Synthesis and Structural Properties of NIR-Absorbing Pyridine-Containing Heptaphyrins

Four examples of stable nonaromatic pyridine containing heteroheptaphyrins (pyrithiaheptaphyrins) 2-5 were synthesized in 8-13% yields by [5+2] condensation of newly synthesized pyridine-based pentapyrrane 8 and bithiophene diol 9 a-d. The X-ray crystallographic analysis of macrocycle 2 proved that the macrocycle assumes a highly planar structure with two inverted thiophene rings. The heteroheptaphyrins 2-5 are asymmetric and showed a greater number of resonances in H-1 NMR spectra compared to our previously reported symmetric heterohexaphyrin (pyrithiahexaphyrin) 1 c. Most of the macrocyclic core protons in pyrithiahepaphyrins 2-5 experienced upfield/downfield shifts compared to pyrithiahexaphyrin 1 c indicating the alteration of pi-conjugation in the macrocycles. The absorption bands were significantly red-shifted and located in the NIR region in macrocycles 2-5 compared to 1 c supporting the increase of pi-delocalization. The theoretical studies support the experimental findings and NICS(0) value supports the non-aromaticity of the macrocycles.

Automated summary

Four stable nonaromatic pyridine-containing heteroheptaphyrins were successfully synthesized in yields of 8-13% by [5+2] condensation. The macrocycles exhibited asymmetric structures and experienced upfield/downfield shifts in core proton positions compared to a previously reported symmetric heterohexaphyrin. The absorption bands were significantly red-shifted, indicating an increase in pi-delocalization, supported by theoretical studies.