Antiaromatic Supramolecules: F⋅⋅⋅S, F⋅⋅⋅Se, and F⋅⋅⋅π Intermolecular Interactions in 32 π Expanded Isophlorins

Abstract

jugated systems with structural features akin to annulenes, and have contrasting ring current effects. 4np macrocycles can react quickly to lose their antiaromatic character. The isophlorin/porphyrin redox couple further illustrates the unstable nature of isophlorin relative to porphyrin under ambient conditions. Similar to the isophlorin p framework, the inherently antiaromatic tetraepoxy annulenes are susceptible to oxidation/reduction to form the corresponding aromatic macrocyclic ions. Their large ring size induces structural flexibility leading to inseparable conformational isomers. Interestingly, steric hindrance favor macrocycles to undergo structure-induced loss of p delocalization. Giant porphyrinoids are well studied examples for macrocycles which adopt figure-eight conformations. Such nonplanar cyclic conjugated systems lack ring current effects and are considered to be nonaromatic in nature. Relative to aromatic systems, the experimental evidence reported for antiaromaticity is found to be far less than satisfactory. There are very limited reports to bridge theoretical predictions with experimental data, particularly for large antiaromatic systems. In this context, the synthesis of stable quintessential planar 4n p molecules is crucial not only to understand the electronic effects of p conjugation but also imperative to the development of novel materials for applications in molecular electronics. Expanding the p framework of either 1 or the modified isophlorin 2 is an attractive strategy for generating a variety of expanded isophlorins (3 and 4, respectively). Its effective conjugation and rigid structural skeleton affords a unique opportunity to explore the chemistry of antiaromatic molecules. But the synthesis of stable antiaromatic isophlorinoids is hindered by the inherent unstable nature of 4np systems. Amongst a few isophlorins known so far, the synthetic strategy employed for 2 is probably the simplest to obtain stable antiaromatic macrocycles from easy-to-make precursors. Adopting a similar synthetic strategy, with modified precursors, we describe the syntheses and structural features of novel and stable 32p vinylogous expanded isophlorins which display unprecedented short F···X (X=p/S/Se) and p–p intermolecular contacts for antiaromatic systems. Replacing two methine carbon bridges in 3 by an equal number of ethylene bridges can convert a (4n+ 2)p system into a 4np system. McMurry coupling is widely used to couple dialdehydes of thiophene oligomers into macrocycles with ethylene bridges. We adopted a modified Rothemund-type synthesis, (Scheme 1), wherein diols of five-membered heterocycles were condensed with bis(thiophene)s under