Theoretical study of the electronic structure of 2,2'-bisilole in comparison with 1,1'-Bi-1,3-cyclopentadiene: sigma*-pi* conjugation and a low-lying LUMO as the origin of the unusual optical properties of 3,3',4,4'-tetraphenyl-2,2'-bisilole

The purpose of this work is the theoretical elucidation of the origin of the unusually long UV-vis absorption maximum, lambda(max) = 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 sigma*-pi* conjugation between a pi-symmetry sigma* orbital of the exocyclic sigma bonds on silicon and a pi* 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 sigma*-pi* 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 sigma*-pi* conjugation and the low-lying LUMO would thus be the origin of the unusual optical properties of bisilole derivatives.