Introduction: aromaticity

Revisiting the quasi-aromaticity in polynuclear metal chalcogenide clusters and their derivative "cluster-assembly" crystalline structures

The concept of aromaticity is primarily invented to account for the high stability of conjugated organic compounds that possess a specific structural and chemical stability with (4n + 2) π electrons. In 1988, quasi-aromaticity was theoretically proposed for the Mo3S44+ core in the Mo3(μ3-S)(μ-S)3(χ-dtp)3(μ-dtp) L compound (χ: chelating ligand; dtp: (EtO)2PS2-) illustrated by canonical molecular orbitals. However, the origin of the quasi-aromaticity and chemical bonding remains ambiguous, lacking a thorough analysis in terms of stability and quantitative measurement of the aromatic character. Thus, in this work, we systematically reported the electronic structure and aromaticity of a series of polynuclear metal chalcogenide clusters [M3X4(H2O)9]4+ (M = Cr, Mo, W, and Sg; X = O, S, Se, and Te) to explore an efficient tool of NICS index values at specific points to measure the quasi-aromaticity and to figure out the (d-p-d) π three-center bonding as the predominant origin from the arrangement of three Mo atoms and three bridged X atoms. Interestingly, derived from the Mo3⋯S3 quasi-plane, the extended sandwich cluster model of a S3⋯Mo3⋯S3 (Mo3S6) structure can be seen as the seed unit of the popular MoS2 nanomaterials, with the resemblance between both molecular and periodic systems regarding geometries, electronic structures, and chemical bonding. Additionally, the highly symmetric Mo3S4 core in [Mo3X4(H2O)9]4+ can be arranged in a staggered and stacked manner to create the Mo6S82- building block, corresponding to the crystalline structures in BaMo6S8 Chevrel phases, albeit with slight deformations. But the neutral Mo6S8 cluster can be seen as the seed structure for the Mo3S4 periodic materials for the high resemblance in terms of geometry, electronic structures and chemical bonding. Drawing upon the observed similarities between cluster models and materials, we propose a new concept termed "cluster-assembly" materials. This concept involves the expansion from a high-symmetry and/or aromatic stable cluster seed unit to form the corresponding derivative materials, presenting an alternative paradigm for investigating crystals and enriching our comprehension of the stabilities exhibited by both gas-phase clusters and solid-state materials. The concept of "cluster-assembly" materials not only contributes to the formulation of design strategies for novel materials or stable clusters but also provides valuable insights into the extension of periodic aromaticity.

Aromaticity of six-membered nitro energetic compounds through molecular electrostatic potential, magnetic, electronic delocalization and reactivity-based indices

The electron density depletion associated with π-hole at the ring center typical of energetic compounds was clearly revealed by the molecular electrostatic potential (ESP). In addition, the spatial arrangement of NO2 groups appears to affect the ESP value in the ring center, and therefore sensitivity to detonation. Indeed, for monocyclic nitrobenzene compounds with the same number of NO2 groups, the ESP value in the ring center decreases as the NO2 groups are more closely spaced. As expected, the central rings become less aromatic as NO2 groups are added. The MCI, PDI, PLR, NICSzz(1), FLU indices are all strongly correlated with the ESP values observed in the ring center of the set of nitrobenzenes. Aromaticity indices based on electron delocalization criteria appear to be very sensitive to small variations in aromaticity. Among magnetic-based indices, only NICSzz(1) is capable to predict small changes in aromaticity. The PLR index derived from conceptual DFT is quite relevant for predicting small variations in aromaticity. According to our results, the most suitable aromaticity index is not based on a single criterion, and that selecting it is more subtle. Therefore, it is important to combine information from several criteria to obtain a more complete description of the aromaticity of the studied compounds.

Supramolecular trapping of a cationic all-metal σ-aromatic {Bi4} ring

Aromaticity in organic molecules is well defined, but its role in metal-only rings remains controversial. Here we introduce a supramolecular stabilization approach of a cationic {Bi4} rhomboid within the symmetric charge sphere of two bowl-shaped dianionic calix[4]pyrrolato indinates. Crystallographic and spectroscopic characterization, quantum chemical analysis and magnetically induced ring currents indicate σ-aromaticity in the formally tetracationic 16-valence electron [Bi4]4+ ring. Computational screening for other p-block elements identifies the planar rhomboid as the globally preferred structure for 16-valence electron four-atomic clusters. The aromatic [Bi4]4+ is isoelectronic to the [Al4]4-, a motif previously observed as antiaromatic in Li3[Al4]- in the gas phase. Thus, subtle factors such as charge isotropy seem to decide over aromaticity or antiaromaticity, advising for caution in debates based on the Hückel model-a concept valid for second-row elements but less deterministic for the heavier congeners.

Aromaticity in Semi-Condensed Figure-Eight Molecules

Electron delocalization and aromaticity was comparatively evaluated in recently synthesized figure-eight molecules made of two condensed U-shaped polycyclic aromatic hydrocarbon moieties connected either by two single bonds or by two para-phenylene groups. The selected examples include molecules that incorporate eight-membered and sixteen-membered rings, as well as a doubly [5]helicene-bridged (1,4)cyclophane. We probe whether some electron delocalization could occur through the stereogenic single bonds in these molecules: Is aromaticity purely (semi-)local, or possibly also global in these molecules? It was concluded that the situation can go from a purely (semi-)local character when the dihedral angle at the connecting single bonds is large, such as in biphenyl, to a predominantly (semi-)local character with a minor global contribution when the dihedral angle is small, such as in the para-phenylene connectors of the [5] helicene-bridged cyclophane.

Ultra-photostable small-molecule dyes facilitate near-infrared biophotonics

Long-wavelength, near-infrared small-molecule dyes are attractive in biophotonics. Conventionally, they rely on expanded aromatic structures for redshift, which comes at the cost of application performance such as photostability, cell permeability, and functionality. Here, we report a ground-state antiaromatic strategy and showcase the concise synthesis of 14 cationic aminofluorene dyes with mini structures (molecular weights: 299-504 Da) and distinct spectra covering 700-1600 nm. Aminofluorene dyes are cell-permeable and achieve rapid renal clearance via a simple 44 Da carboxylation. This accelerates optical diagnostics of renal injury by 50 min compared to existing macromolecular approaches. We develop a compact molecular sensing platform for in vivo intracellular sensing, and demonstrate the versatile applications of these dyes in multispectral fluorescence and optoacoustic imaging. We find that aromaticity reversal upon electronic excitation, as indicated by magnetic descriptors, not only reduces the energy bandgap but also induces strong vibronic coupling, resulting in ultrafast excited-state dynamics and unparalleled photostability. These results support the argument for ground-state antiaromaticity as a useful design rule of dye development, enabling performances essential for modern biophotonics.

Unveiling Möbius/Hückel Topology and Aromaticity in A Core-Expanded [10]Annulene at Different Oxidation States

The exploration of annulene's conformation, electronic properties and aromaticity has generated enduring interest over the years, yet it continues to present formidable challenges for annulenes with more than ten carbon atoms. In this study, we present the synthesis of a stable [10]cyclo-para-phenylmethine derivative (1), which bears a resemblance to [10]annulene. 1 can be readily oxidized into its respective cations, wherein electrons are effectively delocalized along the backbone, resulting in different conformations and aromaticity. Both 1 and its tetracation (14+  ⋅ 4SbF6 - ) exhibit a nearly planar conformation with a rectangular shape, akin to the E,Z,E,Z,Z-[10]annulene. In contrast, the radical cation (1⋅+  ⋅ SbCl6 - ) possesses a doubly twisted Hückel topology. Furthermore, the dication (12+  ⋅ 2SbCl6 - ) displays conformational flexibility in solution and crystalizes with the simultaneous presence of Möbius-twisted (1a2+  ⋅ 2SbCl6 - ) and Hückel-planar (1b2+  ⋅ 2SbCl6 - ) isomers in its unit cell. Detailed experimental measurements and theoretical calculations reveal that: (1) 1 demonstrates localized aromaticity with an alternating benzenoid/quinoid structure; (2) 1a2+  ⋅ 2SbCl6 - and 1b2+  ⋅ 2SbCl6 - with 48π electrons are weakly Möbius aromatic and Hückel antiaromatic, respectively; (3) 14+  ⋅ 4SbF6 - exhibits Hückel aromaticity (46π) and open-shell diradical character.

Nonbonding Electron Delocalization Stabilizes the Flexible N8 Molecular Assembly

Electron delocalization has an important impact on the physical properties of condensed materials. However, the L-electron delocalization in inorganic, especially nitrogen, compounds needs exploitation to improve the energy efficiency, safety, and environmental sustainability of high-energy-density materials (HEDMs). This Letter presents an intriguing N8 molecule, ingeniously utilizing nitrogen's L-electron delocalization. The molecule, exhibiting a unique lollipop-shaped conformation, can fold at various angles with very low energy barriers, self-assembling into environmentally stable, all-nitrogen crystals. These crystals demonstrate unparalleled stability, high energy density, low mechanical sensitivity, and optimal electronic thermal conductivity, outperforming existing HEDMs. The remarkable properties of these designed materials are attributed to two distinct delocalized systems within nitrogen's L-shell: π- and lone pair σ-electrons, which not only stabilize the molecular structure but also facilitate interconnected 3D networks of intermolecular nonbonding interactions. This work might pave the way to the experimental synthesis of environmentally stable all-nitrogen solids.

Antiaromatic Molecules as Magnetic Couplers: A Computational Quest

In this study, we investigate a set of organic diradical structures in which two oxo-verdazyl radicals are selected as radical spin centers that are connected (coupled) via six coupler molecules (CM), resulting in various magnetic (ferromagnetic (FM) or antiferromagnetic (AFM)) characteristics, as reflected by their exchange coupling constants (J). We have designed 12 diradicals with 6-antiaromatic couplers coupled with bis-oxo-verdazyl diradicals with meta-meta (m-m) and para-meta (p-m) positional connectivities. The nature of the magnetic coupling (ferromagnetic, nonmagnetic, or antiferromagnetic) and the magnitude of the exchange constant J depend on the type of coupler, the connecting point between each radical center and CM, the degree of aromaticity of the coupler, and the length of the through-bond distance between radical centers. The computed magnetic exchange coupling constants J for these diradicals at the B3LYP/6-311++G(d,p) and MN12SX/6-311++G(d,p) levels of theory are large for many of these structures, indicating strong ferromagnetic coupling (with positive J values). In some cases, magnetic couplings are observed with J > 1000 cm-1 (B3LYP/6-311++G(d,p)) and strong antiferromagnetic coupling (with negative J values) with J < -1000 cm-1 (B3LYP/6-311++G(d,p)). Similarly, in some cases, magnetic couplings are observed with J > 289 cm-1 (MN12SX/6-311++G(d,p)) and strong antiferromagnetic coupling (with negative J values) with J < -568 cm-1 (MN12SX/6-311++G(d,p)). Furthermore, while numerous studies have reported that the degree of aromaticity of molecular couplers often favors strong ferromagnetic coupling, displaying the high-spin character of diradicals in their ground states, the couplers chosen in this study are characterized as antiaromatic or nonaromatic. The current investigation provides evidence that, remarkably, antiaromatic couplers are able to enhance stability by favoring electronic diradical structures with very strong ferromagnetic coupling when the length of the through-bond distance and connectivity pattern between radical centers are selected in such a way that the FM coupling is optimized. The findings in this study offer new strategies in the design of novel organic materials with interesting magnetic properties for practical applications.

Metallaantiaromaticity of 10-Platinacorrole Complexes

The aromaticity of cyclic π-conjugated organometallic compounds is known as metallaaromaticity. π-Conjugated metallacycles, such as metallabenzenes and metallapentalenes, have been investigated in order to understand the involvement of the d electrons from the metal center in the π-conjugated systems of the organic ligands. Here, we report the synthesis of Pd(II) 10-platinacorrole complexes with cyclooctadiene (COD) and norbornadiene (NBD) ligands. While the Pd(II) 10-platinacorrole COD complex adopts a distorted structure without showing appreciable antiaromaticity, the corresponding NBD complex exhibits a distinct antiaromatic character due to its highly planar conformation. Detailed density functional theory (DFT) calculations revealed that two d orbitals are involved in macrocyclic π-conjugation. We furthermore demonstrated that Craig-Möbius antiaromaticity is not present in the studied system. The synthesis of 10-platinacorroles enables a systematic comparison of the antiaromaticity and aromaticity of closely related porphyrin analogues, providing a better understanding of π-conjugation that involves d orbitals.

Aromaticity Reversal Induced by Vibrations in Cyclo[16]carbon

Aromaticity is typically regarded as an intrinsic property of a molecule, correlated with electron delocalization, stability, and other properties. Small variations in the molecular geometry usually result in small changes in aromaticity, in line with Hammond's postulate. For example, introducing bond-length alternation in benzene and square cyclobutadiene by modulating the geometry along the Kekulé vibration gradually decreases the magnitude of their ring currents, making them less aromatic and less antiaromatic, respectively. A sign change in the ring current, corresponding to a reversal of aromaticity, typically requires a gross perturbation such as electronic excitation, addition or removal of two electrons, or a dramatic change in the molecular geometry. Here, we use multireference calculations to show how movement along the Kekulé vibration, which controls bond-length alternation, induces a sudden reversal in the ring current of cyclo[16]carbon, C16. This reversal occurs when the two orthogonal π systems of C16 sustain opposing currents. These results are rationalized by a Hückel model which includes bond-length alternation, and which is combined with a minimal model accounting for orbital contributions to the ring current. Finally, we successfully describe the electronic structure of C16 with a "divide-and-conquer" approach suitable for execution on a quantum computer.

The role of aromaticity in the cyclization and polymerization of alkyne-substituted porphyrins on Au(111)

Aromaticity is an established and widely used concept for the prediction of the reactivity of organic molecules. However, its role remains largely unexplored in on-surface chemistry, where the interaction with the substrate can alter the electronic and geometric structure of the adsorbates. Here we investigate how aromaticity affects the reactivity of alkyne-substituted porphyrin molecules in cyclization and coupling reactions on a Au(111) surface. We examine and quantify the regioselectivity in the reactions by scanning tunnelling microscopy and bond-resolved atomic force microscopy at the single-molecule level. Our experiments show a substantially lower reactivity of carbon atoms that are stabilized by the aromatic diaza[18]annulene pathway of free-base porphyrins. The results are corroborated by density functional theory calculations, which show a direct correlation between aromaticity and thermodynamic stability of the reaction products. These insights are helpful to understand, and in turn design, reactions with aromatic species in on-surface chemistry and heterogeneous catalysis.

Crystalline 2π Aromatic Azadiboriridinylium: A BN Analogue of Cyclopropenylium Cation

N-Substitution of a thermally unstable diboratriazole 1 with a trimethylsilyl group affords a remarkably stable diboratriazole derivative 2. Ring contraction of 2 with an N-heterocyclic carbene accompanied by the release of N2 as well as 1,4-hydrogen shift affords a carbene-stabilized azadiboriridine 3. Abstraction of the H-B3mem hydride in 3 with methyl trifluoromethanesulfonate leads to the isolation of a hitherto unknown azadiboriridinylium 4, the first BN analogue of cyclopropenylium cation. X-ray diffraction analysis and computational studies confirmed the delocalization of π electrons over the B2 N three-membered ring, indicating the 2π aromatic feature. Compound 4 undergoes ring expansion reactions with azobenzene and pyridazine to furnish triazadiborolidinylium species 5 and 6, the latter of which possesses a cationic B2 N3 ring with a pronounced 6π aromatic property. Moreover, the reaction of 4 with a diazo compound produces a cationic B2 N3 C pentafulvene derivative 7.

Exact two-component theory becoming an efficient tool for NMR shieldings and shifts with spin-orbit coupling

We present a gauge-origin invariant exact two-component (X2C) approach within a modern density functional framework, supporting meta-generalized gradient approximations such as TPSS and range-separated hybrid functionals such as CAM-B3LYP. The complete exchange-correlation kernel is applied, including the direct contribution of the field-dependent basis functions and the reorthonormalization contribution from the perturbed overlap matrix. Additionally, the finite nucleus model is available for the electron-nucleus potential and the vector potential throughout. Efficiency is ensured by the diagonal local approximation to the unitary decoupling transformation in X2C as well as the (multipole-accelerated) resolution of the identity approximation for the Coulomb term (MARI-J, RI-J) and the seminumerical exchange approximation. Errors introduced by these approximations are assessed and found to be clearly negligible. The applicability of our implementation to large-scale calculations is demonstrated for a tin pincer-type system as well as low-valent tin and lead complexes. Here, the calculation of the Sn nuclear magnetic resonance shifts for the pincer-type ligand with about 2400 basis functions requires less than 1 h for hybrid density functionals. Further, the impact of spin-orbit coupling on the nucleus-independent chemical shifts and the corresponding ring currents of all-metal aromatic systems is studied.

On-Surface Synthesis and Real-Space Visualization of Aromatic P3 N3

On-surface synthesis is at the verge of emerging as the method of choice for the generation and visualization of unstable or unconventional molecules, which could not be obtained via traditional synthetic methods. A case in point is the on-surface synthesis of the structurally elusive cyclotriphosphazene (P3 N3 ), an inorganic aromatic analogue of benzene. Here, we report the preparation of this fleetingly existing species on Cu(111) and Au(111) surfaces at 5.2 K through molecular manipulation with unprecedented precision, i.e., voltage pulse-induced sextuple dechlorination of an ultra-small (about 6 Å) hexachlorophosphazene P3 N3 Cl6 precursor by the tip of a scanning probe microscope. Real-space atomic-level imaging of cyclotriphosphazene reveals its planar D3h -symmetric ring structure. Furthermore, this demasking strategy has been expanded to generate cyclotriphosphazene from a hexaazide precursor P3 N21 via a different stimulation method (photolysis) for complementary measurements by matrix isolation infrared and ultraviolet spectroscopy.

Cyclo[n]carbons and catenanes from different perspectives: disentangling the molecular thread

All-carbon atomic rings, cyclo[n]carbons, have recently attracted vivid attention of experimentalists and theoreticians. Among them, cyclo[18]carbon is the most studied system. In this paper, we summarize and review various properties of cyclo[n]carbons, emphasising the aspects of their aromaticity/antiaromaticity. In the first part, the trends in bonding patterns and selected aromaticity indices with the increasing size of the rings are discussed. In the second part we explore the properties of catenane models based on interlocked cyclo[18]carbon rings from different perspectives and investigate their behaviour under the action of external force using computational experiments.

Adaptive σ aromaticity in the rhenacyclopropene rings

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.

Hyperconjugative Aromaticity-Based Circularly Polarized Luminescence Enhancement in Polyaurated Heterocycles

Hyperconjugative aromaticity (HA) frequently appears in metalla-aromatics, but its effect on photophysical properties remains unexplored to date. Herein, we reveal two different HA scenarios in nearly isostructural triaurated indolium and benzofuranylium compounds. The biased HAs show a discernible effect on the spatial arrangement of metal atoms and thus tailor metal parentage in frontier orbitals and the HOMO-LUMO energy gap. Theoretical calculations and structural analyses demonstrate that HA not only influences the degree of electron delocalization over the trimetalated aromatic rings but also affects π-coordination of Au(I) and intercluster aurophilic interaction. Consequently, the triaurated benzofuranylium complex shows better photoluminescence performance (quantum yield up to 49.7%) over the indolium analogue. Furthermore, four pairs of axially chiral bibenzofuran-centered trinuclear and hexanuclear gold clusters were purposefully synthesized to correlate their HA-involved structures with the chiroptical response. The triaurated benzofuranylium complexes exhibit strong circular dichroism (CD) response in solution but CPL silence even in solid film. In contrast, the hexa-aurated homologues display strong CD and intense CPL signals in both aggregated state and solid film (luminescence anisotropy factor glum up to 10-3). Their amplified chiroptical response is finally ascribed to the dominant intermolecular exciton couplings of large assemblies formed through the HA-tailored aggregation of hexanuclear compounds.

Thioxobimanes

Dioxobimanes, colloquially known as bimanes, are a well-established family of N-heterobicyclic compounds that share a characteristic core structure, 1,5-diazabicyclo[3.3.0]octadienedione, bearing two endocyclic carbonyl groups. By sequentially thionating these carbonyls in the syn and anti isomers of the known (Me,Me)dioxobimane, we were able to synthesize a series of thioxobimanes, representing the first heavy-chalcogenide bimane variants. These new compounds were extensively characterized spectroscopically and crystallographically, and their aromaticity was probed computationally. Their potential role as ligands for transition metals was demonstrated by synthesizing a representative gold(I)-thioxobimane complex.

Current-Density Calculations on Zn-Porphyrin40 Nanorings

Two porphyrinoid nanorings have been studied computationally. They were built by linking 40 Zn-porphyrin units with butadiyne bridges. The molecular structures belonging to the D40h point group were fully optimized with the Turbomole program at the density functional theory (DFT) level using the B3LYP functional and the def2-SVP basis sets. The aromatic character was studied at the DFT level by calculating the magnetically induced current-density (MICD) susceptibility using the GIMIC program. The neutral molecules are globally non-aromatic with aromatic Zn-porphyrin units. Charged nanorings could not be studied because almost degenerate frontier orbitals led to vanishing optical gaps for the cations. Since DFT calculations of the MICD are computationally expensive, we also calculated the MICD using three pseudo-π models. Appropriate pseudo-π models were constructed by removing the outer hydrogen atoms and replacing all carbon and nitrogen atoms with hydrogen atoms. The central Zn atom was either replaced with a beryllium atom or with two inner hydrogen atoms. Calculations with the computationally inexpensive pseudo-π models yielded qualitatively the same magnetic response as obtained in the all-electron calculations.

Adaptive Aromaticity in 18e Metallapentalenes

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.

Probing Near-infrared Absorbance of E and Z Diazene Isomers via Antiaromaticity

The photoswitching behaviors of heteroaryl azos and azobenzenes have attracted considerable interest due to their applications from material science to pharmacology. However, the use of UV light limits their application, especially in biomedicine and photopharmacology. In this work, using several aromaticity descriptors, including anisotropy of the induced current density analysis and nucleus-independent chemical shifts, we systematically investigate the relationship between anti-aromaticity and the absorption of a series of heterocyclic azos. We have demonstrated that the antiaromatic heterocycles substituted with diazenes enable the significant red shifts of the n → π* and π → π* transition bands of E and Z isomers via density functional theory calculations. Moreover, introducing substituents into heterocycles could further tune the absorption. Finally, the λmax of the first transition bands of the E (ca. 1026 nm) and Z isomers (ca. 1167 nm) of azos is achieved in the near-infrared region.

Substituent effects on paratropicity and diatropicity in π-extended hexapyrrolohexaazacoronene

Research into the application of antiaromatic compounds as molecular materials is an attractive strategy in the development of electronic materials. Antiaromatic compounds have traditionally been considered to be unstable, and thus, the creation of stable antiaromatic compounds has been sought in the field of organic chemistry. Recently, some studies have been reported on the synthesis, isolation, and elucidation of the physical properties of compounds with stability and definitive antiaromatic properties. In general, antiaromatic compounds are considered to be more susceptible to substituents due to their inherently narrow HOMO-LUMO gap compared to aromatic compounds. However, there have been no studies examining substituent effects in antiaromatic compounds. In this study, we have developed a synthetic method to introduce various substituents into π-extended hexapyrrolohexaazacoronene (homoHPHAC⁺), one of the stable and clearly antiaromatic compounds, and investigated the substituent effects on the optical, redox, and geometrical properties and paratropicity of a series of compounds. In addition, the properties of the two-electron oxidized form, homoHPHAC³⁺, were investigated. Control of electronic properties by introducing substituents into antiaromatic compounds provides a new design guideline for molecular materials.

Aromaticity: Quo Vadis

Aromaticity is one of the most deeply rooted concepts in chemistry. But why, if two-thirds of existing compounds can be classified as aromatic, is there no consensus on what aromaticity is? σ-, π-, δ-, spherical, Möbius, or all-metal aromaticity… why are so many attributes needed to specify a property? Is aromaticity a dubious concept? This perspective aims to reflect where the aromaticity community is and where it is going.

Benzo[1,2-d:4,5-d']bis([1,2,3]thiadiazole) and Its Bromo Derivatives: Molecular Structure and Reactivity

Benzo[1,2-d:4,5-d']bis([1,2,3]thiadiazole) (isoBBT) is a new electron-withdrawing building block that can be used to obtain potentially interesting compounds for the synthesis of OLEDs and organic solar cells components. The electronic structure and delocalization in benzo[1,2-d:4,5-d']bis([1,2,3]thiadiazole), 4-bromobenzo[1,2-d:4,5-d']bis([1,2,3]thiadiazole), and 4,8-dibromobenzo[1,2-d:4,5-d']bis([1,2,3]thiadiazole) were studied using X-ray diffraction analysis and ab initio calculations by EDDB and GIMIC methods and were compared to the corresponding properties of benzo[1,2-c:4,5-c']bis[1,2,5]thiadiazole (BBT). Calculations at a high level of theory showed that the electron affinity, which determines electron deficiency, of isoBBT was significantly smaller than that of BBT (1.09 vs. 1.90 eV). Incorporation of bromine atoms improves the electrical deficiency of bromobenzo-bis-thiadiazoles nearly without affecting aromaticity, which increases the reactivity of these compounds in aromatic nucleophilic substitution reactions and, on the other hand, does not reduce the ability to undergo cross-coupling reactions. 4-Bromobenzo[1,2-d:4,5-d']bis([1,2,3]thiadiazole) is an attractive object for the synthesis of monosubstituted isoBBT compounds. The goal to find conditions for the selective substitution of hydrogen or bromine atoms at position 4 in order to obtain compounds containing a (het)aryl group in this position and to use the remaining unsubstituted hydrogen or bromine atoms to obtain unsymmetrically substituted isoBBT derivatives, potentially interesting compounds for organic photovoltaic components, was not set before. Nucleophilic aromatic and cross-coupling reactions, along with palladium-catalyzed C-H direct arylation reactions for 4-bromobenzo[1,2-d:4,5-d']bis([1,2,3]thiadiazole), were studied and selective conditions for the synthesis of monoarylated derivatives were found. The observed features of the structure and reactivity of isoBBT derivatives may be useful for building organic semiconductor-based devices.

Engineering magic number Au19 and Au20 cage structures using electron withdrawing atoms

Gold cages are a subset of gold nanoparticles and these structures are of major interest due to their favourable physiochemical properties. In order for these structures to be useful in applications, they must be chemically stable. The objective of this research is to transform non-magic number cage structures into magic number cage structures by the addition of electron-withdrawing groups on the cages. The electronic properties for Au₁₉X and Au₂₀X₂ (X = F, Cl, Br, I) are calculated and observed. It is expected that the electron-withdrawing groups will remove the electron density from the gold cages and leave them positively charged. We first optimize the geometries of the initial gold cages and verify the structures are a local minima. From there, we attach our halogens to the gold cages and optimize the structures to determine the NICS values and HOMO-LUMO gaps. NICS values were found to be more negative when a more electronegative halogen was used. Calculations have found that Au₁₉F and Au₂₀F₂ have the most negative NICS values, indicating greater spherical aromaticity. Iodine, being the least electronegative atom, had the most positive NICS value and smallest HOMO-LUMO gap. All calculations are compared to the magic cluster Au₁₈ which satisfies Hirsh's 2(N + 1)² rule for n = 2.

Electrostatic Potential for Exploring Electron Delocalization in Infinitenes, Circulenes, and Nanobelts

The π-conjugation, aromaticity, and stability of the newly synthesized 12-infinitene and of other infinitenes comprising 8-, 10-, 14-, and 16-arene rings are investigated using density functional theory. The π-electron delocalization and aromatic character rooted in infinitenes are quantified in terms of molecular electrostatic potential (MESP) topology. Structurally, the infinitene bears a close resemblance of its helically twisted structure to the infinity symbol. The MESP topology shows that infinitene possesses an infinity-shaped delocalization of the electron density that streams over the fused benzenoid rings. The parameter ∑i=13Δλi, derived from the eigenvalues (λ_i) corresponding to the MESP minima, is used for quantifying the aromatic character of arene rings of infinitene. The structure, stability, and MESP topology features of 8-, 10-, 12-, 14-, and 16-infinitenes are also compared with the corresponding isomeric circulenes and carbon nanobelts. Further, the strain in all such systems is evaluated by considering the respective isomeric planar benzenoid hydrocarbons as reference systems. The 12-infinitene turns out to be the most aromatic and the least strained among all the systems examined.

Ultrafast Excited State Aromatization in Dihydroazulene

Excited-state aromatization dynamics in the photochemical ring opening of dihydroazulene (DHA) is investigated by nonadiabatic molecular dynamics simulations in connection with the mixed-reference spin-flip (MRSF)-TDDFT method. It is found that, in the main reaction channel, the ring opening occurs in the excited state in a sequence of steps with increasing aromaticity. The first stage lasting ca. 200 fs produces an 8π semiaromatic S₁ minimum (S_1, min) through an ultrafast damped bond length alternation (BLA) movement synchronized with a partial planarization of the cycloheptatriene ring. An additional ca. 200 fs are required to gain the vibrational energy needed to overcome a ring-opening transition state characterized by an enhanced Baird aromaticity. Unlike other BLA motions of ππ* state, it was shown that their damping is a characteristic feature of aromatic bond-equalization process. In addition, some minor channels of the reaction have also been discovered, where noticeably higher barriers of the S₁ non/antiaromatic transition structures must be surmounted. These anti-Baird channels led to reformation of DHA or other closed-ring products. The observed competition between the Baird and anti-Baird channels suggests that the quantum yield of photochemical products can be controllable by tipping their balance. Hence, here we suggest including the concept of anti-Baird, which would expand the applicability of Baird rule to much broader situations.

Unraveling the Photoelectron Spectrum of 1-Phospha-2,3,4-triazolate Anion, HCPN3-, A Theoretical Approach

The first five low-lying electronic states of HCPN₃ are probed through extensive ab initio electronic structure and quantum dynamics studies to reproduce the 193 nm photoelectron spectrum. Vibronic Hamiltonian is constructed and availed for time-dependent (TD) and time-independent (TI) quantum dynamical studies. The presence of numerous conical intersections (CIs) and crossings among electronic states yielded interesting nonadiabatic effects in the photoelectron bands of the overall spectrum. Moreover, the theoretical bands corresponding to five electronic states have reproduced all three experimental spectral bands. Among these, the first two bands originated due to a combination of four electronic states as predicted by previous studies. The third band corresponds to the fifth electronic state. The results calculated via TD and TI approaches exhibited satisfying agreement with the experimental results.

Magnetic response properties of carbon nano-onions

The magnetic response of a number of double- and triple-layer carbon nano-onions (CNOs) is analyzed by calculating the magnetically induced current density and the induced magnetic field using the pseudo-π model. Qualitatively the same magnetic response was obtained in calculations at the all-electron level. The calculations show that the CNOs exhibit strong net diatropic (paratropic) ring currents when the external magnetic field points perpendicularly to one of the six-membered (five-membered) rings. They are deshielded inside and shielded outside the CNO; the latter dominates for larger CNOs. The magnetic response originates from a combination of spherical paratropic current densities on the inside of each carbon layer and diatropic current densities on the outside of them. The quantitative differences in the aromaticity of the CNOs as compared to single fullerenes are discussed in terms of ring-current strengths. The magnetic response of some of the CNOs is approximately the sum of the magnetic response of the individual layers, whereas deviations are significant for CNOs containing C₈₀. For the largest CNOs, the deviation from the sum of the fullerene contributions is larger, especially when the external magnetic field is perpendicular to a six-membered ring.

Rigid Enhanced Electrochemiluminescence of 1,2,3-Triaryl Indenes as an Ultrasensitive Sensor for D2O in H2O

The intriguing aggregation-induced emission has recently been applied in the electrochemiluminescence, called aggregation-induced electrochemiluminescence (AIE-ECL), which is conducive to solving the water insolubility and aggregation-caused quenching for most organic luminescence probes. However, AIE-ECL still has the problems of low luminous efficiency and limited practical application. In this work, we disclosed the AIE-ECL properties of 1,2,3-triaryl-substituted indenes containing rigid structures. Experimental and theoretical investigations demonstrated that such a rigid structure could significantly enhance the aromaticity and stability and thereby the luminescence performance of these indenes. Moreover, according to the finding of hydrogen/deuterium exchange for active hydrogen in indene under electrical excitation, ultrasensitive detection for D₂O in H₂O was realized by such an indene-based AIE-ECL system. Our research not only provided an attractive strategy to enhance the luminescence property for an AIE-active luminophore but also established a superior sensor toward D₂O.

Core-electron contributions to the molecular magnetic response

Orbital contributions to the magnetic response depend on the method used to compute them. Here, we show that dissecting nuclear magnetic shielding tensors using natural localized molecular orbitals (NLMOs) leads to anomalous core contributions. The arbitrariness of the assignment might significantly affect the interpretation of the magnetic response of nonplanar molecules such as C₆₀ or [14]helicene and the assessment of their aromatic character. We solve this problem by computing the core- and σ-components of the induced magnetic field (and NICS) and the magnetically induced current density by removing the valence electrons (RVE). We estimate the core contributions to the magnetic response by performing calculations on the corresponding highly charged molecules, such as C₆H₆³⁰⁺ for benzene, using gauge-including atomic orbitals and canonical molecular orbitals (CMOs). The orbital contributions to nuclear magnetic shielding tensors are usually estimated by employing a natural chemical shielding (NCS) analysis in NLMO or CMO bases. The RVE approach shows that the core contribution to the magnetic response is small and localized at the nuclei, contrary to what NCS calculations suggest. This may lead to a completely incorrect interpretation of the magnetic σ-orbital response of nonplanar structures, which may play a major role in the overall magnetic shielding of the system. The RVE approach is thus a simple and inexpensive way to determine the magnetic response of the core- and σ-electrons.

Aromaticity and Extrusion of Benzenoids Linked to [o-COSAN]- : Clar Has the Answer

Benzene and pyrene can be synthetically linked to [o-COSAN]^- keeping their aromaticity. In contrast, naphthalene and anthracene are extruded in the same reaction. We have proven that extrusion is only favorable if the number of Clar's π-sextets remains constant. Thus, Clar has the answer to whether an attached polycyclic aromatic hydrocarbon to [o-COSAN]^- is extruded or not.

Glossary of terms used in physical organic chemistry (IUPAC Recommendations 2021)

This Glossary contains definitions, explanatory notes, and sources for terms used in physical organic chemistry. Its aim is to provide guidance on the terminology of physical organic chemistry, with a view to achieving a consensus on the meaning and applicability of useful terms and the abandonment of unsatisfactory ones. Owing to the substantial progress in the field, this 2021 revision of the Glossary is much expanded relative to the previous edition, and it includes terms from cognate fields.

Retaining Hückel Aromaticity in the Triplet Excited State of Azobenzene

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.

Organocatalytic Asymmetric Dearomatizing Hetero-Diels-Alder Reaction of Nonactivated Arenes

Nonactivated arenes, such as benzene derivatives, are chemically inert due to their intrinsic aromaticity and low polarity. The catalytic asymmetric dearomatization (CADA, coined by You and co-workers) of the nonactivated arenes represents a formidable challenge. We herein demonstrated an organocatalytic asymmetric dearomatizing hetero-Diels-Alder reaction of benzene derivatives. The tunable regioselectivity of this strategy allowed delivery of a diversity of stereochemically complex polycyclic compounds and oxahelicenes with excellent stereoselectivity. The high complexity and three-dimensionality of the products are crucial for their potential applications in materials science and drug discovery. Mechanistic studies suggested that this reaction proceeds through a chiral tetra-substituted vinylidene ortho-quinone methide (VQM) intermediate, which is extremely active to overcome the loss of aromaticity of benzene derivatives with concomitant chirality transfer.

Aromaticity-Enhanced pH-Responsive Electrochemiluminescence of Cyclopentadienols

Due to significantly tackling the problems of aggregation-caused quenching and water insolubility, aggregation-induced emission electrochemiluminescence (AIE-ECL) has emerged as a research highlight in aqueous detection and sensing. Herein, we reported a series of cyclopentadienols featuring excellent AIE-ECL properties on the basis of an enhanced aromaticity strategy. In detail, substituents profoundly determined ECL emission by affecting the characteristic absorption peak intensity ratio in UV-vis spectra and lowest unoccupied molecular orbital (LUMO)-highest occupied molecular orbital (HOMO) energies. It was found that 1,2,3,4,5-pentafluorophenyl cyclopentadienol (PFCD) containing an electron-withdrawing fluorine substituent, the maximum R/B band ratio, and a smaller LUMO-HOMO band gap demonstrated the best ECL performance. Meanwhile, such an AIE-ECL system displayed a wide response range toward pH (4-12) with a good linear relationship. Our research not only enriched polycyclic aromatic hydrocarbon-based AIE-ECL systems but also established an efficient pH sensor in the aqueous phase.

Symmetrical cyclo-N5- hydrogen bonds: stabilization mechanism of four non-metallic cyclo-pentazolate energetic salts

Pairing different cations (R⁺) to stabilize cyclo-N₅^- is the main synthesis path for non-metallic cyclo-pentazolate (cyclo-N₅^-) salts. As novel energetic materials (EMs), crystalline packing-force of cyclo-N₅^- salts has been a puzzle, and whether cyclo-N₅^- is protonated also is a controversial issue. In this paper, four non-metallic cyclo-N₅^- salts, PHAC, N₂H₅N₅, NH₃OHN₅, and NH₄N₅, are quantitatively studied by coupling first-principle method and bond-strength analyzing technology. Different from the traditional CHON-EMs (molecular crystal) and azide-EMs (ionic crystal), the four salts are stabilized by 3D hydrogen bond (HB) networks. One new type of hydrogen bond, protonated HB (p-H, R-H⋯N₅^-), is discovered to be a key stabilizing factor for cyclo-N₅^-. Proton competition mechanism between R and cyclo-N₅^- in p-H HB showed that cyclo-N₅^- cannot be protonated into HN₅. In general, p-H HB can be adopted to estimate the stability of novel non-metallic cyclo-N₅^- EMs. Such findings have great significance for future design and performance prediction of novel cyclo-N₅^- EMs in both theoretical and experimental aspects.

Aryl Transfer Strategies Mediated by Photoinduced Electron Transfer

Radical aryl migrations are powerful techniques to forge new bonds in aromatic compounds. The growing popularity of photoredox catalysis has led to an influx of novel strategies to initiate and control aryl migration starting from widely available radical precursors. This review encapsulates progress in radical aryl migration enabled by photochemical methods─particularly photoredox catalysis─since 2015. Special attention is paid to descriptions of scope, mechanism, and synthetic applications of each method.

Tropo(thio)ne-Embedded HomoHPHACs: Does the Tropylium Cation Induce Global Antiaromaticity in Expanded Hexapyrrolohexaazacoronene?

Tropo(thio)ne-embedded homoHPHACs and their dications were synthesised by an electrophilic annulation of secoHPHAC and successive oxidation. 13C NMR spectra of the dications represented global 22π homoaromaticity via homoconjugation, while alkylation...

Conjugation between 3D and 2D aromaticity: does it really exist? The case of carborane-fused heterocycles

Although several synthesized icosahedral carborane fused 2D π-ring systems are known, and even considerable conjugation has been noted between them in some cases, the phenomenon itself is not fully understood. Based on the results of our computational study, it can be concluded that the 2D aromatic character of the fused (exo) five-membered ring is low, even in cases where significant conjugation was proposed in previous studies. Moreover, the carborane moiety constricts the bonding properties of the exo ring, thus prohibiting or promoting different Lewis resonance structures. These results will shed further light on the design and electronic modulation of new carborane-based materials.

We have demonstrated that carborane fused heterocycles can definitely be considered as non-2D aromatic compounds; therefore, their aromatic character was significantly overestimated in earlier studies.

Antiaromaticity-Promoted Radical Anion stability in α-vinyl Heterocyclics

As an electron-rich species, radical anions have a wide range of applications in organic synthesis. In addition, aromaticity is an essential concept in chemistry that has attracted considerable attention from...

Computational predictions of adaptive aromaticity for the design of singlet fission materials

The concept of adaptive aromaticity has been demonstrated as an alternative strategy for the design of singlet fission materials.