Chiroptical Polymer Functionalized by Chiral Nanofibrillar Network

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 ) [7]. 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 . G-SorPy⁺has no asymmetric carbon or optical activity in the sorbyl group. Therefore, the CD signal is not observed in the molecularly dispersed state. On the other hand, when the molecules form an ordered orientation state, a chiral stacking state is created in the sorbyl group ( Figure 5b ), because of which, a very large CD signal is observed around the absorption band of the sorbyl group. Such amplified chirality is often referred to as secondary chirality to distinguish it from chiral-source chirality. That is, the nanofibers produced by self-assembly are likely to exhibit amplified secondary chirality, and this supramolecular function is directly linked to the principle of chiroptical functionalization for polymers, which we will focus on in this section.

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. G-Pyrin Figure 3 is an amino acid derivative in which pyrene is introduced. Since G-Pyris a hydrophobic compound, it is insoluble in water but soluble in various organic solvents. When G-Pyris dissolved in hot cyclohexane or toluene to a concentration of approximately 5 mM and returned to room temperature, it forms a transparent gel [25, 26]. Figure 6 shows a transparent gel state and a xerogel prepared by freeze drying. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) indicated that clear gel and white solid were formed from fibrous components. When benzene was added to 5 mM G-Pyrgel to dilute it to a 0.2 mM mixture, which was heated to form a solution, no gelation was observed even when it was cooled to room temperature. However, the formation of fibrous aggregates was confirmed by TEM observation. These facts conclude that G-Pyrforms nanofibers, even under low concentration conditions (apparently in a solution state), that do not form a physical gel state.

When a 0.2 mM benzene solution of G-Pyrwas excited at 350 nm, corresponding to the absorption maximum of a pyrene group, a fluorescent spectrum with an emission maximum at 455 nm could be obtained, as shown by the solid line in Figure 7a . This is characteristic of the excimer from pyrene. No similar spectrum is observed for the starting material, pyrenebutanoic acid (dotted lines in Figure 7a ), which can be attributed to the monomeric state. These facts suggest that under the condition that G-Pyrforms nanofibrillar aggregates, the pyrenyl groups are concentrated from each other, wherein excimers are likely to be formed [26, 27, 28].

The CD spectrum also shows the formation of an oriented state by the aggregation of G-Pyr. As shown in Figure 7b , G-Pyrproduced from the L-enantiomer produces an extremely large positive signal (Cotton effect). Because there is no chirality in the pyrene group itself, this suggests chirality induction from an amino acid moiety to the pyrene group. No similar Cotton effect is observed under conditions where G-Pyris in a molecularly dispersed state. To explain this large Cotton effect, it is necessary to consider a phenomenon in which nanofiber-like aggregates are formed by chiral stacking with pyrene groups, as shown in the inset of Figure 7b . In G-Pyr, three amide bonds around the chiral carbon should be focused on as the driving force for producing a chiral stacking state because when the amide bond is changed to an ester bond, a spherical aggregate morphology called a vesicle is preferentially formed over the nanofibrillar aggregates [26].

G-BTis an amino acid derivative with a benzothiophene group as a photofunctional group [82]. When G-BTwas dissolved in methylcyclohexane, the fluorescent color changed significantly from yellowish green to bluish purple, depending on the temperature ( Figure 8a ). The confocal laser photomicrograph in Figure 8c shows that G-BTis dispersed in the solution as a fluorescent fibrillar aggregate. Moreover, when observing the fluorescent spectrum, an emission maximum can be observed around 550 nm at room temperature ( Figure 8b ). Since the emission wavelength of the monomer is approximately 330 nm, the Stokes shift differs by nearly 220 nm. This difference is accompanied by that in the fluorescence lifetimes. In fact, it was found that the lifetime in this new emission band around 550 nm was on the order of milliseconds ( Figure 8d ). Presumably, the nanofibrillar aggregates form a stacking state close to the nanocrystals around the benzothiophene group. When heating is used to promote relaxation of this orientation state or trifluoroacetic acid is added as a hydrogen bond inhibitor, the emission around 550 nm disappears and shifts to 330 nm, and the fluorescence lifetime is of the order of nanoseconds, which corresponds to that of the monomeric state of the benzothiophene group.

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 [84]. A similar phenomenon is observed in the interaction with polysaccharides that form secondary structures [85]. 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, G-Py⁺, which has a pyridinium-based cationic group that promotes electrostatic interaction, is adopted as an amino acid-based molecular gel-forming substance, and it is combined with a selected anionic cyanine-based dye [21]. It was clarified that such induced chirality can be observed even for a single atom such as bromine. Figure 11 shows an example in which induced chirality is expressed by the binding of bromine ions to a chiral molecular gel generated from a gemini surfactant [86].

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 G-SorPy⁺in Figure 5 ; G-SorPy⁺exhibits a large CD intensity in water, but not in ethanol [12]. Similar lyotropic behavior can be observed in G-Pyr. Its fluorescent color in cyclohexane is remarkably different from that in chloroform, and this is attributed to the difference in their solubilities.

An example of thermotropic behavior is the amino acid derivative G-Py⁺with a pyridinium group [4, 81]. As shown in Figure 12a , the CD intensity varies significantly depending on the temperature. This is because the G-Py⁺aggregates have two phase transition temperatures. Beyond these two-phase transition temperatures, the lateral diffusion rate of the molecule increases even in the aggregated state, and the secondary chirality based on the molecular orientation disappears.

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 G-Porwith a porphyrin as a functional group. In cyclohexane, a very large CD intensity is observed near the absorption band of porphyrin, but since there is no asymmetric carbon or chirality at the porphyrin site, the expression of the large CD signal is attributed to chiral orientation among the porphyrin moieties in cyclohexane gel. The addition of an imidazole derivative to this G-Porgel remarkably changes the CD pattern, but such a change does not occur with the addition of pyridine at the same concentration, [87] which is attributed to the different coordination abilities of the guest molecules.

Figure 12c shows the CD pattern of a molecular gel with amino acids having a terpyridyl group, G-TPy. The intensity and pattern of the CD spectra change significantly depending on the type of metal coordinated to the terpyridyl group [67].

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, [90] 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, G-Pyris a fluorescent substance with chirality; therefore, it is considered as a good candidate, but the resultant CPL intensity (g_lum value) was of the order 10⁻³ [66]. It is not superior to the chiral organic fluorescent molecules reported so far. One reason for this has emerged as an inference that there is no optical activity in the excimer emission of pyrene.

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 (|g_lum| > 0.1) was obtained using a molecular gel from G-Py⁺as a chiral template with a hydrophobic fluorescent dye such as anthracene ( Figure 14a ) [81] and an anionic cyanine-based fluorescent dye ( Figure 14b ) [24] combined. Figure 14b also shows that the emission wavelength can be easily tuned by selecting pyrene sulfonic acid or yellow dye as the fluorescent dye and that the positive and negative can be reversed by the enantiomers of the template [24].

The other example of CPL generation is presented by Figure 15 [111]. In this case, the G-COOH-based molecular gel having a carboxyl group is used as a chiral template, considering solubility in a non-aqueous solvent. It was confirmed that the induced CD and the induced CPL were expressed in the binary system combined with the cationic dye. However, in this case, it was also confirmed that the coexistence of a base such as triethylamine in the solution significantly improved its strength. Probably because of the insufficient acidity of the carboxyl group, neutralization of the anions in the cationic dye promoted a strong interaction between the dye and the chiral template.

3.1 Encapsulation of chiral nanofiber into polymer

G-Pyrforms nanofibers in various polymerizable monomers, such as styrene, divinylbenzene, methylmethacrylate, and methylacrylate ( Figure 16 ) [27]. For example, when G-Pyris 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-Pyris 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, 113, 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].

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-Pyris 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-COOHis 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-BTand EVA and forming a film by the casting 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-COOHand a cationic cyanine dye [111]. Nanofibrils are detected using confocal microscopy in the polymer film, as shown in Figure 22b . Figure 22c shows 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 (Eu^{2 +}) and cerium ions (Ce^{3 +}) 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 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 G-Pyris 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 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.