Why Are the Kinked Polyacenes More Stable than the Straight Ones? A Topological Study and Introduction of a New Topological Index of Aromaticity

Acenes and phenacenes in their lowest-lying triplet states. Does kinked remain more stable than straight?

The larger stability of phenacenes compared to their acene isomers in their ground states is attributed to the larger aromaticity of the former. To our knowledge the relative stability of acenes and phenacenes in their lowest-lying triplet states (T1) has not been discussed yet. Using unrestricted density functional theory calculations, our results show that for the smallest members of the series, acenes in their T1 states are more stable than the corresponding phenacenes. However, when the number of the rings (n) involved increases, the energy difference is reduced and for n > 12, phenacenes become more stable than acenes in their T1 states. To rationalize this trend, we analyze the aromaticity of acenes and phenacenes using a set of aromaticity descriptors. We find that in the T1 states of both acenes and phenacenes, the outer rings form aromatic Clar π-sextets. In acenes, delocalization of spin density in the central rings leads to the preferred formation of the largest antiaromatic diradical. Resonant structures in the form of antiaromatic diradical Baird π-octadectets and π-tetradectets are the major contributors, while the smaller ones, such as π-doublets and π-sextets, contribute the least. In phenacenes, structures with diradical antiaromatic Baird π-sextets in some of the central rings contribute the most. These results are relevant to understand the (anti)aromaticity of larger polycyclic aromatic hydrocarbons in their triplet states.

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.

Quantitative Structure-Property Relationships for the Electronic Properties of Polycyclic Aromatic Hydrocarbons

This study presents a development in quantitative structure-property relationships (QSPRs) for research in organic semiconductor materials by introducing a new structural descriptor called "degree of π-orbital overlap" based on two-dimensional structure information of aromatic molecules. Application of this method to predict the electronic properties of polycyclic aromatic hydrocarbon (PAH) molecules, which are known to be the core component of many organic semiconductor materials, is presented. Results demonstrated that QSPRs based on the new descriptor can predict rather accurate band gaps, ionization potentials and electron affinities for a large number of PAHs compared to those explicitly calculated by density functional theory method. This research opens new possibilities for developing QSPRs for other organic semiconductor classes in future.

Aromaticity of acenes: the model of migrating π-circuits

In this work we extend the concept of migrating Clar's sextets to explain local aromaticity trends in linear acenes predicted by theoretical calculations and experimental data. To assess the link between resonance and reactivity and to rationalize the constant-height AFM image of pentacene we used the electron density of delocalized bonds and other functions of the one-electron density from conceptual density functional theory. The presented results provide evidence for migration of Clar's π-sextets and larger circuits in these systems, and clearly show that the link between the theoretical concept of aromaticity and the real electronic structure entails the separation of intra- and inter-ring resonance effects, which in the case of [n]acenes (n = 3, 4, 5) comes down to solving a system of simple linear equations.

The role of the long-range exchange corrections in the description of electron delocalization in aromatic species

In this article, we address the role of the long-range exchange corrections in description of the cyclic delocalization of electrons in aromatic systems at the density functional theory level. A test set of diversified monocyclic and polycyclic aromatics is used in benchmark calculations involving various exchange-correlation functionals. A special emphasis is given to the problem of local aromaticity in acenes, which has been a subject of long-standing debate in the literature. The presented results indicate that the noncorrected exchange-correlation functionals significantly overestimate cyclic delocalization of electrons in heteroaromatics and aromatic systems with fused rings, which in the case of acenes leads to conflicting local aromaticity predictions from different criteria. © 2017 Wiley Periodicals, Inc.

A quantitative metric for conjugation in polyene hydrocarbons having a single classical structure

A quantitative measure of the extent of conjugation is introduced. The number of Dewar resonance structures (DS) in a conjugated hydrocarbon quantitatively determines its extent of conjugation interaction. The greater the degree of conjugation the more stable is the conjugated hydrocarbon. The number of single bonds joining the exo-double bonds of a purely cross-conjugated hydrocarbon is equal to its number of Dewar resonance structures (DS). Polyene cross-conjugated hydrocarbons are minimally conjugated and possess only one classical structure with a succession of fixed single and exo-double bonds. There are three increasing levels of conjugation for systems with a single classical structure according to whether they are cross-conjugated, linear extended two-way conjugated, and ring-enhanced conjugated. Numerous analytical expressions have been derived. Our valence-bond (VB) approach involves studying the interrelationship of several well-known series of conjugated hydrocarbons having only a single classical structure, i.e., the number of Kekulé resonance structures is one (K = 1). This work provides a totally new perspective to the concept of conjugation.

Forty years of Clar's aromatic π-sextet rule

In 1972 Erich Clar formulated his aromatic π-sextet rule that allows discussing qualitatively the aromatic character of benzenoid species. Now, 40 years later, Clar's aromatic π-sextet rule is still a source of inspiration for many chemists. This simple rule has been validated both experimentally and theoretically. In this review, we select some particular examples to highlight the achievement of Clar's aromatic π-sextet rule in many situations and we discuss two recent successful cases of its application.