How do the Hückel and Baird Rules Fade away in Annulenes?

Casademont‐Reig, Irene; Ramos‐Cordoba, Eloy; Torrent‐Sucarrat, Miquel; Matito, Eduard

doi:10.3390/molecules25030711

Cited by 58 publications

(102 citation statements)

References 107 publications

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“…For the [4 n ]annulenes in their T 1 states, the bond length alternations is smallest with UB3LYP and largest with UCAM-B3LYP ( Tables S3 and S6 ), in line with the observation by Matito and co-workers. 82 Here it can be noted that spin delocalization has earlier been observed experimentally in porphyrin nanorings by ENDOR spectroscopy, 83 in line with the UB3LYP result. Yet, the time scale of the EPR measurements (up to 2 μs) may not capture the situation with a rapid interconversion between conformers with semilocalized spin densities.…”

Section: Results and Discussionsupporting

confidence: 82%

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Triplet State Baird Aromaticity in Macrocycles: Scope, Limitations, and Complications

et al. 2021

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The aromaticity of cyclic 4 n π-electron molecules in their first ππ* triplet state (T 1 ), labeled Baird aromaticity, has gained growing attention in the past decade. Here we explore computationally the limitations of T 1 state Baird aromaticity in macrocyclic compounds, [ n ]CM ’s, which are cyclic oligomers of four different monocycles (M = p -phenylene (PP), 2,5-linked furan (FU), 1,4-linked cyclohexa-1,3-diene (CHD), and 1,4-linked cyclopentadiene (CPD)). We strive for conclusions that are general for various DFT functionals, although for macrocycles with up to 20 π-electrons in their main conjugation paths we find that for their T 1 states single-point energies at both canonical UCCSD(T) and approximative DLPNO-UCCSD(T) levels are lowest when based on UB3LYP over UM06-2X and UCAM-B3LYP geometries. This finding is in contrast to what has earlier been observed for the electronic ground state of expanded porphyrins. Yet, irrespective of functional, macrocycles with 2,5-linked furans ( [ n ]CFU ’s) retain Baird aromaticity until larger n than those composed of the other three monocycles. Also, when based on geometric, electronic and energetic aspects of aromaticity, a 3 [ n ]CFU with a specific n is more strongly Baird-aromatic than the analogous 3 [ n ]CPP while the magnetic indices tell the opposite. To construct large T 1 state Baird-aromatic [ n ]CM ’s, the design should be such that the T 1 state Baird aromaticity of the macrocyclic perimeter dominates over a situation with local closed-shell Hückel aromaticity of one or a few monocycles and semilocalized triplet diradical character. Monomers with lower Hückel aromaticity in S 0 than benzene (e.g., furan) that do not impose steric congestion are preferred. Structural confinement imposed by, e.g., methylene bridges is also an approach to larger Baird-aromatic macrocycles. Finally, by using the Zilberg–Haas description of T 1 state aromaticity, we reveal the analogy to the Hückel aromaticity of the corresponding closed-shell dications yet observe stronger Hückel aromaticity in the macrocyclic dications than Baird aromaticity in the T 1 states of the neutral macrocycles.

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Section: Results and Discussionsupporting

confidence: 82%

“…In T 1 , the extent of aromaticity is more or less constant according to FLU at UB3LYP level while there is a variation with UM06-2X with 3 C 8 H 8 as the most and 3 C 20 H 20 as the least Baird-aromatic, in line with similar findings by Matito and co-workers on [4 n ]annulenes in their T 1 states up until C 16 H 16 . 82 …”

Section: Results and Discussionmentioning

confidence: 99%

Triplet State Baird Aromaticity in Macrocycles: Scope, Limitations, and Complications

et al. 2021

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“…Since the latter functional approximations do not suffer from large electron delocalization errors, one is prompted to conclude that B3LYP overestimates the symmetry of c-P6 6+ (and, hence, its aromaticity, see below), whereas it overestimates the asymmetry of c-P6 4+ (RMSD=0.661) resulting in a spurious enhancement of the antiaromatic features of this molecule (see below). Similar results have been recently reported by our group 15,[34][35][36][37] and others. [38][39][40][41] We have also characterized the structure of c-P6 6+ using the LC-wHPBE 42,43 method with w=0.1 and w=0.2 in combination with the 6-31G* basis set.…”

Section: Table S2supporting

confidence: 93%

How Aromatic Are Molecular Nanorings? The Case of a Six-Porphyrin Nanoring

Casademont‐Reig¹,

Guerrero‐Avilés²,

Ramos‐Cordoba³

et al. 2021

Preprint

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<div> <div> <div> Large conjugated rings give rise to novel promising structures that can sustain persistent currents at low temperatures even in the presence of strong magnetic fields. One of the most interesting such molecules was recently synthesized [Anderson et al., Nature, 2017, 541, 3512] in the form of a six-porphyrin nanoring structure, which, according to the authors, in its +6-oxidation state (c-P66+) sustained an aromatic ring current involving 78π electrons; one of the largest aromatic rings ever produced. In this paper, we have provided compelling evidence that this molecule is not aromatic, as it was incorrectly inferred from computational calculations that suffer from large delocalization errors. A thorough analysis of four oxidation states of the six-porphyrin nanoring re- veals that the main reason behind the poor aromaticity of these nanorings is the low delocalization in the transition from the porphyrins to the bridging butadiyne linkers, which disrupts the overall conjugated circuit. These results highlight the importance of choosing an adequate computational method to study large conjugated molecules and the appropriate aromaticity descriptors to identify the part of the molecule that is responsible for the loss of aromaticity. We believe the strategy here employed will be helpful in designing new large aromatic molecular nanorings. </div> </div> </div>

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“…Compounds with polyyne segments were not considered as their FLU values deviate from the others due to and T1 states decrease when going to larger cycles but this is primarily a result of reductions in the values for the S0 state when the annulenes become larger. In T1, the extent of aromaticity is more or less constant according to FLU at UB3LYP level while there is a variation with UM06-2X with3 C8H8 as the most and 3 C20H20 as the least Baird-aromatic, in line with similar findings by Matito and co-workers on [4n]annulenes in their T1 states up until C16H16 46.…”

supporting

confidence: 87%

“…[4n]annulenes in their T1 states, the bond lengths alterations is smallest with UB3LYP and largest with UCAM-B3LYP (Tables S3 and S6), in line with the observation by Matito and coworkers. 46 Here it can be noted that spin delocalization has earlier been observed experimentally in porphyrin nanorings by ENDOR spectroscopy, 47 in line with the UB3LYP…”

Section: Initial Guess Blyp Rohfsupporting

confidence: 64%

Triplet State Baird-Aromaticity in Macrocycles: Scope, Limitations and Complications

Ayub¹,

Bakouri²,

Smith³

et al. 2020

Preprint

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The aromaticity of cyclic 4np-electron molecules in their first pp* triplet state (T1), labelled Baird-aromaticity, has gained growing attention in the last decade. Here we explore computationally the limitations of T1 state Baird-aromaticity in macrocyclic compounds, [n]CM’s, which are cyclic oligomers of four different monocycles (M = para-phenylene (PP), 2,5-linked furan (FU), 1,4-linked cyclohexa-1,3-diene (CHD), and 1,4-linked cyclopentadiene (CPD)). We strive for conclusions that are general for various DFT functionals, although for macrocycles with up to 20 p-electrons in their main conjugation paths we find that for their T1 states single-point energies at both canonical UCCSD(T) and approximative DLPNO-UCCSD(T) levels are lowest when based on UB3LYP over UM06-2X and UCAM-B3LYP geometries. This finding is in contrast to what has earlier been observed for the electronic ground state of expanded porphyrins. Yet, irrespective of functional, macrocycles with 2,5-linked furans ([n]CFU’s) retain Baird-aromaticity until larger n than those composed of the other three monocycles. Also, when based on geometric, electronic and energetic aspects of aromaticity, a 3[n]CFU with a specific n is more strongly Baird-aromatic than the analogous 3[n]CPP while the magnetic indices tell the opposite. To construct large T1 state Baird-aromatic [n]CM’s the design should be such that the T1 state Baird-aromaticity of the macrocyclic perimeter dominates over a situation with local closed-shell Hückel-aromaticity of one or a few monocycles and semi-localized triplet diradical character. Monomers with lower Hückel-aromaticity in S0 than benzene (e.g., furan) that do not impose steric congestion are preferred. Structural confinement imposed by, e.g., methylene bridges is also an approach to larger Baird-aromatic macrocycles. Finally, by using the Zilberg-Haas description of T1 state aromaticity we reveal the analogy to the Hückel-aromaticity of the corresponding closed-shell dications, yet, observe stronger Hückel-aromaticity in the macrocyclic dications than Baird-aromaticity in the T1 states of the neutral macrocycles.

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How do the Hückel and Baird Rules Fade away in Annulenes?

Cited by 58 publications

References 107 publications

Triplet State Baird Aromaticity in Macrocycles: Scope, Limitations, and Complications

Triplet State Baird Aromaticity in Macrocycles: Scope, Limitations, and Complications

How Aromatic Are Molecular Nanorings? The Case of a Six-Porphyrin Nanoring

Triplet State Baird-Aromaticity in Macrocycles: Scope, Limitations and Complications

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