In supramolecular chemistry, the self-assembly state of molecules plays a major role in determining their tangible properties. Controlling the self-assembled state has garnered significant attention as it may possibly be exploited to design materials with desired properties like charge transport capability and fluorescence wavelength. For years, scientists have been attempting to decipher how molecular organization impacts the properties of supramolecular assemblies which are within the nano (<10 nm) and mesoscopic (10-1000 nm) scales. Nevertheless, the study of structures with supramolecular polymer assemblies derived from the identical monomer is commonly hindered by dynamic structural changes and immature control over self-assemblies.
A recent study published on January 1, 2024, within the Journal of the American Chemical Society, investigated the properties of one-dimensional mesoscale supramolecular assemblies of two different structures composed of the identical luminescent molecule. It showed how two structures showed very different properties depending on whether or not they had their molecules arranged in a closed circular pattern or not. The study was led by Prof. Shiki Yagai from Chiba University, with Sho Takahashi, a doctoral course student on the Graduate School of Science and Engineering at Chiba University, as the primary writer. It also included Prof. Martin Vacha from the Department of Materials Science and Engineering at Tokyo Institute of Technology, and Dr. Hikaru Sotome from the Graduate School of Engineering Science at Osaka University as corresponding authors.
“The geometrical great thing about a circular structure, which has no termini and no corners, has fascinated people. Chemists have realized the synthesis of giant cyclic molecules using various approaches not only to create beautiful structures but additionally to compete within the elegance of the strategy of synthesizing such beautiful structures,” says Prof. Yagai, speaking of the inspiration behind this study. “The most effective example of nature utilizing the functional great thing about circular structures can be the light-harvesting antenna organ (LH2, LH1) of purple photosynthetic bacteria. LH2 has a stupendous circular structure resulting from the protein’s outstanding self-organizing ability, and it is assumed that by arranging chlorophyll dyes in a circular array based on this framework, lean light collection and excitation energy transfer are achieved.”
Through the self-assembly of luminescent molecules synthesized based on their very own molecular design, the team obtained a combination of two one-dimensional π?conjugated molecular aggregates with different structures, namely terminus-free cyclic structures (toroids) and randomly coiled structures. The mixture exhibited low-energy and low-intensity luminescence.
The 2 structures were separated using a novel dialysis technique that exploited the difference of their kinetic stability. Post-separation, it was shown that the terminus-free closed toroidal structure led to higher energy and more efficient luminescence in comparison to random coils. The team carried out ultrafast laser spectroscopy to analyze the mechanism of their topology-dependent fluorescence properties. The outcomes indicated random coils with termini lost excitation energy resulting from defects generated by fluctuations in solution, unlike toroids that weren’t easily deformed and exhibited fluorescence without energy loss. Moreover, it was found that within the mixed solution of toroids and random coils, the excitation energy was transferred from the toroid to the random coil resulting from the agglomeration of each assemblies, and only the random coil-derived luminescence was observed.
This study establishes morphological control of materials on the mesoscale as a possible latest guideline for the design of functional materials. It also highlights that within the case of materials which are vulnerable to supramolecular polymorphism, resembling the toroid and random coil, it is crucial to purify the assemblies before analyzing their photophysical properties. If not separated, the outcomes obtained might only reflect biased properties as an alternative of distinct ones resulting from energy transfer between different structures.
The researchers are hopeful that these insights can encourage the event of high-performance flexible devices using cyclic molecular assemblies. “We are able to gladly say that a correlation between structural beauty and functional beauty has been found here, even in meso-scale molecular assemblies. We imagine that the insights from our study could help improve the performance of solar cell devices and light-emitting devices in the long term, thereby facilitating their widespread acceptability and enriching people’s lives along the way in which,” concludes Prof. Yagai.
