Spin-Delocalization in a Helical Open-Shell Hydrocarbon

Abstract

Neutral open-shell molecules, in which spin density is delocalized through a helical conjugated backbone, hold promise as models for investigating phenomena arising from the interplay of magnetism and chirality. Apart from a handful of examples, however, the chemistry of these compounds remains largely unexplored. Here, we examine the prospect of extending spin-delocalization over a helical backbone in a model compound naphtho[3,2,1-no]tetraphene, the first helically chiral open-shell hydrocarbon, in which one benzene ring is fused to [5]helicene, forming a phenalenyl subunit. The unpaired electron in this molecule is delocalized over the entire helical core composed of six rings, albeit in a nonuniform fashion, unlike in phenalenyl. In the case of a monosubstituted derivative, the uneven spin-distribution results in a selective σ-dimer formation in solution, as confirmed by 2D NMR spectroscopy. In contrast, the dimerization process is suppressed entirely when four substituents are installed to sterically hinder all reactive positions. The persistent nature of the tetrasubstituted derivative allowed its characterization by EPR, UV-vis, and CD spectroscopies, validating spin-delocalization through a chiral backbone, in accord with DFT calculations. The nonuniform spin-distribution, which dictates the selectivity of the σ-dimer formation, is rationalized by evaluating the aromaticity of the resonance structures that contribute to spin-delocalization.

Unveiling the Hidden σ‐Dimerization of a Kinetically Protected Olympicenyl Radical

“…[23] Among organic π-radicals, the study on self-association mode of spin 1 = 2 open-shell graphene fragments (OGFs) [24] is of substantial interest on account of their spin-delocalized nature and predominately topology-based properties. [25,26] So far, the smallest member in this family, phenalenyl radical (PR), has been investigated extensively in both theoretical and experimental aspects [27][28][29][30] regarding its dimerization mode. The D 3hsymmetric radical could form a 12-center-2-electron π-dimer and three σ-dimers connected at the α-position as stereoisomers, with chiral RR/SS and meso RS configurations (Figure 1a).…”

Dimethylcethrene: A Chiroptical Diradicaloid Photoswitch

“…In the pursuit of identifying structural features that would allow for fine tuning of key electronic parameters of diradicaloids, namely, the HOMO–LUMO and ST gaps, we turned our attention to systems that feature 8 a helical π-conjugated backbone. On account of the helical structure of these systems, through-space orbital interactions arise 9 within their FMOs, which can increase or decrease the FMO energies.…”

Platforms for Stable Carbon‐Centered Radicals

“…[69] This radical was stable in the solid state open to air at À30 8 8Cand in adegassed solution for af ew weeks.J uríček and co-workers reported ah elical hydrocarbon radical (48)w ith its CD spectra. [70] In 2018, Osuka and co-workers extended their stable radical 40Ni to helical systems. [71] Although their initial designs to introduce methyl and methoxy groups inside the [4]helicene moieties led to inseparable mixtures of regioisomers, rac-49Ra,b,o wing to unexpected rearrangement, an oxidative cyclization reaction furnished the desired p-extended radicals rac-50 a,b in pure forms.The obtained radicals were highly stable both in the solid state and in solution open to air,a llowing X-ray crystallographic analysis of rac-50 a and solid-state magnetometry for rac-50 b.Notably,the optical resolution of 50 b was accomplished on ac hiral column and the separated enantiomers displayed an ear-infrared CD response up to 1400 nm, indicating that the spin is well delocalized over the entire helically conjugated network.…”