Three-Dimensional Fully π-Conjugated Macrocycles: When 3D-Aromatic and When 2D-Aromatic-in-3D?

Area of Science:

• Organic Chemistry
• Theoretical Chemistry
• Materials Science

Background:

• Fully π-conjugated macrocycles with complex 3D structures have shown aromatic properties.
• Distinguishing between 2D and 3D aromaticity in these novel molecular architectures is crucial.

Purpose of the Study:

• To investigate and classify the aromatic character of puckered and cage-type π-conjugated macrocycles.
• To differentiate between true 3D aromaticity and 2D aromaticity within 3D molecular frameworks.

Main Methods:

• Qualitative theoretical analysis.
• Quantum chemical calculations.
• Comparison with established aromaticity models (e.g., Hückel's rule, closo-boranes).

Main Results:

• Macrocycles with 3D structures are classified as 2D-aromatic-in-3D, not truly 3D-aromatic.
• Their aromaticity stems from Hückel's 4n+2 rule applied to macrocyclic paths.
• Even molecules with 6n+2 electron counts and significant delocalization do not qualify as 3D-aromatic.

Conclusions:

• The studied macrocycles exhibit 2D aromaticity despite their three-dimensional conformations.
• True 3D aromaticity requires high symmetry and specific electronic structures, unlike these macrocycles.
• This work clarifies the classification of aromaticity in complex, non-planar conjugated systems.