How to build fully π-conjugated architectures with thienylene and phenylene fragments

Article abstract of DOI:10.1002/ejoc.200601114

A series of small-sized model π-conjugated oligomers have been prepared from thienylene and phenylene or dimethyl-or dimethoxy-substituted phenylene units. Crystallographic data for the methoxylated compound show a quasi-planar conformation with a non-covalent S-O interaction. The resulting strong conjugation in the gas phase has also been highlighted by UV/photoelectron spectroscopy and theoretical calculations (DFT). Indeed, for these compounds there is a large energy gap ΔE^π arising from the interaction between the molecular orbitals of the isolated thienylene-phenylene species. This can be explained in terms of the energies of the two π orbitals of the dimethoxyphenylene unit, the shape of these molecular orbitals in a three-orbital interaction diagram and by the presence of the S...O interaction which reduces the inter-ring angle between the two aromatic cycles. The nature of the sulfur-oxygen interaction, discussed from a theoretical point of view, is mainly electrostatic, the orbital contribution from the only correctly directed orbitals n_O^σ and σ*_S-C being slightly stabilising. These results show extensive conjugation of the π system corroborated by a small HOMO-LUMO gap. These studies, carried out in the solid state and in the gas phase, show how important it is to combine thienylene and dialkoxyphenylene fragments to obtain oligomers with a strong electronic delocalisation. Thus, these compounds are of interest in the fields of electronic and optoelectronic devices. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.