A recent study published in Nature Communications discussed the development of host-guest molecular recognition motifs between methyl-β-cyclodextrin and para-hydroxybenzoic acid for fabricating transparent supramolecular glass. The optical characteristics of the developed bulk material are comparable to those of modern glasses.
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Background
Supramolecular glass is a non-covalently cross-linked amorphous material exhibiting exceptional optical behavior and distinctive intrinsic structural features. Like its gel counterpart, supramolecular glass can be assembled from organic components.
However, supramolecular glass is a relatively novel concept compared to the extensively developed artificial inorganic/organic glass. It has not yet been sufficiently recognized and investigated, especially for its inherent structure, driving force, and physical properties.
The fabrication of transparent supramolecular glass requires a critical selection of suitable building blocks. Although macrocycles and the associated recognition motifs (host-guest complexes) have been widely used in different applications, they are generally not preferred for glass formation. Thus, the researchers developed a feasible strategy for constructing supramolecular glass, a new type of transparent material, via the self-assembly of small organic molecules.
Methods
This study used the molecular recognition motif of methyl-β-cyclodextrin (M) and para-hydroxybenzoic acid (H) as the basic unit to construct transparent supramolecular glass (MH).
The two-step synthesis process began by preparing a supramolecular polymer by evaporating a mixed solution of M and H at 80 °C. The newly formed raw material was then hot-pressed for 10 minutes at a constant temperature (80 °C) and pressure (20 MPa) to obtain MH.
Multiple experimental and theoretical investigations were employed to examine the behavior and intrinsic structure of MH. Since MH formation involved a transition from an aqueous to a bulk state, the solution state was studied using nuclear magnetic resonance (NMR), electron spray ionization mass (ESI-MS), and two-dimensional diffusion-ordered (DOSY) spectroscopy.
The bulk state of MH was analyzed by temperature-dependent Fourier-transform infrared (FTIR) spectroscopy. The interior structure of MH was investigated using powder X-Ray diffraction (PXRD), while atomic force microscopy (AFM) was performed to examine its microscopic mechanical properties.
Since solvent evaporation was used to prepare MH, thermogravimetric analysis (TGA) was performed to detect water molecules that remained in the bulk materials. These water molecules were further investigated by broadband dielectric spectroscopy.
For theoretical investigations of the MH formation process and its properties, molecular dynamic simulations were performed using the Materials Studio software package, and molecular docking was conducted using Autodock 4.0. The latter helped elaborate the molecular mechanisms and interactions of supramolecular components in MH.
Results and Discussion
In this study, different M and H molar ratios were tested, but only limited ratios resulted in glass formation. At these ratios, the recognition behavior between M and H yields stable host-guest structures on a restricted scale. It actuates the supramolecular polymerization of these M/H complexes into isotropic MH in bulk.
NMR, ESI-MS, and DOSY experiments and simulation results evidenced this complexation between M and H, with H located in the cavity of M to form a threaded structure.
Non-covalent polymerization resulting from the host-guest complexation and hydrogen bonding formation imparts high transparency and bulk state to MH. Short-range order (host-guest complexation) and long-range disorder (three-dimensional polymeric network) structures were also identified simultaneously in PXRD analysis, thus demonstrating the typical structural characteristics of glass.
TGA revealed that MH contains approximately 4 wt.% water. Such a small number of water molecules function as vital monomers in supramolecular polymerization. They facilitate glass formation by providing additional hydrogen bonding sites and increasing the crosslinking density of supramolecular glass.
A colorless and transparent MH was formed after hot-pressing and cooling (size: >10 cm, thickness: <1 mm). Its excellent transparency (>85 %) over a wide wavelength range (300 to 1000 nm; visible and near-infrared regions) is comparable to that of commercially available glass materials. Alternatively, it becomes opaque after vacuum drying, demonstrating the significance of water in its optical properties.
Other characteristics of the fabricated MH include a compact and nonporous structure and smooth surface morphology, which can be attributed to the hot-pressing preparation technology. It also exhibits high moisture resistance and low glass transition temperature compared with some supramolecular bulk glasses and adhesives.
Conclusion
The fabricated MH exhibits excellent performance in terms of transmittance, thermal processability, compatibility, and recyclability, which can be attributed to its dynamic assembly process. Utilizing host-guest complexation and structural water, MH demonstrates an optical behavior comparable to modern glass.
These characteristics expand the applicability of bulk supramolecular glass.
The supramolecular strategy developed in this work is effective for constructing transparent materials from organic components. Considering the diversity of macrocycles and host-guest complexes, this strategy can assist in the selection of available recognition motifs for glass formation. It could serve as a universal design concept for transparent supramolecular materials.
Journal Reference
Cai, C., Wu, S., Zhang, Y., Li, F., Tan, Z., Dong, S. (2024). Bulk transparent supramolecular glass enabled by host-guest molecular recognition. Nature Communications. doi.org/10.1038/s41467-024-48089-4
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