COMMUNICATION Journal of Materials Chemistry www.rsc.org/materials Synthesis, physical properties, and field-effect transistors of novel thiophene/thiazolothiazole co-oligomers Shinji Ando, a Jun-ichi Nishida, a Youji Inoue, b Shizuo Tokito b and Yoshiro Yamashita* a a Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama, 226-8502, Japan. E-mail: yoshiro@echem.titech.ac.jp; Fax: 181-45-924-5489; Tel: 181-45-924-5571 b NHK Science and Technical Research Laboratories, Kinuta, Setagaya-ku, Tokyo, 157-8510, Japan. E-mail: tokito.s-eu@nhk.or.jp; Fax: 181-3-5494-3297; Tel: 181-3-5494-3284 Received 10th March 2004, Accepted 22nd April 2004 First published as an Advance Article on the web 17th May 2004 p-Channel OFETs using thiophene/thiazolothiazole co- oligomers as active layers have been successfully fabricated, and the field-effect mobilities for these oligomers were higher than those for thiophene/thiazole co-oligomers. Development of organic semiconductors for active layers of field effect transistors (FETs) has been intensively studied because the performance of the FET is strongly influenced by the active layer. 1–4 In FET devices, the semiconductor layer supports holes or electrons between the source and drain electrode. The density of charge carriers in the channel is modulated by the voltage applied through the gate electrode. Organic semiconductors for FET devices with high perfor- mance are required to have high charge carrier mobilities, on/ off current ratios, stability, and processability. p-Conjugated molecules such as thiophene oligomers are an important class of organic semiconductors and sexithiophene (a-6T) is a typical compound which shows good field-effect characteristics (hole mobility: 0.07–0.1 cm 2 V 21 s 21 and on/off ratio 10 2 –10 3 ). 5 In order to improve the FET performance, many modifications have been carried out on thiophene oliogmers so far. 6 Among them, replacement of the thiophene units with thiazole rings in the thiophene oligomers is reported to be effective to reduce the steric interactions due to the absence of one hydrogen atom. 7 In addition, thiazole is an electron-withdrawing heterocycle and the replacement enhances stability to oxygen. 8 Some thiazole oligomers form p-stacked structures which are necessary for efficient intermolecular p–p overlap. 9 In this context, we have now designed novel thiophene oligomers containing a thiazo- lothiazole unit, which have the following advantages for semiconductors of FETs. First, thiazolothiazole is an analogue of thiazole and the electron-accepting property is considered to be useful to enhance stability to oxygen. Second, these oligo- mers are a kind of donor–acceptor compounds (donor: thio- phene oligomers; acceptor: thiazolothiazole). The energy gap of these donor–acceptor oligomers is reduced compared to those of thiophene oligomers and the lower energy gap is favorable for high carrier mobilities. 10 Ambipolar properties are also expected in such donor–acceptor compounds. 10 Third, the ring has a rigid planar structure due to the fused ring system like acenes. This would lead to efficient intermolecular p–p interactions. Fourth, the derivatives are easily prepared start- ing from the corresponding aldehydes. We report here the synthesis, structure and characterization of novel thiophene/ thiazolothiazole co-oligomers (TFZT, TTFZTT) and the dihexyl derivative (DH-TTFZTT). The OFETs based on these novel oligomers have been fabricated, and their FET performances (TTFZTT, DH-TTFZTT) as p-type semi- conductors are presented here as first examples of FET behavior of thiazolothiazole derivatives. The thiophene derivative (TFZTT) was obtained by the reaction of 2-thiophenecarboxyaldehyde with dithiooxamide. 11 Bithiophene derivatives TTFZTT and DH-TTFZTT were pre- pared by the similar reaction from the corresponding aldehydes (Scheme 1). The thiophene/thiazolothiazole co-oligomers puri- fied by sublimation are bright-yellow (TFZT) and bright-red (TTFZTT, DH-TTFZTT) crystalline solids, respectively. 12 The structures were determined by spectral data along with elemental analysis. Differential scanning calorimetry (DSC) measurements of TFZT and TTFZTT showed sharp melting endotherms at 237 uC and 280 uC, respectively. DSC mea- surement of DH-TTFZTT showed a melting endotherm at 195 uC, indicating that DH-TTFZTT is thermally less stable than TTFZTT. The absorption and emission data of these co-oligomers are summarized in Table 1. The absorption maxima are red-shifted compared to the corresponding thiophene oligomers. This is attributed to an intramolecular charge-transfer interaction between the electron-withdrawing thiazolothiazole unit and the electron-donating thienyl groups. Bithiophene derivatives TTFZTT and DH-TTFZTT show maxima at longer wave- lengths than TFZT due to the extended p-conjugation as well Scheme 1 Synthesis of thiophene/thiazolothizole co-oligomers. DOI: 10.1039/b403699a J. Mater. Chem. , 2004, 14 , 1787–1790 1787 This journal is ß The Royal Society of Chemistry 2004