Synthesis of conjugated 2 and 2,5-(ethenyl) and (ethynyl)phenylethynyl thiophenes: fluorescence properties J. Gonzalo Rodriguez, * Jorge Esquivias, Antonio Lafuente and Laura Rubio Dpto. Quı ´mica Orga ´nica, Universidad Auto ´noma, Cantoblanco, 28049 Madrid, Spain Received 21 November 2005; revised 20 December 2005; accepted 10 January 2006 Available online 13 February 2006 Abstract—Nano conjugated thienylethenyl and thienylethynyl compounds with controlled structure and dimensions have been efficiently prepared, by heterocoupling reaction between 1,4-(thienylethynyl)phenylacetylene (or thienylethenyl)phenylacetylene and 2- or 2,5-dihalothiophene. Conjugated 1,4-di(2-thienylethynylphenyl)- (or 2-thienylethenylphenyl)-1,3-butadiyne were obtained by the homocoupling of the terminal acetylenes in excellent yield. The end-capped (N,N-dimethylaminophenyl)- and [3,5-di(trimethylsilylethynyl)- 1-ethynyl]-2,5-di(phenylethynyl) n thiophene were obtained by the heterocoupling between the corresponding terminal acetylene and 2,5-di(iodo)thiophene, catalyzed by the bis(triphenylphosphine)palladium and cuprous iodide system in excellent yield. q 2006 Elsevier Ltd. All rights reserved. 1. Introduction Nanometer-sized molecules of precise length showing p extended conjugation, exhibit in general high thermal stability and can present electroconductive, magnetic and optical properties. 1 Several potential applications such as artificial photosynthesis, 2 photocatalysis, 3 molecular photo- voltaic cells, 4 molecular informatics, 5 and optoelectronic devices 6,7 are beginning to emerge from this new field of research. Earlier studies on the heteroaromatic systems containing the thiophene ring shows three significant details: (i) an increasing of the first molecular hyperpolarizability (b) 8 (ii) the substitution of the thiophene ring by an aryl one, changes the donor versus the acceptor effect, 9 (iii) the electronic nature of the heteroaromatic ring affects the donor or acceptor strength through the inductive effects. 9 Therefore, polythiophene has been obtained as a conjugated polymer with many interesting optical and electronic properties such as electrochromism and near-metallic conductivity. 10 Oligomers of thiophene are also technologi- cally important materials and have been used in prototype organic thin-film transistors. 11 Interest in materials with these properties has directed substantial efforts towards the preparation of conjugated substituted oligo- and polythiophenes, 12 which exhibit an electronic conductivity that was dependent on the resonance along the polymeric chain. 10 Thiophene dendrimers with p-extended conjugated chains, shown that the p–p* conduction band decreases from the periphery to the heart, increasing the fluorescence quantum yield. 13 The conjugated thiophene chains show interesting solvatochromic properties. 14 Recently, we have developed p-extended conjugated structures with important optical properties. 15 Now, we report the synthesis of conjugated end-capped (N,N- dimethylaminophenyl)- and [3,5-di(trimethylsilylethynyl)- 1-ethynyl]-2,5-di(phenylethynyl) molecules containing the thiophene ring. Moreover, the conjugation effect through the double or triple bond linking the thiophene ring and the conjugated chains are also considered. 2. Results and discussion The synthesis of (E,E)-1,4-di(2 0 -thienyl)-1,3-butadiene, (E,E)-2 16 and 1,4-di(2-thienyl)-1,3-butadiyne (4), 17 was carried out to compare the polarity-polarizability produced by the double or triple bond of the thiophene conjugated systems. Thus, compound (E,E)-2 was obtained by the homocoupling reaction of 2-(2 0 -chlorovinyl)thiophene (1), catalyzed by the zerovalent nickel(triphenylphosphine) n complexes. The mixture (Z/E)-2-chlorovinylthiophene (1), 18 (Z/E, 43:57), was obtained by the Wittig reaction between furfural and Tetrahedron 62 (2006) 3112–3122 0040–4020/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2006.01.032 Keywords: Arylethynylthiophenes; p-Extended conjugation; Eglinton– Glaser reaction; Cadiot–Chodkiewicz reaction; Sonogashira reaction; Fluorescence. * Corresponding author. Tel.: C34 914974715; fax: C34 914973966; e-mail: gonzalo.rodriguez@uam.es