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.