Ethynylazulenes — Building Blocks for Novel Oligoazulenes with Ethynyl and Butadiynyl Bridges

The azulenes 3a,b, 6a,b, 9, 12 and 15 ethynylated in the five-membered ring were prepared by Pd-catalysed cross-coupling of the corresponding iodoazulenes 1a,b, 4a,b, 7, 10 and 13, respectively, with trimethylsilylacetylene and subsequent desilylation. The synthesis of some acyclic oligomers 23–30, 32–36 and 38–40 from these ethynylazulenes could be accomplished by oxidative Eglinton coupling as well as Pd/Cu-catalysed cross-coupling reactions with the appropriate iodoazulenes 1a,b, 4b, 31 and 34. The UV/Vis spectra of the novel oligomers 32, 35 and 36 allow an estimation of the bandgap (Eg) of poly(1,3-azulenylethynylene)s lower than 2 eV.