**3.1 Encapsulation of chiral nanofiber into polymer**

*G-***Pyr** forms nanofibers in various polymerizable monomers, such as styrene, divinylbenzene, methylmethacrylate, and methylacrylate (**Figure 16**) [27]. For example, when *G-***Pyr** is dissolved in methyl methacrylate at a concentration of 1 wt% and photopolymerized in a sandwich cell in the presence of a suitable photosensitizer, a transparent solid film can be obtained (**Figure 17**). When observing the CD spectrum of the obtained solid thin film, a CD signal showing a chiral orientation of pyrene groups was observed (**Figure 17**, red line). Since the strength and pattern are similar to those before polymerization (**Figure 17**, blue line), it is clear that the chiral orientation state of *G-***Pyr** is fixed in the solid thin film. On the other hand, when photopolymerization was performed at 70 °C, a colorless and transparent solid thin film was also obtained, but almost no CD signal was detected (**Figure 17**, black line). That is, it is shown that the molecular orientation state cannot be obtained even if polymerization is carried out under the condition that the molecular orientation is not formed (nanofibers are not formed at 70 °C).

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, 112–114]. **Figure 18** shows the CD and fluorescent spectra of a cast film prepared by spin coating from a PSt solution containing *G-***Pyr**. **Figure 18** clearly shows that the excimer state (b) and the chiral stacking state (c) observed in the solution are reproduced in the polymer thin film [115].

#### **Figure 16.**

*Aggregation morphology of G***-Pyr** *in various polymerizable monomers: [27] (a) styrene; (b) divinylbenzene; (c) methylmethacrylate; and (d) methyl acrylate.*

**Figure 17.** *CD spectra of G***-Pyr** *in methylmethacrylate before and after polymerization [27].*

#### **Figure 18.**

*UV–visible (a), fluorescent (b) and CD (c) spectra of G***-Pyr***-containing polystyrene film prepared by casting from polymer solution. [115] - reproduced by permission of John Wiley and Sons.*

The formation of nanofibers derived from the molecular gelation phenomenon in the polymer can also be detected by direct observation with an electron microscope [112]. As shown in **Figure 19a**, when an EVA film containing *G-***Pyr** is stained with an osmium plasma coater, it can be confirmed that nanofiber-like aggregates with a diameter of 10 nm or less are formed in the EVA film. On the other hand, for highly polar polymers, amphipathic *G-***Py+** can be composited, and hollow nanofibers (**Figure 19b**) are formed in the polymer.

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 *G-***COOH** is used as the chiral nanofiber source. After preparing a transparent cast film consisting of a dye-nanofiber–polymer system to which three kinds of cationic dyes were added, distinct CD signals were detected around each absorption band of the added dyes (**Figure 20**). Because the dye does not have any chiral carbon or chirality by itself, the obtained CD signals cause the added dye to electrostatically bind to the chiral nanofibers in the polymer film. Therefore, they are induced CD [50].

#### **3.2 Organic room temperature phosphorescent film**

A transparent film, as shown in **Figure 21**, can be produced by preparing a mixed solution containing *G-***BT** and EVA and forming a film by the casting

**123**

**Figure 19.**

**Figure 20.**

**3.3 CPL polymer film**

*Tuning of induced CD in dye-polymer mixed films [50].*

*Chiroptical Polymer Functionalized by Chiral Nanofibrillar Network*

method. The obtained film maintains a high Stokes shift and millisecond-order fluorescence lifetimes, likely to that of the solution systems. That is, the orientation state in the solution is also formed in the polymer. The formation of nanofibers in

**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 *G-***COOH** and a cationic cyanine dye [111]. Nanofibrils are detected using confocal microscopy in the polymer film, as shown in **Figure 22b**. **Figure 22c** shows

the polymer can be confirmed by laser microscopy in **Figure 21c** [83].

*TEM images of (a) G-Pyr aggregates in poly(ethylene-vinylacetate) [112] and (b) G-Py<sup>+</sup>*

*polyvinylpyrrolidone [5]. Stained with (a) osmium and (b) uranyl acetate.*

 *in* 

*DOI: http://dx.doi.org/10.5772/intechopen.96853*

*Chiroptical Polymer Functionalized by Chiral Nanofibrillar Network DOI: http://dx.doi.org/10.5772/intechopen.96853*

#### **Figure 19.**

*Current Topics in Chirality - From Chemistry to Biology*

The formation of nanofibers derived from the molecular gelation phenomenon in the polymer can also be detected by direct observation with an electron microscope [112]. As shown in **Figure 19a**, when an EVA film containing *G-***Pyr** is stained with an osmium plasma coater, it can be confirmed that nanofiber-like aggregates with a diameter of 10 nm or less are formed in the EVA film. On the other hand, for

*UV–visible (a), fluorescent (b) and CD (c) spectra of G***-Pyr***-containing polystyrene film prepared by casting* 

*from polymer solution. [115] - reproduced by permission of John Wiley and Sons.*

*CD spectra of G***-Pyr** *in methylmethacrylate before and after polymerization [27].*

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 *G-***COOH** is used as the chiral nanofiber source. After preparing a transparent cast film consisting of a dye-nanofiber–polymer system to which three kinds of cationic dyes were added, distinct CD signals were detected around each absorption band of the added dyes (**Figure 20**). Because the dye does not have any chiral carbon or chirality by itself, the obtained CD signals cause the added dye to electrostatically bind to the chiral nanofibers in the polymer film. Therefore, they are induced CD [50].

A transparent film, as shown in **Figure 21**, can be produced by preparing a mixed solution containing *G-***BT** and EVA and forming a film by the casting

can be composited, and hollow nanofi-

highly polar polymers, amphipathic *G-***Py+**

bers (**Figure 19b**) are formed in the polymer.

**3.2 Organic room temperature phosphorescent film**

**122**

**Figure 18.**

**Figure 17.**

*TEM images of (a) G-Pyr aggregates in poly(ethylene-vinylacetate) [112] and (b) G-Py<sup>+</sup> in polyvinylpyrrolidone [5]. Stained with (a) osmium and (b) uranyl acetate.*

**Figure 20.** *Tuning of induced CD in dye-polymer mixed films [50].*

method. The obtained film maintains a high Stokes shift and millisecond-order fluorescence lifetimes, likely to that of the solution systems. That is, the orientation state in the solution is also formed in the polymer. The formation of nanofibers in the polymer can be confirmed by laser microscopy in **Figure 21c** [83].

#### **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 *G-***COOH** and a cationic cyanine dye [111]. Nanofibrils are detected using confocal microscopy in the polymer film, as shown in **Figure 22b**. **Figure 22c** shows

**Figure 21.**

*Example of room temperature phosphorescence by G-BT-containing polymer film. Luminescence decay curves were obtained at 525 nm. The photographs (a) and (b) are the G-BT-containing polymer film on quartz glass under normal and UV lights, respectively. (c) Confocal image of the G-BT-containing polymer film was obtained with excitation at 488 nm [83] - reproduced by permission of the Royal Society of Chemistry.*

the CPL spectrum of this film, in which the spectral shape and emission wavelength are similar to the results observed in its solution system. These facts indicate that the nanofiber and composite structure of the dye formed in the solution system were maintained as they were in the polymer.

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, [111] 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 [116]. On the other hand, rare earth-free and low-toxic dyes are also required, and therefore, many organic fluorescent dyes have been

**125**

**Figure 23.**

**Figure 22.**

*Chiroptical Polymer Functionalized by Chiral Nanofibrillar Network*

developed [112, 115, 117–119]. Lightness, flexibility, and excellent processability are

*Induction of strong CPL from the composite polymer film from various fluorescent dyes with chiral molecular* 

*Aggregation morphology and CPL spectrum of the polystyrene composite film from the dye (NK-77) with* 

*G***-COOH** *system. [111] - reproduced by permission of the Royal Society of Chemistry.*

In this chapter, we will introduce the optical modulation function using selfassembled fluorescent nanofibers. As shown in **Figure 7**, a polymer film in which *G-***Pyr** is embedded in polystyrene absorbing light in the UV-A region, which is not absorbed by ordinary inorganic glass, and it emits visible light. **Figure 24** shows an example in which a polymer system consisting of a *G-***Pyr**-EVA composite is applied to the surface of a CIGS-based solar cell. Comparing the conversion efficiencies using simulated sunlight (AM1.5), it was confirmed that the power generation

essential advantages of organic materials.

*gel system. [111] - reproduced by permission of the Royal Society of Chemistry.*

*DOI: http://dx.doi.org/10.5772/intechopen.96853*

*Chiroptical Polymer Functionalized by Chiral Nanofibrillar Network DOI: http://dx.doi.org/10.5772/intechopen.96853*

#### **Figure 22.**

*Current Topics in Chirality - From Chemistry to Biology*

were maintained as they were in the polymer.

**3.4 Application for wavelength conversion**

the CPL spectrum of this film, in which the spectral shape and emission wavelength are similar to the results observed in its solution system. These facts indicate that the nanofiber and composite structure of the dye formed in the solution system

*Example of room temperature phosphorescence by G-BT-containing polymer film. Luminescence decay curves were obtained at 525 nm. The photographs (a) and (b) are the G-BT-containing polymer film on quartz glass under normal and UV lights, respectively. (c) Confocal image of the G-BT-containing polymer film was obtained with excitation at 488 nm [83] - reproduced by permission of the Royal Society of Chemistry.*

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, [111] ensuring good CPL strength and light resistance.

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 [116]. On the other hand, rare earth-free and low-toxic dyes are also required, and therefore, many organic fluorescent dyes have been

**124**

**Figure 21.**

*Aggregation morphology and CPL spectrum of the polystyrene composite film from the dye (NK-77) with G***-COOH** *system. [111] - reproduced by permission of the Royal Society of Chemistry.*

#### **Figure 23.**

*Induction of strong CPL from the composite polymer film from various fluorescent dyes with chiral molecular gel system. [111] - reproduced by permission of the Royal Society of Chemistry.*

developed [112, 115, 117–119]. Lightness, flexibility, and excellent processability are essential advantages of organic materials.

In this chapter, we will introduce the optical modulation function using selfassembled fluorescent nanofibers. As shown in **Figure 7**, a polymer film in which *G-***Pyr** is embedded in polystyrene absorbing light in the UV-A region, which is not absorbed by ordinary inorganic glass, and it emits visible light. **Figure 24** shows an example in which a polymer system consisting of a *G-***Pyr**-EVA composite is applied to the surface of a CIGS-based solar cell. Comparing the conversion efficiencies using simulated sunlight (AM1.5), it was confirmed that the power generation

#### **Figure 24.**

*Application of the wavelength conversion film (WCF) for CIGS solar cell. [6] - reproduced by permission of John Wiley and Sons.*

efficiency before coating increased from 15.0% to 15.9% due to coating [112]. This increase in conversion efficiency is attributed to the fluorescent nanofibers embedded in the polymer film that absorb ultraviolet (UV-A region) light with low spectral sensitivity of the solar cell and have an emission peak in the visible region (~460 nm by excimer emission) with high spectral sensitivity.

#### **4. Conclusions**

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 selfassembled 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.

#### **Acknowledgements**

This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology, Japan.

**127**

**Author details**

Hirotaka Ihara1,2\*, Makoto Takafuji2

2 Kumamoto University, Kumamoto, Japan

provided the original work is properly cited.

1 National Institute of Technology, Okinawa College, Nago, Japan

\*Address all correspondence to: ihara@kumamoto-u.ac.jp

and Yutaka Kuwahara2

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

*Chiroptical Polymer Functionalized by Chiral Nanofibrillar Network*

*DOI: http://dx.doi.org/10.5772/intechopen.96853*

#### **Conflict of interest**

The authors declare no conflict of interest.

*Chiroptical Polymer Functionalized by Chiral Nanofibrillar Network DOI: http://dx.doi.org/10.5772/intechopen.96853*

*Current Topics in Chirality - From Chemistry to Biology*

efficiency before coating increased from 15.0% to 15.9% due to coating [112]. This increase in conversion efficiency is attributed to the fluorescent nanofibers embedded in the polymer film that absorb ultraviolet (UV-A region) light with low spectral sensitivity of the solar cell and have an emission peak in the visible region

*Application of the wavelength conversion film (WCF) for CIGS solar cell. [6] - reproduced by permission of* 

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

This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology, Japan.

(~460 nm by excimer emission) with high spectral sensitivity.

**4. Conclusions**

**Figure 24.**

*John Wiley and Sons.*

for further development.

**Acknowledgements**

**Conflict of interest**

The authors declare no conflict of interest.

**126**
