Partly saturated polyacene structures: a theoretical study.

Planar molecular edifices obtained by joining polyacene fragments (polyacene stripes) are investigated at tight-binding (i.e., with a Hückel Hamiltonian) and ab initio level. For this kind of system, it is known that the presence of 60-degree angles between two stripes of the polyacene molecular skeleton induces the formation of singly occupied molecular orbitals, whose combination gives rise to quasi-degenerate electronic states. In particular, two types of convex polygons having a unique side length (rhombuses and triangles) are considered in this work. It is shown that the saturation via hydrogen atoms of the apical carbons located on outer borders of the 60-degree angles increases the number of quasi-degenerate orbitals, and hence the maximal multiplicity of the low-lying states of the system. Our tight-binding and ab initio (CAS-CI, NEVPT2) calculations indicate that the spin multiplicity of these molecular structures is in systematical accord with the Ovchinnikov rule.