Superaromatic Stabilization Energy as a Novel Local Aromaticity Index for Polycyclic Aromatic Hydrocarbons

Assessment of the Performance of the Bond Resonance Energy, Circuit Resonance Energy, Magnetic Resonance Energy, Ring Currents, and Aromaticity of Anthracene, Biphenylene, Phenalenyl, and p-Terphenyl

This paper contributes to the further understanding of bond resonance energy (BRE), circuit resonance energy (CRE), and ring currents (RC = I) as indices for the measure of local ring aromaticity and topological resonance energy (TRE) and magnetic resonance energy (MRE) as indices for the measure of global molecular aromaticity. Calculations of ring currents in the 3-ring polycyclic hydrocarbons of anthracene, biphenylene, phenalenyl, and p-terphenyl are demonstrated. This comparative study provides new molecular insights. A number of shortcut techniques are briefly illustrated. p-Terphenyl is a benzenoid total resonant sextet (TRS) and biphenylene is shown to be a nonbenzenoid TRS, i.e., both obey Clar's sextet principle. p-Terphenyl has only ring circuits, whereas anthracene, biphenylene, and phenalenyl have ring circuits and larger circuits engulfing the ring circuits. The perimeter circuits of anthracene, biphenylene, and phenalenyl engulf all other circuits, thus leading to the definition of MRE. By these several aromatic indices, the monocation, monoanion, and monoradical of phenalenyl are equally aromatic. The agreement in the prediction of local ring aromaticity between the three indices of m-BRE, t-BRE, and CRE is evaluated.

On the Aromatic Stabilization of Fused Polycyclic Aromatic Hydrocarbons

The thermodynamic properties and band gap energies were evaluated for six ortho- and peri-fused polycyclic aromatic hydrocarbons (PAHs): triphenylene; benzo[a]pyrene; benzo[e]pyrene; perylene; benzo[ghi]perylene; coronene. The standard molar enthalpies of formation in the crystalline state and the standard molar enthalpies of sublimation were measured by high precision combustion calorimetry and Knudsen effusion methodology, respectively. The combination of the molar enthalpies of formation in the crystalline state with the respective enthalpies of sublimation was used to evaluate the energetics of the progressive peri-fusion of the aromatic moieties from triphenylene to coronene aiming to investigate the hypothetical superaromaticity character of coronene. The linear trend of the enthalpy of formation in crystalline and gaseous phases in the series (from benzo[e]pyrene to coronene) is an irrefutable indication of a non-superaromaticity character of coronene. High accurate thermodynamic properties of sublimation (volatility, enthalpy, and entropy of sublimation) were derived by the measurement of vapor pressures as a function of temperature, using a Knudsen/quartz crystal effusion methodology. Furthermore, the π-electronic conjugation of these compounds was explored by evaluation of the optical band gaps along with this series of compounds. The morphology of perylene, benzo[ghi]perylene, and coronene thin films, deposited by physical vapor deposition onto transparent conductive oxide substrates (ITO and FTO), was used to analyze the nucleation and growth mechanisms. The morphologies observed were found to be related to the cohesive energy and entropy of the bulk.

Triplet State Baird Aromaticity in Macrocycles: Scope, Limitations, and Complications

The aromaticity of cyclic 4nπ-electron molecules in their first ππ* triplet state (T1), labeled Baird aromaticity, has gained growing attention in the past decade. Here we explore computationally the limitations of T1 state Baird aromaticity in macrocyclic compounds, [n]CM's, which are cyclic oligomers of four different monocycles (M = p-phenylene (PP), 2,5-linked furan (FU), 1,4-linked cyclohexa-1,3-diene (CHD), and 1,4-linked cyclopentadiene (CPD)). We strive for conclusions that are general for various DFT functionals, although for macrocycles with up to 20 π-electrons in their main conjugation paths we find that for their T1 states single-point energies at both canonical UCCSD(T) and approximative DLPNO-UCCSD(T) levels are lowest when based on UB3LYP over UM06-2X and UCAM-B3LYP geometries. This finding is in contrast to what has earlier been observed for the electronic ground state of expanded porphyrins. Yet, irrespective of functional, macrocycles with 2,5-linked furans ([n]CFU's) retain Baird aromaticity until larger n than those composed of the other three monocycles. Also, when based on geometric, electronic and energetic aspects of aromaticity, a 3[n]CFU with a specific n is more strongly Baird-aromatic than the analogous 3[n]CPP while the magnetic indices tell the opposite. To construct large T1 state Baird-aromatic [n]CM's, the design should be such that the T1 state Baird aromaticity of the macrocyclic perimeter dominates over a situation with local closed-shell Hückel aromaticity of one or a few monocycles and semilocalized triplet diradical character. Monomers with lower Hückel aromaticity in S0 than benzene (e.g., furan) that do not impose steric congestion are preferred. Structural confinement imposed by, e.g., methylene bridges is also an approach to larger Baird-aromatic macrocycles. Finally, by using the Zilberg-Haas description of T1 state aromaticity, we reveal the analogy to the Hückel aromaticity of the corresponding closed-shell dications yet observe stronger Hückel aromaticity in the macrocyclic dications than Baird aromaticity in the T1 states of the neutral macrocycles.

Global and local aromaticity of acenes from the information-theoretic approach in density functional reactivity theory

In this work, we report a systematic study on the global and local aromaticity of acenes using a series of model structures from 2-acene to 11-acene. A recently developed ansatz, an information-theoretic approach coached into density functional reactivity theory has been employed, which essentially provides different density functionals characterizing the molecular electron density distribution. Based on the correlation analysis of six conventional aromaticity indices with eight information-theoretic quantities, we examined the aromaticity of acenes from both global and local perspectives. From the global aromaticity viewpoint, our results suggest that different descriptors based on various physicochemical properties are intrinsically dependent. A novel laminated feature ruling local aromaticity of acenes has been unveiled, from which we found that the distance from the terminal rings plays the critical role. Based on the shape of the correlation plots between the conventional aromaticity indices and information-theoretic quantities, the latter could be separated into three subgroups. The seemingly contradictory results from global and local aromaticity perspectives not only present us the uniqueness of the acene systems but all demonstrate the effectiveness of the information-theoretic approach from density functional reactivity theory. Besides strengthening the validity of a series of new aromaticity descriptors, our results should lead to more clear insights into the chemical significance of the information-theoretic quantities.

Aromatic Character of Irregular-Shaped Nanographenes

We found that the Clar sextet formula with the maximum number of sextet rings cannot always be defined meaningfully for large irregular-shaped PAHs. It is true that edge structure is always a primary determinant of the PAH aromaticity pattern. In large PAH molecules, every edge structure modifies the aromaticity pattern near the edge, but its influence fades on going away from the edge. It follows that different textures of the aromaticity pattern appear near different edges. As a result, the entire aromaticity pattern does not always match with a single Clar formula or a single weighted superposed Clar formula. Such an unusual feature of aromaticity patterns could not have been observed distinctly if we had not explored the aromaticity patterns of large irregular-shaped PAH molecules systematically. We used the superaromatic stabilization energy (SSE) as a local aromaticity index, which is the only index of this kind not disturbed by the aromaticity of adjacent benzene rings.

Establishing the pivotal role of local aromaticity in the electronic properties of boron-nitride graphene lateral hybrids

Within an attempt to unravel the conundrum of irregular bandgap variations in hybrids of white-graphene (hBN) and graphene (G) observed in both experiment and theory, strong proofs about the decisive role of aromaticity in their electronic properties are brought to light.

“Carbo-aromaticity” and novel carbo-aromatic compounds

Recent advances in experimental and theoretical studies ofcarbo-benzene derivatives, along with the proposition of a generalization of the definition of aromaticity to the two-membered π-rings of triple bonds, suggest relevance for the notion of “carbo-aromaticity”.

Constrained Clar formulas of coronoid hydrocarbons

Aromatic character of coronoid hydrocarbons is greatly influenced by the shapes of outer and inner peripheries. The most aromatic rings in coronoids are jutting benzene rings on the armchair edges, if any. Clar formulas of many coronoids conform to the aromaticity patterns. However, placement of all aromatic sextets on highly aromatic rings is sometimes forbidden by the presence of the central cavity. The magnitude of aromatic stabilization energy due to macrocyclic conjugation [SSE(mc)] and the NICS(1) value at the center of the cavity strongly depend on the structure of the superposed Clar formula. Localization of π-electrons in fixed aromatic sextets effectively suppresses macrocyclic conjugation. The sign of SSE(mc) is determined by the number of carbon atoms that form the hub cycle.