Global Aromaticity Research Articles

Fused Indacene Dimers.

Polycyclic hydrocarbons consisting of two or more directly fused antiaromatic subunits are rare due to their high reactivity. However, it is important to understand how the interactions between the antiaromatic subunits influence the electronic properties of the fused structure. Herein, we present the synthesis of two fused indacene dimer isomers: s-indaceno[2,1-a]-s-indacene (s-ID) and as-indaceno[3,2-b]-as-indacene (as-ID), containing two fused antiaromatic s-indacene or as-indacene units, respectively. Their structures were confirmed by X-ray crystallographic analysis. 1 H NMR/ESR measurements and DFT calculations revealed that both s-ID and as-ID have an open-shell singlet ground state. However, while localized antiaromaticity was observed in s-ID, as-ID showed weak global aromaticity. Moreover, as-ID exhibited a larger diradical character and a smaller singlet-triplet gap than s-ID. All the differences can be attributed to their distinct quinoidal substructures.

Fused Indacene Dimers

AbstractPolycyclic hydrocarbons consisting of two or more directly fused antiaromatic subunits are rare due to their high reactivity. However, it is important to understand how the interactions between the antiaromatic subunits influence the electronic properties of the fused structure. Herein, we present the synthesis of two fused indacene dimer isomers: s‐indaceno[2,1‐a]‐s‐indacene (s‐ID) and as‐indaceno[3,2‐b]‐as‐indacene (as‐ID), containing two fused antiaromatic s‐indacene or as‐indacene units, respectively. Their structures were confirmed by X‐ray crystallographic analysis. 1H NMR/ESR measurements and DFT calculations revealed that both s‐ID and as‐ID have an open‐shell singlet ground state. However, while localized antiaromaticity was observed in s‐ID, as‐ID showed weak global aromaticity. Moreover, as‐ID exhibited a larger diradical character and a smaller singlet‐triplet gap than s‐ID. All the differences can be attributed to their distinct quinoidal substructures.

A Triply [5]Helicene‐Bridged (1,3,5)Cyclophane

AbstractA rigid propeller‐shaped conjugated triple macrocycle consisting of two nearly perfectly stacked benzene rings and three linking [5]helicene moieties has been synthesized using a glyoxylic Perkin approach. Analysis of the electron delocalization in this atypical aromatic molecule revealed global aromaticity and a 78 π‐electron circuit along the edge of its triple loop, to the detriment of the two 6 π‐electron circuits in the two stacked benzene rings.

A Triply [5]Helicene-Bridged (1,3,5)Cyclophane.

A rigid propeller-shaped conjugated triple macrocycle consisting of two near perfectly stacked benzene rings and three linking [5]helicene moieties has been synthesized using a glyoxylic Perkin approach. Analysis of the electron delocalization in this atypical aromatic molecule revealed global aromaticity and a 78 π-electron circuit along the edge of its triple loop, to the detriment of the two 6 π-electron circuits in the two stacked benzene rings.

Bisazapentalene Dication: Global Aromaticity and Open-Shell Singlet Ground State

Antiaromatic moieties fused in polycyclic π-conjugated molecules usually exhibit strong localized antiaromaticiy. Herein, we reported the synthesis and properties of a bisazapentalene dication (BAP2+) obtained from in situ two-electron oxidation of neutral species 8. Noteworthily, it possesses global aromaticity and an open-shell singlet ground state. This study underlines the importance of heteroatoms in determining the delocalization of π-electrons and the aromaticity of molecules in their oxidized states.

Fundamental difference between simple arenes and PAHs found in o-PAH-connected porphyrins

Simple arenes and PAHs are distinguishable, but identifying their unique differences poses a challenge. This manuscript reveals, for the first time, how these differences manifest in global aromaticity within porphyrinoids.

Electronic delocalization in charged macrocycles is associated with global aromaticity.

When oxidized, cycloparaphenylenes and porphyrin nanorings can exhibit macrocyclic ring currents which have been used to assign these molecules as (anti)aromatic. These assignments have been controversial because the presence of ring currents does not always imply cyclic electronic delocalization, which is essential to the definition of aromaticity. Here, we show that the emergence of macrocyclic ring currents in these molecules is correlated with increased electronic delocalization, supporting assignments of these molecules as (anti)aromatic.

Local and global aromaticity under rotation: analysis of two- and three-dimensional representative carbon nanostructures.

Nanoscaled 2D and 3D carbon structures with closed curved π-surfaces are of relevance in the development of desirable building units for materials science. Such species are able to sustain local and global aromatic circuits involving isolated regions or the overall structural backbone, respectively. Here we account for local and global aromaticity under rotation of representative two- and three-dimensional species involving para-connected and fused edge-sharing phenyl rings ([8]CPP, [10]CPP, CNB), and C60 fullerene at different charge states. Our results denote that nanoscaled 2D global aromatics mimic the behaviour of the most prototypical aromatic 6π-circuit, given by benzene, where the shielding cone properties vary along the rotation motion. In contrast, 3D spherical aromatics remain almost invariant under rotation, given the distinctive characteristics of such species, differing from 2D global aromatics. Dissection of orbital contributions reveals that π-orbitals are determinants for shifting from non-aromatic to spherical aromatic species. Under rotation, the variation of the anisotropic effect inherent to such nanoscaled structures is accounted for, which is relevant to rationalize variation in NMR signal shifts upon the formation of host-guest aggregates.

Charge delocalization and aromaticity of doubly reduced double-walled carbon nanohoops.

Cycloparaphenylenes (CPPs) exhibit selective host capabilities, featuring the ability to incorporate smaller CPPs to form double-walled host-guest complexes. Moreover, CPPs can also be stabilized by global aromaticity under twofold oxidation or reduction, involving electronic conjugation along with the overall structural backbone. Herein we explore the structural modifications, bonding, electron delocalization and magnetic properties of doubly reduced double-walled CPP complexes with DFT methods, in the isolated and aggregate [n + 5]CPP⊃[n]CPP2- (n = 5-8) species. Our results show that the hosts undergo structural, bonding and delocalization deformations towards quinoidal configurations and exhibit global long-ranged shielding cones similar to global aromatic free dianionic CPPs, accounting for charge delocalization on the outer nanohoops, whereas the guests preserve local aromatic benzenoid configurations, resulting in global and local aromatic circuits within the host-guest aggregate. This observation suggests that in multi-layered related species electronic delocalization will be retained at the outer structural surface. The aromaticity of the hosts is manifested in the strong upfield shifts of the guests 1H-NMR signals. Hence, CPP complexes can be extended to doubly reduced species stabilized by global host aromaticity expanding our understanding of doubled-walled nanotubes at the nanoscale regime.

Crystallographic evidence for global aromaticity in the di-anion and tetra-anion of a cyclophane hydrocarbon

Anionic forms of a macrocyclic cyclophane were crystallized by treating the neutral hydrocarbon with alkali metals (Li, Na and K). The di-anions show decreased bond-length alternation, consistent with global aromaticity, whereas the anti-aromatic tetra-anion has a low-symmetry geometry.

Infinitene as two fused helicoidal trails of fused rings: evaluation of the magnetic behavior of [12]infinitene and anionic species displaying global aromaticity and antiaromaticity.

The unique formation of an infinity-shaped carbon backbone made exclusively from fused benzene rings has recently been achieved. The structure of [12]infinitene can be viewed as two fused [6]helicene structures with a central crossover section, depicting a global aromatic behavior along with the overall structure, with deshielding regions along both helicoidal axes. In addition, the 13C-NMR characteristics are discussed. The formation of a cumulative region involving the shielding regions from the aromatic rings is depicted along with the overall aesthetically pleasant structural backbone, which is enhanced at the crossover section. For the evaluated dianionic counterpart, the structure shows a deshielding region above the fused-ring trail and a helicoidal shielding region, ascribed to a global antiaromatic counterpart. The aromaticity is recovered and enhanced at the tetranionic state. Thus, the neutral and tetranionic states are able to build up a long-ranged shielding region, given by the global aromatic behavior, with an enhanced shielding region at the center of the crossover section displaying π-π stacked rings.

Synthesis of a π-extended azacoronene composed of naphthalene and acenaphtho[1,2-c]pyrrole toward a chiral curved π structure

[Formula: see text]-Extended azacoronene has been synthesized via an S[Formula: see text]Ar reaction using acenaphthopyrrole and octafluoronaphthalene utilizing successive Scholl reactions. Although the chiral conformers could not be isolated, the new azacoronene analog exhibited longer-wavelength absorption and reversible oxidation properties, which are characteristic of pyrrole-fused azacoronenes. When combined with the results of our DFT calculations, the experimental results indicate weaker global aromaticity than that of previously reported azacoronenes.

Squarephaneic Tetraanhydride: A Conjugated Square-Shaped Cyclophane for the Synthesis of Porous Organic Materials.

Aromatic carboxylic anhydrides are ubiquitous building blocks in organic materials chemistry and have received considerable attention in the synthesis of organic semiconductors, pigments, and battery electrode materials. Here we extend the family of aromatic carboxylic anhydrides with a unique new member, a conjugated cyclophane with four anhydride groups. The cyclophane is obtained in a three-step synthesis and can be functionalised efficiently, as shown by the conversion into tetraimides and an octacarboxylate. Crystal structures reveal the high degree of porosity achievable with the new building block. Excellent electrochemical properties and reversible reduction to the tetraanions are shown for the imides; NMR and EPR measurements confirm the global aromaticity of the dianions and evidence the global Baird aromaticity of the tetraanions. Considering the short synthesis and unique properties, we expect widespread use of the new building block in the development of organic materials.

8]Cyclo-para-phenylmethine as a Super-Cyclooctatetraene: Dynamic Behavior, Global Aromaticity, and Open-Shell Diradical Character in the Neutral and Dicationic States.

An [8]cyclo-para-phenylmethine derivative ([8]CPPM-Mes) was synthesized. X-ray analysis revealed a tub-shaped geometry similar to the cyclooctatetraene, with alternating benzenoid/quinoid structure. Variable-temperature NMR measurements disclosed a slow valence tautomerization process with an interconversion energy barrier of about 11.7 kcal mol-1 at coalescence temperature (273 K), and two more lower-barrier dynamic processes involving flipping of the 1,4-phenyl rings on the backbone and rotation of the mesityl substituents. Its dication ([8]CPPM-Mes2+ ) adopts a bowl-like geometry with a smaller depth of the cavity, and a slow bowl inversion process was observed by dynamic NMR. The bond lengths of the benzenoid/quinoid rings are more averaged via electron delocalization and the molecule shows global aromaticity, which was further validated by NMR and theoretical analysis. [8]CPPM-Mes2+ exhibits open-shell diradical character with a small singlet-triplet energy gap (-1.8 kcal mol-1 ).

8]Cyclo‐para‐phenylmethine as a Super‐Cyclooctatetraene: Dynamic Behavior, Global Aromaticity, and Open‐Shell Diradical Character in the Neutral and Dicationic States

AbstractAn [8]cyclo‐para‐phenylmethine derivative ([8]CPPM‐Mes) was synthesized. X‐ray analysis revealed a tub‐shaped geometry similar to the cyclooctatetraene, with alternating benzenoid/quinoid structure. Variable‐temperature NMR measurements disclosed a slow valence tautomerization process with an interconversion energy barrier of about 11.7 kcal mol−1 at coalescence temperature (273 K), and two more lower‐barrier dynamic processes involving flipping of the 1,4‐phenyl rings on the backbone and rotation of the mesityl substituents. Its dication ([8]CPPM‐Mes2+) adopts a bowl‐like geometry with a smaller depth of the cavity, and a slow bowl inversion process was observed by dynamic NMR. The bond lengths of the benzenoid/quinoid rings are more averaged via electron delocalization and the molecule shows global aromaticity, which was further validated by NMR and theoretical analysis. [8]CPPM‐Mes2+ exhibits open‐shell diradical character with a small singlet‐triplet energy gap (−1.8 kcal mol−1).

A Macrocyclic Furan with Accessible Oxidation States: Switching Between Aromatic and Antiaromatic Global Ring Currents.

Macrocyclic furans are predicted to switch between global aromaticity and antiaromaticity, depending on their oxidation states. However, the macrocyclic furans reported to date are stabilized by electron withdrawing groups, which result in inaccessible oxidation states. To circumvent this problem, a post-macrocyclization approach was applied to introduce methylene-substituted macrocyclic furans, which display an extremely low oxidation potential of -0.23 vs. Fc/Fc+ , and are partially oxidized in ambient conditions. Additional oxidation to the dication results in aromaticity switching to a global 30πe- aromatic state, as indicated by the formation of a strong diatropic current observed in the 1 H NMR spectrum. NICS and ACID calculations support this trend and provide evidence for a different pathway for the global current in the neutral and dicationic states. According to these findings, macrocyclic furans can be rendered as promising p-type materials with stable oxidation states.

Correspondence on “How Aromatic Are Molecular Nanorings? The Case of a Six‐Porphyrin Nanoring”**

AbstractA recent Research Article in this journal by Matito and co‐workers claimed that none of the oxidation states of a butadiyne‐linked six‐porphyrin nanoring exhibit global aromaticity or antiaromaticity. Here we show that this conclusion is incorrect. Experimental data from NMR spectroscopy for a whole family of nanorings provide strong evidence for global ring currents. The NMR data reveal these ring currents directly, without needing analysis by density functional theory (DFT). Furthermore, DFT calculations reproduce the experimental results when a suitable functional is used.

Expanded Azaporphyrins Consisting of Multiple BODIPY Units: Global Aromaticity and High Affinities Towards Alkali Metal Ions.

The one-pot self-coupling reaction of 3-amino-5-bromo-BODIPY 9 under Buchwald-Hartwig amination conditions gave nitrogen-bridged cyclic BODIPY tetramer 10, pentamer 11, and hexamer 12. Oxidation of 10, 11, and 12 in the presence of alkali metal ions provided 13, 15, and 16, respectively, as the first examples of expanded azaporphyrins possessing more than six pyrrole rings. Curiously, the oxidation even in the absence of alkali metal ions gave the same complexes as a result of spontaneous complexation. 10-12 showed large association constants (104 -106 M-1 ) with K+ and Na+ , but the spontaneous complexations were not observed, indicating the extremely high affinities of 13-16 toward alkali metal ions, which can be ascribed to the multiple and effective ion-dipole interaction of the alkali metal ion with the negatively polarized F atoms of the BF2 walls surrounding the cavity. Importantly, 13, 14, and 15 show diatropic ring currents as a result of global aromaticity.

Expanded Azaporphyrins Consisting of Multiple BODIPY Units: Global Aromaticity and High Affinities Towards Alkali Metal Ions

AbstractThe one‐pot self‐coupling reaction of 3‐amino‐5‐bromo‐BODIPY 9 under Buchwald–Hartwig amination conditions gave nitrogen‐bridged cyclic BODIPY tetramer 10, pentamer 11, and hexamer 12. Oxidation of 10, 11, and 12 in the presence of alkali metal ions provided 13, 15, and 16, respectively, as the first examples of expanded azaporphyrins possessing more than six pyrrole rings. Curiously, the oxidation even in the absence of alkali metal ions gave the same complexes as a result of spontaneous complexation. 10–12 showed large association constants (104–106 M−1) with K+ and Na+, but the spontaneous complexations were not observed, indicating the extremely high affinities of 13–16 toward alkali metal ions, which can be ascribed to the multiple and effective ion–dipole interaction of the alkali metal ion with the negatively polarized F atoms of the BF2 walls surrounding the cavity. Importantly, 13, 14, and 15 show diatropic ring currents as a result of global aromaticity.

Planar π-extended cycloparaphenylenes featuring an all-armchair edge topology.

The [n]cycloparaphenylenes ([n]CPPs)-n para-linked phenylenes that form a closed-loop-have attracted substantial attention due to their unique cyclic structure and highly effective para-conjugation leading to a myriad of fascinating electronic and optoelectronic properties. However, their strained topology prevents the π-extension of CPPs to convert them either into armchair nanobelts or planarized CPP macrocycles. Here we successfully tackle this long-standing challenge and present the bottom-up synthesis and characterization of atomically precise in-plane π-extended [12]CPP on Au(111) by low-temperature scanning probe microscopy and spectroscopy combined with density functional theory. The planar π-extended CPP is a nanographene with an all-armchair edge topology. The exclusive para-conjugation at the periphery yields delocalized electronic states and the planarization maximizes the overlap of p orbitals, which both reduce the bandgap compared to conventional CPPs. Calculations predict ring currents and global aromaticity in the doubly charged system. The intriguing planar ring topology and unique electronic properties make planar π-extended CPPs promising quantum materials.