Open-shell Magnetic States in Alternant and Non-alternant Nanographenes: Conceptions and Misconceptions

In contrast to alternant nanographenes (NGRs), in non-alternant NGRs 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 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. This is experimentally supported by sub-molecularly resolved STM images. The misconceptions in the literature are due to insufficient correlation. For non-alternant 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 lead to open shell magnetic states; ferromagnetic in cases of sublattice imbalance (e.g. triangulenes), antiferromagnetic for non-sublattice cases (e.g. SW3x2, SW4x2), and non-magnetic (diamagnetic) for balanced sublattices (e.g. AGNRs). Obviously, similar results are expected for larger NGRs/GNRs obtained by concatenation of such SW-motifs.