Theoretical design of high-spin polycyclic hydrocarbons.

High-spin organic structures can be obtained from fused polycyclic hydrocarbons, by converting selected peripheral HC(sp(2)) sites into H(2)C(sp(3)) ones, guided by Ovchinnikov's rule. Theoretical investigation is performed on a few examples of such systems, involving three to twelve fused rings, and maintaining threefold symmetry. Unrestricted DFT (UDFT) calculations, including geometry optimizations, confirm the high-spin multiplicity of the ground state. Spin-density distributions and low-energy spectra are further studied through geometry-dependent Heisenberg-Hamiltonian diagonalizations and explicit correlated ab initio treatments, which all agree on the high-spin character of the suggested structures, and locate the low-lying states at significantly higher energies. In particular, the lowest-lying state of lower multiplicity is always found to be higher than kT at room temperature (at least ten times higher). Simplification of the ferromagnetic organization based on sets of semilocalized nonbonding orbitals is proposed. Molecular architectures are thus conceived in which the ferromagnetically-coupled unpaired electrons tally up to one third of the involved conjugated carbons. Connecting such building blocks should provide bidimensional materials endowed with robust magnetic properties.