Visible-Near Infrared Absorbing Dithienylcyclopentadienone-Thiophene Copolymers for Organic Thin-Film Transistors Changduk Yang, † Shinuk Cho, ‡ Ryan C. Chiechi, | Wesley Walker, § Nelson E. Coates, ‡ Daniel Moses, ‡ Alan J. Heeger, ‡ and Fred Wudl* ,†,‡ Department of Chemistry and Biochemistry, UniVersity of California, Santa Barbara, California 93106, Center for Polymers and Organic Solids, UniVersity of California, Santa Barbara, California 93106, and Department of Chemistry and Biochemistry, UniVersity of California, Los Angeles, California 90095 Received August 30, 2008; E-mail: wudl@chem.ucsb.edu Solution-processable organic semiconductors with relatively high- mobility are required for printing low-cost organic thin-film transistor (OTFT) circuits for flexible electronics. 1-6 Thiophene- based conjugated polymers have been extensively studied as materials for such applications. 7-10 In particular, as a result of the structural regularity of the polymer backbone, regioregular poly(3- hexylthiophene) (rr-P3HT) exhibits a relatively high charge-carrier (hole) mobility. 11,12 Another important development in the synthesis of π-conjugated polymers has been the utilization of donor-acceptor architectures within the backbone. The donor-acceptor systems cause partial intramolecular charge transfer (ICT) that enables manipulation of the electronic structure (HOMO/LUMO levels), leading to low band gap semiconducting polymers 13-18 with relatively high charge carrier mobilities. 19-26 To create a push-pull system, we have recently prepared a series of polymers based on diaryl-dithienylcyclopentadienone (2,5- dithienyl-3,4-(1,8-naphthylene)cyclopentadienone, DTCPD), re- vealing a broad absorption band covering practically the whole visible region. 27,28 Given the attractive properties of DTCPD and those of the regioregular thiophene-based materials, we have combined the two units into a new polymer repeat unit. Following the ICT strategy, 4,4′-dialkyl-[2,2′]bithiophene (DAT) moieties are selected as an electron-rich comonomer for DTCPD since tail-to-tail regio- positioning of the alkyl chains on the thiophene monomer helps promote self-organization, while minimizing any steric interactions between neighboring alkyl groups, thus preserving backbone planarity. 29 Besides, it is expected that the unsubstituted thiophene units in DTCPD can increase the ionization potential (IP) via improving rotational freedom, 30,31 possibly resulting in enhanced oxidative stability when compared to that of the poly(alkylth- iophene)s. Herein, we present the synthesis of a series of alternating DTCPD-DAT copolymers (DTCPD-alt-DHT and DTCPD-alt- DDT) and the initial characterization of these copolymers. We report the first examples of the performance of organic field-effect transistors (OFETs) fabricated from DTCPD-based copolymers (Figure 1). The synthesis of the donor-acceptor copolymers is depicted in Scheme 1. First, 2,5-dithienyl-3,4-(1,8-naphthylene)cyclopentadi- enone (DTCPD) was prepared by procedures described earlier (DCC-mediated and double Knoevenagel condensation). 27,28 The desired 2,5-di(5-bromothienyl)-3,4-(1,8-naphthylene)cyclopentadi- enone (1) was obtained after treatment with bromine in acetic acid/ dichloromethane (95%). 4,4′-Dialkyl[2,2′]bithiophenes (DAT; 4,4′- dihexyl[2,2′]bithiophene (DHT) and 4,4′-didodecyl[2,2′]bithiophenes (DDT)) 32 were converted to the corresponding distannyl comono- mers via dilithiation and subsequent reaction with trimethylstannyl chloride. 33 The DTCPD units were incorporated with the DAT comonomer into the polymer backbone via palladium(0)-mediated Stille-type polycondensation, affording a series of DTCPD-alt-DAT copolymers (DTCPD-alt-DHT and DTCPD-alt-DDT). This syn- thetic protocol utilizes two symmetrical monomers, thus avoiding the regio-irregularities that can occur during the polymerization of asymmetric monomers. 7 Gel-permeation chromatography (GPC) analysis against a PS standard yields a number-averaged molecular mass (M n ) of 4700 and 6200 gmol -1 for DTCPD-alt-DHT and DTCPD-alt-DDT respectively with a polydispersity (PDI) of 1.39 and 1.71. There are many examples of the application of microwave irradiation as a heat source to reduce reaction times and increase | Current address: Harvard University, Department of Chemistry and Chemical Biology, 12 Oxford St., Cambridge, MA 02138. † Department of Chemistry and Biochemistry, University of California, Santa Barbara. ‡ Center for Polymers and Organic Solids, University of California, Santa Barbara. § Department of Chemistry and Biochemistry, University of California, Los Angeles. Figure 1. Chemical structures of DTCPD, DTCPD-alt-DHT, and DTCPD- alt-DDT. Scheme 1 a a Reagent and conditions: (i) Br 2 in AcOH/CH 2 Cl 2 , 95% (ii) Pd(PPh 3 ) 2 Cl 2 with conventional heating or Pd 2 (dba) 3 /(o-tolyl) 3 P with microwave heating in chlorobenzene. Published on Web 11/17/2008 10.1021/ja806784e CCC: $40.75  2008 American Chemical Society 16524 9 J. AM. CHEM. SOC. 2008, 130, 16524–16526