Two-dimensional (2D) chiral nanomaterials possessing an extremely large surface-to-volume ratio and good dispersibility in solution are of promising applications in chiral optoelectronics, enantiomeric separation, and asymmetric catalysis. Preparation of 2D nanostructures from direct assembly of homopolymers could greatly simplify the production process. However, in comparison to block copolymers, 2D nanostructures of homopolymers in absence of soluble blocks are not colloidally stable and face unsolved problems of agglomeration in solution and irregular anisotropic orientation of polymers. It is still a critical challenge to construct the 2D materials which simultaneously exhibit good regularity and stability.
The teams led by Jie Zhang and Xinhua Wan at the College of Chemistry and Molecular Engineering, Peking University, have reported a unique assembly strategy of free-standing two-dimensional chiral nanomaterials based on amphiphilic poly(phenylacetylene) (PPA) homopolymers, which bear hydrophobic and hydrophilic dendritic side-chains and adopt a DNA-like double helical conformation, as shown in Fig. 1. The assemblies acquire modulable 2D hexagonal nanosheets directly in solution, with enhanced CPL and chiral self-sorting behaviour. The research results were published online by the journal Angew. Chem. Int. Ed., (https://onlinelibrary.wiley.com/doi/10.1002/anie.202214293), with the title “2D Hexagonal Assemblies of Amphiphilic Double-Helical Poly(phenylacetylene) Homopolymers with Enhanced Circularly Polarized Luminescence (CPL) and Chiral Self-Sorting”.
Fig. 1 Chemical constitution and self assembly of amphiphilic PPA homopolymers.
Computer simulations afforded a compressed helix with a cis-cisoid backbone in the PPAs. The main chain and amide bonds were completely shielded by the hydrophobic chain (the red region) and hydrophilic chain (the blue region). Through nucleation and epitaxial growth, PPAs could spontaneously assembled into regular hexagonal nanosheets in THF/EtOH. The sizes of the nanosheets can be modulated from nanometers to submillimeters upon varying the ratio of binary solvents, while the thickness is linearly correlated with the molecular weights.
The hexagonal nanosheets formed by amphiphilic PPAs is stably dispersed in solution. In contrast, the PPAs with all hydrophobic alkyl side chains only form small nanosheets that cannot further grow up through Oswald ripening mechanism and are easy to agglomerate. The results confirm that the importance of amphiphilic structure on the formation and growth of 2D nanosheets. This work provides a new molecular design strategy for the preparation of 2D chiral nanomaterials by using homopolymers.
Furthermore, the 2D architecture can significantly enhance the CPL of polymers with a high dissymmetry factor ~0.1. After mixing two kinds of PPAs with compacted cis-cisoid helical conformation and stretched cis-transoid helical conformation, driven by a chiral discrimination of helical conformation, the PPAs can be self-recognized and self-sorted into homochiral 2D nanosheets, as directly visualized by using fluorescent microscopy (Fig. 2).
Fig. 2 Enhanced CPL and chiral self-sorting behavior of amphiphilic PPAs
Siliang Cai, a Ph.D. student at the College of Chemistry and Molecular Engineering, Peking University, is the first author, and Prof. Jie Zhang and Prof. Xinhua Wan from the College of Chemistry and Molecular Engineering, Peking University are the corresponding authors. This research was financially supported by the National Natural Science Foundation of China and Beijing National Laboratory for Molecular Sciences.
Original link: https://onlinelibrary.wiley.com/doi/10.1002/anie.202214293