A Spiropentasiladiene Radical Cation: Spin and Positive Charge Delocalization across Two Perpendicular SiSi Bonds and UV–vis–NIR Absorption in the IR‑B Region

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 1 as a dark-green solid via the one-electron oxidation of the stable spiropentasiladiene 2. 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 1. 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 1 (λmax = 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 Si5 skeleton.