Honda Shunya, Sugawara Ryutaro, Ishida Shintaro, Iwamoto Takeaki
Department of Chemistry, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578, Japan.
J Am Chem Soc. 2021 Feb 24;143(7):2649-2653. doi: 10.1021/jacs.0c12426. Epub 2021 Feb 10.
Spiroconjugation, that is, through-space orbital interactions between two perpendicular π orbitals, is a key concept in the contemporary molecular design of spirocyclic π-electron systems. We synthesized spiropentasiladiene radical cation salt as a dark-green solid via the one-electron oxidation of the stable spiropentasiladiene . Characterization of the molecular structure combined with theoretical studies indicated that the spin and positive charge are delocalized across the two perpendicular Si═Si double bonds of . Two π(Si═Si) orbitals are split into HOMO and SOMO with a small energy gap owing to the second-order Jahn-Teller distortion and steric repulsion between bulky alkyl groups upon one-electron oxidation. In the UV-vis-NIR spectrum, the longest-wavelength absorption band of (λ = 1972 nm) covers the IR-B region (1400-3000 nm; 0.89-0.41 eV) despite having the smallest possible spiroconjugation motif. The unprecedented absorption band in the IR region was assigned to the HOMO → SOMO transition that arises from the delocalized π-orbitals in the spirocyclic Si skeleton.
螺共轭，即两个垂直π轨道之间的空间轨道相互作用，是当代螺环π电子体系分子设计中的一个关键概念。我们通过稳定的螺戊硅二烯的单电子氧化反应，合成了作为深绿色固体的螺戊硅二烯自由基阳离子盐。分子结构表征与理论研究表明，自旋和正电荷在的两个垂直Si═Si双键间离域。由于单电子氧化后二阶 Jahn-Teller 畸变以及庞大烷基之间的空间排斥作用，两个π(Si═Si)轨道分裂为具有小能量间隙的HOMO和SOMO。在紫外-可见-近红外光谱中，尽管具有尽可能小的螺共轭基序，但的最长波长吸收带（λ = 1972 nm）覆盖了红外-B区域（1400 - 3000 nm；0.89 - 0.41 eV）。红外区域中前所未有的吸收带被归因于螺环Si骨架中离域π轨道产生的HOMO→SOMO跃迁。
