Stability of the high-spin ground state in alternant pi -conjugated organic molecules.

Abstract

Alternant quantum cell models with unequal numbers of atoms on the two sublattices have been predicted to have a high-spin ground state. In this paper, we examine the stability of this high-spin ground state with respect to breaking the alternancy symmetry and distortion of the backbone conjugation. We find that in the Pariser-Parr-Pople (PPP) models and the Hubbard models with weak correlations, the ground state continues to be the high-spin state, even when alternancy symmetry is broken by introducing large site-energy differences. In the Hubbard model, for strong correlation strengths, the ground state switches from a high-spin to a low-spin state when large site-energy differences are introduced. The bond-order calculations in all these models shows that the low-spin state is susceptible to dimerization of the backbone. In the distorted chains, the low-spin state stabilizes to a greater extent leading to low-spin ground states at least in ‘‘soft’’ lattices. However, experience with one-dimensional systems suggests that the lattice distortion could occur unconditionally leading to low-spin ground state in infinitely long polymers. Thus, realization of organic ferromagnetics via high-spin polymers could be elusive.