The apparent and hidden variables for optimizing and functionalizing non-alternant nanographenes: A comprehensive study

Abstract

Atomically precise non-alternant isomers of short armchair graphene nanoribbons (AGNRs) with A = 2 armchair and Z = 3, 4 zigzag rings have been recently synthesized by introducing Stone-Wales (SW) moieties, pursuing open shell magnetic states for spintronic applications. Hereby, to support the synthesis of new SW-AGNRs, we develop new and efficient principles and rules, based on the fundamental but largely misunderstood concept of (anti)aromaticity. We find that the driving force is increasing global aromaticity (rather than decreasing local antiaromaticity) through preserving the original aromaticity pattern (AP) of the parent alternant AGNR. The resulting APs are practically identical for both open-singlet and triplet states, leading to nearly isoenergetic open shell magnetic states. We uncover that the optimum arrangement of the SW moieties, involving minimum energy cost, is their closer proximity to the “inert” empty rings in the parent AP, which is found identical to the daughter AP, both determined by Z through the generalized rule: Z = 3n, 3n±1 (n = 1, 2…). As proof of concept, we study in detail the SW3×4 and SW4×4, which have similar spintronic, but distinct optoelectronic and structural characteristics. SW4×4, is a better candidate for optoelectronic, in addition to spintronic applications. For the Z = 2 SW2×3AGNRs, which are special cases with mixed and incomplete APs, our current preliminary results suggest possible applications as three-stage magnetic switches (diamagnetic, antiferromagnetic, and ferromagnetic). These results, obtained using aromaticity as a simple and efficient tool rather than a fuzzy concept, apply also to the already synthesized SW-AGNRs, validating our earlier and present results and predictions.