Quest for the Most Aromatic Pathway in Charged Expanded Porphyrins

Despite the central role of aromaticity in the chemistry of expanded porphyrins, the evaluation of aromaticity remains difficult for these extended macrocycles. The presence of multiple conjugation pathways and different planar and nonplanar π-conjugation topologies makes the quantification of global and local aromaticity even more challenging. In neutral expanded porphyrins, the predominance of the aromatic conjugation pathway passing through the imine-type nitrogens and circumventing the amino NH groups is established. However, for charged macrocycles, the question about the main conjugation circuit remains open. Accordingly, different conjugation pathways in a set of neutral, anionic, and cationic expanded porphyrins were investigated by means of several aromaticity indices rooted in the structural, magnetic, and electronic criteria. Overall, our results reveal the predominance of the conjugation pathway that passes through all nitrogen atoms to describe the aromaticity of deprotonated expanded porphyrins, while the outer pathway through the perimeter carbon atoms becomes the most aromatic in protonated macrocycles. In nonplanar and charged macrocycles, a discrepancy between electronic and magnetic descriptors is observed. Nevertheless, our work demonstrates AVmin remains the best tool to determine the main conjugation pathway of expanded porphyrins. M.A. and I.C.R. wish to acknowledge the VUB for a Strategic Research Program awarded to ALGC. The resources and services used in this work were provided by the Flemish Supercomputer Center (VSC), funded by the Research Foundation - Flanders (FWO), and the Flemish Government. I.C.R. acknowledges co-funding from the European Union′s Horizon 2020 research and innovation Maria Skłodowska-Curie Actions, under grant agreement number 945380. It has been also supported by grants from the Spanish government MICINN (PGC2018-098212-B-C21, PID2019-104772GB, PID2019-105488GB-I00, and PCI2019-103657), Diputación Foral de Gipuzkoa (2019-CIEN-000092-01), Gobierno Vasco (IT1346-19, IT1254-19, and PIBA19-0004), and the DIPC (DIPC_INV_003132). Open Access funding provided by University of Basque Country.