Conformational Fixation of a Rectangular Antiaromatic [28]Hexaphyrin Using Rationally Installed Peripheral Straps.

Area of Science:

• Organic Chemistry
• Supramolecular Chemistry
• Materials Science

Background:

• Hexaphyrins are macrocyclic compounds with interesting photophysical and electronic properties.
• Controlling the conformation of macrocycles is crucial for tuning their characteristics.
• Antiaromaticity in organic systems remains an area of active research.

Purpose of the Study:

• To synthesize a conformationally constrained rectangular hexaphyrin.
• To investigate the electronic and photophysical properties of the resulting molecule.
• To confirm the predicted antiaromatic character.

Main Methods:

• Synthesis via SNAr reaction of a [26]hexaphyrin with allyl alcohol.
• Intramolecular olefin metathesis using Hoveyda-Grubbs second-generation catalyst.
• Reduction using sodium borohydride (NaBH4).
• Characterization using X-ray crystallography, UV/Vis/NIR spectroscopy, and electrochemistry.

Main Results:

• Successful synthesis of a rectangular [28]hexaphyrin bearing outer straps.
• The peripheral straps enforced a planar, rectangular conformation.
• Experimental evidence confirmed Hückel antiaromatic character, including a strong paratropic ring current.
• Observed characteristic UV/Vis/NIR absorption features and a small HOMO-LUMO gap.
• Demonstrated very fast S1 decay, consistent with antiaromaticity.

Conclusions:

• The synthesized rectangular hexaphyrin provides a new platform for studying antiaromatic systems.
• Conformational control through peripheral strapping is an effective strategy for accessing specific macrocyclic geometries.
• The findings contribute to the understanding of structure-property relationships in conjugated organic molecules.