Comparative studies of the geometric and electronic properties of 1,1-disubstituted-2,3,4,5-tetraphenylsiloles and 1,1,2,2-tetramethyl-3,4,5,6- tetraphenyl-1,2-disila-3,5-cyclohexadiene

We present an experimental and density functional theory comparison of the geometric structure, electronic characteristics and optical properties of 1,1-dimethyl-2,3,4,5-tetraphenylsilole (I), 1,1-(propane-1,3-diyl)-2,3,4,5- tetraphenylsilole (II), 1,1-dimethoxy-2,3,4,5-tetraphenylsilole (III) and 1,1,2,2-tetramethyl-3,4,5,6-tetraphenyl-1,2-disila-3,5-cyclohexadiene (IV). The molecular structures of II-IV have been determined using X-ray crystallography and are compared to the previously reported structure of I. Many characteristics of IV are significantly different from those of I-III, due to the presence of the additional silicon atom. In contrast to I-III, which have planar central rings, IV has a non-planar ring conformation due to the presence of two approximately tetrahedrally coordinated silicon atoms. Cyclic voltammetry suggests that I-III are reduced at similar potentials to that for tris(8-hydroquinolinato)aluminium and that IV is more readily oxidizable than I-III. Additionally, while IV is non-emissive in both solution and the solid state, the three siloles are fluorescent in the solid state. For the four compounds, we have calculated the intramolecular reorganization energies for electron- and hole-transfer reactions, ionization potentials and electron affinities.