Triplet State Baird Aromaticity in Macrocycles: Scope, Limitations, and Complications

Area of Science:

• Computational Chemistry
• Organic Chemistry
• Quantum Chemistry

Background:

• Baird aromaticity, concerning cyclic 4nπ-electron systems in the triplet state (T1), is an area of increasing research interest.
• Understanding the limitations and design principles for achieving T1 state aromaticity in macrocycles is crucial for developing novel materials.

Purpose of the Study:

• To computationally investigate the limitations of T1 state Baird aromaticity in macrocyclic compounds ([n]CMs).
• To identify structural and electronic factors that promote or hinder Baird aromaticity in macrocycles.
• To provide design guidelines for constructing large T1 state Baird-aromatic macrocycles.

Main Methods:

• Density Functional Theory (DFT) calculations were employed to explore Baird aromaticity.
• Various DFT functionals (UB3LYP, UM06-2X, UCAM-B3LYP) were assessed for geometry optimization and single-point energy calculations.
• Canonical UCCSD(T) and DLPNO-UCCSD(T) methods were used for high-level energy evaluations.

Main Results:

• Macrocycles incorporating 2,5-linked furans ([n]CFUs) exhibit sustained Baird aromaticity over a wider range of sizes compared to other monocycles.
• The choice of DFT functional for geometry optimization impacts the calculated T1 state energies, with UB3LYP often providing the lowest energies.
• While geometric and electronic criteria favor [n]CFUs for Baird aromaticity, magnetic indices suggest otherwise when compared to [n]CPPs.
• T1 Baird aromaticity in macrocycles is favored when it dominates over localized Hückel aromaticity and diradical character.

Conclusions:

• Monomers with lower ground-state Hückel aromaticity (like furan) and reduced steric hindrance are preferable for designing large T1 Baird-aromatic macrocycles.
• Structural confinement, such as through methylene bridges, can enhance Baird aromaticity in macrocyclic systems.
• A strong analogy exists between T1 Baird aromaticity in neutral macrocycles and Hückel aromaticity in their dicationic counterparts, though dications show stronger aromaticity.