Bis-periazulene: a simple Kekul� biradical with a triplet ground state

Nendel, Maja; Goldfuß, Bernd; Houk, K. N.; Häfner, Klaus; Grieser, Udo

doi:10.1007/s002140050511

Cited by 16 publications

(17 citation statements)

References 8 publications

(10 reference statements)

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“…The only unsynthesized isomer, bis-periazulene (cyclohepta[ def ]fluorene) 1 , remains elusive despite many synthetic efforts. The first synthetic attempt for 1a was reported in 1955 by Reid .…”

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confidence: 99%

Bis-periazulene (Cyclohepta[def]fluorene) as a Nonalternant Isomer of Pyrene: Synthesis and Characterization of Its Triaryl Derivatives

et al. 2022

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Bis-periazulene (cyclohepta[def ]fluorene), which is an unknown pyrene isomer, was synthesized as kinetically protected forms. Its triaryl derivatives 1c−e exhibited the superimposed electronic structures of peripheral, polarized, and open-shell π-conjugated systems. In contrast to previous theoretical predictions, bis-periazulene derivatives were in the singlet ground state. Changing an aryl group controlled the energy gap between the lowest singlet−triplet states.

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confidence: 99%

Bis-periazulene (Cyclohepta[def]fluorene) as a Nonalternant Isomer of Pyrene: Synthesis and Characterization of Its Triaryl Derivatives

et al. 2022

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“…Furthermore, Konishi and Yasuda et al. synthesized a bis‐periazulene (cyclohepta[ def ]fluorene) derivative 5 , [19] whose ground state was calculated to be triplet, [20] but experimentally elucidated as singlet with a small singlet‐triplet energy gap (Δ E ST ) of −3.78 kJ mol −1 . Similarly, Ma, Bogani, and Feng et al.…”

Section: Figurementioning

confidence: 99%

“…In addition, Haley et al synthesized an indeno[1,2-a]fluorene derivative 4, [18] whose ground state was not experimentally confirmed due to the insufficient ESR measurement at low temperatures. Furthermore, Konishi and Yasuda et al synthesized a bis-periazulene (cyclohepta[def]fluorene) de- rivative 5, [19] whose ground state was calculated to be triplet, [20] but experimentally elucidated as singlet with a small singlet-triplet energy gap (ΔE ST ) of À 3.78 kJ mol À 1 . Similarly, Ma, Bogani, and Feng et al reported the synthesis of benzo-fused cyclohepta[def]fluorenes possessing a singlet ground state with a very low-lying triplet excited state.…”

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confidence: 99%

Synthesis and Isolation of a Kekulé Hydrocarbon with a Triplet Ground State

Shintani

Shimizu²,

Morikoshi³

et al. 2022

Angewandte Chemie

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Design, synthesis, and isolation of a Kekulé hydrocarbon with a triplet ground state is described. Its triplet ground state was unambiguously confirmed by ESR experiments, and the structure and fundamental physical properties were also revealed. The key feature of the molecular design is the decrease in the bonding interaction in the singlet state by aromatic stabilization of benzene rings and the increase of the exchange interaction of unpaired electrons which are favorable for the triplet state. These results contribute to the development of hydrocarbon-based organic magnetic materials.

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“…These systems all have quinoidal core structures derived conceptually by linking simple radical-containing units through one or more p -phenylenes − or thiophenes, by linking delocalized phenalenyl radicals via an aromatic core, − or by incorporation of the same basic design features into more complex π-frameworks. , In one case a thermally accessible quintet state of a quinoidal dinitrene was observed . Triplet ground states in neutral biradicaloid species have been predicted on the basis of PPP calculations, − and more recently based on density functional calculations for acenes, cyclacenes, and derivatives, − bisperiazulene, , an oligoazulene, and large fluoranthene-based hydrocarbons. , To date, only one such biradicaloid, 1 ( n = 2), has been demonstrated experimentally to have a triplet ground state. , …”

Section: Introductionmentioning

confidence: 99%

“…28,29 In one case a thermally accessible quintet state of a quinoidal dinitrene was observed. 30 Triplet ground states in neutral biradicaloid species have been predicted on the basis of PPP calculations, 31−34 and more recently based on density functional calculations for acenes, cyclacenes, and derivatives, 35−37 bisperiazulene, 38,39 an oligoazulene, 40 and large fluoranthene-based hydrocarbons. 41,42 To date, only one such biradicaloid, 1 (n = 2), has been demonstrated experimentally to have a triplet ground state.…”

Section: ■ Introductionmentioning

confidence: 99%

Rational Design of High-Spin Biradicaloids in the Isobenzofulvene and Isobenzoheptafulvene Series

Snyder

2012

J. Phys. Chem. A

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The relative energies of singlet biradicaloid and of triplet and singlet biradical electronic states for a series of benzannelated isobenzofulvenes and isobenzoheptafulvenes were calculated at the (u-)B3LYP/6-31G(d), full π-space CASSCF-CASPT2 (≤14 π-e(-)s), and full π-space RASSCF-RASPT2 (≤24 π-e(-)s) levels of theory. Both absolute and relative CASPT2 energies were reproduced quite well by the RASPT2 approach, which can be extended to much larger active spaces. RASPT2 (and DFT) calculations find that increasing benzannelation leads to triplet ground states in both hydrocarbon series, in violation of the classical principle of maximum bonding. This confirmed the expectations that the combined effects of resonance energy and aromaticity could compensate for the extra formal π-bond of the biradicaloid singlet, and that the strong exchange coupling inherent to the embedded trimethylenemethane (TMM) would manifest itself in the biradicaloids. The relative energy of the biradicaloid singlet rises rapidly upon benzannelation, as π-bonding between the high-energy delocalized GVB orbitals decreases. The underlying π-orbital topology is revealed when this weak π-bonding is artificially eliminated by a 1:1 mixing of the nondegenerate HOMO and LUMO to produce an overcorrelated valence bond (OCVB) orbital pair. For members of both biradicaloid series, the OCVB pairs are nondisjoint, revealing a limiting triplet preference with increasing benzannelation. Within the two-electron, two-orbital approximation, the effects of π-bonding in the singlet biradicaloids and orbital localization away from the acene π-system in the triplet biradicals can be analyzed as perturbations of the singlet OCVB biradicals. The application of a VB-based spin coupling scheme is discussed, in which the unpaired electrons of these species can be considered both ferromagnetically and antiferromagnetically coupled, with the strength of the latter strongly dependent on the acene subunit.

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Bis-periazulene: a simple Kekul� biradical with a triplet ground state

Abstract: B3LYP/6-31G* calculations on bis-periazulene (cyclohepta[def ]-¯uorene) predict a triplet ground state for this molecule. The singlet has an aromatic 14pelectron periphery but is 2 kcal/mol higher in energy. The results agree with earlier predictions by Heilbronner.

Cited by 16 publications

References 8 publications

Bis-periazulene (Cyclohepta[def]fluorene) as a Nonalternant Isomer of Pyrene: Synthesis and Characterization of Its Triaryl Derivatives

Bis-periazulene (Cyclohepta[def]fluorene) as a Nonalternant Isomer of Pyrene: Synthesis and Characterization of Its Triaryl Derivatives

Synthesis and Isolation of a Kekulé Hydrocarbon with a Triplet Ground State

Rational Design of High-Spin Biradicaloids in the Isobenzofulvene and Isobenzoheptafulvene Series

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