Conjugated Polymers DOI: 10.1002/anie.200901231 Controlling Rigidity and Planarity in Conjugated Polymers: Poly(3,4-ethylenedithioselenophene)** Yair H. Wijsboom, Asit Patra, SanjioS. Zade, Yana Sheynin, Mao Li, Linda J.W. Shimon, and Michael Bendikov* Conjugated oligomers and polymers [1, 2] attract considerable interest owing to their application in photovoltaic cells, [3, 4] organic light-emitting diodes (OLEDs), [5, 6] organic field- effect transistors (OFETs), [7] and electrochromic devices. [8] Generally, planarity and good conjugation are required so that organic materials can achieve band gaps in the semi- conductor region, high conductivity, high mobility, and an electrooptical response. Polythiophenes are among the most promising and best-studied conducting polymers. [1, 2] How- ever, even parent bithiophene is not planar in the gas phase (according to both experiment and theory), [9] and crystal packing forces are responsible for the planarity of oligothio- phenes in the solid state. [10] Various small substituents (such as two adjacent alkyl chains on the same or neighboring rings: 3,4 or 3,3’-substitution) cause oligothiophene to become nonplanar, and the availability of oligo- and polythiophenes with substituents that do not disturb planarity is very limited (for example, poly(3-hexylthiophene) is planar). [10, 11] Although twisting of the oligothiophene backbone requires very little energy, it results in a significant increase in the HOMO–LUMO gap. [12] The fact that small conformational changes to conjugated polymers may produce large band-gap effects has been utilized in the development of polythio- phene-based sensors. [13, 14] Poly(3,4-ethylenedioxythiophene) (PEDOT) [15] has many advantages over other conducting polymers in organic electronics applications. However, it cannot be applied as a light-absorbing donor in organic solar cells, for example, owing to its very low oxidation potential and, consequently, very low work function. PEDOT is believed to be planar; however, its analogue, poly(3,4-ethylenedithiothiophene) (PEDTT), [16–20] in which oxygen atoms are replaced by sulfur atoms, is assumed to be twisted, as manifested by its significantly wider band gap (2.2 eV for PEDTT vs. 1.6 eV for PEDOT). [18, 21] Indeed, the dimer of 3,4-ethylenedithiothio- phene (bis-EDTT) has an inter-ring twist angle of 458, [18] whereas bis-EDOT has a planar structure in the solid state. [17, 18, 20, 22, 23] Recently, we obtained the first conductive polyseleno- phene, poly(3,4-ethylenedioxyselenophene) (PEDOS), which has a relatively narrow band gap and excellent electrochromic properties. [24, 25] Synthesis of stable and conductive PEDOS enables the development of applications of polyselenophenes as organic electronic materials. Designing such materials demands the identification of more rigid conjugated systems capable of bearing various substituents on their backbone whilst retaining their planarity. Herein, we report that the range of substituents that polyselenophenes can bear whilst still maintaining their planarity is wider than that of polythiophenes, and is mostly due to the more rigid backbone of the polyselenophenes. Poly(3,4-ethylenedithioseleno- phene) (PEDTS) has a significantly narrower optical band gap (0.6–0.8 eV) than PEDTT, which can be attributed to its planarity. Moreover, PEDTS is a conducting polymer that is not as electron-rich as PEDOS and PEDOT. The top of the valence band of PEDTS is about 0.7 eV (0.64 eV experimen- tal, 0.81 eV calculated) lower than that of PEDOT, which makes PEDTS a very attractive material for organic solar cell applications. The energy required to twist around inter-ring bonds in decaselenophene is small ; however, it is notably greater (by a factor of 1.2–1.8; Supporting Information, Figure S7) [26] than in decathiophene. Twisting to a 608 inter-ring dihedral angle requires only 2.6 kcal mol À1 per inter-ring bond for decasele- nophene (2.1 kcal mol À1 for decathiophene) and twisting to a [*] Y. H. Wijsboom, Dr. A. Patra, Dr. S. S. Zade, [+] Dr. Y. Sheynin, Dr. M. Li, Dr. M. Bendikov Department of Organic Chemistry Weizmann Institute of Science, Rehovot 76100 (Israel) Fax: (+ 972) 8934-4142 E-mail: michael.bendikov@weizmann.ac.il Homepage: http://www.weizmann.ac.il/oc/bendikov/ Dr. L. J. W. Shimon Chemical Research Support Unit Weizmann Institute of Science (Israel) [ + ] Current address: School of Chemistry Indian Institute of Science, Education and Research Kolkata-700106 (India) [**] We thank Prof. Dmitrii F. Perepichka (McGill University) for helpful discussions. We thank the MINERVA Foundation and the Israel Science Foundation for financial support. M.B. is the incumbent of the Recanati career development chair, a member ad personam of the Lise Meitner-Minerva Center for Computational Quantum Chemistry, and acknowledges DuPont for a Young Professor’s Award. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.200901231. Angewandte Chemie 5443 Angew. Chem. Int. Ed. 2009, 48, 5443 –5447  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim