Theoretical comparison of ethylene-, disilane- and ethynylene-bonded aromatic compounds from the viewpoint of conjugation formation

Abstract

Oligosilanes, which contain Si–Si σ bonds in the molecules, exhibit properties of σ conjugation analogous to the π conjugation observed in CC π bonds. Similarly, Si–Si σ-bonded aromatic compounds display interactions between Si–Si σ and aromatic π bonds, resulting in the unique optical properties of conjugated systems such as intense absorption in the UV-vis region. This phenomenon is known as σ–π conjugation. While σ-conjugation and π-conjugation are well understood and various reviews have been published, σ–π conjugation in Si–Si σ-bonded aromatic compounds has not received much attention. In this paper, quantum chemical calculations were performed on representative compounds to examine the interaction of C–C σ, Si–Si σ, and CC π orbitals with aromatic π orbitals. Their Frontier orbitals and electronic transitions were analyzed to elucidate their similarities and differences. The conformation of the phenyl group in the Si–Ph moiety plays a crucial role in aromatic disilanes, and conjugation is the most effective when the overlap between Si–Si σ and phenyl π orbitals is maximized. The relationship between the silicon chain length in oligosilanes and their optical properties was also examined. The results indicated that σ–π conjugations increase the HOMO energy level and σ*–π* conjugation decreases the LUMO energy level in Si–Si σ-bonded aromatic molecules, leading to a reduction in the HOMO−LUMO gap. NBO analysis further supports the presence of modest π*–σ and σ*–π conjugations in aromatic disilanes. The results presented in this work provide fundamental insights into the design and application of functional Si–Si σ-bonded aromatic molecules.