Pyridone-Fused Aromatic Belts: Synthesis and Properties of Amide-Bridged [6]Cycloparaphenylenes

Pyridone-fused aromatic belts, namely amide-bridged [6]cycloparaphenylenes, were successfully synthesized to investigate how polar amide groups govern symmetry and electronic structure in π-conjugated nanobelt molecules. The polar orientations of the six amide bonds give rise to two discrete structural isomers: S₆- and D₃-symmetric aromatic belts. These compounds were obtained through an efficient macrocyclization directed by the cis-preference of tertiary aromatic amides, followed by Ni-mediated intramolecular aryl-aryl coupling. Single-crystal x-ray diffraction analyses revealed that both isomers adopt rigid belt-shaped architectures with enhanced π-conjugation along the framework. The D₃-symmetric isomer was successfully resolved into (M,M)- and (P,P)-enantiomers using chiral HPLC and the isomer exhibited a distinct chiroptical response in electronic circular dichroism spectroscopy (|g_abs| = 2.7 × 10^-3). Notably, cyclic voltammetry was used to demonstrate that the D₃-symmetric isomer possesses a narrow HOMO-LUMO energy gap of 2.35 eV together with considerably stabilized frontier molecular orbitals. This work establishes a versatile molecular design concept for constructing aromatic belts with well-defined chiroptical properties and high oxidative robustness, expanding opportunities in nanobelt science and chiral functional materials.