Exploiting the Aromatic Chameleon Character of Fulvenes for Computational Design of Baird‐Aromatic Triplet Ground State Compounds

Yadav, Sangeeta; Bakouri, Ouissam El; Jorner, Kjell; Tong, Hui; Dahlstrand, Christian; Solà, Miquel; Ottosson, Henrik

doi:10.1002/asia.201801821

Cited by 20 publications

(43 citation statements)

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“…35 Thus, they adapt to the different electron counting rules for aromaticity in different states, and accordingly, act as "aromatic chameleons". 36 However, the CT character in the excited states is rather small, except for fulvenes with strong acceptor substituents at the exocyclic carbon and donor substituents at the endocyclic positions. In such compounds one can, according to computations, drive the triplet state to be the ground state (a T0 state) with a Baird-aromatic cyclopentadienyl cationic ring.…”

Section: Introductionmentioning

confidence: 99%

“…In such compounds one can, according to computations, drive the triplet state to be the ground state (a T0 state) with a Baird-aromatic cyclopentadienyl cationic ring. 36 Yet, despite the rather modest CT character of most fulvenes in their lowest excited states, they still display significant solvatochromism. 19 Numerous pro-aromatic molecules can be designed which on paper can be labelled as Hückel-Baird hybrids in their lowest triplet states (Figure 1).…”

Section: Introductionmentioning

confidence: 99%

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Guidelines for Tuning the Excited State Hückel–Baird Hybrid Aromatic Character of Pro‐Aromatic Quinoidal Compounds**

et al. 2021

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The present study aims to provide a solid ground for the identification, characterization and controlled design of pro-aromatic quinoidal organic compounds holding conjugated rings with Hückel and/or Baird (singlet and triplet) excited state aromatic character, and expects to become a reference work for future studies on Baird-aromaticity. Concretely, we explore a wide range of compounds with a central conjugated ring of different sizes and symmetric exocyclic substitutions. We employ a combination of different computational tools and we also introduce a new energy-based approach for the quantification of the Baird-aromatic character. The key findings of this study indicate that Baird aromaticity is achieved in pro-aromatic quinoidal compounds having small anionic rings or in electron deficient rings with the proper exocyclic substitution. Low-lying states of these systems show very strong hole/electron overlaps, implying that their aromaticity cannot be related to intramolecular charge transfer. File list (2) download file view on ChemRxiv manuscript.pdf (5.62 MiB) download file view on ChemRxiv supp_info.pdf (38.90 MiB)

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Guidelines for Tuning the Excited State Hückel–Baird Hybrid Aromatic Character of Pro‐Aromatic Quinoidal Compounds**

et al. 2021

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“…For the fulvene with X = NH 2 and Y = CN, NICS(1) zz,S0 = −24.8 ppm and NICS(1) zz,T1 = 23.8 ppm, while for the fulvene with X = CN and Y = NH 2 , NICS(1) zz,S0 = 27.4 ppm and NICS(1) zz,T0 = −11.9 ppm. Noteworthy, the strongly Baird-aromatic cyclopentadienyl cation in its T 0 state, 77,78 representing the limiting Baird-aromatic resonance structure of a fulvene in its triplet state, 49 has a NICS(1) zz,T0 value of −26.3 ppm. In its lowest singlet state, the cyclopentadienyl cation is also strongly diradical, as evidenced by a y 0 value of 0.96.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

Strategies for Design of Potential Singlet Fission Chromophores Utilizing a Combination of Ground State and Excited State Aromaticity Rules

Bakouri¹,

Smith²,

Ottosson

2020

Preprint

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Singlet exciton fission photovoltaics requires chromophores with their lowest excited states arranged so that 2E(T1) < E(S1) and E(S1) < E(T2). Herein, qualitative theory and quantum chemical calculations are used to develop explicit strategies on how to use Baird’s 4n rule on excited state aromaticity, combined with Hückel’s 4n+2 rule for ground state aromaticity, to tailor new potential chromophores for singlet fission. We first analyze the E(T1), E(S1) and E(T2) of benzene and cyclobutadiene (CBD) as, respectively, excited state antiaromatic and aromatic archetypes, and reveal that CBD fulfils the criteria on the state ordering for a singlet fission chromophore. We then look at fulvenes, a class of compounds that can be tuned by choice of substituents from Baird-antiaromatic to Baird-aromatic in T1 and S1, and from Hückel-aromatic to Hückel-antiaromatic in S0. The T1 and S1 states of most substituted fulvenes (159 of 225) are described by singly excited HOMO→LUMO configurations, providing a rational for the simultaneous tuning of E(T1) and E(S1) along an approximate (anti)aromaticity coordinate. Key to the tunability is the exchange integral (KH,L), which ideally is constant throughout the compound class, providing a constant DE(S1-T1). This leads us to a geometric model for identification of singlet fission chromophores, and we explore what factors limit the model. Candidates with calculated E(T1) of ~1 eV or higher are identified among benzannelated 4npi-electron compound classes and siloles. In brief, it is clarified how the joint utilization of Baird’s 4n and Hückel’s 4n+2 rules, together with substituent effects (electronic and steric) and benzannelation, can be used to tailor new chromophores with potential use in singlet fission photovoltaics.

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“…43 We first test our design strategy on substituted fulvenes (pentafulvenes) being a compound class with E(T1) and E(S1) that span a large part of the region between the corresponding excited state energies of benzene and CBD. [44][45][46][47][48] This feature is a result of the "aromatic chameleon" character of fulvenes, meaning that they can adapt to the different aromaticity rules in different electronic states; Baird's rule in T1 and S1, and Hückel's rule in S0. 49 π-Electron withdrawing groups (EWGs) at the exocyclic position lead to low-lying T1 and S1 states as they enhance the Baird-aromatic character of these states (Figure 2A).…”

Section: Design Strategiesmentioning

confidence: 99%

Strategies for Design of Potential Singlet Fission Chromophores Utilizing Baird's Rule on Excited State Aromaticity

O¹,

Smith²,

Ottosson

2019

Preprint

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In singlet exciton fission one photon of light is used to create two excitons of triplet multiplicity. This process requires chromophores with their lowest excited states arranged so that 2E(T1) < E(S1) and E(S1) < E(T2). To match different technology platforms there is a high need for new candidate chromophores with the desired excited state orderings. Herein, qualitative theory and quantum chemical calculations are used to develop explicit strategies on how to use Baird’s 4n rule on excited state aromaticity to tailor new potential chromophores for singlet fission. We first analyze the E(T1), E(S1) and E(T2) of benzene and cyclobutadiene (CBD) as, respectively, excited state antiaromatic and aromatic archetypes, and reveal that CBD fulfils the criteria on the state ordering for a singlet fission chromophore. We then look at fulvenes, a class of compounds that can be tuned from Baird-antiaromatic to Baird-aromatic in T1 and S1 by choice of substituents. The T1 and S1 states of fulvenes are both described by singly excited HOMO→LUMO configurations, which provides a rational for the simultaneous and similar tuning of E(T1) and E(S1) along an approximate (anti)aromaticity coordinate. This leads us to a geometric model for identification of singlet fission chromophores. Candidates with calculated E(T1) of ~1 eV or higher are also identified among benzannelated 4npi-electron compound classes and among siloles influenced to various extents by Baird-(anti)aromaticity in T1 and S1. Finally, we explore the limitations of the design approach. In brief, it is clarified how Baird’s 4n rule together with substituent effects (electronic and steric) and benzannelation can be used to tailor new chromophores with potential use in singlet fission photovoltaics.

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Exploiting the Aromatic Chameleon Character of Fulvenes for Computational Design of Baird‐Aromatic Triplet Ground State Compounds

Cited by 20 publications

References 81 publications

Guidelines for Tuning the Excited State Hückel–Baird Hybrid Aromatic Character of Pro‐Aromatic Quinoidal Compounds**

Guidelines for Tuning the Excited State Hückel–Baird Hybrid Aromatic Character of Pro‐Aromatic Quinoidal Compounds**

Strategies for Design of Potential Singlet Fission Chromophores Utilizing a Combination of Ground State and Excited State Aromaticity Rules

Strategies for Design of Potential Singlet Fission Chromophores Utilizing Baird's Rule on Excited State Aromaticity

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