ExCage.

Area of Science:

• Supramolecular Chemistry
• Organic Synthesis
• Host-Guest Chemistry

Background:

• Cyclophanes with face-to-face pyridinium units exhibit strong binding with polycyclic aromatic hydrocarbons (PAHs).
• Noncovalent interactions like [π···π] stacking and [C-H···π] interactions are key to high binding affinities.

Purpose of the Study:

• To synthesize a novel, three-fold symmetric, extended pyridinium-based cage-like host (ExCage(6+)).
• To investigate the binding affinities and complexation dynamics of ExCage(6+) with various PAHs.

Main Methods:

• Template-directed, catalyst-assisted synthesis of the ExCage(6+) host molecule.
• Isothermal titration calorimetry (ITC) to measure association constants (K(a)) of host-PAH complexes.
• Dynamic (1)H NMR spectroscopy to determine decomplexation barriers and rapid injection (1)H NMR spectroscopy for complexation barriers.

Main Results:

• ExCage(6+) demonstrated high binding affinities for PAHs, with association constants ranging from 2.82 × 10(3) M(-1) for naphthalene to 5.5 × 10(6) M(-1) for perylene.
• Stepwise increases in decomplexation barriers were observed for larger PAHs (phenanthrene, pyrene, triphenylene, coronene), indicating enhanced complex stability.
• Kinetic data showed smaller PAHs complex faster, while thermodynamic data revealed larger PAHs form stronger complexes, challenging traditional molecular recognition models.

Conclusions:

• The ExCage(6+) host effectively binds a range of PAHs with high affinity and stability.
• The study provides valuable kinetic and thermodynamic data on host-guest complexation, advancing the understanding of molecular recognition.
• The findings suggest that for larger PAHs, the Rebek 55% solution formula for molecular recognition becomes less applicable.