Due to their distinctive optical activity, CPL materials have attracted much attention. To address the needs of optoelectronic applications in organic light-emitting diodes (OLEDs), chiral sensing, information encryption, etc., an increasing number of research projects have been concentrated on the molecular design of superior CPL materials. This article discusses the impact of structure on the design of CPL polymers.
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Circularly Polarized Luminescence (CPL) Polymers
CPL-active polymeric materials are superior in terms of scalability, flexibility, 3D stacking ability, and film-forming properties. CPL can be produced directly using chiral luminescent materials, avoiding the drawbacks of conventional techniques. Two widely used methods for analyzing chiroptical characteristics are circular dichroism (CD) and the CPL spectrum.
Because of their naturally magnetic dipole-allowed transitions, lanthanide (III) complexes have long been a popular candidate for researching the characteristics of CPL. Conjugated polymers, the most widely reported polymer-based CPL materials, could frequently produce strong fluorescence emission in a diluted solution by carefully designing electron-rich and electron-deficient repeating units. Many polymer-based chiral systems with good CPL properties have been discovered owing to the development of diverse chiral monomers and unique manufacturing methods for chiral systems.
Applications of Polymer-Based CPL Materials
Several reviews on polymer-based CPL materials have been published recently, summarizing their findings and focusing on topics like induced CPL for block copolymers, helical polymers, chiroptical properties in thin films of p-conjugated systems, aggregation-induced emission (AIE) polymers, colloidal polymers, and helically assembled p-conjugated polymers.
Recent studies reported the generation of chiral binaphthyl derivatives, glucose-linked biphenyl units, 1,2-diaminocyclohexane, triptycene, optically stable helical aromatics, and natural bio-architectures to conventional chiral monomers for the synthesis of CPL-active polymers.
One of the recent studies reported the development of a polylactide-based crystalline chiral that displayed full-color CPL by integrating fluorophores in the film and allowed for easy film tilting to adjust the handedness and magnitude of CPL with a maximum value of 9.5 x 10-3. Another study discussed the development of a polymer based on biphenyl with a rigid torsion skeleton, which demonstrated strong ultralong phosphorescent emission for up to 33 s at low temperatures.
The covalent bonding of chiral units into fluorescent polymer structures is a universal technique for producing CPL signals. However, additional chiral monomer or polymer synthesis could reduce costs and complexity. In most cases, chiral polymers must be created using chiral catalysts or chiral monomers. Another flexible method to produce CPL emission is the chiral induction of achiral polymer through the chiral polymer. The density, hardness, and mechanical strength of polymer spherulites with radiation structure are higher than those of randomly aggregated polymers, giving them additional potential mechanical and optical qualities.
CPL-active polymers have been actively utilized in asymmetric photosynthesis, biomolecular sensing, cell differentiation, information technology, chiral optoelectronics, etc. Chiral polymers are a subset of CPL materials that are primarily used in information processing, chemical recognition, and OLEDs.
It has remained difficult to manage CPL electrically because the majority of CPL materials are non-electroactive and non-electrosensitive. Recently, a study showed that applying an alternating voltage to helical microfibers made from the co-assembly of chiral camphor sulfonic acid, conducting polyaniline, and tetraphenylethene (TPE), CPL signals could be switched on and off in a reversible manner and be used for double-layer information encryption. Moreover, the universality of this technique might be demonstrated by switching CPL signals in the same manner, utilizing quantum dots as a light source.
Recent Studies
In a recent study published in the Coordination Chemistry Reviews, the authors reviewed and discussed the creation of CPL-active polymers, including chiral side chain polymers, chiral main chain polymers, chiral coordination polymers, and chiral induction systems using polymers, with a focus on the polymer structure-dependent design technique. They also discussed recent applications of CPL-active polymers.
The design approach of CPL-active polymers was provided from four perspectives, namely, chiral coordination polymers and introduction to its application in OLEDs, molecular recognition, and information processing, to further this field's advancement. They also discussed the functioning chiral units in the polymer side chain and in the polymer backbone of CPL-active polymers based on noncovalent interaction and chiral coordination polymers.
The authors categorized the side-chain polymers into four groups: poly(fluorene), polyacetylenes, poly(quinoxaline-2,3-diyl), and other different side-chain chiral polymers. The design technique for incorporating chiral moiety in the polymer backbone achieved the required CPL property easily compared to the fluorescent polymer with the chiral unit in the pendant of polymer.
Conclusion and Future Perspective
The research to date has been primarily focused on investigating CPL property in its ordered states, such as in films, liquid crystal (LC) systems, and aggregates, because of possible greater glum in the ordered structure. The most prevalent polymer-based CPL materials are fluorescent polymers with chiral units in polymer side chains. Because of their stable chiral conformation and high chiral induction, polymers with chiral units in the main chain are easier to create CPL signals than polymers with chiral units in the side chain.
The non-covalent interaction between achiral polymers and chiral environments greatly enriches CPL materials systems. It could lessen the challenges and costs associated with the additional synthesis of chiral monomers or fluorescent polymers. However, the key lies in the careful planning of the chiral perturbation generation process. Although creating chiral coordination polymers can be a useful way to obtain better CPL signals, CPL-active coordination polymers are still relatively hard to come by.
Many potential chiral monomers with complicated structures and polymerization techniques have been developed with the advancement of synthesis technology, such as porous CPL polymer, based on the development of asymmetric catalysis. Moreover, chiral supramolecular polymers with predicted helicity and well-controlled dimensions could be created using the ground-breaking synthesis technique known as asymmetric live supramolecular polymerization. It has enormous promise for use in the search for CPL-active polymers with intricate structural patterns and diverse functional properties.
Also, dynamic CPL switching and glum amplification have seen significant advancements in chiral LC devices. Lyotropic N*-LC systems have received less attention than thermotropic N*-LC systems, although they could be a viable alternative for the study of CPL optoelectronic devices.
In conclusion, despite significant advancements in the study of CPL, its use is still in its early stages. The primary applications for CPL materials are optoelectronic devices. The development of CPL materials will benefit from additional research into their uses in various disciplines, particularly life science.
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References and Further Reading
Yang, S., et al. (2023). Circularly polarized luminescence polymers: From design to applications. Coordination Chemistry Reviews, 485, 215116. https://doi.org/10.1016/j.ccr.2023.215116
Yang, K., et al. (2022). Helix-Sense-Selective Polymerization of Achiral Monomers for the Preparation of Chiral Helical Polyacetylenes Showing Intense CPL in Solid Film State. Macromolecular Rapid Communications, 43(11), 2200111. https://doi.org/10.1002/marc.202200111
Khorloo, M., et al. (2021). Enantiomeric Switching of the Circularly Polarized Luminescence Processes in a Hierarchical Biomimetic System by Film Tilting. ACS Nano, 15, 1, 1397–1406. https://doi.org/10.1021/acsnano.0c08665
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