Helical twisting properties of new chiral dopants with double (S)-1,2-propanediol units for nematic liquid crystals

Article abstract of DOI:10.1016/j.cclet.2009.09.007

A novel series of chiral dopants synthesized from (S)-1,2-propanediol and mesogenic carboxylic acids were characterized by FT-IR, ¹H NMR, elemental analysis and their helical twisting properties were investigated by doping the chiral dopants into a nematic liquid crystal host (SLC-1717). The results show that, the helical pitch of N*-LC mixture exhibited a terminal alkyl chain length dependence and the molecular twisting power β also exhibited a temperature dependence (increasing β with increasing temperature).

Full text of DOI:10.1016/j.cclet.2009.09.007

Available online at www.sciencedirect.com
Chinese Chemical Letters 21 (2010) 139–142
www.elsevier.com/locate/cclet
Helical twisting properties of new chiral dopants with double
(S)-1,2-propanediol units for nematic liquid crystals
a
a
b
Hai Quan Zhang a, , Wen Ke Liu , Li Cheng Zhao , Wei Huang ,
*
Shuai Zhang a, Ying Li a, Huai Yang b,
*
a State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China
b Department of Materials Physics and Chemistry, School of Materials Science and Engineering,
University of Science and Technology Beijing, Beijing 100083, China
Received 5 June 2009
Abstract
A novel series of chiral dopants synthesized from (S)-1,2-propanediol and mesogenic carboxylic acids were characterized by
1
FT-IR, H NMR, elemental analysis and their helical twisting properties were investigated by doping the chiral dopants into a
nematic liquid crystal host (SLC-1717). The results show that, the helical pitch of N*-LC mixture exhibited a terminal alkyl chain
length dependence and the molecular twisting power b also exhibited a temperature dependence (increasing b with increasing
temperature).
# 2009 Hai Quan Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Chiral dopants; Chiral nematic; Pitch; Molecular twisting power
The thermotropic liquid crystals have attracted the keenest interest of scientists for their important technological
applications [1–3], and chiral nematic liquid crystals having macro helical structure are currently used in liquid crystal
display (LCD) devices [4–5]. The induction of chirality in mesophases of non-chiral mesogens can be achieved by
adding a small quantity of chiral dopant (up to 10%) to non-chiral liquid-crystalline hosts [6], and molecular chirality
is transferred to macroscopic chirality resulting in the formation of new phases with helical ordering. The N*-LC so
obtained is characterized by pitch and helical twisting sense. Nowadays, many more investigation on the temperature
dependence of the pitch of the chiral nematic phase and structural design of the chiral molecule has been conducted on,
but the category of chiral dopants is almost amino chiral compound [6–9]. Recently, Mitov and Dessaud have
investigated the polymorphism and optical behavior of chiral dopants with single (S)-1,2-propanediol unit [10] and we
have also researched the helical twisting behavior of chiral dopants containing single chiral centre ((S)-1,2-
propanediol derivatives) [11,12].
In this paper, in pursuit of a clearer understanding between molecular structure and helical twisting behavior of
chiral dopants inducing N*-LCs, a new kind of chiral dopants with double chiral centres (optically active (S)-1,2-
propanediol) were synthesized and investigated.
* Corresponding authors.
E-mail address: hqzhang@ysu.edu.cn (H.Q. Zhang).
1001-8417/$ – see front matter # 2009 Hai Quan Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2009.09.007

140
H.Q. Zhang et al. / Chinese Chemical Letters 21 (2010) 139–142
Scheme 1. The synthetic routes to CnTC: (a) DCC/DMAP, THF, rt; (b) p-phthaloyl chloride, Et3N/DMAP, THF, 0 8C.
The synthesis of the chiral dopants is outlined in Scheme 1. First, compound Cn was synthesized by esterification of
the commercially available corresponding 4-alkyloxybenzoic acid added dropwise and (S)-1,2-propanediol with
dicyclohexyl carbodiimide (DCC) and 4-dimethylaminopyridine (DMAP). Further, a series of CnTC were synthesized
by acylation of terephthaloyl dichloride and Cn with DMAP and triethylamine (Et3N). The results of FT-IR, 1H NMR
and elemental analyses [FT-IR (KBr, cmÀ1): 2931, 2871 (–CH3, –CH2–), 1721 (C O), 1605, 1510; 1H NMR (CDCl3):
d 8.05–8.09, 7.94–7.98, 6.86–6.90 (m, 12H, Ph–H), 5.51–5.55 (m, 2H, C*H), 4.46–4.51 (d, 4H, C*–CH2–O–), 1.37–
1.41 (d, 6H, C*–CH3), 3.97–4.00, 1.77–1.81, 1.44–1.48, 0.91–0.94 (m, 22H, alkyl-H); elemental analysis (%) for
C38H46O10: calcd: C 68.86, H 7.00, O 24.14; found: C 68.91, H 7.06, O 24.03] indicated that the chemical structure
was found to be consistent with the structure CnTC. (Electronic Supplementary Information (ESI) available: general
experimental procedure, synthesis and characterization.)
The nematic LC host of SLC-1717 (TN-I = 91.8 8C) and chiral dopant (CnTC) were dissolved in acetone with the
concentration of chiral substance in about 2.0 wt.%, and then sonicated for at least 2.0 h to ensure that an even N*-LC
mixture was obtained after removing the solvent. The Cano wedge technique [13] was employed to measure the pitch
lengths of the N*-LC mixture at different temperatures.
Fig. 1 shows the dependence of helical pitch on the terminal alkyl chain length at 30 8C. It could be seen clearly that
the pitch decreased with terminal alkly chain length increasing. Besides, an odd–even effect could be observed in the
helical pitches of N*-LC doping by the chiral dopants with different terminal alkyl chain length. For the even-member
terminal alkyl chain, a weaker chirality transfer to nematic LC molecules might suggest a larger pitch.
Fig. 2 shows the temperature dependence of the molecular twisting power b of CnTC (n = 3–8). As seen, the chiral
dopants with different terminal alkyl chain length exhibited a similar temperature dependence of the induced helical
structure, namely, increasing b values with increasing temperature. However, increasing degree is different slightly.
Enhanced mutual interactions with nematic LC molecules and promoted chirality transfer in LCs could well explain
the b values increasing [14]. In addition, as can be seen in Fig. 2, compared to C3TC and C4TC with a higher b, C5TC,
Fig. 1. The dependence of helical pitch on the terminal alkyl chain length at 30 8C.

H.Q. Zhang et al. / Chinese Chemical Letters 21 (2010) 139–142
141
Fig. 2. The temperature dependence of the molecular twisting power b.
Fig. 3. The POM photos of the SLC-1717 and N*-LC induced by C8TC: (a) SLC-1717, 25 8C; (b) N*-LC (C8TC/SLC-1717), 20 8C; (c) N*-LC
(C8TC/SLC-1717), 86 8C.
C6TC and C8TC exhibited a lower b values due to much longer alkyl diminishing the ‘‘biaxiality’’ by increasing the
conformational flexibility of the molecule according to the reported theory [15]. But C7TC is specific leading to our in-
depth investigation in the future. Here the signs ‘+’ and ‘À’ indicate right-handed and left-handed helices, respectively.
Compared to the former research done [12], this kind of chiral dopants with more higher molecular twisting power
were obtained by increasing the amounts of chiral centre. Therefore, introducing more chiral centres into molecular
structure could be an effective way to improve the molecular twisting b.
The polarized optical microscopy (POM) photos of the N*-LC mixture and host nematic LC (SLC-1717) are
showed in Fig. 3. At room temperature 25 8C, the typical schlieren texture to the nematic phase for the SLC-1717.
Fingerprint texture to a chiral nematic phase (N*) at 20 8C and texture of a schlieren defect with twofold brushes to the
nematic phase at 86 8C for the N*-LC mixture (C8TC/SLC-1717) could suggest that the helical pitch increases
gradually with the increasing temperature.
To sum up, a novel series of chiral dopants with double (S)-1,2-propanediol units were synthesized and the twisting
power of chiral dopants was investigated. The results showed that an odd–even effect was observed in the helical
pitches with different terminal alkyl chain length, the even ones suggested a larger pitch due to a weaker chirality
transfer to nematic LC molecules. Moreover, the chiral dopants exhibited a temperature dependence of the helical
twisting behavior, typically increasing molecular twisting power with increasing temperature. And for the compounds
with longer alkyl length (C5TC, C6TC, C8TC) showed a lower b values due to the diminishing ‘‘biaxiality’’. At last,
this kind of chiral dopants with two chiral centres exhibited more higher b than the one with single chiral centre.
Acknowledgments
Financial support from the National Science Foundation (No. 20674005), the Flat-Panel Display Special Project of
China 863 Plan (No. 2008AA03A318) and Projects of Chinese National Science and Technology Tackling Key
Problems (No. 2007BAE31B02).

142
H.Q. Zhang et al. / Chinese Chemical Letters 21 (2010) 139–142
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version, at doi:10.1016/
j.cclet.2009.09.007.
References
[1] M.R. Huang, X.G. Li, G. Lin, Sep. Sci. Technol. 30 (1995) 449.
[2] X.G. Li, M.R. Huang, L. Hu, et al. Eur. Polym. J. 35 (1999) 157.
[3] M.R. Huang, X.G. Li, Gas Sep. Purif. 9 (1995) 87.
[4] S.S. Choi, S.M. Morris, W.T.S. Huck, Soft Matter 5 (2009) 354.
[5] R.A. Van Delden, B.L. Feringa, Angew. Chem. Int. Ed. 40 (2001) 3198.
[6] G.W. Gray, M. Hird, D. Lacey, J. Chem. Soc. Perkin Trans. 2 (1989) 2041.
[7] R.A. Van Delden, B.L. Feringa, Angew. Chem. 40 (2001) 3198.
[8] H.G. Kuball, H. Bruning, Chirality 9 (1997) 407.
[9] N.I. Shkolnikova, L.A. Kutulya, N.S. Pivnenko, Liq. Cryst. 50 (2005) 108.
[10] M. Mitov, N. Dessaud, Liq. Cryst. 34 (2007) 183.
[11] X.T. Yuan, H. Cao, H. Yang, Chin. J. Liq. Cryst. Displays 23 (2008) 404.
[12] W. Huang, H.Q. Zhang, H. Yang, Liq. Cryst. 36 (2009) 531.
[13] R. Cano, Bull. Soc. Fr. Miner. 91 (1968) 20.
[14] G. Gottarelli, P. Mariani, G.P. Spada, Tetrahedron 39 (1983) 1337.
[15] D.J. Earl, M.R. Wilson, J. Chem. Phys. 119 (2003) 10280.