Topological edge states in a double isomeric Class-II oligo(indenoindene)

Abstract: I report the theoretical prediction of non-trivial physics in a one dimensional multiradical system consisting in fused six and five membered ${\pi}$-conjugated carbon rings, known as oligo(indenoindene) (OInIn). Topologically protected electronic states may emerge in fermionic chains if there is an alternation in the coupling of adjacent unpaired electrons, being described effectively by the Su-Schrieffer-Heeger (SSH) model. Class-II OInIn isomers act as tight-binding chains in the non-interacting regime, thus we can expect the emergence of SSH physics in an OInIn produced by the combination of two isomers that belong to this class. That is the case of the system studied in this manuscript, whose calculated non-interacting band structure shows a gap opening compared to the gapless pure isomeric forms, hosting ingap localized states at the chain termini depending on the termination, and a non-zero Zak phase that confirms the non-trivial topology. These results were consistent with spin unrestricted mean-field Hubbard and density functional theory calculations, showing antiferromagnetic unquenched local magnetic moments at the pentagons, and strong edge localization depending on the termination. This work advances in the understanding of the physics of non-alternant multiradical ${\pi}$-conjugated hydrocarbons.