Theoretical Study of the Electronic Structure of 2,2′-Bisilole in Comparison with 1,1′-Bi-1,3-cyclopentadiene: σ*–π* Conjugation and a Low-Lying LUMO as the Origin of the Unusual Optical Properties of 3,3′,4,4′-Tetraphenyl-2,2′-bisilole

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Abstract
The purpose of this work is the theoretical elucidation of the origin of the unusually long UV-vis absorption maximum, λmax = 398 nm, of 3,3′,4,4′-tetraphenyl-2,2′-bisilole, which we have observed recently. Several semiempirical and ab initio calculations have been performed for some model compounds, silole and 2,2′-bisilole, in comparison with their carbon analogs, cyclopentadiene and 1,1′-bi-1,3-cyclopentadiene, respectively. The PM3 calculations indicate that the silole ring has a considerably low-lying LUMO, arising from σ*–π* conjugation between a π-symmetry σ* orbital of the exocyclic σ bonds on silicon and a π* orbital of the butadiene skeleton. Ab initio calculations at the CIS/6-31G* level of theory suggest that the experimental large differences in the absorption maximum in the UV-vis spectra between the bisilole and the bicyclopentadiene derivatives are ascribed also to the low-lying LUMO level of the bisilole compared with the bicyclopentadiene. It is further demonstrated that the σ*–π* conjugation is enhanced by the molecular distortions, twisting and folding, of the bisilole skeleton present in the X-ray structure, resulting in the lowering of the LUMO level. The calculated data are supported by the redox potentials of some representative compounds, as determined by cyclic voltammetry measurements. The σ*–π* conjugation and the low-lying LUMO would thus be the origin of the unusual optical properties of bisilole derivatives.