Theoretical study on conformation and electronic state of Hückel-aromatic multiply N-confused [26]hexaphyrins

Conformations and electronic states of Hückel-aromatic regular, singly, doubly, and triply N-confused [26]hexaphyrins were investigated using density functional theory (DFT) calculations. A comparison of the molecular energies of 754 structures in all revealed that the most stable conformers depend on the degree of confusion, where ring strain and intramolecular hydrogen bonding would play a critical role. Consequently, regular and singly N-confused hexaphyrins prefer a dumbbell conformation, doubly N-confused hexaphyrin prefers a rectangular conformation, and triply N-confused hexaphyrin prefers a triangular conformation. Introduction of N-confused pyrrole rings into the hexaphyrin framework causes narrower HOMO-LUMO energy gaps, while it does not affect the NICS values or aromaticity significantly. The steric repulsion imposed by meso-aryl substituents largely affects the relative energies among the conformers.