Acid-base controlled multiple conformation and aromaticity switches in tren-capped hexaphyrins

Area of Science:

• Supramolecular Chemistry
• Organic Chemistry
• Photochemistry

Background:

• Hexaphyrins are macrocyclic compounds with unique electronic and photophysical properties.
• Conformational flexibility and aromaticity in macrocycles are crucial for their function.
• Protonation is a common method to tune the properties of nitrogen-containing macrocycles.

Purpose of the Study:

• To investigate the conformational changes and aromaticity shifts in tren-capped hexaphyrin upon protonation.
• To characterize the resulting supramolecular structure and its host-guest chemistry.
• To understand the relationship between molecular conformation, electronic properties, and ion binding.

Main Methods:

• Protonation of tren-capped hexaphyrin using acidic conditions.
• Spectroscopic analysis (NMR, UV-Vis, fluorescence) to monitor conformational changes and aromaticity.
• X-ray crystallography to determine the solid-state structure of the protonated species.
• Computational modeling to support experimental observations.

Main Results:

• Protonation induced sequential conformational isomerizations: rectangular-to-Möbius and Möbius-to-triangular.
• Concomitant reversal of antiaromaticity to aromaticity was observed during these transformations.
• A "crypto-bowl-shape" hexaphyrin conformation was achieved, effectively hosting a trifluoroacetate counterion.
• The cage environment and electronic properties were significantly altered by protonation and isomerization.

Conclusions:

• Protonation is a powerful tool to control the conformation and aromaticity of tren-capped hexaphyrins.
• The observed isomerizations lead to a novel host-guest complex with potential applications in molecular recognition.
• Understanding these dynamic processes provides insights into the design of functional macrocyclic systems.