Zwitterionic Aromaticity on Azulene Extrapolated to <i>carbo</i>‐Azulene

Poater, Jordi; Heitkämper, Juliane; Poater, Albert; Maraval, Valérie; Chauvin, Rémi

doi:10.1002/ejoc.202101228

Cited by 7 publications

(6 citation statements)

References 74 publications

(20 reference statements)

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“…The 10π-aromaticity of azulene was predicted in previous studies. , However, the relevance of this finding has been largely overlooked. For example, the permanent dipole moment of azulene is usually explained by resonance structures that invoke delocalization through the transannular bond, but these resonance structures do not significantly contribute to the net electronic structure of azulene (Figure a), as evidenced by the negligible Δ C 10 value. As a case in point, we found that homoazulene, the homoannelated counterpart of azulene without a conjugated transannular bond, has a similar permanent dipole moment (Chapter S13.1).…”

Section: Resultsmentioning

confidence: 99%

Excited-State (Anti)Aromaticity Explains Why Azulene Disobeys Kasha’s Rule

Dunlop,

Ludvíková,

Banerjee

et al. 2023

J. Am. Chem. Soc.

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Fluorescence exclusively occurs from the lowest excited state of a given multiplicity according to Kasha’s rule. However, this rule is not obeyed by a handful of anti-Kasha fluorophores whose underlying mechanism is still understood merely on a phenomenological basis. This lack of understanding prevents the rational design and property-tuning of anti-Kasha fluorophores. Here, we propose a model explaining the photophysical properties of an archetypal anti-Kasha fluorophore, azulene, based on its ground- and excited-state (anti)aromaticity. We derived our model from a detailed analysis of the electronic structure of the ground singlet, first excited triplet, and quintet states and of the first and second excited singlet states using the perturbational molecular orbital theory and quantum-chemical aromaticity indices. Our model reveals that the anti-Kasha properties of azulene and its derivatives result from (i) the contrasting (anti)aromaticity of its first and second singlet excited states (S1 and S2, respectively) and (ii) an easily accessible antiaromaticity relief pathway of the S1 state. This explanation of the fundamental cause of anti-Kasha behavior may pave the way for new classes of anti-Kasha fluorophores and materials with long-lived, high-energy excited states.

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Section: Resultsmentioning

confidence: 99%

Excited-State (Anti)Aromaticity Explains Why Azulene Disobeys Kasha’s Rule

Dunlop,

Ludvíková,

Banerjee

et al. 2023

J. Am. Chem. Soc.

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“…For 2a , the 5MR moiety is concave, which indicates no magnetic shielding in the inner region of the 5MR moiety. In azulyne, the aromaticity can be justified by the presence of an ionic resonance structure that has a negative charge on the 5MR and a positive charge on the 7MR . In contrast, the ICSS zz magnetic shielding in 3a is similar to azulynes I – III , suggesting its aromaticity.…”

Section: Resultsmentioning

confidence: 99%

Stabilizing a 20-Electron Metallaazulyne by Aromaticity

Qiu

Zhu

2022

Inorg. Chem.

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The 18-electron rule states that metal complexes with 18 valence electron metal centers are thermodynamically stable because nine valence orbitals of transition metals including one s orbital, three p orbitals, and five d orbitals can collectively accommodate 18 electrons, achieving the same electron configuration as the noble gas in the period. Thus, 20-electron compounds are extremely rare due to a violation of such a rule. Here, we demonstrate a 20-electron metallaazulyne via density functional theory calculations stabilized by aromaticity, which was supported by various aromaticity indices including nucleus-independent chemical shift, anisotropy of the induced current density, the isochemical shielding surface, and electron density of delocalized bonds. Interestingly, when a transition metal fragment is first introduced into the aromatic azulyne molecule, the resulting osmaazulyne becomes antiaromatic, in sharp contrast to the previous transformation from pentalyne to metallapentalyne. More interestingly, when osmaazulyne is reduced by two electrons, the resulting 20e osmaazulyne becomes aromatic. Our findings highlight an important application of aromaticity in stabilizing 20e species, inviting experimental verification.

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“…3 times less according to energetic criteria). 24 The rigid diameter of the π-system expands from 24.3 Å for 9 to 29.3 Å for 2 . While the frontier MOs of the parent molecule 9 spread over the antennas (HOMO over the thiophene rings, LUMO over the BTD units), those of the carbo -benzene 2 concentrate over the C 18 core (Table S3, ESI†).…”

Section: Resultsmentioning

confidence: 99%