Search

We present herein the synthesis of novel pseudo‐metalla‐carbaporphyrinoid species (1M: M=Pd and Pt) achieved through the inner coordination of palladium(II) and platinum(II) with an acyclic N‐confused tetrapyrrin analogue. Despite their tetrapyrrole frameworks being small, akin to well‐known porphyrins, these species exhibit an unusually narrow HOMO–LUMO gap, resulting in an unprecedentedly low‐energy absorption in the second near‐infrared (NIR‐II) region. Density functional theory (DFT) calculations revealed unique dπ‐pπ‐conjugated electronic structures involving the metal dπ‐ligand pπ hybridized molecular orbitals of 1M. Magnetic circular dichroism (MCD) spectroscopy confirmed distinct electronic structures. Remarkably, the complexes feature an open‐metal coordination site in the peripheral NN dipyrrin site, forming hetero‐metal complexes (1Pd‐BF2 and 1Pt‐BF2) through boron difluoride complexation. The resulting hetero metalla‐carbaporphyrinoid species displayed further redshifted NIR‐II absorption, highly efficient photothermal conversion efficiencies (η; 62–65 %), and exceptional photostability. Despite the challenges associated with the theoretical and experimental assessment of dπ‐pπ‐conjugated metalla‐aromaticity in relatively larger (more than 18π electrons) polycyclic ring systems, these organometallic planar tetrapyrrole systems could serve as potential molecular platforms for aromaticity‐relevant NIR‐II dyes. [ABSTRACT FROM AUTHOR]

Incorporating heteroatoms can effectively modulate the molecular optoelectronic properties. However, the fundamental understanding of BN doping effects in BN‐embedded polycyclic aromatic hydrocarbons (PAHs) is underexplored, lacking rational guidelines to modulate the electronic structures through BN units for advanced materials. Herein, a concise synthesis of novel B2N2‐perylenes with BN doped at the bay area is achieved to systematically explore the doping effect of BN position on the photophysical properties of PAHs. The shift of BN position in B2N2‐perylenes alters the π electron conjugation, aromaticity and molecular rigidness significantly, achieving substantially higher electron transition abilities than those with BN doped in the nodal plane. It is further clarified that BN position dominates the photophysical properties over BN orientation. The revealed guideline here may apply generally to novel BN‐PAHs, and aid the advancement of BN‐PAHs with highly‐emissive performance. [ABSTRACT FROM AUTHOR]

A 14‐electron ternary anionic CBe2H5− cluster containing a planar tetracoordinate carbon (ptC) atom is designed herein. Remarkably, it can be stabilized by only two beryllium atoms with both π‐acceptor/σ‐donor properties and two hydrogen atoms, which means that the conversion from planar methane (transition state) to ptC species (global minimum) requires the substitution of only two hydrogen atoms. Moreover, two ligand H atoms exhibit alternate rotation, giving rise to interesting dynamic fluxionality in this cluster. The electronic structure analysis reveals the flexible bonding positions of ligand H atoms due to C−H localized bonds, highlighting the rotational fluxionality in the cluster, and two CBe2 3c–2e delocalized bonds endow its rare 2σ/2π double aromaticity. Unprecedentedly, the fluxional process exhibits a conversion in the type of bonding (σ bond↔π bond), which is an uncommon fluxional mechanism. The cluster can be seen as an attempt to apply planar hypercoordinate carbon species to molecular motors. [ABSTRACT FROM AUTHOR]

First synthesized in 1868, alizarin became one of the first synthetic dyes and was widely used as a red dye in the textile industry, making it more affordable and readily available than the traditional red dyes derived from natural sources. Despite extensive both experimental and computational analyses on the electronic effects of substituents on the shape of the visible spectrum of alizarin and alizarin Red S, no previous systematic work has been undertaken with the aim to fine tune the dominant absorption region defining its color by introducing other electron‐withdrawing or electron‐donor groups. For such, we have performed a comprehensive study of electronic effects of substituents in position C3 of alizarin by means of a time dependent DFT approach. These auxochromes attached to the chromophore are proven to alter both the wavelength and intensity of absorption. It is shown that the introduction of an electron‐donor group in alizarin causes the transition bands to be significantly red‐shifted whereas electron‐withdrawing groups cause a minor blue‐shifting. [ABSTRACT FROM AUTHOR]

Pyriporphyrinoids have recently attracted a significant proliferation of attention due to their versatile characters, which stem from structural motifs in which the pyridine moiety is involved. The evolution of pyriporphyrin chemistry revealed the subtle modifications of the macrocyclic core that tweak the electronic structure as compared to the parental macrocycle. The amendment of π-electronic organization inside the core manifests exceptional photophysical and coordination properties that cover a vast range of seemingly contradictory fields. In fundamental chemistry, the pyridine unit acts as a modulator of π-conjugated porphyrinoid systems, resulting in aromaticity swapping. From the applied chemistry perspective, these macrocycles are primarily utilized as (i) sensors, (ii) NIR absorbing photoacoustic dyes, (iii) electrochemical catalysts, (iv) singlet biradicaloid generation and (v) contributors to generate metal complexes with intriguing binding modes. Surprisingly, despite their prominence, pyriporphyrinoids are inadequately investigated, while pyridine unit-embedded calixphyrin, calixpyridinopyrrole and calixpyridine are barely reported. This review article illustrates the controlled formation of specific porphyrinic scaffolds with pyridine unit(s) and diverse functionalized heterocyclic and/or carbocyclic building block(s), and demonstrates a substantial influence on the macrocyclic properties. [ABSTRACT FROM AUTHOR]

One‐pot syntheses of new π‐extended metallaaromatic compounds have been developed by utilizing Ir‐mediated CH bond activation of ethylene‐ or ethylidene‐bridged diphenol derivatives. Depending on the bridging alkyl groups, two types of iridaoxabenzenes, both of which are doubly fused with benzo and benzofuran units, have been obtained. Studies on their structures and electronic characters indicate that both complexes have an aromatic character on the iridaoxacycles, and their π‐conjugated systems are fully delocalized over the whole molecular skeletons. These novel metallaaromatic complexes exhibited some reactivities which are distinct from those reported for the non‐fused metallaaromatic compounds. [ABSTRACT FROM AUTHOR]

Intramolecular π‐aromatization is a trait of many organic compounds that enhances the stability of their structures and polarizes related C−C π bonds. In contrast, rare study is focused on this phenomenon in metal clusters. Many existing homometallic clusters exhibit aromaticity, often characterized by nonpolar metal‐metal bonds and a high degree of symmetry. However, synthesizing low‐symmetric homometallic clusters with high‐polar metal‐metal bonds is challenging due to their limited thermodynamic stability. Herein, we report a facile strategy for the synthesis of [Au(μ2‐ER2)]3−AuPMe3 (E=Ge, Sn; R2=1,1,4,4‐tetrakis(trimethylsilyl)butane‐1,4‐diyl) clusters and reveal a novel stabilization mode, intramolecular σ‐aromatization. Our electronic structure analyses show that these low‐symmetric clusters possess a ten‐electron σ‐aromatic system, which is achieved via intramolecular σ‐aromatization. Moreover, the strength of σ‐aromaticity gives rise to a polarity‐tunable exo‐Au−Au bond. [ABSTRACT FROM AUTHOR]

Intramolecular π‐aromatization is a trait of many organic compounds that enhances the stability of their structures and polarizes related C−C π bonds. In contrast, rare study is focused on this phenomenon in metal clusters. Many existing homometallic clusters exhibit aromaticity, often characterized by nonpolar metal‐metal bonds and a high degree of symmetry. However, synthesizing low‐symmetric homometallic clusters with high‐polar metal‐metal bonds is challenging due to their limited thermodynamic stability. Herein, we report a facile strategy for the synthesis of [Au(μ2‐ER2)]3−AuPMe3 (E=Ge, Sn; R2=1,1,4,4‐tetrakis(trimethylsilyl)butane‐1,4‐diyl) clusters and reveal a novel stabilization mode, intramolecular σ‐aromatization. Our electronic structure analyses show that these low‐symmetric clusters possess a ten‐electron σ‐aromatic system, which is achieved via intramolecular σ‐aromatization. Moreover, the strength of σ‐aromaticity gives rise to a polarity‐tunable exo‐Au−Au bond. [ABSTRACT FROM AUTHOR]

Diradicaloid helicenes constructed formally by non‐benzenoid double π‐extension of phenanthrene were synthesized by a common strategy involving double electrophilic benzannulation. Steric effects in the second benzannulation step led to considerable structural diversity among the products, yielding a symmetrical dinor[7]helicene 1 and two isomeric unsymmetrical double helicenes 2 and 3, containing a nor[5]helicene and [4]helicene fragment, respectively, in addition to a common nor[6]helicene motif. Geometries, configurational dynamics, and electronic structure of these helicenes were analyzed using solid‐state structures, spectroscopic methods, and computational analyses. The open‐shell character of the singlet states of these helicenes increases in the order 3<1<2, with strongly varying diradicaloid indexes and singlet–triplet gaps. Compounds 1–3 displayed narrow optical gaps of 0.79–1.25 eV, resulting in significant absorption in the near infrared (NIR) region. They also exhibit reversible redox chemistry, each of them yielding stable radical cations, radical anions, and dianions, in some cases possessing intense NIR absorptions extending beyond 2500 nm. [ABSTRACT FROM AUTHOR]

Diradicaloid helicenes constructed formally by non‐benzenoid double π‐extension of phenanthrene were synthesized by a common strategy involving double electrophilic benzannulation. Steric effects in the second benzannulation step led to considerable structural diversity among the products, yielding a symmetrical dinor[7]helicene 1 and two isomeric unsymmetrical double helicenes 2 and 3, containing a nor[5]helicene and [4]helicene fragment, respectively, in addition to a common nor[6]helicene motif. Geometries, configurational dynamics, and electronic structure of these helicenes were analyzed using solid‐state structures, spectroscopic methods, and computational analyses. The open‐shell character of the singlet states of these helicenes increases in the order 3<1<2, with strongly varying diradicaloid indexes and singlet–triplet gaps. Compounds 1–3 displayed narrow optical gaps of 0.79–1.25 eV, resulting in significant absorption in the near infrared (NIR) region. They also exhibit reversible redox chemistry, each of them yielding stable radical cations, radical anions, and dianions, in some cases possessing intense NIR absorptions extending beyond 2500 nm. [ABSTRACT FROM AUTHOR]

Circumacenes (CAs) are a distinctive type of benzenoid polycyclic aromatic hydrocarbons where an acene unit is completely enclosed by a layer of outer fused benzene rings. Despite their unique structures, the synthesis of CAs is challenging, and until recently, the largest CA molecule synthesized was circumanthracene. In this study, we report the successful synthesis of an extended circumpentacene derivative 1, which represents the largest CA molecule synthesized to date. Its structure was confirmed by X‐ray crystallographic analysis and its electronic properties were systematically investigated by both experiments and theoretical calculations. It shows a unique open‐shell diradical character due to the existence of extended zigzag edges, with a moderate diradical character index (y0=39.7 %) and a small singlet‐triplet energy gap (ΔES‐T=−4.47 kcal/mol). It exhibits a dominant local aromatic character with π‐electrons delocalized in the individual aromatic sextet rings. It has a small HOMO–LUMO energy gap and displays amphoteric redox behavior. The electronic structures of its dication and dianion can be considered as doubly charged structures in which two coronene units are fused with a central aromatic benzene ring. This study provides a new route toward stable multizigzag‐edged graphene‐like molecules with open‐shell di/polyradical character. [ABSTRACT FROM AUTHOR]

Circumacenes (CAs) are a distinctive type of benzenoid polycyclic aromatic hydrocarbons where an acene unit is completely enclosed by a layer of outer fused benzene rings. Despite their unique structures, the synthesis of CAs is challenging, and until recently, the largest CA molecule synthesized was circumanthracene. In this study, we report the successful synthesis of an extended circumpentacene derivative 1, which represents the largest CA molecule synthesized to date. Its structure was confirmed by X‐ray crystallographic analysis and its electronic properties were systematically investigated by both experiments and theoretical calculations. It shows a unique open‐shell diradical character due to the existence of extended zigzag edges, with a moderate diradical character index (y0=39.7 %) and a small singlet‐triplet energy gap (ΔES‐T=−4.47 kcal/mol). It exhibits a dominant local aromatic character with π‐electrons delocalized in the individual aromatic sextet rings. It has a small HOMO–LUMO energy gap and displays amphoteric redox behavior. The electronic structures of its dication and dianion can be considered as doubly charged structures in which two coronene units are fused with a central aromatic benzene ring. This study provides a new route toward stable multizigzag‐edged graphene‐like molecules with open‐shell di/polyradical character. [ABSTRACT FROM AUTHOR]

Quantum chemical calculations reveal that ethyne (C2H2) can be a very good fragment to create strong Be−Be and Zn−Zn interactions. The complexes are thermodynamically stable in which Be2 and Zn2 fragments act as electron donors towards C2H2 fragment acquiring Be2δ+ and Zn2δ+ states. The partial positive nature of Be2 and Zn2 fragments made their electronic structure to be described adequately by single reference character. Introduction of electronegative F atom as in C2F2 results in shorter Be−Be and Zn−Zn distances. Bonding analyses reveal that all these complexes feature delocalized π bond resulting in aromaticity. [ABSTRACT FROM AUTHOR]

An interesting chiral molecule with a double half-twisted π-electron system has been investigated with theoretical calculations. To investigate the geometry and electronic structure, the size of macrocyclic cavity, electrostatic potential (ESP) and density-of-states (DOS) were calculated. The multi-center bond order (MCBO) and AV1245 index were calculated to compare the aromaticity of phenylene groups. To investigate the spectroscopic properties, Raman, Raman Optical Activity (ROA), UV–Vis spectrum, fluorescence, CPL (circularly polarized luminescence) and ECD (electronic circular dichroism) spectra have been calculated and analyzed. The oscillator strengths, rotatory strengths and dissymmetry factor (glum) have been discussed. [ABSTRACT FROM AUTHOR]

In designing three‐dimensional (3‐D) molecular stars, it is very difficult to enhance the molecular rigidity through forming the covalent bonds between the axial and equatorial groups because corresponding axial groups will generally break the delocalized π bond over equatorial frameworks and thus break their star‐like arrangement. In this work, exemplified by designing the 3‐D stars Be2©Be5E5+ (E = Au, Cl, Br, I) with three delocalized σ bonds and delocalized π bond over the central Be2©Be5 moiety, we propose that the desired covalent bonding can be achieved by forming the delocalized σ bond(s) and delocalized π bond(s) simultaneously between the axial groups and equatorial framework. The covalency and rigidity of axial bonding can be demonstrated by the total Wiberg bond indices of 1.46–1.65 for axial Be atoms and ultrashort Be‐Be distances of 1.834–1.841 Å, respectively. Beneficial also from the σ and π double aromaticity, these mono‐cationic 3‐D molecular stars are dynamically viable global energy minima with well‐defined electronic structures, as reflected by wide HOMO‐LUMO gaps (4.68–5.06 eV) and low electron affinities (4.70–4.82 eV), so they are the promising targets in the gas phase generation, mass‐separation, and spectroscopic characterization. [ABSTRACT FROM AUTHOR]

Herein, the electronic structure and bonding character of the stable cyclo[18]carbon (C18) precursor, C18Br6, are thoroughly characterized by molecular orbital (MO), density of states (DOS), bond order (BO), and interaction region indicator (IRI) analyses. The delocalization characters of out‐of‐plane and in‐plane π‐electrons (labeled as πout‐ and πin‐electrons, respectively) in bonding regions were examined using localized orbital locator (LOL) and electron localization function (ELF). The aromaticity was investigated, studying the molecular magnetic response to external magnetic field by computing the magnetically induced current density (Jind), iso‐chemical shielding surface (ICSS), anisotropy of the induced current density (AICD), and the induced magnetic field (Bind). All these analyses indicate that C18Br6 is a globally aromatic species with lower aromaticity than C18, and the blocking of in‐plane π‐conjugation (labeled as πin‐conjugation) by the presence of ‐Br substituents in it is the underlying cause for the weakening of molecular aromaticity. [ABSTRACT FROM AUTHOR]

We herein present the synthetic approach to a new antiaromatic double aza[7]helicene C that features NN‐embedded polycyclic aromatic hydrocarbons (PAHs). This heteroatom‐doped helicene showed a rarely obtained long‐wavelength emission and far‐red circularly polarized luminescence (CPL) in the solid state. These optical and chiroptical properties could be ascribed to both the NN‐PAH core structure and the further extension through angular ring fusions. Such a unique electronic structure also culminated in facile chemical oxidations of neutral C to the positively charged chiral radical (C⋅+) and dication species (C2+). Interestingly, DFT computations revealed that the pyridazine central core showed an antiaromaticity‐to‐aromaticity switching, in contrast to the inversed transition for the helical periphery in cationic states. The reported approaches are anticipated to lead to the development of further redox‐active chiral systems for potential applications in chiroptoelectronics, spintronics as well as fluorescent bioimaging. [ABSTRACT FROM AUTHOR]

We herein present the synthetic approach to a new antiaromatic double aza[7]helicene C that features NN‐embedded polycyclic aromatic hydrocarbons (PAHs). This heteroatom‐doped helicene showed a rarely obtained long‐wavelength emission and far‐red circularly polarized luminescence (CPL) in the solid state. These optical and chiroptical properties could be ascribed to both the NN‐PAH core structure and the further extension through angular ring fusions. Such a unique electronic structure also culminated in facile chemical oxidations of neutral C to the positively charged chiral radical (C⋅+) and dication species (C2+). Interestingly, DFT computations revealed that the pyridazine central core showed an antiaromaticity‐to‐aromaticity switching, in contrast to the inversed transition for the helical periphery in cationic states. The reported approaches are anticipated to lead to the development of further redox‐active chiral systems for potential applications in chiroptoelectronics, spintronics as well as fluorescent bioimaging. [ABSTRACT FROM AUTHOR]

The relationship between the overall electronic structure of π‐conjugated molecules and the arrangement of their constituent elements is of fundamental importance. Establishing rational design guidelines for conjugated hydrocarbons with narrow HOMO‐LUMO gaps is useful to develop near‐infrared (NIR) responsive dyes and redox‐active materials. This study describes the synthesis and properties of three conjugated hydrocarbons, i. e. an indenonaphthoperylene, an indenoterrylene, and a diindenoterrylene. These molecules exhibit NIR absorption despite the absence of significant antiaromaticity and diradical character. Notably, the indenonaphthoperylene exhibits red‐to‐NIR emission in the 620–850 nm region. The indenoterrylene and the diindenoterrylene exhibit NIR absorption tailing to 870 and 940 nm, respectively. Moreover, the effect of the π‐extension of indenoperylene is disclosed in order to propose guidelines for achieving a narrow HOMO‐LUMO gap with negligible antiaromaticity and diradical character. [ABSTRACT FROM AUTHOR]

Both graphene and hexagonal boron nitride (h‐BN) have found their application in catalysis and electronic devices owing to their unique π electronic nature. However, it remains unclear on how their sizes and growing modes influence their electronic properties. In order to extensively understand their structure–property relationship to stimulate and assist potential applications, in this work, the electronic structures of the polyaromatic hydrocarbons (PAHs) and polyborazines (boron nitride nanosheets, BNNSs) were analyzed by means of density functional theory calculations to investigate how the one‐ and two‐dimensional growth of their building blocks, benzene and borazine, respectively, may modulate/govern their aromaticity and reactivities. The analysis shows that the aromaticity and reactivity of PAHs exhibit significant response to their geometries, while the properties of BNNSs show strong localization nature and are nearly size‐independent. This unique nature of BNNSs is different from both the organic polyaromatic molecules and the inorganic metal/metal oxides whose reactivity may be significantly enhanced upon modulating their sizes, and offers implications in the design of new materials grafted by BNNS for manipulatable electronic properties. This study also brings caution on the application of widely used aromaticity descriptors, and shows that in general the descriptors derived from electronic structure are more reliable. [ABSTRACT FROM AUTHOR]

Bis-periazulene (cyclohepta[def]fluorene) is one of the seven possible non-alternant isomers of pyrene. Different from the other isomers, bis-periazulene had remained an uncharacterized hydrocarbon until 2022, since it was first reported in 1955. This paper documents our recent achievements in synthesizing and characterizing bis-periazulene derivatives. Its triaryl derivatives 5c‒e exhibited the superimposed electronic structures of peripheral, polarized, and open-shell π-conjugated systems. Furthermore, in contrast to previous theoretical predictions, bis-periazulene derivatives showed the singlet ground state. [ABSTRACT FROM AUTHOR]

Sapphyrin is a pentapyrrolic expanded porphyrin with a 22π aromatic character. Herein, we report the synthesis of a 20π antiaromatic sapphyrin isomer 1 by oxidative cyclization of a pentapyrrane precursor P5 with a terminal β‐linked pyrrole. The resulting isomer 1, containing a mis‐linked bipyrrole unit in the skeleton, exhibits a reactivity for further oxidation due to the distinct antiaromatic electronic structure, affording a fused macrocycle 2, possessing a spiro‐carbon‐containing [5.6.5.6]‐tetracyclic structure. Subsequent treatment with an acid afforded a weakly aromatic pyrrolone‐appended N‐confused corrole 3, and thermal fusion gave a [5.6.5.7]‐tetracyclic‐ring‐embedded 14π aromatic triphyrin(2.1.1) analog 4. The cyclization at the mis‐linked pyrrole moiety of P5 played a crucial role in synthesizing the antiaromatic porphyrinoid susceptible to facile transformation to novel porphyrinoids with variable aromaticity. [ABSTRACT FROM AUTHOR]

Sapphyrin is a pentapyrrolic expanded porphyrin with a 22π aromatic character. Herein, we report the synthesis of a 20π antiaromatic sapphyrin isomer 1 by oxidative cyclization of a pentapyrrane precursor P5 with a terminal β‐linked pyrrole. The resulting isomer 1, containing a mis‐linked bipyrrole unit in the skeleton, exhibits a reactivity for further oxidation due to the distinct antiaromatic electronic structure, affording a fused macrocycle 2, possessing a spiro‐carbon‐containing [5.6.5.6]‐tetracyclic structure. Subsequent treatment with an acid afforded a weakly aromatic pyrrolone‐appended N‐confused corrole 3, and thermal fusion gave a [5.6.5.7]‐tetracyclic‐ring‐embedded 14π aromatic triphyrin(2.1.1) analog 4. The cyclization at the mis‐linked pyrrole moiety of P5 played a crucial role in synthesizing the antiaromatic porphyrinoid susceptible to facile transformation to novel porphyrinoids with variable aromaticity. [ABSTRACT FROM AUTHOR]

We predict the geometries, electronic properties, and aromaticity of thiophene analogues of anti-kekulene with six to nine thiophene rings 1–4, together with those of cyclobutadithiophenes (CDTs) and anti-kekulene as reference compounds, using density functional theory calculations. Investigation of the simplest reference compounds, CDTs, reveals that the local aromaticity of their thiophene rings is influenced by their fused position (b- or c-bond) to the four-membered ring (4MR). A thiophene ring fused at the b-position (b-TR) retains its aromatic character to some extent, whereas the aromatic character of one fused at the c-position is attenuated. The 4MR with two fused b-TRs retains a strong anti-aromatic character. Thiophene analogues of anti-kekulene with six to eight thiophene rings 1–3 favor bowl-shaped structures, in contrast to the planar structure of anti-kekulene, because of the shorter distances of the sulfur bridges. Compound 4, with nine thiophene rings, adopts a planar structure. The local aromaticity and anti-aromaticity of the thiophene ring and 4MR are significantly attenuated in 1–4 compared with the reference compounds, the CDTs and anti-kekulene. This can be attributed to the considerable contribution of the quinoidal electronic structure in 1–4. The present study provides new insight into the aromatic and electronic nature of systems containing cyclobutadienothiophene. [ABSTRACT FROM AUTHOR]

Comprehensive Summary: Carbolong compounds as a metal‐heteroaromatic compound with both organometallic properties and π‐conjugated systems exhibit great potential in organic catalysis and optoelectronic devices. In this work, for the first time, the "Bucket Effect" is revealed to promote the third‐order nonlinear optical (NLO) performance in metal‐heteroaromatic compounds. We have successfully constructed and investigated a series of novel metallapentalenes with higher third‐order NLO performance benefited from the "Bucket Effect". Meanwhile, aromaticity and electron−hole analysis further confirm the internal homogeneity of organometallic rings, reduced bandgap, and enhanced low‐energy peak response resulted in the enhanced third‐order NLO effects. The success of this work is discovering an emerging material library of high third‐order NLO effects, and illustrating the feasibility of engineering the high response metal‐heteroaromatic optical devices at the electronic structure level. [ABSTRACT FROM AUTHOR]

The 20π porphyrinoids are immediate higher homologues of 18π porphyrins and differ from porphyrins in aromaticity which in turn affects the structure, properties and chemical reactivities. Research over the years indicated that the 20π porphyrinoids can be stabilized as non‐aromatic/anti‐aromatic or Mobius aromatic macrocycles using different strategies such as core‐modification of porphyrins, non‐metal/metal complexation of porphyrins, peripheral modification of porphyrins and expanded porphyrinoids. The structural properties such as aromaticity of the macrocycle can be controlled by choosing the right synthetic strategy. This review will provide an overview of the development in the chemistry of 20π porphyrinoids giving emphasize on the synthesis, structure and electronic properties of these macrocycles which have huge potential for various applications. [ABSTRACT FROM AUTHOR]

The aromatic character of the platinabenzene molecule has been under discussion since it was synthesized in 2002. Previous reports conclude that it has an aromatic character but without total evidence. The lack of all-electron relativistic analysis, the correct electron counting, and the spin–orbit effects were some of the missing points here fulfilled. It was found that platinabenzene is not 6-π electron Hückel aromatic nor 8-π electron Möbius aromatic but 10π electron Hückel aromatic. The aromatic character was tackled from two different points of view. First, from an electronic structure analysis, and second via the local and global magnetic descriptors, i.e., nucleus independent chemical shift (NICS), and magnetically induced current density (MICD) respectively. That exposes the role of the Pt core electrons in the strength of the current density and chemical shifts, as well as the role of the paramagnetic and diamagnetic component of the induced current density. The huge spin–orbit contribution (9.3 nA T−1) to the strength of current density is also explained. Finally from the analysis of the role of 5d orbitals a classical explanation of the aromaticity based on Hückel's 4n + 2 rule with n = 2 was done. This analysis based on the electronic structure was previously not taken into account for this system nor the spin–orbit contribution to the current density. [ABSTRACT FROM AUTHOR]

We disclose a successive π‐expansion of pyracylene towards boat‐shaped polycyclic scaffolds. The unique structural features of the resulting compounds were revealed by X‐ray crystallographic analysis. Depending on the extent of π‐expansion the compounds display intense bathochromically shifted absorption bands in their UV/Vis spectra and are prone to several redox events as documented by cyclic voltammetry. The experimental observations are in line with the computational studies based on density functional theory, suggesting progressive narrowing of the HOMO–LUMO gap and distinct evolution of the electronic structure and aromaticity. [ABSTRACT FROM AUTHOR]

A concave indolo[3,2,1‐jk]carbazole‐based compound was synthesized and characterized, which was further compared with the property of its planar congener. The results reveal that the change in molecular shape alters the molecular electronic structure, thus leading to the reduced aromaticity and divergent energy levels of frontier molecular orbitals of the concave compound. Moreover, the bowl‐shaped molecule demonstrates a blue‐shift in the UV/Vis absorption and a red‐shift in the weakened fluorescence emission due to the vibration of the concave face. [ABSTRACT FROM AUTHOR]

Photoacids show a strong increase in acidity in the first electronic excited state, enabling real‐time studies of proton transfer in acid‐base reactions, proton transport in energy storage devices and biomolecular sensor protein systems. Several explanations have been proposed for what determines photoacidity, ranging from variations in solvation free energy to changes in electronic structure occurring along the four stages of the Förster cycle. Here we use picosecond nitrogen K‐edge spectroscopy to monitor the electronic structure changes of the proton donating group in a protonated aromatic amine photoacid in solution upon photoexcitation and subsequent proton transfer dynamics. Probing core‐to‐valence transitions locally at the amine functional group and with orbital specificity, we clearly reveal pronounced electronic structure, dipole moment and energetic changes on the conjugate photobase side. This result paves the way for a detailed electronic structural characterization of the photoacidity phenomenon. [ABSTRACT FROM AUTHOR]

This Review surveys the experimental and synthetic history of three classic antiaromatic cores: cyclobutadiene, borole, and pentalene. The coverage extends from the bare molecular cores to fused or substituted variants that impart thermodynamic or kinetic stability to the antiaromatic systems. Recent studies to utilize the unusual properties of these antiaromatic cores in the context of advanced electronic materials are discussed. [ABSTRACT FROM AUTHOR]

A quantum chemical calculation was performed to investigate the structural and bonding properties of B6C8− anion and B6C8 neutral. The geometries of the most stable isomers of both B6C8− anion and B6C8 neutral were found to be a planar dodecagon with four C2 units separated by four B atoms, which can be further reinforced by the central strong B−B single bond and aromaticity. The bond lengths, Wiberg bond orders, constant charge density surfaces, MOs, and AdNDP analyses were used to give insight into the chemical bonds in the C2h symmetric planar dodecagon of B6C8 neutral. The results showed that the most stable isomer of B6C8 has a closed-shell electronic structure with HOMO–LUMO gap of 2.20 eV and has σ plus π double delocalized bonding characters. More interestingly, the most stable isomer of B6C8 exhibits σ aromaticity and π antiaromaticity. [ABSTRACT FROM AUTHOR]

For the creation of next‐generation organic electronic materials, the integration of π‐systems has recently become a central theme. Such functional materials can be assembled by supramolecular polymerization when aromatic π‐systems are used as monomers, and the properties of the resulting supramolecular polymer strongly depend on the electronic structure of the monomers. Here, we demonstrate the construction of a supramolecular polymer consisting of an antiaromatic π‐system as the monomer. An amide‐functionalized NiII norcorrole derivative formed a one‐dimensional supramolecular polymer through π‐π stacking and hydrogen‐bonding interactions, ensuring the persistency of the conducting pathway against thermal perturbation, which results in higher charge mobility along the tightly bound linear aggregates than that of the aromatic analogue composed of ZnII porphyrins. [ABSTRACT FROM AUTHOR]

For the creation of next‐generation organic electronic materials, the integration of π‐systems has recently become a central theme. Such functional materials can be assembled by supramolecular polymerization when aromatic π‐systems are used as monomers, and the properties of the resulting supramolecular polymer strongly depend on the electronic structure of the monomers. Here, we demonstrate the construction of a supramolecular polymer consisting of an antiaromatic π‐system as the monomer. An amide‐functionalized NiII norcorrole derivative formed a one‐dimensional supramolecular polymer through π‐π stacking and hydrogen‐bonding interactions, ensuring the persistency of the conducting pathway against thermal perturbation, which results in higher charge mobility along the tightly bound linear aggregates than that of the aromatic analogue composed of ZnII porphyrins. [ABSTRACT FROM AUTHOR]

The electronic structure and stability of M′@Mn systems is considered using DFT. The metal atom M′ is a rare earth, actinide, or a d transition metal, and M is a coinage (Group 11) metal. The σ aromaticity along the Mn ring is emphasized. As a further indicator for it, the magnetically induced currents along the ring are studied, and found to be considerably larger than the aromatic π currents in benzene. A name saturnenes is suggested for these atom‐inside‐ring systems. Attention is also called for the known analogous stick‐inside‐ring systems. [ABSTRACT FROM AUTHOR]

This review summarizes the results on the aromaticity of a series of synthesized and hypothetical neutral heterocirculene molecules and their double charged ions. The aromaticity of heterocirculenes is a direct reflection of their electronic structure responsible for the specific optoelectronic and photophysical properties. We show how the presence of a heteroatom in the outer macrocycle affects the aromaticity of hetero[8]circulenes. In addition, we also describe the change in aromaticity and strain energy for a series of the “lower” (n < 8) and “higher” (n > 8) hetero[n]circulenes. It was demonstrated that the loss of planarity with increased strain leads to an increased antiaromaticity of the lower hetero[n]circulenes, whereas higher hetero[n]circulenes demonstrate a more pronounced aromatic nature because of the small departure from planarity of each heteroarene ring in hetero[n]circulene molecule. Finally, we discuss the aromatic nature of the first examples of π-extended hetero[8]circulenes. [ABSTRACT FROM AUTHOR]

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 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 %. [ABSTRACT FROM AUTHOR]

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 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 %. [ABSTRACT FROM AUTHOR]

Structurally authenticated free B‐alkyl boroles are presented and electronic implications of alkyl substitution were assessed. Deprotonation of a boron‐bound exocyclic methyl group in a B‐methyl borole yields the first 5‐boratafulvene anion—an isomer to boratabenzene. Boratafulvene was structurally characterized and its electronic structure probed by DFT calculations. The pKa value of the exocyclic B−CH3 in a set of boroles was computationally approximated and confirmed a pronounced acidic character caused by the boron atom embedded in an anti‐aromatic moiety. The non‐aromatic boratafulvene reacts as a C‐centered nucleophile with the mild electrophile Me3SnCl to give a stannylmethyl borole, regenerating the anti‐aromaticity. As nucleophilic synthons for boroles, boratafulvenes thus open an entirely new avenue for synthetic strategies toward this highly reactive class of heterocycles. Boratafulvene reacts as a methylene transfer reagent in a bora‐Wittig‐type reaction generating a borole oxide. [ABSTRACT FROM AUTHOR]

Main observation and conclusion: Borane clusters and their derivatives have attracted extensive attention in inorganic chemistry due to their fascinating multi‐center bonding patterns and physicochemical properties. Here we report a systematic theoretical investigation on the geometry, electronic structure and chemical bonding of BnXn0/2‐ (n = 4, 6; X = H, F, Cl, Br, I, At, Ts) clusters, attempting to explore their bonding features, exceptional stability and the ligand effect. We find that the electronic structure and stability of BnXn0/2‐ clusters can be tuned by the size of the boron cage and electronegativity of the ligand. Fragment orbital energy matching and orbital overlap are of great importance to the covalency of the cluster. In addition to the ionic electrostatic interaction that dominates the bonding interaction and decreases as the ligand becomes heavier, multi‐center bonding covalency determined by the orbital interaction increases accordingly, attributing to the reducing electronegativity. The σ‐donations from the ligand to the boron cage as well as multi‐center two‐electron (nc‐2e) bonding contribute to the σ aromaticity and superb stability. This work reveals the foremost factors that determine the structure and stability of boron clusters, and provides insights into the nature of chemical bonding for species with boron cages and even bulk boron. [ABSTRACT FROM AUTHOR]

Two‐electron reduction of carbene‐supported 9‐bromo‐9‐borafluorenes with excess KC8, Na, or Li‐naphthalenide affords six N‐heterocyclic carbene (NHC)‐ or cyclic(alkyl)(amino) carbene (CAAC)‐stabilized borafluorene anions (3–8)‐the first isolated and structurally authenticated examples of the elusive 9‐carbene‐9‐borafluorene monoanion. The electronic structure, bonding, and aromaticity of the boracyclic anions were comprehensively investigated via computational studies. Compounds 5 and 8 react with metal halides via salt elimination to give new B‐E (E=Au, Se, Ge)‐containing materials (9–12). Upon reaction with diketones, the carbene ligand cleanly dissociates from 5 or 8 to yield new B‐O containing spirocycles (13–14) that cannot be easily obtained using "normal" valent borafluorene compounds. Collectively, these results support the notion that carbene‐stabilized monoanionic borafluorenes may serve as a new platform for the one‐step construction of higher‐value boracyclic materials. [ABSTRACT FROM AUTHOR]

Two‐electron reduction of carbene‐supported 9‐bromo‐9‐borafluorenes with excess KC8, Na, or Li‐naphthalenide affords six N‐heterocyclic carbene (NHC)‐ or cyclic(alkyl)(amino) carbene (CAAC)‐stabilized borafluorene anions (3–8)‐the first isolated and structurally authenticated examples of the elusive 9‐carbene‐9‐borafluorene monoanion. The electronic structure, bonding, and aromaticity of the boracyclic anions were comprehensively investigated via computational studies. Compounds 5 and 8 react with metal halides via salt elimination to give new B‐E (E=Au, Se, Ge)‐containing materials (9–12). Upon reaction with diketones, the carbene ligand cleanly dissociates from 5 or 8 to yield new B‐O containing spirocycles (13–14) that cannot be easily obtained using "normal" valent borafluorene compounds. Collectively, these results support the notion that carbene‐stabilized monoanionic borafluorenes may serve as a new platform for the one‐step construction of higher‐value boracyclic materials. [ABSTRACT FROM AUTHOR]

π-electron conjugation and aromaticity are commonly associated with delocalization and especially high mobility of the π electrons. We investigate if also the electron correlation (pair density) exhibits signatures of the special electronic structure of conjugated systems. To that end the shape and extent of the pair density and derived quantities (exchange-correlation hole, Coulomb hole, and conditional density) are investigated for the prototype systems ethylene, hexatriene, and benzene. The answer is that the effects of π electron conjugation are hardly discernible in the real space representations of the electron correlation. We find the xc hole to be as localized (confined to atomic or diatomic regions) in conjugated systems as in small molecules. This result is relevant for density functional theory (DFT). The potential of the electron exchange-correlation hole is the largest part of vxc, the exchange-correlation Kohn-Sham potential. So the extent of the hole directly affects the orbital energies of both occupied and unoccupied Kohn-Sham orbitals and therefore has direct relevance for the excitation spectrum as calculated with time-dependent DFT calculations. The potential of the localized xc hole is comparatively more attractive than the actual hole left behind by an electron excited from a delocalized molecular orbital of a conjugated system. [ABSTRACT FROM AUTHOR]

With the help of the recently developed SIESTA-pole (Spanish Initiative for Electronic Simulations with Thousands of Atoms) - PEXSI (pole expansion and selected inversion) method [L. Lin, A. García, G. Huhs, and C. Yang, J. Phys.: Condens. Matter 26, 305503 (2014)], we perform Kohn-Sham density functional theory calculations to study the stability and electronic structure of hydrogen passivated hexagonal graphene nanoflakes (GNFs) with up to 11700 atoms. We find the electronic properties of GNFs, including their cohesive energy, edge formation energy, highest occupied molecular orbital-lowest unoccupied molecular orbital energy gap, edge states, and aromaticity, depend sensitively on the type of edges (armchair graphene nanoflakes (ACGNFs) and zigzag graphene nanoflakes (ZZGNFs)), size and the number of electrons. We observe that, due to the edge-induced strain effect in ACGNFs, large-scale ACGNFs' edge formation energy decreases as their size increases. This trend does not hold for ZZGNFs due to the presence of many edge states in ZZGNFs. We find that the energy gaps Eg of GNFs all decay with respect to 1/L, where L is the size of the GNF, in a linear fashion. But as their size increases, ZZGNFs exhibit more localized edge states. We believe the presence of these states makes their gap decrease more rapidly. In particular, when L is larger than 6.40 nm, we find that ZZGNFs exhibit metallic characteristics. Furthermore, we find that the aromatic structures of GNFs appear to depend only on whether the system has 4N or 4N + 2 electrons, where N is an integer. [ABSTRACT FROM AUTHOR]

photoelectron spectra, electronic structure, anti-aromaticity. [Display omitted] • Electronic structures of pentalene and derivatives. • Simulated UV photoelectron spectra pentalene and derivatives in the gas phase. • Determination of bandgaps in pentalene derivatives. • Study of substituent effects annulation on aromaticity/anti-aromaticity. The vertical, valence ionization energies for pentalene (C 8 H 6) and some of its derivatives have been calculated as free molecules in the gas phase using high-level ab initio coupled-cluster method: EOM-DLPNO-CCSD. Their valence electronic structures depend on the number, type and position of substituents. We have used the calculated vertical, valence ionization energies to simulate UV photoelectron spectra. HOMO-LUMO gaps and the influence of substituents on ionization energies and electronic structure were also studied. We discuss the antiaromaticity and bis-annulation effects in pentalenes. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The first example of a BN‐doped cycloparaphenylene BN‐[10]CPP was synthesized and characterized. Its reactivity and photophysical properties were evaluated in direct comparison to its carbonaceous analogues Mes‐[10]CPP and [10]CPP. While the photophysical properties of BN‐[10]CPP remains similar to its carbonaceous analogues, the electronic structure changes associated with the introduction of a 1,2‐azaborine BN heterocycle into a CPP scaffold enables facile and selective late‐stage functionalizations that cannot be accomplished with carbonaceous CPPs. Specifically, Ir‐catalyzed hydrogenation of BN‐[10]CPP selectively reduces the BN heterocyclic ring, which upon hydrolysis produces a rare example of a macrocyclic paraphenylene 6 incorporating the versatile ketone functionality within the macrocyclic ring. [ABSTRACT FROM AUTHOR]

The first example of a BN‐doped cycloparaphenylene BN‐[10]CPP was synthesized and characterized. Its reactivity and photophysical properties were evaluated in direct comparison to its carbonaceous analogues Mes‐[10]CPP and [10]CPP. While the photophysical properties of BN‐[10]CPP remains similar to its carbonaceous analogues, the electronic structure changes associated with the introduction of a 1,2‐azaborine BN heterocycle into a CPP scaffold enables facile and selective late‐stage functionalizations that cannot be accomplished with carbonaceous CPPs. Specifically, Ir‐catalyzed hydrogenation of BN‐[10]CPP selectively reduces the BN heterocyclic ring, which upon hydrolysis produces a rare example of a macrocyclic paraphenylene 6 incorporating the versatile ketone functionality within the macrocyclic ring. [ABSTRACT FROM AUTHOR]

Two new structural motifs of the B24 clusters are constructed by use of the leapfrog transformation. The resulting leapfrog B24 has either a bowl shape with a square vacancy or a quasi‐planar 2D close‐packed triangular boron sheet. The neutral and ionic forms of the latter are found to be more stable than their homologous leapfrog bowl clusters, with the exception of the dicationic B24+2. While the leapfrog isomer is less stable than the tubular double ring in the neutral state, it becomes competitive in some ionic states. The nucleus independent chemical shift, electron localization function, ring current maps and the electronic structure of leapfrog B24 clusters reveal them to behave as aromatics. [ABSTRACT FROM AUTHOR]

We report the design, synthesis, structure, bonding, and reaction of a neutral 2π aromatic three‐membered disilaborirane. The disilaborirane is synthesized by a facile one‐pot reductive dehalogenation of amidinato‐silylene chloride and dibromoarylborane with potassium graphite. Despite the tetravalent arrangement of atoms around silicon, the three‐membered silicon‐boron‐silicon ring is aromatic, as evidenced by NMR spectroscopy, nucleus independent chemical shift calculations, first‐principles electronic structure studies using density functional theory (DFT) and natural bond orbital (NBO) based bonding analysis. Trimethylsilylnitrene, generated in situ, inserts in the Si−Si bond of disilaborirane to obtain a four‐membered heterocycle 1‐aza‐2,3‐disila‐4‐boretidine derivative. Both the heterocycles are fully characterized by X‐ray crystallography. [ABSTRACT FROM AUTHOR]