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(..) (lambda(ET)=450 nm) are calculated to be different from that of (1)7(..) (lambda(ET)=673 nm). These differences are attributed to interactions between orbitals of the two allyl radical moieties, especially, the existence of double sigma*-pi* conjugation, that is, pi*-sigma*-pi* conjugation, that takes place in the silole subunit. Copyright (C) 2011 John Wiley & Sons, Ltd.