Stable Tetrabenzo-Chichibabin’s Hydrocarbons: Tunable Ground State and Unusual Transition between Their Closed-Shell and Open-Shell Resonance Forms

Stable open-shell polycyclic aromatic hydrocarbons (PAHs)
are of
fundamental interest due to their unique electronic, optical, and
magnetic properties and promising applications in materials sciences.
Chichibabin’s hydrocarbon as a classical open-shell PAH has
been investigated for a long time. However, most of the studies are
complicated by their inherent high reactivity. In this work, two new
stable benzannulated Chichibabin’s hydrocarbons 1-CS and 2-OS were prepared, and their electronic structure
and geometry in the ground state were studied by various experiments
(steady-state and transient absorption spectra, NMR, electron spin
resonance (ESR), superconducting quantum interference device (SQUID),
FT Raman, X-ray crystallographic etc.) and density function theory
(DFT) calculations. 1-CS and 2-OS exhibited
tunable ground states, with a closed-shell quinoidal structure for 1-CS and an open-shell biradical form for 2-OS. Their corresponding excited-state forms 1-OS and 2-CS were also chemically approached and showed different
decay processes. The biradical 1-OS displayed an unusually
slow decay to the ground state (1-CS) due to a large
energy barrier (95 ± 2.5 kJ/mol) arising from severe steric hindrance
during the transition from an orthogonal biradical form to a butterfly-like
quinoidal form. The quick transition from the quinoidal 2-CS (excited state) to the orthogonal biradicaloid 2-OS (ground state) happened during the attempted synthesis of 2-CS. Compounds 1-CS and 2-OS can
be oxidized into stable dications by FeCl3 and/or concentrated
H2SO4. The open-shell 2-OS also
exhibited a large two-photon absorption (TPA) cross section (760 GM
at 1200 nm).