The Combination of Cross‐Hyperconjugation and σ‐Conjugation in 2,5‐Oligosilanyl Substituted Siloles

Reaction of a 2,5‐dilithiated silole with excess dichlorodimethylsilane gives the respective 2,5‐bis(chlorodimethylsilyl) substituted silole. This compound can be converted to 2,5‐bis(oligosilanyl) substituted siloles by addition of a suitable oligosilanide. In the UV spectra of the thus obtained compounds the lowest energy absorptions are bathochromically shifted compared to the absorptions of the two constituents, namely the 2,5‐disilyl substituted silole and a trisilane. The bathochromic shift is interpreted as being caused by a mixed σ‐conjugation/cross‐hyperconjugation. This assumption is supported by TD‐DFT calculations, which show a significant contribution from Si−Si bonds to the HOMO of the molecule. Attaching oligosilanyl units to the 2‐ and 5‐positions of a silole was found to cause a ca. 30 nm bathochromic shift of the low energy absorption band. Computational analysis shows hyperconjugational interaction of σ (Si−Si) bonds in the oligosilanyl units with the occupied π‐orbitals of the butadiene part of the silole ring, which leads to higher energy of the HOMO and consequently to a smaller HOMO/LUMO energy difference.