Aromatic Pathways in Twisted Hexaphyrins

Abstract

The aromatic pathways and the degree of aromaticity of expanded porphyrins have been determined by explicit calculations of the routes and strengths of the magnetically induced currents using the gauge-including magnetically induced current (GIMIC) approach. Density functional theory calculations show that the doubly twisted hexaphyrins fulfilling Hückel's (4n + 2) pi-electron rule for aromaticity and those obeying the 4n pi-electron rule for antiaromaticity are aromatic and antiaromatic, respectively. The investigated [26]hexaphyrin (2) and (3) and [30]hexaphyrin (5) isomers are aromatic, and [28]hexaphyrin (4) is antiaromatic. The formally antiaromatic [24]hexaphyrin (1) does not sustain any strong ring current and must be considered nonaromatic. A detailed analysis of the current pathways of the hexaphyrins is presented. It was found that the current pathways of the investigated aromatic hexaphyrins are not always dominated by the flow along the inner route through the non-hydrogenated C-N-C moieties, as previously proposed. The current flow is often split into two branches at the pyrrole rings, but sometimes it takes the outer route via the C=C bond of the pyrrole. The current pathway of the weak paratropic ring current of [24]hexaphyrin is dominated by the outer C=C route. The calculations show that the routes of the current transport cannot be assessed merely by inspection or from nucleus independent chemical shifts; explicit calculations of the current pathways are compulsory. The current-density studies also show that the pyrrole rings do not sustain any strong ring currents of their own.