Aromaticity of the completely annelated tetraphenylenes: NICS and GIMIC characterization

Construction of a tetrabenzotetrathia[8]circulene by a "fold-in" oxidative fusion reaction: synthesis and optical properties

Herein we report the first synthesis of a tetrabenzotetrathia[8]circulene by a "fold-in" type oxidative fusion reaction. Compared to the pristine tetrathia[8]circulene, the four-fold benzoannulation slightly weakened the antiaromatic character of the central COT ring. The tetrabenzotetrathia[8]circulene exhibited fluorescence at room temperature, and phosphorescence at 77 K with a phosphorescence quantum yield of 11.7%.

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.

Synthesis of Polycyclic Arenes Composed of Four-, Five-, Six-, and Eight-Membered Rings via an Unexpected Four-Membered Ring Formation Reaction

The synthesis of polycyclic arenes composed of four-, five-, six-, and eight-membered rings via an unexpected four-membered ring formation reaction is reported. The carbonylation of an octalithiated tetraphenylene yielded a tricarbonylated arene containing a four-membered ring or a tetracarbonylated one, depending on the carbonyl reagents. The structures were determined by X-ray crystallography, and their electronic properties were examined by using absorption spectroscopy and cyclic voltammetry. The antiaromaticity of the four-, five-, and eight-membered rings of these compounds was studied by theoretical calculations.

From six to eight Π-electron bare rings of group-XIV elements and beyond: can planarity be deciphered from the "quasi-molecules" they embed?

Ab initio molecular orbital theory is used to study the structures of six and eight π-electron bare rings of group-XIV elements, and even larger [n]annulenes up to C₁₈H₁₈, including some of their mono-, di-, tri-, and tetra-anions. While some of the above rings are planar, others are nonplanar. A much spotlighted case is cyclo-octatetraene (C₈H₈), which is predicted to be nonplanar together with its heavier group-XIV analogues Si₈H₈ and Ge₈H₈, with the solely planar members of its family having the stoichiometric formulas C₄Si₄H₈ and C₄Ge₄H₈. A similar situation arises with the six π-electron bare rings, where benzene and substituted ones up to C₃Si₃H₆ or so are planar, while others are not. However, the explanations encountered in the literature find support in ab initio calculations for such species, often rationalized from distinct calculated features. Using second-order Møller-Plesset perturbation theory and, when affordable (particularly tetratomics, which may allow even higher levels), the coupled-cluster method including single, double, and perturbative triple excitations, a common rationale is suggested based on a novel concept of quasi-molecules or the (3+4)-atom partition scheme. Any criticism of tautology is therefore avoided. The same analysis has also been successfully applied to even larger [n]annulenes, to their mixed family members involving silicon and germanium atoms, and to the C₁₈ carbon ring. Furthermore, it has been extended to annulene anions to check the criteria of the popular Hückel rule for planarity and aromaticity. Exploratory work on cycloarenes is also reported. Besides a partial study of the involved potential energy surfaces, equilibrium geometries and harmonic vibrational frequencies have been calculated anew, for both the parent and the actual prototypes of the quasi-molecules.

Theoretical study on the structures, electronic properties, and aromaticity of thia[4]circulenes

The structures, electronic properties, and aromaticity of a series of thia[4]circulenes are predicted using quantum chemical calculations.

Aromaticity of Heterocirculenes

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

Topological Analysis of Magnetically Induced Current Densities in Strong Magnetic Fields Using Stagnation Graphs

Stagnation graphs provide a useful tool to analyze the main topological features of the often complicated vector field associated with magnetically induced currents. Previously, these graphs have been constructed using response quantities appropriate for modest applied magnetic fields. We present an implementation capable of producing these graphs in arbitrarily strong magnetic fields, using current-density-functional theory. This enables us to study how the topology of the current vector field changes with the strength and orientation of the applied magnetic field. Applications to CH4, C2H2 and C2H4 are presented. In each case, we consider molecular geometries optimized in the presence of the magnetic field. The stagnation graphs reveal subtle changes to this vector field where the symmetry of the molecule remains constant. However, when the electronic state and symmetry of the corresponding equilibrium geometry changes with increasing field strength, the changes to the stagnation graph are extensive. We expect that the approach presented here will be helpful in interpreting changes in molecular structure and bonding in the strong-field regime.

Dianthracenylazatrioxa[8]circulene: Synthesis, Characterization and Application in OLEDs

A soluble, green-blue fluorescent, π-extended azatrioxa[8]circulene was synthesized by oxidative condensation of a 3,6-dihydroxycarbazole and 1,4-anthraquinone by using benzofuran scaffolding. This is the first circulene to incorporate anthracene within its carbon framework. Solvent-dependent fluorescence and bright green electroluminescence accompanied by excimer emission are the key optical properties of this material. The presence of sliding π-stacked columns in the single crystal of dianthracenylazatrioxa[8]circulene is found to cause a very high electron-hopping rate, thus making this material a promising n-type organic semiconductor with an electron mobility predicted to be around 2.26 cm²  V^-1  s^-1 . The best organic light-emitting diode (OLED) device based on the dianthracenylazatrioxa[8]circulene fluorescent emitter has a brightness of around 16 000 Cd m^-2 and an external quantum efficiency of 3.3 %. Quantum dot-based OLEDs were fabricated by using dianthracenylazatrioxa[8]circulene as a host matrix material.

Hetero[8]circulenes: synthetic progress and intrinsic properties

Synthetic route for hetero[8]circulenes are classified into the annulative construction of eight-membered ring and the peripheral ring fusion of eight-membered ring. Post-transformation enables the tuning of structural features and electronic nature.

Structure, stability and electronic properties of one-dimensional tetrathia- and tetraselena[8]circulene-based materials: a comparative DFT study

Computations reveal how the electronic and optical properties can be controlled in nanostructures.

Aromaticity and photophysics of tetrasila- and tetragerma-annelated tetrathienylenes as new representatives of the hetero[8]circulene family

The electronic structure, absorption and emission spectra, aromaticity and photophysical behavior of the recently synthesized tetrasilatetrathia[8]circulene and tetragermatetrathia[8]circulene compounds have been studied computationally.

Impact of heteroatoms (S, Se, and Te) on the aromaticity of heterocirculenes

Computations reveal the structural and energetic aspects of aromaticity in heterocirculenes.

Relations between the aromaticity and magnetic dipole transitions in the electronic spectra of hetero[8]circulenes

Magnetically induced current densities have been calculated at the second-order Møller-Plesset perturbation theory (MP2) level for seven hetero[8]circulenes and their dicationic and dianionic forms. Calculations of the magnetic dipole transition moments have also been carried out at the algebraic diagrammatic construction (ADC(2)) and the second-order approximate coupled-cluster (CC2) levels. The calculations show that the degree of aromaticity and the size of the magnetic dipole transition moment of the lowest magnetic-dipole allowed excited state are related. We show that neutral hetero[8]circulenes are weakly antiaromatic when the first excited state with a large magnetic dipole transition moment of 10-16 a.u. lies at high energies (∼2.8-3.5 eV). For the dications, this transition often lies at much lower energies. Hetero[8]circulene dications with large magnetic dipole transition moments are strongly antiaromatic. The lowest excited states of the hetero[8]circulene dianions have very small magnetic dipole transition moments implying that they are aromatic.

Chemistry and Properties of Indolocarbazoles

The indolocarbazoles are an important class of nitrogen heterocycles which has evolved significantly in recent years, with numerous studies focusing on their diverse biological effects, or targeting new materials with potential applications in organic electronics. This review aims at providing a broad survey of the chemistry and properties of indolocarbazoles from an interdisciplinary point of view, with particular emphasis on practical synthetic aspects, as well as certain topics which have not been previously accounted for in detail, such as the occurrence, formation, biological activities, and metabolism of indolo[3,2- b]carbazoles. The literature of the past decade forms the basis of the text, which is further supplemented with older key references.

A study of the aromaticity of heteroannelated cyclooctatetraene derivatives

The aromaticity of the rings of thiophene, pyrrole, furan, and benzene annelated cyclooctatetraene (COT) derivatives and of their double charged ions was studied using the graph-theoretical theory of aromaticity. On the basis of topological resonance energy, it was found that the global aromaticity is dependent upon on the arrangement of heteroatoms in the given molecule. Relative stability of these molecules when in different charged states can been explained in terms of the topological charge stabilization rule. We expect that fusing the COT ring with an increasing number of aromatic rings will lead to an increase in the aromaticity of the molecule. According to the bond resonance energy (BRE) and circuit resonance energy (CRE) indices, local antiaromaticity of the COT ring is weakened as the number of fused rings increases, and these changes play a significant role in the global aromaticity of the molecule. For some compounds, our BRE and CRE indices do not predict the same order of magnitude of the local aromatic character of certain rings that the nucleus independent chemical shift (NICS(0) and (NICS(1)) methods predict. Finally, for the available compounds, correlations between the diatropic and paratropic chemical shifts of the protons and our ring current results were analyzed and good agreement was found.

A computational study of aromaticity and photophysical properties of unsymmetrical azatrioxa[8]circulenes

In the present work we have studied a series of unsymmetrical azatrioxa[8]circulenes in order to explain the impact of outer substituents and benzoannelation on photophysical constants and aromaticity of these compounds.

Ab initio investigation of electric and magnetic dipole electronic transitions in the complex of oxygen with benzene

The electric dipole transitions between pure spin and mixed spin electronic states are calculated at the XMC-QDPT2 and MCSCF levels of theory, respectively, for different intermolecular distances of the C6H6 and O2 collisional complex. The magnetic dipole transition moment between the mixed-spin ground ("triplet") and the first excited ("singlet") states is calculated by quadratic response at MCSCF level of theory. The obtained results confirm the theory of intensity borrowing and increasing the intensity of electronic transitions in the C6H6 + O2 collision. The calculation of magnetically induced current density is performed for benzene molecule being in contact with O2 at the distances from 3.5 to 4.5 Å. The calculation shows that the aromaticity of benzene is rising due to the conjugation of π-MOs of both molecules. The C6H6 + O2 complex becomes nonaromatic at the short distances (r < 3.5 Å). The computation of static polarizability in the excited electronic states of the C6H6 + O2 collisional complex at various distances supports the theory of red solvatochromic shift of the a → X band. Graphical abstract The C6H6+ O2 collisional complex.

Aromaticity of the doubly charged [8]circulenes

Magnetically induced current densities and current pathways have been calculated for a series of fully annelated dicationic and dianionic tetraphenylenes, which are also named [8]circulenes.

Benzoannelated aza-, oxa- and azaoxa[8]circulenes as promising blue organic emitters

In the present study, we studied the synergetic effect of benzoannelation and NH/O-substitution for enhancing the absorption intensity in a series of novel designed benzoannelated aza- and oxa[8]circulenes.

A DFT and QTAIM study of the novel d-block metal complexes with tetraoxa[8]circulene-based ligands

The transition metal complexes with the tetraoxa[8]circulene-based ligand are designed. The complexation process strictly depends on matching of the d-metal cation size to the size of the tetraoxa[8]circulene formed cavity.