24π Core-Modified Nonfused Pentaphyrin with Möbius Aromaticity.

Area of Science:

• Organic Chemistry
• Supramolecular Chemistry
• Materials Science

Background:

• Porphyrins and related macrocycles are crucial in various scientific fields.
• Understanding the relationship between macrocycle structure and electronic properties is key.
• Exploring novel macrocyclic topologies like Möbius aromaticity is an active research area.

Purpose of the Study:

• To synthesize and characterize core-modified 24π pentaphyrin 7 and 20π homoporphyrin 8.
• To investigate the electronic and structural properties of these macrocycles in their freebase and protonated forms.
• To determine the aromaticity and topological characteristics of the synthesized compounds.

Main Methods:

• Synthesis of pentaphyrin 7 and homoporphyrin 8.
• Optical and 1H NMR spectroscopy at variable temperatures.
• Single-crystal X-ray diffraction analysis.
• Calculation of nucleus-independent chemical shift (NICS) values.
• Anisotropy-induced current density (AICD) plotting.

Main Results:

• Freebase pentaphyrin 7 exhibits nonaromatic behavior at room temperature, with weak aromaticity at lower temperatures.
• Protonation of pentaphyrin 7 induces significant spectral changes, structural distortion, and Möbius aromaticity.
• Single-crystal X-ray structure of protonated 7a reveals severe macrocycle twisting.
• NICS calculations and AICD plots support the Möbius aromatic topology of protonated 7.
• Homoporphyrin 8 remains nonaromatic in both freebase and protonated states.

Conclusions:

• Protonation triggers a transition to Möbius aromaticity in core-modified pentaphyrin 7.
• The study demonstrates the influence of macrocycle conformation on aromaticity.
• Homoporphyrin 8 serves as a nonaromatic control, emphasizing the unique behavior of pentaphyrin 7.