Preparation of conjugated gels of regioregular HT sexi(3-n-octylthiophene)and related star molecules

We present new kinds of conducting gels of octylthiophene. Contrary to the gels previously obtained in our laboratory, they present two "high" gelatio n thresholds (i.e., related to high crosslink ratios). Consequently, system s close to the gelation threshold can be prepared with accuracy. Furthermor e, they are calibrated and regioregular: The length and the regioregularity of the chain segments between adjacent cross-links are fully controlled. T hey are prepared by chemical polycondensation of monodisperse regioregular [(head-to-tail (HT)] oligo(octylthiophene)s with tribromobenzene. For the p reparation of these gels, two kinds of regioregular oligomers are synthesiz ed: linear (from dimer to hexamer) and "star" oligo(octylthiophene)s. The t wo star oligomers are made of a benzene ring trisubstituted in meta positio ns by a regioregular (HT) segment of respectively six (E6) and twelve (E12) octylthiophenes. The structural properties of the oligomers are studied by H-1 and C-13 NMR spectroscopy; information concerning their conformation i s reported. The optical properties (UV-vis absorption and fluorescence) of the star oligomers are compared with that of the linear oligomers. A linear evolution of the energy of the fluorescence transitions of the linear regi oregular oligo(octylthiophene)s with the reverse of thiophene number is dem onstrated for the first time. The Stille coupling chosen to prepare the gel s from the oligomers is studied to determine the experimental conditions al lowing both the good solubility of large clusters formed during the gel pre paration and the good yield of the coupling reaction. A first series of 14 networks is reported. It is characterized by two gelation thresholds that a re very high, as expected, but very close one from another. A strategy is p roposed to increase the distance between the two gelation thresholds in the coming series.