Density functional theory study of silole‐fused tetramethyleneethane biradicals with orbital interactions

The geometries, energy gap between singlet and triplet states (S–T gap), electronic transitions, and molecular orbitals of the diphenyl derivative of the silole‐fused tetramethyleneethane (TME) (3,4‐dimethylenesilole) biradical 7 •• were determined by using the density functional theory method at the unrestricted Becke, three‐parameter, Lee–Yang–Parr/cc‐pVDZ level. In a manner similar to that of the parent TME biradical, the S–T gap of 7 •• is small. The wavelengths of electronic transitions of 3 7 •• ( λ ET  = 450 nm) are calculated to be different from that of 1 7 •• ( λ ET  = 673 nm). These differences are attributed to interactions between orbitals of the two allyl radical moieties, especially, the existence of double σ*–π* conjugation, that is, π*–σ*–π* conjugation, that takes place in the silole subunit. Copyright © 2011 John Wiley & Sons, Ltd.