Azabuckybowl-Based Molecular Tweezers as C₆₀ and C₇₀ Receptors

Area of Science:

• Supramolecular Chemistry
• Organic Chemistry
• Materials Science

Background:

• Fullerenes (C60 and C70) are important carbon allotropes with unique electronic properties.
• Molecular tweezers are supramolecular hosts capable of encapsulating guest molecules.
• Designing selective hosts for fullerenes remains a challenge in host-guest chemistry.

Purpose of the Study:

• To design and synthesize novel molecular tweezers based on nitrogen-embedded buckybowl subunits.
• To investigate the binding affinities and selectivity of these tweezers towards C60 and C70 fullerenes.
• To explore the structural, electronic, and optical properties of the resulting fullerene-supramolecular complexes.

Main Methods:

• Synthesis of nitrogen-embedded buckybowl-based molecular tweezers with varying covalent linkers.
• Solution-state binding studies using optical spectroscopy and proton nuclear magnetic resonance (1H NMR) titration.
• Solid-state structural analysis via X-ray diffraction.
• Electrochemical analysis and Density Functional Theory (DFT) calculations.
• Nonlinear optical (NLO) measurements.

Main Results:

• Successfully synthesized molecular tweezers with tunable binding strengths for C60 and C70.
• Established 1:1 complex stoichiometry and elucidated solid-state structures showing tweezers encapsulating fullerenes.
• Demonstrated selective binding: carbazole-bridged tweezers favor C70, while phenanthrene-bridged tweezers favor C60.
• Observed enhanced stability of fullerene complexes against redox reactions and chromatographic purification.
• Identified intermolecular charge-transfer interactions and enhanced two-photon absorption cross-sections upon complexation.

Conclusions:

• Nitrogen-embedded buckybowl tweezers offer a versatile platform for selective fullerene recognition.
• The linker design is crucial for tuning the cavity size and achieving complementary binding to C60 or C70.
• Complexation enhances the stability and modifies the electronic and optical properties of both the tweezers and fullerenes.