Abstract
A supramolecular complex, formed by encapsulation of C60 fullerene in a molecular container built from two resorcin[4]arene rims zipped together by peptidic arms hydrogen bonded into a cylindrical β-sheet, was studied by X-ray crystallography, solid-state and solution NMR, EPR spectroscopy and differential scanning calorimetry (DSC). The crystal structure, determined at 100 K, reveals that the complex occupies 422 site symmetry, which is compatible with the molecular symmetry of the container but not of the fullerene molecule, which has only 222 symmetry. The additional crystallographic symmetry leads to a complicated but discrete disorder, which could be resolved and modelled using advanced features of the existing refinement software. Solid-state NMR measurements at 184-333 K indicate that the thermal motion of C60 in this temperature range is fast but has different activation energies at different temperatures, which was attributed to a phase transition, which was confirmed by DSC. Intriguingly, the activation energy for reorientations of C60 in the solid state is very similar for the free and encaged molecules. Also, the rotational diffusion coefficients seem to be very similar or even slightly higher for the encaged fullerene compared to the free molecule. We also found that chemical shift anisotropy (CSA) is not the main relaxation mechanism for the 13C spins of C60 in the studied complex.
Highlights
An inherent feature of molecular capsules is the presence of a cavity that can host and sequester other molecules from the external environment
The encapsulated C60 molecule resides at this site, but since it has only C2 molecular symmetry in the [001]
We have described the synthesis of a molecular container formed by zipping together two resorcin[4]arene hemispheres via a system of circular -sheet hydrogen bonds between their peptidic arms
Summary
An inherent feature of molecular capsules is the presence of a cavity that can host and sequester other molecules (guests) from the external environment. Molecular capsules find application in the storage of unstable and reactive molecules, and their controlled release (Mal et al, 2009), as catalytic nanovessels that provide a constrictive environment for single reactions (Zhang & Tiefenbacher, 2015; Zhang et al, 2017) and domino-type processes (Salles et al, 2013). Interactions between a capsule and its cargo are of particular interest to molecular engineers because these are the very factors that control the thermodynamics and kinetics of the encapsulation process. Precise characterization of the mode of such interactions is in most cases troublesome, especially for spherical guests residing in spherical cavities. Because the occupation factor (percentage of the internal volume of the cavity that is occupied by the guest molecule) is often low. For non-polar guests, an optimal occupation factor is claimed to be 55%, which is similar to the occupation factor of a non-polar liquid phase
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