Intense Circularly Polarized Luminescence Contributed by Helical

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Intense Circularly Polarized Luminescence Contributed by Helical Chirality of Monosubstituted Polyacetylenes 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: Introducing fluorescent chromophores to helical architectures can generate circularly polarized luminescence (CPL). Herein, a series of fluorescent chiral helical monosubstituted polyacetylenes showing both intense optical activity and CPL are prepared by copolymerizing achiral fluorescent acetylenic monomer and a chiral monomer. The luminescence dissymmetry factor (glum) values of the obtained copolymers can reach up to +0.136 and −0.264. The formation of predominantly one-handed helical structures in the copolymer backbones has proved to play crucial roles in achieving the intense CPL. The dynamic helical polymer chains further enable the CPL-active materials to exhibit both solvent sensitivity and temperature sensitivity. Moreover, the helical polymer structures can also helpfully restrain the aggregation-caused quenching (ACQ) effect of fluorescent groups. Therefore, the helical polymers play double roles: providing helical chirality for generating CPL and simultaneously circumventing ACQ effect of chromophores. The present study provides new insights into designing and preparing advanced functional chiroptical materials.

1. INTRODUCTION Functional materials with circularly polarized luminescence (CPL) have recently constituted an active research area in both materials science and chirality-related areas for their significant potentials in 3D optical displays, optical storage devices, and photoelectric devices.1−7 Through the continuous efforts and development in the past few years, various CPLactive materials covering small molecules, polymers, supramolecular aggregates, lanthanide complexes, and chiral liquid crystals have been designed and prepared.8−20 Normally, chirality and chromophores are two essential components for constructing CPL materials, in which chirality strongly affects the intensity of CPL. On the other hand, synthetic helical polymers are remarkably attractive due to their unique macromolecular architectures and the well-known chiral amplification effect.21−36 Exploiting novel CPL materials based on synthetic helical polymers will not only significantly increase new members of CPL family but also hopefully lead to unprecedented findings. Unfortunately, in spite of the numerous artificial helical polymers already prepared, there have been only a few reports concerning CPL-active materials constructed by synthetic helical polymers.37−41 As a proof-ofconcept work, the present article reports the CPL emission derived from chiral helical monosubstituted polyacetylenes (mono-PAs). Chiral helical substituted polyacetylenes, as typical synthetic helical polymers, have attracted extensive attention due to their intriguing optical activity and significant potentials in chiral related areas.21,22,32,33,42 Recently, Akagi et al. creatively © XXXX American Chemical Society

prepared CPL-active materials using disubstituted polyacetylenes in chiral liquid crystals.43−45 Nonetheless, the contribution of helical polymer structures to CPL performance has not been in depth explored yet. Besides, definitely different from disubstituted polyacetylenes, monosubstituted acetylenes generally require the use of different catalyst systems for polymerization, and the resulting mono-PAs usually show no photoluminescence behavior.32 We hypothesize that introducing luminophores to chiral helical polymers may yield novel CPL-active materials. Furthermore, the helical chirality of the helical polymer chains may lead to CPL with high dissymmetry factors. In the above context, we in this study report our exciting success in achieving CPL emission based on monoPAs via copolymerization of achiral fluorescent substituted acetylene monomer with a chiral monomer. The glum of the prepared materials can reach up to 10−1. More importantly, the CPL signals can be changed by solvent and temperature effects due to the dynamic helical conformation of mono-PAs.

2. RESULTS AND DISCUSSION The fluorescent chiral helical mono-PAs were synthesized by a well-investigated solution polymerization approach,46 as presented in Scheme 1. Nonfluorescent chiral monomer (RSA or S-SA) underwent solution copolymerization with an achiral fluorescent monomer (DA) in the presence of Received: July 17, 2018 Revised: August 15, 2018

A

DOI: 10.1021/acs.macromol.8b01545 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Schematic Illustration for Preparing Fluorescent Chiral Helical Monosubstituted Polyacetylenes with CPL Performance

Table 1. Data for the Prepared Fluorescent Chiral Helical Monosubstituted Polyacetylenes gabsd 4a

sample

Mn/10

R-P73 S-P73 R-P82 R-P91

1.44 1.60 1.44 1.47

Mw/Mna

[α]Db

ΦFc

375 nm

460 nm

glume

1.23 1.23 1.24 1.19

−756 +1202 −3156 −7436

25.3 17.6 12.9 6.3

+0.0039 −0.0077 +0.0115 +0.0218

−0.0783 +0.0837 −0.0605 −0.0629

+0.009 −0.017 −0.033 −0.073

a

Determined by GPC with dimethyformamide (DMF) as eluent. b[α]D values of the polymers measured in CHCl3 with c = 0.1 g/dL. cAbsolute fluorescence quantum yield obtained using the calibrated integrating sphere system. dDetermined by the CD and UV−vis spectra. eDetermined by the CPL spectra.

Figure 1. (a) CD and UV−vis spectra of R-P73 and S-P73 in CHCl3 solution. [R-P73] = [S-P73] = 1 mM. (b) Fluorescence spectra of the corresponding polymers (1 mM, λex = 369 nm). The inset presents the photo images of the corresponding polymer solution samples under UV 365 nm light irradiation.

positive CD signal at 375 nm, while S-P73 shows opposite result. Referring to the literature, split-type CD signals are familiar in substituted polyacetylenes due to the their preferred-handed helical structures.47,48 Meanwhile, no Cotton effect is found in the homopolymer of monomer DA due to the absence of chiral factor in DA (Figure S2). Specific optical rotations ([α]D) also provide a significant support for the above findings. As shown in Table 1, the [α]D values of R-P73 and S-P73 are −756 and +1202, respectively. The gabs (Kuhn dissymmetry factor,49,50 defined as gabs = [CD/32980]/Abs) values of R-P73 and S-P73 were further calculated to quantitatively evaluate the degree of preferential helicity, as listed in Table 1. Taking R-P73 as an example, the polymer shows large gabs values at both the first and second CD signals: gabs = −0.0783 at 460 nm and gabs = +0.0039 at 375 nm. The above results demonstrate that the obtained copolymers exhibit intense optical activity derived from the predominantly one-handed helical conformation in polymer chains, referring to our earlier intensive studies dealing with helical polyacetylenes.31,46,51,52

rhodium-based catalyst with the SA/DA molar ratio of 7/3. Specific details are presented in the Experimental Section. The monomers are illustrated in Scheme 1. The polymerization proceeded smoothly, providing the copolymer in a quantitative yield. The obtained copolymer (defined as P73) was identified by FT-IR spectroscopy, as shown in Figure S1. The disappearance of vibrational absorption peak at 2118 cm−1, which belongs to the CC group, indicates the successful polymerization of SA and DA in the presence of rhodiumbased catalyst. The obtained P73 (R- and S-P73) was subjected to GPC measurement to analyze the molecular weight and molecular weight distribution, as listed in Table 1. Moderate number-average molecular weight (Mn) and narrow molecular weight distribution (Mw/Mn) are obtained in the obtained polymers. The results convincingly confirm the successful preparation of P73. Figure 1a presents the CD and UV−vis spectra of the obtained copolymers. The copolymers show strong mirror CD signals with split peaks in the polymer backbone region, in which R-P73 exhibits negative CD signal around 460 nm but B

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Figure 2. (a, c) CPL spectra of R-P73 and S-P73 in CHCl3 solution excited at 369 nm. (b, d) CPL dissymmetry factor glum versus wavelength of RP73 and S-P73 excited at 369 nm. In (a) and (b), [R-P73] = [S-P73] = 1 mM; in (c) and (d), [R-P73] = [S-P73] = 10 mM.

Because monomer DA is fluorescent, the corresponding polymers R-P73 and S-P73 are expected to show photoluminescence (PL) behavior. To confirm this hypothesis, the polymers were characterized by fluorescent measurement, and the obtained spectra are depicted in Figure 1b. The two copolymers both exhibit strong green emission with a maximum at 500 nm (Figure 1b), while no fluorescent emission is found in the homopolymers of monomer R-SA (Figure S3). Furthermore, R-P73 and S-P73 show a satisfactory quantum yield of 25.3% and 17.6%, respectively (Table 1). The CPL performance of the prepared helical polymers (RP73 and S-P73) was subsequently investigated, as presented in Figure 2a. Amazingly, strong mirror-image CPL signals with maximum emission at 470 nm can be observed, in which RP73 shows a plus signal while S-P73 shows a minus signal. For comparison, the CPL spectra of monomer DA and its homopolymer were also recorded (Figure S4); however, no CPL signal is found in the range of 430−650 nm in the two cases because DA is achiral compound. The correlation between the CD effects and CPL signs was further studied for better understanding of the ground-state chirality and photoexcited-state chirality. Figure S5 presents the PL excitation spectra monitored at 430, 450, 480, 500, 530, and 550 nm. The maximum of the excitation wavelength is found to be constant at 370 nm at varied emission wavelengths. The excitation spectra keep similar to the absorption of fluorescent dansyl group (Figure 1a and Figure S2). Besides, obvious CD signals were observed at 375 nm in P73 (Figure 1a). It indicates that the CPL signs may arise from the second Cotton band of P73 at 375 nm. In more detail, S-P73 with a negative

CD signal at 375 nm shows a negative CPL, while R-P73 with a positive CD sign at 375 nm displays a positive CPL. Luminescence dissymmetry factor glum (defined as 2 × (IL − IR)/(IL + IR), where IL and IR denote the intensity of left- and right-handed CPL, respectively) is used to evaluate the magnitude of CPL.53 Experimentally, glum = [ellipticity (mdeg)/(32980/ln10)]/PL (DC in volts). Obviously, the magnitude of glum depends on both the intensity of CPL and PL. Thus, the maximum of glum maybe not necessarily appear at the same position of CPL or PL extremum. Herein, we noticed the maximum of glum value occurs at 435 nm (Figure 2b), while the maximum CPL exists at 470 nm (Figure 2a). The dissymmetry factors of R-P73 and S-P73 are found to be +0.009 and −0.017, respectively (Table 1). It should be noted that the polymer concentration has a big influence on the intensity of CPL signals and the magnitude of glum values. Figure 2c presents the CPL spectra of R-P73 and S-P73 in 10 mM CHCl3 solutions. The intensity of CPL signals increases significantly when compared with Figure 2a (the polymers, 1 mM). Moreover, the glum values also significantly increase, as shown in Figure 2d. The glum value comes up to +0.136 in RP73 and −0.264 in S-P73. Besides, the fluorescent intensity decreases with increase of polymer concentration probably due to the ACQ effect54 (Figure S6). When the polymer concentration was increased in CHCl3, the solubility of it decreased. As a result, the fluorescent groups trended to aggregate and ACQ phenomenon took place. A further attempt to obtain higher glum by increasing polymer concentration was failed due to the limited solubility of P73 in CHCl3. To date, there are only a few reports concerning chiral polymers with glum values up to 10−1,55−57 while most of the previously C

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Figure 3. (a) CD and UV−vis spectra of R-P73 in DMSO (1 mM) and monomer R-SA/DA mixture in CHCl3 (1 mM). (b) CPL spectra of R-P73 in DMSO (1 mM) and R-SA/DA mixture in CHCl3 (1 mM) excited at 369 nm. (c, d) Schematic illustration presents the relationship between the induced CPL signals and helical structures.

reported chiral polymers possessed the glum values in the range 10−5−10−2.38−41,58−60 The formation of predominantly one-handed helical structures of polymer chains is considered being the major driving force for the CPL effects as observed above. To elucidate this consideration, we subsequently conducted specific experiments by dissolving R-P73 in dimethyl sulfoxide (DMSO). From the data in Figure 1a, we know that R-P73 can keep stable its one-handed helical conformation in CHCl3. However, in contrast to the results in CHCl3 (Figures 1a and 2a), R-P73 shows no Cotton effect around 450 nm in DMSO (Figure 3a), accompanied by the disappearance of CPL signal (Figure 3b). The results clearly demonstrate the solvent sensitivity of the obtained CPL. The cause of this phenomenon should be due to the high polarity of DMSO and its good affinity with the conjugated polymer backbone. As a result, the predominantly one-handed polymer helicity in R-P73 transforms into random conformation in DMSO.46,61 The PL emission spectrum of R-P73 in DMSO was further measured, as illustrated in Figure S7. The maximum of emission peak shifts to 520 nm in DMSO together with the decrease of PL intensity. This phenomenon indicates that intramolecular charge transfer (ICT) effect62−64 exists in R-P73. Besides, the CD and CPL spectra of monomer mixture containing R-SA and DA in CHCl3 are also recorded in Figure 3a,b. As expected, neither the Cotton effect nor the CPL sign is found in the control sample. The above result indicates that charge transfer between the camphor and dansyl group may have made a certain contribution to the CPL; however, as convincingly demonstrated by the experiments above, the formation of predominantly one-handed helical conformation in polymer main chains plays the crucial role for generating CPL signals. Schematic diagrams are illustrated in Figure 3c,d for better understanding the essential driving force for the generation of CPL. In the obtained fluorescent helical polymers, fluorescent groups are arranged along the polymer backbone via covalent bonds. CPL is generated when the polymer chains form predominantly one-handed helical

structures (Figure 3c); however, no CPL is obtained when the predominantly one-handed helical structures transform to random coils (Figure 3d). Following the same preparation strategy above, we subsequently prepared more helical copolymers by changing the proportion of the two monomers. Specifically, increasing the ratio of R-SA/DA (from 8/2 to 9/1 in mol), copolymers RP82 and R-P91 were prepared. The obtained copolymers were further characterized by FT-IR and GPC techniques, as presented in Figure S8 and Table 1. Fluorescence spectra (Figure S9) indicate that the PL intensity decreases with the increase of chiral monomer SA. The fluorescence quantum yields of the polymers are listed in Table 1. The polymers were further characterized by CD and UV−vis techniques, as shown in Figure S10. The intensity of CD signals increases intensely with the increase of SA proportion, demonstrating the enhancement of optical activity along with increasing chiral monomer ratio. The [α]D and gabs values further support this conclusion (Table 1). The intensity of CPL signs shows a similar trend, as shown in Figure S11. The glum values of R-P73, R-P82, and R-P91 are +0.009, +0.033, and +0.073, respectively (Figure S12). These results demonstrate that optically active helical mono-PAs can act as an excellent candidate for constructing CPL materials due to their helical structures. Notably, because of the feature of copolymerization strategy, the changes in the intensity of CPL signal and fluorescence always show an opposite trend. Thus, it is difficult to find the optimum balance between CPL intensity and fluorescent intensity. Our next target is to prepare CPL-active polymers using chiral fluorescent monomers, which will hopefully solve the above problem. To explore the potentials of the prepared chiral helical polymers in chiroptical device, we further used the above chiral helical polymer (taking P73 as an example) to prepare composite films taking poly(methyl methacrylate) (PMMA) as supporting material. After being dried under ambient conditions, free-standing and transparent films with a uniform thickness of ∼0.3 mm were obtained (Figure 4). The film D

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Figure 4. (a) Photograph of the chiral helical polymer/PMMA composite film under UV 365 nm light (top) and room light (bottom). (b) Mirrorimage CPL spectra of the composite films excited at 350 nm. (c, d) CPL spectra of the composite films at various temperatures.

exhibits strong aquamarine blue fluorescence under UV light. Intense Cotton effects are found in the composite films (Figure S13), similar to the CD spectra in polymer solutions (Figure 1a). Moreover, strong mirror-image CPL signals are also observed in the prepared films, in which the R-P73/PMMA film shows positive signs while the S-P73/PMMA film shows negative signs (Figure 4b). The signs of CPL signals in composite films are consistent with the results in the corresponding polymer solutions (Figure 2a,c). The dissymmetry factor glum values in the composite films are about +0.024 and −0.047, respectively. The helical conformation of dynamic substituted polyacetylenes is sensitive to external stimuli due to their low helix inversion barrier.33 With the prepared composite films in hand, we subsequently explored the temperature effect on the CPL performance. As shown in Figure 4c, the CPL intensity decreased obviously when temperature increased from 30 to 80 °C. The cause of this phenomenon is due to the occurrence of helix-to-coil conformational transition of polymer backbones.46,61,65,66 Moreover, the fluorescent intensity also decreased with the increase of temperature (Figure 4d). When temperature was raised, the polymer main chains began to transform from helix to random coils. As a consequence, the pendent fluorescent groups had more opportunity to aggregate, and ACQ effects took place. In other words, the ACQ effect can be restrained when chromophores are spirally arranged along the helical skeleton via covalent bonds. This finding is significant for further designing and preparing more CPL materials using routine fluorescent chromophores. Besides, the

PL and glum values as a function of temperature are plotted in Figure S14. It is found that the glum value in the S-P73/PMMA film trends to decrease with the increase of temperature while the glum value in the R-P73/PMMA film decreases slightly when increasing temperature, probably due to the higher stereoregularity in S-P73 main chains. The results indicate that high temperature is unfavorable for stabilizing CPL performance. Further enhancing temperature would result in softening and deformation of the films. Besides, the CPL spectra could not recover to the original intensity when cooling temperature to 30 °C. The results in Figures 3a and 4c convincingly demonstrate that the formation of predominantly one-handed helical structures plays a decisive role in the generation of CPL signals. Furthermore, because of the thermosensitivity of CPL performance, the composite films are considered having potential applications as advanced functional chiroptical materials.

3. CONCLUSIONS A series of CPL-active chiral helical monosubstituted polyacetylenes were synthesized. The prepared copolymers all exhibited strong CD and CPL signals due to the helical chirality of polymer backbones. Based on the polymers’ dynamic helical conformations, the obtained CPL signals can be switched upon changing solvent and varying temperature. The present investigations are not only important in enhancing our understanding of the contribution of predominantly onehanded helical polymer structures in fabricating CPL but also E

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

provide a simple and universal strategy for preparing CPLactive materials. The study will open up new research opportunities for chiral helical polymers and the prepared CPL materials are of considerable potential applications.

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4. EXPERIMENTAL SECTION

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01545. Additional FT-IR, PL, CPL, CD, and UV−vis spectra of

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REFERENCES

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Materials. Solvents were distilled under reduced pressure. Fluorescent achiral substituted acetylene monomer (DA)67 and chiral substituted acetylene monomer (SA)68 were synthesized according to our previous reports. (nbd)Rh+B−(C6H5)4 (nbd = 2,5-norbornadiene) was prepared as reported in the literature.69 Poly(methyl methacrylate) (PMMA, average Mw = 550000) was purchased from Alfa Aesar. Measurements. FT-IR spectra were recorded using a Nicolet NEXUS 670 infrared spectrometer (KBr tablet). The number-average molecular weight (Mn) and molecular weight polydispersity (PDI, Mw/Mn) of prepared polymers were determined by GPC (Waters Styragel HT4 columns) with dimethylformamide (DMF) as eluent. Fluorescence spectra were measured on a Varian Cary Eclipse spectrophotometer (Varian, USA). Circular dichroism (CD) and UV−vis absorption measurements were conducted on a Jasco 810 spectropolarimeter. Specific rotations were measured on a JASCO P1020 digital polarimeter with a sodium lamp as the light source at room temperature. CPL spectra were measured on JASCO CPL-200. For polymer solutions, the CPL spectra were recorded at room temperature; for composite films, the CPL spectra were recorded at varied temperatures. Preparation of Fluorescent Chiral Helical Monosubstituted Polyacetylenes. Fluorescent chiral helical substituted polyacetylenes were prepared by a typical solution copolymerization strategy. Taking P73 as example, predetermined amounts of chiral substituted acetylene monomer SA (0.07 g, 0.26 mmol), achiral fluoresent monomer DA (0.032 g, 0.11 mmol), and (nbd)Rh+B−(C6H5)4 (0.0019 g, 0.0037 mmol) were dissolved in 2 mL of chloroform (CHCl3). The polymerization was performed under N2 at 30 °C for 6 h. Then the solution containing the resulting polymer was poured into a large amount of methanol to precipitate out the polymer. Similar solution polymerizations were performed with R-P82 and R-P91. Preparation of Composite Films. The composite films were prepared by dissolving copolymer P73 into the PMMA solution. In detail, P73 (0.02 g) was dissolved in 10 mL of PMMA (0.6 g) CHCl3 solution and cast onto a glass dish. The CHCl3 was then evaporated under ambient conditions, and films with a uniform thickness of ∼0.3 mm were obtained.

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the polymers, glum curves of the polymers (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.P.). *E-mail: [email protected] (J.D.). ORCID

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

The authors declare no competing financial interest. F

DOI: 10.1021/acs.macromol.8b01545 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.8b01545 Macromolecules XXXX, XXX, XXX−XXX