Editor’s Choice: Chiral Functional Materials

Yong Cui

Yong Cui received his PhD in 1999 from Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, and conducted postdoctoral research at the University of Science and Technology of China, the University of North Carolina at Chapel Hill and the University of Chicago from 1999 to 2005. He joined the School of Chemistry & Chemical Engineering, Shanghai Jiao Tong University in 2005, where he is now a chair professor of chemistry. He is a Fellow of the Royal Society of Chemistry. His research interests focus on the design and synthesis of porous materials, especially framework materials and chiral materials, for catalysis, adsorption, separation, sensing and optics.

Chirality is one of the core research topics in the multidisciplinary fields of chemistry, biology, medicine, and physics. Chiral molecules often play an essential role in many biological structures. For example, L-amino acids are favoured to construct enzymes and proteins, while D-sugars are the main components of DNA and RNA. Hence, rationally designing synthetic chiral molecules or supermolecules with enzyme-like properties has been one of the most exciting recent developments in molecular chemistry over the past decade, with chemists finding a huge difference in pharmacological properties between enantiomers. From a materials science perspective, self-assembly of pre-designed chiral molecules to synthesize structurally diverse, well-defined functional materials across differing length scales would be one of the most straightforward strategies to mimic and replicate the functions of natural biomacromolecules. Such crystallization-driven assembly models also provide good opportunity for the development of highly sophisticated materials [for example, chiral metal–organic frameworks, chiral covalent organic frameworks (CCOFs), and chiral hybrid organic–inorganic perovskites] for applications in enantioselective recognition and separation, asymmetric catalysis, and circularly polarized luminescence (CPL) etc. Perhaps most importantly, the fundamental understanding of the translation of chirality from molecular to supramolecular to macroscopic scales is crucial to unveil biological mechanisms.

In this Editor’s Choice collection, I have selected outstanding papers published in the past three years in Materials Horizons, that expand the design and development of chiral functional materials. These articles provide fascinating insights and perspectives with regards to chiral recognition, nonlinear optical effects, circularly-polarized luminescence and chiral-induced spin selectivity (CISS) effects.

The article by Xu et al. (https://doi.org/10.1039/d2mh01429g) reports that the introduction of chiral α-methylbenzyl ammoniums within hybrid tin(IV) halides can transfer their chirality to the inorganic frameworks. This enables broken symmetry and polar arrangements in the crystals and leads to outstanding second-order nonlinear optical responses, thereby highlighting that the lattice distortion plays a role in tuning the optical properties of low-dimensional hybrid organic–inorganic metal halides. This work demonstrates the advantages of the chemical variability of hybrid organic–inorganic metal halides, which offer a promising option to synthesize chiral tin(IV) halides for applications in optoelectronic materials and devices.

The construction of artificial nanozymes with high activity and enantioselectivity is still a great challenge. Qu et al. (https://doi.org/10.1039/d0mh01535k) report the preparation of a well-defined CCOF that comprises an L-histidine (L-His) residue as the binding site. This material can act as a chiral nanozyme that exhibits a higher activity for chiral recognition relative to natural enzyme Horseradish peroxidase (HRP). The straightforward design of the chiral COF nanozyme is a fine example of an enantioselective nanozyme with higher activity than the natural enzyme, opening a new avenue for the better design of chiral biomimetic materials with high activity.

The measurement and understanding of chirality transfer to a chemically non-chiral bulk is one of the most important and fascinating challenges impacting on all areas of materials science. Mehl et al. (https://doi.org/10.1039/d0mh01274b) report the synthesis of gold nanoparticles (AuNPs) functionalized both with chiral ligands based on the binaphthol motif and with nematogenic groups (ChirAuLC). It has been found that the chiral transfer efficiency of ChirAuLC is higher than NPs functionalized only with chiral groups (ChirAuNP). This work demonstrates a novel approach that can be used for the synthesis of macro-chiral liquid crystal nanoparticles with unusual chirality enhancement, which may be beneficial for the development of other high-performance chiral materials.

The work by Zheng et al. (https://doi.org/10.1039/d0mh01303j) reports a distinct type of CPL-active material with an extremely high dissymmetry factor (g_lum) (up to 0.61). This material was achieved by hierarchical assembly of self-assembled superhelices based on the helical TPE macrocycle, which provides a new approach to synthesize highly CPL-active materials using the new concept of further assembly of helical self-assemblies.

The realization of CPL in three-dimensional (3D) hybrid perovskites and the precise design of hybrid perovskites with a large piezoelectric response have always been significant challenges but highly desired. The work by Duan, Wang, Zhang et al. (https://doi.org/10.1039/d2mh00698g) reports new 3D hybrid rare-earth double perovskites (HREDPs) through a chirality induction strategy. Note that the introduction of chirality gives rise to the coupling of multiaxial ferroelectricity and ferroelasticity, which brings about a satisfactory large piezoelectric response. Moreover, CPL was realized for the first time in HREDPs. Thus, this work emphasizes a design strategy for the development of molecule-based chiral materials with excellent CPL activity.

A comprehensive review by Xu and Mi (https://doi.org/10.1039/d3mh00024a) discusses the CISS effects in biomolecules, hybrid organic–inorganic perovskites and inorganic materials. The review summarizes the important discoveries and recent experiments performed during the development of the CISS effect, analyzes the spin polarized transport in various types of materials and discusses the mechanisms, theoretical calculations, experimental techniques and biological significance of the CISS effect.

In most cases, the type of chiral units used are mainly focusing on point chirality and axis chirality. In the study by Li, Chen et al. (https://doi.org/10.1039/d1mh01404h), planar chirality-based molecules were rationally designed for the development of chiral thermally activated delayed fluorescence (TADF) materials and the synthesis of [2.2]paracyclophane-type molecules with a D–π*–A type structure were reported. Such a design strategy suppresses the racemization of planar chirality, making it possible to fabricate circularly polarized organic light-emitting diodes. This work provides a promising approach for the development of planar chiral TADF materials.

Pan, Wang and Yu (https://doi.org/10.1039/d1mh01154e) show strong magnetic field manipulation of photoluminescence, also called magneto-photoluminescence, through the circularly polarized photoexcitation of chiral lead halide perovskites (LHPs), such as (R-MBA)₂PbI₄ and (S-MBA)₂PbI₄. This work implies that the magnetic field provides an effective means to manipulate both the polarization and intensity of CPL in chiral LHPs, which can be exploited for novel device applications.

Takimiya et al. (https://doi.org/10.1039/d1mh01119g) reported the introduction of an enantiopure 2-ethylhexyl group on to dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) through a Negishi-coupling reaction to synthesize 2-(R)-(2-ethylhexyl)- and 2-(S)-(2-ethylhexyl)-DNTT (namely R- and S-EH-DNTT, respectively). The crystallinity, thin-film structures, and organic field-effect transistors based on R-, S- and racemic EH-DNTT (rac-EH-DNTT) were studied to elucidate the effect of stereoisomerism in the 2-ethylhexyl group. Highlighting the relationship between the stereoisomerism and the solid-state structure of enantiopure and racemic molecules.

Finally, Xu, Cao et al. (https://doi.org/10.1039/d0mh01207f) developed a reliable method to achieve chiral photoluminescence which is implemented from silver nanostructures with optical duality in one go via a two-fold three-dimensional laser printing scheme. This method holds great promise for future versatile applications in chiroptical nanodevices.

I hope this collection summarises some key insights over recent years into the timely progress of research and varied applications of chiral functional materials, and inspires researchers to further delve into the fundamentals and applications of these materials in future work.

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