The Interplay of Aromaticity and Antiaromaticity in N-Doped Nanographenes

The aromaticity of three non-planar, fully conjugated aza-nanographenes built around a pyrrolo[3,2-b]pyrrole core is assessed through application of two different computational procedures - GIMIC and NICS. We examine the calculated magnetically induced current densities (GIMIC) and nucleus independent chemical shifts (NICS). The structural differences between these three apparently similar molecules lead to significantly different aromatic properties. GIMIC analysis indicates that the peripheral diatropic ring current of 3.9 nA/T for the studied bowl-shaped diaza-nanographene is the strongest, followed by the double [6]helicene which lacks seven-membered rings, and is practically nonexistent for the double [5]helicene possessing seven-membered rings. The biggest difference however is that in the two not-fully-fused molecules the central pyrrole rings possess a significant diatropic current of about 4.1 nA/T, whereas there is no such current in the diaza-nanographene. Moreover, the antiaromaticity of the s...