A Tale of Two Isomers: Enhanced Antiaromaticity/Diradical Character versus Deleterious Ring‐Opening of Benzofuran‐fused s‐Indacenes and Dicyclopenta[b,g]naphthalenes

Abstract

We examine the effects of fusing two benzofurans to s‐indacene (indacenodibenzofurans, IDBFs) and dicyclopenta[b,g]naphthalene (indenoindenodibenzofurans, IIDBFs) to control the strong antiaromaticity and diradical character of these core units. Synthesis via 3‐functionalized benzofuran yields syn‐IDBF and syn‐IIDBF. syn‐IDBF possesses a high degree of paratropicity, exceeding that of the parent hydrocarbon, which in turn results in strong diradical character for syn‐IIDBF. In the case of the anti‐isomers, synthesized via 2‐substituted benzofurans, these effects are decreased; however, both derivatives undergo an unexpected ring‐opening reaction during the final dearomatization step. All the results are compared to the benzothiophene‐fused analogues and show that the increased electronegativity of oxygen in the syn‐fused derivatives leads to enhancement of the antiaromatic core causing greater paratropicity. For syn‐IIDBF increased diradical character results from rearomati‐zation of the core naphthalene unit in order to relieve this paratropicity.

Designing molecules with a high-spin (quintet,S= 2) ground state for magnetic and spintronic applications

“…However, the majority of the PCHs (with or without substituted heteroatoms) have a small-to-moderate open-shell character and possess a singlet ( S = 0) ground state due to the double-spin polarization 26 with a low-lying triplet excited state. 5,25,27–42 Recent studies on high-spin substituted PCHs report a triplet ( S = 1) ground state (Fig. 1d and e), showing the possibilities to design hydrocarbon-based materials with magnetic properties.…”

“…32,45 Optimization of s -indacene's molecular structure and properties involving the substitution of bulky tert -butyl groups to kinetically block the reactive sites 46 and fusion of external five-/six-membered rings to extend the π-conjugation path served as the most proficient strategies. 30–32,34,40,47–49 The fusion of external rings modulates the paratropicity of the s -indacene core, reducing the highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) energy gap. 31,32,40 Our recent study, 50 where we have thoroughly investigated the relationship between the HOMO–LUMO energy gap and open-shell character, showed that molecules with a smaller HOMO–LUMO energy gap provide a more open-shell character.…”

“…51 The open-shell character of the molecules with an s -indacene core indicates a moderate diradical character obtained using the spin-projected unrestricted Hartree–Fock (PUHF) method (see Table S1†). 34 Due to a large spin-contamination inherent to the PUHF method, the open-shell diradical character obtained is largely overestimated. 50,52 Therefore, the electronic ground state of the available materials with an s -indacene core lies in between the closed-shell and open-shell low-spin singlet state, with a thermally accessible high-spin triplet excited state.…”

“…50,52 Therefore, the electronic ground state of the available materials with an s -indacene core lies in between the closed-shell and open-shell low-spin singlet state, with a thermally accessible high-spin triplet excited state. 34,40,49…”

“…53–59 On the other hand, the addition of the five-membered carbonaceous rings can modulate the aromatic character along the molecular backbone, generating an open-shell polyradical character. 60–64 Although there is a significant amount of work available on molecular-based diradicals, 5,14,16–19,27–29,33,34,36–38,50,51,55,65–67 very few studies report open-shell materials with a large polyradical character. 60–64,68–73 In the open-shell diradicaloid form, the benzenoid ring at the s -indacene core can regain aromatic stabilization energy at the expense of the double-bond breaking of the dicyanomethylene groups (Fig.…”