New4,4 0 -oligophenylenevinylenefunctionalized-[2,2 0 ]-bipyridyl chromophores:synthesis,opticalandthermalproperties Lydie Viau, Olivier Maury and Hubert Le Bozec * ‘‘Organom etalliques et Catalyse: Chimie et Electrochimie Mol eculaires’’, UMR 6509 CNRS-Universit e de Rennes 1, Institut de Chimie de Rennes, Campus de Beaulieu, 35042 Rennes, France Received 17 September 2003; revised 13 October 2003; accepted 20 October 2003 Abstract—The synthesis and characterization of new bipyridyl-based chromophores featuring extended oligophenylenevinylene p-conjugated backbones are reported. Their absorption and emission properties as well as their thermal stabilities are discussed in comparison to those of the parent ligand. Ó 2003 Elsevier Ltd. All rights reserved. Organic molecules bearing extended p-conjugated sys- tems are attracting much attention for their properties in the field of molecular electronics 1 and photonics. 2 In particular, oligophenylenevinylene derivatives (OPV) have been extensively investigated in regard to applica- tions in the fields of photo- and electroluminescence, 3 photovoltaism, 4 or nonlinear optics (NLO). 5 In this context, our group has been involved in the design of 4,4 0 -disubstituted-[2,2 0 ]-bipyridines as fluorophores and precursors to molecular, 6a;b macromolecular 6c and supramolecular 6d metallo-octupoles for second order nonlinear optics. From the wide range of 4,4 0 -p-con- jugated bipyridyl ligands already prepared, 7 such as 4,4 0 -bis(dibutylaminostyryl)-[2,2 0 ]-bipyridine 1, we have previously shown how simple modification of the p linker enables the generation of tunable chromophores and fluorophores. Continuing our effort to probe the structural factors that could improve the optical and nonlinear optical properties of p-conjugated bipyridyl ligands and complexes, we report herein the synthesis of new bipyridines featuring extended oligophenylenevin- ylene p-conjugated backbones (OPV-bpy) 2–3 which are the styrene-based homologues of 1 (Chart 1). Their optical properties (absorption and emission) as well as their thermal stability will be discussed with regard to those of the parent ligand 1. Several methodologies were tested for the preparation of OPV-bpy 2–3. The first attempts, involving a classical Knoevenagel condensation between 4,4 0 -dimethyl-[2,2 0 ]- bipyridine and the corresponding aldehydes, in the presence of lithium diisopropylamide or potassium-tert- butoxide, failed. 7 A Heck cross-coupling reaction between 4,4 0 -bis-vinyl-[2,2 0 ]-bipyridine 8 and 4-dibutyl- amino-4 0 -bromostilbene was also carried out by analogy with the preparation of analogous push–pull OPV- pyridines, 9 but in our case no reaction occurred. Further investigations involving Heck cross-coupling between 4,4 0 -bis(bromostyryl)-[2,2 0 ]-bipyridine 10 and p-dibutyl- aminostyrene under various conditions always gave mixtures of 2 and the corresponding monofunctional- ized derivative which could not be separated by crys- tallization or chromatography. Finally, the synthesis of OPV-bpyÕs 2–3 was achieved by means of a double Wadsworth–Emmons reaction, as depicted in Scheme 1. The preparation of the aldehyde precursors 7 11 and 11 was carried out using different routes. Reaction of 4- bromobenzyl bromide 4 in refluxing P(OEt) 3 gave the corresponding phosphonate 5; subsequent Wittig-type reaction with p-dibutylaminobenzaldehyde resulted in the formation of the stilbene 6. Further halogen–lithium exchange, followed by quenching of the lithiated inter- mediate with dimethylformamide led to the formation of the desired aldehyde 7 in 36% overall yield after purifi- cation by column chromatography. The use of a similar methodology failed for the preparation of the ‘‘styrylo- gous’’ compound 11. However, this could be prepared in 21% overall yield by using an alternative four-step Keywords: bipyridines; chromophores; fluorophores. * Corresponding author. Tel.: +33-2-23236544; fax: +33-2-23236939; e-mail: lebozec@univ-rennes1.fr 0040-4039/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2003.10.099 Tetrahedron Letters 45 (2004) 125–128 Tetrahedron Letters