Chirality is one of the basic factors that influence a wide range of activities from chemical synthesis to tissue construction in life phenomena. Recently, researchers have attempted to use chirality as an optical signal. In animals, it is used to transmit information to insects and crustaceans, and it has also been confirmed that it promotes growth in plants. This chapter presents a new organic system that produces a chiral optical signal, that is, circularly polarized luminescence (CPL), which has been attracting attention in recent years. In particular, the chapter is focused on the generating CPL through chirality induction with the chiral self-assembling phenomenon and explaining its application as an optical film.
- supramolecular gel
- circular dichroism
- circularly polarized luminescence
Fiber materials have applications in various industrial fields. One of these is compounding with bulk polymer materials to improve and strengthen their physical strength by controlling factors such as the bending elastic modulus, tensile strength, and thermal expansion coefficient. Generally, the higher the dispersion of fibers in polymer, the higher the effect on polymer.
Typical examples of fibers with industrial applications are carbon fibers, aramid fibers, and whiskers. In recent years, cellulose nanofibers  having a nano-sized diameter have attracted attention not only because they can be manufactured using inexhaustible and inexpensive raw materials but also because the strength of each cellulose nanofiber is 1/5th that of iron and 5 times the strength [2, 3]. Therefore, if the process cost and the method of dispersion of cellulose nanofibers in bulk materials can be improved and established, it will be used in various materials and fields, such as automobiles and home appliances.
In this chapter, we focus on the low light-scattering property due to the sufficiently small diameters as optical materials because transparency is one of the most important physical elements. Inorganic glass is a transparent material with excellent heat resistance, light resistance, and chemical resistance. Therefore, it is almost universally used as a partition plate during material conversion and energy conversion using light as an energy source. However, inorganic glass is heavy, inflexible, and fragile, and to improve impact resistance, laminated glass with an organic polymer as an interlayer film is often used. Therefore, there is a need for a transparent material that is light, soft, and has good processability. Transparent polymers are suitable to replace glass because organic polymers are essentially light at the elemental level and can be adjusted for hardness and flexibility. In addition, it is attractive that expansive functionality can be tailor-made at the monomer level.
One of the methods for imparting functions while taking advantage of such characteristics of organic polymers is to blend fillers. This method is particularly applicable to general-purpose polymers and is widely used for function enhancement and function conversion. Among them, nano-sized fillers are attracting particular attention in applications that require transparency because they have low light-scattering properties. However, there are several challenges in the use of nanofillers, and at present, reducing process costs and simplifying the compounding process are the main focus issues. In particular, simplification of the compounding process is a universal problem that results from the high specific surface area of nanomaterials and is thus often a barrier to development and research. This problem also exists in the cellulose nanofibers mentioned above.
This chapter outlines an approach ( Figure 1 ) that utilizes nanofiber formation [4, 5, 6] using the self-assembly of small molecules as a new strategic method for functionalizing polymer materials; also, an example of its application as an optical material is introduced.
2. Opt functional nanofibers by self-assembly
While various methods for creating nanofibers have been developed, it is difficult to unravel a mass of nanofibers once entangled by a process that is complicated and requires high energy. A typical example is cellulose nanofibers (CNFs). CNFs are environmentally friendly and have excellent potential functions; therefore, ensuring efficient dispersion of CNFs is essential to develop their applications. The method introduced in this chapter is not used for unraveling entangled nanofibers, but rather a for growing nanofibers in a bulk polymer ( Figure 1 ). Therefore, because the material used is a low-molecular-weight organic substance, a nanofiber-forming material that is structurally compatible with the polymer material can be selected. This approach promises a surfactant-free process in compounding polymeric materials.
To grow nanofibers in polymers, we utilize self-assembling gelation, which is a novel method in solution systems. Well-known molecular materials for this purpose are cholesterol derivatives , sugar derivatives , and amino acid derivatives [9, 10] ( Figure 2 ). These materials associate in a self-organizing manner by promoting weak intermolecular interactions and orientation states that are attributed to the molecular design. It is also known that the presence of chirality in the molecular structure tends to make the association form a nanofiber-like structure. After a certain concentration, nanofibers become entangled, resulting in gelation of the solution, which is called a molecular gel or supramolecular gel and is different from a typical polymer gel. Figure 3 summarizes amino acid-derived molecular gel-forming materials [5, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76] discussed in this chapter.
2.1 Principle of opt functionalization by molecular gelation
Following are some essential requirements for small molecules to assemble in solution to form nanofibers. (1) The molecule has a part that is dispersible in a solvent, (2) a part that exhibits a relatively strong interaction among molecules, (3) a rigid and non-bulky structure that promotes intermolecular stacking, which is useful as an auxiliary function, and finally, (4) a mechanism that facilitates one-dimensional growth of the association structure set in the molecular structure. The existence of molecular chirality that promotes twisted molecular stacking can be considered the most effective mechanism. A molecular gel comprising a cholesterol derivative, reported by Weiss et al. in 1987, is one of the small molecules that is suitable for this application ( Figure 2 ) . However, it has been reported that nanofiber-like aggregates are formed by various low-molecular-weight substances when not limited to the apparent gel state. For example, the authors found that nanofiber-like aggregates could be formed by fibrous bilayer membrane structures in aqueous systems [9, 77]. Since Weiss’s report, non-cholesterol derivatives, such as derivatives from amino acid, peptide and polysaccharide, are known as molecular gel-forming substances, and they have led to remarkable developments in this field [78, 79, 80].
Figure 3 shows typical amino acid derivatives that produce molecular gels. These molecules satisfy all four requirements described above. These derivatives are more advantageous than cholesteryl derivatives because hydrogen bonds are based on intermolecular interactions, and therefore, by properly tuning the molecular structure, nanofiber structures can be formed in a wide range of solvents, such as water and organic solvents. Further, as shown in the electron micrograph of Figure 4 , they all form nanofibrillar aggregates although the detailed aggregation morphology differs based on the structure. The formed aggregates have a nano-sized diameter and are rarely bundled, even if they are mixed in the polymer as described later; therefore, there are few whitening due to light scattering. This is an extremely important property for producing a transparent opt functional film.
Molecular gels made from amino acid derivatives often exhibit amplified chirality. Amino acids have an asymmetric center, because of which they act as chiral materials by themselves, and a quite low circular dichroism (CD) intensity, which is a measure of the magnitude of chirality. When these amino acid derivatives are aggregated with hydrogen bonds and a twist in a certain direction occurs in the orientation state, a very large CD signal (Cotton effect) can be obtained; a typical example is shown in Figure 5 .
2.2 Luminous nanofiber
There are roughly two methods for imparting optical functions to self-assembled nanofibers. One is the single-component system, which is a method for introducing a luminescent functional group during molecular design. The other method is the binary system, which is a method for combining self-assembled nanofibers as a template with a luminophore as a guest molecule by molecular interactions such as electrostatic interaction.
2.2.1 Single-component system for chirality enhancement
First, a molecular gel system introduced using pyrene, which is widely known as an organic fluorescent dye, is described.
When a 0.2 mM benzene solution of
The CD spectrum also shows the formation of an oriented state by the aggregation of
2.2.2 Binary system for chirality enhancement
While the method of obtaining chirality enhancement by the single-molecule system outlined in Section 2.2.1 has a high degree of freedom in molecular design, there are practical limits in the synthetic process. As a solution to this problem, a binary system has been proposed in which a chiral molecular gel is used as a template material with a highly-ordered microenvironment and is combined with a non-chiral luminescent material. The advantage of this approach is that even a luminescent dye with a complicated structure has few synthetic chemical restrictions because introducing chirality into it is not necessary.
In this method, the wavelength of the amplified CD signals can be easily controlled based on the dye selected by incorporating an appropriate interaction system between the chiral template orientation and the luminescent dye. This idea has been attempted by several researchers over the years. For example, when a polyamino acid with an ionic functional group in the side chain interacts with a particular dye, dimer formation and the accompanying formation of secondary chirality of the dye are observed . A similar phenomenon is observed in the interaction with polysaccharides that form secondary structures . Figure 9 shows the CD spectra of hyaluronic acid and cyanine dyes. Because there is no asymmetric carbon in the cyanine dye used, it can be concluded that the CD spectra in Figure 9 are induced.
There are many examples of chirality induction due to the combination of molecular gels and non-chiral dyes. Figure 10 shows an example of induced chirality as the binary system. In this case,
2.2.3 Stimuli-responsive chirality
Since molecular gelation by a low-molecular-weight compound is a phenomenon induced by molecular assembly, there is a critical gelation concentration, and it varies depending on the kind of solvent. Moreover, the hydrocarbon chain in the molecular structure has a phase transition phenomenon corresponding to the melting point. Therefore, it is easy to understand that molecular gels can have both lyotropic and thermotropic properties.
A typical example of a lyotropic property is given in
An example of thermotropic behavior is the amino acid derivative
Molecular gels are sensitive not only to solvent types and temperatures but also to other external factors. Figure 12b shows the CD spectra of the amino acid derivative
Figure 12c shows the CD pattern of a molecular gel with amino acids having a terpyridyl group,
As described above, chiral molecular gels can significantly change chirality with respect to various external factors, so their use for sensing using this responsiveness is also attractive.
2.3 Circularly polarized luminescent molecular gel
Circularly polarized luminescence (CPL) is angle-independent and contains light information not found in normal light, and therefore, it is expected to be used in various applications. Potential applicability is found in many industrial fields such as biometric recognition systems, light sources for plant factories, storage memories, and 3D displays. On the other hand, considering organic materials that generate CPL, although there are numerous research results, [88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110] their strength (optical purity) is low and they have not yet been put into practical use.
Figure 13 summarizes the structural formula of single-molecule CPL-generating dyes [90, 92, 94, 96, 99, 103, 104, 105, 106, 107, 108, 109, 110]. Most conventional organic materials that generate CPL are chiral fluorescent dyes that utilize the twisting and straining of molecules,  and therefore, require complicated chemical synthesis and purification processes. To overcome these problems and fabricate a CPL generation system with higher optical purity, the authors utilized self-assembled nanofiber systems formed from amino acid-derived molecular gels.
As mentioned in Section 2.1,
On the basis of this finding, we applied a binary method wherein an existing non-chiral fluorescent dye was incorporated into a molecular gel as a chiral template for CPL generation. This method is advantageous because it is not necessary to introduce chirality into the fluorescent dye, because of which, the degree of freedom in synthetic chemistry is greatly increased. In addition, the light emitting region of the CPL can be easily tuned by proper selection of the dye. As a result, the highest value in CPL (|
The other example of CPL generation is presented by Figure 15 . In this case, the
3. Application for chiroptical polymer film
3.1 Encapsulation of chiral nanofiber into polymer
A more convenient method for producing a nanofiber composite polymer film is casting method that uses a polymer solution of general-purpose polymers such as polystyrene (PSt), polymethylmethacrylate (PMMA), and poly (ethylene-vinyl acetate) copolymers (EVA) [24, 82, 112, 113, 114]. Figure 18 shows the CD and fluorescent spectra of a cast film prepared by spin coating from a PSt solution containing
The formation of nanofibers derived from the molecular gelation phenomenon in the polymer can also be detected by direct observation with an electron microscope . As shown in Figure 19a , when an EVA film containing
In the casting method, it is also possible to entrust the optical function to the added dye. Figure 20 shows an example when an anionic molecular gel with
3.2 Organic room temperature phosphorescent film
A transparent film, as shown in Figure 21 , can be produced by preparing a mixed solution containing
3.3 CPL polymer film
Figure 22a shows a glass plate in which polystyrene with a composite CPL source is cast on one side. The CPL source is a binary system using a molecular gel from
Cyanine-based dyes, such as NK77 and 2012, are attractive as they combine with molecular gels to ensure good CPL strength; however, they are fragile due to light resistance. Therefore, binarization with a more chemically stable fluorescent dye is required. Figure 23 shows the CPL spectra of a polymer film fabricated in combination with a more light-resistant dye,  ensuring good CPL strength and light resistance.
3.4 Application for wavelength conversion
The wavelength of sunlight and artificial lights cannot always be suitable for their applications, and light management using methods such as shading, wavelength cutting, and polarization is required depending on the application. In silicon-based solar cells, the spectral sensitivity to ultraviolet and near-infrared lights is low. Therefore, high energy levels of ultraviolet light are cut by glass. In order to effectively utilize unused light, it is necessary to introduce a technology that converts unnecessary ultraviolet and near-infrared light into visible light. Various fluorescent materials that use rare earths are leading the way in this field of application: nitride systems containing europium ions (Eu2 +) and cerium ions (Ce3 +) as activators, and rare earth materials such as garnet-based materials are used as fluorescent materials for lamps and white LEDs . On the other hand, rare earth-free and low-toxic dyes are also required, and therefore, many organic fluorescent dyes have been developed [112, 115, 117, 118, 119]. Lightness, flexibility, and excellent processability are essential advantages of organic materials.
In this chapter, we will introduce the optical modulation function using self-assembled fluorescent nanofibers. As shown in Figure 7 , a polymer film in which
In this chapter, we describe the characteristics of self-assembled nanofibers generated from amino acid-derived molecules, the expression principle of their unique optical properties, and their complexing with polymers. Since self-assembled nanofibers are structures formed by non-covalent bonds, one side that is physically fragile remains. However, conventional problems related to the dispersion of nanofibers are solved, and complicated dispersion techniques and surface modification processes that cause harmful effects are not required. Therefore, it is a material that enables higher-order functionalization of the polymer material while maintaining characteristics of the bulk polymer. In addition, various techniques for complementing vulnerabilities have been proposed. The optical management film with the optical modulation function introduced in this chapter is expected to be used not only for solar cells but also for various applications such as housing, automobiles, displays, artificial lighting, and plant factory lighting. We hope that the methodology using molecular gel-based functionalization can provide findings for further development.
This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology, Japan.
Conflict of interest
The authors declare no conflict of interest.