10 We reported the preparation of poly(thieno[3,4-b]thiophene) (PT34bT) from thieno[3,4-b]thiophene via oxidative electro- chemical polymerization. 11 We have also demonstrated that PT34bT can be prepared via oxidative chemical polymerization and that the polymer can be reduced and then sulfonated to levels of 56-65% to yield variable gap water-processable PT34bT. Thin films were prepared via the layer-by-layer technique. 12 We have also reported the oxidative chemical polymerization of T34bT in water in the presence of poly- (styrenesulfonic acid) (PSSA) to prepare water-dispersible PT34bT-PSS. 13 This methodology permitted the formation of optically transparent and conductive films for potential use in low-conductivity applications. Oxidative polymerization of T34bT yields a conjugated polymer exhibiting a band gap of 0.85 eV. 11 We have reported the stability of PT34bT-PSS in water for almost a year. Hence, to our knowledge, this is the lowest band gap polymer to exhibit such stability in water and stored under ambient conditions. Films of PT34bT were found to be sky blue in the neutral form and highly transparent in the oxidized form. PT34bT prepared by electrochemical oxidative polymerization was found to be capable of being both p- and n-doped at low positive and negative potentials, respectively, with excellent stability toward p-doping and moderate stability to n-doping. * Corresponding author. E-mail: sotzing@mail.ims.uconn.edu. 3118 Macromolecules 2006, 39, 3118-3124 10.1021/ma0526746 CCC: $33.50 © 2006 American Chemical Society Published on Web 04/08/2006