Open-shell magnetic states in alternant and non-alternant nanographenes: Conceptions and misconceptions

Abstract

In contrast to alternant, in non-alternant nanographenes (NGRs) and graphene nanoribbons (GNRs) no “sublattice structure” can be defined, associated with significant conceptual and computational simplifications. This leads to some fundamental differences between the two. We uncover here the broken electron-hole symmetry in non-alternant NGRs as one fundamental difference closely related to distorted Dirac points (cones) and their diradical open-shell character. We also show by higher level calculations beyond common DFT, based on many-body (MPn) and coupled clusters (CCSD(T)) theory, that the alternant series of peri‑acenes (bisanthene, peri‑tetracene, peri‑pentacene, … etc.), contrary to opposite reports in the literature, have clearly closed singlet ground states, in contrast to their non-alternant isomers based on Stone-Wales (SW) defects. We suggest that this can be experimentally verified by sub-molecularly resolved STM images. The misconceptions in the literature are due to insufficient correlation. For non-alternant SW-NGRs/GNRs with antiaromatic rings the driving force for open-shell states and distorted Dirac points (involving localized electrons and delocalized holes) is antiaromaticity, which is a sufficient but not always necessary condition. This is in juxtaposition to the aromaticity of the alternant isomers with closed shell states. Thus, in both cases sublattice problems, such as sublattice imbalance or complete lack of sublattice would lead to open shell magnetic states; ferromagnetic (e.g. triangulenes), or antiferromagnetic respectively (e.g. SW3 × 2, SW4 × 2), in contrast to non-magnetic (diamagnetic) states for balanced sublattices (e.g. armchair GNRs, AGNRs). Obviously, similar results, regarding the role of antiaromaticity and the broken electron-hole symmetry would be expected for larger NGRs/GNRs obtained by concatenation of such SW-motifs.