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Tuning the helical twisting power of nematic liquid crystals induced by chiral 1,2-propanediol derivatives using varied substituents
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Chinese Chemical Letters 23 (2012) 37–40 www.elsevier.com/locate/cclet
Tuning the helical twisting power of nematic liquid crystals induced by chiral 1,2-propanediol derivatives using varied substituents Hai Quan Zhang *, An Lei Qin State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China Received 13 June 2011 Available online 8 November 2011
Abstract In this study, a novel series of chiral 1,2-propanediol derivatives with different electron-donating and electron-withdrawing groups were synthesized and characterized by FT-IR and 1H NMR. The helical twisting properties of all the chiral dopants were investigated by doping the chiral dopants into a nematic liquid crystal host (SLC-1717). The results indicate that the donor–acceptor electron effect have a prominent influence on helical twisting property of the chiral nematic phase induced by the chiral dopants. Introducing electron-withdrawing groups into the terminal ends of chiral 1,2-propanediol can decrease the absolute values of the helical twisting power. In addition, the helix inversion temperatures of the induced chiral nematic phase are variational with the change of terminal groups. # 2011 Hai Quan Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Chiral dopants; Chiral nematic phase; Helical twisting power; Electron-donating group; Electron-withdrawing group
Chiral nematic liquid crystals (N*-LCs) with macro helical structures are currently used both in liquid crystal display (LCD) devices and chirality research [1–3]. Chiral molecules, which have the property to transfer and magnify the molecular chirality, can induce a chiral nematic phase by dissolving into an achiral liquid crystal mesophase [4,5]. Generally, the ability of a chiral dopant to generate a helical structure is measured as helical twisting power (HTP), b, is defined as: b = (Pcwr) 1, where P is the helical pitch of the resultant phase, cw is the weight concentration and r is the purity of chiral dopants [6]. The dominant factors affecting the b value of the chiral nematic phase and design of novel chiral molecule has been attracted attention, but the categories of chiral dopants are mainly chiral amino [7] and chiral biphenyls compounds [8]. However, few research to report the helical twisting behavior of the nematic liquid crystals induced by chiral 1,2-propanediol derivatives besides our group and collaborator. Recently, we reported the influences of terminal alkyl chain length, numbers of the chiral centre on the helical twisting behavior of chiral nematic phase induced by chiral 1,2-propanediol derivatives [9–12]. In order to get a further understanding of the relationship between the chemical structure and the helical twisting property, here we designed a series of 1,2-propanediol derivatives with different electron-donating and electronwithdrawing groups (chemical structure see Fig. 1), and the influence of donor–acceptor electron effect on helical twisting behavior and the helix inversion temperatures in a chiral nematic phase were also investigated and discussed.
* Corresponding author. E-mail address: [email protected] (H.Q. Zhang). 1001-8417/$ – see front matter # 2011 Hai Quan Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2011.09.019
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H.Q. Zhang, A.L. Qin / Chinese Chemical Letters 23 (2012) 37–40
Fig. 1. Chemical structure of the chiral 1,2-propanediol derivatives.
1. Experimental Synthetic routes and structural characterizations of the chiral 1,2-propanediol derivatives were described in Supporting information. The desired chiral dopants were doped into the nematic LC host of SLC-1717 (TN-I = 91.8 8C, Shijiazhuang Yongsheng Huatsing Liquid Crystal Material Co., Ltd.) with the concentration about 5.0 wt.%. The pitch, P, which is the distance of the helix corresponding to a 2p molecular rotation, was measured by Cano wedge technique [13]. To characterize the twisting power of the chiral dopants, the molecular twisting power b = 1/PNn was used (Nn is the number density of the chiral dopant in mol/m3). The values of b were calculated assuming a density of the LC solution of 1 g/cm3 [14]. 2. Results and discussion Fig. 2 shows the temperature dependence of the molecular twisting power curves of N*-LCs induced by NSF, NSCF3, NSNO2, SC5, and SNO2, respectively. It can be seen that the molecular twisting power of the induced N*-LCs decreased strongly with increasing temperature. However, a helix inversion was observed at about 54 8C for the NSCF3, which is in agreement with textural observations of the N*-LCs. Here ‘‘+’’ and ‘‘ ’’ indicate right-handed and left-handed helices, respectively. Fig. 3 shows POM photos of the N*-LCs induced by NSCF3 at different temperatures in the wedge-shaped cell (c = 5.0 wt.%). As can be seen, the distance of parallel disclination lines increased with increasing temperature in the investigated temperature range from 16 8C to 35 8C. The homogeneous alignment of nematic LCs could be observed at about 54 8C, with the transmitted light intensity of the cell changing periodically while rotating between the polarizer and the analyzer. That is to say, the pitch, P, of N*-LCs induced by NSCF3 became infinite at about 54 8C. Then a helix inversion occurred with the temperature variation. With temperature increasing further, the distance of parallel disclination lines decreased in the temperature range from 60 8C to the temperature of the clearing point. Table 1 lists the molecular twisting power at 20.0 8C and the helix inversion temperatures (TH-I) of the chiral dopants were estimated from the b curve. It can be seen that the absolute values of the helical twisting power and TH-I depend on the characterization of electron-donating and electron-withdrawing groups strongly. When the two pentyloxy groups (–OC5H11) in the both sides of dopant (SC5) were replaced by two nitro-groups (SNO2), the helical
a
b
NSF NSCF3
200
-1
β m mol
2
-1 2
β m mol
-8
-400
-12
-600
-16
-800
-20 -24
-1000 20
30
40
50
60 o
Temperature ( C)
70
80
90
-1200 10
-28 20
30
40 50 60 70 o Temperature ( C)
Fig. 2. Temperature dependence of the molecular twisting power.
80
90
100
-1
-600
-4
SNO2
2
-400
SC5
β m mol
-200
-800 10
0
-200
NSNO2
0
0
H.Q. Zhang, A.L. Qin / Chinese Chemical Letters 23 (2012) 37–40
39
Fig. 3. POM photos of the N*-LCs induced by NSCF3 at different temperatures in the wedge-shaped cell: (a) 16.0 8C; (b) 35.0 8C; (c) 54.0 8C; (d) 60.0 8C; (e) 75.0 8C; (f) 92.0 8C.
Table 1 Molecular twisting power at 20.0 8C and the helix inversion temperatures of the N*-LCs induced by different chiral dopants. Chiral dopants 2
1
b (20.0 8C) (m mol ) TH-I (8C)
NSCF3
NSF
NSNO2
SC5
SNO2
456.5 54.0
535.2 84.2
678.4 73.6
1123.6 79.8
20.5 94.3
twisting power values of SNO2 is about 54–55 times lower than that of SC5, but the TH-I increased from 79.8 8C to 94.3 8C. In addition, when the two pentyloxy groups (–OC5H11) in dopant (SC5) were replaced just at one side by a nitro-group (NSNO2), the b values decreased just about 1 times, and the TH-I increased about 5 8C. These results indicate that introducing electron-withdrawing groups into the chiral 1,2-propanediol derivatives can decrease the helical twisting power. In summary, a series of chiral 1,2-propanediol derivatives with different terminal substituent were synthesized and characterized. The compounds NSF, NSCF3, NSNO2, SC5 and SNO2 exhibit a similar temperature dependence of the helical twisting behavior that the molecular twisting power b decreased with the temperature increasing. When introducing electron-withdrawing groups into the two sides of chiral dopant, the absolute values of b decreased strongly compared with the dopant with two pentyloxy groups (–OC5H11) and the helix inversion temperatures TH-I also changed, that is, the b values and TH-I depend on the nature of the terminal substituents strongly. Acknowledgments This work was supported in part by the National Natural Science Foundation of China (No. 51173155) and the Hebei Province Science Foundation of China (No. E2010001182). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.cclet.2011.09.019. References [1] H.S. Kitzerow, C. Bahr, Chirality in Liquid Crystals, Springer-Verlag, New York, Berlin, Heidelberg, 2001, p. 67. [2] A.V. Emelyanenko, Phys. Rev. E 67 (2003) 1.
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H.Q. Zhang, A.L. Qin / Chinese Chemical Letters 23 (2012) 37–40
[3] S. Furumi, S. Yokoyama, A. Otomo, et al. Thin Solid Films 423 (2003) 438. [4] R. Eelkema, B.L. Feringa, Org. Biomol. Chem. 4 (2006) 3729. [5] (a) S. Pieraccini, M.I. Donnoli, A. Ferrarini, et al. J. Org. Chem. 68 (2003) 519; (b) S. Pieraccini, A. Ferrarini, G.P. Spada, Chirality 20 (2008) 749. [6] (a) M.R. Wilson, D.J. Earl, J. Mater. Chem. 11 (2001) 2672; (b) D.J. Earl, M.R. Wilson, J. Chem. Phys. 19 (2003) 10280. [7] K. Fukuda, H. Suzuki, M. Tokita, et al. J. Mol. Struct.: Theochem. 821 (2007) 95. [8] (a) R. Holzwarth, R. Bartsch, Z. Cherkaoui, et al. Chem. Eur. J. 10 (2004) 3931; (b) R. Holzwarth, R. Bartsch, Z. Cherkaoui, et al. Eur. J. Org. Chem. (2005) 3536. [9] W. Huang, X.G. Zhang, H. Yang, et al. Chin. Chem. Lett. 20 (2009) 1435. [10] H.Q. Zhang, W.K. Liu, L.C. Zhao, et al. Chin. Chem. Lett. 21 (2010) 139. [11] H.Q. Zhang, W.K. Liu, A.L. Qin, et al. Liq. Cryst. 37 (2010) 317. [12] X.G. Zhang, W. Huang, H. Yang, et al. Liq. Cryst. 36 (2009) 531. [13] R. Cano, Bull. Soc. Fr. Miner. 91 (1968) 20. [14] M. Mitov, N. Dessaud, Liq. Cryst. 34 (2007) 183.
Chinese Chemical Letters 23 (2012) 37–40 www.elsevier.com/locate/cclet
Tuning the helical twisting power of nematic liquid crystals induced by chiral 1,2-propanediol derivatives using varied substituents Hai Quan Zhang *, An Lei Qin State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China Received 13 June 2011 Available online 8 November 2011
Abstract In this study, a novel series of chiral 1,2-propanediol derivatives with different electron-donating and electron-withdrawing groups were synthesized and characterized by FT-IR and 1H NMR. The helical twisting properties of all the chiral dopants were investigated by doping the chiral dopants into a nematic liquid crystal host (SLC-1717). The results indicate that the donor–acceptor electron effect have a prominent influence on helical twisting property of the chiral nematic phase induced by the chiral dopants. Introducing electron-withdrawing groups into the terminal ends of chiral 1,2-propanediol can decrease the absolute values of the helical twisting power. In addition, the helix inversion temperatures of the induced chiral nematic phase are variational with the change of terminal groups. # 2011 Hai Quan Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Chiral dopants; Chiral nematic phase; Helical twisting power; Electron-donating group; Electron-withdrawing group
Chiral nematic liquid crystals (N*-LCs) with macro helical structures are currently used both in liquid crystal display (LCD) devices and chirality research [1–3]. Chiral molecules, which have the property to transfer and magnify the molecular chirality, can induce a chiral nematic phase by dissolving into an achiral liquid crystal mesophase [4,5]. Generally, the ability of a chiral dopant to generate a helical structure is measured as helical twisting power (HTP), b, is defined as: b = (Pcwr) 1, where P is the helical pitch of the resultant phase, cw is the weight concentration and r is the purity of chiral dopants [6]. The dominant factors affecting the b value of the chiral nematic phase and design of novel chiral molecule has been attracted attention, but the categories of chiral dopants are mainly chiral amino [7] and chiral biphenyls compounds [8]. However, few research to report the helical twisting behavior of the nematic liquid crystals induced by chiral 1,2-propanediol derivatives besides our group and collaborator. Recently, we reported the influences of terminal alkyl chain length, numbers of the chiral centre on the helical twisting behavior of chiral nematic phase induced by chiral 1,2-propanediol derivatives [9–12]. In order to get a further understanding of the relationship between the chemical structure and the helical twisting property, here we designed a series of 1,2-propanediol derivatives with different electron-donating and electronwithdrawing groups (chemical structure see Fig. 1), and the influence of donor–acceptor electron effect on helical twisting behavior and the helix inversion temperatures in a chiral nematic phase were also investigated and discussed.
* Corresponding author. E-mail address: [email protected] (H.Q. Zhang). 1001-8417/$ – see front matter # 2011 Hai Quan Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2011.09.019
38
H.Q. Zhang, A.L. Qin / Chinese Chemical Letters 23 (2012) 37–40
Fig. 1. Chemical structure of the chiral 1,2-propanediol derivatives.
1. Experimental Synthetic routes and structural characterizations of the chiral 1,2-propanediol derivatives were described in Supporting information. The desired chiral dopants were doped into the nematic LC host of SLC-1717 (TN-I = 91.8 8C, Shijiazhuang Yongsheng Huatsing Liquid Crystal Material Co., Ltd.) with the concentration about 5.0 wt.%. The pitch, P, which is the distance of the helix corresponding to a 2p molecular rotation, was measured by Cano wedge technique [13]. To characterize the twisting power of the chiral dopants, the molecular twisting power b = 1/PNn was used (Nn is the number density of the chiral dopant in mol/m3). The values of b were calculated assuming a density of the LC solution of 1 g/cm3 [14]. 2. Results and discussion Fig. 2 shows the temperature dependence of the molecular twisting power curves of N*-LCs induced by NSF, NSCF3, NSNO2, SC5, and SNO2, respectively. It can be seen that the molecular twisting power of the induced N*-LCs decreased strongly with increasing temperature. However, a helix inversion was observed at about 54 8C for the NSCF3, which is in agreement with textural observations of the N*-LCs. Here ‘‘+’’ and ‘‘ ’’ indicate right-handed and left-handed helices, respectively. Fig. 3 shows POM photos of the N*-LCs induced by NSCF3 at different temperatures in the wedge-shaped cell (c = 5.0 wt.%). As can be seen, the distance of parallel disclination lines increased with increasing temperature in the investigated temperature range from 16 8C to 35 8C. The homogeneous alignment of nematic LCs could be observed at about 54 8C, with the transmitted light intensity of the cell changing periodically while rotating between the polarizer and the analyzer. That is to say, the pitch, P, of N*-LCs induced by NSCF3 became infinite at about 54 8C. Then a helix inversion occurred with the temperature variation. With temperature increasing further, the distance of parallel disclination lines decreased in the temperature range from 60 8C to the temperature of the clearing point. Table 1 lists the molecular twisting power at 20.0 8C and the helix inversion temperatures (TH-I) of the chiral dopants were estimated from the b curve. It can be seen that the absolute values of the helical twisting power and TH-I depend on the characterization of electron-donating and electron-withdrawing groups strongly. When the two pentyloxy groups (–OC5H11) in the both sides of dopant (SC5) were replaced by two nitro-groups (SNO2), the helical
a
b
NSF NSCF3
200
-1
β m mol
2
-1 2
β m mol
-8
-400
-12
-600
-16
-800
-20 -24
-1000 20
30
40
50
60 o
Temperature ( C)
70
80
90
-1200 10
-28 20
30
40 50 60 70 o Temperature ( C)
Fig. 2. Temperature dependence of the molecular twisting power.
80
90
100
-1
-600
-4
SNO2
2
-400
SC5
β m mol
-200
-800 10
0
-200
NSNO2
0
0
H.Q. Zhang, A.L. Qin / Chinese Chemical Letters 23 (2012) 37–40
39
Fig. 3. POM photos of the N*-LCs induced by NSCF3 at different temperatures in the wedge-shaped cell: (a) 16.0 8C; (b) 35.0 8C; (c) 54.0 8C; (d) 60.0 8C; (e) 75.0 8C; (f) 92.0 8C.
Table 1 Molecular twisting power at 20.0 8C and the helix inversion temperatures of the N*-LCs induced by different chiral dopants. Chiral dopants 2
1
b (20.0 8C) (m mol ) TH-I (8C)
NSCF3
NSF
NSNO2
SC5
SNO2
456.5 54.0
535.2 84.2
678.4 73.6
1123.6 79.8
20.5 94.3
twisting power values of SNO2 is about 54–55 times lower than that of SC5, but the TH-I increased from 79.8 8C to 94.3 8C. In addition, when the two pentyloxy groups (–OC5H11) in dopant (SC5) were replaced just at one side by a nitro-group (NSNO2), the b values decreased just about 1 times, and the TH-I increased about 5 8C. These results indicate that introducing electron-withdrawing groups into the chiral 1,2-propanediol derivatives can decrease the helical twisting power. In summary, a series of chiral 1,2-propanediol derivatives with different terminal substituent were synthesized and characterized. The compounds NSF, NSCF3, NSNO2, SC5 and SNO2 exhibit a similar temperature dependence of the helical twisting behavior that the molecular twisting power b decreased with the temperature increasing. When introducing electron-withdrawing groups into the two sides of chiral dopant, the absolute values of b decreased strongly compared with the dopant with two pentyloxy groups (–OC5H11) and the helix inversion temperatures TH-I also changed, that is, the b values and TH-I depend on the nature of the terminal substituents strongly. Acknowledgments This work was supported in part by the National Natural Science Foundation of China (No. 51173155) and the Hebei Province Science Foundation of China (No. E2010001182). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.cclet.2011.09.019. References [1] H.S. Kitzerow, C. Bahr, Chirality in Liquid Crystals, Springer-Verlag, New York, Berlin, Heidelberg, 2001, p. 67. [2] A.V. Emelyanenko, Phys. Rev. E 67 (2003) 1.
40
H.Q. Zhang, A.L. Qin / Chinese Chemical Letters 23 (2012) 37–40
[3] S. Furumi, S. Yokoyama, A. Otomo, et al. Thin Solid Films 423 (2003) 438. [4] R. Eelkema, B.L. Feringa, Org. Biomol. Chem. 4 (2006) 3729. [5] (a) S. Pieraccini, M.I. Donnoli, A. Ferrarini, et al. J. Org. Chem. 68 (2003) 519; (b) S. Pieraccini, A. Ferrarini, G.P. Spada, Chirality 20 (2008) 749. [6] (a) M.R. Wilson, D.J. Earl, J. Mater. Chem. 11 (2001) 2672; (b) D.J. Earl, M.R. Wilson, J. Chem. Phys. 19 (2003) 10280. [7] K. Fukuda, H. Suzuki, M. Tokita, et al. J. Mol. Struct.: Theochem. 821 (2007) 95. [8] (a) R. Holzwarth, R. Bartsch, Z. Cherkaoui, et al. Chem. Eur. J. 10 (2004) 3931; (b) R. Holzwarth, R. Bartsch, Z. Cherkaoui, et al. Eur. J. Org. Chem. (2005) 3536. [9] W. Huang, X.G. Zhang, H. Yang, et al. Chin. Chem. Lett. 20 (2009) 1435. [10] H.Q. Zhang, W.K. Liu, L.C. Zhao, et al. Chin. Chem. Lett. 21 (2010) 139. [11] H.Q. Zhang, W.K. Liu, A.L. Qin, et al. Liq. Cryst. 37 (2010) 317. [12] X.G. Zhang, W. Huang, H. Yang, et al. Liq. Cryst. 36 (2009) 531. [13] R. Cano, Bull. Soc. Fr. Miner. 91 (1968) 20. [14] M. Mitov, N. Dessaud, Liq. Cryst. 34 (2007) 183.