Renaissance of Annulene Chemistry

How do the Hückel and Baird Rules Fade away in Annulenes?

Two of the most popular rules to characterize the aromaticity of molecules are those due to Hückel and Baird, which govern the aromaticity of singlet and triplet states. In this work, we study how these rules fade away as the ring structure increases and an optimal overlap between p orbitals is no longer possible due to geometrical restrictions. To this end, we study the lowest-lying singlet and triplet states of neutral annulenes with an even number of carbon atoms between four and eighteen. First of all, we analyze these rules from the Hückel molecular orbital method and, afterwards, we perform a geometry optimization of the annulenes with several density functional approximations in order to analyze the effect that the distortions from planarity produce on the aromaticity of annulenes. Finally, we analyze the performance of three density functional approximations that employ different percentages of Hartree-Fock exchange (B3LYP, CAM-B3LYP and M06-2X) and Hartree-Fock. Our results reveal that functionals with a low percentage of Hartree-Fock exchange at long ranges suffer from severe delocalization errors that result in wrong geometrical structures and the overestimation of the aromatic character of annulenes.

Global aromaticity at the nanoscale

Aromaticity can be defined by the ability of a molecule to sustain a ring current when placed in a magnetic field. Hückel’s rule states that molecular rings with [4n+2] π-electrons are aromatic, with an induced magnetisation that opposes the external field inside the ring, whereas those with 4n π-electrons are antiaromatic, with the opposite magnetisation. This rule reliably predicts the behaviour of small molecules, typically with fewer than 22 π-electrons (n = 5). It is not clear whether aromaticity has a size limit, or whether Hückel’s rule extends to much larger macrocycles. Here, we present evidence for global aromaticity in porphyrin nanorings with circuits of up to 162 π-electrons (n = 40); aromaticity is controlled by changing the constitution, oxidation state and conformation. Whenever a ring current is observed, its direction is correctly predicted by Hückel’s rule. The largest ring currents occur when the porphyrins units have fractional oxidation states.

Which isomer is it, 1,2,5,6- or 1,4,5,8-tetrasubstituted cycloocta-1,3,5,7-tetraene? Synthesis of symmetrically tetrasubstituted cycloocta-1,3,5,7-tetraene derivatives

Ni(0) catalyzed cyclotetramerization [(2 + 2 + 2 + 2) cycloaddition] of propargyl alcohol gave D2 symmetric 1,4,5,8-tetrakis(hydroxymethyl)cycloocta-1,3,5,7-tetraene (1) as the major product contrary to what was reported earlier to be the corresponding C2v symmetric isomer, namely 1,2,5,6-tetrakis(hydroxymethyl)cycloocta-1,3,5,7-tetraene. The structure of 1 has been unambiguously assigned on the basis of single crystal XRD data which showed a very interesting "densely" intermolecularly hydrogen bonded solid state supramolecular structure. Tetrabromide (3) prepared from tetra-alcohol (1) reacted with a variety of nucleophiles, namely carbon, nitrogen, sulfur and phosphorus nucleophiles, to give the corresponding derivatives in good yields. The tetraphosphonate derivative (9) underwent the Wittig-Horner reaction with aromatic aldehydes to give the corresponding tetra-styryl derivatives (10a-i). The reaction of tetrabromide with aniline, benzyl amine and R(+)-α-methylbenzyl amine gave the corresponding C2v symmetric pyrroline fused 1,2,5,6-tetra-substituted cycloocta-1,3,5,7-tetraene derivatives, while all the other derivatives belonged to D2 symmetric 1,4,5,8-tetra-substituted cycloocta-1,3,5,7-tetraene. Quantum mechanical calculations support the above conclusion based on the relative stabilities of the two isomers. The fusion of a 5-membered pyrroline ring with the tub-shaped cyclooctatetraene (COT) ring leads to flattening of the tub (shallow tub) making it easier for the ring inversion of COT through a planar intermediate to take place readily resulting in the formation of C2v symmetric pyrroline fused COT derivatives.

Aromatic and Antiaromatic Pathways in Triphyrin(2.1.1) Annelated with Benzo[b]heterocycles

Understanding of the aromatic properties and magnetically induced current densities of highly conjugated chromophores is important when designing molecules with strongly delocalized electronic structure. Linear extension of the triphyrin(2.1.1) skeleton with an annelated benzo[b]heterocycle fragment modifies the aromatic character by extending the electron delocalization pathway. Two-electron reduction leads to an antiaromatic triphyrin(2.1.1) ring and an aromatic benzo[b]heterocycle subunit. Current-density calculations provide detailed information about the observed pathways and their strengths.

Shape-Persistent π-Conjugated Macrocycles with Aggregation-Induced Emission Property: Synthesis, Mechanofluorochromism, and Mercury(II) Detection

Shape-persistent conjugated macrocycles are fundamentally important because of their unique structure and properties. Herein, a series of π-conjugated macrocycles with a shape-persistent architecture, an adaptive backbone, and aggregation-induced emission (AIE) properties are synthesized via oxidative coupling of acetylene-terminated tetraphenylethylene precursor with a half-ring topology and following transformation from butadiynylene linkers into thienylene ones. Characterization by NMR spectroscopy and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry provided unambiguous proofs for the macrocyclic structures. In particular, the free rotation of aromatic rings in the rigid macrocyclic backbone was validated by two-dimensional NMR spectroscopy, variable-temperature NMR measurements, and theoretical calculations. Moreover, these shape-persistent macrocyclic chromophores all exhibited obvious AIE phenomena and remarkable mechanofluorochromism behaviors with a red-shifted luminescence upon grinding and blue-shifted emission after solvent annealing. Also, the introduction of S atoms into the macrocyclic frameworks endowed the macrocyclic luminogen the capability to selectively detect mecury(II) ions in aqueous media among other metal ions.

C-N Coupling via Antiaromatic Endocyclic Nitrenium Ions

Herein, we report a C-N coupling reaction via antiaromatic endocyclic nitrenium ions. The nitrenium ion intermediate was generated by a combination of iodine(III) reagent PhI(OCOCF₃)₂ and the N-H center of benzimidazole units at ambient laboratory conditions. Metal-free synthesis of benzimidazole-fused phenanthridine derivatives was achieved in good to excellent yields.

Global Aromaticity in Macrocyclic Polyradicaloids: Hückel's Rule or Baird's Rule?

Aromaticity is one of the most important concepts in organic chemistry to understand the electronic properties of cyclic π-conjugated molecules. Over a century, different aromaticity rules have been developed and validated. For planar monocyclic conjugated polyenes (also known as [n]annulenes), they will be aromatic if they contain [4N + 2] π electrons according to Hückel's rule, or antiaromatic if they have [4N] π electrons. Topological change from a planar to a half-twisted Möbius strip will lead to [4N] ([4N + 2]) aromaticity (antiaromaticity), which is just inverse to Hückel's rule. When the molecules are excited into the first triplet excited state, the Hückel (anti)aromaticity observed in the ground state will become reversed according to Baird's rule. Strictly speaking, these basic rules are only applicable for monocyclic conjugated systems, but some polycyclic systems such as porphyrinoids may also follow these rules if there is a dominant [n]annulene-like conjugation pathway. On the other hand, all-benzenoid polycyclic aromatic hydrocarbons usually display local aromaticity with π electrons predominantly localized at certain benzene rings according to Clar's aromatic sextet rule. In recent years, some proaromatic and antiaromatic molecules with even number of paired electrons have been found to exhibit open-shell diradical character and unique optical, electronic, and magnetic activities. One of the major driving forces is their intrinsic tendency to become aromatic in the open-shell diradical/polyradical forms. A number of stable diradicaloids and linear polyradicaloids have been successfully synthesized by using thermodynamic and kinetic stabilizing strategies. Herein, our particular interest is a type of macrocyclic polyradicaloid in which multiple frontier π-electrons are antiferromagnetically coupled with each other in a cyclic mode. Formally, these free electrons may behave like normal π-electrons in the [n]annulenes, and thus, it raises questions about their possible global aromaticity and which rule they will follow. In the past 5 years, our group has synthesized a series of macrocyclic polyradicaloids and systematically investigated their global aromaticity and electronic properties. Some important findings include: (1) global (anti)aromaticity is generally observed, but there is a balance between local aromaticity and global aromaticity; (2) most of these molecules follow Hückel's rule in the singlet state and display respective (anti)aromatic characteristics; (3) in some special cases, both Hückel's rule and Baird's rule can be applicable, and a unique annulene-within-an-annulene super-ring structure was demonstrated for the first time; (4) global antiaromaticity in the transition state is also important and a slow valence tautomerization process was observed in a supercyclobutadiene tetraradicaloid. These studies demonstrate how these open-shell macrocyclic polyradicaloids adapt their geometry and spin state to reach the lowest-energy state (aromatic). In this Account, we will mainly discuss their synthesis, global aromaticity, and the fundamental structure-radical character-aromaticity-properties relationships. Various experimental methods (e.g., NMR, X-ray crystallographic analysis, and electronic absorption spectroscopy) and theoretical calculations (e.g., anisotropy of the induced current density, nucleus independent chemical shift, and isochemical shielding surface) have been used to elaborate their (anti)aromatic character. At the end, a perspective on the possible three-dimensional global aromaticity in fully conjugated cagelike diradicaloids or polyradicaloids will be also discussed.

Cyclobis(7,8-(para-quinodimethane)-4,4'-triphenylamine) and Its Cationic Species Showing Annulene-Like Global (Anti)Aromaticity

π-Conjugated macrocycles containing all-benzenoid rings usually show local aromaticity, but reported herein is the macrocycle CBQT, containing alternating para-quinodimethane and triphenylamine units displaying annulene-like anti-aromaticity at low temperatures as a result of structural rigidity and participation of the bridging nitrogen atoms in π-conjugation. It was easily synthesized by intermolecular Friedel-Crafts alkylation followed by oxidative dehydrogenation. X-ray crystallographic structures of CBQT, as well as those of its dication, trication, and tetracation were obtained. The dication and tetracation exhibited global aromaticity and antiaromaticity, respectively, as confirmed by NMR measurements and theoretical calculations. Both the dication and tetracation possess open-shell singlet ground states, with a small singlet-triplet gap.

Synthesis of a Macrocycle Bearing a Carbon Framework of [16]Cyclophenacene as a Carbon Nanobelt

A synthetic pathway to a functionalized tetrahydro[5]phenacene was developed, which served as a precursor, leading to a dehydrobenzo[32]annulene macrocycle containing four carbon-carbon triple bonds. The high efficiency of the macrocyclization step can be attributed to the structural rigidity of its immediate precursor. Hydrogenation of the four carbon-carbon triple bonds produced a macrocycle bearing a carbon framework of [16]cyclophenacene as the shortest macrocyclic belt-like structure of an (8,8)armchair carbon nanotube.

A study on the aromatic conjugation pathways and the ring currents of bridged [18]annulenes

In this article, the topological resonance energy (TRE) method was used to investigate the global aromaticity of a set of [18]annulene-derived compounds which were obtained by replacing either two, four, or all six of the inner hydrogen atoms of [18]annulene with bridges (oxygen, imino-, sulfur, or combinations of the three). Our TRE results show that all the mono-bridged, di-bridged, and tri-bridged [18]annulenes are globally aromatic systems with positive TREs and show relatively larger aromaticity in comparison with the [18]annulene. The aromaticity of each compound was explained using the topological charge stabilization (TCS) rule. The bond resonance energy (BRE) and circuit resonance energy (CRE) methods were used to evaluate local aromaticity. Our BRE and CRE results show that incorporation of five-membered heterocyclic rings changes the main conjugation pathway of the bridged [18]-annulenes. The local aromaticities arising from the five-membered heterocyclic rings (6π) contribute strongly to global aromaticity. However, the ring current (RC), which arises from the 18π annulene-like pathway structures, is the primary determinant of the magnetic properties of the molecule. Our RC results are in good agreement with available ¹H-NMR chemical shift data.

The topological resonance energy method was used to investigate the global aromaticity of a set of [18]annulene-derived compounds obtained by replacing two, four, or all six of the inner hydrogen atoms with bridges (oxygen, imino-, sulfur, or combinations of the three).

Spectroscopic characterisation of centropolyindanes

A highly promising class of three-dimensional polyaromatic hydrocarbons comprises the centropolyindanes. The characteristic feature of these compounds is the mutual fusion of several molecules of indane along the saturated C-C bonds of their cyclopentane rings. Among the polycyclic aromatic hydrocarbons, the centropolyindanes are special because of the saturated core of sp3-hybridised carbon atoms embedded in a three-dimensional environment of aromatic building blocks. While the centropolyindanes and their numerous derivatives have been studied in detail by NMR spectroscopy, mass spectrometry and X-ray diffraction, investigation of their vibrational features, and especially those of the neopentane core present in most cases, have not been performed so far. In the present paper, we report the first systematic study of a set of centropolyindanes by vibrational spectroscopy, using inelastic neutron scattering (INS), infrared and Raman spectroscopies.

Phospha- and arsa-bridged cyclononatetraenides: novel zwitterionic 10π aromatic hemispheres

To probe the limits of the internal charge compensation approach for the generation of 10π aromatic hemispheres derived from hetero-acepentalenide zwitterions, the phospha- and arsa-bridged cyclononatetraenides were studied computationally by DFT at the B3LYP/6-311++G(d,p) level, and compared with the known aza-analog.

Carbolong Chemistry: A Story of Carbon Chain Ligands and Transition Metals

The construction of metal-carbon bonds is one of the most important issues of organometallic chemistry. However, the chelation of polydentate ligands to a metal via several metal-carbon bonds is rare. Metallapentalyne, which can be viewed as a 7-carbon (7C) chain coordinated to a metal via three metal-carbon bonds, was first reported in 2013. Although metallapentalyne contains a metal-carbon triple bond in a five-membered ring (5MR) and the bond angle around the carbyne carbon is only 129.5°, metallapentalyne exhibits excellent stability to air, moisture, and heat. Metallapentalyne possesses the rare planar Möbius aromaticity, which is in sharp contrast to the Hückel antiaromaticity in pentalyne. The metal fragment not only relieves the large ring strain present in pentalyne but also results in the transformation of the antiaromaticity in pentalyne to aromaticity in metallapentalyne. With the extension of the carbon chain from 7 to 12 carbon atoms, a series of novel polycyclic frameworks were constructed via the formation of several metal-carbon bonds. Some interesting phenomena were observed for these complexes. For instance, (1) the carbyne carbon of the 7C framework could react with both nucleophilic and electrophilic reagents, leading to the formation of 16- and 18-electron metallapentalenes; (2) σ aromaticity was first observed in an unsaturated system in the 8C framework; (3) two classical antiaromatic frameworks, cyclobutadiene and pentalene, were simultaneously stabilized in the 9C framework for the first time; (4) three fused 5MRs bridged by a metal are coplanar in the 10C framework; (5) the first [2 + 2 + 2] cycloaddition of a late transition metal carbyne complex with alkynes was realized during the construction of an 11C framework; (6) the largest number of carbon atoms coordinated to a metal atom in the equatorial plane was observed in the 12C framework; and (7) sharing of the transition metal by multiple aromatic units has seldom been observed in the metalla-aromatics. Therefore, the term carbolong chemistry has been used to describe the chemistry of these novel frameworks. More interestingly, carbolong complexes exhibit diverse properties, which could lead to potential future applications. As the discovery and creation of molecular fragments lead to advancements in chemistry, medical science, and materials chemistry, these novel polydentate carbon chain chelates might have important influences in these fields due to their facile synthesis, high stability, and unique properties.

Molecular dynamics investigation of the mechanical behavior of multi-layered graphyne and its family under tensile loading

This paper aims to study the mechanical properties of the multi-layered graphyne and other members of the graphyne family under the uniaxial tensile loading. For this purpose, molecular dynamics simulations are used. The effects of the size and number of layers on the fracture and elastic properties are studied. It is shown that Young's modulus of the zigzag multi-layered graphyne is slightly larger than armchair one. Comparing the stress-strain curves of the multi-layered graphynes with different number of layers, it is observed that the fracture stress and strain of the nanosheets are inversely related to the number of layers. Investigating the influence of the number of acetylene linkage in the structure of the graphyne-n family on their mechanical properties, it is shown increasing the number of triple bonds leads to weakening the fracture stress and Young's modulus of the nanosheet.

Effective stabilization of a planar phosphorus(iii) center embedded in a porphyrin-based fused aromatic skeleton

Planar phosphorus(iii) centers were effectively stabilized by structural constraint as well as aromatic stabilization in porphyrin-based fused π-conjugated frameworks.

Organophosphorus(iii) compounds usually take on stable pyramidal structures with a large inversion barrier of 30–35 kcal mol^–1. In contrast, diphenylphosphine-fused Ni(ii) porphyrin, where the phosphorus atom is directly attached at the meso-position and embedded in a rigid skeleton, exhibits a considerably planarized phosphorus center. Here we report the synthesis of a mesityl-substituted Ni(ii) porphyrin analogue, 6, which allowed an evaluation of the inversion barrier (ΔG‡203) by variable temperature ¹H NMR spectroscopy which showed it to be exceptionally small, at 14.0 kcal mol^–1. The observed small inversion barrier has been attributed to conformational constraint imposed by the fused structure. In addition, it was thought that the planar transition state is stabilized by the Ni(ii) porphyrin network that allows the contribution of a 22π-aromatic circuit involving phosphorus lone-pair electrons. Along this postulate, we attempted to engineer diarylphosphine-fused porphyrins with smaller inversion barriers by replacing the fused benzene rings with five-membered heterocyclic rings such as thiophene, benzothiophene, benzofuran, indole, benzothiophene 1,1-dioxide, and thiophene 1,1-dioxide. In that order, the aromatic character of the heterocycle decreases, which leads to increasing contribution of the 22π-aromatic circuit. Actually, the inversion barrier of the phosphorus center becomes smaller in this order and reaches 8.7 kcal mol^–1 for thiophene 1,1-dioxide-fused Ni(ii) porphyrin 15, supporting the postulate.

Single-Acetylene Linked Porphyrin Nanorings

The synthesis of ethyne-linked porphyrin nanorings has been achieved by template-directed Sonogashira coupling. The cyclic hexamer and octamer are predicted by density functional theory to adopt low symmetry conformations, due to dihedral twists between neighboring porphyrin units, but their symmetries are effectively D_6h and D_8h, respectively, in solution by ¹H NMR. The fluorescence spectra indicate that the singlet excited states of these nanorings are highly delocalized.

A Zwitterionic, 10 π Aromatic Hemisphere

A new concept in anionic 10 π aromaticity is described by the embedding of a compensating charge within an aromatic cyclononatetraenide ring by the symmetric superposition of an alkyl ammonium bridge. This is accomplished by the methylation of azatriquinacene to give a quaternary ammonium salt, followed by oxidation to the tetraene and final deprotonation. The resulting zwitterion is a stable [9]annulene with strong aromaticity as shown by its degree of C-C bond equalization and a nucleus-independent chemical shift value lower than that of benzene. The solid-state structure shows an eclipsed stacking motif with the electron-poor ammonium methyl groups occupying the electron-rich cavity of the aromatic bowl.

Energetics of Baird aromaticity supported by inversion of photoexcited chiral [4n]annulene derivatives

For the concept of aromaticity, energetic quantification is crucial. However, this has been elusive for excited-state (Baird) aromaticity. Here we report our serendipitous discovery of two nonplanar thiophene-fused chiral [4n]annulenes Th4 COT Saddle and Th6 CDH Screw , which by computational analysis turned out to be a pair of molecules suitable for energetic quantification of Baird aromaticity. Their enantiomers were separable chromatographically but racemized thermally, enabling investigation of the ring inversion kinetics. In contrast to Th6 CDH Screw , which inverts through a nonplanar transition state, the inversion of Th4 COT Saddle , progressing through a planar transition state, was remarkably accelerated upon photoexcitation. As predicted by Baird's theory, the planar conformation of Th4 COT Saddle is stabilized in the photoexcited state, thereby enabling lower activation enthalpy than that in the ground state. The lowering of the activation enthalpy, i.e., the energetic impact of excited-state aromaticity, was quantified experimentally to be as high as 21-22 kcal mol-1.Baird's rule applies to cyclic π-conjugated molecules in their excited state, yet a quantification of the involved energetics is elusive. Here, the authors show the ring inversion kinetics of two nonplanar and chiral [4n]annulenes to support Baird's rule from an energetic point of view.

Explorations of the Indenofluorenes and Expanded Quinoidal Analogues

Highly conjugated hydrocarbons have attracted interest for use as active materials in electronic devices such as organic field effect transistors (OFET) and organic photovoltaics (OPV). In this Account, we review our progress in synthesizing and studying a new class of small molecules for potential use as organic semiconductors. The idea originated from prior research on octadehydrodibenz[12]annulene, as the system can undergo double transannular cyclization to yield the indeno[1,2-b]fluorene skeleton. Subsequent functionalization afforded the first stable, well-characterized indeno[1,2-b]fluorene derivatives, albeit in minute quantities (tens of milligrams). The preparation of these formally antiaromatic compounds has since been optimized: the new synthetic routes utilize inexpensive starting materials, involve robust and high-yielding transformations, and are amenable to considerably larger scale reaction. We have since researched the chemical space of indeno[1,2-b]fluorenes and related quinoidal structures by substitution with a number of functional groups and by permutation of the indenofluorene scaffold. These modifications have allowed us to explore fundamental concepts such as biradical character and antiaromaticity, important considerations when tuning electronic properties to yield functional organic materials. Altering the outer rings by exchange of carbocycles for heterocycles or by inclusion of additional rings as part of the fully conjugated skeleton is one strategy we have examined. Fusing these different aryl groups to s-indacene revealed a dependence of the antiaromaticity of the indacene core upon the outer group. Computational analysis of a series of indeno[1,2-b]fluorene derivatives uncovered an array of different levels of antiaromaticity in the core of the indeno[1,2-b]fluorene derivatives, with one of the benzothiophene derivatives calculated to be as antiaromatic as the parent s-indacene itself. Conversely, we have prepared compounds with expanded cores, starting with the naphthalene-based fluoreno[4,3-c]fluorene, which was produced through a similar route as the indeno[1,2-b]fluorene, using a dione as the key intermediate. Similar to indeno[1,2-b]fluorene, fluoreno[4,3-c]fluorene showed a closed shell ground state, with no evidence of open shell character even upon heating to 170 °C. Increasing the size of the quinoidal core to three rings afforded a diindeno[b,i]anthracene (DIAn) derivative, a compound with a much more complex electronic picture. To produce DIAn, a new synthetic route was devised involving a Friedel-Crafts alkylation to form the five-membered ring and a DDQ oxidation to produce the final compound. DIAn displayed NMR signals that were broadened at room temperature and disappeared when heated, indicative of a molecule with significant biradical character. Extensive computational and experimental investigation verified the controllable expression of its biradical character, with DIAn best described with a ground state that lies between a closed shell compound and a open-shell singlet biradical, with ready access to a thermally populated triplet state.

Implications of the final ring closure to 10b-aza-10c-borapyrene for aryl–alkyne ring-closing mechanisms

Through a combined computational and isotopic labeling study, it has been observed that the activation energy for the aryl–alkyne ring closure of the azaborine containing 4-ethynyl-4a-aza-4b-boraphenanthrene is dramatically lower, and it appears to proceed via an alternate mechanism than that of its hydrocarbon analog. This catalyst-free reaction proceeds at modest reaction conditions compared with traditional pyrolytic synthetic methods and holds promise for the efficient construction of fused ring systems containing azaborine functional groups that may not be accessible in the purely hydrocarbon form.

Tunable thermal conductivities of graphene and graphyne under in-plane torsion

Thermal conductivities of graphene and graphyne are revealed to be tunable under in-plane circular rotations caused by the evolution of surface morphology using non-equilibrium molecular dynamics method.