Kinetically Blocked Stable Heptazethrene and Octazethrene: Closed-Shell or Open-Shell in the Ground State?

Abstract

Polycyclic aromatic hydrocarbons with an open-shell singlet biradical ground state are of fundamental interest and have potential applications in materials science. However, the inherent high reactivity makes their synthesis and characterization very challenging. In this work, a convenient synthetic route was developed to synthesize two kinetically blocked heptazethrene (HZ-TIPS) and octazethrene (OZ-TIPS) compounds with good stability. Their ground-state electronic structures were systematically investigated by a combination of different experimental methods, including steady-state and transient absorption spectroscopy, variable temperature NMR, electron spin resonance (ESR), superconducting quantum interfering device (SQUID), FT Raman, and X-ray crystallographic analysis, assisted by unrestricted symmetry-broken density functional theory (DFT) calculations. All these demonstrated that the heptazethrene derivative HZ-TIPS has a closed-shell ground state while its octazethrene analogue OZ-TIPS with a smaller energy gap exists as an open-shell singlet biradical with a large measured biradical character (y = 0.56). Large two-photon absorption (TPA) cross sections (σ((2))) were determined for HZ-TIPS (σ((2))(max) = 920 GM at 1250 nm) and OZ-TIPS (σ((2))(max) = 1200 GM at 1250 nm). In addition, HZ-TIPS and OZ-TIPS show a closely stacked 1D polymer chain in single crystals.

Recent Progress in Quinoidal Singlet Biradical Molecules

“…87 This derivative 42 showed biradicaloid behavior; broad 1 H NMR signals in the aromatic region, small ¦E ST (16.2 kJ mol ¹1 ), and an ESR signal in the solid state. The amount of singlet biradical character (y) was estimated to be 43% by a broken symmetry UCAM-B3LYP calculation.…”

Modern zethrene chemistry

“…20 Bulky triisopropylsilyl (TIPS) ethynyl groups are attached to the most reactive sites by nucleophilic addition of the corresponding quinones followed by reduction with SnCl 2 . The kinetical blocking by TIPS group and the extended spin delocalization to the ethynyl unit make the compounds 36 and 38 as very stable materials.…”

Recent Advances in Organic Radicals and Their Magnetism

“…Zethrenes with six, seven and eight rings (136, 137, and 138) have been synthesized and studied for their dynamic open-shell to close-shell properties (Chart 9) [156,157] (140) [156]. Compound 139 was found to be a closed-shell Kekule hydrocarbon as it exhibited sharp 1 H-NMR signals at room temperature due to a large singlet-triplet energy gap and only a minute quantity of triplet species.…”

“…Zethrenes with six, seven and eight rings (136, 137, and 138) have been synthesized and studied for their dynamic open-shell to close-shell properties (Chart 9) [156,157]. Recently, Wu et al reported a series of derivatives of zethrenes: zethrenebis(dicarboximide) (139), and heptazethrenebis(dicarboximide) (140) [156].…”