Synthesis and properties of N-carbazole end-capped conjugated molecules Vinich Promarak a, * and Somsak Ruchirawat b a Advanced Organic Materials and DevicesLaboratory, Department of Chemistry, Faculty of Science, Ubon Ratchathani University, Warinchumrap, Ubon Ratchathani 31490, Thailand b Chulabhorn Research Institute, Vipavadee Rangsit Highway, Bangkok 10210, Thailand Received 4 September 2006; revised 20 November 2006; accepted 7 December 2006 Abstract—A series of novel N-carbazole end-capped p-conjugated molecules were synthesized by a divergent approach with the use of bro- mination, Suzuki cross-coupling, and Ullmann reactions and their physical properties were investigated. In dilute solution, UV–vis absorption spectra displayed bathochromic shift with respect to their conjugated backbones, and photoluminescence spectra showed emission maxima in the blue region. Thermal analysis revealed that they are thermally stable semi-crystalline and amorphous materials. All molecules exhibited good electrochemical stability with high-lying HOMO energy levels and have potential applications as hole-transporting and light-emitting layers in organic light-emitting diodes or as host materials for electrophosphorescent applications. Ó 2006 Elsevier Ltd. All rights reserved. 1. Introduction p-Conjugated small molecules have attracted much atten- tion in the area of organic chemistry and material science. They have some very interesting electronic and optical prop- erties and have been investigated as advanced molecular electronic materials. 1–3 p-Conjugated thiophene-based oligo- mers have earlier been used as model compounds for poly- mers 4 and have recently attracted interest as the sole active components in devices. 5,6 The advantages of oligomers are that their physical properties can be easily tuned to the de- sired properties by changing the structure e.g. solubilizing chains, end-capping groups, insertion of certain groups, and different oligomer lengths. Much synthetic effort has been focused on improving physical characteristics of the oligothiophene-based compounds. 7,8 Due to its unique photo, electrical, and chemical properties, carbazole has been widely used as a functional building block or substituent in the construction of organic photoconductors, 9 non-linear optic (NLO) materials, 10 hole-transport, and light-emissive materials for OLED devices 11 and as host materials for phos- phorescence applications. 12 Moreover, thermal stability and glassy state durability of organic molecules were found to be significantly improved upon incorporation of a carbazole moiety in the structure. 13 To our best knowledge, there is no report on N-carbazole end-capped bisthiophenyl derivatives, the synthesis and physical properties of which still remain to be explored. In this paper, we accomplished the synthesis of novel N-car- bazole end-capped p-conjugated bisthiophenyl derivatives, aiming at investigating how their optical, thermal, and elec- trochemical properties were affected by the molecular struc- ture of the conjugated backbones. 2. Results and discussion 2.1. Synthesis and characterization Scheme 1 shows a synthetic route to the carbazole end- capped p-conjugated molecules CTB, CTC, CTF , and CT using a divergent approach in which the desired mole- cules were constructed in an outward direction stepwise from the cores, end-capping with the carbazole units in the final step. Different cores of dialkoxybenzene 1, alkylcarb- azole 6, and dialkylfluorene 11, were used and synthesized as described in the literature. The alkyl substituents would provide good solubility to the desired products in common organic solvents. They were first treated with either Br 2 solution or N-bromosuccinimide (NBS) as brominating reagents to give their corresponding dibromo compounds 2, 7, and 11 in high yields. Attachment of thiophene units to both ends of each molecule was accomplished by coupling of the resultant dibromo compounds 2, 7, and 11 with Keywords: Carbazole end-cap; p-Conjugated molecules; Ullmann cou- pling; Hole-transport; Organic light-emitting diodes. * Corresponding author. Tel.: +66 15 930005; fax: +66 45 288379; e-mail: pvinich@sci.ubu.ac.th 0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2006.12.011 Tetrahedron 63 (2007) 1602–1609