A comparative study of one and two dimensional π-conjugated systems

Torres, Ana E.; Flores, R. M.; Fomine, Serguei

doi:10.1016/j.synthmet.2016.01.005

Cited by 10 publications

(6 citation statements)

References 32 publications

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“…It seems as if electron distributions always elected those configurations, ensuring a large number of Clar sextets (if not the maximum one since 5d would require a ⟨ S 2 ⟩ value close to 3.0 for the m s = 0 UDFT solution). In this example, two unpaired electrons can be seen to impinge on six-membered rings ( 5b ), but it is wiser to see trimethylenemethane (TMM) diradicals at each end of the molecule, according to 5c . , As an extension of this work, we show here how the same tendency to preserve a maximum or large number of sextets may result in the appearance of unpaired electrons, even in the absence of any extracyclic methylene group …”

Section: Introductionmentioning

confidence: 75%

Predicting the Open-Shell Character of Polycyclic Hydrocarbons in Terms of Clar Sextets

Trinquier

Malrieu

2018

J. Phys. Chem. A

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Rather unexpected spin-symmetry breakings of mean-field single determinants occur in singlet ground states of many families of alternating conjugated hydrocarbons which accept a full on-bond electron pairing. These symmetry breakings may be seen as an indication of the existence of unpaired electrons. Although qualitative, the concept of disjoint electronic sextets proposed by Clar (hereafter called CS) is at least a heuristic tool for predicting various features of fused polybenzenic hydrocarbons. The present work shows that identifying the preferred CS distribution enables one to rationalize the existence of one or several spin-symmetry breakings, i.e., the existence and the number of unpaired electrons in alternant fused polycyclic hydrocarbons via a simple recipe for the prediction of these features. This recipe is based on comparison between various distributions of CS on the molecular frame, subject to a restriction concerning the fragments of the graph that do not belong to the CS. This rule is successfully confronted to UDFT calculations and to a recently proposed criterion predicting the possible occurrence of spin-symmetry breaking from the topological Hückel Hamiltonian. The confrontation runs on series of rhombuses, periacenes, anthenes, and other graphene flakes or nanoribbons. The CS distribution definitely offers a qualitative guide to look at the possible occurrence of (multiple) symmetry breakings in polycyclic architectures which are commonly seen as closed-shell singlets.

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Section: Introductionmentioning

confidence: 75%

Predicting the Open-Shell Character of Polycyclic Hydrocarbons in Terms of Clar Sextets

Trinquier

Malrieu

2018

J. Phys. Chem. A

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“…Performing UDFT calculations with B3LYP exchange potentials, Bendikov et al found that a symmetry breaking takes place in hexacene and larger analogues . This finding, confirmed by several similar calculations, − led to attribute diradical character to long polyacenes . It might however be considered as questionable .…”

Section: Numerical Testsmentioning

confidence: 96%

Can a Topological Approach Predict Spin-Symmetry Breaking in Conjugated Hydrocarbons?

Malrieu

Trinquier

2016

J. Phys. Chem. A

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The closed-shell mean-field single determinants of large alternant hydrocarbons are frequently unstable with respect to a possible spin-symmetry breaking which produces different orbitals for the α and β electrons, either in Hartree-Fock or in Kohn-Sham DFT calculations. The present work shows that one may easily predict whether such a symmetry breaking will take place from the elementary topological Hückel Hamiltonian which introduces a simple hopping integral t. The demonstration makes use of the simplest representation of the bielectronic repulsion, namely, the Hubbard bielectronic operator, reduced to an on-site repulsion U, and takes benefit of the mirror theorem. A recipe is proposed to determine the relevant t/U ratio for a given exchange-correlation potential. The symmetry-breaking phenomenon first concerns the mixing between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), but it may eventually run on other pairs of mirror orbitals. These symmetry breakings may take place while the other molecular orbitals keep a closed-shell character. The spin polarization of these MOs, appearing in typical unrestricted mean-field calculations, is an induced and amplifying effect, which has to be distinguished from the symmetry breaking itself. Special attention is paid to the possible appearance of multiple symmetry breakings, leading to a polyradical character. The model is tested on six series of polycyclic hydrocarbons. This elementary approach sheds new arguments on the debate concerning the di- or polyradical character of polyacenes.

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“…Quite recently, as suggested by Fürst et al [5], the possibility of creating periodically perforated graphene structures (the so-called graphene antidot lattice) has been evoked to obtain even much more enhanced and diverse electronic and magnetic properties. Indeed complex graphene structures, having hexagonal, rhomboidal, or triangular shape, can exhibit extremely complex, yet predictable, electronic, and magnetic properties that make them ideal candidates, to rationally design organic giant nanomagnets, also exploiting the possibility offered by the different connectivity of different graphene units [6,7]. These kinds of structure have recently attracted both experimental and theoretical consideration.…”

Section: Introductionmentioning

confidence: 99%

The Electronic Structure of Graphene Nanoislands: A CAS-SCF and NEVPT2 Study

Cusinato

Evangelisti

Leininger

et al. 2018

Advances in Condensed Matter Physics

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This paper presents a tight binding and ab initio study of finite graphene nanostructures. The attention is focused on three types of regular convex polygons: triangles, rhombuses, and hexagons, which are the most simple high-symmetry convex structures that can be ideally cut out of a graphene layer. Three different behaviors are evidenced for these three classes of compounds: closed-shells for hexagons; low-spin open-shells for rhombuses; high-spin open-shells for triangles.

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A comparative study of one and two dimensional π-conjugated systems

Cited by 10 publications

References 32 publications

Predicting the Open-Shell Character of Polycyclic Hydrocarbons in Terms of Clar Sextets

Predicting the Open-Shell Character of Polycyclic Hydrocarbons in Terms of Clar Sextets

Can a Topological Approach Predict Spin-Symmetry Breaking in Conjugated Hydrocarbons?

The Electronic Structure of Graphene Nanoislands: A CAS-SCF and NEVPT2 Study

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