The Apparent and Hidden Variables for Optimizing and Functionalizing Non-alternant Nanographenes

Motivated by the recent synthesis of non-alternant isomers of small peri acenes through the introduction of Stone-Wales (SW) and “SW-like” (azulene) moieties, on their alternant isomers, we set here to examine the roots, and profound characteristics of such nanographenes (NGRs) to systematically optimize and functionalize their spintronic and optoelectronic properties. Setting aside for the moment (longitudinal) quantum confinement, which can be taken care by longer NGRs, we find that the deeper reason for these properties is either decreasing antiaromaticity or increasing aromaticity, the two not always been equivalent. Although decreasing antiaromaticity determines the spin state for a given geometry, increasing aromaticity is the decisive factor for selecting the ground state geometry. Yet, it is not necessarily associated with larger negative values of nucleus-independent chemical shifts (NICS). Actually, “higher aromaticity” maybe accompanied by highly antiaromatic rings with positive NICS numbers. It is demonstrated that “increasing aromaticity” in fact describes the topological capability of the non-alternant system to accommodate the full (or the largest part of the) aromaticity pattern of the corresponding alternant NGR, meaning that “defects” should be concentrated as much as possible around the empty rings. The aromaticity pattern is fully determined by the number Z of zigzag rings at the two ends, which follows the generalized width rule: Z= 3n±1 (n=1, 2,…). similarly to alternant AGNRs. Thus, the Z=3 SW-AGNRs compared to those of Z=4, have different aromaticity patterns and (consequently) different (opto)electronic and magnetic properties. These results and conclusions are in accord with earlier predictions of the present author about the “topological/aromatic” role of “empty” rings. Furthermore, they are also validated by the already synthesized non-alternant NGRs (with Z=3 and 4).