Polycyclic Hydrocarbons from [4<i>n</i>]Annulenes: Correlation versus Hybridization Forces in the Formation of Diradicaloids

Quintero, Sergio Moles; Haley, Michael M.; Kertész, Miklós; Casado, Juan

doi:10.1002/anie.202209138

Cited by 41 publications

(18 citation statements)

References 89 publications

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“…Internal connections of the 4n π-electron system: Covalent bonds that connect atoms of the 4n π-electron system directly, so without an additional structural unit, can conceal the antiaromaticity / reduce the diradical character. 14 This is particularly effective if it creates locally aromatic units (in which all π-electrons of these locally aromatic units also form part of the formal 4n π-electron system), as in biphenylene (Figure 2c left). However, internal connections can considerably reduce the ability of the molecules to further conceal the antiaromaticity by conformational changes.…”

Section: Structural Motifs Of Concealed Antiaromaticitymentioning

confidence: 99%

“…Besides monocyclic antiaromatic molecules with two degenerate SOMOs and a diradical character y0 of 1 (referred to as truly antiaromatic molecules in this article), there are molecules with a formal 4n π-electron system but reduced diradical character (between 0 and 1). 14 These molecules -including pentalene, dibenzopentalene, s-indacene, and biphenylene (Figure 1c) and their derivatives as well as different cyclooctatetraene derivatives -are often also referred to as antiaromatic, [15][16][17][18][19][20][21][22][23] even when the diradical character is low. In contrast to truly antiaromatic molecules, which may only be isolated under cryogenic conditions, these polycyclic molecules are more stable and isolated more readily.…”

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confidence: 99%

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Concealed antiaromaticity

Glöcklhofer

2023

Preprint

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The literature reports numerous molecules claimed to be antiaromatic because of a formal 4n pi-electron system. However, this neglects the actual local aromaticity of the molecules, which often feature multiple subunits with [4n+2] pi-electrons besides the formal 4n pi-electron system. This has led to considerable criticism from those who believe that the term antiaromatic should not be used for any molecule with a formal 4n pi-electron system but should be reserved for truly antiaromatic molecules. To reconcile the different viewpoints, the concept of concealed antiaromaticity is introduced here. Concealed antiaromaticity acknowledges that many molecules claimed to be antiaromatic are not truly antiaromatic, but they can exhibit behaviour under certain conditions that would normally be expected for antiaromatic molecules. Three types of concealed antiaromaticity are distinguished based on the conditions under which the molecules can behave like antiaromatic molecules: concealed antiaromaticity revealable in redox reactions (Type I-CA), upon photoexcitation (Type II-CA), and in intermolecular interactions (Type III-CA). The concept of concealed antiaromaticity will enable the rational design of molecules that show the desirable properties of antiaromatic molecules under the different conditions, with broad applications from organic electronics to supramolecular architectures, while avoiding the low stability of truly antiaromatic molecules.

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Section: Structural Motifs Of Concealed Antiaromaticitymentioning

confidence: 99%

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confidence: 99%

Concealed antiaromaticity

Glöcklhofer

2023

Preprint

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“…The concept of antiaromaticity was established many decades ago for prototypical monocyclic hydrocarbons of 4n πelectrons. 1,2 However, polycyclic antiaromatic hydrocarbons (PAAHs) have attracted renewed interest during the last two decades in view of new criteria of antiaromaticity 3−5 and small energy gaps between HOMO/LUMO levels and the Fermi level, which are relevant to open-shell character 6,7 and carrier mobility in organic field-effect transistors (OFETs). 8−17 Among PAAHs, dibenzo[a,e]pentalene (DBP) and indeno- [1,2-b]fluorene ([1,2-b]IF) (Figure 1a), which belong to nonalternant hydrocarbons bearing five-membered rings, have been intensively investigated as stable singlet diradicals 18−20 and building blocks 17,21−25 for conjugated polymers and macrocyclic molecules of unique topology and for optoelectronic applications.…”

Section: ■ Introductionmentioning

confidence: 99%

s-Indacene Revisited: Modular Synthesis and Modulation of Structures and Molecular Orbitals of Hexaaryl Derivatives

et al. 2023

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Though s-indacene is an intriguing antiaromatic hydrocarbon of 12 π-electrons, it has been underrepresented due to the lack of efficient and versatile methods to prepare stable derivatives. Herein we report a concise and modular synthetic method for hexaaryl-s-indacene derivatives bearing electron-donating/-accepting groups at specific positions to furnish C 2h -, D 2h -, and C 2v -symmetric substitution patterns. We also report the effects of substituents on their molecular structures, frontier molecular orbital (MO) levels, and magnetically induced ring current tropicities. Both theoretical calculations and X-ray structure analyses indicate that the derivatives of the C 2h -substitution pattern adopt different C 2h structures with significant bond length alternation depending on the electronic property of the substituents. Due to the nonuniform distribution of the frontier MOs, their energy levels are selectively modulated by the electron-donating substituents. This leads to the inversion of the HOMO and HOMO–1 sequences with respect to those of the intrinsic s-indacene as theoretically predicted and experimentally proven by the absorption spectra at visible and near-infrared regions. The NICS values and the 1H NMR chemical shifts of the s-indacene derivatives indicate their weak antiaromaticity. The different tropicities are explained by the modulation of the HOMO and HOMO–1 levels. In addition, for the hexaxylyl derivative, weak fluorescence from the S2 excited state was detected due to the large energy gap between the S1 and S2 states. Notably, an organic field-effect transistor (OFET) fabricated using the hexaxylyl derivative exhibited moderate hole carrier mobility, a result which opens the door for optoelectronic applications of s-indacene derivatives.

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“…9 Besides antiaromatic molecules with two degenerate SOMOs and a diradical character y0 of 1 (referred to as truly antiaromatic molecules in this article), there are molecules with a formal 4n πelectron system but reduced diradical character (between 0 and 1). 10 These molecules -including pentalene, dibenzopentalene, s-indacene, and biphenylene (Figure 1c) and their derivatives as well as different cyclooctatetraene derivatives -are often also referred to as antiaromatic, [11][12][13][14][15][16][17][18] even when the diradical character is low. However, the classification of such molecules as antiaromatic has been criticized, particularly for polycyclic molecules that feature both formal 4n and 4n+2 πelectron systems (such as dibenzopentalene and biphenylene, Figure 1c right).…”

mentioning

confidence: 99%

“…• Connecting the 4n π-electron system internally (Figure 2d): Introducing covalent bonds that connect atoms of the 4n π-electron system directly, so without an additional structural unit, can conceal the antiaromaticity / reduce the diradical character. 10 This can be an effective strategy if it leads to the creation of locally aromatic units in which all π-electrons of the locally aromatic units also form part of the formal 4n π-electron system, as in biphenylene (Figure 2d top). However, connecting the 4n π-electron system internally can considerably reduce the ability of the molecules to further conceal the antiaromaticity by conformational changes.…”

mentioning

confidence: 99%

Concealed antiaromaticity

Glöcklhofer

2023

Preprint

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Numerous articles in the recent literature report molecules that are claimed to be antiaromatic, as they feature a formal 4n π-electron system. The purported antiaromaticity often serves well as an explanation for some of the observed properties, but it neglects the actual local aromaticity of the molecules, which often feature multiple subunits with 4n+2 π-electrons besides the formal 4n π-electron system. This has led to considerable criticism from those who believe that the term antiaromatic should not be used for any molecule with a formal 4n π-electron system but should be reserved for truly antiaromatic molecules. To reconcile the different viewpoints, the term and concept of concealed antiaromaticity is introduced here. The concept accepts that many of the molecules claimed to be antiaromatic are not truly antiaromatic. However, it acknowledges that this does not prevent those molecules from exhibiting behaviour under certain conditions that would normally be expected for antiaromatic molecules, due to the formal, concealed 4n π-electron system that characterizes these molecules. Based on the conditions under which the molecules can behave like antiaromatic molecules, three types of concealed antiaromaticity are distinguished here: concealed antiaromaticity revealable in redox reactions (Type I-CA), upon excitation (Type II-CA), and in intermolecular interactions (Type III-CA). The concept of concealed antiaromaticity will enable the conscious, rational design of molecules that show the desirable properties of antiaromatic molecules while avoiding or diminishing the undesirable properties, namely the low stability of truly antiaromatic molecules. This will yield molecules and materials with highly interesting properties for a broad range of applications, from organic electronics to supramolecular chemistry.

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Polycyclic Hydrocarbons from [4n]Annulenes: Correlation versus Hybridization Forces in the Formation of Diradicaloids

Cited by 41 publications

References 89 publications

Concealed antiaromaticity

Concealed antiaromaticity

s-Indacene Revisited: Modular Synthesis and Modulation of Structures and Molecular Orbitals of Hexaaryl Derivatives

Concealed antiaromaticity

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