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

A series of small-sized model pi-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 Delta E-pi, 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 pi 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)(sigma) and (sigma (*) (S-C) being slightly stabilising. These results show extensive conjugation of the pi system corroborated by a small HOMOLUMO 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. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007).