Sterically Frustrated Aromatic Enes with Various Colors Originating from Multiple Folded and Twisted Conformations in Crystal Polymorphs**

Nishiuchi, Tomohiko; Aibara, Seito; Yamakado, Takuya; Kimura, Ryo; Saito, Shohei; Sato, Hiroyasu; Kubo, Takashi

doi:10.1002/chem.202200286

Cited by 11 publications

(4 citation statements)

References 91 publications

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“…In nature, double-bond isomerization forms the functional basis of the retinoid cycle responsible for human vision 6 , with fine-tuning provided by molecular distortions of the chromophore 7 . In the laboratory, twistable double bonds have been incorporated into molecular switches and motors 5 , switchable chromophores 2 , 8 – 10 and organic magnetic materials 11 – 17 . The performance of such molecular devices critically depends on the electronic and steric features of the π system containing the switchable bond.…”

Section: Mainmentioning

confidence: 99%

Tetrafluorenofulvalene as a sterically frustrated open-shell alkene

Prajapati,

Ambhore,

Dang

et al. 2023

Nat. Chem.

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Electronic and steric effects are known to greatly influence the structure, characteristics and reactivity of organic compounds. A typical π bond is weakened by oxidation (corresponding to the removal of electrons from bonding orbitals), by reduction (through addition of electrons to antibonding orbitals) and by unpairing of the bonding electrons, such as in the triplet state. Here we describe tetrafluorenofulvalene (TFF), a twisted, open-shell alkene for which these general rules do not hold. Through the synthesis, experimental characterization and computational analysis of its charged species spanning seven redox states, the central alkene bond in TFF is shown to become substantially stronger in the tri- and tetraanion, generated by chemical reduction. Furthermore, although its triplet state contains a weaker alkene bond than the singlet, in the quintet state its bond order increases substantially, yielding a flatter structure. This behaviour originates from the doubly bifurcated topology of the underlying spin system and can be rationalized by the balancing effects of benzenoid aromaticity and spin pairing.

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Section: Mainmentioning

confidence: 99%

Tetrafluorenofulvalene as a sterically frustrated open-shell alkene

Prajapati,

Ambhore,

Dang

et al. 2023

Nat. Chem.

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“…The geometries other than the folded and twisted forms are frozen in crystal lattices and can be analyzed by X-ray . The study of 14 demonstrates that highly congested and hindered OCEs and quinodimethanes can adopt a variety of geometries in the crystal lattice when the energy difference is small enough to be compensated by the packing force …”

Section: Difference Between the Open-shell Perpendicular Form And The...mentioning

confidence: 99%

Carbon-based Biradicals: Structural and Magnetic Switching

Ishigaki,

Harimoto,

Shimajiri

et al. 2023

Chem. Rev.

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Sterically hindered C�C double bonds often deform into a bent or twisted geometry. Thus, many overcrowded ethylenes or anthraquinodimethanes can adopt multiple conformations, such as a folded form or a twisted form, which are interconvertible under the application of external stimuli. A perpendicular form with biradical character can also be adopted when designed to incorporate a stable carbon-based radical unit, which is involved in stimuliresponsive magnetic switching accompanied by a structural change. This review focuses on recent advances in the development of such strained π-electron systems and reveals the factors that affect the mutual interconversion and switching behavior. The energy barrier for the interconversion of conformational isomers is affected by the tricyclic skeleton or bulky substituents on the C�C double bonds, whereas the relative stability of the perpendicular biradical form increases with the additional insertion of 9,10-anthrylene units into the C�C double bonds. CONTENTS 1. Introduction 13952 2. Difference between the Open-shell Perpendicular Form and the Closed-shell Twisted Form 13953 3. Interconversion between the Biradical Perpendicular Form and Closed-shell Twisted Form of Overcrowded Ethylene 13955 4. Interconversion between the Biradical Perpendicular Form and Closed-shell Folded Form of Anthraquinodimethanes 13956 5. Biradical Perpendicular Form Derived from the Closed-shell Folded Form of Bianthraquinodimethanes 13958 6. Stable Biradical Perpendicular Form with an Oligo(9,10-anthrylene) Unit 13960 7. Biradical Perpendicular Form

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“…Upon applying the pressure, the color of single crystal 1 gradually changed from red (0 GPa) to dark red (5.0 GPa), that is, piezochromic behavior even though 1 has no flexible π-skeletons which readily reflect the structural change by its color. [30][31][32] (Figure 12A), X-ray crystallography under high pressure revealed that the C-C distances of C9-C9 0 and C10-C10 0 are slightly shortened from 2.777 Å to 2.755 Å and 2.826 Å to 2.788 Å, respectively (Figure 12B). TD-DFT calculations of 1 showed that the longer-wavelength shift of the HOMO-LUMO transition by applying pressure from 543 nm (f = 0.0017) to 592 nm (f = 0.0029) was observed (Figure S18).…”

Section: Attempt To Enhance the Antiaromaticity Of 1 Using Hydrostati...mentioning

confidence: 99%

Stacked antiaromaticity in the π‐congested space between the aromatic rings in the anthracene dimer

Nishiuchi

Makihara

Kishi

et al. 2022

J of Physical Organic Chem

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Substance containing two π-congested aromatic systems are attractive targets in synthetic studies as well as efforts designed to explore the unique properties that originate from π-congestion. Since the time of the computational studies by Schleyer and Warner, the concept of stacked aromaticity created by the encounter of two antiaromatic rings has received much attention. A structure containing π-congestion between two antiaromatic norcorrole rings was prepared by Shinokubo et. al. In contrast, questions about what happens when two aromatic rings are closely stacked have remained unanswered. Specifically, the electronic consequences of stacking of two aromatic π-conjugated systems have not been fully evaluated. To the best of our knowledge, only one computational study has been performed by Herges, the results of which suggest that [2.2]paracyclophane, possessing short distance (<3.00 Å) between two aromatic ring planes, has a paratropic ring current between two the benzene rings. This observation is consistent with the conclusion that stacked antiaromaticity exists within the π-congested space between aromatic rings. Herein, we investigated the stacked antiaromaticity of highly -congested anthracene dimers using anthracenophane, which possesses a short inter-plane carbon-carbon distance of ca. 2.80 Å. The absorption spectrum of this substance contains a weak broad band from 450 to 550 nm that is attributed from HOMO-LUMO transition. These properties exist in planar cyclooctatetraene derivatives that having 8π-electron antiaromaticity. The results of nucleus-independent chemical shift and anisotropy of the induced current density calculations indicate that relatively weak antiaromatic character exists between the central six-membered rings of the two anthracene moieties in anthracenophane. In addition, an attempt to enhance the stacked antiaromaticity of anthracenophane using pressure to enhance π-congestion was unsuccessful.

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Sterically Frustrated Aromatic Enes with Various Colors Originating from Multiple Folded and Twisted Conformations in Crystal Polymorphs**

Cited by 11 publications

References 91 publications

Tetrafluorenofulvalene as a sterically frustrated open-shell alkene

Tetrafluorenofulvalene as a sterically frustrated open-shell alkene

Carbon-based Biradicals: Structural and Magnetic Switching

Stacked antiaromaticity in the π‐congested space between the aromatic rings in the anthracene dimer

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