A theoretical study of closed polyacene structures.

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.