Helically assembled π-conjugated polymers with circularly polarized luminescence

Polymers P5−7 exhibited enantiotropic main-chain liquid crystallinity at high temperatures and across a wide range of elevated temperatures. Polarized optical microscopy (POM) revealed the presence of an enantiotropic LC phase in all of the polymers. Figure 4 shows the optical textures of the LC phases of (R)-P6 and (R)-P7. (R)-P6 exhibited an oily streak texture (figure 4(a)) and a fingerprint texture (figure 4(b)) at approximately 200 and 150 °C, respectively, during the cooling process; both of these textures can be assigned to the N* phase. Meanwhile, (R)-P7 (figure 4(c)) exhibited a focal conic texture with a unique red color at approximately 156 °C during the cooling process, which is also attributed to the N* phase. (S)-P6 and (S)-P7 exhibited optical textures similar to those of the corresponding (R)-type polymers. P5s showed no distinct optical textures attributable to the N* phase. P5s were found to possess high clearing points (greater than 300 °C), which may be due to the higher molecular weight of the polymers.

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CD spectra of the P5s, P6s, and P7s in chloroform (c = 1.0 × 10⁻⁴ m) are shown in figures 5(a)–(c). Cotton effects are observed in the π−π* transition bands of the conjugated main chains. (R)-P5 exhibited a positive bisignate Cotton effect¹ that appeared in the π−π* transition band of the conjugated main chain that consisted of biphenylene-thienylene moieties, first with positive and then negative components at longer (420 nm) and shorter (370 nm) wavelengths, respectively. At the same time, (S)-P5 exhibited a completely reversed CD behavior, which was first negative and then positive (figure 5(a)). This behavior implies that a strong exciton coupling occurs in the main chains, leading to the formation of an interchain π-stacked structure [66]. The P6s and P7s also exhibited bisignate Cotton effects, in which the signals for the (R)- and (S)-configurations were mirror images (figures 5(b) and (c)). The observation of the bisignate Cotton effect for the polymers with three aromatic rings in the repeating unit suggests that the polymers are stacked with twisting through π−π interactions, resulting in a helical π-stacked assembly. Figure 6 presents a schematic representation of the helical π-stacked structure of the polymer assembly.

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The Cotton effects in the CD spectra decreased in intensity with increasing temperature and finally disappeared at 70 °C for the P5s and P6s and at 80 °C for the P7s. Additionally, the absorption bands of the polymers shifted toward shorter wavelengths with increasing temperature. These results also imply that the polymer assembly with a helical π-stacked structure exists even in dilute solution at room temperature.

The (R)- and (S)-polymer thin films also exhibited mirror-opposite Cotton effects in the region of the π−π* transition (figures 5(d)−(f)). At the same time, the CD intensity of the film increased by approximately 2−10 times that of the solution. These results are attributed to strengthening of the helical π-stacking between the polymer main chains in solid states, such as films.

Figure 7(a) presents the photoluminescence (PL) and CPL spectra of the P5s, P6s, and P7s in chloroform (1.0 × 10⁻⁴ m), and these polymers exhibit circular polarization in their fluorescence bands. The CPL signals for the polymers with (R)- and (S)-configurations appear in opposite directions, resulting in almost mirror images in solution and cast films; for example, the (R)-P6 exhibits a positive signal, whereas the (S)-P6 exhibits a negative signal.

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Figure 7(b) presents the CPL spectra of the polymer films. The g_lum values of the polymer films for the P6s and P7s increased relative to the corresponding solution by approximately 1 order of magnitude, whereas the g_lum values for the P5s only increased by approximately 2−3 times relative to the corresponding solution. This increase may be attributed to the strong π−π interactions between polymer backbones in the solid state, as mentioned above. The P6s and P7s, which have a bithienylene moiety in the repeating unit, possessed larger g_lum values than the P5s, which have biphenylene moieties rather than bithienylene moieties. This difference may be explained by the difference in coplanarity between bithienylene and bithienylene. Incorporating bithiophene into the polymer backbone leads to a higher degree of coplanarity and stronger π−π interactions, which result in larger g_lum values relative to the P5s.

The polymer films were annealed for 30 min at 150 °C, which falls in the LC temperature range, and were then subjected to CPL measurements. Figure 7(c) presents the CPL and g_lum spectra of the solutions, as-cast films, and annealed films for the P7s. After annealing the as-cast films, the absolute g_lum values increased by at least 1 order of magnitude and produced g_lum values as high as 10⁻¹, which is similar to those of chiral polyfluorenes [67, 68]. These g_lum values are very high despite the regiorandom structure of the polymers. This value may be due to the helical π-stacking during annealing at the LC temperature. Interestingly, the CPL signals inverted after annealing the P7s; that is, the g_lum of the as-cast (S)-P7 film showed a positive signal of +4.1 × 10⁻² at 680 nm that became a negative signal of −1.5 × 10⁻¹ at 666 nm after annealing. Annealing in the LC phase encourages the self-assembly of the polymer backbone and the formation of macroscopic π−π stacks, which lead to high absolute g_lum values [67, 68]. Annealing may also lead to helical packing of the polymer backbones [69] in the N* phase, which may be responsible for the inversion of the g_lum signal for the P5s and P7s. However, the P6s did not exhibit an inversion of the g_lum signal. In the P6s, the chiral substituent is linked to phenylene, whereas it is linked to thiophene in the P5s and P7s, as indicated above. Although the structural changes in the polymer thin films caused by annealing are not yet understood, any differences in the chemical structure may be related to the inversion of the g_lum signals caused by annealing. The hierarchical arrangement of higher-ordered structures by chiral liquid crystallinity, such as the self-ordering of the polymer aggregates in the N* phase by annealing at the LC temperature, is a promising route for highly enhancing the dissymmetry factor up to 10⁻¹ in the CPL of the chiral CPs.

Finally, the authors mixed solutions of the P5s, P6s, and P7s, which exhibit blue, green, and orange fluorescence, respectively. The actual ratio of P5, P6, and P7 employed in the mixture was 1:1:5. Figure 8(a) presents the CPL spectra of the polymer mixtures. System 1 is the mixture of ( R )-P5, ( R )-P6, and ( S )-P7, which displays a positive g_lum, and system 2 is the mixture of ( S )-P5, ( S )-P6, and ( R )-P7, which displays a negative g_lum. These mixtures presented mirrored CPL spectra from 400−700 nm and a g_lum value on the order of 10⁻³, indicating white CPL. Figure 8(b) presents a photograph of the polymer mixture in chloroform, which emits white light.

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However, the cast films prepared using the above mixtures exhibited red, not white, fluorescence due to polymer aggregation, which allows either an energy transfer from the excited states of P5 and P6 to that of P7 or the reabsorption of the fluorescence of P5 and P6 by P7. To suppress both energy transfer and reabsorption, the interchain distances in the polymers need to be increased. Therefore, the polymer mixture in system 1 was dispersed in an excess of polystyrene by dissolving both solutions in chloroform and then casting them onto a quartz substrate. This cast film sufficiently separated the polymers from each other in terms of their interchain distances to create a so-called 'pseudodilute solution' state.

Figure 8(c) depicts the white fluorescence of the film on a quartz substrate. The white luminescence is circularly polarized emission. The g_lum values increased by a factor of 1.5−5.8 compared with the corresponding values in solution but remained on the order of 10⁻³ because the polymers were dispersed in an excess of polystyrene to avoid aggregation by decreasing the interchain interactions, which results in conditions similar to those observed in the solutions.

2.2. Part 2: Reversible switching of circularly polarized luminescence by external stimuli

For the purpose of functionalizing CPL, it is very interesting to control chiroptical properties, especially CPL, through the use of external stimuli [70].

The conversion ratio from the open to closed forms of DE moiety was confirmed by ¹H NMR analysis. The ratios in a photostationary state (PSS) were determined to be 10% and 7−9% for the cast films of P13 and P14, respectively. PSS is an equilibrium state between the open and closed forms of DE moiety under UV irradiation.

The cast films of the polymers exhibited Cotton effects in the corresponding absorption region. The CD spectra of the P13 and P14 films are shown in figures 13(c) and (f), respectively. The (R)- and (S)-P13 films exhibited bisignate CD couplets that were mirror images of each other. This result implies that the polymers in the film are assembled to form a one-handed helical interchain π-stacked structure. On the other hand, the (R)-P14 and (S)-P14 films showed CD bands that were mirror images of each other. However, the CD bands exhibited no splitting shape and relatively weak signals compared with those of the P13 films. This result suggests that the P14 polymers form loosely π-stacked helical structures. The non-straight and rather bent backbone of P14, which consists of bithienylene-monophenylene repeating unit, might be less suitable for the formation of interchain π-stacking, thereby resulting in a weak CD signal. Interestingly, there are no differences in the CD spectra between the open state and PSS, irrespective of the P13 and P14 films and of (R)- and (S)-configuration. This result indicates that the helical π-stacked structure of the P13 film and the chiral structure of the P14 film remain unchanged even after the reversible photoisomerization between the open form and the closed one in the side chain.

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A fluorescence quenching behavior was observed in the cast polymer films, as shown in figures 14(a) and (c). The fluorescence band at 413 nm for (R)-P13 and that at 570 nm for (R)-P14 in the open state drastically decreased in intensity and then quenched upon UV irradiation. However, the quenched fluorescence was regenerated by irradiation with visible light, leading to a reversible photoswitching in fluorescence, as shown in figures 14(a) and (c). The ratios of the fluorescence intensity between the open state and PSS (I_open/I_PSS) were evaluated as follows: I_open/I_PSS (at 413 nm) = 240 for the (R)-P13 film and I_open/I_PSS (at 570 nm) = 80 for the (R)-P14 film. The fluorescence quenching might be due to an efficient energy transfer from the excited polymer main chain to the closed form of the DE moiety. However, the excited DE of the closed form changes to its ground state through a nonradiative transition.

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Notably, the cast films of the polymers show CPL and CD in the wavelength region corresponding to the transitions of the polymer main chains. The formation of helical π-stacked structures of P13 and P14 is supported by the observation of CPL in the cast films, as shown in figures 14(b) and (d). The open forms of P13 and P14 exhibit Cotton effects in CPL spectra. Figure 14(b) and (d) also show that the CPL intensities of the P13 and P14 films drastically decrease, irrespective of the (R)- and (S)-configuration, upon photoisomerization of DE moiety from the open form to the closed one. Namely, the quenching of CPL occurs in the PSS, which is associated with the quenching of fluorescence itself. The reverse photoisomerization from the closed form to the open one causes the CPL band to reappear. This photoswitching behavior was observed over 10 cycles.

The ratio of the CPL intensities (ΔI_open/ΔI_PSS) between the open state and the PSS of the (R)-P13 film at 414 nm and those of the (R)-P14 film at 650 nm were 13 and 7, respectively, which are smaller than the corresponding ratios of the fluorescence intensities (240 and 80 in I_open/I_PSS for (R)-P13 and (R)-P14, respectively). The degree of circular polarization of luminescence was evaluated using g_lum. As shown in figure 15, the g_lum values for the P13 and P14 films were estimated to be on the order of 10⁻² at 414 nm and 650 nm, respectively. Finally, note that the helically π-stacked structure formed in the films of the present conjugated polymers is so rigid that it is not affected by the photochemical opening and closing isomerization reaction of the DE moiety in the solid state. Thus, it was possible to control the reversible switching of CPL between emission and quenching through photochemical irradiations by retaining the chirality of the polymers.

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2.3. Part 3. Supramolecular structures of ionic ACPs with circularly polarized luminescence

Ionic ACPs are amphiphilic polymers that contain hydrophilic ionic groups in the side chains and a hydrophobic aromatic conjugated backbone. In addition, the conjugated backbones easily aggregate with each other due to their high π-stacking ability, and therefore, ionic ACPs can form a hierarchical nanophase-separated structure. Thus, ionic ACPs are considered to be promising candidates as blocks to construct highly ordered supramolecular nanostructures.

Figure 18(a) presents the UV−vis absorption and CD spectra of P16, BNP, and a mixture of P16 and BNP (abbreviated as P16−BNP) in methanol−water (50:50). P16 exhibited an absorption band at 340 nm, which corresponds to the π−π* transition of the π-conjugated backbone. BNP showed an absorption band at 228 nm, which corresponds to the π−π* transition of the binaphthalene. BNP exhibited a bisignate Cotton effect in the absorption band region due to the axial chirality of the binaphthalene. P16−BNP showed an additional absorption band at a wavelength (384 nm) that was 44 nm longer than that of P16. This result implies that the effective conjugation length of the P16 backbone increases, accompanied by an increase in the coplanarity of the polymer backbone, which could be due to the formation of an interchain π-stacked structure. Furthermore, P16−BNP exhibited an induced CD band in the π−π* transition region of the π-conjugated backbone (figure 18(a)). The strong bisignate Cotton effects observed at 376 and 341 nm imply the presence of an exciton-coupling phenomenon between the main chains [66]. The induced CD band, as well as the corresponding absorption band, increased in intensity with increasing concentrations of BNP (figure 18(c)). These results imply that P16 forms an assembly with BNP in methanol−water and that the assembly has induced chirality of the polymer moiety, which is caused by chirality transfer from the axially chiral compound (BNP) to the achiral polymer (P16). That is, the cationic P16 polymers are able to come sufficiently close to each other with the aid of the anionic BNP to yield an interchain helical π-stacked assembly [46, 80–86]. Notably, the absorption band at 384 nm and the CD band with the bisignate Cotton effect decreased in intensity with increasing temperature, and both absorption features finally disappeared at 75 °C (figure 18(d)). This result indicates that the helically π-stacked assembly is gradually destroyed upon heating to afford free P16s and BNPs. Because the aliphatic chiral compound, which bears two anionic sulfonate groups, gives no ICD even when it is mixed with P16, the π−π interaction between the conjugated polymer and the aromatic chiral compound, in addition to the presence of electrostatic interactions between the compounds, is indispensable for the formation of the π-stacked assembly.

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Figure 18(b) presents the PL and CPL analyses of P16, BNP, and P16−BNP in methanol−water (50:50). Not only did P16 exhibit blue fluorescence, but P16−BNP did as well, although the fluorescence intensity of P16−BNP was weaker than that of P16. This may be due to the presence of the so-called 'concentration quenching in fluorescence' phenomenon resulting from the formation of a polymer assembly with an interchain π-stacked structure [46, 84, 85]. It is intriguing that both P16−( R )-BNP and P16−( S )-BNP exhibited negative and positive monosignate Cotton effects at 414 nm in the CPL spectra. This result indicates that the former and the latter have right- and left-handed CPL, respectively. Note that the PL and CPL bands at approximately 370 nm arise from the chiral inducer, BNP.

Both Cotton effects in the CD and CPL spectra of P16−( R )-BNP and P16−( S )-BNP show mirror images in sign, implying that the helical sense of the polymer assembly strictly depends on the chirality of the chiral inducer, BNP. According to exciton coupling theory [66], positive and negative bisignate Cotton effects in CD spectra indicate right- (P-helicity) and left-handed (M-helicity) screw structures, respectively. Therefore, P16−( R )-BNP, which shows a negative bisignate Cotton effect in the CD spectrum, has a left-handed π-stacked structure with M-helicity. This is consistent with the negative monosignate Cotton effect observed in the CPL spectrum, which indicates the emission of circularly polarized light with M-helicity. On the other hand, P16−( S )-BNP, which shows a positive bisignate Cotton effect in the CD spectrum and a positive monosignate Cotton effect in the CPL spectrum, has a right-handed π-stacked structure with P-helicity. The absorption dissymmetry factor (g_abs) values were −9.8 × 10⁻² (at 376 nm) and +2.1 × 10⁻² (at 341 nm) for P16−( R )-BNP and +9.9 × 10⁻² (at 378 nm) and −2.8 × 10⁻² (at 342 nm) for P16−( S )-BNP. The g_lum values were −2.4 × 10⁻² (at 414 nm) for P16−( R )-BNP and +3.1 × 10⁻² (at 415 nm) for P16−( S )-BNP. The values of g_abs and g_lum increased with increasing amounts of BNP until the molar ratio of [BNP]/[P16] reached 2.0. The observation of dissymmetry factors on the order of 10⁻²−10⁻¹ in the CD and CPL spectra indicate that the polymer assembly has a considerably strong helicity due to the interchain π-stacked structure.

Figures 19(a) and (b) present scanning electron microscopy (SEM) images of the P16−BNP (1.0:2.0 mol/ mol) assembly. The SEM images reveal a spherical morphology formed by the polymer assemblies (figures 19(a) and (b)). The sphere sizes range from 140 to 255 nm, with an average size of approximately 200 nm. This result is in good agreement with the results of dynamic light scattering measurements, which indicated an average sphere size of 220 ± 57 nm. Figure 19(c) displays a fluorescence microscopy image of polymer spheres prepared by spin-coating at 1000 rpm on a glass plate. A mercury-vapor lamp with a wavelength of 365 nm was used as the excitation light, and a low-pass filter was employed to remove wavelengths shorter than 420 nm. As shown in figure 19(c), each sphere exhibits blue fluorescence.

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To elucidate the inner structure, the polymer spheres were observed using POM at room temperature. To obtain finer POM images of the polymer spheres, it is desirable to use a sample of large particles for the POM measurement. To increase the particle size, the polymer spheres were prepared by adding the BNP solution to the P16 solution very slowly and then leaving the mixture at room temperature for 7 days without stirring. Note that Maltese Cross patterns with extinction rings are observed in all spheres, as shown in figure 19(d). This result indicates that each sphere is constructed from a polymer spherulite [87–89]. The polymer spherulite is a spherical aggregate that consists of crystalline lamellae and interspaced noncrystalline sites, where the former (crystalline lamellae) radiate from the center of the spherulite. The appearance of a Maltese Cross pattern implies that the axis of the interchain π-stacked structure coincides with the polarization directions of the analyzer or the polarizer of a polarized optical microscope (figures 20(a)–(c)) [89]. Furthermore, the extinction rings indicate the formation of twisted lamellar structures along the radial axis of the spherulite. In this study, the twisted lamellae consist of helical π-stacked conjugated polymers. Plausible models of the polymer assemblies and spherulite are depicted in figures 20(c) and (d). The spherulites observed in POM are spherical aggregates of the polymer assemblies bearing the interchain helically π-stacked structure, whose helical axes are parallel to the radial direction of the spherulite, as shown in figure 20(c). The present system is the first example of a chiroptical spherulite that is hierarchically constructed from π-conjugated polymers. Note that the Maltese Cross pattern disappeared without a change in the pitch of the extinction rings upon thermal heating. At the same time, the absorption and CD bands at approximately 384 nm decreased in intensity with a slight blue-shift and disappeared (figure 18(d)). These results suggest that the decomposition of the polymer spherulite might occur simultaneously with that of the polymer assembly, although the spherulite is a hierarchically higher-ordered structure than the polymer assembly.

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