Clar Sextets Research Articles

Two Different Clar's Sextets: Clar's Rule Does Not Necessarily Contradict the Global Pathway of Conjugated Macrocycles.

Two contradictory theories, Clar's sextet and resonance theory, explain the stability of conjugated macrocycles using localized and delocalized models, respectively. To reconcile these theories and gain better insights, PAH-containing porphyrinoids were chosen as a representative class of conjugated macrocycles. Two types of Clar's sextets were identified and proposed for the first time based on their interaction with global conjugated pathways: (i) shared sextets that integrate with and perturb the global resonance pathway, and (ii) independent sextets that are separate from the pathway and potentially stabilize or do not perturb it. To test our hypothesis, we synthesized precise regioisomers with and without an independent sextet to exclude variables such as steric hindrance, different PAH characteristics, and other elements. This task is challenging due to the extensive synthetic efforts needed for regioisomeric PAH-containing porphyrinoids. We focused on 2,3-vinylnaphthiporphyrin, which features shared sextets, because its o-regioisomers are readily accessible. Two distinct regioisomers (1,2-vinylnaphthiporphyrins) with an independent sextet were synthesized along with unexpected nonaromatic N-fused porphyrinoids. Our analysis shows that 1,2-vinylnaphthiporphyrins with an independent sextet are more aromatic than 2,3-vinylnaphthiporphyrin (nonaromatic). This sextet analysis was also applied to other reported and yet-to-be-synthesized PAH-containing porphyrinoids, and these results are consistent with our current study.

Peri-Benzo-Diindenotetracenyl: Helically π-Extended Allyl Radical with Robust Stability.

Here is described the synthesis and characterization of a stable hydrocarbon radical, peri-benzo-diindenotetracenyl, with a helical structure. Although the helical π-radical has no peripheral substituents, it was stable in the solid and solutions. According to the X-ray diffraction analysis and quantum chemical calculations, the radical was best described as an allyl radical fused by five Clar's sextets. The optically resolved enantiomers exhibited mirror image CD spectra with |gCD| of 2.4×10-4 at 522 nm. The racemization barrier was determined to be 95.9 kJ/mol at 298 K, which is compatible with that of [5]helicene (108 kJ/mol).

6,6'-Biindeno[1,2-b]fluorene: an open-shell indenofluorene dimer.

Nakano et al. reported that the antiaromatic indenofluorene (IF) isomers are diradicaloid molecules having varying degrees of open-shell character, with indeno[1,2-b]fluorene displaying a weaker diradical character index (y 0 = 0.072). Unlike 6,12-trimethylsilylethynyl disubstituted [1,2-b]IF, the 6,12-aryl disubstituted [1,2-b]IF derivatives did not show any experimental evidence of diradical properties. This raised the question of whether a [1,2-b]IF dimer would prefer a closed-shell or an open-shell ground state. To address this, herein we report the synthesis of a 6,6'-biindeno[1,2-b]fluorene derivative, which is a [1,2-b]IF dimer, constructed by linking two [1,2-b]IF units with a C-C single bond at carbons 6 and 6' bearing the largest orbital coefficients for the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO). The C6-C6' linkage effectively narrowed the HOMO-LUMO gap while the strong desire to avoid s-indacene antiaromaticity restored two Clar sextets in two proaromatic para-quinodimethane subunits, resulting in an open-shell bifluorenylidene-type diradicaloid (y 0 = 0.268) ground state with minor tetraradical character index (y 1 = 0.007). The open-shell nature was confirmed by single crystal X-ray and electron paramagnetic resonance analyses, and supported by theoretical calculations.

Peculiar electronic properties of wider armchair graphene nanoribbons: Multiple topological end-states and “phase transitions”

In contrast to the thinner 5,6,7-AGNRs in which one pair of topological end-states is generated at a critical length Lc in a form of a metal-insulator-like “phase transition”, in wider AGNRs (with Z = 3n, 3n±1, n = ,2, 3 … zigzag rings) n ≥ 2 pairs of end-states (associated with n ≥ 2 transitions) can be produced, with very peculiar properties, such as quantum interference (QI), stemming from the interaction between (multiple) pairs of end states localized in the same narrow region. At the level of density functional theory such AGNRs at large enough lengths would be misleadingly appear as highly spin polarized with spin s = n. Our results are fully supported by experiment, wherever available. We verify that the Z = 3n-1 AGNRs are always metallic with near zero bandgap, as was originally believed, contrary to some recent suggestions, and do not strictly follow the above pattern. It is shown that in the 13-AGNRs (Z = 3n, n = 2) a unique and intense (close to the theoretical limit) constructive QI (CQI) appears, which covers a relatively wide region of lengths from about L∼ 50 Å to L∼80 Å. This peculiar L-dependent CQI is strongly related to their aromaticity pattern consisting of migrating Clar sextets. The rest of AGNRs exhibit only small kinks (for n = 2) or dips (for n = 3) in conductivity, which could be considered as weak and narrow CQI, or DQI (destructive QI), respectively.

Pentagon‐Containing Doublet Graphene Fragments with Edge‐Dependent Spin/Charge Distribution

Comprehensive SummaryThe synthesis of stable open‐shell hydrocarbon system comprising of pentagon defects and zigzag edges remains unexplored, which can be considered as model compounds for unraveling the spin‐localized states in graphene edges. With concise synthetic approaches, two π‐extended fluorenyl radicals were synthesized and isolated in their crystalline state. X‐ray crystallographic analysis and studies on the magnetic, optical and electrochemical properties of neutral and charged species have revealed an interesting edge‐dependence of the spin/charge distribution, that is, the spin and charge distribution shifted from the pentagon defects to the zigzag edges with the elongation of conjugation. Such phenomenon was unprecedented for hydrocarbon radicals and can be rationalized by the recovery of Clar Sextets in the dominant resonance structures. Remarkably, one of the radicals exhibited exceptional stability in air‐saturated solutions with the half‐life time up to 260 d, thus providing opportunities for the applications as electronic materials.

Quantitative Resonance Theory Based on the Clar Sextet Model.

Despite its great explanatory power in understanding the chemistry of polycyclic aromatic hydrocarbons (PAHs) and related systems, the Clar sextet rule still remains an intuitive and qualitative model with notable exceptions in some cases. Here we develop a quantitative theory of chemical resonance based on semilocalized Clar-type resonance structures (named the Clar resonators) consisting of variable numbers of Clar sextets and C═C bonds. The constructed wave functions of the Clar resonators are used to expand the actual wave function of the π-conjugated system obtained from a DFT or Hartree-Fock calculation. The resultant weights and one-electron energies of the Clar resonators can serve as a quantitative measure of the importance of these resonators. Implementing the theory in our open-source python code EzReson and applying it to over a thousand PAH molecules of different sizes and shapes, we show that the weight of the Clar resonators increases exponentially with increasing number of sextets and that their energy decreases linearly with the latter, thus confirming the general validity of the Clar rule. On the basis of such a large-scale resonance analysis, we propose three extended Clar rules, along with a unified quantitative model, that are able to evaluate the importance of all Clar resonators and the ring aromaticity for PAHs. Using the present theories, we uncover the essential role that the minor Clar resonators may play in correctly understanding the resonance stabilization and local aromaticity of rings, which was totally overlooked in the original Clar model.

Exploitation of Baird Aromaticity and Clar’s Rule for Tuning the Triplet Energies of Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons (PAH) are a prominent substance class with a variety of applications in molecular materials science. Their electronic properties crucially depend on the bond topology in ways that are often highly non-intuitive. Here, we study, using density functional theory, the triplet states of four biphenylene-derived PAHs finding dramatically different triplet excitation energies for closely related isomeric structures. These differences are rationalised using a qualitative description of Clar sextets and Baird quartets, quantified in terms of nucleus independent chemical shifts, and represented graphically through a recently developed method for visualising chemical shielding tensors (VIST). The results are further interpreted in terms of a 2D rigid rotor model of aromaticity and through an analysis of the natural transition orbitals involved in the triplet excited states showing good consistency between the different viewpoints. We believe that this work constitutes an important step in consolidating these varying viewpoints of electronically excited states.

Adsorption of organic molecules on graphene and fluorographene: An unresolved discrepancy between experiment and theory

AbstractHerein, we studied the adsorption of 15 organic and three polar molecules onto pristine and fluorinated graphene. Two levels of fluorination were selected, CF fluorographene for which all carbon atoms are fluorinated and C4F fluorinated graphene, which maximizes the number of Clar sextets. Five different density functionals were employed: M06‐L, M06‐2X, PBE‐D2 and PBE‐D3BJ. In contrast with recent experimental investigations, we found that the adsorption energies for organic molecules onto perfect graphene are significantly lower than those determined for fluorographene. This situation is exacerbated when enthalpic and free energy corrections are included. However, a different scenario was found for polar molecules such as H2O, H2O2 and HF. Partial fluorination following a C4F pattern leads to significantly lower adsorption energies for these three molecules, and the superhydrophilicity of C4F is maintained after thermodynamic corrections are included. In fact, even the removal of a single fluorine atom results in a dramatic change in the adsorption energy of water on fluorographene. We discuss the performance of the density functionals assayed and the importance of using finite and infinite models. This work demonstrates that it is possible to adjust the adsorption energy of a given polar molecule onto graphene by controlling the fluorination level and in doing so, that partially fluorinated graphene models may open new avenues for the separation of molecules with different polarity.

A Stable [4,3]Peri-acene Diradicaloid: Synthesis, Structure, and Electronic Properties.

The synthesis of peri-fused acenes (peri-acenes) with two or more rows is challenging due to their intrinsic open-shell diradical character. Herein, we report the isolation of a derivative (4) of [4,3]peri-acene in crystalline form. The parent [4,3]peri-acene, containing three rows of tetracene, has a large diradical character (y0 =94.8 %) originating from aromatic stabilization. Due to kinetic blocking, 4 showed a reasonable stability with a half-life time of ≈157 h under ambient conditions. Its structure was determined by X-ray crystallographic analysis, and bond-length analysis revealed eight localized Clar's sextets. 4 exhibited an open-shell singlet ground state with a narrow electrochemical energy gap (1.13 eV) and a small singlet-triplet energy gap (-0.57 kcal mol-1 from SQUID measurements). Its electronic properties are compared with previously reported peri-tetracene and teranthene derivatives.

A Stable [4,3]Peri‐acene Diradicaloid: Synthesis, Structure, and Electronic Properties

AbstractThe synthesis of peri‐fused acenes (peri‐acenes) with two or more rows is challenging due to their intrinsic open‐shell diradical character. Herein, we report the isolation of a derivative (4) of [4,3]peri‐acene in crystalline form. The parent [4,3]peri‐acene, containing three rows of tetracene, has a large diradical character (y0=94.8 %) originating from aromatic stabilization. Due to kinetic blocking, 4 showed a reasonable stability with a half‐life time of ≈157 h under ambient conditions. Its structure was determined by X‐ray crystallographic analysis, and bond‐length analysis revealed eight localized Clar's sextets. 4 exhibited an open‐shell singlet ground state with a narrow electrochemical energy gap (1.13 eV) and a small singlet–triplet energy gap (−0.57 kcal mol−1 from SQUID measurements). Its electronic properties are compared with previously reported peri‐tetracene and teranthene derivatives.

Tuning the UV spectrum of PAHs by means of different N-doping types taking pyrene as paradigmatic example: categorization via valence bond theory and high-level computational approaches.

Tuning of the electronic spectra of carbon dots by means of inserting heteroatoms into the π-conjugated polycyclic aromatic hydrocarbon (PAH) system is a popular tool to achieve a broad range of absorption and emission frequencies. Especially nitrogen atoms have been used successfully for that purpose. Despite the significant progress achieved with these procedures, the prediction of specific shifts in the UV-vis spectra and the understanding of the electronic transitions is still a challenging task. In this work, high-level quantum chemical methods based on multireference (MR) and single-reference (SR) methods have been used to predict the effect of different nitrogen doping patterns inserted into the prototypical PAH pyrene on its absorption spectrum. Furthermore, a simple classification scheme based on valence bond (VB) theory and the Clar sextet rule in combination with the harmonic oscillator measure of aromaticity (HOMA) index was applied to arrange the different doping structures into groups and rationalize their electronic properties. The results show a wide variety of mostly redshifts in the spectra as compared to the pristine pyrene case. The most interesting doping structures with the largest red shifts leading to absorption energies below one eV could be readily explained by the occurrence of diradical VB structures in combination with Clar sextets. Moreover, analysis of the electronic transitions computed with MR methods showed that several of the low-lying excited states possess double-excitation character, which cannot be realized by the popular SR methods and, thus, are simply absent in the calculated spectra.

Synthesis of Circumpyrene by Alkyne Benzannulation of Brominated Dibenzo[hi,st]ovalene.

A transition-metal catalyzed alkyne benzannulation allowed an unprecedented synthesis of circumpyrene, starting from 3,11-dibromo-6,14-dimesityldibenzo[hi,st]ovalene (DBOV). The circumpyrene was characterized by a combination of NMR, mass spectrometry, and single-crystal X-ray diffraction analysis, revealing its multizigzag-edged structure. Two newly introduced C═C bonds in circumpyrene strongly perturbed the electronic structures of DBOV, as evidenced by increased optical and electrochemical energy gaps. This is in good agreement with an increased number of Clar's sextets as well as a decreased number of π-electrons in the conjugation pathway of circumpyrene, according to anisotropy of the induced current density (ACID) calculations. The present approach opens a new avenue to multizigzag-edged nanographenes and offers insights into their (opto)electronic properties.

Edge Effects in Benzenoid and Total Resonant Sextet Benzenoid Hydrocarbons and Clar's Sextet Principle.

Because the effect of a benzenoid perimeter edge on a sextet pattern is of paramount importance, trends in the degree of sextet aromaticity are determined. The spectacular structural isomorphism between the total set of benzenoid hydrocarbons and its strain-free total resonant sextet (TRS) subset as described herein is an unequivocal demonstration of the validity of the Clar sextet principle. Aromaticity and the Clar's sextet principle are intricately related which form the basis of this isomorphism. Hexagons on the edge of a polycyclic benzenoid hydrocarbon tend to have higher spin density, have a higher probability of sextet residence, be more reactive to substitution reactions, and have a perimeter topology described by four distinct parameters by means of a very simple relation. It is shown that because the location of Clar sextets in TRS benzenoids is totally fixed, the relative order of ring aromaticity of the peripheral benzene hexagons can be predicted from their topology. Using Bosanac and Gutman's energy effect ( ef) and Aihara's bond resonance energy, we show for the first time that the relative order of decreasing ring aromaticity for perimeter edge hexagons in any specific TRS benzenoid is quarto > trio > duo > solo > empty. TRS benzenoid hydrocarbons are those isomers in benzenoids having their number of formula carbons divisible by six and a maximum number of perimeter bay regions (ηo) given by η0(TRS) = (1/2) NH(sextet) - 3 + F. NH(sextet) is the number of formula hydrogens and F is the number of perimeter fjords (triplet of adjacent bay regions); for strain-free TRS benzenoids, F = 0.

Topological Ring-Currents and Clar Sextets in Fully Benzenoid Hydrocarbons. II. Large Structures Containing More than 18 Rings.

In previous work, π-electron ring currents in several fully benzenoid hydrocarbons were calculated by means of the quasi-graph-theoretical, parameter-free Hückel-London-Pople-McWeeny (HLPM) method. It was found that for fully benzenoid hydrocarbons formed from fewer than about 17 rings all ring currents in full rings are larger in intensity than the ring current in benzene, while ring currents in the empty rings are smaller than the benzene value. There was an indication that this distinction might break down when larger systems are considered. Accordingly, these studies are here extended to a selection of fully benzenoid hydrocarbons containing up to 43 rings. It is still found that all full rings in all structures bear ring currents that are greater than the ring current in benzene but, in several of the larger systems, empty rings are found to bear ring currents greater than the benzene value. In the very largest structures examined, the ring currents in some of the empty rings are greater than the currents in as many as half of the symmetrically nonequivalent full rings. Despite this, because every full ring bears a ring current of intensity greater than that of any of the empty rings that are immediately adjacent to it, all of the full rings are regions of diatropic circulation; there is also strong diamagnetic circulation around the perimeters of each of the fully benzenoid systems studied. Both of these observations, based on the topological HLPM approach, are qualitatively in agreement with the predictions of sophisticated ab initio calculations, earlier performed on some of these fully benzenoid structures by Steiner et al.

Topological Ring-Currents and Clar Sextets in Fully Benzenoid Hydrocarbons I. Structures with Fewer than 18 Rings.

It is well-known that the very special family of conjugated systems described as "fully benzenoid hydrocarbons" have a unique Clar structure that causes all of their rings to be classified as either "full" or "empty". In this work, π-electron ring-currents in more than two dozen such structures are calculated by means of the quasi-graph-theoretical, parameter-free Hückel-London-Pople-McWeeny (HLPM) method. In the case of benzenoid hydrocarbons, the calculated values of such "topological" ring-currents depend on only the carbon-carbon connectivity of the system in question. It is found that, until a certain fully benzenoid hydrocarbon comprising 17 rings is examined, all ring currents in full rings are seen to be larger in intensity than the ring current in benzene, while such currents in empty rings are smaller than the benzene value. There is an indication that this distinction might break down when "giant" fully benzenoid structures are considered. These conclusions are consistent with chemical intuition, with earlier studies examining Clar aromatic sextets by means of NICS calculations, and with previous qualitative ab initio predictions of the patterns of π-electron current-flow in the fully benzenoid hydrocarbons.

Qualitative Views on the Polyradical Character of Long Acenes.

Spin-symmetry breaking appears in the DFT treatment of polyacenes, beyond a certain length, the critical length depending on the exchange-correlation potential. This phenomenon may be attributed to an instability with respect to HOMO-LUMO mixing, which suggests a diradical character of long acenes. However, the increase of the S2 operator with acene length questions this simple view. It is shown that this increase cannot be attributed to spin polarization of the inner MOs, and that a second symmetry breaking takes place for the pentadecacene, with four unpaired electrons centered at the first and third quarters of the chain. The spin density distributions of broken-symmetry solutions support a qualitative picture in terms of tetra-methylene hexacenes separated by Clar sextets. A strategy is proposed to identify local symmetry breaking in polyradical systems.

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.

Theoretical investigation of curved π‐conjugated fullerene flakes: open‐shell character, aromaticity, and third‐order nonlinear optical property

Open‐shell singlet nature, aromaticity, and second hyperpolarizabilities of two kinds of curved π‐conjugated fragments of C60 fullerene, called C60 flakes, together with their corresponding planar analogues are theoretically investigated. It is found that one curved system, A-1, exhibits intermediate open‐shell singlet nature, while the other structure, B-1, and the two corresponding planar systems possess a closed‐shell ground state. The variation of the open‐shell singlet nature is explained by the number of Clar's sextets in the resonance structures, which is associated with the local aromaticity, as quantified by the NICS0 values. The calculated orientationally averaged second hyperpolarizabilities show that the intermediate open‐shell singlet system A‐1 exhibits the best performance, and that the other curved system B‐1 is the worst among the four C60 flakes, whereas all systems exhibit better performance than the original spherical C60 system. This tendency suggests that a lower dimensionality in the π‐conjugated structure is beneficial to the construction of efficient nonlinear optical molecules. The present results not only show that the open‐shell singlet system A‐1 is a performant nonlinear optical molecule, but they also contribute to a deeper understanding of the electronic structure of curved π‐conjugated molecules. Copyright © 2016 John Wiley & Sons, Ltd.

Bistetracene: an air-stable, high-mobility organic semiconductor with extended conjugation.

We report the synthesis and characterization of "bistetracene", an unconventional, linearly extended conjugated core with eight fused rings. The annellation mode of the system allows for increased stability of the conjugated system relative to linear analogues due to the increased number of Clar aromatic sextets. By attaching the appropriate solubilizing substituents, efficient molecular packing with large transfer integrals can be obtained. The electronic structure calculations suggest these large polycyclic aromatic hydrocarbons (PAHs) exhibit excellent intrinsic charge transport properties. Charge carrier mobilities as large as 6.1 cm(2) V(-1) s(-1) and current on/off ratios of 10(7) were determined experimentally for one of our compounds. Our study provides valuable insight into the design of unconventional semiconductor compounds based on higher PAHs for use in high-performance devices.

Antidot-dependent bandgap and Clar sextets in graphene antidot lattices

By using first-principles calculations, the bandgap of two types of graphene antidot lattices (GALs) with triangular lattice, anthrancene-removed GALs and phenanthrene-removed GALs, have been studied. One third of either GALs are semiconductors as lattice parameter varies, while the lattice parameters corresponding to semiconductors are different in the two GALs, indicating the bandgap opening of GALs depends on the antidot configuration as well as lattice parameter. The bandgap opening or not is interpreted with Clar's aromatic sextet theory. A nonzero bandgap appears in phenanthrene-removed GALs, only when the number of Clar sextets exceeds one third of the total number of hexagons in the unit cell. However, due to a long zigzag edge existing in anthrancene-removed GALs, the bandgap can be open without meeting the condition. Our results suggest that the antidot configuration is a non negligible factor on the bandgap of graphene antidot lattices.