# Stability of the high-spin ground state in alternant $\pi$-conjugated organic molecules

Sinha, Bhabadyuti and Albert, IDL and Ramasesha, S (1990) Stability of the high-spin ground state in alternant $\pi$-conjugated organic molecules. In: Physical Review B: Condensed Matter and Materials Physics, 42 (14). pp. 9088-9097.

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## 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.

Item Type: Journal Article Copyright of this article belongs to The American Physical Society Division of Chemical Sciences > Solid State & Structural Chemistry Unit CA Sharada 07 Feb 2008 19 Sep 2010 04:41 http://eprints.iisc.ac.in/id/eprint/12650