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Combining Chiral Helical Polymer with Achiral Luminophores for Generating Full-Color, On−Off, and Switchable Circularly Polarized Luminescence Biao Zhao,†,‡ Kai Pan,‡ and Jianping Deng*,†,‡ †

State Key Laboratory of Chemical Resource Engineering and ‡College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China

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S Supporting Information *

ABSTRACT: Circularly polarized luminescent (CPL) materials are currently drawing ever-increasing interest. This contribution reports the first success in simply combining chiral helical substituted polyacetylenes (HSPAs) with achiral luminophores to fabricate CPL materials demonstrating a high dissymmetry factor (glum) up to 10−1, despite neither covalent nor noncovalent interactions occurring between the two components. Circularly polarized scattering and fluorescence-selective absorption mechanisms are proposed for the generation of CPL, and a “matching rule” is further established for selecting chiral polymers and achiral luminophores for the purpose. Taking advantage of the circularly polarized scattering effect, full-color tunable CPL materials are prepared from the combination of achiral fluorescent dyes and chiral HSPAs. Following the fluorescence-selective absorption mechanism, functional composite films with on−off and switchable CPL performance are fabricated. Also, remarkably, the glum value in the prepared materials can reach up to +0.323. The present study provides a simple, powerful, and universal strategy for constructing novel CPL materials. To justify the hypothesis above, i.e., whether fluorescent light can be handed-selectively separated by chiral matters, we in this study designed a process to let fluorescent light pass through chiral matters using chiral helical polymers as a model. Excitingly, we found that such a process can result in the expected CPL signals. Over the past decade, artificial chiral helical polymers have been intensively investigated due to their unique macromolecular helical architectures and outstanding optical activity.30−45 They have proved to hold significant potentials in chiral-related areas such as asymmetric catalysis,46−48 chiral recognition/resolution,49 enantioselective crystallization,50,51 and enantioselective-controlled release.52,53 CPL-active materials based on chiral helical polymers are in particular attractive.54−57 Very recently, we have prepared CPL-active materials based on chiral helical substituted polyacetylenes (HSPAs) by introducing fluorescent groups to helical polymer backbones.58 The present contribution reports our success in achieving tunable CPL emissions by taking HSPAs as chiral filter, in which high glum values up to 10−1 can be realized even when there is no interaction between chiral moieties and fluorescent chromophores. Following this strategy, full-color, on−off, and switchable CPL materials were further prepared. Herein, two important points are worthy to be highlighted: (1) The first one is that the generation mechanism of CPL in

1. INTRODUCTION Today, functional materials with circularly polarized luminescence (CPL) performance have been becoming a rapidly rising star in both material science and chiral-related areas for their significant potentials in 3D optical displays, optical storage devices, photoelectric devices, and so forth.1−8 To date, the reported CPL materials have been mainly prepared by combining chiral moieties with chromophores through covalent bonds or noncovalent interactions (such as metal− ligand coordination, electrostatic, and π−π interaction).9−18 However, the routine methods generally need tedious preparation processes, and the interactions between chiral component and fluorescent component have been regarded as indispensable factors for successfully achieving chirality transfer. To obtain CPL without any interaction between chiral units and fluorescent units is advantageous but highly challenging. Fortunately, inspired by the circularly polarized reflection effect in nature and living organisms,19−22 scientists have achieved CPL derived from chiral liquid crystals23−27 and crystalline chiral polymers.28,29 Thus, even if the incident light is unpolarized, the transmitted light from these chiral matters can be highly polarized. We further hypothesize that fluorescent light can be handed-selectively separated by general chiral matters even without crystalline structures. If so, CPL can be directly constructed by letting racemic fluorescent light pass through chiral matters. This strategy will provide more alternatives for preparing CPL-active materials and significantly increas their varieties due to the numerous kinds of chiral matters and luminophores. © XXXX American Chemical Society

Received: October 26, 2018 Revised: December 11, 2018

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number-average molecular weight (Mn) of the prepared PSA is 4200. PDA was prepared by solution polymerization of achiral fluorescent monomer DA.60 Specific details are presented in the Supporting Information. Circular dichroism (CD) and UV−vis absorption spectra of PSA are illustrated in Figure 1. The polymers show intense mirror-CD signals around 425 nm in the polymer backbone region (Figure 1a), in agreement with our previous studies dealing with the same polymer.59,61 The Cotten effects have been well explored in substituted polyacetylenes due to the selective absorption of light by their preferred-handed helical structures.62,63 The results demonstrate that the obtained PSA possesses intense optical activity derived from predominantly one-handed helical conformations in polymer main chains. Just as expected, no Cotton effect is found in the wavelength range of 300−600 nm in PDA due to the absence of chiral factor in DA (Figure S1 in the Supporting Information). That is PDA is a racemic helical polymer and possesses no optical activity.60 Specific optical rotations ([α]D) also provide a significant support for the above findings. The [α]D values of R-PSA and S-PSA are −1402 and +1390, respectively, while the corresponding [α]D value of PDA is ∼0. Besides, the Kuhn dissymmetry factor gabs of PSA was further calculated. The gabs values of R-PSA and S-PSA at 425 nm are −0.017 and +0.011, respectively. The results definitely substantiate the strong optical activity of PSA. We carefully examined the CD spectra and observed that the PSA solutions show intense CD bands longer than 465 nm (Figure 1b), while no obvious absorption band is found in the corresponding UV−vis region (Figure 1a). When rotating the front and back face of PSA solutions for CD measurement, we found that the CD spectra hardly show any change. This finding indicates that the Cotten band longer than 465 nm in Figure 1b cannot be simply due to linear dichroism (LD) and linear birefringence circular dichroism, which strongly depend on the sample orientation.64,65 Instead, the CD bands may be contributed by circularly polarized reflection and circularly polarized scattering effects.66,67 Circularly polarized reflection effect is well-known to exist in chiral liquid crystals.19−22 Recently, Yashima and co-workers have prepared chiral liquid crystalline polyacetylenes in concentrated solution.68−70 We next investigated whether PSA solution could form chiral liquid crystals (LCs). If this is the case, the CD signals longer than 465 nm may be derived from circularly polarized reflection. The polarized optical micrograph of PSA solution

this work is due to circularly polarized scattering and fluorescence-selective absorption effects of chiral HSPAs, which will be discussed in detail below. We envisage this mechanism will provide more inspirations for the design of novel chiroptical materials. (2) The other is that we have established a quite simple and universal strategy for preparing CPL materials. Particularly, besides artificial chiral polymers, the established strategy is also potentially applicable to natural chiral macromolecules and chiral supramolecular self-assembly systems for developing new CPL materials.

2. RESULTS AND DISCUSSION Demonstration of Chirality and Fluorescence. The strategy for preparing CPL-active materials is illustrated in Scheme 1. The CPL was accomplished in chloroform (CHCl3) Scheme 1. Schematic Illustration for Preparing CPL by Taking Helical Substituted Polyacetylenes as Chiral Filter and Chemical Structures of PSA, DA, and PDA

by taking chiral helical polymer (PSA) as the chiral source, with achiral fluorescent molecules (DA) and racemic fluorescent polymer (PDA) as the emissive source. PSA was prepared by precipitation polymerization of chiral substituted acetylenes (SA) according to our previous literature.59 The precipitation polymerization product of SA exhibited good solubility in CHCl3 solution due to the relatively low numberaverage molecular weight, while the solution polymerization product was found to be only partly soluble in CHCl3. The

Figure 1. (a) CD and UV−vis spectra of R-PSA and S-PSA in CHCl3. [R-PSA] = [S-PSA] = 1 mM. (b) Magnified CD spectra. (c) Fluorescence spectra of the PSA, DA, and PDA (1 mM, λex = 369 nm). The inset presents the photo images of the corresponding solution samples under UV 365 nm light irradiation. B

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Figure 2. (a-1, b-1) Schematic illustration presents the relative position between chiral helical polymers and achiral fluorescent substance in the CPL testing process and (a-2, b-2) the corresponding CPL spectra excited at 369 nm. [R-PSA] = [S-PSA] = [DA] = [PDA] = 1 mM. In (a-1) and (a-2), CPL signals are observed. In (b-1) and (b-2), no CPL signal is observed.

is presented in Figure S2. However, no fingerprint texture is observed, indicating that no LCs phase exists in the PSA solution. It is reasonable that LCs state generally cannot be formed in dilute solutions.68−70 Combining all the above considerations, the intense Cotten effect longer than 465 nm in PSA strongly suggests the possibility of a circularly polarized scattering effect, which probably derived from the aggregation behavior of PSA chains in CHCl3 solution.71 Besides, the dynamic light scattering (DLS) result of PSA solution (Figure S3) and the relatively high baseline of UV−vis values in Figure 1a further confirm this analysis. Accordingly, on the basis of the above results and discussion, we can conclude that circularly selective absorption and scattering simultaneously exist in PSA solutions. Based on the two effects, a series of tunable CPL emissions were realized, as will be discussed below. The fluorescent spectra of PSA, DA, and PDA were further measured, as presented in Figure 1c. No fluorescent emission is found in PSA while DA and PDA show intense green emission at 500 nm. Compared with the fluorescent spectrum of DA, the decrease of photoluminescence (PL) intensity in PDA may be due to the partial energy transfer from chromophore DA to polymer backbone. The results also clearly confirm that the helical polymers PSA possess optical activity but without fluorescence property. Circularly Polarized Scattering for CPL. To explore whether chiral helical polymers can serve as a “filter” to generate CPL, we next conducted experiments by arranging the solutions of achiral fluorescent molecules and chiral helical polymers side-by-side for CPL measurement. The detailed operation is illustrated in Figure 2, including two situations: situation 1fluorescent substance first and then helical polymersand situation 2the opposite to situation 1. We find that in situation 1 strong CPL signals with different handedness and emission maximum at 500 nm are observed (Figures 2a-1 and 2a-2). Moreover, the position of CPL signals keeps consistent with the fluorescent emission. The magnitude of CPL can be evaluated by the luminescence dissymmetry

factor glum, which is defined as equal to 2 × (IL − IR)/(IL + IR), where IL and IR denote the intensity of left- and right-handed CPL, respectively.72 Generally, the glum of small organic compounds is within the range of 10−5−10−3.73 Herein, a dissymmetry factor of |glum| = 0.015 is obtained (Figure S4), much higher than the reported organic molecules. The CPL enhancement is possibly derived from the optofluidic medium effect.8,56,74 The correlation between CD effect and CPL performance was further studied for better understanding the induced CPL. It is found that the induced CPL signals are just opposite to the corresponding CD signals at the same wavelength around 500 nm (Figure 1b vs Figure 2a-2). In detail, the S-PSA sample shows a negative Cotton effect but a positive CPL signal, while the R-PSA sample shows a positive Cotton effect but a negative CPL signal. More interestingly, in situation 1 discussed above, when we replaced achiral fluorescent molecules (DA) with racemic fluorescent polymer (PDA), intense mirror-CPL can be also obtained (Figure 2a-2). For comparison, the CPL spectra of DA and PDA were also recorded (Figure S5); however, no CPL signal is found in the range of 430−650 nm in the two samples. These results convincingly demonstrate that the presence of chiral helical polymers is crucial for the generation of CPL. In contrast, in situation 2, no CPL signal is found when reversing the position of fluorescent substance and chiral helical polymers (Figures 2b-1 and 2b-2). It indicates that the relative position between fluorescent substance and chiral helical polymers is of significant importance for successfully inducing CPL. To more evidently confirm the CPL-induction behavior of chiral helical polymers, we next blended DA and PSA in CHCl3 solution for CPL measurement, as shown in Figure S6. Even though this system is more complicated than situation 1 in Figure 2, strong mirror-image CPL signals with maximum emission at 500 nm are also observed in Figure S6, in which the R-PSA/DA mixture shows the minus signal while the SPSA/DA mixture shows the plus signal. These findings are consistent with the results in Figure 2a-2. Moreover, when C

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Figure 3. (a) Chemical structures of the achiral fluorescent dyes. (b) Photograph of the mixtures of various fluorescent dyes and PSA under UV 365 nm light irradiation. (c) Fluorescent spectra of the corresponding mixtures. (d) Mirror-image CPL spectra of the corresponding mixtures. The solid lines were the CPL spectra obtained from dyes/S-PSA mixtures; the dashed lines were the CPL spectra obtained from dyes/R-PSA mixtures. (e) glum values of the CPL spectra as a function of the fluorescent wavelength.

10−3 with the increase of fluorescent wavelength), similar to the changing tendency of CD signals in Figure 1b. Moreover, the maximum emission of CPL and PL is at the same position, in accordance with the results in Figure 2a-2. Furthermore, the signals of induced CPL are also opposite to the CD signals (Figure S10 versus Figure 1b). According to the above data, we find an interesting rule: the intensity of the induced CPL depends on the CD intensity at the same position of fluorescence emission. The generation mechanism of the induced CPL cannot be selective absorption of fluorescent light by chiral helical polymers because of the following two points: (1) The polymers cannot absorb light above 465 nm, which can be confirmed by the fact that no UV−vis absorption appeared in the corresponding region (Figure 1a). (2) The maximum emission of the induced CPL does not appear at the polymer backbone region (Figure 1a vs Figure 2a-2 and Figure S10). Accordingly, we think the generation of the above CPL is attributed to the circularly polarized scattering in chiral helical polymers. To date, constructing full-color CPL by circularly polarized scattering effect has not been reported. Recently, Rizzo et al. have reported CPL emission by coupling fluorescein with chiral syndiotactic polystyrene films,29 clearly demonstrating the feasibility of our strategy reported herein. In this work, by taking advantage of the circularly polarized scattering effect, we established an universal and simple strategy for constructing full-color tunable CPL-active materials based on chiral helical substituted polyacetylenes and achiral luminophores.

increase the concentration of R-PSA, the intensity of the induced CPL signal increased obviously (Figure S7), and the glum value could reach up to −0.102 (Figure S8). Furthermore, for the mixture of PDA and PSA, similar results are also obtained (Figure S9). Following the same strategy established above, we further chose a series of achiral fluorescent dyes with different colors to prepare full-color CPL materials. The molecule structures of the achiral fluorescent dyes are illustrated in Figure 3a, including 2-(prop-2-yn-1-yl)-1H-benzo[de]isoquinoline1,3(2H)-dione (1), 4-(pyren-1-yl)butanoic acid (2), coumarin 6 (3), rhodamine 6G (4), rhodamine B (5), and Nile red (6). By simply mixing fluorescent dyes with R-RSA or S-PSA, we could obtain full-color tunable materials with emission ranging from 470 to 610 nm (Figure 3b,c). Figure 3d depicts the CPL spectra of these mixtures. All the tested samples exhibit mirrorimage CPL signals from 470 to 610 nm, demonstrating the successful preparation of full-color tunable CPL-active materials. The mixtures obtained from S-PSA show left-handed CPL (L-CPL) while the mixtures derived from R-PSA exhibit right-handed CPL (R-CPL). Following our strategy, more novel CPL materials with diverse colors or even white color will be highly expected. With the prepared full-color CPL materials in hand, we further compared their CPL intensity (Figure S10) and glum values (Figure 3e) with different spectroscopic ranges. It is found that the intensity of CPL signals and the corresponding glum values tend to decrease with the increase of fluorophore wavelength (glum value decreases from 1.41 × 10−2 to 2.5 × D

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Figure 4. (a) Fluorescence spectra of the PSA-based and DA-based composite films (λex = 350 nm). The inset presents the photo images of the corresponding film samples under room light (top) and UV 365 nm light (bottom). (b) CD and UV−vis spectra of the chiral composite films.

Figure 5. (a) Schematic illustration presents the chiroptical device for preparing “on−off” CPL and (b, c) the corresponding CPL spectra excited at 320 nm.

Fluorescence-Selective Absorption for CPL. Besides circularly polarized scattering, one may notice that the selective absorption of light due to the predominantly one-handed helical structures in PSA also makes a significant contribution to the Cotten effect (see intense CD signals at 425 nm in Figure 1a). Accordingly, we further raised another question: Can fluorescent light be handed-selectively absorbed by chiral helical structures, thus leading to CPL? If so, CPL emission

with a high glum value may be obtained due to the helical chirality of polymer main chains. To prove this hypothesis, we subsequently used the above chiral helical polymer PSA and fluorescent monomer DA to prepare composite films by taking poly(methyl methacrylate) (PMMA) as supporting material. After being dried under ambient conditions, free-standing and transparent films with a uniform thickness were obtained (Figure 4). The DA-based fluorescent film exhibits strong E

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Figure 6. (a) Schematic illustration presents the chiroptical device for preparing switchable CPL and (b, c) the corresponding CPL spectra excited at 320 nm.

emission of CPL and PL appears at the same wavelength. The difference is attributed to the different generation mechanisms of CPL in the two cases. In Figure 2, the CPL signal is generated by circularly polarized scattering effect in PSA solutions, as discussed above. Nonetheless, the CPL signal in Figure 5b is generated by fluorescence-selective absorption of PSA. This conclusion can be further confirmed by comparing the CD spectra of PSA-based composite films (Figure 4b) and the CPL spectra (Figure 5b). The maximum CPL appears at the same position of CD extremum but not the PL extremum. Besides, the glum value of the induced CPL is +0.114. Notably, the intensity of CPL signs and the corresponding magnitude of glum values depend on the concentration of chiral helical polymer in composite film. When increasing the amount of RPSA in chiral composite film, the intensity of CPL signal increases significantly (Figure S12). Moreover, the glum value comes up to +0.323. However, no CPL signal is observed when incident light first irradiated the R-PSA face (Figure 5a,c). Therefore, “on−off” CPL can be easily realized by just reversing the irradiation orientation of incident light. Inspired by the results in Figure 5, we further achieved switchable CPL based on a chiroptical device with a sandwich structure. As shown in Figure 6a, the device contains three separate films with fluorescent film inside and chiral films on both outsides. By changing the irradiation direction of incident light, intense CPL signals with different handedness were obtained (Figure 6b,c). Moreover, the maximum of the induced CPL is also at 431 nm, in good agreement with the

aquamarine blue emission with a maximum at 470 nm while no fluorescent emission is found in the PSA-based chiral films (Figure 4a). The fluorescent film was further characterized by CD and UV−vis measurement. Just as expected, no Cotton effect is found in the wavelength range of 300−600 nm because DA and PMMA are achiral substances (Figure S11). However, strong split-type CD signals are observed in the chiral composite films (Figure 4b), in which the R-PSA-based composite film exhibits a negative Cotton effect at 439 nm while the S-PSA-based composite film shows a positive Cotton effect at the same position. Slightly different from the results in PSA solutions (Figure 1a), there is no apparent CD band above 500 nm in the chiral composite films (Figure 4b). The cause of this phenomenon is probably due to the presence of PMMA chains, which restrain the aggregation of PSA chains.71 As a consequence, circularly polarized scattering effect becomes not obvious in the composite films. To explore whether fluorescent light can be selectively absorbed by predominantly one-handed helical structures, we designed a device by attaching R-PSA film and DA film together for CPL test, as illustratively shown in Figure 5. When incident light first irradiated the DA face, intense left-handed CPL is observed (Figure 5b). By carefully examining the CPL spectra, we noticed that the maximum CPL signal and PL emission are not at the same position; in detail, the CPL sign shows emission maximum at 431 nm while the fluorescence exhibits emission maximum at 470 nm. This finding is not consistent with the results in Figure 2, in which the maximum F

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measurement (Figure S14). However, no CPL signal is observed in this case because the CD and PL spectra do not obey the “matching rule”. In addition, we also highlight that even though the specific generation mechanism for CPL still needs further more investigations, the established strategy in this study is quite simple and universal, potentially applicable to various material systems. Furthermore, our investigations have demonstrated that the as-obtained CPL materials generally possess high glum values.

results in Figure 5. The key point to achieve tunable CPL in the present work is to separate the chiral composite film and fluorescent composite film. To further prove this point, we subsequently dissolved R-PSA and DA in PMMA solution to prepare chirally fluorescent composite film for CPL measurement (Figure S13). The intense CPL signal can be also observed at 431 nm in this case, further supporting the fluorescence-selective absorption mechanism discussed above. However, the induced CPL signals show no change when changing the irradiation orientation of incident light (Figure S13b,c). Besides, we excitingly found a new CPL band around 490 nm, which may be contributed by the self-assembly of fluorescent molecules in chiral film.75 On the basis of the above results (Figures 4−6 and Figure S13), we conclude that the CPL was generated by the fluorescence-selective absorption mechanism of chiral helical polymers. So far, to the best of our knowledge, there has been no report dealing with the fluorescence-selective absorption mechanism for constructing CPL materials. We further analyze the essential driving force for the phenomena, and a schematic diagram for the fluorescence-selective absorption mechanism is illustrated in Scheme 2. Achiral luminophore is first excited by

3. CONCLUSIONS We have succeeded in preparing CPL-active materials with high luminescence dissymmetry factor by taking helical substituted polyacetylenes as chiral filter. Circulaly polarized scattering and fluorescence-selective absorption mechanisms were proposed and justified for the generation of CPL. Based on the mechanisms, full-color, on−off, and switchable CPL was easily achieved. The present study not only enhances our understanding of chiral matters but also provides simple and universal approaches to prepare CPL-active materials. We are strongly convinced that the study will open up a new research area for chiral polymers, and the established strategy is promising for constructing numerous CPL materials with significant potential applications.

Scheme 2. Schematic Illustration for the FluorescentSelective Absorption Mechanism

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b02305. Materials, experimental details, and supplementary data (DOC)

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UV light, thus generating fluorescent light containing equal left-handed and right-handed components. Then the produced racemic fluorescent light passes through the chiral helical polymer. Because of the predominant one-handed helical structures in polymer chains, the fluorescent light with a certain handedness is absorbed. The remaining fluorescent light with the opposite handedness is emitted and finally generates CPL. According to this mechanism, tunable CPL could be easily prepared, as has been demonstrated in this work. It should be especially pointed out that there is a “matching rule” in both the circularly polarized scattering and fluorescence-selective absorption mechanisms. As shown in Figure 7, CPL can be generated in the overlapping area of CD spectra and PL spectra. This rule can guide us to design and prepare CPL materials, in which no interaction between chiral matters and fluorescent matters is required. To more convincingly prove this point, we further designed a sample composed by a chiral helical polymer with CD extremum at 350 nm and DA with PL extremum at 500 nm for CPL

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kai Pan: 0000-0003-4449-9766 Jianping Deng: 0000-0002-1442-5881 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21774009 and 21474007). REFERENCES

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Figure 7. Schematic illustration for the “matching rule”. The overlapping area indicates the wavelength range. G

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