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

[Image: see text] The aromaticity of cyclic 4nπ-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, w...

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Autores principales: Ayub, Rabia, El Bakouri, Ouissam, Smith, Joshua R., Jorner, Kjell, Ottosson, Henrik
Formato: Online Artículo Texto
Lenguaje:English
Publicado: American Chemical Society 2021
Acceso en línea:https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7884009/
https://www.ncbi.nlm.nih.gov/pubmed/33427474
http://dx.doi.org/10.1021/acs.jpca.0c08926
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author Ayub, Rabia
El Bakouri, Ouissam
Smith, Joshua R.
Jorner, Kjell
Ottosson, Henrik
author_facet Ayub, Rabia
El Bakouri, Ouissam
Smith, Joshua R.
Jorner, Kjell
Ottosson, Henrik
author_sort Ayub, Rabia
collection PubMed
description [Image: see text] The aromaticity of cyclic 4nπ-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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spelling pubmed-78840092021-02-16 Triplet State Baird Aromaticity in Macrocycles: Scope, Limitations, and Complications Ayub, Rabia El Bakouri, Ouissam Smith, Joshua R. Jorner, Kjell Ottosson, Henrik J Phys Chem A [Image: see text] The aromaticity of cyclic 4nπ-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. American Chemical Society 2021-01-11 2021-01-21 /pmc/articles/PMC7884009/ /pubmed/33427474 http://dx.doi.org/10.1021/acs.jpca.0c08926 Text en © 2021 The Authors. Published by American Chemical Society This is an open access article published under a Creative Commons Attribution (CC-BY) License (http://pubs.acs.org/page/policy/authorchoice_ccby_termsofuse.html) , which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited.
spellingShingle Ayub, Rabia
El Bakouri, Ouissam
Smith, Joshua R.
Jorner, Kjell
Ottosson, Henrik
Triplet State Baird Aromaticity in Macrocycles: Scope, Limitations, and Complications
title Triplet State Baird Aromaticity in Macrocycles: Scope, Limitations, and Complications
title_full Triplet State Baird Aromaticity in Macrocycles: Scope, Limitations, and Complications
title_fullStr Triplet State Baird Aromaticity in Macrocycles: Scope, Limitations, and Complications
title_full_unstemmed Triplet State Baird Aromaticity in Macrocycles: Scope, Limitations, and Complications
title_short Triplet State Baird Aromaticity in Macrocycles: Scope, Limitations, and Complications
title_sort triplet state baird aromaticity in macrocycles: scope, limitations, and complications
url https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7884009/
https://www.ncbi.nlm.nih.gov/pubmed/33427474
http://dx.doi.org/10.1021/acs.jpca.0c08926
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