Frontier Orbital Views of Stacked Aromaticity

Area of Science:

• Organic Chemistry
• Quantum Chemistry
• Materials Science

Background:

• Antiaromatic molecules with 4n π electrons show stacked aromaticity via π-π stacking.
• The underlying mechanism of stacked aromaticity remains unclear.
• Pseudo-Jahn-Teller distortions influence antiaromatic molecule stability.

Purpose of the Study:

• Investigate the mechanism of stacked aromaticity in antiaromatic molecules.
• Utilize cyclobutadiene as a model system.
• Clarify the role of orbital interactions and molecular geometry.

Main Methods:

• Theoretical and experimental approaches.
• Analysis of orbital interactions in stacked molecules.
• Computational modeling of cyclobutadiene stacking.

Main Results:

• Face-to-face stacking of antiaromatic molecules alters orbital energy gaps.
• Cyclobutadiene's SOMOs split into HOMO and LUMO due to bond alternation.
• Stacking leads to a smaller HOMO-LUMO gap in cyclobutadiene dimers.
• Orbital interchange at specific distances enhances inter-unit bond strength, enabling stacked aromaticity.

Conclusions:

• Orbital interactions and geometric distortions are key to stacked aromaticity.
• The HOMO-LUMO gap engineering of monomer units controls stacking distance.
• Stacked aromaticity in antiaromatic systems is a tunable phenomenon.