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

Abstract

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 FeCl(3) and/or concentrated H(2)SO(4). The open-shell 2-OS also exhibited a large two-photon absorption (TPA) cross section (760 GM at 1200 nm).

Recent Advances in Organic Radicals and Their Magnetism

“…Wu et al reported derivatives of Chichibabin's hydrocarbon, 133-CS/133-OS and 134-CS/134-OS ( Figure 17) [155]. The time-dependent UV-vis-NIR absorption spectra of the 133-OS biradical showed that the absorption spectrum of the biradical diminished with time and the final spectrum after the decay entirely matched with the stable quinoidal form 133-CS (Figure 18a).…”

Recent Progress in Quinoidal Singlet Biradical Molecules

“…Recently two types of tetrabenzo derivatives 15 and 16 of 2 were isolated by Wu and their biradicaloid behaviors were thoroughly investigated by means of X-ray crystallographic and spectroscopic measurements. 38 The interesting point of the benzannulated compounds lies in that the electronic structure of the ground state is determined by the energy balance between structural strain and π-bond formation. Due to the steric hindrance around the anthracene moiety, 15 and 16 adapt not a planar form but an orthogonal or folded form as the most stable form.…”

N‐Heterocyclic Carbene Analogues of Thiele and Chichibabin Hydrocarbons

“…[4e, 8] Over the last few years,v arious derivatives and analogues of TH and CH with enhanced stability and diradical character have been prepared using kinetic and/or thermodynamic stabilization approaches. [9a,10] Spin delocalization with extended p-conjugated systems using a" benzannulation" approach [11] has also been used to stabilize carbon-centered diradicals such as III.Nevertheless, the number of carbon-centered diradicals remains limited. [9a,10] Spin delocalization with extended p-conjugated systems using a" benzannulation" approach [11] has also been used to stabilize carbon-centered diradicals such as III.Nevertheless, the number of carbon-centered diradicals remains limited.…”