Iterative synthesis and characterization of cross-conjugated iso-polydiacetylenes.

An iterative synthesis of cross-conjugated iso-polydiacetylenes (iso-PDAs) is reported based on three fundamental building blocks: vinyl triflates 8, 9, and 25. An efficient sequence of palladium-catalyzed cross-coupling reactions of vinyl triflates and terminal alkynes has been employed to extend the chain length of these oligomers. The longest member of the series, nonamer 19, spans 3.4 nm from Si to Si atom. The stability and solubility of the conjugated oligomers have been evaluated as a function of pendant substitution. Assessment of solid-state structural properties was achieved via X-ray crystallographic analyses of monomer 12 and trimer 14. The electronic characteristics of the monodisperse oligomers have been fully analyzed by UV-vis spectroscopy in solution and as thin films, and these studies suggest that pi-electron communication is present along the enyne framework, but quickly reaches saturation by the stage of nonamer 19.