Search for singlet-triplet bistabilities in conjugated hydrocarbons

The purpose of this paper is to search for bistable hydrocarbons for which the lowest singlet and triplet states are nearly degenerate in their substantially different equilibrium geometries, the two wells being separated by a significant energy barrier. For non-alternant hydrocarbons with fused odd-membered rings, two different ways to anticipate the spin multiplicity are in contradiction. A series of such molecules accepts a Kekule structure, which suggests that the ground state is a singlet, while extended use of the Ovchinnikov rule predicts a triplet ground state. Calculations using both a geometry-dependent Heisenberg Hamiltonian and ab initio methods indicate that most of these hydrocarbons actually have a singlet ground state and a low-lying excited triplet state. Different ways to stabilize the diradical structure are studied. It is found that the presence of external fused six-membered rings on the non-alternant skeleton stabilizes the triplet state, leading to a bistability phenomenon. Such compounds should have interesting magnetic properties.