Porphyrin/Quinoidal Bithiophene Based Macrocycles and Their Dications: Template-free Synthesis and Global Aromaticity

Abstract

We report the template-free synthesis and characterization of a new type of porphyrin/quinoidal-bithiophene-based conjugated macrocycle. X-ray crystallographic analysis of the dimer (2MC) revealed a cyclophane-like geometry with large dihedral angles between the porphyrin and the neighboring thiophene rings, and NMR measurements and theoretical calculations confirmed a localized aromatic character of the porphyrin/thiophene rings and quinoidal character of the bithiophene linkers. Restricted rotation of the thiophene rings linked to the porphyrin unit was observed by variable-temperature NMR measurements. The dication (2MC2+) adopts a chair-shaped conformation to facilitate π-electron delocalization around the whole macrocycle. As a result, the molecule is globally aromatic, with a dominant 54 π conjugation pathway. The trimer (3MC) also shows localized aromatic character of porphyrin rings and conformational flexibility, but its dication (3MC2+) is rigid and globally aromatic with a dominant 82 π conjugation pathway.