Ouissam El Bakouri | Universitat de Girona (original) (raw)

Papers by Ouissam El Bakouri

Journal of the American Chemical Society, Feb 28, 2020

Singlet exciton fission photovoltaic technology requires chromophores with their lowest excited s... more 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 Bairdantiaromatic 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,L), which ideally is constant throughout the compound class, providing a constant ΔE(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 ∼1 eV or higher are identified among benzannelated 4nπelectron 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.

Journal of Organic Chemistry, Mar 19, 2020

According to the currently accepted structure− property relationships, aceno-pentalenes with an a... more 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, 8π 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 1 H 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 sixmembered 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]π 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.

Organometallics, Aug 28, 2013

Biophysical Journal, Feb 1, 2021

Journal of the American Chemical Society, May 6, 2022

Several fully π-conjugated macrocycles with puckered or cage-type structures were recently found ... more 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 3Daromatic 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 Huckel'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.

Several fully -conjugated macrocycles with puckered or cage-type structures were recently found ... more Several fully -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 thus far should be described as 2D-aromatic with three-dimensional molecular structures (abbr. 2Daromatic-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 molecules with tetrahedral or octahedral structures an aromatic closed-shell electronic structure results with 6n + 2 electrons. On the other hand, 2D-aromatic-in-3D structures exhibit aromaticity that results from the fulfillment of Hückel's 4n + 2 rule for each individual 2 macrocyclic path, yet, their -electron counts are coincidentally 6n + 2 numbers for macrocycles with three tethers of equal lengths. The 3D-macrocyclic molecules, suggested to be 3D-aromatic, are instead similar to naphthalene. It is notable that 2D-aromatic-in-3D macrocyclic cages can be aromatic with tethers of different lengths, i.e., when their -electron counts differ from 6n + 2. Finally, we identify tetrahedral and cubic -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 that these tetrahedral and cubic molecules show substantial -electron delocalization they cannot be classified as true 3D-aromatics. 8(HL)4(L)8 12-, L = tetrazole dianion) metal cluster which additionally can be described as cubic aromatic since it exhibits an electron delocalization over the entire Zn I 8 cube. 38

Chemical Communications, 2014

An efficient method for the synthesis of N-arylated pyridazinones from potassium 2-furantrifluoro... more An efficient method for the synthesis of N-arylated pyridazinones from potassium 2-furantrifluoroborate and aryldiazonium salts is described. The reaction was run in water at 0-5 0 C in short reaction times and without any catalyst or additive. A mechanistic proposal is made based on 10 the experimental data and DFT calculations. Pyridazin-3(2H)-ones and their N-aryl substituted derivatives are building blocks found in many biologically active compounds such as pharmaceuticals and pesticides. 1 As an example, compound 1 (Figure 1), which is used in the treatment of central

ChemInform, Mar 6, 2014

ABSTRACT

ACS omega, Mar 3, 2022

The synthesis and properties of a series of unsymmetrical thienopentalenes are explored, includin... more The synthesis and properties of a series of unsymmetrical thienopentalenes are explored, including both monoareno and diareno derivatives. For the synthesis of monoareno pentalenes, a carbopalladation cascade reaction between alkynes and gem-dibromoolefins was applied. Diareno pentalene derivatives were accessed via gold-catalyzed cyclization of diynes. Thiophene was fused to pentalene in two different geometries via its 2,3 and 3,4 bonds. 2,3-Fusion resulted in increased antiaromaticity of the pentalene unit compared to the 3,4-fusion both in the monoareno and diareno framework. Monothienopentalenes that contained the destabilizing 2,3-fusion could not be isolated. For diareno derivatives, the aromatic character of the different aryl groups fused to the pentalene was not independent. Destabilizing fusion on one side resulted in alleviated aromaticity on the other side and vice versa. The synthesized molecules were characterized experimentally by 1H NMR and UV–vis spectroscopies, cyclic voltammetry, and X-ray crystallography, and their aromatic character was assessed using magnetic (NICS and ACID) and electronic indices (MCI and FLU).

Journal of Physical Chemistry A, Jan 11, 2021

The aromaticity of cyclic 4nπ-electron molecules in their first ππ* triplet state (T 1), labeled ... more 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 Huckel aromaticity of one or a few monocycles and semilocalized triplet diradical character. Monomers with lower Huckel 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 Huckel aromaticity of the corresponding closed-shell dications yet observe stronger Huckel aromaticity in the macrocyclic dications than Baird aromaticity in the T 1 states of the neutral macrocycles.

Green Chemistry, 2022

Please note that technical editing may introduce minor changes to the text and/or graphics, which... more Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal's standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.

Journal of the American Chemical Society

Several fully π-conjugated macrocycles with puckered or cage-type structures were recently found ... more 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 3Daromatic 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 Huckel'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.

Tetrahedron, 2013

Aryleheteroaryl and heteroaryleheteroaryl compounds are obtained through the SuzukieMiyaura cross... more Aryleheteroaryl and heteroaryleheteroaryl compounds are obtained through the SuzukieMiyaura crosscoupling reactions between diazonium salts and potassium trifluoroborates using PdCl 2 (CH 3 CN) 2 as the catalyst in water and in the absence of any additive, ligand or base. In addition, the same mild catalytic system is effective for the MatsudaeHeck reaction between several monoolefins with aryldiazonium salts.

Chemical Communications, 2014

An efficient method for the synthesis of N-arylated pyridazinones from potassium 2-furantrifluoro... more An efficient method for the synthesis of N-arylated pyridazinones from potassium 2-furantrifluoroborate and aryldiazonium salts is described. The reaction was run in water at 0-5 0 C in short reaction times and without any catalyst or additive. A mechanistic proposal is made based on 10 the experimental data and DFT calculations. Pyridazin-3(2H)-ones and their N-aryl substituted derivatives are building blocks found in many biologically active compounds such as pharmaceuticals and pesticides. 1 As an example, compound 1 (Figure 1), which is used in the treatment of central

Several fully pi-conjugated macrocycles with puckered or cage-type structures were recently found... more Several fully pi-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 thus far 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 molecules with tetrahedral or octahedral structures an aromatic closed-shell electronic structure results with 6n + 2 electrons. On the other hand, 2D-aromatic-in-3D structures exhibit aromaticity that results from the fulfillment of Hückel’s 4n + 2 rule for each in...

Chemical Science

Cibalackrot-type compounds are Hückel instead of Baird aromatic in their first triplet states (T1... more Cibalackrot-type compounds are Hückel instead of Baird aromatic in their first triplet states (T1). By choice of substituents and additional benzannelation we adjust the T1 energies, providing a new strategy for singlet fission chromophore design.

The hemiterpene isoprene is a volatile C5 hydrocarbon, with industrial applications. It is genera... more 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 simulated or natural solar light, can be dimerized into limonene, paradiprene, and isomeric C10H16 hydrocarbons (monoterpenes) in very high yields (above 90% after 44 hours) under sensitized conditions. The optimal sensitizer in our experiments is di(naphth-1-yl)methanone which we can use with a loading of merely 0.1 mol%, and it is easily 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 jet fuel drop-in. Importantly, the photodimerization can be carried out at ambient condition...

The exact energies of the lowest singlet and triplet excited states in organic chromophores are c... more The exact energies of the lowest singlet and triplet excited states in organic chromophores are crucial to their performance in optoelectronic devices. The possibility of utilizing singlet fission to enhance the performance of photovoltaic devices has resulted in a wide demand for tuneable, stable organic chromophores with wide S1 – T1 energy gaps (>1 eV). Cibalackrot-type compounds were recently considered to have favorably positioned excited state energies for singlet fission, and they were found to have a degree of aromaticity in the lowest triplet excited state (T1). This work reports on a revised and deepened theoretical analysis taking into account the excited state Hückel-aromatic (instead of Baird-aromatic) as well as diradical characters, with the aim to design new organic chromophores based on this scaffold in a rational way starting from qualitative theory. We demonstrate that the substituent strategy can effectively adjust the spin populations on the chromophore moiet...

Journal of the American Chemical Society, Feb 28, 2020

Singlet exciton fission photovoltaic technology requires chromophores with their lowest excited s... more 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 Bairdantiaromatic 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,L), which ideally is constant throughout the compound class, providing a constant ΔE(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 ∼1 eV or higher are identified among benzannelated 4nπelectron 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.

Journal of Organic Chemistry, Mar 19, 2020

According to the currently accepted structure− property relationships, aceno-pentalenes with an a... more 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, 8π 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 1 H 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 sixmembered 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]π 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.

Organometallics, Aug 28, 2013

Biophysical Journal, Feb 1, 2021

Journal of the American Chemical Society, May 6, 2022

Several fully π-conjugated macrocycles with puckered or cage-type structures were recently found ... more 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 3Daromatic 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 Huckel'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.

Several fully -conjugated macrocycles with puckered or cage-type structures were recently found ... more Several fully -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 thus far should be described as 2D-aromatic with three-dimensional molecular structures (abbr. 2Daromatic-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 molecules with tetrahedral or octahedral structures an aromatic closed-shell electronic structure results with 6n + 2 electrons. On the other hand, 2D-aromatic-in-3D structures exhibit aromaticity that results from the fulfillment of Hückel's 4n + 2 rule for each individual 2 macrocyclic path, yet, their -electron counts are coincidentally 6n + 2 numbers for macrocycles with three tethers of equal lengths. The 3D-macrocyclic molecules, suggested to be 3D-aromatic, are instead similar to naphthalene. It is notable that 2D-aromatic-in-3D macrocyclic cages can be aromatic with tethers of different lengths, i.e., when their -electron counts differ from 6n + 2. Finally, we identify tetrahedral and cubic -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 that these tetrahedral and cubic molecules show substantial -electron delocalization they cannot be classified as true 3D-aromatics. 8(HL)4(L)8 12-, L = tetrazole dianion) metal cluster which additionally can be described as cubic aromatic since it exhibits an electron delocalization over the entire Zn I 8 cube. 38

Chemical Communications, 2014

An efficient method for the synthesis of N-arylated pyridazinones from potassium 2-furantrifluoro... more An efficient method for the synthesis of N-arylated pyridazinones from potassium 2-furantrifluoroborate and aryldiazonium salts is described. The reaction was run in water at 0-5 0 C in short reaction times and without any catalyst or additive. A mechanistic proposal is made based on 10 the experimental data and DFT calculations. Pyridazin-3(2H)-ones and their N-aryl substituted derivatives are building blocks found in many biologically active compounds such as pharmaceuticals and pesticides. 1 As an example, compound 1 (Figure 1), which is used in the treatment of central

ChemInform, Mar 6, 2014

ABSTRACT

ACS omega, Mar 3, 2022

The synthesis and properties of a series of unsymmetrical thienopentalenes are explored, includin... more The synthesis and properties of a series of unsymmetrical thienopentalenes are explored, including both monoareno and diareno derivatives. For the synthesis of monoareno pentalenes, a carbopalladation cascade reaction between alkynes and gem-dibromoolefins was applied. Diareno pentalene derivatives were accessed via gold-catalyzed cyclization of diynes. Thiophene was fused to pentalene in two different geometries via its 2,3 and 3,4 bonds. 2,3-Fusion resulted in increased antiaromaticity of the pentalene unit compared to the 3,4-fusion both in the monoareno and diareno framework. Monothienopentalenes that contained the destabilizing 2,3-fusion could not be isolated. For diareno derivatives, the aromatic character of the different aryl groups fused to the pentalene was not independent. Destabilizing fusion on one side resulted in alleviated aromaticity on the other side and vice versa. The synthesized molecules were characterized experimentally by 1H NMR and UV–vis spectroscopies, cyclic voltammetry, and X-ray crystallography, and their aromatic character was assessed using magnetic (NICS and ACID) and electronic indices (MCI and FLU).

Journal of Physical Chemistry A, Jan 11, 2021

The aromaticity of cyclic 4nπ-electron molecules in their first ππ* triplet state (T 1), labeled ... more 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 Huckel aromaticity of one or a few monocycles and semilocalized triplet diradical character. Monomers with lower Huckel 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 Huckel aromaticity of the corresponding closed-shell dications yet observe stronger Huckel aromaticity in the macrocyclic dications than Baird aromaticity in the T 1 states of the neutral macrocycles.

Green Chemistry, 2022

Please note that technical editing may introduce minor changes to the text and/or graphics, which... more Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal's standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.

Journal of the American Chemical Society

Several fully π-conjugated macrocycles with puckered or cage-type structures were recently found ... more 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 3Daromatic 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 Huckel'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.

Tetrahedron, 2013

Aryleheteroaryl and heteroaryleheteroaryl compounds are obtained through the SuzukieMiyaura cross... more Aryleheteroaryl and heteroaryleheteroaryl compounds are obtained through the SuzukieMiyaura crosscoupling reactions between diazonium salts and potassium trifluoroborates using PdCl 2 (CH 3 CN) 2 as the catalyst in water and in the absence of any additive, ligand or base. In addition, the same mild catalytic system is effective for the MatsudaeHeck reaction between several monoolefins with aryldiazonium salts.

Chemical Communications, 2014

An efficient method for the synthesis of N-arylated pyridazinones from potassium 2-furantrifluoro... more An efficient method for the synthesis of N-arylated pyridazinones from potassium 2-furantrifluoroborate and aryldiazonium salts is described. The reaction was run in water at 0-5 0 C in short reaction times and without any catalyst or additive. A mechanistic proposal is made based on 10 the experimental data and DFT calculations. Pyridazin-3(2H)-ones and their N-aryl substituted derivatives are building blocks found in many biologically active compounds such as pharmaceuticals and pesticides. 1 As an example, compound 1 (Figure 1), which is used in the treatment of central

Several fully pi-conjugated macrocycles with puckered or cage-type structures were recently found... more Several fully pi-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 thus far 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 molecules with tetrahedral or octahedral structures an aromatic closed-shell electronic structure results with 6n + 2 electrons. On the other hand, 2D-aromatic-in-3D structures exhibit aromaticity that results from the fulfillment of Hückel’s 4n + 2 rule for each in...

Chemical Science

Cibalackrot-type compounds are Hückel instead of Baird aromatic in their first triplet states (T1... more Cibalackrot-type compounds are Hückel instead of Baird aromatic in their first triplet states (T1). By choice of substituents and additional benzannelation we adjust the T1 energies, providing a new strategy for singlet fission chromophore design.

The hemiterpene isoprene is a volatile C5 hydrocarbon, with industrial applications. It is genera... more 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 simulated or natural solar light, can be dimerized into limonene, paradiprene, and isomeric C10H16 hydrocarbons (monoterpenes) in very high yields (above 90% after 44 hours) under sensitized conditions. The optimal sensitizer in our experiments is di(naphth-1-yl)methanone which we can use with a loading of merely 0.1 mol%, and it is easily 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 jet fuel drop-in. Importantly, the photodimerization can be carried out at ambient condition...

The exact energies of the lowest singlet and triplet excited states in organic chromophores are c... more The exact energies of the lowest singlet and triplet excited states in organic chromophores are crucial to their performance in optoelectronic devices. The possibility of utilizing singlet fission to enhance the performance of photovoltaic devices has resulted in a wide demand for tuneable, stable organic chromophores with wide S1 – T1 energy gaps (>1 eV). Cibalackrot-type compounds were recently considered to have favorably positioned excited state energies for singlet fission, and they were found to have a degree of aromaticity in the lowest triplet excited state (T1). This work reports on a revised and deepened theoretical analysis taking into account the excited state Hückel-aromatic (instead of Baird-aromatic) as well as diradical characters, with the aim to design new organic chromophores based on this scaffold in a rational way starting from qualitative theory. We demonstrate that the substituent strategy can effectively adjust the spin populations on the chromophore moiet...