Doubly Silicon‐Bridged Planar 2,2′‐Bipyridine Ligands: Neutral Intramolecular Coordination to Silicon Centers and σ–π Conjugation–Dependent Optical Properties

Although many silicon‐bridged planar conjugation compounds have been reported and extensively studied as optical and electronic materials, there are limited examples of conjugated compounds bridged through coordination bonds to silicon atoms. We designed and synthesized 2,2′‐bipyridine ligands with silicon atoms at the 3‐ and 3′‐positions and obtained a planar bipyridine moiety stabilized by double intramolecular coordination of the nitrogen atoms of the 2,2′‐bipyridine moiety to the silicon atoms. The crystal structures of 3,3′‐disilyl‐2,2′‐bipyridines were determined by X‐ray diffraction analysis. The geometry of the pentacoordinated silicon centers was similar to trigonal bipyramidal because of the intramolecular Si–N coordination. In addition to the crystal structures, 29 Si NMR spectra, electron‐ionization mass spectroscopy, and theoretical calculations supported the presence of Si–N coordination. 3,3′‐Disilyl‐2,2′‐bipyridines showed much weaker absorptions than 2,2′‐bipyridine. Time‐dependent density functional theor (TD‐DFT) calculations indicate that these extraordinary optical properties are due to the conjugation of the bipyridine π‐orbital and silicon–carbon σ‐bond. Our comprehensive studies of planar conjugation compounds bridged by intramolecular Si–N coordination provide a new conjugation system comprising π‐orbitals and silicon σ‐bonds.