6,6′-Biindeno[1,2-b]anthracene: An Open-Shell Biaryl with High Diradical Character

Abstract

We report in situ generation of a 6,6′-biindeno [1,2-b]anthracene (BIA) derivative as an open-shell biaryl with high diradical character, which could be identified by mass spectrometry, NMR spectroscopy, single-crystal X-ray analysis, UV−vis−NIR absorption spectroscopy, and electron paramagnetic resonance (EPR) spectroscopy. Theoretical calculations by various methods and variable-temperature EPR analyses were performed to tackle the elusive ground state of BIA diradical, suggesting a singlet ground state with a nearly degenerate triplet state. These results provide insight into the design of unique open-shell biaryls.

Unlocking Biradical Character in Diborepins

“…Initially, at room temperature, the characteristic half-field transition of Δ m s = ±2 was not detected in these compounds. This observation aligns with behaviors seen in other disjointed main-group biradicals, characterized by large interspin distances and minimal zero-field splitting (ZFS) parameters . In such cases, the half-field transition intensity inversely correlates with the sixth power of the interspin distance ( I (Δ m s = ±2) ∝ r –6 ).…”

Bis(9-Boraphenanthrene) and Its Stable Biradical

“…The paramagnetic nature of 4 was confirmed by solution and solid-state EPR spectroscopy (Figure ). In THF, the EPR spectrum of 4 displays a single broad signal centered at g = 2.004, with no observable hyperfine splitting, in agreement with EPR spectra observed for carbon-based PAH biradical systems. ,, The powder EPR spectrum of 4 displays low and high field transitions (Δ m s = ±1) characteristic of organic spin triplets ( S = 1), coinciding with a central transition determined through simulation to be a monoradical ( S = ) impurity originating from a single N– carbene C–B spin system . The assignment of this impurity is consistent with our previously reported CAAC-stabilized borepin and borafluorene radicals, where the spin system is localized to the N– carbene C–B moiety. , The point-dipole approximation employing the simulated zero-field splitting parameter | D′ | (2.62 mT) determined an average interspin distance of 10.2 Å, consistent with crystallographic measurements indicating a distance of approximately 9.57 Å between the centroids of the two N– carbene C–B fragments.…”

“…Conversion of 1 into diradical 4 leads to a bathochromic shift in absorbance. Compound 4 exhibits low energy absorption features at 520 and 472 nm which resemble the low-lying singlet excited states typical for singlet biradicaloids and are suggested to originate from a doubly excited electronic configuration …”

“…Most of the reported boron-doped PAHs can be characterized by a closed-shell electron configuration. − In contrast, boron-doped PAHs exhibiting an open-shell singlet configuration are rare (Figure f,g). , This biradical character emerges only when the biradical resonance structure overcomes the “classical” closed-shell preference due to a combination of strong electronic factors and stabilization energy. − The stability and reactivity of closed-shell PAHs can be determined by the number of aromatic π-sextet rings they possess (i.e., Clar’s aromatic sextet rule). , Benzenoid PAHs with the same chemical composition with more aromatic sextet rings are more stable and less reactive. In contrast, for open-shell biradical species, the biradical character (γ) increases significantly as more aromatic sextet rings form .…”

Carbon-based Biradicals: Structural and Magnetic Switching

“…There was no sign of isomerization to the folded form (Scheme 29). 115 Furthermore, for the biradical form of 33, both the singlet and triplet states were obtained as energyminimized structures by DFT calculation (CASSCF(12e,12o)/ def2-TZVPP) with the triplet state as a ground state. The VT-ESR study showed that the signal intensity of ΔM s = ±2 increased with a decrease in temperature, thus demonstrating a triplet ground state.…”