Antiaromatic Compounds Research Articles

Antiaromaticity in molecular assemblies and materials.

Antiaromatic rings are infamously unstable and difficult to work with but they possess unusual electronic properties that make them interesting for fundamental and applied research. This perspective presents reports on discrete or polymeric assemblies made from antiaromatic building blocks, bound by either covalent linkages or supramolecular interactions. Compared to polymeric materials, discrete assemblies are more commonly studied, but most efforts have been devoted to their preparation and fundamental property studies, whereas applications are scarcely suggested. Future research in the field should focus on developing applications that benefit from the specific properties of antiaromatic rings. On the other hand, the few reports on antiaromatic-based materials hint at a promising future for this class of materials in organic electronics. To guide non-experts, different antiaromatic compounds are evaluated for their suitability as building blocks for larger assemblies.

Effects of benzoheterocyclic annelation on the s-indacene core: a computational analysis.

Aromaticity and antiaromaticity are pivotal concepts in chemistry, with significant implications for molecular properties and reactivity. In particular, thanks to their increased conductivity and small HOMO-LUMO energy gaps, antiaromatic molecules are promising for use in organic electronics. The inherent instability of such molecules is often addressed by carbocyclic fusion, which also reduces the antiaromaticity of the core structure. Herein, we have employed a computational approach to explore the effects of heterocyclic fusion on the s-indacene core, focusing on three main aspects: the impact of the heteroatom, the heterocycle, and extended conjugation. We found that the heteroatoms themselves can substantially modulate antiaromaticity, and that the site of substitution plays a large role in the extent of stabilization afforded. Heterocycle fusion further modulates antiaromaticity, though to a lesser extent than the heteroatom effect. This effect diminishes upon benzannelation, highlighting the complexity of aromatic and antiaromatic interplay. Our findings offer a nuanced understanding of the factors affecting antiaromaticity in s-indacene-based polycyclic systems, providing a conceptual framework for predicting and tuning these properties for applications in organic electronics. This work underscores the importance of both substitution position and heterocyclic fusion in designing stable antiaromatic compounds.

Hückel Molecular Orbital Analysis for Stability and Instability of Stacked Aromatic and Stacked Antiaromatic Systems.

Face-to-face stacking of aromatic compounds leads to stacked antiaromaticity, while that of antiaromatic compounds leads to stacked aromaticity. This is a prediction with a long history; in the late 2000s, the prediction was confirmed by high-precision quantum chemical calculations, and finally, in 2016, a π-conjugated system with stacked aromaticity was synthesized. Several variations have since been reported, but essentially, they are all the same molecule. To realize stacked aromaticity in a completely new and different molecular system and to trigger an extension of the concept of stacked aromaticity, it is important to understand the origin of stacked aromaticity. The Hückel method, which has been successful in giving qualitatively correct results for π-conjugated systems despite its bold assumptions, is well suited for the analysis of stacked aromaticity. We use this method to model the face-to-face stacking systems of benzene and cyclobutadiene molecules and discuss their stacked antiaromaticity and stacked aromaticity on the basis of their π-electron energies. By further developing the discussion, we search for clues to realize stacked aromaticity in synthesizable molecular systems.

Synthesis and characterization of Craig-type antiaromatic species with [4n + 2] π electrons

Antiaromaticity is extended from aromaticity as a complement to describe the unsaturated cyclic molecules with antiaromatic destabilization. To prepare antiaromatic species is a particularly challenging goal in synthetic chemistry because of the thermodynamic instability of such molecules. Among that, both Hückel and Möbius antiaromatic species have been reported, whereas the Craig one has not been realized to date. Here, we report the first example of planar Craig antiaromatic species. Eight Craig antiaromatic compounds were synthesized by deprotonation-induced reduction process and were fully characterized as follows. Single-crystal X-ray crystallography showed that these complexes have planar structures composed of fused five-membered rings with clearly alternating carbon-carbon bond lengths. In addition, proton NMR (1H NMR) spectroscopy in these structures showed distinctive upfield shifts of the proton peaks to the range of antiaromatic peripheral hydrogens. Experimental spectroscopy observations, along with density-functional theory (DFT) calculations, provided evidence for the Craig antiaromaticity of these complexes. Further study experimentally and theoretically revealed that the strong exothermicity of the acid-base neutralization process was the driving force for this challenging transformation forming Craig antiaromatic species. Our findings complete a full cycle of aromatic chemistry, opening an avenue for the development of new class of antiaromatic systems.

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 controls the excited-state properties of host-guest complexes of nanohoops.

π-Conjugated organic molecules have exciting applications as materials for batteries, solar cells, light-emitting diodes, etc. Among these systems, antiaromatic compounds are of particular interest because of their smaller HOMO-LUMO energy gap compared to aromatic compounds. A small HOMO-LUMO gap is expected to facilitate charge transfer in the systems. Here we report the ground and excited-state properties of two model nanohoops that are nitrogen-doped analogs of recently synthesized [4]cyclodibenzopentalenes - tetramers of benzene-fused aromatic 1,4-dihydropyrrolo[3,2-b]pyrrole ([4]DHPP) and antiaromatic pyrrolo[3,2-b]pyrrole ([4]PP). Their complexes with C60 fullerene show different behavior upon photoexcitation, depending on the degree of aromaticity. [4]DHPP acts as an electron donor, whereas [4]PP is a stronger electron acceptor than C60. The ultrafast charge separation combined with the slow charge recombination that we found for [4]PP⊃C60 indicates a long lifetime of the charge transfer state.

Spontaneous Assembly and Three‐Dimensional Stacking of Antiaromatic 5,15‐Dioxaporphyrin on HOPG

Antiaromatic compounds have recently received considerable attention because of their novel properties such as narrow HOMO-LUMO gaps and facile formation of mutual stacking. Here, the spontaneous assembly of antiaromatic meso-2-thienyl-substituted 5,15-dioxaporphyrin (DOP-1) is scrutinized at the liquid-solid interface by scanning tunneling microscopy (STM). Polymorphism in monolayers characterized by the orthogonal and parallel assemblies is found at the low concentration of 0.05 mM. The parallel assembly is more stable and dominantly formed at higher concentrations. Aggregation was observed at concentrations >0.2 mM, and the STM images of the aggregates implied the formation of stacked layers. The intrinsic electronic structures of the mutually stacked bilayer generated by applying an electric pulse to the monolayer were probed by scanning tunneling spectroscopy to reveal the narrowing of the HOMO-LUMO gap by about 20 % compared with the monolayer, thus suggesting significant molecular orbital interactions.

Tuning the antiaromatic character and charge transport of pentalene-based antiaromatic compounds by substitution

Substituting electron-donating and electron-accepting substituents on antiaromatic pentalene cores can significantly tune the optical absorption, energy levels, antiaromaticity, and transistor switch-on behavior.

An Air‐ and Water‐Stable B4N4‐Heteropentalene Serving as a Host Material for a Phosphorescent OLED

AbstractReplacement of the carbon–carbon bonds of antiaromatic compounds with polar boron–nitrogen bonds often provides isoelectronic BN compounds with excellent thermodynamic stability and interesting photophysical properties. By this element‐substitution strategy, we synthesized a new B4N4‐heteropentalene derivative, 1, which is fully substituted with mesityl groups. Owing to kinetic protection by the sterically bulky substituents, 1 is remarkably stable toward air and even water. Single‐crystal X‐ray analysis of 1 revealed the bonding characteristics of the B4N4‐heteropentalene structure. In a glassy matrix, 1 emitted short‐wavelength phosphorescence with an onset at 350 nm, indicating that the triplet energy is substantially high. DFT calculations reasonably explained the ground‐ and excited‐state electronic structures of 1 as well as its emission properties. Motivated by the high‐energy triplet state of 1, we used it as a host material to fabricate a phosphorescent organic light‐emitting diode with an external quantum efficiency of 15 %.

An Air- and Water-Stable B4 N4 -Heteropentalene Serving as a Host Material for a Phosphorescent OLED.

Replacement of the carbon-carbon bonds of antiaromatic compounds with polar boron-nitrogen bonds often provides isoelectronic BN compounds with excellent thermodynamic stability and interesting photophysical properties. By this element-substitution strategy, we synthesized a new B4 N4 -heteropentalene derivative, 1, which is fully substituted with mesityl groups. Owing to kinetic protection by the sterically bulky substituents, 1 is remarkably stable toward air and even water. Single-crystal X-ray analysis of 1 revealed the bonding characteristics of the B4 N4 -heteropentalene structure. In a glassy matrix, 1 emitted short-wavelength phosphorescence with an onset at 350 nm, indicating that the triplet energy is substantially high. DFT calculations reasonably explained the ground- and excited-state electronic structures of 1 as well as its emission properties. Motivated by the high-energy triplet state of 1, we used it as a host material to fabricate a phosphorescent organic light-emitting diode with an external quantum efficiency of 15 %.

Enhancing the Antiaromaticity of s-Indacene through Naphthothiophene Fusion.

Addressing the instability of antiaromatic compounds often involves protection with bulky groups and/or fusion of aromatic rings, thus decreasing paratropicity. We report four naphthothiophene-fused s-indacene isomers, one of which is more antiaromatic than parent s-indacene. This surprising result is examined computationally through nucleus-independent chemical shift XY calculations and experimentally via nuclear magnetic resonance spectroscopy, X-ray crystallography, ultraviolet-visible spectrophotometry, and cyclic voltammetry, with the latter two indicating that this molecule possesses the lowest highest occupied molecular orbital-lowest unoccupied molecular orbital energy gap observed for heterocycle-fused s-indacene.

A Volumetric Analysis of the 1H NMR Chemical Shielding in Supramolecular Systems.

The liquid state NMR chemical shift of protons is a parameter frequently used to characterize host–guest complexes. Its theoretical counterpart, that is, the 1H NMR chemical shielding affected by the solvent (1H CS), may provide important insights into spatial arrangements of supramolecular systems, and it can also be reliably obtained for challenging cases of an aggregation of aromatic and antiaromatic molecules in solution. This computational analysis is performed for the complex of coronene and an antiaromatic model compound in acetonitrile by employing the GIAO-B3LYP-PCM approach combined with a saturated basis set. Predicted 1H CS values are used to generate volumetric data, whose properties are thoroughly investigated. The 1H CS isosurface, corresponding to a value of the proton chemical shift taken from a previous experimental study, is described. The presence of the 1H CS isosurface should be taken into account in deriving structural information about supramolecular hosts and their encapsulation of small molecules.

Antiaromaticity-aromaticity transition of cyclo[16]carbon upon metal encapsulation.

In contrast to aromatic compounds with particular stability, antiaromatic compounds are usually less stable due to their high reactivity and unfavorable formation energies. Cyclo[16]carbon (C16) is a carbon ring molecule with a dual antiaromatic character. In this study, we demonstrate that C16 can be transformed into highly aromatic molecules upon metal encapsulation. The geometrical characteristics, electronic properties and thermodynamic stability of MC16 compounds (M = Ca, Sc, Ti, V, Ce, U) are fully investigated from a theoretical perspective. Based on natural population analysis, atom-in-molecules theory and localized molecular orbital analysis, the nature of the metal-carbon interaction in the MC16 compounds is investigated. It has been proved that the bonding between Ca and C16 corresponds to a typical ionic interaction, while other metal atoms form polar covalent bonds with C16. By analyzing the frontier molecular orbitals and magnetic response of MC16, we have found that all the encapsulated metal atoms donate two electrons to the in-plane π orbitals via either electron transfer or orbital hybridization, which makes the in-plane π orbitals completely satisfy the 4n + 2 (n = 4) Hückel aromaticity rule. The U atom formally transfers four electrons to the carbon ring in total, two to the in-plane π orbitals and two to the out-of-plane π orbitals, which results in the remarkable dual aromaticity feature of UC16. The transformation of aromaticity can be utilized to develop new strategies for the synthesis of novel carbon ring molecules.

Dithieno[a,e]pentalenes: Highly Antiaromatic Yet Stable π-Electron Systems without Bulky Substituents.

Dithieno[a,e]penalenes (DTPs) with various substituents were synthesized as a class of antiaromatic compounds. Annulation of thiophene rings imparts the pentalene moiety with high thermal stability even without bulky substituents, while retaining antiaromaticity. The higher magnitude of antiaromaticity in DTPs, in addition to the differences in the electronic structures of the fused aromatic rings, resulted in a narrower HOMO-LUMO gap than that of the corresponding dibenzo[a,e]pentalene analog, giving rise to red-shifted electronic absorption that reaches the near-infrared region. Moreover, systematic investigations on the solid-state packing structure revealed that DTPs prefer offset face-to-face packing motifs rather than face-centered π-π stacking. In particular, the thienyl-substituted DTP bearing hydrophilic side chains exhibited thermochromic behavior in polar solvents, which was ascribed to the formation of aggregates.

Synthesis and Characterization of Ion Pairs between Alkaline Metal Ions and Anionic Anti-Aromatic and Aromatic Hydrocarbons with π-Conjugated Central Seven- and Eight-Membered Rings.

The synthesis, isolation and full characterization of ion pairs between alkaline metal ions (Li+, Na+, K+) and mono-anions and dianions obtained from 5H-dibenzo[a,d]cycloheptenyl (C15H11 = trop) is reported. According to Nuclear Magnetic Resonance (NMR) spectroscopy, single crystal X-ray analysis and Density Functional Theory (DFT) calculations, the trop‒ and trop2−• anions show anti-aromatic properties which are dependent on the counter cation M+ and solvent molecules serving as co-ligands. For comparison, the disodium and dipotassium salt of the dianion of dibenzo[a,e]cyclooctatetraene (C16H12 = dbcot) were prepared, which show classical aromatic character. A d8-Rh(I) complex of trop− was prepared and the structure shows a distortion of the C15H11 ligand into a conjugated 10π -benzo pentadienide unit—to which the Rh(I) center is coordinated—and an aromatic 6π electron benzo group which is non-coordinated. Electron transfer reactions between neutral and anionic trop and dbcot species show that the anti-aromatic compounds obtained from trop are significantly stronger reductants.

The Use of D’ Index in the Determination of the Aromatic Character of Organic Compounds. Comparison with HOMHED and NICS.

AbstractThe comparison of D index with HOMHED gave a good correlation (R2= 0.8221). However, a modification of D index, namely D’, where the number of cycles involved in the tested molecule is substituted by the ratio between the number of atoms involved in π orbitals in the tested molecule and in reference molecule, gave a better correlation with a larger number of compounds (R2=0.8819), The correlation of D’ with calculated HOMHED for pentatomic heterocyclic compounds did not give a unique correlation but a series of possible correlations. This behavior depends on the parameters used for the calculation of HOMHED that allow some negative values. In fact, the comparison of D’ with NICS for selected pentatomic heterocycles gave good correlation. Good correlations were observed also in the comparison of D’ with HOMHED and NICS for antiaromatic compounds.

An antiaromatic-walled nanospace.

Over the past few decades, several molecular cages, hosts and nanoporous materials enclosing nanometre-sized cavities have been reported1-5, including coordination-driven nanocages6. Such nanocages have found widespread use in molecular recognition, separation, stabilization and the promotion of unusual chemical reactions, among other applications3-10. Most of the reported nanospaces within molecular hosts are confined by aromatic walls, the properties of which help to determine the host-guest behaviour. However, cages with nanospaces surrounded by antiaromatic walls have not yet been developed, owing to the instability of antiaromatic compounds; as such, the effect of antiaromatic walls on the properties of nanospaces remains unknown. Here we demonstrate the construction of an antiaromatic-walled nanospace within a self-assembled cage composed of four metal ions with six identical antiaromatic walls. Calculations indicate that the magnetic effects of the antiaromatic moieties surrounding this nanospace reinforce each other. This prediction is confirmed by 1H nuclear magnetic resonance (NMR) signals of bound guest molecules, which are observed at chemical shift values of up to 24 parts per million(ppm), owing to the combined antiaromatic deshielding effect of the surrounding rings. This value, shifted 15 ppm from that of the free guest, is the largest 1H NMR chemical shift displacement resulting from an antiaromatic environment observed so far. This cage may thus be considered as a type of NMR shift reagent, moving guest signals well beyond the usual NMR frequency range and opening the way to further probing the effects of an antiaromatic environment on a nanospace.

Vorticity: Simplifying the analysis of the current density.

The induced current density (J(r)) provides useful information about the electronic structure of molecules under a magnetic field (B). However, the analysis of its topology is cumbersome because of its vectorial nature. We show that its tropicity (direction of rotation) and its strength can be compressed in the triple product B ⋅ ∇ × J(r) (tpJ(r)) that is a scalar field. The topology of tpJ(r) has clear similarities to the Laplacian of the electron density. Additionally, the topology of aromatic and antiaromatic compounds is notoriously different. The vorticity of J(r) is helpful to define the circulation of the current density, C, that contrary to other methods, can be easily defined for individual rings in polycyclic molecules. This allows tpJ(r) to clearly reproduce the Clar's structure of polycyclic aromatic hydrocarbons. © 2019 Wiley Periodicals, Inc.

Experimental and theoretical study on spontaneous intermolecular charge transfer features and antiaromaticities of unusual bisazo compounds with antiaromatic cores

Antiaromatic compounds have been applied in optoelectronic devices. In this paper, acenaphthoquinone bisphenylhydrazone (1a) and acenaphthoquinone bis(4-bromophenylhydrazone) (2a), were synthesized by a Schiff base reaction of acenaphthoquinone with phenylhydrazine or 4-bromophenylhydrazine. Two bisazo compounds with antiaromatic five-membered rings, 1,2-bisphenylazoacenaphthylene (1b) and 1,2-bis(4-bromophenylazo)acenaphthylene (2b) were respectively synthesized by oxidation reactions of compound 1a or 2a with PbO2. The crystal structures show that these compounds would be aggregated to H-aggregation structures, which were caused by the antiaromatic cores. Based on crystal structures of 1a, 2a, 1b and 2b, nucleus independent chemical shift (NICS) values were calculated. The obtained NICS (0) values for the five-membered rings located at cores of molecules 1a (3.64), 1b (4.64), 2a (4.42) and 2b (3.97) suggest that those core rings would exhibit antiaromatic nature. Crystal geometries and thermodynamic data of Gibbs free energies and entropies suggest that the bisazo compounds would easily accept and delocalize electrons in these crystals due to the antiaromatic cores and H-aggregation. This paper suggests a way of design of electron-accepting materials with spontaneous charge transfer nature.

Recent Advances of Biphenylene: Synthesis, Reactions and Uses

Biphenylene is an antiaromatic compound that has a strained butadiene skeleton that joins two benzene rings. Various methods for synthesizing the biphenylene core have been developed. In addition, the reactivity of biphenylene has attracted much attention because C–C bonds of biphenylene can be cleaved by various organometallic species. Furthermore, the biphenylene motif not only acts as a spacer in a variety of functional molecules, but also serves as a backbone for catalysts and ligands. This minireview summarizes how to construct the biphenylene structure, how to react biphenylene, and how to use the biphenylene skeleton in functional materials.