Article pubs.acs.org/Macromolecules
Liquid Crystallinity Enforced Chirality Transfer from Chiral Monosubstituted Polyacetylene Copolymer to Poly(p‑phenylene ethynylene) Benedict A. San Jose, Satoshi Matsushita, and Kazuo Akagi* Department of Polymer Chemistry, Kyoto University, Katsura, Kyoto 615-8510, Japan S Supporting Information *
ABSTRACT: Chiral liquid crystalline (LC) monosubstituted polyacetylene (mono-PA) copolymers, consisting of an LC acetylene monomer unit and an asymmetric center-containing chiral acetylene monomer unit, have been synthesized. The former has a phenylcyclohexyl (PCH) mesogen linked with a trimethylene spacer, and the latter has an (R)-/(S)-1methylpropargyl undecanoate structure. By varying the feed ratio of the acetylene monomers, a facile method for tuning the LC and the chiral functionality in the mono-PA copolymers was developed. The synergistic effects between the LC and the chiral monomer units were employed to enable the chirality transfer from the mono-PA copolymer to the achiral LC conjugated polymer, poly(p-phenylene ethynylene) [LC-PPE]. The utilization of chiral LC mono-PA copolymers for the chirality transfer of LC-PPE is reported, wherein the LC monomer units enhance the interpolymer interactions through increased miscibility in the LC phase, while the chiral monomer units induce chirality in the LC-PPE main chain. levels.16−19 Copolymers with alternating donor and acceptor monomer units are useful materials for photovoltaic devices and organic transistors due to their tunable electroptical properties. Interestingly, chirality can also be incorporated into multicomponent conjugated polymers, in which one or more of the components are chiral. There have been previous reports of copolymers bearing structurally competitive monomer units, where polyisocyanates with chiral moieties of opposite chirality show chiral inversion with a change in temperature or phase.20,21 The copolymerization of two types of monomer units bearing LC and chiral moieties could be a promising method for the generation of chiral LC conjugated polymers. In addition, by varying the feed ratios of the monomer units, tuning of the functional properties of the copolymers can be achieved. We herein report the synthesis of various monosubstituted polyacetylene (mono-PA) copolymers, which have both LC and chiral moieties. By varying the feed ratio of the monomers, we have developed a facile method for tuning the LC and chiral functionalities in mono-PA. We focused on the synergistic effects between the LC and chiral monomer units to enable chirality transfer from the mono-PA copolymer to the achiral LC poly(p-phenylene ethynylene) [LC-PPE]. We present a synergistic approach for utilizing chiral LC mono-PA for the chiral transcription on LC-PPE, wherein the LC
1. INTRODUCTION Chiral conjugated polymers have attracted much attention due to their chiral supramolecular structures, which give rise to chiroptical functionalities, such as circular dichroism (CD) and circularly polarized luminescence (CPL). These classes of conjugated polymers have been used in enantioselective synthesis and enantioselective sensing1,2 and have been utilized as materials in light-emitting diodes, optical amplifiers, and optical information storage.3−8 The phenomenon of chirality transfer, where chiral molecules become achiral macromolecules, has been widely studied. For example, chiral supramolecules control the polymerization of achiral monomers to yield achiral macromolecules,9,10 e.g., the chiral transcription of chiral amines to polyisocyanate,11 the chiral induction by chiral liquid crystalline (LC) dopants to LC polymers,12,13 the chiral induction of ionic chiral dopants to ionic poly(p-phenylene) [PPP],14 and the chiral induction of achiral cationic polythiophene by DNA.15 However, a chiral transfer from a chiral conjugated polymer to an achiral polymer has not yet been reported. Conjugated copolymers, such as block copolymers and random copolymers, can be regarded as multifunctional macromolecular systems. These copolymers combine structurally different monomer units, with different functional groups, into one material, giving rise to properties that are not accessible in homopolymers. Varying the monomer structures of copolymers bearing alternating donor and acceptor monomer units influences numerous properties, such as hole mobility, absorption band, electrical conductivity, and energy © XXXX American Chemical Society
Received: September 3, 2016 Revised: October 7, 2016
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DOI: 10.1021/acs.macromol.6b01952 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Table 1. Results of Polymerization of the mono-PA Copolymers and LC-PPE
monomer unit enhances the interpolymer interactions through increased miscibility in the LC phase, while the chiral monomer unit induces chirality on the LC-PPE main chain.
2. RESULTS AND DISCUSSION The acetylene monomers, substituted with LC and chiral groups, were copolymerized to form mono-PA derivatives (Scheme 1). The monomers were randomly copolymerized at
PA1 (R)-PA0.75−0.25* (S)-PA0.75−0.25* (R)-PA0.50−0.50* (S)-PA0.50−0.50* (R)-PA0.25−0.75* (S)-PA0.25−0.75* (R)-PA2 (S)-PA2 LC-PPE
Scheme 1. Syntheses of the Monosubstituted Polyacetylene (mono-PA) Copolymers and LC-Substituted Poly(pphenylene ethynylene) [LC-PPE] a
Mn
Mw
Mw/Mn
DPa
21000 28000 16000 28000 24000 29000 46000 27000 34000 7400
30000 45000 42000 44000 70000 81000 102000 93000 100000 15400
1.5 1.6 2.7 1.6 2.9 2.8 2.2 3.4 3.0 2.1
67 97 54 103 88 113 180 116 142 18
Degree of polymerization.
synthesis of the precursors, monomers, and polymers are described in the Supporting Information. 2.1. Liquid Crystallinity. The mono-PA polymer (PA1) and copolymers (PA0.75−0.25* and PA0.50−0.50*) exhibited enantiotropic liquid crystallinity (Table 2). POM analysis revealed that PA1 has a fan-shaped texture, which is characteristic of a smectic A (SmA) LC phase (Figure 1a). The smectic LC temperature ranges were 74−183 and 60−157 °C in the heating and cooling processes, respectively. The LC phase of PA1 was further investigated using X-ray diffraction (XRD) measurements. The XRD pattern shows a sharp peak in the small angle region (2θ = 2.3°) and a broad diffraction peak in the wide angle region (2θ = 18.6°) (Figure 1d). This type of XRD profile is usually observed for a SmA phase.25,26 The diffraction angle (18.6°) in 2θ corresponds to a value of 4.8 Å, which is assigned to be the distance between the mesogenic groups. The reflection angle (2.3°) in 2θ corresponds to a value of 38.4 Å, which is assigned to be the smectic interlayer distance between polymer chains.27 Interesting trends in the LC properties can be observed when the monomer feed ratio for the mono-PA copolymers is changed. The transition temperatures between the glassy and LC phase, as well as those between the LC and isotropic phase, decrease from PA1 to (R)-PA0.50−0.50*. Narrowing of the LC temperature range can also be observed when the monomer feed ratio of the LC monomer decreases from PA1 to (R)PA0.50−0.50*. The LC POM textures of the mono-PA copolymers also changed. The POM image of (R)-PA0.75−0.25* shows a filament texture, which is composed of smectic batonnets, characteristic of the SmA phase (Figure 1b).25,26 (R)PA0.50−0.50* exhibits a sandy granular texture; the smectic LC domains decrease in size compared to PA1 and (R)-PA0.75−0.25* (Figure 1c). Furthermore, XRD patterns show that the copolymers exhibit the same SmA phase diffraction peaks, but with decreasing intensities and the absence of higher ordered diffraction peaks (Figure 1d). However, (R)PA0.25−0.75* and (R)-PA2 exhibited no LC behavior in the POM and XRD. When the LC monomer ratio was decreased, the smectic interlayer arrangement of the mono-PA copolymer became less stable. Therefore, the XRD peaks assigned to the smectic interlayer decreased in intensity, and the domains of the LC textures decreased in size. As a consequence, the copolymer still maintained a stable mesophase, even for LC monomer feed ratios up to 50%. These trends in the XRD and POM observations imply that the LC and chiral monomers are in structural competition to exert their respective effects on the copolymer main chain.
various monomer feed ratios, which resulted in copolymers with varying LC and chiral properties. The LC monomer is composed of a phenylcyclohexyl (PCH) mesogen core, a trimethylene chain linked to an ether type spacer [−(CH2)3O], and a terminal pentyl chain. The LC monomer is abbreviated as M1. The chiral monomers, (R)- and (S)-1-methylpropargyl undecanoate, are named (R)- and (S)-M2, respectively. The copolymers are named based on the mole ratio within the monomer feeding and the chiral configuration of their monomers. The homopolymer consisting of only the LC monomer is named PA1, while its chiral counterpart is named as (R)- or (S)-PA2. For instance, a copolymer written as (S)PA0.75−0.25* describes a mono-PA copolymer with a monomer feed ratio of 75% LC side chain (M1) and 25% chiral side chain [(S)-M2]. Gel permeation chromatography (GPC) results showed that the mono-PA copolymers have Mn values ranging from 16 000 to 46 000, Mw/Mn values ranging from 1.5 to 3.4, and degrees of polymerization (DP) ranging from 54 to 180 (Table 1).22 The PPE main chain was substituted with LC groups as side chains, where the LC group is a cyanobiphenyl (CB) mesogen core linked to an ester type hexamethylene [−COO(CH2)6O−] spacer. The polymer is abbreviated as LC-PPE. The PPE monomers were polymerized via Sonogashira− Hagihara coupling.23,24 GPC results showed that LC-PPE has a Mn of 7400 and Mw/Mn of 2.1 (Table 1). Details of the B
DOI: 10.1021/acs.macromol.6b01952 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Table 2. LC Transition Temperatures of mono-PA Copolymers
Figure 1. POM images of (a) PA1 at 100 °C in the cooling process, (b) (R)-PA0.75−0.25* at 97 °C in the cooling process, and (c) (R)PA0.50−0.50* at 40 °C in the cooling process. (d) XRD patterns of PA1, (R)-PA0.75−0.25*, and (R)-PA0.50−0.50* showing a broad reflection at 4.8 Å (18.6° in 2θ) and a sharp peak at 38.4 Å (2.3° in 2θ); the inset shows the Laue pattern of PA1.
Figure 2. UV−vis (above) and CD spectra (below) of the monosubstituted polyacetylenes in CHCl3 solution (c = 1.2 × 10−4 M). Solid lines and dashed lines indicate (S)-mono-PA and (R)-monoPA copolymers, respectively.
2.2. Chirality. The UV−vis and circular dichroism (CD) spectra of the mono-PA polymers and copolymers, which were prepared in a chloroform (CHCl3) solution (c = 1.2 × 10−4 M) (Figure 2) and as an annealed cast film (Figure S1 in the Supporting Information), were measured. The UV−vis spectrum of PA1 in solution showed an absorption band at 320 nm which corresponded to the π−π* transition of the polyene main chain and a band at 280 nm which corresponded to the π−π* transition of the PCH side chain. The UV−vis spectra of (R)- and (S)-PA2 showed broad absorption bands which were attributed to the π−π* transition of the polyene main chain (∼320 nm). Clearly, as the chiral monomer [(R)- and (S)-M2] feed ratio increases, the relative
intensity of the main chain absorption (∼320 nm) increases, while the PCH side chain absorption (∼280 nm) decreases. The CD spectra of the chiral mono-PAs, which bear (R)- and (S)-chiral moieties, showed monosignate Cotton effects at approximately 320 nm, which corresponds to absorption by the polyene main chain. These results imply that the mono-PAs have a one-handed helical excess conformation and intrachain helicity.28 Annealed cast films showed similar CD spectra (Figure S1), which indicates that the mono-PA copolymers exhibit chirality in the LC phase. The degree of CD was evaluated using the absorption dissymmetry factor, gabs. This factor is defined by the following C
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Macromolecules equation: gabs = (εL − εR)/[(εL + εR)/2] = Δε/ε, where gabs, −2 < gabs < 2, where εL and εR are the absorption coefficients of left- and right-handed polarized light, respectively. An increase in gabs at 320 nm, which was on the order of 10−4−10−3, was observed as the feed ratio of the chiral monomer unit increased (Table 3).
functional monomers, we developed a facile method for tuning the LC and the chiral functionalities in mono-PA and imparting chirality to the smectic LC phase of an LC conjugated polymer. 2.4. Chirality Transfer. We have previously reported that aromatic conjugated polymers such as PPP with LC moieties, upon the addition of LC chiral dopants, form into helical πstacking structures in aggregated states due to chiral induction.13 We investigated the potential of chirality transfer on an achiral LC polymer by chiral LC mono-PA copolymers. PPE has a planar main chain structure that easily forms into πstacking structures in the aggregate state. Interactions between the mono-PA copolymers and LC-PPE were further enhanced by improved miscibility of the polymer main chains due to LC−LC interactions brought about by their LC side chains (Scheme 2). It should be remembered that among the homopolymers and copolymers of monosubstituted PAs synthesized here, only PA1, PA0.75−0.25*, and PA0.50−0.50* exhibited enantiotropic smectic LC phase. However, PA0.50−0.50* showed the narrower LC temperature range, particularly in the cooling process (Table 2), and less stability of the smectic interlayer arrangement compared with those of PA0.75−0.25* (Figure 1). Therefore, only PA0.75−0.25* was used as a suitable copolymer for the examination of the liquid crystallinity-enforced chirality transfer from chiral polymer to achiral polymer. (R)-/(S)-PA0.75−0.25* was dissolved in CHCl3, and then an achiral LC polymer, LC-PPE, was added to the solution at a 1:1 mole ratio. The polymer mixture was then drop-cast onto a quartz substrate, and the film was subjected to thermal annealing at the LC temperature (140 °C) for 1 h. Note that LC-PPE exhibits a nematic LC (N-LC) phase from 100 to 180 °C under heating process and 120 to 180 °C under cooling process (Figure 3). The annealed cast films of the polymer
Table 3. Absorption Dissymmetry Factor (gabs) Values of the mono-PA Copolymers in CHCl3 Solution (c = 1.2 × 10−4 M) gabs at 320 nm (R)-PA0.75−0.25* (S)-PA0.75−0.25* (R)-PA0.50−0.50* (S)-PA0.50−0.50*
−7.69 7.85 −1.42 1.44
× × × ×
10−4 10−4 10−3 10−3
gabs at 320 nm (R)-PA0.25−0.75* (S)-PA0.25−0.75* (R)-PA2 (S)-PA2
−1.71 1.74 −1.75 1.75
× × × ×
10−3 10−3 10−3 10−3
2.3. Structural Competition and Collaboration. Based on the LC and chiroptical properties of the mono-PA copolymers, the effects of structural competition and collaboration between the monomer units of the mono-PA copolymers can be ascribed to a mono-PA main chain consisting of distinct LC and chiral segments. At the LC temperature of the mono-PA copolymers, there is a spontaneous arrangement of the copolymer chains at their LC phase. At LC monomer feed ratios greater than or equal to 50% (p ≥ 50%), the LC segments, which have PCH moieties linked with flexible spacers, direct the copolymer main chains to a SmA arrangement.27,29 However, at LC monomer feeding ratios less than 50% (p < 50%), there are not enough LC segments to stabilize the SmA phase. In this case, the copolymers are arranged in a random isotropic manner and therefore exhibit no LC behavior. The chiral segments of the mono-PA copolymer exhibit intrachain helicity and are responsible for the Cotton effects observed in the CD spectra. Therefore, as the number of chiral segments increases in the copolymer main chain, the gabs values also increase. When the LC monomer feed ratios decrease, we also observe a decrease in the XRD peak intensity and a narrowing of the LC phase temperature range. In this situation, the increase in the amount of chiral monomer units imparts more helicity to the mono-PA main chain, which perturbs the interlayer arrangement of the LC segments. This effect coincides with a decrease in the LC domain size and an increase in CD intensity. Therefore, the LC and chiral segments of the main chain behave as “competitive structural factors” when generating their effects in the mono-PA copolymer. When suitable feed ratios between the LC and chiral molecules are employed for copolymerization, which was observed for PA0.75−0.25* and PA0.50−0.50*, the LC and chiral segments contribute cooperatively to afford both LC and chiral functionalities in the copolymer (Tables 2 and 3). By varying the feed ratio of the
Figure 3. (a) POM image of LC-PPE annealed at 150 °C in cooling process, showing a Schlieren texture of the nematic phase. (b) XRD pattern and profile of LC-PPE, showing a broad reflection at 4.4 Å corresponding to the distance between LC side chains.
Scheme 2. Chirality Transfer from Chiral Polymer, (R)-/(S)-PA0.75−0.25* to Achiral (Racemic) Polymer, LC-PPE
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DOI: 10.1021/acs.macromol.6b01952 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
have a SmA phase, while the chiral monomer units impart chirality to the main chain. Furthermore, for chirality transfer on achiral LC-PPE, the LC monomer units enhance the interpolymer interactions through increased miscibility in the LC phase, while the chiral monomer units induce the chirality on the LC-PPE main chain. These results prompt us to further investigate the phenomenon of chirality transfer of other achiral conjugated polymers, which will be reported in the near future.
mixtures were then evaluated by measuring CD spectra. The CD spectra of a (R)-PA0.75−0.25*/LC-PPE annealed cast film showed a negative Cotton effect on the main chain absorption region of mono-PA from 270 to 390 nm and an induced positive Cotton effect on the main chain absorption region of PPE from 390 to 475 nm (Figure 4).
4. EXPERIMENTAL SECTION The materials, methods, and syntheses of the mono-PA monomers M1 and (R)-/(S)-M2; LC-PPE precursors (1), (2), and (3); and the polymers PA1, (R)-/(S)-PA0.75−0.25*, (R)-/(S)-PA0.50−0.50*, (R)-/(S)PA0.25−0.75*, (R)-/(S)-PA2, and LC-PPE are described in the Supporting Information.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01952. Description of materials and methods used in this research, synthetic routes of the monomers and polymers, representative results of polarizing optical microscope (POM), and chiroptical properties (CD) (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail
[email protected] (K.A.). Notes
The authors declare no competing financial interest.
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Figure 4. UV−vis (above) and CD spectra (below) of (R)-/(S)PA0.75−0.25*/LC-PPE annealed films. Shaded regions represent absorption regions of (R)-/(S)-PA0.75−0.25* (green) and LC-PPE (pink).
ACKNOWLEDGMENTS This work was supported by Grants-in-Aid for Science Research (A) (No. 25246002), that for Challenging Exploratory Research (No. 15K13706), and that for Young Scientists (A) (No. 16H06051) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
A (S)-PA0.75−0.25*/LC-PPE annealed cast film exhibited an opposite sign of the Cotton effect, relative to (R)-PA0.75−0.25*/ LC-PPE. The UV−vis spectra of the (R)-/(S)-PA0.75−0.25*/LCPPE exhibited an absorption band at 310 nm, which corresponds to the absorptions of the polymer main chain of (R)-/(S)-PA0.75−0.25* and the CB LC moiety of LC-PPE. An absorption shoulder band from the LC-PPE main chain was also observed at 400 nm. For reference, we measured the absorption spectra of LC-PPE in CHCl3 and cast film (Figure S2). The spectra exhibit PPE main chain and CB absorption bands at 400 and 300 nm, respectively. The induced Cotton effect of LC-PPE was observed in the region of its main chain absorption band, which indicates that the chiral transfer on LCPPE occurs through the chiral LC mono-PA copolymers. It is found that the synergistic effects between the LC and the chiral monomer units enabled the chirality transfer from the chiral LC PA copolymer to the achiral LC-PPE, wherein the LC monomer units enhance the interpolymer interactions through increased miscibility in the LC phase, while the chiral monomer units induce chirality in the LC-PPE main chain.30
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REFERENCES
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3. CONCLUSION We present the synergistic use of the LC and the chiral functionality of the monomer units of mono-PA copolymers. The LC monomer units enable the mono-PA main chain to E
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(24) Sonogashira, K. Development of Pd−Cu catalyzed crosscoupling of terminal acetylenes with sp2-carbon halides. J. Organomet. Chem. 2002, 653, 46−49. (25) Textures of Liquid Crystals; Dierking, I., Ed.; Wiley-VCH: Weinheim, 2003. (26) Liquid Crystalline and Mesomorphic Polymers; Shibaev, V. P., Lam, L., Eds.; Springer-Verlag: New York, 1994. (27) The lengths of the mono-PA side chains were evaluated using molecular mechanics calculations. The side chain with the PCH503 moiety has a length (L1) of 18.0 Å. The chiral side chain has a length (L2) of 14.6 Å. The LC phase of the mono-PAs is arranged in a smectic A phase (SmA) with the interlayer distance (d) approximate to 2L1. (28) Berova, N.; Nakanishi, K. In Circular Dichroism: Principles and Applications, 2nd ed.; Berova, N., Nakanishi, K., Woody, R., Eds.; John Wiley and Sons, Inc.: New York, 2000; Chapter 12, pp 337−382. (29) The present analyses enabled us to draw a polymer structure where in the LC segments the LC side chains are alternatively located at both sides of polyene chain to form a cis-transoid stereoregular configuration of head−tail−head−tail linkages. In the case of Rh-based catalysts, polymerization proceeds via an insertion mechanism and provides cis-rich mono-PA with a cis-transoidal backbone. The LC distance of 4.8 Å between LC side chains measured by XRD is close to the calculated value of ∼4.4 Å. (30) The miscibility between PA0.75−0.25* and LC-PPE in their LC states is indispensable for the efficient chirality transfer. Although the LC phases of PA0.75−0.25* and LC-PPE are different from each other, i.e., smectic and nematic, respectively, the miscibility is ensured by LC−LC interactions between the LC-substituted polymers. In addition, PA0.75−0.25* has a lower smectic interlayer ordering than PA1 because chiral monomer units in the copolymer perturb the interlayer arrangement of the LC segments. PA0.75−0.25* could be miscible with LC-PPE having a nematic ordering due to the less stable interlayer arrangement. The chirality on the LC-PPE main chain is induced by the chiral monomer units in a PA0.75−0.25* structure. Although the detail mechanism of the chirality transfer still remains unclear, it could be clarified through the future investigation by applying the present approach to other achiral conjugated polymers.
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DOI: 10.1021/acs.macromol.6b01952 Macromolecules XXXX, XXX, XXX−XXX
