|Posted on March 12, 2018 at 8:45 AM|
The role of aromaticity in porphyrinoids is a current subject of debate due to the intricate structure of these macrocycles, which can adopt Hückel, Möbius and even figure-eight conformers. One of the main challenges in these large π-conjugated structures is identifying the most conjugated pathway because, among aromaticity descriptors, there are very few that can be applied coherently to this variety of conformers.
As a result of a joint colaboration with Miquel Torrent-Sucarrat, Mercedes Alonso and Julia Contreras-Garcia, we have recently published a paper on Phys. Chem. Chem. Phys. studying the most aromatic circuits in porphyrinoids. The main authors of the paper are Irene Casademont (PhD student at the UPV/EHU and the DIPC) and Tatiana Woller (PhD student at the Vrije Universiteit Brussel). In this work, we have found that two new electronic aromaticity indices AV1245 and AVmin developed in our group, provide a reliable description of the aromatic pathways in a series of nine porphyrinoids. Not many indices can be used to identify the most aromatic pathway in a macrocyclic (for instance, NICS which is probably the most popular index cannot be calculated for particular circuits) and in our study we have also used BLA, BOA, FLU and HOMA. All these indices agree on the general features of these compounds, such as the fulfillment of Hückel's rule or which compounds should be more or less aromatic from the series. However, only AVmin can identify the most aromatic circuit in all the molecules. Our results evince the difficulty of finding the most aromatic pathway in the macrocycle for large porphyrinoids.
We study the effect of the exchange in DFT functionals on the description of the aromaticity of the porphyrinoids. The amount of exact exchange quantitatively changes the picture for most aromaticity descriptors, AVmin being the only exception that shows the same qualitative results in all cases.
The paper has been published in Phys. Chem. Chem. Phys.