Oligomers of “Extended Viologen”, <i>p</i>-Phenylene-bis-4,4‘-(1-aryl-2,6-diphenylpyridinium), as Candidates for Electron-Dopable Molecular Wires

We report the synthesis and spectral characterization of the first five members of an oligomeric series built from alternating <i>p</i>-connected 1,4-benzene and 1,4-pyridinium rings, <b>1[</b><b><i>n</i></b><b>]</b>−<b>4[</b><b><i>n</i></b><b>]</b>, <i>n</i> = 1−5, with <i>p</i>-phenylene-bis-4,4‘-(1-aryl-2,6-diphenylpyridinium) (“extended viologen”) as the repeating unit. The lengths of these rodlike molecules range from 2 to 9 nm. The monomer was obtained from <i>p-</i>phenylene-bis-4,4‘-(2,6-diphenylpyrylium) (<b>5</b>) and <i>p-</i>phenylenediamine (<b>6</b>) or <i>p</i>-aminoacetanilide (<b>9</b>). Higher oligomers were synthesized by stepwise elaboration of the monomer by reactions with the appropriate bis-pyrylium (<b>5</b>) or pyrylium-phenylene-pyridinium (<b>8</b>) salts. Eight different counterions were used, and dodecamethylcarba-<i>closo</i>-dodecaborate was found to offer particularly favorable solubility characteristics. Ultraviolet absorption spectra of the oligomers suggest that the individual extended viologen segments interact only weakly, as a result of the strongly twisted orientation of the benzene rings that separate them. The UV spectrum of the monomer was interpreted by comparison with semiempirical INDO/S calculations performed at a DFT optimized geometry.