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Cite this: Chem. Commun., 2014, 50, 365
Helical polydiacetylene prepared in the liquid crystal phase using circular polarized ultraviolet light†
Received 22nd September 2013, Accepted 28th October 2013
Yangyang Xu, Hao Jiang, Qijin Zhang, Feng Wang* and Gang Zou*
DOI: 10.1039/c3cc47245k www.rsc.org/chemcomm
Herein the enantio-selective polymerization of diacetylene (DA) units in the discotic hydrogen-bonding complex is carried out in an asymmetric reaction field consisting of the lamellar columnar LC phase and CPUL stimulus.
Helical conformation is a representative secondary structure of biomacromolecules in nature, such as proteins and deoxyribonucleic acids. In particular, the introduction of chirality into conjugated polymer materials is expected to result in novel optical, electrical and magnetic properties, and tremendous potential applications in organic devices with special electromagnetic functions and chirality sensing.1,2 In the past few decades, various helical conjugated polymers, including polythiophene,3 polypyrrole,4 polyaniline,5 and polydiacetylene (PDA),6 have been prepared by various methods including the polymerization of optically active monomers, supramolecular assembly, enzyme catalysed, template synthesis, and electrochemical polymerization.7,8 Recently, professor Akagi et al. have developed a novel method for preparing helical conjugated polymers in a chiral nematic liquid crystal reaction field.9,10 A chiral nematic liquid crystal (LC) has a quasi-layer structure, which can effectively provide chiral order in the propagating step of a polymerization reaction. The handedness of the helix can be reversed by using the chiral dopant with opposite chirality.9 However, this method is not suitable for preparing helical PDA via topo-polymerization processes, since the presence of a liquid crystal and the chiral dopant as the reaction media might annihilate the topo-polymerization of diacetylene (DA) monomers.11,12 To the best of our knowledge, attempts to synthesize helical PDA chains in the LC phase, particularly the columnar LC phase, have not been described. Various mesogenic DA monomers have been synthesized, and the topo-polymerization reactions have been carried out in liquid-crystal media including nematic and smetic phases.13,14 The disc-shaped CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, Key Laboratory of Optoelectronic Science and Technology in Anhui Province, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China. E-mail: [email protected], [email protected] † Electronic supplementary information (ESI) available: XRD, POM, TEM, UV and CD characterization techniques. See DOI: 10.1039/c3cc47245k
This journal is © The Royal Society of Chemistry 2014
mesogenic DA monomers are of particular interest as they stack in columns, and zipping of diacetylenes along the column axis by polymerization will lead to supramolecules with columnar structures. The design of discotic LC can be accomplished by supramolecular approaches, among which hydrogen-bonding (HB) assembly could be the most effective way to build a rigid discotic mesogenic DA monomers by considering bonding directionality and strength. Herein, we present a novel HB complex consisting of a clicked C3-symmetric triazole derivative (TTB) with 10,12pentacosadiynoic acid (PCDA) in a 1 : 3 stoichiometry. Notably, this HB complex is the first example of DA LC materials prepared by the supramolecular assembly method. The photo-polymerization reactions of the above discotic HB complex were performed in the crystal phase, the LC phase and the isotropic phase, respectively, and the effect of the LC reaction field has been evaluated. To date, the inducement of a chiral dopant has been a necessary condition for the preparation of the helix conjugated polymers in the LC reaction field. Whether helical PDA can be synthesized in the LC reaction field in the absence of a chiral dopant remains a question to be answered. Recently, we reported the successful enantio-selective polymerization of PDA Langmuir–Blodgett (LB) films by means of circular polarized UV light (CPUL).15 The helical direction of PDA chains in the LB films could be controlled and modulated by CPUL treatment. However, the enantio-selective polymerization mechanism of DA monomers with CPUL is not clear and attempts to synthesize helical PDA in the columnar LC phase with CPUL have not been described to date.16,17 We demonstrate herein the possibility of controlling the formation of helical PDA chains in the LC field by using an external chiral stimulus such as CPUL. 10,12-Pentacosadiynoic acid (PCDA) is purchased from Tokyo Chemical Industry Co., Ltd., and purified to remove the polymerized part before use. 1,3,5-Tris(1-alkyl-1H-1,2,3-triazol-4-yl) benzene (TTB) was synthesized in analogy to the previous procedure.18 A HB complex was obtained by mixing TTB with PCDA in a 1 : 3 molar ratio in chloroform solution followed by slow evaporation of the solvent at reduced pressure. Taking into account the core structure of the HB complex, we suggested that the carbonyl oxygen atoms in PCDA could be a hydrogen-acceptor with respect to the donor from
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Fig. 1 (a) Molecular structure and the synthetic route of the discotic HB complex; (b) FT-IR spectra of (i) TTB, (ii) PCDA, and (iii) PCDA–TTB complex; (c) 1H-NMR spectra of (i) TTB and (ii) HB complex in CDCl3.
the TTB unit. The formation of the above HB complex was confirmed by FT-IR and 1H-NMR spectroscopy characterization. The CQO stretching vibration in pure PCDA samples appeared at 1693 cm 1, which indicated that the carboxyl group was not in a free but an associated state.19 Conversely, the carbonyl band in the HB complex shifted to 1703 cm 1 (as shown in Fig. 1b), lower than that of the free carboxylic acid, suggesting that the carbonyl group still interacted with a different H-bonding donor, presumably CHb of the triazole in TTB. To corroborate this, we examined the 1H-NMR signals in deuterated chloroform solutions. As shown in Fig. 1c, the signal from the hydrogen atom CHb of triazole in TTB appeared at 7.98 ppm, but shifted to 8.06 ppm in the HB complex. The signal for CHa of the central benzene unit also shifted downfield, confirming the formation of the discotic HB complex.18 Then the thermotropic LC behaviour of the HB complex was analysed by using differential scanning calorimetry (DSC), polarized optical microscopy (POM) and X-ray diffraction (XRD) techniques. The results are summarized in Fig. 2. As the reference sample, TTB or PCDA alone exhibited no LC phase. However, the discotic HB complex showed a monotropic LC phase exclusively upon heating (Fig. 2a) before polymerization. To identify the LC phase, we investigated the POM textures of the HB complex, which melted at 37.8 1C and exhibited a lamellar texture structure upon heating from the crystal phase (Fig. 2b). The morphology of above film was further evaluated by TEM characterization. As shown in Fig. S2 (ESI†), closely parallel fibers (about 13 nm in diameter) could be observed in the monolayer of the HB complex. The XRD pattern at 50 1C for the sample revealed five reflections at 4.54, 2.38, 1.54, 1.15, 0.94 nm in the low angle regime with the reciprocal spacing ratio of 1 : 1/2 : 1/3 : 1/4 : 1/5, assigned as the (001), (002), (003), (004) and (005) reflections, indicating a layered structure with a periodicity of 4.54 nm (Fig. 2c). Actually, the lateral spacing was shorter than that of the fully extended disk diameter (6.5 nm) calculated by Chemical 3D, suggesting the lamellar columnar packing of the HB complex with interpenetration structure. In addition, the diffraction at 0.41 nm corresponds to the mean
366 | Chem. Commun., 2014, 50, 365--367
ChemComm
Fig. 2 (a) DSC curve of the HB complex; (b) POM texture and (c) Small angle X-ray diffraction profile of the complex in the LCol liquid crystal phase before polymerization; (d) the model of a highly ordered lamellar columnar mesophase.
distance between the fluid alkyl chains, suggesting the formation of a lamellar columnar mesophase (LCol) as shown in the model proposed in Fig. 2d. A characteristic absorption at 245 nm for the TTB chromophore could be observed for the films of the HB complex, obviously blue-shifted compared with that in chloroform solution (Fig. S3, ESI†). This blue-shift for excitonic absorption could be understood as an indication that under the given condition, TTB units formed H-aggregates by face to face stacking in the assemblies. Photo-triggered polymerization of DA units could be detected optically as the conjugated p electron system of PDA chains showed typical intense absorption maximum at about 640 and 580 nm. As shown in Fig. 3a, the discotic HB complex could be photopolymerized in the crystal phase and the LCol liquid crystal phase, while not in the isotropic phase. After polymerization, the fractional conversion values for the samples polymerized in the crystal and the LCol phase were about 0.35 and 0.31, respectively. The polymerization kinetic process for the HB complex in the crystal phase and the LCol phase could both be described as a first order reaction, and the rate constants k1 for the HB complex in the crystal phase and the LCol phase were 0.0032 and 0.43 s 1 (as shown in Fig. S4, ESI†). The polymerization rate for the HB complex in the LCol phase was considerably faster than that in the crystal phase. The possible explanation could be proposed as follows: since the molecular motion in the crystal phase is restricted by the crystal lattice, the reaction during the induction time is very slow. After the local concentration of polymers increases to a certain extent, a sort of
Fig. 3 (a) UV-Vis absorption and (b) CD spectra of the HB complex after CPUL irradiation in (i) crystal, (ii) liquid crystal, and (iii) isotropic phase, respectively.
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structural transition occurs and the mother lattice is replaced by the daughter lattice. The reaction, thereafter, proceeds smoothly.20 While in the LCol phase, the HB complex is free from the restriction of the crystal lattice and the DA units react immediately when they are aligned in a suitable orientation for 1,4-addition in the fluid environment. The chance for the monomers to encounter such a relative orientation may be related to the local order in the lamellar columnar LC phase. While in the isotropic phase, DA units in the HB complex are aligned disorderly in the fluid environment, which is not suitable for 1,4-addition reaction. Therefore the photopolymerization of the HB complex in the isotropic phase would not occur. Besides, after polymerization with CPUL, the packing structure of the films in the crystal phase and the LCol phase could be preserved, respectively. The samples polymerized in the LCol liquid crystal phase exhibited lamellar columnar packing structure, totally different from that polymerized in the crystal phase (as shown in Fig. S5, ESI†). Probably the most important result of this investigation is the synthesis of helical PDA chains in the LCol state with CPUL. The films of the HB complex in the crystal phase and the lamellar columnar LC state were polymerized with unpolarized UV light or CPUL, respectively. When above films were subjected to CD measurement, it was interesting to note that only the sample polymerized in the LCol state with CPUL exhibited significant CD signals at the corresponding absorption band of PDA chains, while not in all other cases (as shown in Fig. 3b). To investigate the origin of the CD signal, measurements were carried out by rotating the films about the normal of the film surface. The signal intensity hardly changed with the rotation angle (Fig. S6, ESI†), indicating that the main origin of CD signals should be the helix formation of PDA chains. All the above results indicated that both LCol state and the CPUL played essential roles in the preparation of helical PDA chains. Herein the chiral source is only CPUL, and the irradiation of left- or right-handed CPUL was expected to definitely yield opposite helical PDA chains. When subjected to left-handed CPUL irradiation for 20 s, a positive and negative Cotton effect appeared at 550 and 660 nm with a crossover at 640 nm, which indicated the formation of left-handed helical structures for PDA backbones. While subjected to right-handed CPUL irradiation for 20 s, an opposite Cotton effect appearing at 550 and 660 nm could be detected, suggesting the formation of right-handed helical structures for PDA backbones (as shown in Fig. S7, ESI†). In order to further quantify the magnitude of CD of the chiral PDAs, we evaluated the CD data using absorption dissymmetry factors (gabs), and a positive Cotton effect (gabs value, 0.0003) appeared at 550 and a negative Cotton effect (gabs value, 0.0001) appeared at 660 nm, respectively (Fig. S8, ESI†). Since there is a long flexible spacer between the PDA main chain and the TTB core, the formation of helical PDA chains would hardly affect the arrangement of the TTB core. All the above results indicated that the screw direction of PDA chains could be rigorously controlled with the handedness direction of the CPUL. In the crystal phase, the molecular motion of the HB complex in the films is restricted by the crystal lattice, and the DA units could not be aligned in a suitable helical orientation, preventing the formation of helical PDA chains. While in the LCol state, the molecular motion of the HB complex is relatively free, which could effectively provide chiral order in the propagating step of a polymerization reaction due to the interaction between the external CPUL and the DA dimer.
This journal is © The Royal Society of Chemistry 2014
Communication
Scheme 1 The formation mechanism of the helical PDA chain in the LCol liquid crystal state.
Therefore, helical PDA chains could be prepared. On the basis of these experimental results, the formation mechanism of the helical PDA chains in the LCol state with CPUL was illustrated in Scheme 1. In summary, the enantio-selective polymerization of DA units had been carried out in an asymmetric reaction field consisting of the lamellar columnar LC phase and CPUL stimulus. This work not only creates a new way for preparing helical conjugated polymers but also is of great fundamental value for the understanding of the enantio-selective polymerization mechanism of helical polymers prepared in the liquid crystal phase. This work is financed by the National Natural Science Foundation of China (No: 51173176, 51273186, 21074123, 91027024), and the Fundamental Research Funds for the Central Universities (WK2060200008).
Notes and references 1 J. Rubio-Magnieto, A. Thomas, S. Richeter, A. Mehdi, P. Dubois, R. Lazzaroni, S. Clement and M. Surin, Chem. Commun., 2013, 49, 5483. 2 F. Wang, M. A. J. Gillissen, P. J. M. Stals, A. R. A. Palmans and E. W. Meijer, Chem.–Eur. J., 2012, 18, 11761. 3 G. Fukuhara and Y. Inoue, Chem.–Eur. J., 2012, 18, 11459. 4 P. Paik, A. Gedanken and Y. Mastai, J. Mater. Chem., 2010, 20, 4085. 5 T. Moriuchi, S. D. Ohmura, K. Morita and T. Hirao, Chem. Asian J., 2011, 6, 3206. 6 Y. Xu, J. Li, W. Hu, G. Zou and Q. Zhang, J. Colloid Interface Sci., 2013, 400, 116. 7 X. d. Hatten, D. Asil, R. H. Friend and J. R. Nitschke, J. Am. Chem. Soc., 2012, 134, 19170. 8 K. Kawabata, M. Takeguchi and H. Goto, Macromolecules, 2013, 46, 2078. 9 K. Akagi, G. Piao, S. Kaneko, K. Sakamaki, H. Shirakawa and M. Kyotani, Science, 1998, 282, 1684. 10 K. Akagi, Chem. Rev., 2009, 109, 5354. 11 X. Sun, T. Chen, S. Huang, L. Li and H. Peng, Chem. Soc. Rev., 2010, 39, 4244. 12 B. Yoon, S. Lee and J.-M. Kim, Chem. Soc. Rev., 2009, 38, 1958. 13 J. Y. Chang, J. R. Yeon, Y. S. Shin, M. J. Han and S.-K. Hong, Chem. Mater., 2000, 12, 1076. 14 S. H. Kang, K. S. Jang, P. Theato, R. Zentel and J. Y. Chang, Macromolecules, 2007, 40, 8349. 15 G. Zou, H. Jiang, H. Kohn, T. Manakab and M. Iwamoto, Chem. Commun., 2009, 5627. 16 F. Vera, R. M. Tejedor, P. Romero, J. Barbera, M. B. Ros, J. L. Serrano and T. Sierra, Angew. Chem., Int. Ed., 2007, 46, 1873. 17 F. Vera, J. Barbera, P. Romero, J. L. Serrano, M. B. Ros and T. Sierra, Angew. Chem., Int. Ed., 2010, 49, 4910. 18 M.-H. Ryu, J.-W. Choi, H.-J. Kim, N. Park and B. K. Cho, Angew. Chem., Int. Ed., 2011, 50, 5737. 19 H. Jiang, X. Chen, X. Pan, G. Zou and Q. Zhang, Macromol. Rapid Commun., 2012, 33, 773. 20 T. Okuno, A. Izuoka, T. Ito, S. Kubo, T. Sugawara, N. Sato and Y. Sugawara, J. Chem. Soc., Perkin Trans. 2, 1998, 889.
Chem. Commun., 2014, 50, 365--367 | 367
Published on 29 October 2013. Downloaded by Selcuk University on 11/02/2015 13:08:16.
COMMUNICATION
View Article Online View Journal | View Issue
Cite this: Chem. Commun., 2014, 50, 365
Helical polydiacetylene prepared in the liquid crystal phase using circular polarized ultraviolet light†
Received 22nd September 2013, Accepted 28th October 2013
Yangyang Xu, Hao Jiang, Qijin Zhang, Feng Wang* and Gang Zou*
DOI: 10.1039/c3cc47245k www.rsc.org/chemcomm
Herein the enantio-selective polymerization of diacetylene (DA) units in the discotic hydrogen-bonding complex is carried out in an asymmetric reaction field consisting of the lamellar columnar LC phase and CPUL stimulus.
Helical conformation is a representative secondary structure of biomacromolecules in nature, such as proteins and deoxyribonucleic acids. In particular, the introduction of chirality into conjugated polymer materials is expected to result in novel optical, electrical and magnetic properties, and tremendous potential applications in organic devices with special electromagnetic functions and chirality sensing.1,2 In the past few decades, various helical conjugated polymers, including polythiophene,3 polypyrrole,4 polyaniline,5 and polydiacetylene (PDA),6 have been prepared by various methods including the polymerization of optically active monomers, supramolecular assembly, enzyme catalysed, template synthesis, and electrochemical polymerization.7,8 Recently, professor Akagi et al. have developed a novel method for preparing helical conjugated polymers in a chiral nematic liquid crystal reaction field.9,10 A chiral nematic liquid crystal (LC) has a quasi-layer structure, which can effectively provide chiral order in the propagating step of a polymerization reaction. The handedness of the helix can be reversed by using the chiral dopant with opposite chirality.9 However, this method is not suitable for preparing helical PDA via topo-polymerization processes, since the presence of a liquid crystal and the chiral dopant as the reaction media might annihilate the topo-polymerization of diacetylene (DA) monomers.11,12 To the best of our knowledge, attempts to synthesize helical PDA chains in the LC phase, particularly the columnar LC phase, have not been described. Various mesogenic DA monomers have been synthesized, and the topo-polymerization reactions have been carried out in liquid-crystal media including nematic and smetic phases.13,14 The disc-shaped CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, Key Laboratory of Optoelectronic Science and Technology in Anhui Province, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China. E-mail: [email protected], [email protected] † Electronic supplementary information (ESI) available: XRD, POM, TEM, UV and CD characterization techniques. See DOI: 10.1039/c3cc47245k
This journal is © The Royal Society of Chemistry 2014
mesogenic DA monomers are of particular interest as they stack in columns, and zipping of diacetylenes along the column axis by polymerization will lead to supramolecules with columnar structures. The design of discotic LC can be accomplished by supramolecular approaches, among which hydrogen-bonding (HB) assembly could be the most effective way to build a rigid discotic mesogenic DA monomers by considering bonding directionality and strength. Herein, we present a novel HB complex consisting of a clicked C3-symmetric triazole derivative (TTB) with 10,12pentacosadiynoic acid (PCDA) in a 1 : 3 stoichiometry. Notably, this HB complex is the first example of DA LC materials prepared by the supramolecular assembly method. The photo-polymerization reactions of the above discotic HB complex were performed in the crystal phase, the LC phase and the isotropic phase, respectively, and the effect of the LC reaction field has been evaluated. To date, the inducement of a chiral dopant has been a necessary condition for the preparation of the helix conjugated polymers in the LC reaction field. Whether helical PDA can be synthesized in the LC reaction field in the absence of a chiral dopant remains a question to be answered. Recently, we reported the successful enantio-selective polymerization of PDA Langmuir–Blodgett (LB) films by means of circular polarized UV light (CPUL).15 The helical direction of PDA chains in the LB films could be controlled and modulated by CPUL treatment. However, the enantio-selective polymerization mechanism of DA monomers with CPUL is not clear and attempts to synthesize helical PDA in the columnar LC phase with CPUL have not been described to date.16,17 We demonstrate herein the possibility of controlling the formation of helical PDA chains in the LC field by using an external chiral stimulus such as CPUL. 10,12-Pentacosadiynoic acid (PCDA) is purchased from Tokyo Chemical Industry Co., Ltd., and purified to remove the polymerized part before use. 1,3,5-Tris(1-alkyl-1H-1,2,3-triazol-4-yl) benzene (TTB) was synthesized in analogy to the previous procedure.18 A HB complex was obtained by mixing TTB with PCDA in a 1 : 3 molar ratio in chloroform solution followed by slow evaporation of the solvent at reduced pressure. Taking into account the core structure of the HB complex, we suggested that the carbonyl oxygen atoms in PCDA could be a hydrogen-acceptor with respect to the donor from
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Fig. 1 (a) Molecular structure and the synthetic route of the discotic HB complex; (b) FT-IR spectra of (i) TTB, (ii) PCDA, and (iii) PCDA–TTB complex; (c) 1H-NMR spectra of (i) TTB and (ii) HB complex in CDCl3.
the TTB unit. The formation of the above HB complex was confirmed by FT-IR and 1H-NMR spectroscopy characterization. The CQO stretching vibration in pure PCDA samples appeared at 1693 cm 1, which indicated that the carboxyl group was not in a free but an associated state.19 Conversely, the carbonyl band in the HB complex shifted to 1703 cm 1 (as shown in Fig. 1b), lower than that of the free carboxylic acid, suggesting that the carbonyl group still interacted with a different H-bonding donor, presumably CHb of the triazole in TTB. To corroborate this, we examined the 1H-NMR signals in deuterated chloroform solutions. As shown in Fig. 1c, the signal from the hydrogen atom CHb of triazole in TTB appeared at 7.98 ppm, but shifted to 8.06 ppm in the HB complex. The signal for CHa of the central benzene unit also shifted downfield, confirming the formation of the discotic HB complex.18 Then the thermotropic LC behaviour of the HB complex was analysed by using differential scanning calorimetry (DSC), polarized optical microscopy (POM) and X-ray diffraction (XRD) techniques. The results are summarized in Fig. 2. As the reference sample, TTB or PCDA alone exhibited no LC phase. However, the discotic HB complex showed a monotropic LC phase exclusively upon heating (Fig. 2a) before polymerization. To identify the LC phase, we investigated the POM textures of the HB complex, which melted at 37.8 1C and exhibited a lamellar texture structure upon heating from the crystal phase (Fig. 2b). The morphology of above film was further evaluated by TEM characterization. As shown in Fig. S2 (ESI†), closely parallel fibers (about 13 nm in diameter) could be observed in the monolayer of the HB complex. The XRD pattern at 50 1C for the sample revealed five reflections at 4.54, 2.38, 1.54, 1.15, 0.94 nm in the low angle regime with the reciprocal spacing ratio of 1 : 1/2 : 1/3 : 1/4 : 1/5, assigned as the (001), (002), (003), (004) and (005) reflections, indicating a layered structure with a periodicity of 4.54 nm (Fig. 2c). Actually, the lateral spacing was shorter than that of the fully extended disk diameter (6.5 nm) calculated by Chemical 3D, suggesting the lamellar columnar packing of the HB complex with interpenetration structure. In addition, the diffraction at 0.41 nm corresponds to the mean
366 | Chem. Commun., 2014, 50, 365--367
ChemComm
Fig. 2 (a) DSC curve of the HB complex; (b) POM texture and (c) Small angle X-ray diffraction profile of the complex in the LCol liquid crystal phase before polymerization; (d) the model of a highly ordered lamellar columnar mesophase.
distance between the fluid alkyl chains, suggesting the formation of a lamellar columnar mesophase (LCol) as shown in the model proposed in Fig. 2d. A characteristic absorption at 245 nm for the TTB chromophore could be observed for the films of the HB complex, obviously blue-shifted compared with that in chloroform solution (Fig. S3, ESI†). This blue-shift for excitonic absorption could be understood as an indication that under the given condition, TTB units formed H-aggregates by face to face stacking in the assemblies. Photo-triggered polymerization of DA units could be detected optically as the conjugated p electron system of PDA chains showed typical intense absorption maximum at about 640 and 580 nm. As shown in Fig. 3a, the discotic HB complex could be photopolymerized in the crystal phase and the LCol liquid crystal phase, while not in the isotropic phase. After polymerization, the fractional conversion values for the samples polymerized in the crystal and the LCol phase were about 0.35 and 0.31, respectively. The polymerization kinetic process for the HB complex in the crystal phase and the LCol phase could both be described as a first order reaction, and the rate constants k1 for the HB complex in the crystal phase and the LCol phase were 0.0032 and 0.43 s 1 (as shown in Fig. S4, ESI†). The polymerization rate for the HB complex in the LCol phase was considerably faster than that in the crystal phase. The possible explanation could be proposed as follows: since the molecular motion in the crystal phase is restricted by the crystal lattice, the reaction during the induction time is very slow. After the local concentration of polymers increases to a certain extent, a sort of
Fig. 3 (a) UV-Vis absorption and (b) CD spectra of the HB complex after CPUL irradiation in (i) crystal, (ii) liquid crystal, and (iii) isotropic phase, respectively.
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structural transition occurs and the mother lattice is replaced by the daughter lattice. The reaction, thereafter, proceeds smoothly.20 While in the LCol phase, the HB complex is free from the restriction of the crystal lattice and the DA units react immediately when they are aligned in a suitable orientation for 1,4-addition in the fluid environment. The chance for the monomers to encounter such a relative orientation may be related to the local order in the lamellar columnar LC phase. While in the isotropic phase, DA units in the HB complex are aligned disorderly in the fluid environment, which is not suitable for 1,4-addition reaction. Therefore the photopolymerization of the HB complex in the isotropic phase would not occur. Besides, after polymerization with CPUL, the packing structure of the films in the crystal phase and the LCol phase could be preserved, respectively. The samples polymerized in the LCol liquid crystal phase exhibited lamellar columnar packing structure, totally different from that polymerized in the crystal phase (as shown in Fig. S5, ESI†). Probably the most important result of this investigation is the synthesis of helical PDA chains in the LCol state with CPUL. The films of the HB complex in the crystal phase and the lamellar columnar LC state were polymerized with unpolarized UV light or CPUL, respectively. When above films were subjected to CD measurement, it was interesting to note that only the sample polymerized in the LCol state with CPUL exhibited significant CD signals at the corresponding absorption band of PDA chains, while not in all other cases (as shown in Fig. 3b). To investigate the origin of the CD signal, measurements were carried out by rotating the films about the normal of the film surface. The signal intensity hardly changed with the rotation angle (Fig. S6, ESI†), indicating that the main origin of CD signals should be the helix formation of PDA chains. All the above results indicated that both LCol state and the CPUL played essential roles in the preparation of helical PDA chains. Herein the chiral source is only CPUL, and the irradiation of left- or right-handed CPUL was expected to definitely yield opposite helical PDA chains. When subjected to left-handed CPUL irradiation for 20 s, a positive and negative Cotton effect appeared at 550 and 660 nm with a crossover at 640 nm, which indicated the formation of left-handed helical structures for PDA backbones. While subjected to right-handed CPUL irradiation for 20 s, an opposite Cotton effect appearing at 550 and 660 nm could be detected, suggesting the formation of right-handed helical structures for PDA backbones (as shown in Fig. S7, ESI†). In order to further quantify the magnitude of CD of the chiral PDAs, we evaluated the CD data using absorption dissymmetry factors (gabs), and a positive Cotton effect (gabs value, 0.0003) appeared at 550 and a negative Cotton effect (gabs value, 0.0001) appeared at 660 nm, respectively (Fig. S8, ESI†). Since there is a long flexible spacer between the PDA main chain and the TTB core, the formation of helical PDA chains would hardly affect the arrangement of the TTB core. All the above results indicated that the screw direction of PDA chains could be rigorously controlled with the handedness direction of the CPUL. In the crystal phase, the molecular motion of the HB complex in the films is restricted by the crystal lattice, and the DA units could not be aligned in a suitable helical orientation, preventing the formation of helical PDA chains. While in the LCol state, the molecular motion of the HB complex is relatively free, which could effectively provide chiral order in the propagating step of a polymerization reaction due to the interaction between the external CPUL and the DA dimer.
This journal is © The Royal Society of Chemistry 2014
Communication
Scheme 1 The formation mechanism of the helical PDA chain in the LCol liquid crystal state.
Therefore, helical PDA chains could be prepared. On the basis of these experimental results, the formation mechanism of the helical PDA chains in the LCol state with CPUL was illustrated in Scheme 1. In summary, the enantio-selective polymerization of DA units had been carried out in an asymmetric reaction field consisting of the lamellar columnar LC phase and CPUL stimulus. This work not only creates a new way for preparing helical conjugated polymers but also is of great fundamental value for the understanding of the enantio-selective polymerization mechanism of helical polymers prepared in the liquid crystal phase. This work is financed by the National Natural Science Foundation of China (No: 51173176, 51273186, 21074123, 91027024), and the Fundamental Research Funds for the Central Universities (WK2060200008).
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