Recently, it has been discovered that ultrasound can control the self-assembly and gelation processes, resulting in huge changes in nanoscale morphology and material properties. Apart from crystals that form racemic aggregates, few ultrasound-induced symmetry breakdowns have been reported.
In this work, Institute of Chemistry, Chinese Academy of Sciences Liu Minghua researcher and Associate Researcher Zhang Li et al. reported a detailed study on the process of ultrasound-induced supramolecular gelation with mirror symmetric breaking of . Sonication induced a transition from the initial achiral sphere to the helical nanofiber , forming a metal gel with strong supramolecular chirality. Experimental characterization and theoretical simulations show that chirality comes from the helical stacking between Ag(I) and achiral molecules, and the position of pyridyl is the key to the generation of supramolecular chirality. In addition, the prepared spiral nanofibers can be used to guide chiral symmetric breakdown under ultrasound, showing more robust and repeatable results than chiral induction of photoactive solvents. The authors also measured the spin polarization of charge transport of injected current after mirror symmetric breakage through the spin-correlation transmission of nanofibers. authors used helical nanofibers as spin filters to achieve up to 45% spin polarization at room temperature. The work was published in Angew. Chem. Int. Ed., titled "Ultrasound-Directed Symmetry Breaking and Spin Filtering of Supramolecular Assembly from only Achiral Building Blocks".
[Preparation and Characterization of Materials]
Figure 1. Preparation and Characterization of Materials
Achiral monomer is designed based on the benzene-1, 3, 5-tricarboxyamide (BTA) motif, and each of the three arms has an aminopyridine group. BTAM-oP forms single crystal , which is a scanning electron microscope (SEM) image showing a typical hexagonal microtubule structure. Single crystal X-ray diffraction analysis shows that the molecular accumulation of BTAM-oP single crystal is a three-line model. Due to the accumulation of hydrogen bonds, two adjacent molecules accumulate face to face. The authors found that due to the strong interaction of silver ion (Ag+) and Ag (I) with aminopyridine groups, self-assembly of BTAM-oP can be quickly triggered. Ag (I) was added at 288 K and the transparent solution turned into a milky white suspension within 60 seconds. morphology analysis showed that the suspension consisted of uniform microspheres with a diameter of 1.7 ± 0.2 μm. To avoid precipitation, the authors sonicated the system. As shown in Figure 1c, the suspension gradually clarified under 288 K ultrasound, and after 1800 W treatment for 10 min, a translucent supramolecular gel was finally formed.
[Performance Characterization]
Figure 2. Morphological evolution analysis
High resolution atomic force microscope studies showed that nanofibers are not straight, but have clear spiral morphology (Figure 2a). The circular dichroism (CD) spectroscopy further verified the chirality of the spiral nanofibers (Fig. 2b). Samples from different batches showed near-perfect mirror CD spectra. Further statistical distribution of CD amplitudes shows that the effect of ultrasound is highly repeatable: the absolute value of CD intensity is almost the same, and the optical activity can be sustained unless it disintegrates after heating. The author uses simulation research to provide possible clues for structural transformation. In the absence of sonication, silver ions bind to BTAM-oP molecules, promoting the formation of spherical structures in an amorphous manner, thereby reducing surface free energy. However, with the help of π-π stacking and hydrogen bonding, the high-frequency mechanical wave propagation of ultrasonic plays a role in cracking and reshaping the coordination stacking mode, making the fiber morphology stable.
Figure 3. Chiral properties characterization
Add 10% BTAM-oP/Ag nanofibers to the sample (a predetermined signal was obtained by CD analysis) and then sonication was applied. The authors found that the final CD signal and intensity of the sample were consistent with the added nanofibers. More importantly, the authors found that this approach is still effective even if the spiral nanofibers are in a solid state. For example, chiral fibers are spin-coated onto a clean silicon substrate and then evaporated to dryness. This solid nanofiber has the ability to control chirality even after being stored at room temperature for two years.
[Spin Selective Transmission]
Figure 4. Spin Selective Transmission Characterization
Finally, the authors evaluated spin selective transport of nanofibers by magnetically conductive atomic force microscopy (mc-AFM), as shown in Figure 4a. The average current-voltage (I-V) curve of chiral nanofibers under different magnetization directions is shown in Figure 4b. For (+) nanofibers, higher currents can be observed when electrons are injected from a substrate magnetized in the "downward" direction, relative to the "upward" direction (Fig. 4b). In the case of (−) nanofibers, the opposite behavior can be observed, and higher currents are observed when electrons are injected from the magnetized substrate in the upward direction (Fig. 4c). According to the mc-AFM measurement results of different batches, the spin polarization calculated by the author is shown in Figure 4d. The spin polarizations of (−)-nanofibers and (+)-nanofibers were +45±5% and −42±6%, respectively. It is worth noting that this spin polarization is even higher than some chiral self-assembly systems and is comparable to chiral solvent-induced supramolecular polymers (38%-46%). In order to function in spin electronic devices, the polarity of the spin transfer through the chiral material must be adjusted. This is usually achieved by using two enantiomeric forms, so cumbersome synthesis and purification processes are sometimes inevitable. In this work, chiral nanofibers of the left or right hand can be easily obtained after mirror symmetry breaking, providing a promising alternative to adjusting the self-spin transfer.
summary, This article reports a completely achiral molecular system capable of transitioning between achiral microspheres and chiral nanofibers. More importantly, the spiral nanofibers prepared by sonication have more robust and repeatable controls on nanoscale assembly than commonly used optically active solvents. The authors further demonstrate that spiral nanofibers can serve as a spin filter with similar spin polarization (45%) at room temperature, thus providing a new avenue for the development of novel chiral organic spin electronic devices.
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Source: Frontiers of Polymer Science
