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| 2. |
- Ayub, Rabia, et al.
(author)
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Triplet State Baird Aromaticity in Macrocycles : Scope, Limitations, and Complications
- 2021
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In: Journal of Physical Chemistry A. - : American Chemical Society. - 1089-5639 .- 1520-5215. ; 125:2, s. 570-584
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Journal article (peer-reviewed)abstract
- The aromaticity of cyclic 4nπ-electron molecules in their first ππ∗ triplet state (T1), labeled Baird aromaticity, has gained growing attention in the past 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 = 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 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 semilocalized 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. © 2021 The Authors.
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| 3. |
- El Bakouri, Ouissam, et al.
(author)
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Strategies for Design of Potential Singlet Fission Chromophores Utilizing a Combination of Ground-State and Excited-State Aromaticity Rules
- 2020
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In: Journal of the American Chemical Society. - : AMER CHEMICAL SOC. - 0002-7863 .- 1520-5126. ; 142:12, s. 5602-5617
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Journal article (peer-reviewed)abstract
- Singlet exciton fission photovoltaic technology requires chromophores with their lowest excited states arranged so that 2E(T-1) < E(S-1) and E(S-1) < E(T-2). 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 Huckel's 4n + 2 rule for ground-state aromaticity, to tailor new potential chromophores for singlet fission. We first analyze the E(T-1), E(S-1), and E(T-2) of benzene and cyclobutadiene (CBD) as excited-state antiaromatic and aromatic archetypes, respectively, and reveal that CBD fulfills 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 T-1 and S-1 and from Huckel-aromatic to Huckel-antiaromatic in S-0. The T-1 and S-1 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(T-1) and E(S-1) along an approximate (anti)aromaticity coordinate. Key to the tunability is the exchange integral (K-H,K-L), which ideally is constant throughout the compound class, providing a constant.E(S-1 - T-1). This leads us to a geometric model for the identification of singlet fission chromophores, and we explore what factors limit the model. Candidates with calculated E(T-1) values of similar to 1 eV or higher are identified among benzannelated 4npelectron compound classes and siloles. In brief, it is clarified how the joint utilization of Baird's 4n and Huckel'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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| 4. |
- El Bakouri, Ouissam, et al.
(author)
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Three-Dimensional Fully π-Conjugated Macrocycles : When 3D-Aromatic and When 2D-Aromatic-in-3D?
- 2022
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In: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 144:19, s. 8560-8575
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Journal article (peer-reviewed)abstract
- Several fully π-conjugated macrocycles with puckered or cage-type structures were recently found to exhibit aromatic character according to both experiments and computations. We examine their electronic structures and put them in relation to 3D-aromatic molecules (e.g., closo-boranes) and to 2D-aromatic polycyclic aromatic hydrocarbons. Using qualitative theory combined with quantum chemical calculations, we find that the macrocycles explored hitherto should be described as 2D-aromatic with three-dimensional molecular structures (abbr. 2D-aromatic-in-3D) and not as truly 3D-aromatic. 3D-aromatic molecules have highly symmetric structures (or nearly so), leading to (at least) triply degenerate molecular orbitals, and for tetrahedral or octahedral molecules, an aromatic closed-shell electronic structure with 6n + 2 electrons. Conversely, 2D-aromatic-in-3D structures exhibit aromaticity that results from the fulfillment of Hückel’s 4n + 2 rule for each macrocyclic path, yet their π-electron counts are coincidentally 6n + 2 numbers for macrocycles with three tethers of equal lengths. It is notable that 2D-aromatic-in-3D macrocyclic cages can be aromatic with tethers of different lengths, i.e., with π-electron counts different from 6n + 2, and they are related to naphthalene. Finally, we identify tetrahedral and cubic π-conjugated molecules that fulfill the 6n + 2 rule and exhibit significant electron delocalization. Yet, their properties resemble those of analogous compounds with electron counts that differ from 6n + 2. Thus, despite the fact that these molecules show substantial π-electron delocalization, they cannot be classified as true 3D-aromatics.
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| 5. |
- Mayer, Peter J., et al.
(author)
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Structure–Property Relationships in Unsymmetric Bis(antiaromatics) : Who Wins the Battle between Pentalene and Benzocyclobutadiene?
- 2020
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In: Journal of Organic Chemistry. - : American Chemical Society (ACS). - 0022-3263 .- 1520-6904. ; 85:8, s. 5158-5172
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Journal article (peer-reviewed)abstract
- According to the currently accepted structure-property relationships, aceno-pentalenes with an angular shape (fused to the 1,2-bond of the acene) exhibit higher antiaromaticity than those with a linear shape (fused to the 2,3-bond of the acene). To explore and expand the current view, we designed and synthesized molecules where two isomeric, yet, different, 8 pi antiaromatic subunits, a benzocyclobutadiene (BCB) and a pentalene, are combined into, respectively, an angular and a linear topology via an unsaturated six-membered ring. The antiaromatic character of the molecules is supported experimentally by H-1 NMR, UV-vis, and cyclic voltammetry measurements and X-ray crystallography. The experimental results are further confirmed by theoretical studies including the calculation of several aromaticity indices (NICS, ACID, HOMA, FLU, MCI). In the case of the angular molecule, double bond-localization within the connecting six-membered ring resulted in reduced antiaromaticity of both the BCB and pentalene subunits, while the linear structure provided a competitive situation for the two unequal [4n]pi subunits. We found that in the latter case the BCB unit alleviated its unfavorable antiaromaticity more efficiently, leaving the pentalene with strong antiaromaticity. Thus, a reversed structure-antiaromaticity relationship when compared to aceno-pentalenes was achieved.
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| 6. |
- Pati, Avik K., et al.
(author)
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Tuning the Baird aromatic triplet-state energy of cyclooctatetraene to maximize the self-healing mechanism in organic fluorophores
- 2020
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In: Proceedings of the National Academy of Sciences of the United States of America. - : National Academy of Sciences (PNAS). - 0027-8424 .- 1091-6490. ; 117:39, s. 24305-24315
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Journal article (peer-reviewed)abstract
- Bright, photostable, and nontoxic fluorescent contrast agents are critical for biological imaging. "Self-healing" dyes, in which triplet states are intramolecularly quenched, enable fluorescence imaging by increasing fluorophore brightness and longevity, while simultaneously reducing the generation of reactive oxygen species that promote phototoxicity. Here, we systematically examine the self-healing mechanism in cyanine-class organic fluorophores spanning the visible spectrum. We show that the Baird aromatic triplet-state energy of cyclooctatetraene can be physically altered to achieve order of magnitude enhancements in fluorophore brightness and signal-to-noise ratio in both the presence and absence of oxygen. We leverage these advances to achieve direct measurements of large-scale conformational dynamics within single molecules at submillisecond resolution using wide-field illumination and camera-based detection methods. These findings demonstrate the capacity to image functionally relevant conformational processes in biological systems in the kilohertz regime at physiological oxygen concentrations and shed important light on the multivariate parameters critical to self-healing organic fluorophore design.
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| 7. |
- Rana, Anup, et al.
(author)
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A combined photobiological-photochemical route to C10 cycloalkane jet fuels from carbon dioxide via isoprene
- 2022
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In: Green Chemistry. - : Royal Society of Chemistry. - 1463-9262 .- 1463-9270. ; 24, s. 9602-
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Journal article (peer-reviewed)abstract
- The hemiterpene isoprene is a volatile C5 hydrocarbon with industrial applications. It is generated today from fossil resources, but can also be made in biological processes. We have utilized engineered photosynthetic cyanobacteria for direct, light-driven production of bio-isoprene from carbon dioxide, and show that isoprene in a subsequent photochemical step, using either near-UV or simulated or natural solar light, can be dimerized into limonene, paradiprene, and isomeric C10H16 hydrocarbons (monoterpenes) in high yields under photosensitized conditions (above 90% after 44 hours with near-UV and 61% with simulated solar light). The optimal sensitizer in our experiments is di(naphth-1-yl)methanone which we use with a loading of 0.1 mol%. It can also easily be recycled for subsequent photodimerization cycles. The isoprene dimers generated are a mixture of [2 + 2], [4 + 2] and [4 + 4] cycloadducts, and after hydrogenation this mixture is nearly ideal as a drop-in jet fuel. Importantly the photodimerization can be carried out at ambient conditions. However, the high content of hydrogenated [2 + 2] dimers in our isoprene dimer mix lowers the flash point below the threshold (38 °C); yet, these dimers can be converted thermally into [4 + 2] and [4 + 4] dimers. When hydrogenated these monoterpenoids fully satisfy the criteria for drop-in jet fuels with regard to energy density, flashpoint, kinematic viscosity, density, and freezing point. Life-cycle assessment results show a potential to produce the fuel in an environmentally sustainable way.
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| 8. |
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| 9. |
- Shoji, Y., et al.
(author)
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An Element-Substituted Cyclobutadiene Exhibiting High-Energy Blue Phosphorescence
- 2021
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In: Angewandte Chemie International Edition. - : John Wiley and Sons Inc. - 1433-7851 .- 1521-3773. ; 60:40, s. 21817-21823
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Journal article (peer-reviewed)abstract
- 1,3,2,4-Diazadiboretidine, an isoelectronic heteroanalogue of cyclobutadiene, is an interesting chemical species in terms of comparison with the carbon system, whereas its properties have never been investigated experimentally. According to Baird's rule, Hückel antiaromatic cyclobutadiene acquires aromaticity in the lowest triplet state. Here we report experimental and theoretical studies on the ground- and excited-state antiaromaticity/aromaticity as well as the photophysical properties of an isolable 1,3,2,4-diazadiboretidine derivative. The crystal structure of the diazadiboretidine derivative revealed that the B2N2 ring adopts a planar rhombic geometry in the ground state. Yet, theoretical calculations showed that the B2N2 ring turns to a square geometry with a nonaromatic character in the lowest triplet state. Notably, the diazadiboretidine derivative has the lowest singlet and triplet states lying at close energy levels and displays blue phosphorescence.
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| 10. |
- Shoji, Yoshiaki, et al.
(author)
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Inside Cover : An Element-Substituted Cyclobutadiene Exhibiting High-Energy Blue Phosphorescence (Angew. Chem. Int. Ed. 40/2021)
- 2021
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In: Angewandte Chemie International Edition. - : John Wiley & Sons. - 1433-7851 .- 1521-3773. ; 60:40, s. 21598-21598
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Journal article (peer-reviewed)abstract
- The ground- and excited-state properties of a diazadiboretidine (DADBT) derivative, an isoelectronic heterocyclic analogue of cyclobutadiene, are reported by Henrik Ottosson, Hiromi Nakai, Tadaaki Ikoma, Takanori Fukushima, and co-workers in their Research Article on page 21817. While the B2N2 ring of the DADBT derivative adopts a rhombic geometry in the ground state, it turns to a square geometry with a nonaromatic character in the lowest triplet state. Notably, the DADBT derivative displays blue phosphorescence in solution, providing insight into the photophysical properties of heterocyclic pi-systems.+
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