Helicenes have attracted considerable attention due to their inherent helical chirality and extended π-conjugation. Recently, rapid progress has been witnessed in the preparation of double, triple, quadruple, quintuple, and sextuple helicenes, where plural helicene moieties are symmetrically arranged in a single molecule. While synthetic efforts and X-ray crystallographic analyses devoted to these multiple helicenes and theoretical investigations on their isomerization and racemization behaviors have been relatively well documented and reviewed in the literature, the chiroptical properties of the multiple helicities have been somewhat overlooked. This review discourses the cumulative and systematic investigations on the chiroptical properties such as the circular dichroism (CD) and circularly polarized luminescence (CPL) of multiple helicenes. Although the number and structural variations of multiple helicenes reported to date have been fairly limited, this review overviews the current status of the chemistry of multiple helicenes from the viewpoint of chiroptical properties and provides insights into the design principle for advanced chiroptical materials through the proper arrangement of multiple helices, highlighting the impact of the molecular symmetry on the chiroptical responses.

Chiroptical Properties of Symmetric Double, Triple, and Multiple Helicenes.

The research in circularly polarized luminescence has attracted wide interest in recent years. Efforts on one side are directed toward the development of chiral materials with both high luminescence efficiency and dissymmetry factors, and on the other side, are focused on the exploitations of these materials in optoelectronic applications. This review summarizes the recent frontiers (mostly within five years) in the research in circularly polarized luminescence, including the development of chiral emissive materials based on organic small molecules, compounds with aggregation-induced emissions, supramolecular assemblies, liquid crystals and liquids, polymers, metal-ligand coordination complexes and assemblies, metal clusters, inorganic nanomaterials, and photon upconversion systems. In addition, recent applications of related materials in organic light-emitting devices, circularly polarized light detectors, and organic lasers and displays are also discussed.

Frontiers in circularly polarized luminescence: molecular design, self-assembly, nanomaterials, and applications

Circularly polarized luminescence (CPL) based on aggregation-induced emission luminogens (AIEgens) has received increasing attention owing to the increase of applications to generate CPL devices and biologically active probes. CPL-active materials fabricated from AIEgens exhibit higher photoluminescence quantum yields and larger dissymmetry factors in the solid state in comparison with single molecules in solution. We called the phenomenon of enhanced CPL in an aggregated state aggregation-induced circularly polarized luminescence (AICPL). This review highlights recent research in the CPL generation of AIEgens from (macro)molecular luminogens and supramolecular assemblies based on the AIE mechanism of restriction of intramolecular motion. In addition, some cases in which CPL-active AIEgens have enabled advancement in stimulus response materials and application in CPL devices, especially circularly polarized electroluminescence. Thus, CPL-active AIEgens are highly promising candidates for a wide range of technologically oriented multidisciplinary fields.

https://pubs.rsc.org/en/content/articlepdf/2020/tc/c9tc07022b

Circularly polarized luminescence from AIEgens

The construction of circularly polarized luminescence (CPL) switches with multiple switchable emission states and high dissymmetry factors (glum ) has attracted increasing attention due to their broad applications in diverse fields such as the development of smart devices and sensors. Herein, a new family of AIE-active chiral [3]rotaxanes were designed and synthesized, from which a novel CPL switching system was successfully constructed. The switching process was realized through the controlled motions of the chiral pillar[5]arene macrocycles along the axle through the addition or removal of the acetate anions, which not only modulated the chirality information transfer but also tuned the aggregations of the integrated [3]rotaxanes, thus resulting in reversible transformations between two emission states with both high photoluminescence quantum yields (PLQYs) and high dissymmetry factors (glum ) values.

AIE‐active Chiral [3]Rotaxanes with Switchable Circularly Polarized Luminescence

We report the synthesis of one-dimensional supramolecular polymers composed of one-handed helical macromolecules bearing fluorescent pendant groups and the generation of circularly polarized light on the basis of hierarchical chiral amplification starting from a tiny amount of chiral substituent. Copolymerization of benzo[1,2-b:4,5-b']dithiophene-appended achiral/chiral isocyanides (99:1, mol/mol) with a solid-state photoluminescence feature afforded submicrometer supramolecular fibers, in which almost perfect single-handed helical polyisocyanides were noncovalently connected end to end. The resulting helical supramolecular polymers were further helically assembled to form a cholesteric liquid crystal film with an intense circularly polarized luminescence (CPL) signal. Surprisingly, the supramolecular system containing only 0.01 mol % of the chiral monomer unit also emitted the observable circularly polarized light owing to multiple chiral amplification from an infinitesimal point chirality to helical chirality and then to supramolecular chirality. Furthermore, chiral information was efficiently transferred from the helically assembled supramolecular system containing 1 mol % of the chiral unit to achiral dye molecules blended in the film, allowing full-color tunable induced CPL with luminescence dissymmetry factors greater than 1.0 × 10-2. This unprecedentedly strong chiral amplification enables the creation of helical supramolecular polymers and chirally assembled systems with various chiral functions based solely on an infinitesimal chiral source.

Helical Assemblies of One-Dimensional Supramolecular Polymers Composed of Helical Macromolecules: Generation of Circularly Polarized Light Using an Infinitesimal Chiral Source.

The circularly polarized luminescence (CPL) switching is of significant interest for applications in security technologies and sensing devices. Small organic molecules (SOMs) show several advantages over metal complexes, supramolecular assemblies, and polymers. Therefore, the recent progress on the CPL switching in SOMs is here reviewed. The results are summarized based on the strategies used to tune factors that influence the emission properties, and thus, to realize CPL switching. The strategies that have been adopted include promoting the excimer formation of fluorescent units, changing the conformation of fluorophores, tuning the electronic structure of the π-skeleton/substituent, and modulating the intramolecular charge-transfer dynamics.

Circularly Polarized Luminescence Switching in Small Organic Molecules.

https://chemistry-europe.onlinelibrary.wiley.com/doi/pdf/10.1002/chem.201986861

Jia‐Liang Ma

Papers

Circularly Polarized Luminescence Switching in Small Organic Molecules.

Frontispiece: Circularly Polarized Luminescence Switching in Small Organic Molecules