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.