Synthesis of Quinoidal Fused Oligosiloles by Rhodium-Catalyzed Stitching Reaction and Theoretical Investigation of Their Properties

New quinoidal fused oligosiloles containing an even number of silole units have been synthesized by a rhodium-catalyzed stitching reaction. Employing [RhCl­(tfb)]2 as the catalyst significantly improved the stitching efficiency, and up to six siloles could be fused in quinoidal form. A systematic comparison of the physical properties of Si1–Si6′ confirmed the unique trend in their LUMO levels, which become higher with longer π conjugation. To understand the origin of this unusual trend, theoretical calculations were also carried out using various model compounds, and the results indicated that the terminal indenylidene (cyclopentadienylidene) moieties in Si1–Si6 (Si1a–Si6a) are primarily responsible for this phenomenon through their frontier orbital correlations with the HOMO of the central polyene unit, which becomes higher in energy with longer π conjugation.