Aromatic Expanded Isophlorins: Stable 30π Annulene Analogues with Diverse Structural Features

Abstract

Synthesis and structural diversity of novel aromatic expanded isophlorins are described. Expanded isophlorins are higher analogues of 20π isophlorin; 30π isophlorins are the simplest examples of expanded isophlorins. They are synthesized from easy to make precursors. Owing to the sp2 carbons along the conjugated network, they represent higher analogues of annulenes which are not realized until date. In contrast to the parent isophlorin 2, expanded isophlorins 7−9 and 11−13 are aromatic (4n + 2)π systems and also differ in their optical properties and structural features. They exhibit ring inverted structures and the number of ring inversions is dependent on the nature of the heteroatoms present in the core of the macrocycle. This was confirmed by both 1H NMR spectroscopy and single crystal-ray diffraction analysis. The upfield chemical shifts for the protons of the inverted rings confirmed the diatropic ring current effects as expected for (4n + 2)π aromatic systems. Solid state analysis revealed a planar conformation for 7 and near planar conformations for other macrocycles. They also show strong intermolecular interactions through C(sp2)−H···F−C(sp2) hydrogen bonds.