Synthesis and characterization of novel N-carbazole end-capped oligothiophene-fluorenes Vinich Promarak, a, * Auradee Pankvuang a and Somsak Ruchirawat b a Advanced Organic Materials and Devices Laboratory, Department of Chemistry, Faculty of Science, Ubon Ratchathani University, Warinchumrap, Ubon Ratchathani 34190, Thailand b Chulabhorn Research Institute, Vipavadee Rangsit Road, Bangkok 10210, Thailand Received 24 October 2006; revised 6 December 2006; accepted 13 December 2006 Available online 8 January 2007 Abstract—A series of novel N-carbazole end-capped oligothiophene-fluorenes was synthesized using Ullmann coupling, bromina- tion, and Suzuki coupling reactions. The optical, thermal, and electrochemical properties of these materials can be tuned by varying the conjugation length of the oligothiophene segment. The terminal carbazole and central fluorene moieties of the resulting materials are beneficial for their morphology, conjugation length, and solubility. Ó 2006 Elsevier Ltd. All rights reserved. p-Conjugated oligothiophene molecules have attracted much attention in the area of organic chemistry and material sciences. They possess very interesting elec- tronic and optical properties and have been investigated as advanced molecular electronic materials, including, for example, organic field effect transistors (OFETs) 1 and organic light-emitting diodes (OLEDs). 2 The advantages of oligomers are that their physical proper- ties can be easily tuned to the desired properties by changing the structure, for example, solubilizing chains, end-capping groups, insertion of various functional groups and changing the oligomer lengths. Examples in- clude 2-n-hexylfluorene, 3 diphenylamine, 4 cyclophane, 5 pyrene, 6 ethylenedithio, 7 9,9-diphenylfluorene, 8 2,3,4,5- tetrahydrobenzo[b]thiophene, 9 dimesitylboryl, 10 and bis(4-methylphenyl)aminophenyl 2 end-capped oligothio- phenes, and oligothiophene-functionalized truxene, 11 tetraphenylmethane, 12 and thienyl-S,S-dioxide. 13 Due to its unique optical, electrical, and chemical properties, carbazole has been used widely as a functional building block or substituent in the construction of organic materials for optoelectronic devices. 14 Moreover, the thermal stability and glassy state durability of the organic molecules were found to be significantly improved upon incorporation of a carbazole moiety into the structure. 15 To our best knowledge, there are no reports on N-carbazole end-capped oligothiophene- fluorenes, whose synthesis and physical properties still remain to be explored. In this Letter, we report our success in accomplishing the synthesis of a novel series of N-carbazole end- capped p-conjugated oligothiophene-fluorenes and investigate their optical, thermal, and electrochemical properties. The synthetic strategy to the desired molecules is illus- trated in Scheme 1. Dibromofluorene 1 bearing solubi- lizing n-hexyl substituents was treated with an excess of 2-thiophene boronic acid under Suzuki coupling con- ditions using Pd(PPh 3 ) 4 as catalyst and Na 2 CO 3 as base to afford compound 2 in 58% yield. The resulting com- pound was chemoselectively brominated with NBS, in which only the most nucleophilic 5-positions of both thio- phene rings were substituted to give the corresponding dibromo compound 3 in 71% yield. By repeating the bromination and Suzuki coupling reaction sequences, dibromo oligothiophene-fluorenes 5 and 7 were formed in good yields. Final coupling of each dibromo compound 3, 5, and 7 with an excess of carbazole under Ullmann coupling conditions using copper powder as catalyst and K 2 CO 3 as base in nitrobenzene at reflux afforded the corresponding N-carbazole end-capped target oligomers CTnF (n = 1–3) in good yields. For 0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2006.12.073 Keywords: Oligothiophenes; Carbazole; Fluorene; Ullmann coupling; Organic light-emitting diode. * Corresponding author. Tel.: +66 81 5930005; fax: +66 45 288379; e-mail: pvinich@sci.ubu.ac.th Tetrahedron Letters 48 (2007) 1151–1154