Functionalized [3 + 3]cycloalkynes: substituent effect on self-aggregation by nonplanar pi-pi interactions

Area of Science:

• Organic Chemistry
• Supramolecular Chemistry
• Materials Science

Background:

• Benzo[c]phenanthrenes are polycyclic aromatic hydrocarbons with a unique helical structure.
• Chiral macrocycles offer potential in molecular recognition and self-assembly.
• Understanding aggregation behavior is crucial for designing functional materials.

Purpose of the Study:

• To synthesize optically active benzo[c]phenanthrene-based macrocycles.
• To investigate the influence of functional groups on aggregation properties.
• To explore the self-assembly behavior of electron-rich helicenes.

Main Methods:

• Synthesis of optically active (M)-2,11-dihydroxy-1,12-dimethylbenzo[c]phenanthrene-5,8-dicarbonitrile and its derivatives.
• Functionalization with oxygen-containing groups (-OH, -OSiMe2-t-Bu, -OAc, -OTf, -ONf).
• Aggregation studies using Nuclear Magnetic Resonance (NMR), Circular Dichroism (CD), and vapor pressure osmometry (VPO) in various solvents.

Main Results:

• Synthesized a series of oxygen-functionalized [3 + 3]cycloalkynes based on a chiral benzo[c]phenanthrene core.
• Observed varying aggregation strengths in different solvents (CHCl3, THF, acetone).
• Found that electron-withdrawing substituents generally promote stronger aggregation.
• Synthesized (M)-1,12-dimethylbenzo[c]phenanthrene-2,5,8,11-tetraol, which readily oxidized to a quinone forming self-charge-transfer complexes.

Conclusions:

• The aggregation of these chiral macrocycles is tunable via functional group modification.
• Solvent polarity significantly impacts the aggregation behavior.
• Electron-withdrawing groups enhance supramolecular assembly in these helicene-based systems.
• The electron-rich tetraol derivative and its quinone form interesting solid-state complexes.