A theoretical study of closed polyacene structures

Abstract

The electronic structure of planar molecular edifices obtained by joining polyacene fragments (polyacene stripes) is investigated at tight-binding and ab initio levels. It is shown that the presence of 60° angles in the molecular skeleton induces the formation of singly-occupied molecular orbitals, whose combination gives rise to quasi-degenerate electronic states. The ab initio investigation requires therefore the use of CAS-SCF and MR-PT approaches. The three types of possible convex polygons having a unique side length (hexagons, rhombuses and triangles) have been considered in this work. The spin multiplicity of these quasi-flat molecular structures is found to be in systematical accord with the Ovchinnikov rule.

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

“…For large enough edifices (as it is the case here), the interaction between these two orbitals vanishes and they become degenerate at the Fermi level. This is very similar to the behavior of the open rhombuses studied in our previous paper [4]. It can be seen that for H 5 the coincidence with the extrapolated infinite graphene case (red curve on the plots, obtained from the analytical solution for a graphene sheet with periodic boundary conditions of size 100 × 100 hexagons) is almost perfect.…”

“…Our experience with this kind of systems is that one obtains, in this way, orbitals that are close to the CAS-SCF ground-state orbitals, at a much lower computational cost [4,26]. Such a procedure is often used in order to find optimum orbitals in magnetic systems, and it gives MO that are of CAS-SCF quality if the occupation numbers in the open-shell manifold are close to one.…”

“…Of course, one can not expect to obtain quantitative results using this minimal basis set paragon, but it has been widely used to get qualitative trends for many families of molecules, both organic and inorganic. In particular, its ability to give the right tendencies concerning the behavior of low dimensional systems has been evidenced by some of us [4,24]. In order to further assess this statement, we compared the HartreeFock and NEVPT2 energies (at the STO-3G geometries) for the H 2, R 3, and T 3 nanoflakes using STO-3G and different extended ANO basis sets optimized by Widmark and coworkers [25].…”

“…Indeed, finite nanostructures can exhibit novel and fascinating properties due to the confining effects and the finite size. First of all graphene nanostructures, ribbons or island, show an accumulation of electronic density at the border, giving raise to the so-called edge states [4]. The presence of such states can strongly modify the physical and chemical behavior of such materials and has been demonstrated by different research groups, both theoretically and computationally.…”

“…Tight binding calculations have been performed using a dedicated code written by the authors which has already been described in a previous paper [4]. The Hamiltonian matrix, based on the Hückel formalism, has been constructed from the connectivity of every carbon atom in each nanoisland.…”