Aromatic and magnetic properties in a series of heavy rare earth-doped Ge6 cluster anions

A series of pentagonal bipyramidal anionic germanium clusters doped with heavy rare earth elements,REGe 6 - (RE = Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu), have been identified at the PBE0/def2-TZVP level using density functional theory (DFT). Our findings reveal that the centrally doped pentagonal ring structure demonstrates enhanced stability and heightened aromaticity due to its uniform bonding characteristics and a larger charge transfer region. Through natural population analysis and spin density diagrams, we observed a monotonic decrease in the magnetic moment from Gd to Yb. This is attributed to the decreasing number of unpaired electrons in the 4f orbitals of the heavy rare earth atoms. Interestingly, the system doped with Er atoms showed lower stability and anti-aromaticity, likely due to the involvement of the 4f orbitals in bonding. Conversely, the systems doped with Gd and Tb atoms stood out for their high magnetism and stability, making them potential building blocks for rare earth-doped semiconductor materials.

Unraveling the effect of aromaticity for the dynamics of excited states of single benzene fluorophores

The photophysical properties of a series of recently synthesized single benzene fluorophores were investigated using ensemble density functional theory calculations. The energetic stability of the ground and excited state species were counterposed against the aromaticity index derived from local vibrational modes. It was found that the large Stokes shift of the fluorophores (up to ca. 5800 cm  - 1 ) originates from the effect of electron donating and electron withdrawing substituents rather than π -delocalization and related (anti-)aromaticity. On the basis of nonadiabatic molecular dynamics simulations, the absence of fluorescence from one of the regioisomers was explained by the occurrence of easily accessible S  1 /S  0 conical intersections below the vertical excitation energy level. It is demonstrated in the manuscript that the analysis of local mode force constants and the related aromaticity index represent a useful tool for the characterization of π -delocalization effects in π -conjugated compounds.

From Stilbenes to carbo-Stilbenes: an Encouraging Prospect

Beyond previously described carbo-naphthalene and carbo-biphenyl, a novel type of bis-carbo-benzenic molecules is envisaged from the stilbene parent. The synthesis, structure, spectroscopic and electrochemical properties of two such carbo-stilbenes are described at complementary experimental and computational DFT levels. In the selected targets, the bare skeletal carbo-mer of carbo-stilbene is decorated by 8 or 10 phenyl groups, 0 or 2 tert-butyl groups, and 2 n-octyl chains, the later substituents being introduced to compensate anticipated solubility issues. As in the parent stilbene series, isomers of the phenylated carbo-stilbenes are characterized. The cis- and trans-isomers are, however, formed in almost equal amounts and could not be separated by either chromatography or crystallization. Nevertheless, due to a slow interconversion at the NMR time scale (up to 55 °C) the 1H NMR signals of both isomers of the two carbo-stilbenes could be tentatively assigned. The calculated structure of the cis-isomer exhibits a helical shape, consistent with the observed magnetic shielding of phenyl p-CH nuclei residing inside the shielding cone of the facing C18 ring. The presence of the two isomers in solution also gives rise to quite broad UV-vis absorption spectra with main bands at ca 460, 560 and 710 nm, and a significant bathochromic shift for the decaphenylated carbo-stilbene vs the di-tert-butyl-octaphenylated counterpart. Square wave voltammograms do not show any resolution of the two isomers, giving a reversible reduction wave at -0.65 or -0.58 V/SCE, and an irreversible oxidation peak at 1.11 V/SCE, those values being classical for most carbo-benzene derivatives. Calculated NICS values (NICS(1)=-12.5±0.2 ppm) also indicate that the aromatic nature of the C18 rings is not markedly affected by the dialkynylbutatriene (DAB) connector between them.

Synthesis, Spectroscopic Properties, and Metalation of 3-Alkoxybenziporphyrins

A series of 5-alkoxy-1,3-benzenedicarbaldehydes and related dimers were prepared in three steps from dimethyl 5-hydroxyisophthalate. Acid catalyzed condensation of the dialdehydes with a tripyrrane dicarboxylic acid, followed by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone, afforded good yields of 3-alkoxybenziporphyrins, although dimeric tetraaldehydes failed to give isolatable porphyrinoid products. Proton NMR spectroscopy gave no indication of an aromatic ring current, but addition of trifluoroacetic acid resulted in the formation of dications that exhibited weakly diatropic characteristics. Spectroscopic titration with TFA demonstrated that stepwise protonation took place, generating monocationic and dicationic species. 3-Alkoxybenziporphyrins reacted with nickel(II) or palladium(II) acetate to give the related nickel(II) or palladium(II) complexes. These stable organometallic derivatives showed increased diatropic properties that were most pronounced for the palladium(II) complexes. These unique porphyrinoids provide further insights into the properties of benziporphyrins.

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.

Unveiling the Reaction Mechanisms of the Synthesis and the Excited State Intramolecular Proton Transfer of 2-(2'-hydroxyphenyl)imidazo[1,2-a]pyridine

Given its wide variety of applications in the pharmaceutical industry, the synthesis of imidazo[1,2-a]pyridines has been extensively studied since the beginning of the last century. Here, we disclose the mechanism for the synthesis of imidazo[1,2-a]pyridines by means of the Ortoleva-King reaction. We also reveal the reaction pathway leading to the formation of a iodinated byproduct, demonstrating the challenge of preventing the formation of such a byproduct because of the low energy barrier to access it. Moreover, quantum chemistry tools were employed to investigate the mechanism of intramolecular proton transfer in the excited state, and connections with aromaticity were explored.

A Triply Linked Porphyrin-Norcorrole Hybrid with Singlet Diradical Character

Norcorrole Ni(II) complexes have recently received considerable attention because they are readily accessible antiaromatic molecules. Their high stability under ambient conditions and ease of synthesis have enabled the exploration of the intrinsic properties of antiaromatic molecules. Here, we report the synthesis and properties of meso-meso singly linked porphyrin-norcorrole hybrids and a triply linked porphyrin-norcorrole hybrid. The singly linked and triply linked porphyrin-norcorrole hybrids were fully characterized, including an X-ray structural analysis. Due to their orthogonal conformation, the singly linked hybrids maintain the individual electronic properties of their porphyrin and norcorrole subunits, while the triply linked hybrid shows a significantly smaller electrochemical HOMO-LUMO gap (0.45 eV) than that of Ni(II) dimesitylnorcorrole (1.08 eV). Furthermore, the triply linked hybrid exhibits singlet diradical characteristics, as confirmed by VT NMR, ESR, and SQUID experiments.

Aromaticity in the Electronic Ground and Lowest Triplet States of Molecules with Fused Thiophene Rings

Analysis of the variations of the off-nucleus isotropic magnetic shielding, σiso(r), around thiophene, thienothiophenes, dithienothiophenes and sulflowers in their electronic ground (S0) and lowest triplet (T1) states reveals that some of the features of aromaticity and bonding in these molecules do not fit in with predictions based on the popular Hückel's and Baird's rules. Despite having 4n π electrons, the S0 states of the sulflowers are shown to be aromatic, due to the local aromaticities of the individual thiophene rings. To reduce its T1 antiaromaticity, the geometry of thiophene changes considerably between S0 and T1: In addition to losing planarity, the carbon-carbon two 'double' and one 'single' bonds in S0 turn into two 'single' and one 'double' bonds in T1. Well-defined Baird-style aromaticity reversals are observed between the S0 and T1 states of only three of the twelve thiophene-based compounds investigated in this work, in contrast, the sulflower with six thiophene rings which is weakly aromatic in S0 becomes more aromatic in T1. The results suggest that the change in aromaticity between the S0 and T1 states in longer chains of fused rings is likely to affect mostly the central ring (or the pair of central rings); rings sufficiently far away from the central ring(s) can retain aromatic character.

Helicene Aromaticity Deviates from the Clar Rule-On the Electronic Dissimilarity of Large Isomeric Fibonacenes

This combined experimental and theoretical study illustrates the profound consequences of non-planarity on the electronic properties of polycyclic arenes. Three isomeric [10]fibonacene tetraesters were synthesized through a robust and regiocontrolled Perkin/Mallory approach: a nearly planar [10]phenacene derivative, a moderately twisted [10]semicircle derivative, and a 3D non-planar [10]helicene derivative. The photophysical properties of the 3D [10]helicene isomer were found to be dramatically different from the comparable ones of the [10]phenacene and [10]semicircle isomers. The aromatic properties of the [10]phenacene and [10]semicircle isomers conform well with their predictive Kekulé and Clar analyses, but the [10]helicene isomer deviates from these general topological rules, which appears to be a general phenomenon for [n]fibonacenes with n≥9.

Indenoannulated Tridecacyclene: An All-Carbon Seven-Stage Redox-Amphoter

We disclose an indenoannulated tridecacyclene comprising a central cyclooctatetraene moiety with multiple adjacent pentagonal rings which is accessible in a concise synthetic sequence. The saddle-shaped geometry of the non-benzenoid polycyclic scaffold and its unique packing behavior in the solid state were characterized by X-ray crystallography. In electrochemical studies, the compound undergoes seven reversible redox events comprising five reductions and two oxidations. The dicationic and dianionic species obtained by chemical oxidation and reduction, respectively, were characterized spectroscopically in solution. Density functional theory calculations were applied to provide insights into aromaticity evolution in the respective charged species, highlighting the beneficial effect of the non-benzenoid moieties on charge stabilization.

MB16 - (M=Sc, Y, La): Perfect Bowl-Like Boron Clusters

The introduction of transition-metal doping has engendered a remarkable array of unprecedented boron motifs characterized by distinctive geometries and bonding, particularly those heretofore unobserved in pure boron clusters. In this study, we present a perfect (no defects) boron framework manifesting an inherently high-symmetry, bowl-like architecture, denoted as MB16 - (M=Sc, Y, La). In MB16 -, the B16 is coordinated to M atoms along the C5v-symmetry axis. The bowl-shaped MB16 - structure is predicted to be the lowest-energy structure with superior stability, owing to its concentric (2 π+10 π) dual π aromaticity. Notably, the C5v-symmetry bowl-like B16 - is profoundly stabilized through the doping of an M atom, facilitated by strong d-pπ interactions between M and boron motifs, in conjunction with additional electrostatic stabilization by an electron transfer from M to the boron motifs. This concerted interplay of covalent and electrostatic interactions between M and bowl-like B16 renders MB16 - a species of exceptional thermodynamic stability, thus making it a viable candidate for gas-phase experimental detection.

Tuning (Anti)Aromaticity: Variations on the [8]-Circulene Framework

Optoelectronic properties of organic molecules are underpinned by delocalisation and delocalisability of π-electrons. These properties are sensitive to small changes in electron count, whether achieved by heteroatom substitution or redox chemistry. One measure of the delocalisability of π-electrons is the current induced by an external magnetic field, which is diagnostic of (anti)aromaticity. The ab initio ipsocentric method is used here to model diverse ring-current patterns in the family of [8]-circulenes based on tetracyclopenta[def,jkl,pqr,vwx]tetraphenylene (TCPTP), in different charge states, with disjoint hetero-atom substitution, and with CC units systematically replaced by BN pairs. Maps calculated at the CHF/CTOCD-DZ2/6-31G** level reveal that these modifications of the TCPTP framework access the full range of possibilities for current from concentric global circulations (typically counter rotating) to full (non-aromatic) localisation. In the ipsocentric approach, induced current density is partitioned into robust orbital contributions that obey selection rules based on orbital symmetry, energy and nodal character. The selection rules are applied here to interpret current-density and exploit insights gained from simpler models to suggest design strategies for fine-tuning of π-delocalisability (aromaticity and antiaromaticity) in macrocyclic frameworks.

[2,2]Paracyclophane Bridged, Thiophene Based Macrocycles: Synthesis and Electronic Properties in Different Redox States

The study of through-space electronic coupling in π-conjugated systems remains an underexplored area. In this work, we present the facile synthesis of two isomeric macrocycles (1 and 2) bridged by [2,2]paracyclophane (pCp) and based on thiophene. The structures of these macrocycles have been confirmed through X-ray crystallographic analysis. Our investigation centers on their electronic properties across various redox states, with a specific focus on potential through-space electronic coupling and global aromaticity. Experimental measurements, including UV-vis-NIR electronic absorption, NMR, ESR spectra, and X-ray diffraction, combined with theoretical calculations, reveal that both the neutral compounds and their tetracations exhibit a closed-shell ground state. However, their dications manifest as diradical dications with a subtle magnetic exchange interaction. Consequently, the through-space electronic coupling facilitated by the pCp unit in their respective ground states appears to be weak.

Effect of Benzo-Annelation on Triplet State Energies in Polycyclic Conjugated Hydrocarbons

In a series of earlier studies, the effect of benzo-annelation was found to be a useful tool for tuning the aromaticity in polycyclic conjugated compounds to desired level. In this work we studied the (anti)aromaticity of benzo-annelated derivatives of three conjugated hydrocarbons (anthracene, fluoranthene and biphenylene) in their lowest lying singlet (S0) and triplet (T1) states by means of the energy effect (ef), harmonic oscillator model of aromaticity (HOMA), multicentre delocalization indices (MCI), magnetically induced current densities (MICDs) and nucleus independent chemical shifts (NICS). We showed that benzo-annelation is a topology-based effect which can be used to modify the T1 state excitation energies (E(T1)). A quantitative model was established being able to accurately predict the E(T1) based only on the numbers of angularly, linearly and geminally annelated benzene rings. In addition, it was demonstrated that the E(T1) can be directly related to the (anti)aromatic character of the central ring in the studied molecules in their S0 state.

Substitution Effect of a Single Nitrogen Atom on π-Electronic Systems of Linear Polycyclic Aromatic Hydrocarbons (PAHs): Theoretically Visualized Coexistence of Mono- and Polycyclic π-Electron Delocalization

We theoretically investigated the nitrogen substitution effect on the molecular structure and π-electron delocalization in linear nitrogen-substituted polycyclic aromatic hydrocarbons (N-PAHs). Based on the optimized molecular structures and magnetic field-induced parameters of fused bi- and tricyclic linear N-PAHs, we found that the local π-electron delocalization of subcycles (e.g., mono- and bicyclic constituent moieties) in linear N-PAHs is preserved, despite deviation from ideal structures of parent monocycles. The introduction of a fused five-membered ring with a pyrrolic N atom (N-5MR) in linear N-PAHs significantly perturbs the π-electronic condition of the neighboring fused six-membered ring (6MR). Monocyclic pyrrole exhibits substantial bond length alternations, strongly influencing the π-electronic systems of both the fused N-5MR and 6MR in linear N-PAHs, depending on the location of shared covalent bonds. A fused six-membered ring with a graphitic N atom in an indolizine moiety cannot generate monocyclic π-electron delocalization but instead contributes to the formation of polycyclic π-electron delocalization. This is evidenced by bifurcated diatropic ring currents induced by an external magnetic field. In conclusion, the satisfaction of Hückel's 4n + 2 rule for both mono- and polycycles is crucial for understanding the overall π-electron delocalization. It is crucial to consider the unique characteristics of the three types of substituted N atoms and the spatial arrangement of 5MR and 6MR in N-PAHs.

Unraveling the Push-Pull Effect in Acenes, Polyenes and Polyynes

Substituent effects (SEs) are fundamental for predicting molecular reactivity, while polyene, polyyne and acene derivatives are precursors to compounds with diverse applications. Computations were performed for Y-R-X systems, where reaction sites Y=NO2 and O- , substituents X=NO2 , CN, Cl, H, OH, NH2 , and spacers R=polyene, polyyne (n=1-5, 10 repeating units) and acene (up to tetracene). The cSAR (charge of the substituent active region) approach allowed to present, for the first time, quantitative relations describing the spacer's electron-donating and withdrawing properties as a function of n and the spacer type. The electronic properties of the X substituents depend on the type of spacer, its length and the Y group, which is an example of the reverse SE. To describe how the SE between Y and X weakens with n, two approaches were compared: cSAR and SESE (SE stabilization energy). The EDDB (electron density of delocalized bonds) characterize changes in electron delocalization in spacers due to the SE. A new approach - EDDB differential maps - allow to extract the effect of X substitution on the electron delocalization. The charges at spacer's C atoms correlate with cSAR; changes in the slopes confirm the charge transfer by resonance.

Metal Influence on Cyaphide-Azide 1,3-Dipolar Cycloaddition Reactions: Aromaticity and Activation Strain

The factors governing 1,3-dipolar cycloaddition reactions involving C≡P-containing compounds are computationally explored in detail using quantum chemical tools. To this end, the parent process involving tBuN3 and tBuCP is analyzed and compared to the analogous reaction involving organometallic cyaphide complexes (metal=Au, Pt, Ge, Mg), in order to understand the role of the metal fragment in such transformations. It is found that while the metal fragment does not significantly influence the aromaticity of the corresponding concerted transition states or the regioselectivity of the transformation, it may modify the reactivity of the cyaphide complexes (i. e. Ge and Mg cyaphide complexes are comparatively more reactive). The computed reactivity trends and the factors behind the regioselectivity of the cycloaddition reaction are quantitatively analyzed with the help of the activation strain model in combination with the energy decomposition analysis method.

On the aromaticity and stability of benzynes in the ground and lowest-lying triplet excited states

In this work, we have revisited the aromaticity of benzyne isomers at the unrestricted density functional theory level (UDFT) using the energetic, magnetic, and delocalization criteria. In addition, this last criterion has also been analyzed employing complete active space (CASSCF) calculations. The results show conservation of aromaticity in these monocycles. Additionally it is observed that this trend is maintained in polycyclic aromatic hydrocarbon derivatives such as biradical didehydrophenanthrenes. Do these results imply a violation of Baird's rule? The answer is No, because this conservation in aromaticity is due to the loss of hydrogen atoms affects only the electronic σ skeleton and exerts a minor influence on the π cloud. Additionally, we have analyzed the relative stability of benzyne isomers and their relationship with experimental ΔES-T values. According to the literature, the stability of the benzynes in the singlet state is due to an effective interaction between the electrons of the biradical centers; however, this effect is completely reversed in the triplet state, which explains why the para isomer has the lowest ΔES-T gap.

Aromaticity and Magnetic Behavior in Benzenoids: Unraveling Ring Current Combinations

Nowadays, an active research topic is the connection between Clar's rule, aromaticity, and magnetic properties of polycyclic benzenoid hydrocarbons. In the present work, we employ a meticulous magnetically induced current density analysis to define the net current flowing through any cyclic circuit, connecting it to aromaticity based on the ring current concept. Our investigation reveals that the analyzed polycyclic systems display a prominent global ring current, contrasting with subdued semi-local and local ring currents. These patterns align with Clar's aromatic π-sextets only in cases where migrating π-sextet structures are invoked. The results of this study will enrich our comprehension of aromaticity and magnetic behavior in such systems, offering significant insights into coexisting ring current circuits in these systems.

Di(p-dibenzi)[40]decaphyrin(1.0.0.0.0.1.0.0.0.0) PdII Complex: A Weakly Hückel 38π-Aromatic Macrocycle

Di(p-benzi)[40]decaphyrin(1.0.0.0.0.1.0.0.0.0) BF2 complex and tris(p-benzi)[60]pentadecaphyrin(1.0.0.0.0.1.0.0.0.0.1.0.0.0.0) BF2 complex were synthesized by Suzuki-Miyaura coupling of α,α'-diborylated tetrapyrrole BF2 -complex with 1,4-diiodobenzene. Bis-BF2 complex was converted to bis-PdII complex via its free base. Macrocycles bis-BF2 and tris-BF2 complex take Möbius topology but are nonaromatic, since the macrocyclic conjugation is disrupted by the locally aromatic 1,4-phenylene units. In contrast, bis-PdII complex is a weakly Hückel 38π-aromatic macrocycle as evinced by its red-shifted, enhanced, and structured Q-like bands and a small electrochemical HOMO-LUMO gap. Interestingly, one 1,4-pheylene part of bis-PdII complex takes a quinonoidal distorted structure and the other takes a usual benzene structure in a figure-eight conformation with Hückel topology.

Synthesis and Photochromic Properties of Diaryl [5]Helicene Derivatives

Photoinduced tuning of aromaticity and correlative molecular properties has attracted enormous interest in recent years both for modulating photochromism properties and designing novel photochromic materials. Here, we report the synthesis and photochemical characterization of diaryl[5]helicene-based diarylethene molecular switches. 3,4-Bis(2,4-dimethyl-5-phenylthiophen-3-yl)dibenzo[c,g]phenanthrene derivative 1 a showed no photochromic reaction, whereas 3,4-bis(2-methyl-5-phenylthiophen-3-yl)dibenzo[c,g]phenanthrene derivative 2 a and 3,4-bis(5-methyl-2-phenylthiazol-4-yl)dibenzo[c,g]phenanthrene derivative 3 a exhibited reversible photochromism in different aprotic solvents with specific light irradiation. Meanwhile, the diarylethene compounds 2 a and 3 a underwent turn-off mode fluorescence photoswitching together with photoisomerization upon light irradiation. Remarkably, the photoinduced changes in the aromaticity of [5]helicene as a central ethene bridge along with the relative smaller activation energy and higher frequency factor facilitated the thermal bleaching rates of diarylethene switches 2 and 3 in solution. This research provides new insight for designing aromatic diarylethene photoswitches for reversible fluorescence switching, photoinduced changes in aromaticity and further fast thermal back reactions.

Experimental and theoretical evidences for the formation of transition metal complexes with five coplanar metal-carbon σ bonds

The σ bond is an important concept in chemistry, and the metal-carbon (M-C) σ bond in particular is a central feature in organometallic chemistry. Synthesis of stable complexes with five coplanar M-C σ bonds is challenging. Here, we describe the synthesis of two different types of stable complexes with five coplanar M-C σ bonds, and examine the stability of such complexes which use rigid conjugated carbon chains to chelate with the metal center. Density functional theory (DFT) calculations show that the M-C σ bonds in these complexes have primarily a covalent character. Besides the σ nature, there are also a π conjugation component among the metal center and carbons, which causes delocalization. This work expanded the coplanar M-C σ bonds to five.

Ring Strain Energy of Diheteropnictogeniranes El2 Pn (Pn=N, P, As, Sb)- Accurate versus Additive Approaches

Accurate ring strain energy (RSE) values for sixty-six parent pnictogeniranes having two other identical p-block elements, El2 Pn, have been reported. A decrease in RSE was observed to correlate with an increase in the p character of the AO used in endocyclic bonds, which is particularly remarkable on descending the groups 15 and 16. The latter also parallels higher -NICS(1) values, which seems not to be related with an increase in aromaticity, as pointed out by other NICS-related criteria, but to atom-centred diatropic currents mostly arising from the presence of lone pairs. Only in case of pnictogenaditrieliranes Tr2 Pn (Tr=B, Al, Ga), the decrease of -NICS(1) is related to a lower Hückel-type 2π-electron aromaticity on descending group 13. The use of an additive methodology based on atom-strain contributions enables estimation of RSEs for a large majority of all possible three-membered rings containing group 13-16 elements with modest accuracy (RMSE=4.371 kcal/mol), that could be remarkably improved by using bond-strain contributions (RMSE=1.183 kcal/mol) instead.

Synthesis of Naphthalene- and Phenanthrene-Fused Smaragdyrins and Their BF2 Complexes

Naphthalene- and phenanthrene-fused [22]smaragdyrin BF2 -complexes were synthesized by 1) Suzuki-Miyaura coupling of β-brominated [22]smaragdyrin BF2 complexes with 2-formylarylboronates, 2) Witting-type methoxymethylenation of the formyl group, and 3) methanesulfonic acid-catalyzed cyclization reaction. Subsequently these BF2 complexes were deboronized and oxidized to the corresponding antiaromatic [20]smaragdyrin free bases. The installed fused structures led to decrease of the aromatic characters of the [22]smaragdyrin BF2 complexes and the antiaromatic characters of the [20]smaragdyrin free bases.

Spin-Injection in Graphene: An EPR and Raman Study

In this article, the enrichment of graphene and graphene oxide with free radicals through their functionalization with tyrosine is studied. In contrast with what is commonly observed in the functionalization of graphene with organic species the addition of tyrosine radicals on to the graphene substrate led to a remarkable increase of the aromatic character as indicated by the spectroscopic data. Similar behaviour was observed for the functionalization of graphene oxide. In addition, a brief analysis of the tyrosine functionalized graphene with EPR spectroscopy showed a remarkable enhancement of the spin density that could be useful in spintronics.

One-pot Syntheses of Benzo- and Benzofuran-fused Iridaoxabenzenes via CH Bond Activations of Alkyl-bridged Diphenol Derivatives

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.

Actinide-Actinide Bonding: Electron Delocalisation and σ-Aromaticity in the Tri-Thorium Cluster [{Th(η8 -C8 H8 )(μ-Cl)2 }3 K2 ]

The tri-thorium cluster [{Th(η8 -C8 H8 )(μ3 -Cl)2 }3 {K(THF)2 }2 ]∞ (Nature 2021, 598, 72-75) was reported to feature intriguing σ-aromatic bonding between the thorium atoms, a mode of metal-metal bonding unique in the actinide series. However, the presence of this bonding motif has since been challenged by others. Here, we computationally explore electron delocalisation in a molecular cluster fragment of [{Th(η8 -C8 H8 )(μ3 -Cl)2 }3 {K(THF)2 }2 ]∞ and examine its responses to an applied magnetic field using a variety of methods. We also discuss the importance of the choice of basis set for the Th atoms and issues regarding locating QTAIM bond critical points. When taken together, the computed data consistently suggest the presence of delocalised Th-Th bonding and Th3 σ-aromaticity.

Structures and Properties of Planar Ge3O3 Cluster and Its Buckled Honeycomb Two-Dimensional Nanostructure

The discovery of graphene and its excellent properties inspired the search for more two-dimensional (2D) materials. Understanding the structures and properties of the smallest repeating units as well as crystal 2D materials is helpful for designing 2D materials. As germanium tends to form three-dimensional structures, the preparation of germanium-based 2D nanomaterials is still a challenge. Herein, we report a Ge3O3 cluster with the potential to construct a germanium oxide 2D nanostructure. We conduct a combined anion photoelectron spectroscopy and theoretical study on Ge3O3-/0. The structure of Ge3O3- is a Cs symmetric nonplanar six-membered ring, while that of Ge3O3 is a D3h symmetric planar six-membered ring. Chemical bonding analyses reveal that Ge3O3 exhibits π aromaticity. First-principle results suggest that a buckled honeycomb 2D nanostructure with a wide band gap of 3.14 eV may be produced based on Ge3O3, which is promising in optoelectronic applications especially in blue, violet, and ultraviolet regions.

Predicting a New Δ-Proton Sponge-Base of 4,12-Dihydrogen-4,8,12-triazatriangulene through Proton Affinity, Aromatic Stabilization Energy, and Aromatic Magnetism

Herein, we report designing a new Δ (delta-shaped) proton sponge base of 4,12-dihydrogen-4,8,12-triazatriangulene (compound 1) and calculating its proton affinity (PA), aromatic stabilization, natural bond orbital (NBO), electron density ρ(r), Laplacian of electron density ∇2 ρ(r), (2D-3D) multidimensional off-nucleus magnetic shielding (σzz (r) and σiso (r)), and scanning nucleus-independent chemical shift (NICSzz and NICS). Density functional theory (DFT) at B3LYP/6-311+G(d,p), ωB97XD/6-311+G(d,p), and PW91/def2TZVP were used to compute the magnetic shielding variables. In addition, relevant bases like pyridine, quinoline, and acridine were also studied and compared. The protonation of compound 1 yields a highly symmetric carbocation of three Hückel benzenic rings. Comparing our findings of the studied molecules showed that compound 1 precedes others in PA, aromatic isomerization stabilization energy, and basicity. Therefore, the basicity may be enhanced when a conjugate acid gains higher aromatic features than its unprotonated base. Both multidimensional σzz (r) and σiso (r) off-nucleus magnetic shieldings outperformed electron-based techniques and can visually monitor changes in aromaticity that occur by protonation. The B3LYP/6-311+G(d,p), ωB97XD/6-311+G(d,p), and PW91/def2TZVP levels showed no significant differences in detailing isochemical shielding surfaces.

FP(μ-N)2 S: A Sulfur-Pnictogen Four-Membered Ring with 6π Electrons

The new 6π-electron four-membered ring compound 3-fluoro-1λ2 ,2,4,3λ3 -thiadiazaphosphetidine, FP(μ-N)2 S, has been generated in the gas phase through high-vacuum flash pyrolysis (HVFP) of thiophosphoryl diazide, FP(S)(N3 )2 , at 1000 K. Subsequent isolation of FP(μ-N)2 S in cryogenic matrices (Ar, Ne, and N2 ) allows its characterization with matrix-isolation IR and UV-vis spectroscopy by combination with 15 N-isotope labeling and computations at the CCSD(T)-F12a/VTZ-F12 level of theory. Upon visible-light irradiation at 550 nm, this cyclic compound undergoes ring-opening to the thiazyl isomer FPNSN, followed by dissociation to FP and SN2 under subsequent UV-irradiation at 365 nm. In sharp contrast to the square planar structure for the isolobal four-membered ring S2 N2 , a puckered structure with significant biradical character has been found for FP(μ-N)2 S.

Main-Group Metal Complexes of Benzene: Predicted Features of Stabilization and Isomerization

Complexes of metal atoms with organic molecules represent a broad variety of systems with many important applications, e.g., in metal-organic interfaces and organometallic chemistry. One class involves aromatic species like benzene (Bz). Here, such complexes with second-group metals are investigated systematically in terms of structure and shape, stability and isomerization, charge distribution and aromaticity, and polarity and IR spectra. Three groups of isomers are identified, varying from metastable to stable ones, in effect featuring "physisorption" or "chemisorption". In particular, the high polarity of binary complexes and nonadditive stabilization of ternary systems for some isomers are found. Also, the Bz component's shape alteration for different isomers and system sizes and related aromaticity variations are predicted to be considerable. Property evolution for the series of metal components is analyzed.

On Integral INICS Aromaticity of Pyridodiazepine Constitutional Isomers and Tautomers

The structure, energetics, and aromaticity of c.a. 100 constitutional isomers and tautomers of pyrido[m,n]diazepines (m = 1, 2; n = 2, 3, 4, 5; m ≠ n) were studied at the B3LYP/cc-pVTZ level. The pyrido[1,3]diazepines appear the most, while pyrido[2,4]diazepines are the least stable (ca. 26 kcal/mol). In the pyrido[1,n]diazepine group (n = 2-5), the [1,5] isomers are higher in energy by ca. 4.5 kcal/mol and the [1,4] ones by ca. 7 kcal/mol, and the pyrido[1,2]diazepines are the least stable (ca. 20 kcal/mol). All the most stable pyrido[1,n]diazepines have N-atoms near the ring's junction bond but on opposite sites. The most stable [2,n]-forms are also those with the pyridine ring N6-atom near the junction bond. Surprisingly, for the [1,2]-, [1,3]-, and [1,4]-isomer condensation types of pyridine and diazepine rings, the same N9 > N7 > N6 > N8 stability pattern obeys. The stability remains similar in a water medium simulated with the Polarizable Continuum Model of the solvent and is conserved when calculated using the CAM-B3LYP or BHandHlyp functionals. The ring's aromaticity in the pyridine[m,n]diazepines was established based on the integral INICS index resulting from the NICSzz-scan curves' integration. The integral INICS index is physically justified through its relation to the ringcurrent as demonstrated by Berger, R.J.F., et al. Phys. Chem. Chem. Phys. 2022, 24, 624. The six-membered pyrido rings have negative INICSZZ indices and can be aromatic only if they are not protonated at the N-atom. All protonated pyrido and seven-membered rings exhibit meaningful positive INICSZZ values and can be assigned as antiaromatic. However, some non-protonated pyrido rings also have substantial positive INICSZZ indices and are antiaromatic. A weak linear correlation (R2 = 0.72) between the INICSZZ values of the pyridine I(6) and diazepine I(7) rings exists and is a consequence of the communication between the π-electron systems of the two rings. The juxtaposition of the INICS descriptor of the six- and seven-membered rings and diverse electron density parameters at the Ring Critical Points (RCP) revealed good correlations only with the Electrostatic Potentials from the electrons and nuclei (ESPe and ESPn). The relationships with other RCP parameters like electron density and its Laplacian, total energy, and the Hamiltonian form of kinetic energy density were split into two parts: one nearly constant for the six-membered rings and one linearly correlating for the seven-membered rings. Thus, most of the electron density parameters at the RCP of the six-membered rings of pyridodiazepines practically do not change with the diazepine type and the labile proton position. In contrast, those of the seven-membered rings display aromaticity changes in the antiaromatic diazepine with its ring structural modifications.

Bismuth Infrared Star: Being at a Glance

Bismuth polycations have garnered significant attention from researchers due to their extraordinary and counter-intuitive structures and stoichiometries. Despite extensive experimental and theoretical investigations, understanding of the bonding in such clusters remains insufficient. An AdNDP bonding analysis was conducted to elucidate the bonding characteristics using both homoatomic and heteroatomic bismuth clusters with various stoichiometries. Analysis of the calculated nucleus-independent chemical shift data confirmed the aromatic nature of these species. Universal bonding patterns were identified that can be applied to a range of homoatomic and heteroatomic bismuth clusters. Additionally, calculations of absorbance and fluorescence spectra were performed to gain insights into the near-infrared emission and establish a potential correlation between absorbance and the identified bonding patterns.

B₇Be₆B₇: A Boron-Beryllium Sandwich Complex

Planar boron clusters have often been regarded as "pi-analogous" to aromatic arenes because of their similar delocalized pi-bonding. However, unlike arenes such as C5H5- and C6H6, boron clusters have not previously shown the ability to form sandwich complexes. In this study, we present the first sandwich complex involving beryllium-and boron, B7Be6B7. The global minimum of this combination adopts a unique architecture having a D6h geometry, featuring an unprecedented monocyclic Be6 ring sandwiched between two quasi-planar B7 motifs. The thermochemical and kinetic stability of B7Be6B7 can be attributed to strong electrostatic and covalent interactions between the fragments. Chemical bonding analysis shows that B7Be6B7 can be considered as a [B7]3-[Be6]6+[B7]3- complex. Moreover, there is a significant electron delocalization within this cluster, supported by the local diatropic contributions of the B7 and Be6 rings.

Enhanced Production of Organosulfur Species during a Severe Winter Haze Episode in the Guanzhong Basin of Northwest China

The molecular composition of organic aerosols in ambient PM_2.5 was investigated in an urban area in the Guanzhong basin of northwest China during a severe regional haze episode in the winter of 2018/2019. Organic matter, accounting for 20-35% of PM_2.5 mass concentration, was characterized using direct infusion and electrospray ionization coupled with high-resolution Orbitrap mass spectrometry. The number of organic molecular formula assignments was primarily dominated by organosulfur species (OrgS, including CHOS and CHONS) in negative ion mode. The number and peak signal intensity of OrgS distinctly increased during the severe haze episode. Organosulfates and nitrooxy-organosulfates constituted the majority number (72-94%) of OrgS over the entire period. Although the OrgS were mostly present in aliphatic molecular structures, an increase in the number of polycyclic aromatic OrgS on haze days revealed the enhanced contribution from anthropogenic sources. The number of OrgS strongly correlated with ambient relative humidity and the oxidation ratios of sulfur and nitrogen, suggesting the important roles of aqueous phase chemistry and atmospheric oxidation in the formation of OrgS. A thorough understanding of the significance of OrgS will be essential to assess and mitigate the adverse impacts of haze pollution.

Isolation, Structural and Physical Characterization as well as Reactivity of Persistent Acenium Radical Cation Salts

The unsubstituted acenium radical cations (ARCs) are extremely sensitive and were hitherto only studied in situ, i.e. in the gas phase, as dilute solutions in strong acids or by matrix isolation spectroscopy at about 10 K. In this study, we prepared, structurally, electro-chemical-ly and spectroscopically characterized room temperature stable ARC salts with the weakly coordi-nating anion [F{Al(ORF)3}2]- (ORF = -OC(CF3)3) supported by the weakly coordi-nating sol-vent 1,2,3,4-tetrafluorobenzene (TFB). Reaction of the neutral acenes with Ag+[F{Al(ORF)3}2]- led, non-innocent, to intermediate [Ag2(acene)2]2+ complexes, which decompose over time to Ag0 and the corresponding (impure) ARC salts. By contrast, direct deelectronation with the recently developed innocent deelectronator radical cation salt [anthraceneHal]+∙[F{Al(ORF)3}2]- led to phase-pure products [acene]+∙[F{Al(ORF)3}2]- (anthraceneHal = 9,10-dichlorooctafluoro-anthracene; acene = anthra-, tetra-, pentacene). For the first time, a homogenous set of spectroscopic data on analytically pure ARC salts was obtained. In addition, cyclo-voltammetric measure-ments of the acenes connected the potentials in solution with those in the gas-phase. Hence, the data complement the existing isolated gas-phase, strong acid or matrix isolation studies. A first entry to follow-up chemistry of the acenium radical cations as ligand forming oxidizers was demonstrated by reaction with ½ Co2(CO)8 giving [Co(anthracene)(CO)2]+.

Synthesis of a Benzotrisazulene via Trioxobenzotrisazulene

Polycyclic aromatic hydrocarbons (PAHs) containing odd-membered rings (such as pentagons or heptagons) are fascinating synthetic targets. A special case is the introduction of five- and seven-membered rings in form of an azulene unit at the same time. Azulene is an aromatic compounds known for its deep blue color, which is a result of its internal dipole moment. When azulene is embedded into PAHs it can change the optoelectronic properties of the PAH significantly. Here we report the synthesis and characterization of a PAH containing three azulene units via reduction and elimination of its trioxo derivative.

Etheno-bridged Azaacene Spiro Dimers

We present the synthesis of two sets of spiro-connected azaacene dimers. Their geometry and electronic coupling are critically determined by a secondary linker, i.e. an etheno- and an ethano-bridge. The core fragment of the etheno-bridged dimer corresponds to a conformationally locked cis-stilbene. Optoelectronic properties, single crystal X-ray structures and stability with respect to oxidation of the conjugated and non-conjugated dimers are reported and compared. The conjugated dimers exhibit smaller optical gaps and bathochromically shifted absorption maxima, but are prone to unexpected oxygen addition, dearomatizing one of the azaacene substituents.

Synthesis of Homo-Metallic Heavier Analogues of Cyclobutene and the Cyclobutadiene Dianion

The reduction of the boryl-substituted SnII bromide {(HCDippN)2 B}Sn(IPrMe)Br with 1.5 equivalents of potassium graphite leads to the generation of the cyclic tetratin tetraboryl system K2 [Sn4 {B(NDippCH)2 }4 ], a homo-metallic heavier analogue of the cyclobutadiene dianion. This system is non-aromatic as determined by Nucleus Independent Chemical Shift Calculations (NICS(0)=-0.28, NICS(1)=-3.17), with the primary contributing resonance structures shown by Natural Resonance Theory (NRT) to involve a Sn=Sn double bond and 1,2-localized negative charges. Abstraction of the K+ cations or oxidation leads to contraction or cleavage of the Sn4 unit, respectively, while protonation generates the neutral dihydride 1,2-Sn4 {B(NDippCH)2 }4 H2 (a heavier homologue of cyclobutene) in a manner consistent with the predicted charge distribution in the [Sn4 {B(NDippCH)2 }4 ]2- dianion.

Aromatic Character and Relative Stability of Pyrazoloporphyrin Tautomers and Related Protonated Species: Insights into How Pyrazole Changes the Properties of Carbaporphyrinoid Systems

Pyrazoloporphyrins (PzPs), which are porphyrin analogues incorporating a pyrazole subunit, are examples of carbaporphyrin-type structures with a carbon atom within the macrocyclic cavity. DFT calculations were used to assess a series of 17 PzP tautomers, nine monoprotonated species and four related diprotonated PzP dications. The geometries of the structures were optimized using M06-2X/6-311++G(d,p), and the relative stabilities computed with the cc-PVTZ functional. Nucleus independent chemical shifts, both NICS(0) and NICS(1)_zz, were calculated, and the anisotropy of the induced current density (AICD) plots were generated for all of the species under investigation. The results for free base PzPs show that fully aromatic PzP tautomers are not significantly more stable than weakly aromatic cross-conjugated species. In addition, strongly aromatic structures with internal CH_2's are much less stable, a feature that is also seen for protonated PzPs. The degree of planarity for the individual macrocycles does not significantly correlate with the stability of these structures. The results allow significant aromatic conjugation pathways to be identified in many cases, and provide insights into the aromatic properties of this poorly studied system. These investigations also complement experimental results for PzPs and emphasize the need for further studies in this area.

Electronic Structure and Aromaticity of an Unusual Cyclo[18]carbon Precursor, C18Br6

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 are examined using localized orbital locator (LOL) and electron localization function (ELF). Then, the aromaticity is addressed via 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.

Single-Benzene Dual-Emitters Harness Excited-State Antiaromaticity for White Light Generation and Fluorescence Imaging

Molecular emitters simultaneously generating light at different wavelengths have wide applications. With a small molecule, however, it is challenging to realize two independent radiative pathways. We invented the first examples of dual-emissive single-benzene fluorophores (SBFs). Two emissive tautomers are generated by synthetic modulation of the hydrogen bond acidity, which opens up pathways for excited-state proton transfer. White light is produced by a delicate balance between the energy and intensity of the radiative decay from each tautomer. We show that the excited-state antiaromaticity of the benzene core itself dictates the proton movements driving the tautomer equilibrium. Using this simple benzene platform, a fluorinated SBF was synthesized with a record high solubility in perfluorocarbon solvents. White light-emitting devices and multicolor imaging of perfluorocarbon nanodroplets in live cells demonstrate the practical utility of these molecules.

Modular Synthesis of Structurally Diverse Azulene-embedded Polycyclic Aromatic Hydrocarbons by Knoevenagel-type Condensation

The research interest in azulene-embedded polycyclic aromatic hydrocarbons (PAHs) has significantly increased recently, but the lack of efficient synthetic strategy impedes the investigation of their structure-property relation and further opto-electronic applications. Here we report a novel modular synthetic strategy towards diverse azulene-embedded PAHs via a tandem Suzuki coupling and base-promoted Knoevenagel-type condensation with good yields and great structural versatility, including non-alternant thiophene-rich PAHs, butterfly- or Z-shaped PAHs bearing two azulene units, and the first example of two-azulene-embedded double [5]helicene. The structural topology, aromaticity and photophysical properties were investigated by NMR, X-ray crystallography analysis and UV/Vis absorption spectroscopy assisted by DFT calculations. This strategy provides a new platform for rapidly synthesizing unexplored non-alternant PAHs or even graphene nanoribbons with multiple azulene units.

N,N-Diphenyl Dihydrophenazines: Using π-Extension to Access Dicationic Multifunctional Materials

Dihydrophenazines are receiving increasing attention due to applications in numerous fields of chemistry, from light emission to organo-photocatalysis. Despite this growing interest and numerous works involving the preparation of radical cations based on this scaffold, the isolation and study of the aromatic dications obtained by 2 electron oxidation of dihydrophenazines is still mostly unexplored. From this point of view, along with the substitution at the N atoms generally used to tune dihydrophenazine properties, the π-extension of the phenazine core could play a crucial role in making dicationic states accessible. This could result in an extension of the knowledge on these elusive dications and in potentially highly interesting applications ranging from material science to molecular actuators.

Screening Carbon-Boron Frustrated Lewis Pairs for Small-Molecule Activation including N2 , O2 , CO, CO2 , CS2 , H2 O and CH4 : A Computational Study

Dinitrogen (N₂ ) activation is particularly challenging under ambient conditions because of its large highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap (10.8 eV) and high bond dissociation energy (945 kJ mol^-1 ) of the N≡N triple bond, attracting considerable attention from both experimental and theoretical chemists. However, most effort has focused on metallic systems. In contrast, nitrogen activation by frustrated Lewis pairs (FLPs) has been initiated recently via theoretical calculations. Here we perform density functional theory (DFT) calculations to screen a series of experimentally viable FLPs for small-molecule activation including N₂ , O₂ , CO, CO₂ , CS₂ , H₂ O and CH₄ . In addition, aromaticity is found to play an important role in most of these small-molecule activation. The particularly thermodynamic stabilities of the activation products and low reaction barriers could be a step forward for the development of FLP towards small-molecule activation including N₂ , inviting experimental chemists' verification.

Aromatic 1,2-Azaborinin-1-yls as Electron-Withdrawing Anionic Nitrogen Ligands for Main Group Elements

The 2-aryl-3,4,5,6-tetraphenyl-1,2-azaborinines 1-EMe₃ and 2-EMe₃ (E=Si, Sn; aryl=Ph (1), Mes (=2,4,6-trimethylphenyl, 2)) were synthesized by ring-expansion of borole precursors with N₃ EMe₃ -derived nitrenes. Desilylative hydrolysis of 1- and 2-SiMe₃ yielded the corresponding N-protonated azaborinines, which were deprotonated with nBuLi or MN(SiMe₃ )₂ (M=Na, K) to the corresponding group 1 salts, 1-M and 2-M. While the lithium salts crystallized as monomeric Lewis base adducts, the potassium salts formed coordination polymers or oligomers via intramolecular K⋅⋅⋅aryl π interactions. The reaction of 1-M or 2-M with CO₂ yielded N-carboxylate salts, which were derivatized by salt metathesis to methyl and silyl esters. Salt metathesis of 1-M or 2-M with methyl triflate, [Cp*BeCl] (Cp*=C₅ Me₅ ), BBr₂ Ar (Ar=Ph, Mes, 2-thienyl), ECl₃ (E=B, Al, Ga) and PX₃ (X=Cl, Br) afforded the respective group 2, 13 and 15 1,2-azaborinin-2-yl complexes. Salt metathesis of 1-K with BBr₃ resulted not only in N-borylation but also Ph-Br exchange between the endocyclic and exocyclic boron atoms. Solution ¹¹ B NMR data suggest that the 1,2-azaborinin-2-yl ligand is similarly electron-withdrawing to a bromide. In the solid state the endocyclic bond length alternation and the twisting of the C₄ BN ring increase with the sterics of the substituents at the boron and nitrogen atoms, respectively. Regression analyses revealed that the downfield shift of the endocyclic ¹¹ B NMR resonances is linearly correlated to both the degree of twisting of the C₄ BN ring and the tilt angle of the N-substituent. Calculations indicate that the 1,2-azaborinin-1-yl ligand has no sizeable π-donor ability and that the aromaticity of the ring can be subtly tuned by the electronics of the N-substituent.

Facile Formation of Stable Neutral Radicals and Cations from [22]Smaragdyrin BF2 Complexes

meso-Trimesityl-substituted [20]smaragdyrin freebase was synthesized by p-toluenesulfonic acid catalyzed reaction of 5-mesityldipyrromethane and 2,14-dibromodipyrrin in an improved yield of 63 %. Unexpectedly, treatment of the [20]smaragdyrin freebase with BF₃  ⋅ OEt₂ and triethylamine (TEA) gave a stable radical species, in which the BF₂ unit is coordinated at the tripyrrin site, probably by ready release of a hydrogen atom of a [22]smaragdyrin BF₂ complex. Similar treatment of [22]smaragdyrin free base produced another [22]smaragdyrin BF₂ complex, in which the BF₂ unit is coordinated at the dipyrrin site. The tripyrrin site coordinated neutral radical was oxidized with AgSbF₆ to give a stable antiaromatic cation; this was reduced with NaBH₄ to its 22π congener, which was easily oxidized back to the neutral radical in the air and rearranged to thermodynamically stable dipyrrin site coordinated [22]smaragdyrin BF₂ complex upon treatment with BF₃  ⋅ OEt₂ and TEA. Further, the dipyrrin site coordinated [22]smaragdyrin BF₂ complex was similarly oxidized to a stable neutral radical and a stable cation in a stepwise manner. This work demonstrates a rare ability of smaragdyrin BF₂ complexes to exist in multiple redox states, particularly forming a stable neutral radical by facile release of a hydrogen atom.

Nucleus Independent Chemical Shift (NICS) at Small Distances from the Molecular Plane: The Effect of Electron Density

When close to the molecular plane, the behavior of nucleus independent chemical shift (NICS) as a function of the distance from the molecular plane deviates from its behavior at larger distances. By using a dense grid of NICS-probes (BQs) it is shown that, when close to the molecular plane, maximal (absolute) NICS values are obtained above the atoms. These maxima move towards the center as the grid is elevated until the (absolute) maximum NICS is obtained at the center and stay there when the grid is further elevated. It is shown that this behavior is a result of the current density, which is influenced by the electron density, according to the Biot-Savart law, which, in turn, causes the induced magnetic field measured by the NICS. It is thus concluded that if magnetic aromaticity is studied, the NICS calculations should be carried out at a large enough distance so that only the π-ring current affects the NICS. At distances ≥2 Å, NICS(r)_π,zz =A+B*C^r . Using non-linear correlation for obtaining A, B and C and extrapolate to NICS(1)_π,zz and NICS(1.7)_π,zz is recommended as measures for aromaticity.

Conjugation Extension and Halochromic Behaviors of S-Fused Polycyclic Aromatic Hydrocarbons Bearing Cyclopenta[b]thiopyran Moieties

Three S-fused polycyclic aromatic hydrocarbons (PAHs) bearing cyclopenta[b]thiopyran moieties have been designed and successfully synthesized. With the conjugation extension, the absorption onset of the longest PAH reaches 1110 nm. All the three S-fused PAHs exhibit significant halochromic properties in both solution and solid states. Upon protonation, the proton is incorporated on the cyclopentadiene ring while the positive charge is localized on the thiopyrylium ring. Moreover, no significant difference can be found for the two shorter PAHs upon the protonation by different organic acids, such as trifluoroacetic acid (TFA) and trifluoromethanesulfonic acid (TfOH), while the longest PAH can be only mono-protonated by TFA but di-protonated by stronger TfOH. Furthermore, after protonation, the non-emissive S-fused PAHs exhibit strong fluorescence and can be regenerated by simply neutralization with triethylamine. The enhanced emission of mono-protonated products stem from S₂ →S₀ transitions, which disobey the Kasha's rule.

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.