1
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Proos Vedin N, Escayola S, Radenković S, Solà M, Ottosson H. The n,π* States of Heteroaromatics: When are They the Lowest Excited States and in What Way Can They Be Aromatic or Antiaromatic? J Phys Chem A 2024; 128:4493-4506. [PMID: 38787346 PMCID: PMC11163469 DOI: 10.1021/acs.jpca.4c02580] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/20/2024] [Revised: 05/10/2024] [Accepted: 05/10/2024] [Indexed: 05/25/2024]
Abstract
Heteroaromatic molecules are found in areas ranging from biochemistry to photovoltaics. We analyze the n,π* excited states of 6π-electron heteroaromatics with in-plane lone pairs (nσ, herein n) and use qualitative theory and quantum chemical computations, starting at Mandado's 2n + 1 rule for aromaticity of separate spins. After excitation of an electron from n to π*, a (4n + 2)π-electron species has 2n + 2 πα-electrons and 2n + 1 πβ-electrons (or vice versa) and becomes πα-antiaromatic and πβ-aromatic. Yet, the antiaromatic πα- and aromatic πβ-components seldom cancel, leading to residuals with aromatic or antiaromatic character. We explore vertically excited triplet n,π* states (3n,π*), which are most readily analyzed, but also singlet n,π* states (1n,π*), and explain which compounds have n,π* states with aromatic residuals as their lowest excited states (e.g., pyrazine and the phenyl anion). If the πβ-electron population becomes more (less) uniformly distributed upon excitation, the system will have an (anti)aromatic residual. Among isomers, the one that has the most aromatic residual in 3n,π* is often of the lowest energy in this state. Five-membered ring heteroaromatics with one or two N, O, and/or S atoms never have n,π* states as their first excited states (T1 and S1), while this is nearly always the case for six-membered ring heteroaromatics with electropositive heteroatoms and/or highly symmetric (D2h) diheteroaromatics. For the complete compound set, there is a modest correlation between the (anti)aromatic character of the n,π* state and the energy gap between the lowest n,π* and π,π* states (R2 = 0.42), while it is stronger for monosubstituted pyrazines (R2 = 0.84).
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Affiliation(s)
- Nathalie Proos Vedin
- Department
of Chemistry—Ångström Laboratory, Uppsala University, 751 20 Uppsala, Sweden
| | - Sílvia Escayola
- Institut
de Quìmica Computacional i Catàlisi and Departament
de Química, Universitat de Girona, C/Maria Aurèlia Capmany,
69, 17003 Girona, Catalonia, Spain
- Donostia
International Physics Center (DIPC), 20018 Donostia, Euskadi, Spain
| | - Slavko Radenković
- Faculty
of Science, University of Kragujevac, P.O. Box 60, 34000 Kragujevac, Serbia
| | - Miquel Solà
- Institut
de Quìmica Computacional i Catàlisi and Departament
de Química, Universitat de Girona, C/Maria Aurèlia Capmany,
69, 17003 Girona, Catalonia, Spain
| | - Henrik Ottosson
- Department
of Chemistry—Ångström Laboratory, Uppsala University, 751 20 Uppsala, Sweden
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2
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Swain CK, Scheiner S. Comparison of Various Theoretical Measures of Aromaticity within Monosubstituted Benzene. Molecules 2024; 29:2260. [PMID: 38792120 PMCID: PMC11123954 DOI: 10.3390/molecules29102260] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/11/2024] [Revised: 05/08/2024] [Accepted: 05/09/2024] [Indexed: 05/26/2024] Open
Abstract
The effects of monosubstitution on the aromaticity of benzene are assessed using a number of different quantitative schemes. The ability of the mobile π-electrons to respond to an external magnetic field is evaluated using several variants of the NICS scheme which calculate the shielding of points along the axis perpendicular to the molecule. Another class of measures is related to the drive toward the uniformity of C-C bond lengths and strengths. Several energetic quantities are devised to approximate an aromatic stabilization energy and the tendency of the molecule to maintain planarity. There is a lack of consistency in that the various measures of aromaticity lead to differing conclusions as to the effects of substituents on the aromaticity of the ring.
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Affiliation(s)
| | - Steve Scheiner
- Department of Chemistry and Biochemistry, Utah State University, Logan, UT 84322-0300, USA
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3
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Chagas JCV, Milanez BD, Oliveira VP, Pinheiro M, Ferrão LFA, Aquino AJA, Lischka H, Machado FBC. A multi-descriptor analysis of substituent effects on the structure and aromaticity of benzene derivatives: π-Conjugation versus charge effects. J Comput Chem 2024; 45:863-877. [PMID: 38153839 DOI: 10.1002/jcc.27296] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/24/2023] [Revised: 11/29/2023] [Accepted: 12/10/2023] [Indexed: 12/30/2023]
Abstract
This work provides a detailed multi-component analysis of aromaticity in monosubstituted (X = CH3, CH 2 - , CH 2 + , NH2, NH-, NH+, OH, O-, and O+) and para-homodisubstituted (X = CH3, CH2, NH2, NH, OH, and O) benzene derivatives. We investigate the effects of substituents using single-reference (B3LYP/DFT) and multireference (CASSCF/MRCI) methods, focusing on structural (HOMA), vibrational (AI(vib)), topological (ELFπ), electronic (MCI), magnetic (NICS), and stability (S0-T1 splitting) properties. The findings reveal that appropriate π-electron-donating and π-electron-accepting substituents with suitable size and symmetry can interact with the π-system of the ring, significantly influencing π-electron delocalization. While the charge factor has a minimal impact on π-electron delocalization, the presence of a pz orbital capable of interacting with the π-electron delocalization is the primary factor leading to a deviation from the typical aromaticity characteristics observed in benzene.
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Affiliation(s)
- Julio C V Chagas
- Department of Chemistry, Aeronautics Institute of Technology, São José dos Campos, São Paulo, Brazil
| | - Bruno D Milanez
- Department of Chemistry, Aeronautics Institute of Technology, São José dos Campos, São Paulo, Brazil
| | - Vytor P Oliveira
- Department of Chemistry, Aeronautics Institute of Technology, São José dos Campos, São Paulo, Brazil
| | - Max Pinheiro
- Department of Chemistry, Aeronautics Institute of Technology, São José dos Campos, São Paulo, Brazil
| | - Luiz F A Ferrão
- Department of Chemistry, Aeronautics Institute of Technology, São José dos Campos, São Paulo, Brazil
| | - Adelia J A Aquino
- Department of Mechanical Engineering, Texas Tech University, Lubbock, Texas, USA
| | - Hans Lischka
- Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas, USA
| | - Francisco B C Machado
- Department of Chemistry, Aeronautics Institute of Technology, São José dos Campos, São Paulo, Brazil
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4
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Deng Q, Zhu J. Adaptive σ aromaticity in the rhenacyclopropene rings. J Comput Chem 2023; 44:2294-2301. [PMID: 37466308 DOI: 10.1002/jcc.27192] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/30/2023] [Revised: 05/30/2023] [Accepted: 07/03/2023] [Indexed: 07/20/2023]
Abstract
Species generally exhibit one-state aromaticity either in the lowest singlet state (S0 ) or the lowest triplet state (T1 ) according to the Hückel's and Baird's rules. Hence, it is rare for species exhibit two-state aromaticity in both the S0 and T1 states (termed as adaptive aromaticity), let alone adaptive σ aromaticity. Here, we report adaptive σ aromaticity in unsaturated rhenacyclopropene rings via density functional theory (DFT) calculations. Various aromaticity indices including NICS, ACID, EDDB together with isodesmic reactions support the adaptive σ aromaticity in these rhenacyclopropene rings. As the T1 state of these species is formed by the ππ* excitation, the σ-aromaticity of these three-membered rings in the S0 state could hold in the T1 state. In addition, the aromaticity effect of the fused rings is also examined. Our findings expand the family of adaptive σ aromaticity, enriching the metallaaromatic chemistry.
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Affiliation(s)
- Qianqian Deng
- State Key Laboratory of Physical Chemistry of Solid Surfaces and Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, China
| | - Jun Zhu
- State Key Laboratory of Physical Chemistry of Solid Surfaces and Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, China
- School of Science and Engineering, The Chinese University of Hong Kong, Shenzhen, Guangdong, China
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5
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Abstract
Complexes with aromaticity in both the lowest singlet state (S0) and the lowest triplet state (T1) (denoted as adaptive aromaticity) are rare because according to Hückel's and Baird's rules, a species could be aromatic in either the S0 or T1 state in most cases. Thus, it is particularly challenging to design species with adaptive aromaticity. Previous reports on adaptive aromaticity were mainly focused on 16e metallapentalenes. Here, we demonstrate that 18e metallapentalenes could possess adaptive aromaticity supported by a set of aromaticity indices when the nitrido and imido ligands are introduced via density functional theory calculations. Further investigation suggests that the metal-carbon bond strength plays an important role in the S0 state aromaticity and the T1 state aromaticity could be attributed to spin electron localization. All these findings could be useful for the development of metallaaromatic chemistry.
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Affiliation(s)
- Qingfu Ye
- State Key Laboratory of Physical Chemistry of Solid Surfaces, Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, People's Republic of China
| | - Yifei Fang
- State Key Laboratory of Physical Chemistry of Solid Surfaces, Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, People's Republic of China
| | - Jun Zhu
- School of Science and Engineering, The Chinese University of Hong Kong, Shenzhen, Guangdong 518172, China
- State Key Laboratory of Physical Chemistry of Solid Surfaces, Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, People's Republic of China
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6
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Apebende CG, idante PS, Magu TO, Asogwa FC, Onyebuenyi IB, Unimuke TO, Gber TE. Density Functional Theory Study of the Influence of Activating and Deactivating Groups on Naphthalene. RESULTS IN CHEMISTRY 2022. [DOI: 10.1016/j.rechem.2022.100669] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
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7
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Nitu C, Crespi S. Computational study on the reduction and solvolysis of triplet chlorobenzenes. J PHYS ORG CHEM 2022. [DOI: 10.1002/poc.4437] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Affiliation(s)
- Cristina Nitu
- Department of Chemistry, Ångström Laboratory Uppsala University Uppsala Sweden
- Stratingh Institute for Chemistry, Faculty of Mathematics and Natural Sciences, University of Groningen, AG Groningen The Netherlands
| | - Stefano Crespi
- Department of Chemistry, Ångström Laboratory Uppsala University Uppsala Sweden
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8
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Swathi Krishna PE, Dev VV, Ramakrishnan R, Hariharan M. Retaining Hückel Aromaticity in the Triplet Excited State of Azobenzene. Chemphyschem 2022; 23:e202200045. [PMID: 35532154 DOI: 10.1002/cphc.202200045] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/19/2022] [Revised: 03/09/2022] [Indexed: 02/04/2023]
Abstract
The implication of the potential concept of aromaticity in the relaxed lowest triplet state of azobenzene, an efficient molecular switch, using elementary aromaticity indices based on magnetic, electronic, and geometric criteria has been discussed. Azobenzene exhibits a major Hückel aromatic character retained in the diradical lowest relaxed triplet state (T1 ) by virtue of a twisted geometry with partial delocalization of unpaired electrons in the perpendicular p-orbitals of two nitrogen atoms to the corresponding phenyl rings. The computational analysis has been expanded further to stilbene and N-diphenylmethanimine for an extensive understanding of the effect of closed-shell Hückel aromaticity in double-bond-linked phenyl rings. Our analysis concluded that stilbene has Hückel aromatic character in the relaxed T1 state and N-diphenylmethanimine has a considerable Hückel aromaticity in the phenyl ring near the carbon atom while a paramount Baird aromaticity in the phenyl ring near the nitrogen atom of the C=N double bond. The results reveal the application of excited-state aromaticity as a general tool for the design of molecular switches.
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Affiliation(s)
- P E Swathi Krishna
- School of Chemistry, Indian Institute of Science Education and Research Thiruvananthapuram, Thiruvananthapuram, Kerala 695551, India
| | - Vivek V Dev
- School of Chemistry, Indian Institute of Science Education and Research Thiruvananthapuram, Thiruvananthapuram, Kerala 695551, India
| | - Remya Ramakrishnan
- School of Chemistry, Indian Institute of Science Education and Research Thiruvananthapuram, Thiruvananthapuram, Kerala 695551, India
| | - Mahesh Hariharan
- School of Chemistry, Indian Institute of Science Education and Research Thiruvananthapuram, Thiruvananthapuram, Kerala 695551, India
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9
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Báez-Grez R, Pino-Rios R. The hidden aromaticity in borazine. RSC Adv 2022; 12:7906-7910. [PMID: 35424723 PMCID: PMC8982269 DOI: 10.1039/d1ra06457f] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/26/2021] [Accepted: 02/25/2022] [Indexed: 02/05/2023] Open
Abstract
The aromaticity of borazine (B3N3H6), also known as the inorganic benzene, is a controversial issue since this compound has several characteristics that could qualify it as an aromatic compound. However, recent studies using magnetic criteria indicate that this compound should be considered as a non-aromatic system. This assignment is mainly due to diatropic currents in the nitrogen atoms without observation of ring currents. The present work shows by means of the magnetic criteria that borazine has a ring current hidden by the local contributions of degenerate orbitals π1 and π2. Additionally, the study of borazine's first triplet state antiaromaticity using the magnetic and energetic criteria by means of isomerization stabilization energies (ISEs) together with Baird's and Hückel's rules suggests that borazine is best described as an (weakly) aromatic system.
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Affiliation(s)
- Rodrigo Báez-Grez
- Computational and Theoretical Chemistry Group, Departamento de Ciencias Químicas, Facultad de Ciencias Exactas, Universidad Andres Bello República 498 Santiago Chile
| | - Ricardo Pino-Rios
- Laboratorio de Química Teórica, Facultad de Química y Biología, Universidad de Santiago de Chile (USACH) Av. Libertador Bernardo O'Higgins 3363 Santiago Estación Central, Región Metropolitana Chile
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10
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Gu X, Yang L, Jin P. Planar Inorganic Five-Membered Heterocycles with σ+π Dual Aromaticity in Both S0 and T1 States. Phys Chem Chem Phys 2022; 24:22091-22101. [DOI: 10.1039/d2cp03116g] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Cyclic species being aromatic in both the lowest singlet and triplet electronic states (so-called adaptive aromaticity) are scarce. To date, the reported systems are mostly organometallic heterocycles with the aromaticities...
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11
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Lin L, Zhu J. Computational predictions of adaptive aromaticity for the design of singlet fission materials. Inorg Chem Front 2022. [DOI: 10.1039/d1qi01442k] [Citation(s) in RCA: 5] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/17/2023]
Abstract
The concept of adaptive aromaticity has been demonstrated as an alternative strategy for the design of singlet fission materials.
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Affiliation(s)
- Lu Lin
- State Key Laboratory of Physical Chemistry of Solid Surfaces, Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, People's Republic of China
| | - Jun Zhu
- State Key Laboratory of Physical Chemistry of Solid Surfaces, Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, People's Republic of China
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12
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Qiu R, Zhu J. Adaptive aromaticity in 16-valence-electron metallazapentalenes. Dalton Trans 2021; 50:16842-16848. [PMID: 34779463 DOI: 10.1039/d1dt03244e] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
According to Hückel's and Baird's rules, cyclic species are generally aromatic only either in the lowest singlet state (S0) or in the lowest-lying triplet ππ* excited state (T1). Thus, species with aromaticity both in S0 and T1 states (termed as adaptive aromaticity) are particularly rare. Herein, we carry out density functional theory (DFT) calculations to examine the aromaticity of 16e metallapentalenes containing heteroatoms (N, O). Interestingly, metallazapentalenes show adaptive aromaticity whereas metalloxapentalenes display nonaromaticity in the S0 and T1 states, which is supported by structural, magnetic, and electronic indices. In addition, a series of metallazapentalenes containing strong σ- or π-donor ligands are predicted to achieve adaptive aromaticity. Our findings expand the family of adaptive aromatics significantly, inviting experimental chemists to realize more hetero-metallapentalenes with adaptive aromaticity.
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Affiliation(s)
- Rulin Qiu
- State Key Laboratory of Physical Chemistry of Solid Surfaces and Fujian Provincial, Key Laboratory of Theoretical and Computational Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, China.
| | - Jun Zhu
- State Key Laboratory of Physical Chemistry of Solid Surfaces and Fujian Provincial, Key Laboratory of Theoretical and Computational Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, China.
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13
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Di Terlizzi L, Roncari F, Crespi S, Protti S, Fagnoni M. Aryl-Cl vs heteroatom-Si bond cleavage on the route to the photochemical generation of σ,π-heterodiradicals. Photochem Photobiol Sci 2021; 21:667-685. [PMID: 34775550 DOI: 10.1007/s43630-021-00119-6] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/31/2021] [Accepted: 10/16/2021] [Indexed: 11/30/2022]
Abstract
The photochemistry of aryl chlorides having a X-SiMe3 group (X = O, NR, S, SiMe2) tethered to the aromatic ring has been investigated in detail, with the aim to generate valuable ϭ,π-heterodiradicals. Two competitive pathways arising from the excited triplet state of the aromatics have been observed, namely heterolysis of the aryl-chlorine bond and homolysis of the X-silicon bond. The former path is found in chlorinated phenols and anilines, whereas the latter is exclusive in the case of silylated thiophenols and aryl silanes. A combined experimental/computational approach was pursued to explain such a photochemical behavior.Graphical abstract.
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Affiliation(s)
- Lorenzo Di Terlizzi
- Department of Chemistry, PhotoGreen Lab, Viale Taramelli 12, 27100, Pavia, Italy
| | - Francesca Roncari
- Department of Chemistry, PhotoGreen Lab, Viale Taramelli 12, 27100, Pavia, Italy
| | - Stefano Crespi
- Stratingh Institute for Chemistry, Center for Systems Chemistry, University of Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands
| | - Stefano Protti
- Department of Chemistry, PhotoGreen Lab, Viale Taramelli 12, 27100, Pavia, Italy
| | - Maurizio Fagnoni
- Department of Chemistry, PhotoGreen Lab, Viale Taramelli 12, 27100, Pavia, Italy.
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14
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Singlet/Triplet State Anti/Aromaticity of CyclopentadienylCation: Sensitivity to Substituent Effect. CHEMISTRY 2021. [DOI: 10.3390/chemistry3030055] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022] Open
Abstract
It is well known that singlet state aromaticity is quite insensitive to substituent effects, in the case of monosubstitution. In this work, we use density functional theory (DFT) calculations to examine the sensitivity of triplet state aromaticity to substituent effects. For this purpose, we chose the singlet state antiaromatic cyclopentadienyl cation, antiaromaticity of which reverses to triplet state aromaticity, conforming to Baird’s rule. The extent of (anti)aromaticity was evaluated by using structural (HOMA), magnetic (NICS), energetic (ISE), and electronic (EDDBp) criteria. We find that the extent of triplet state aromaticity of monosubstituted cyclopentadienyl cations is weaker than the singlet state aromaticity of benzene and is, thus, slightly more sensitive to substituent effects. As an addition to the existing literature data, we also discuss substituent effects on singlet state antiaromaticity of cyclopentadienyl cation.
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15
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Pino-Rios R, Báez-Grez R, Solà M. Acenes and phenacenes in their lowest-lying triplet states. Does kinked remain more stable than straight? Phys Chem Chem Phys 2021; 23:13574-13582. [PMID: 34109330 DOI: 10.1039/d1cp01441b] [Citation(s) in RCA: 9] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
The larger stability of phenacenes compared to their acene isomers in their ground states is attributed to the larger aromaticity of the former. To our knowledge the relative stability of acenes and phenacenes in their lowest-lying triplet states (T1) has not been discussed yet. Using unrestricted density functional theory calculations, our results show that for the smallest members of the series, acenes in their T1 states are more stable than the corresponding phenacenes. However, when the number of the rings (n) involved increases, the energy difference is reduced and for n > 12, phenacenes become more stable than acenes in their T1 states. To rationalize this trend, we analyze the aromaticity of acenes and phenacenes using a set of aromaticity descriptors. We find that in the T1 states of both acenes and phenacenes, the outer rings form aromatic Clar π-sextets. In acenes, delocalization of spin density in the central rings leads to the preferred formation of the largest antiaromatic diradical. Resonant structures in the form of antiaromatic diradical Baird π-octadectets and π-tetradectets are the major contributors, while the smaller ones, such as π-doublets and π-sextets, contribute the least. In phenacenes, structures with diradical antiaromatic Baird π-sextets in some of the central rings contribute the most. These results are relevant to understand the (anti)aromaticity of larger polycyclic aromatic hydrocarbons in their triplet states.
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Affiliation(s)
- Ricardo Pino-Rios
- Laboratorio de Química Teórica, Facultad de Química y Biología, Universidad de Santiago de Chile (USACH), Región Metropolitana, Chile.
| | - Rodrigo Báez-Grez
- Computational and Theoretical Chemistry Group, Departamento de Ciencias Químicas, Facultad de Ciencias Exactas, Universidad Andres Bello, República 498, Santiago, Chile
| | - Miquel Solà
- Institut de Química Computacional i Catàlisi and Departament de Química, Universitat de Girona, 17003 Girona, Catalonia, Spain.
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16
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Markert G, Paenurk E, Gershoni-Poranne R. Prediction of Spin Density, Baird-Antiaromaticity, and Singlet-Triplet Energy Gap in Triplet-State Polybenzenoid Systems from Simple Structural Motifs. Chemistry 2021; 27:6923-6935. [PMID: 33438296 DOI: 10.1002/chem.202005248] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/08/2020] [Revised: 01/08/2021] [Indexed: 12/15/2022]
Abstract
Triplet-state aromaticity has been recently proposed as a strategy for designing functional organic electronic compounds, many of which are polycyclic aromatic systems. However, in many cases, the aromatic nature of the triplet state cannot be easily predicted. Moreover, it is often unclear how specific structural manipulations affect the electronic properties of the excited-state compounds. Herein, the relationship between the structure of polybenzenoid hydrocarbons (PBHs) and their spin-density distribution and aromatic character in the first triplet excited state is studied. Although a direct link is not immediately visible, classifying the PBHs according to their annulation sequence reveals regularities. Based on these, a set of guidelines is defined to qualitatively predict the location of spin and paratropicity and the singlet-triplet energy gap in larger PBHs, using only their smaller tri- and tetracyclic components, and subsequently tested on larger systems.
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Affiliation(s)
- Greta Markert
- Laboratorium für Organische Chemie, ETH, 8093, Zurich, Switzerland
| | - Eno Paenurk
- Laboratorium für Organische Chemie, ETH, 8093, Zurich, Switzerland
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17
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Affiliation(s)
- Yuanyuan Huang
- State Key Laboratory of Physical Chemistry of Solid Surfaces and Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China
| | - Dandan Chen
- State Key Laboratory of Physical Chemistry of Solid Surfaces and Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China
| | - Jun Zhu
- State Key Laboratory of Physical Chemistry of Solid Surfaces and Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China
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18
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de Souza
Farias SA, da Costa KS, Martins JB. Analysis of Conformational, Structural, Magnetic, and Electronic Properties Related to Antioxidant Activity: Revisiting Flavan, Anthocyanidin, Flavanone, Flavonol, Isoflavone, Flavone, and Flavan-3-ol. ACS OMEGA 2021; 6:8908-8918. [PMID: 33842761 PMCID: PMC8028018 DOI: 10.1021/acsomega.0c06156] [Citation(s) in RCA: 41] [Impact Index Per Article: 13.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/17/2020] [Accepted: 03/01/2021] [Indexed: 06/03/2023]
Abstract
Understanding the antioxidant activity of flavonoids is important to investigate their biological activities as well as to design novel molecules with low toxicity and high activity. Aromaticity is a chemical property found in cyclic structures that plays an important role in their stability and reactivity, and its investigation can help us to understand the antioxidant activity of some heterocyclic compounds. In the present study, we applied the density functional theory (DFT) to investigate the properties of seven flavonoid structures with well-reported antioxidant activity: flavan, anthocyanidin, flavanone, flavonol, isoflavone, flavone, and flavan-3-ol. Conformational, structural, magnetic, and electronic analyses were performed using nuclear magnetic resonance, ionization potentials, electron affinity, bond dissociation energy, proton affinity, frontier molecular orbitals (highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO)), and aromaticity through nucleus-independent chemical shifts to analyze these seven flavonoid structures. We revised the influence of hydroxyl groups on the properties of flavonoids and also investigated the influence of the aromaticity of these seven flavonoids on the antioxidant activity.
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Affiliation(s)
- Sergio Antônio de Souza
Farias
- Laboratory
of Molecular Modeling, Institute of Educational Sciences, Federal University of Western Pará, 68040-255 Santarém, Pará, Brazil
| | - Kauê Santana da Costa
- Institute
of Biodiversity, Federal University of Western
Pará, 68040-255 Santarém, Pará, Brazil
| | - João B.
L. Martins
- Laboratory
of Computational Chemistry, Institute of Chemistry, University of Brasilia, 4478 Brasília, Distrito
Federal, Brazil
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19
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Xu F, Chen D, Zeng J, Zhu J. Osmapentalyne and osmapentalene complexes containing boron monofluoride ligands: structure, bonding and adaptive aromaticity. NEW J CHEM 2021. [DOI: 10.1039/d1nj02464g] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
Abstract
Adaptive aromaticity, being aromatic in both the lowest singlet and triplet states, is found in a BF-containing osmapentalene complex.
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Affiliation(s)
- Fangzhou Xu
- State Key Laboratory of Physical Chemistry of Solid Surfaces and Fujian Provincial, Key Laboratory of Theoretical and Computational Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, China
| | - Dandan Chen
- State Key Laboratory of Physical Chemistry of Solid Surfaces and Fujian Provincial, Key Laboratory of Theoretical and Computational Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, China
| | - Jie Zeng
- State Key Laboratory of Physical Chemistry of Solid Surfaces and Fujian Provincial, Key Laboratory of Theoretical and Computational Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, China
| | - Jun Zhu
- State Key Laboratory of Physical Chemistry of Solid Surfaces and Fujian Provincial, Key Laboratory of Theoretical and Computational Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, China
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20
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Baranac-Stojanović M, Stojanović M, Aleksić J. Triplet state (anti)aromaticity of some monoheterocyclic analogues of benzene, naphthalene and anthracene. NEW J CHEM 2021. [DOI: 10.1039/d1nj00207d] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022]
Abstract
By employing DFT calculations, we show the influence of heteroatom substitution on the triplet state (anti)aromaticity of benzene, naphthalene and anthracene.
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Affiliation(s)
| | - Milovan Stojanović
- University of Belgrade – Institute of Chemistry
- Technology and Metallurgy – Center for Chemistry
- Belgrade
- Serbia
| | - Jovana Aleksić
- University of Belgrade – Institute of Chemistry
- Technology and Metallurgy – Center for Chemistry
- Belgrade
- Serbia
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21
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Huang Y, Dai C, Zhu J. Adaptive σ-Aromaticity in an Unsaturated Three-Membered Ring. Chem Asian J 2020; 15:3444-3450. [PMID: 32856746 DOI: 10.1002/asia.202000900] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/30/2020] [Revised: 08/27/2020] [Indexed: 01/02/2023]
Abstract
Based on Hückel's and Baird's rules, species are aromatic either in the lowest singlet state (S0 ) or the lowest triplet state (T1 ) only. Thus, species with adaptive aromaticity (with aromaticity in both the S0 and T1 states) is particularly rare. On the other hand, σ-aromaticity in the T1 state has been underdeveloped, let alone adaptive σ-aromaticity. Herein, via various aromaticity indices including NICS, ACID and EDDB, we demonstrate adaptive σ-aromaticity in an unsaturated three-membered ring, which is a traditional area dominated by π-aromaticity. The origin of adaptive σ-aromaticity could be attributed to the excitation mode of the T1 state formed from out-of-plane π molecular orbital to the π* orbitals. Thus the σ-aromaticity of the three-membered ring in the S0 state could hold in the T1 state. Our findings extend the concept of adaptive σ-aromaticity into three-membered rings and could be useful to further develop the concept of both σ-aromaticity and adaptive aromaticity.
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Affiliation(s)
- Yuanyuan Huang
- State Key Laboratory of Physical Chemistry of Solid Surfaces and Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), and College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, P. R. China
| | - Chenshu Dai
- State Key Laboratory of Physical Chemistry of Solid Surfaces and Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), and College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, P. R. China
| | - Jun Zhu
- State Key Laboratory of Physical Chemistry of Solid Surfaces and Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), and College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, P. R. China
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22
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Chen D, Szczepanik DW, Zhu J, Solà M. Probing the Origin of Adaptive Aromaticity in 16-Valence-Electron Metallapentalenes. Chemistry 2020; 26:12964-12971. [PMID: 32519777 DOI: 10.1002/chem.202001830] [Citation(s) in RCA: 19] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/15/2020] [Revised: 06/08/2020] [Indexed: 12/19/2022]
Abstract
Species with adaptive aromaticity are aromatic in the ground and lowest-lying triplet excited states and they have normally intermediate singlet-triplet gaps. Few examples of compounds with adaptive aromaticity are known to date, including 16-valence-electron (16e) metallapentalenes. A sweeping search could be conducted to discover new members of this group, but efficient designs with an explicit strategy would facilitate the quest for new members of this elusive family. Density functional theory calculations and aromaticity evaluations have been performed to reveal the nature of triplet-state aromaticity in 16e metallapentalenes. Our results show that coordination of strong σ- or π-donor ligands helps achieving adaptive aromaticity of 16e metallapentalenes by means of a spin delocalization mechanism. These results have important implications for understanding the unusual properties of the organometallic adaptive aromatics, leading the way to efficient design of new compounds with tunable singlet-triplet gaps.
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Affiliation(s)
- Dandan Chen
- State Key Laboratory of Physical Chemistry of Solid Surfaces and Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, P. R. China.,Institute of Computational Chemistry and Catalysis, Department of Chemistry, University of Girona, C/ M. Aurèlia Capmany, 69, 17003, Girona, Catalonia, Spain
| | - Dariusz W Szczepanik
- Institute of Computational Chemistry and Catalysis, Department of Chemistry, University of Girona, C/ M. Aurèlia Capmany, 69, 17003, Girona, Catalonia, Spain.,K. Guminski Department of Theoretical Chemistry, Faculty of Chemistry, Jagiellonian University, Gronostajowa, 2, 30-387, Kraków, Poland
| | - Jun Zhu
- State Key Laboratory of Physical Chemistry of Solid Surfaces and Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, P. R. China
| | - Miquel Solà
- Institute of Computational Chemistry and Catalysis, Department of Chemistry, University of Girona, C/ M. Aurèlia Capmany, 69, 17003, Girona, Catalonia, Spain
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23
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Oruganti B, Pál Kalapos P, Bhargav V, London G, Durbeej B. Photoinduced Changes in Aromaticity Facilitate Electrocyclization of Dithienylbenzene Switches. J Am Chem Soc 2020; 142:13941-13953. [PMID: 32666793 PMCID: PMC7458422 DOI: 10.1021/jacs.0c06327] [Citation(s) in RCA: 26] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
![]()
The concepts of excited-state
aromaticity and antiaromaticity have
in recent years with increasing frequency been invoked to rationalize
the photochemistry of cyclic conjugated organic compounds, with the
long-term goal of using these concepts to improve the reactivities
of such compounds toward different photochemical transformations.
In this regard, it is of particular interest to assess how the presence
of a benzene motif affects photochemical reactivity, as benzene is
well-known to completely change its aromatic character in its lowest
excited states. Here, we investigate how a benzene motif influences
the photoinduced electrocyclization of dithienylethenes, a major class
of molecular switches. Specifically, we report on the synthesis of
a dithienylbenzene switch where the typical nonaromatic, ethene-like
motif bridging the two thienyl units is replaced by a benzene motif,
and show that this compound undergoes electrocyclization upon irradiation
with UV-light. Furthermore, through a detailed quantum chemical analysis,
we demonstrate that the electrocyclization is driven jointly and synergistically
by the loss of aromaticity in this motif from the formation of a reactive,
antiaromatic excited state during the initial photoexcitation, and
by the subsequent relief of this antiaromaticity as the reaction progresses
from the Franck–Condon region. Overall, we conclude that photoinduced
changes in aromaticity facilitate the electrocyclization of dithienylbenzene
switches.
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Affiliation(s)
- Baswanth Oruganti
- Department of Chemistry and Biomedical Sciences, Faculty of Health and Life Sciences, Linnaeus University, SE-45041 Kalmar, Sweden
| | - Péter Pál Kalapos
- MTA-TTK "Lendület" Functional Organic Materials Research Group, Institute of Organic Chemistry, Research Centre for Natural Sciences, 1117 Budapest, Hungary
| | - Varada Bhargav
- Department of Chemistry, GITAM Institute of Science, GITAM (deemed to be University), Visakhapatnam 530045, Andhra Pradesh, India
| | - Gábor London
- MTA-TTK "Lendület" Functional Organic Materials Research Group, Institute of Organic Chemistry, Research Centre for Natural Sciences, 1117 Budapest, Hungary
| | - Bo Durbeej
- Division of Theoretical Chemistry, IFM, Linköping University, SE-58183 Linköping, Sweden
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24
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Wen Z, Karas LJ, Wu CH, Wu JIC. How does excited-state antiaromaticity affect the acidity strengths of photoacids? Chem Commun (Camb) 2020; 56:8380-8383. [PMID: 32462169 PMCID: PMC7390693 DOI: 10.1039/d0cc02952a] [Citation(s) in RCA: 21] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Photoacids like substituted naphthalenes (X = OH, NH3+, COOH) are aromatic in the S0 state and antiaromatic in the S1 state. Nucleus independent chemical shifts analyses reveal that deprotonation relieves antiaromaticity in the excited conjugate base, and that the degree of "antiaromaticity relief" explains why some photoacids are stronger than others.
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Affiliation(s)
- Zhili Wen
- Department of Chemistry, University of Houston, Houston, TX 77204, USA.
| | - Lucas José Karas
- Department of Chemistry, University of Houston, Houston, TX 77204, USA.
| | - Chia-Hua Wu
- Department of Chemistry, University of Houston, Houston, TX 77204, USA.
| | - Judy I-Chia Wu
- Department of Chemistry, University of Houston, Houston, TX 77204, USA.
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25
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Dai C, Chen D, Zhu J. Achieving Adaptive Aromaticity in Cyclo[10]carbon by Screening Cyclo[n]carbon (n=8-24). Chem Asian J 2020; 15:2187-2191. [PMID: 32468684 DOI: 10.1002/asia.202000528] [Citation(s) in RCA: 37] [Impact Index Per Article: 9.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/28/2020] [Revised: 05/27/2020] [Indexed: 11/05/2022]
Abstract
Discovery of species with adaptive aromaticity (being aromatic in both the lowest singlet and triplet states) is particularly challenging as cyclic species are generally aromatic either in the ground state or in the excited state only, according to Hückel's and Baird's rules. Inspired by the recent realization of cyclo[18]carbon, here we demonstrate that cyclo[10]carbon possesses adaptive aromaticity by screening cyclo[n]carbon (n=8-24), which is supported by nucleus-independent chemical shift (NICS), anisotropy of the current-induced density (ACID), π contribution of electron localization function (ELFπ ) and electron density of delocalized bonds (EDDB) analyses. Further study reveals that the lowest triplet state of cyclo[10]carbon is formed by in-plane ππ* excitation. Thus, the major contribution to the aromaticity from out-of-plane π molecular orbitals does not change significantly in the lowest singlet state. Our findings highlight a crucial role of out-of-plane π orbitals in maintaining aromaticity for both the lowest singlet and triplet states as well as the aromaticity dependence on the number of the carbon in cyclo[n]carbon.
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Affiliation(s)
- Chenshu Dai
- State Key Laboratory of Physical Chemistry of Solid Surfaces and Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University
| | - Dandan Chen
- State Key Laboratory of Physical Chemistry of Solid Surfaces and Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University
| | - Jun Zhu
- State Key Laboratory of Physical Chemistry of Solid Surfaces and Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University
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26
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Slanina T, Ayub R, Toldo J, Sundell J, Rabten W, Nicaso M, Alabugin I, Fdez Galván I, Gupta AK, Lindh R, Orthaber A, Lewis RJ, Grönberg G, Bergman J, Ottosson H. Impact of Excited-State Antiaromaticity Relief in a Fundamental Benzene Photoreaction Leading to Substituted Bicyclo[3.1.0]hexenes. J Am Chem Soc 2020; 142:10942-10954. [PMID: 32456426 PMCID: PMC7497645 DOI: 10.1021/jacs.9b13769] [Citation(s) in RCA: 18] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022]
Abstract
![]()
Benzene exhibits a rich photochemistry
which can provide access
to complex molecular scaffolds that are difficult to access with reactions
in the electronic ground state. While benzene is aromatic in its ground
state, it is antiaromatic in its lowest ππ* excited
states. Herein, we clarify to what extent relief of excited-state
antiaromaticity (ESAA) triggers a fundamental benzene photoreaction:
the photoinitiated nucleophilic addition of solvent to benzene in
acidic media leading to substituted bicyclo[3.1.0]hex-2-enes. The
reaction scope was probed experimentally, and it was found that silyl-substituted
benzenes provide the most rapid access to bicyclo[3.1.0]hexene derivatives,
formed as single isomers with three stereogenic centers in yields
up to 75% in one step. Two major mechanism hypotheses, both involving
ESAA relief, were explored through quantum chemical calculations and
experiments. The first mechanism involves protonation of excited-state
benzene and subsequent rearrangement to bicyclo[3.1.0]hexenium cation,
trapped by a nucleophile, while the second involves photorearrangement
of benzene to benzvalene followed by protonation and nucleophilic
addition. Our studies reveal that the second mechanism is operative.
We also clarify that similar ESAA relief leads to puckering of S1-state silabenzene and pyridinium ion, where the photorearrangement
of the latter is of established synthetic utility. Finally, we identified
causes for the limitations of the reaction, information that should
be valuable in explorations of similar photoreactions. Taken together,
we reveal how the ESAA in benzene and 6π-electron heterocycles
trigger photochemical distortions that provide access to complex three-dimensional
molecular scaffolds from simple reactants.
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Affiliation(s)
- Tomáš Slanina
- Department of Chemistry - Ångström Laboratory, Uppsala University, SE-751 20, Uppsala, Sweden.,Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences, Flemingovo námĕstí 2, 16610 Prague 6, Czech Republic
| | - Rabia Ayub
- Department of Chemistry - Ångström Laboratory, Uppsala University, SE-751 20, Uppsala, Sweden
| | - Josene Toldo
- Department of Chemistry - Ångström Laboratory, Uppsala University, SE-751 20, Uppsala, Sweden
| | - Johan Sundell
- Medicinal Chemistry, Research and Early Development Cardiovascular, Renal and Metabolism, BioPharmaceuticals R&D, AstraZeneca, Gothenburg, Sweden
| | - Wangchuk Rabten
- Department of Chemistry - Ångström Laboratory, Uppsala University, SE-751 20, Uppsala, Sweden
| | - Marco Nicaso
- Department of Chemistry - Ångström Laboratory, Uppsala University, SE-751 20, Uppsala, Sweden
| | - Igor Alabugin
- Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306-4390, United States
| | - Ignacio Fdez Galván
- Department of Chemistry - BMC, Uppsala University, SE-751 23 Uppsala, Sweden
| | - Arvind K Gupta
- Department of Chemistry - Ångström Laboratory, Uppsala University, SE-751 20, Uppsala, Sweden
| | - Roland Lindh
- Department of Chemistry - BMC, Uppsala University, SE-751 23 Uppsala, Sweden.,Uppsala Center for Computational Chemistry - UC3, Uppsala University, SE-751 23 Uppsala Sweden
| | - Andreas Orthaber
- Department of Chemistry - Ångström Laboratory, Uppsala University, SE-751 20, Uppsala, Sweden
| | - Richard J Lewis
- Medicinal Chemistry, Research and Early Development Respiratory, Inflammation and Autoimmune, BioPharmaceuticals R&D, AstraZeneca, Gothenburg, Sweden
| | - Gunnar Grönberg
- Medicinal Chemistry, Research and Early Development Respiratory, Inflammation and Autoimmune, BioPharmaceuticals R&D, AstraZeneca, Gothenburg, Sweden
| | - Joakim Bergman
- Medicinal Chemistry, Research and Early Development Cardiovascular, Renal and Metabolism, BioPharmaceuticals R&D, AstraZeneca, Gothenburg, Sweden
| | - Henrik Ottosson
- Department of Chemistry - Ångström Laboratory, Uppsala University, SE-751 20, Uppsala, Sweden
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