2D and 3D Isophlorinoids: Synthesis, Structural Characterization, Redox and Electronic properties

Abstract

In general, anti-aromaticity has not gained significant attention mainly due to the lack of stable 4nπ systems. In this regard, a stable isophlorin and its expanded derivatives provide a hope to glance into the world of anti-aromatic systems. Unlike aromatic porphyrinoids, its counterpart anti-aromatic expanded isophlorinoids are not much celebrated molecules due to lack of straightforward synthetic strategies. Apart from the structural similarity, the electronic and redox properties of these 4nπ and (4n+2)π systems are found to be different with opposite ring current effects. Core modified and expanded isophlorins have been characterized as stable 4nπ macrocycles with characteristic electronic and redox properties those are not observed in aromatic systems. Despite many efforts, few strategies are reported to synthesise planar expanded isophlorins. Although very successful at synthesizing large macrocycles, these strategies add considerable synthetic steps and lack of ring current effects. In this context, the synthesis of stable quintessential planar 4nπ molecules is crucial not only to understand the electronic effects of π-conjugation but also imperative to the development of novel materials for applications in molecular electronics. During the attempted synthesis of expanded isophlorins using acetylene bridges instead of methine carbon, a stable and neutral (4n+1)π radical was discovered. A detailed analysis was carried out to confirm the neutral radical nature of the macrocycle using various analytical techniques and computational studies. Further, the synthesis of novel 40π antiaromatic expanded, core modified isophlorinoids and the diradicaloid nature of the oxidised dication molecules were studied. The non-covalent interaction between 40π isophlorin and fullerene was also studied in the solid-state. Finally, a tetrapod 3D fully π-conjugated molecular cage was synthesised using a simple acid catalysed reaction. The X-Ray crystallography analysis confirmed the tetrapod cage structure and intermediates, which resemble three-fourths or half of the cage structures. Further, by varying the stoichiometric ratio of precursors, significant control over the yield of these products was achieved.