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Three kinds of bent-shaped molecules (A-1, A-2, A-3) were synthesized and doped into the chiral nematic host to induce blue phase. The widest induced blue phase temperature range of 48.6°C is gained for the molecules with acrylate end groups and lateral substituent CN, which has the largest biaxiality and better solubility. After ultraviolet (UV) irradiation, the blue phase range was reduced to 39.1°C, the relationship between the molecular structures and the inducing effect was discussed.
© 2016 Optical Society of America
1. Introduction
The liquid crystal (LC) blue phase (BP), which normally appears between the isotropic and chiral nematic state, showing a colorful platelet texture or a foggy texture under polarized optical microscopy (POM) with two crossed polarizers, is a kind of special liquid crystal phase with double twisted cylinder(DTC) and self-assembled three-dimensional structures, classified as body-centered cubic blue phase(BPI), simple cubic blue phase(BPII), and isotropic symmetry phase(BPIII) according to the nanostructures of DTCs [1–6]. Such arrangement makes BPs possessing many excellent physical properties, such as optical isotropy, fast response, and selective reflection of circular polarized light. Because of these attractive properties, the blue phase shows a great applicable prospect in wide viewing angle, high optical efficiency, fast response and low energy consumption liquid crystal displays [7]. However, its instinct defect, namely, the narrow temperature range (usually a few Kevin), limits their practical application in the electro-optic devices [1–7].
Actually some progresses have been obtained in attempting to widen the blue phase range in recent decades [2–25]. The BP range is not just a few K, may reaches to 60°C or more. By the well-known good stabilization of polymer network, Kitzerow et al. [16] stabilized the blue phase to widen the BP temperature range by using the liquid crystal molecules which can be polymerized by UV light at the blue phase state, so that the BP 3D structures were fixed; however, this method led to a serious problem, damaging the electrically tunable property of the liquid crystal. Another method differing from that of Kitzerow et al. [16] is the polymer stabilized blue phase (PSBP) [5,6], firstly proposed by Kikuchi et al. [17]. In the blue phase cubic defects photo-sensitive monomers were filled, after being polymerized at the blue phase state, the blue phase was stabilized and the phase temperature range was widen, almost 60°C. But this method has also some drawbacks, such as high driving voltage and hysteresis, which need to be overcome by new methods.
In 2003, the exotic phenomenon, reported by Nakata et al. [18], which was that the blue phase could be induced by doping a small amount of bent-shaped molecules into the chiral nematic liquid crystals(N*LCs), caught much attention of researchers. Although the BP temperature range was only 2-5°C and the mechanism was not clear, it provided new horizon to study the blue phases and widen the blue phase temperature ranges. Based on this work, many different bent-shaped molecules were synthesized by different researching groups to induce the blue phase in order to gain wide temperature range and good stabilization of the blue phase [7,9–11,19–25]. Wang L. et al. [22] reported that 25.9°C temperature range of BPI in the N*LC doped with thiophene-based mesogens was obtained. Wider temperature range BP by doping bent-shaped 1,3,4-oxadiazole derivatives into N*LCs host was obtained by Zheng Z. G. et al. and the physical mechanism of the BP induced by bent-shaped molecules was also clarified [23], the effect of the terminal chain length on the bent-shaped 1,3,4-oxadiazole derivative induced BP was revealed [24]. After that Zhu G. et al. [25] reported that the temperature range of the blue phase induced by bent-shaped monomer with allylic end groups which was polymerized under ultraviolet (UV) irradiation could be further widened to about 4 K than that before UV irradiation. The mechanism of that was also demonstrated.
In this paper, we report a wide BPLC range of 48.6°C induced by doping a kind of bent-shaped 4-cyanoresorcinol molecule with acrylate end groups into a chiral nematic liquid crystal. After UV irradiation, the BP range is still remained 39.1°C. Compared to the bent-shaped molecules with alkyl end groups, the molecules with acrylate end groups have more power to stabilize BP, the relationship between the molecular structure and the inducing effect will be discussed.
2. Experiments
Two kinds of new bent-shaped molecules (chemical structures shown in Fig. 1): 4-cyano-1,3-phenylenebis(4-((4-(acryloyloxy)benzoyl)oxy)benzoate) (A-1), 1,3-phenylene bis(4-((4-(acryloyloxy)benzoyl)oxy)benzoate) (A-2) were synthesized, and one reported bent-shaped molecule: 4-cyano-1,3-phenylene bis(4-((4-heptylbenzoyl)oxy)benzoate (A-3) [26] was synthesized as a contrastive compound used in the experiments. The synthetic route of compound A-1 was shown in Fig. 2. The synthesis of other two compounds A-2, A-3 is similar to the preparation of A-1. The compounds were confirmed by Hydrogen Nuclear Magnetic Resonance (1HNMR, Bruker AVANCE 400MHz) and High-Resolution Mass Spectrometry (HRMS, Micromass GCTTM). The phase transition temperatures were measured by DSC (PerkinElmer Diamond DSC). A-1: melting point: 139.05 °C. 1H NMR (400 MHz, CDCl3) δ/ppm 8.35-8.37 (m, 2H),8.27-8.30(m, 6H), 7.81-7.83 (d, J = 8.56 Hz, 1H), 7.55 (s, 1H), 7.41-7.44 (m, 4H),7.33-7.35(m, 5H),6.65-6.70 (d, J = 17.36 Hz, 2H),6.33-6.40 (dd, J1 = 17.36, J2 = 10.4 Hz, 2H), 6.08-6.11 (d, J = 10.4 Hz, 2H).
A-2: melting point: 158.35 °C. 1HNMR(400MHz, CDCl3) δ/ppm 8.27-8.31(m, 8H), 7.49-7.53 (t, J = 8.08Hz, 1H),7.38-7.40(d, 4H), 7.32-7.34(d, 4H), 7.19-7.21(m, 3H), 6.65-6.69 (d, J = 17.32 Hz, 2H), 6.32-6.39(dd, J1 = 17.32 Hz, J2 = 10.48 Hz, 2H), 6.08-6.11(d, J = 10.48Hz, 2H).A-3: The transition temperatures: Cr 98 N 116 Iso. 1HNMR(400MHz, CDCl3) δ/ppm 8.34 (d, J = 8.72 Hz, 2H), 8.28 (d, J = 8.72 Hz, 2H), 8.18-8.07 (m, 4H), 7.81 (d, J = 8.6 Hz, 1H), 7.54 (d, J = 2.12Hz, 1H), 7.40-7.42 (m, 4H), 7.30-7.35 (m, 5H), 2.71 (t, J = 7.70 Hz, 4H), 1.65-1.68(m, 4H), 1.29-1.34 (m, 16H), 0.89 (t, J = 6.86 Hz, 6H).The samples used in the experiments were prepared by doping bent-shaped compound into the N*LC host [25], consisting of 67.2% eutectic nematic liquid crystals SLC9023 (from Slichem Co. Ltd) and 32.8% chiral dopant R811(from Merck Co. Ltd). The N*LC host was firstly tested, injected into 9.5μm-thick cell without alignment layers, observed by POM, no blue phase exists in the host during cooling procedure from the isotropic state at 0.1°C/min and the transition temperature from isotropic phase to the chiral nematic phase (TIso-N*) is 73.3°C.
Several samples with different weight ratio of the bent-shaped molecule doped into the N*LC host were prepared. The mixtures were heated to isotropic phase on a hot stage and filled into 9.5μm-thick cell with no alignment layers by the capillary effect at isotropic state and held for 15 min., then set on the precisely controlled hot stage (Linkam T95-PE) cooling from isotropic state at a rate of 0.1°C/min. The LC phases and the textures were recorded by POM (Nikon Eclipse LV100POL) with crossed polarizers.
Ultraviolet irradiation to the samples were performed under 365nm UV LED (5.0 mW/cm2) for 600s to initiate the bent-shaped molecule to polymerize. IR spectra (Nicolet 6700) of the before-and-after exposure samples were measured to characterize whether the molecules were polymerized or not.
3. Results and discussion
For the compounds with acrylate end groups (A-1, A-2) there was no liquid crystalline phase observed by POM, at the melting points the compounds became clear but a few minutes later, they became white solid, as the acrylate end group is easy to be polymerized, the white solid could be the polymer of the compounds. The bent-shaped molecule with alkyl end groups (A-3) showed the liquid crystalline property with the transition temperatures Cr 98 N 116 Iso, (by cooling: Iso 116 N 62 SmC 48 Sx 39 Cr).
All three bent-shaped compounds can be mixed in the host mixture at the ratio under 9%. Compound A-2 has the worst solubility because of its larger rigid core, there were a few amount crystal found by cooling at the ratio of 9%. The compound A-1 has also larger rigid core but the lateral cyano(CN) makes the melting point decreasing, it can be good mixed with the host at the ratio of 9%. When the doping amount was under 3%, there was no blue phase observed. When the doping amount was increased to 5%, the blue phase was induced, and the temperature ranges were enlarged with the increasing of the doping amount. As the previous research reported that the larger biaxiality the bent-shaped molecule has, the stronger capabilities it has to induce blue phase [23]. The increasing of doping amount is obviously beneficial to inducing BP. The optimized geometries of bent-shaped molecules are given in Fig. 3 and the bent angles (θ) of the molecules are labeled. The biaxiality of molecules is proportional to their bent angle and molecules with longer rigid core possess larger bent angle [23]. For the bent-shaped molecules A, the bent angles of A-1, A-2 and A-3 are 50°, 48° and 42° respectively. Apparently, the molecule A-1 has the largest bent angle. The lateral substituent CN which can strengthen the polarity of the molecule also enhanced the biaxiality [23], so the molecule A-1 has the largest biaxiality. The emergence of the blue phase is due to the double-twisted arrangement of LCs, which is closely related to the twisted arrangement of bent-shaped molecules. Because of the strong interactions between bent-shaped molecules and LCs, as the previous research reported, the doping of the bent-shaped molecules does not change the twist energy of system [23], but strengthen the ability to form double-twisted structure.
Table 1 shows the blue phase temperature ranges induced by different compounds with different amounts, two compounds with acrylate end groups and one compound with alkyl end groups were used to induce blue phase. The results given in the table are the data of the un-polymerized compounds because the samples S1 to S7 were tested at the temperatures far lower than the melting points of compounds A-1 and A-2, as well as without UV exposure. At the same doping ratio of 7% or 9%, the best effect was found by the compound A-1 at the doping ratio of 9%, 48.6 °C, blue phase was induced at rather low temperature, near room temperature.
Table 1. BP temperature ranges induced by bent-shaped molecules before polymerization.
The typical textures and microscopic pictures of sample S4 (as an example) are presented in Fig. 4. Figure 4(a) exhibits a homogeneously dark state of isotropic phase; 4(b) shows the appearance of the blue phase at 74.9°C; with the temperature decreased, the colorful platelet texture by the selective Bragg reflection of different lattices is very evident shown in the image 4(c); when cooling down to 26.3°C, the blue phase vanished and chiral nematic phase began to emerge, as described in image 4(d). From POM, the colors of the platelets changed from colors with short wavelengths to colors with long wavelengths with decreasing temperature, and the selective reflection spectra of the blue phase during the cooling process exhibited a few nanometers red shift, illustrated that BP induced by compound A-1 belonged to BPI [20,27,28], shown in Fig. 5, this typically opposite to BPII [20,27].
Fig. 4 Microscopic textures of sample S4. (a) Iso state at 90.0°C; (b) Iso state transfers to BP at 74.9°C; (c) BP phase at 62.0°C; (d) BP transfers to N* at 26.3°C, the scale bar is 100μm.
For the samples with dopants with acrylate end groups the blue phase temperature ranges were decreased with the increasing of storage time, possibly because of the unstable acrylate end groups, UV irradiation was used to polymerize the compounds at the BP state to fix the double twisted structure. It was found that the BP temperature range of A-2 was widened to 22.7 °C (shown in Table 2, E-3), further widened 3.6°C after UV irradiation. However, for the compound A-1, the BP temperature ranges were decreased after UV irradiation, sample S4 (Table 1), before UV irradiation, the BP temperature range is 48.6°C, after UV irradiation, the transition temperatures are Iso 74.0 BP 34.9 N* (Table 2, E2), obviously, the transition temperature TBP-N* is upped, possibly the space between molecules was smaller after polymerization; the lateral substituent CN disturbed the double twisted arrangement. However the BP temperature range is still remained 39.1°C. Based on this result, the molecules should be modified to being better soluble with the host in future research work.
Table 2. Transition temperatures and BP temperature ranges after polymerization.
IR spectra were performed to test the polymerization of the acrylate groups [25], Fig. 5 is the result of pure compound A-1 before-after UV curing, the characteristic peaks at 1634.1cm−1, 980.0cm−1 and 855.2cm−1 of acrylate end group decreased obviously after UV exposure, verifies the polymerization of the acrylate groups. Through the calculation of area of double bonds absorption peak in the Fig. 6, we can reasonably deduce the reaction extent up to 88.2% (the corrected area of double bonds absorption peak at 1634.1cm−1 before exposure is 0.161 and after exposure is 0.019, the reaction extent of acrylate groups is 88.2% (1-0.019/0.161 = 0.882)).
4. Conclusion
Three kinds of bent-shaped molecules were synthesized and doped into N*LC to induce blue phase. The widest blue phase temperature range of 48.6°C induced by bent-shaped molecule with acrylate end groups and lateral substituent CN was found, after UV irradiation, the BP range was reduced to 39.1°C. For the BP stabilization the lateral substituent CN enhanced the molecular biaxiality but also disturbed the double twisted arrangement after polymerization.
Acknowledgments
This work is sponsored by the Natural Science Foundation of China (Grant No. 61435008, 61575063).
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