Three-Dimensional Fully π-Conjugated Macrocycles: When 3D-Aromatic and When 2D-Aromatic-in-3D?

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 (, -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.

Unsymmetrical Thienopentalenes: Synthesis, Optoelectronic Properties, and (Anti)aromaticity Analysis.

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 -dibromoolefins was applied. Diareno pentalene derivatives were accessed via gold-catalyzed cyclization of diynes.

An Element-Substituted Cyclobutadiene Exhibiting High-Energy Blue Phosphorescence.

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.

Excited state character of Cibalackrot-type compounds interpreted in terms of Hückel-aromaticity: a rationale for singlet fission chromophore design.

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 S-T 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 (T).

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

The aromaticity of cyclic 4π-electron molecules in their first ππ* triplet state (T), labeled Baird aromaticity, has gained growing attention in the past decade. Here we explore computationally the limitations of T state Baird aromaticity in macrocyclic compounds, 's, which are cyclic oligomers of four different monocycles (M = -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 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.

Tuning the Baird aromatic triplet-state energy of cyclooctatetraene to maximize the self-healing mechanism in organic fluorophores.

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.

Structure-Property Relationships in Unsymmetric Bis(antiaromatics): Who Wins the Battle between Pentalene and Benzocyclobutadiene?†.

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 H NMR, UV-vis, and cyclic voltammetry measurements and X-ray crystallography.

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

Singlet exciton fission photovoltaic technology requires chromophores with their lowest excited states arranged so that 2(T) < (S) and (S) < (T). Herein, qualitative theory and quantum chemical calculations are used to develop explicit strategies on how to use Baird's 4 rule on excited-state aromaticity, combined with Hückel's 4 + 2 rule for ground-state aromaticity, to tailor new potential chromophores for singlet fission. We first analyze the (T), (S), and (T) 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.

Is Excited-State Aromaticity a Driving Force for Planarization of Dibenzannelated 8π-Electron Heterocycles?

Compounds with dibenzannelated heterocycles with eight π-electrons are found in a range of applications. These molecules often adopt a bent structure in the ground state (S ) but can become planar in the first excited states (S and T ) because of the cyclically conjugated 4nπ central ring, which fulfils the requirements for excited state aromaticity. We report on a quantum chemical investigation of the aromatic character in the S and T states of dibenzannelated seven- and six-membered heterocycles with one, two, or three heteroatoms in the 8π-electron ring.

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

Due to the reversal in electron counts for aromaticity and antiaromaticity in the closed-shell singlet state (normally ground state, S ) and lowest ππ* triplet state (T or T ), as given by Hückel's and Baird's rules, respectively, fulvenes are influenced by their substituents in the opposite manner in the T and S states. This effect is caused by a reversal in the dipole moment when going from S to T as fulvenes adapt to the difference in electron counts for aromaticity in various states; they are aromatic chameleons. Thus, a substituent pattern that enhances (reduces) fulvene aromaticity in S reduces (enhances) aromaticity in T , allowing for rationalizations of the triplet state energies (E ) of substituted fulvenes.

Metal Cluster Electrides: A New Type of Molecular Electride with Delocalised Polyattractor Character.

Electrides are ionic substances containing isolated electrons. These confined electrons are topologically characterised by a quasi-atom, that is, a non-nuclear attractor (NNA) of the electron density. The electronic structure of the octahedral A Li and A Be species shows that these species have a large number of NNAs.

On the regioselectivity of the Diels-Alder cycloaddition to C in high spin states.

Controlling the regioselectivity in the exohedral functionalization of fullerenes and endohedral metallofullerenes is essential to produce specific desired fullerene derivatives. In this work, using density functional theory (DFT) calculations, we show that the regioselectivity of the Diels-Alder (DA) cycloaddition of cyclopentadiene to 2S+1C60 changes from the usual [6,6] addition in the singlet ground state to the [5,6] attack in high spin states of C60. Changes in the aromaticity of the five- and six-membered rings when going from singlet to high spin C60 provide a rationale to understand this regioselectivity change.

Correction: Planar vs. three-dimensional X(6)(2-), X(2)(4)(2-), and X(3)Y(3)(2-) (X, Y = B, Al, Ga) metal clusters: an analysis of their relative energies through the turn-upside-down approach.

Correction for 'Planar vs. three-dimensional X, XY, and XY (X, Y = B, Al, Ga) metal clusters: an analysis of their relative energies through the turn-upside-down approach by Ouissam El Bakouri et al., Phys.

Can Baird's and Clar's Rules Combined Explain Triplet State Energies of Polycyclic Conjugated Hydrocarbons with Fused 4nπ- and (4n + 2)π-Rings?

Compounds that can be labeled as "aromatic chameleons" are π-conjugated compounds that are able to adjust their π-electron distributions so as to comply with the different rules of aromaticity in different electronic states. We used quantum chemical calculations to explore how the fusion of benzene rings onto aromatic chameleonic units represented by biphenylene, dibenzocyclooctatetraene, and dibenzo[a,e]pentalene modifies the first triplet excited states (T) of the compounds. Decreases in T energies are observed when going from isomers with linear connectivity of the fused benzene rings to those with cis- or trans-bent connectivities.

Planar vs. three-dimensional X6(2-), X2Y4(2-), and X3Y3(2-) (X, Y = B, Al, Ga) metal clusters: an analysis of their relative energies through the turn-upside-down approach.

Despite the fact that B and Al belong to the same group 13 elements, the B6(2-) cluster prefers the planar D2h geometry, whereas Al6(2-) favours the Oh structure. In this work, we analyse the origin of the relative stability of D2h and Oh forms in these clusters by means of energy decomposition analysis based on the turn-upside-down approach. Our results show that what causes the different trends observed is the orbital interaction term, which combined with the electrostatic component do (Al6(2-) and Ga6(2-)) or do not (B6(2-)) compensate the higher Pauli repulsion of the Oh form.

Octahedral aromaticity in (2S+1)A1g X6(q) clusters (X = Li-C and Be-Si, S = 0-3, and q = -2 to +4).

It is generally observed that quintessential aromatic compounds have delocalised electronic configurations that are of closed-shells or open-shells half-filled with the same spin electrons. Guided by this property, we search for aromatic octahedral clusters of the type X6(q) (X = Li-C and Be-Si, q = -2 to +4) in (2S+1)A1g electronic states with spin multiplicities ranging from the singlet to the septet. With some exceptions, we find that closed-shells or open-shells half-filled with same spin electron systems have large multicentre indices and negative NICS values that are characteristic patterns of aromatic compounds.

A new mild synthetic route to N-arylated pyridazinones from aryldiazonium salts.

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 °C in short reaction times and without any catalyst or additive. A mechanistic proposal is made based on the experimental data and DFT calculations.