LC mixture with promoted electro-optical properties for LCD applications by using a highly UV-resistant dopant

A highly ultraviolet (UV)-resistant dopant 3HBBTfV consisting of an α-trifluomethyl vinylbenzene (TfV) moiety was synthesized and mixed with a commercial LC host (A) to promote its dielectric anisotropy in LC mixture C (A:3HBBTfV = 9:1 w/w). Their electric-optical properties were studied and compared with those of an analogous commercial LC mixture B composed of LC mixture A (as a commercial host) mixed with the same weight ratio (10% wt.) of a commercial component 3HBEBTf containing a 3,4,5-trifluorobenzene (BTf) moiety. The electro-optical properties of voltage holding ratio (VHR) values and voltage-transmission (V-T) curves of LC mixture C were sustained after the UV irradiation. These results showed that dopant 3HBBTfV (acting as an UV stabilizer) demonstrated a higher stability under UV exposure in contrast to commercial component 3HBEBTf. Therefore, LC mixture C (containing UV-resistant dopant 3HBBTfV) exhibited good UV resistance, high dielectric anisotropy, and excellent electro-optical properties. ©2015 Optical Society of America OCIS codes: (230.3720) Liquid-crystal devices; (160.3710) Liquid crystals; (120.5050) Phase measurement; (120.6780) Temperature. References and links 1. Y. H. Jeonga, Y. J. Lima, E. Jeonga, W. G. Jangb, and S. H. Leeb, “Optimal pixel design for low driving, single gamma curve and single cell‐gap transflective fringe‐field switching liquid crystal display,” Liq. Cryst. 35(2), 187–194 (2011). 2. H. Nam and S. W. Lee, “Low-power liquid crystal display television panel with reduced motion blur,” IEEE Trans. Consum. Electron. 56(2), 307–311 (2010). 3. P. Liu, F. Zafar, and A. Badano, “The effect of ambient illumination on handheld display image quality,” J. Digit. Imaging 27(1), 12–18 (2014). 4. S. Yamada, Y. Hirata, R. Ishii, and T. 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Introduction
Recently, liquid crystal displays (LCDs) are very important and indispensable to modern life owing to their various applications, such as notebooks, monitors, mobile phones, televisions, and portable devices. Due to their small sizes, light weights, portabilities, high image qualities, and low-power consumptions, LCDs have become main trends in all display areas [1][2][3][4][5]. The major ones should be twisted nematic [6], in-plane-switching [7], multi-domain vertical alignment [8], and fringing field switching modes [9]. Currently, nematic liquid crystal (LC) mixtures have been widely utilized in the field of LCDs, and blue phase LCs have potentials to be further explored as new materials for future applications [10][11][12]. To achieve the reliable request of liquid crystals (LCs) for display applications, the primary concern of stability for LC materials is as important as their performance. In order to satisfy the requirements of long operational life time and high image quality for the commercial specifications of LCDs, LC materials possessing ultraviolet (UV) resistance also play an important role to reach the long term reliability of the displays [13][14][15]. In most practical cases, UV irradiation causes gradual degradations of LC molecules. Once LC mixtures are deteriorated by the exposure of UV light, the electro-optical (EO) performance of the LC materials might also be declined. Therefore, the promising LC materials with excellent UV-resistant properties may offer enduring stabilities for display devices. The reliabilities of LCDs can be evaluated by two of the major display performance parameters, including voltage holding ratio (VHR) and residual direct current (RDC) [16][17][18][19]. Typically, the durable stabilities of LC mixtures can be estimated by the variations of the VHR and RDC parameters before and after UV treatments [20].
On the other hand, the key requirements of LCDs with high performance must have not only good chemical and thermal stabilities of LC materials, but also involve low viscosities lead to short addressing time and low threshold voltages in displays. Towards fast responsive LCDs, the rotational viscosities of LC materials were known as one of the most important physical properties. For such planar cells, the switching time is proportional to the rotational viscosity coefficient (γ 1 ) of the LC materials, where a lower rotational viscosity could effectively decrease both rising time (τ r ) and falling time (τ f ), which are given by the following Eqs. (1) and (2) where d is the thickness of the LC film, ε a is the anisotropy of dielectric susceptibility, V is the applied voltage, and K 11 is the elastic constant of splay. Many previous papers have reported that the molecular structures of LC materials could be adjusted to have low viscosities and high dielectric anisotropies, which were sufficient to be used as the fast responsive LC mixtures [25][26][27][28][29][30].
A poor UV-resistant performance of a commercial LC mixture B consisting of a LC mixture A (as a commercial host) mixed with 10% wt. of a commercial component 3HBEBTf (see Fig. 1) has been reported previously [31,32], which needs to be improved due to the requirement of photo-alignment under UV process. Due to the un-stable ester linkage [23] of the commercial component 3HBEBTf under UV exposure, LC mixture A (removal of component 3HBEBTf) as a commercial host was used in this study. In addition, the strong absorption bands ca. 254 nm of the two longest conjugated analogous terphenyl compounds in this commercial LC mixture A (as a commercial host listed in Table 1) have been reported previously [33], which also un-stabilize LC host A under UV exposure. Herein, as demonstrated in Fig. 1 we developed a highly UV-resistant dopant material 3HBBTfV consisting of an α-trifluomethyl vinylbenzene (TfV) moiety to be mixed with the commercial host LC mixture A (as a commercial host) to form a LC mixture C (A:3HBBTfV = 9:1 w/w). As illustrated in Fig. 2, the UV resistant dopant 3HBBTfV was designed and synthesized by a four-step synthesis to produce a phenylcyclohexane derivative containing an α-trifluomethyl vinylbenzene moiety (acting as a molecular dipole moment enhancing component to induce a higher dielectric anisotropy). The mesogenic core (phenylcyclohexane unit) of dopant 3HBBTfV has a chemical structure similar to those major components in LC host A, so it (acting as an UV stabilizer) could be miscible with the LC host at such a high ratio (10% wt.). In contrast to the commercial LC mixture B containing 10% wt. of the commercial component 3HBEBTf, the electro-optical properties of LC mixture A (a commercial host) and UV-resistant LC mixture C (containing 10% wt. of our newly designed dopant 3HBBTfV) are compared in this study. LC mixture C containing dopant 3HBBTfV revealed good UV resistance in voltage holding ratio (VHR) values and voltage-transmission (V-T) curves, so its excellent electro-optical properties can improve the performance of the LC mixture in the LCD application.

Materials
Chemicals and solvents were reagent grades and purchased from Aldrich, Acros, and TCI Co. Tetrahydrofuran (THF) and dichloromethane (DCM) were distilled to keep anhydrous before use. The other chemicals were used without further purification. Commercial component 3HBEBTf (Tm = 98.0°C) was purchased from Suzhou chemical Co. and as supplied.

Synthesis
Synthesis of I-A. n-BuLi (14.8 mL, 2.5 M in hexane, 35.6 mmol) was added dropwise into a solution of commercially available starting material 3HHBBr (5.0 g, 17.8 mmol) in anhydrous THF, and the mixture was reacted continuously at −78°C for 1.5 h under nitrogen atmosphere. Then, trimethyl borate (5 mL, 44.5 mmol) was added dropwise to the solution, and the reaction mixture was warmed to room temperature and stirred overnight. The resulting mixture was quenched with 6N HCl and extracted with dichloromethane. The combined organic layers were washed with water and dried over Na 2 SO 4 and evaporated. The

Synthesis of II-A.
A solution of I-A (3.9 g, 15.8 mmol), 1-bromo-4-iodobenzene (4.07 g, 14.4 mmol), Pd(PPh 3 ) 4 (832 mg, 0.72 mmol), and Na 2 CO 3 (12.2 g, 11.5 mmol) in a mixture of 1,2-dimethoxyethane (100 mL) and water (66.7 mL) was stirred at 90°C for 12 h under nitrogen atmosphere. After that, the mixture was cooled to room temperature and extracted with DCM. The combined organic layers were washed with water and dried over Na 2 SO 4 and evaporated. The residual product was purified by flash column chromatography (silica gel, hexane/DCM = 5:1, v/v) to give II-A in a yield of 59% as a white powder. 1

Preparation of LC mixtures A, B, and C
LC mixture A was prepared from eight commercial molecular components, and their chemical structures and composition are shown in Table 1. LC mixture A was mixed with 10 wt% of 3HBEBTf and 3HBBTfV to obtain LC mixtures B and C, respectively. 1 H NMR spectra were recorded on a Bruker DRX-400 (400 MHz) spectrometer using CDCl 3 as solvent. The EI-MS experiment was performed on a Bruker APEX II. All samples of homogeneous LC mixtures were filled into glass cells by capillarity. The LC cells were assembled in standard 90° twisted-nematic liquid crystal (TN-LC) configuration cells (crossed rubbing) and the thickness of liquid crystal layer was 4 μm. The UV aging experiments of LC materials were performed by means of an Atlas UV 2000 test machine (0.55 W/m 2 , 365 nm) or a high power Rayonet UV reactor system (wavelength = 254 nm with an illumination energy > 50W/m 2 ). The UV exposure condition of TN-LC cells was followed by irradiating with UV light (0.55 W/m 2 , 365 nm) at 50°C under 1 atm, in which the glass cells were cooled to room temperature and the VHR values of LCs were obtained. The values of VHR and response time along with voltage-transmission (V-T) characteristic curves of TN-LC cells were carried out by VHRM-105 and DMS-803 measurement systems (Autronic-Melchers GmbH). The clearing temperatures, melting points, and optical textures were studied using a polarizing optical microscope (POM, model: Olympus BX-51) equipped with a hot stage.

Results and discussion
The four-step synthetic route of UV-resistant dopant 3HBBTfV is shown in Fig. 2. The chemical structure and purity of dopant 3HBBTfV were confirmed by 1 H NMR and MS spectroscopies. Detailed synthetic procedures and characterization data are given in the experiment section. Both dopants 3HBEBTf and 3HBBTfV do not exhibit any mesophasic behavior and only show melting points at 98.0°C and 151.8°C, respectively. As demonstrated in Table 1, LC mixture A possessing the nematic phase at room temperature was selected as the LC host, which is composed of eight commercial molecular components. As given in Table 2 As demonstrated in Fig. 3, LC mixture C only displayed the nematic phase with the schlieren texture at room temperature in the POM study during cooling from the clearing temperature to room temperature. However, LC mixture C did not reveal any phase separation between dopant 3HBBTfV and LC mixture A (host), which sufficiently indicated a good miscibility of the dopant material (3HBBTfV) in the host for LC mixture C. On the other hand, similar mesomorphic behavior was also observed in LC mixture B. The optical anisotropy Δn values of these LC mixtures are in the range of 0.1251−0.1275 (see Table 2).   As shown in Fig. 4, the V-T cures of LC mixtures A, B, and C reveal gradual decreases in driving voltages sequentially in TN mode cells. The threshold-voltage (V th ) values of LC mixtures A, B and C are 1.25 V, 1.22 V, and 1.20 V, respectively, at 90% transmittance, as well as 2.25 V, 2.20 V, and 2.10V, respectively, at 10% transmittance. In comparison with the V-T curve of LC mixture A, both LC mixtures B and C have slightly reduced V th values, which indicates that doping LC mixtures with 3HBEBTf and 3HBBTfV, respectively, are useful to improve the V-T curves with reduced V th values. Moreover, due to the highest dielectric anisotropy of LC mixture C among all LC mixtures (Table 2), the lowest V th value was induced by doping LC host with 3HBBTfV (possessing a TfV moiety) in contrast to 3HBEBTf (possessing a BTf moiety). Since no obvious changes of V-T curves by UV irradiations at wavelength = 365 nm in all LC mixtures, the UV-resistant effects on V-T curves were accelerated under 2.5 h of UV exposure at wavelength = 254 nm with an illumination energy > 50W/m 2 to provide important implications regarding long-term UV stabilities of LC mixtures. The V-T curves of LC mixtures A, B, and C before and after UV exposure at wavelength = 254 nm (with a higher energy) are shown in Figs. 5(a), 5(b), and 5(c), respectively. As demonstrated in Fig.  5(b), a large discrepancy was observed in the V-T curves of LC mixture B before and after UV exposure. Comparably, after UV exposures Figs. 5(a) and 5(c) exhibited closer (almost overlapped) V-T curves of both LC mixtures A and C, which indicate more durable UV exposure properties were obtained while LC host A exited alone or mixed with UV-resistant 3HBBTfV. According to the result of V th  π(K 11 /ɛ 0 Δɛ) 1/2 [23], since the potential breakages of 3HBEBTf and 3HBBTfV under UV exposure are the ester linage of fluorobenzene (BTf) and the vinyl group of α-trifluomethyl vinylbenzene (TfV), respectively, the elastic constant reduction (K 11 ) in 3HBEBTf is larger than 3HBBTfV (but with little difference in both anisotropic dielectric constants Δɛ). As illustrated in Fig. 5(b), this result caused the largest reduction shift of V th in LC mixture B containing 3HBEBTf after UV exposure [34]. However, the breakage of the vinyl group in UV-resistant 3HBBTfV maintained its elastic constant value (K 11 ) after UV exposure but reduced its anisotropic dielectric constant Δɛ. Thus, as displayed in Fig. 5(c) a small increasing shift of V th in LC mixture C containing 3HBBTfV was observed after UV exposure. Moreover, the response time (rise time τ r , fall time τ f , and total time τ total ) values of all LC mixtures were measured and compared. As illustrated in Table 3, the values of response time were increased from 6.9 to 7.2 ms to 7.1-7.6 ms, respectively, by UV exposure for all LC mixtures. However, the response time of LC mixture C had the smallest increment after UV exposure, which again suggested the contribution of the UV-resistant dopant 3HBBTfV to LC mixture C. In addition, the voltage holding ratio (VHR) values of all three LC mixtures were evaluated after UV exposure, where the VHR value was defined as the ratio of the relaxed voltage in the non-selected period (V) to the initially selected voltage (V o ) at a specified frequency f (where V o = 3.5 V and f = 60 Hz in this study), i.e., VHR = V/V o [35]. As shown in Fig. 6(a), LC mixture C (mixed with 3HBBTfV) showed a higher VHR value than LC mixtures A and B after UV exposure (365 nm, 0.55 W/m 2 , 12 h). The VHR value of LC mixture C was barely decayed from > 99.5% to 98.9%, but it dropped to 68.7% for LC mixture B after 12 h of UV exposure. To accelerate this experiment, the UV exposure was proceeded again in a Rayonet UV reactor which provided 2.5 h of UV light (wavelength = 254 nm) with an illumination energy > 50W/m 2 . As shown in Fig. 5(b), the result revealed that the VHR value of LC mixture C was hardly decayed from > 99.5% to 91.5% after 2.5 h of UV exposure, but it rapidly dropped to 11.5% for LC mixture B after 0.5 h of UV exposure. According to the previous report [36], the un-stabilities of the ester linkage and the fluorine substituents on the phenyl ring in the commercial component 3HBEBTf containing a 3,4,5-trifluorobenzene (BTf) moiety can reduce their UV resistance [23]. Hence, the largest decays in VHR values of LC mixture B in Figs. 6(a) and 6(b) were due to the dissociation of LC mixtures B containing 3HBEBTf after UV irradiations, which might generate new ionic species to reduce the relaxed voltages in the non-selected period (V) and thus to have smaller VHR values [37]. Therefore, LC mixture C (i.e., LC mixture A mixed with 10% wt. of 3HBBTfV) was confirmed to possess the best UV light resistance among these LC mixtures.

Conclusion
In this study, a novel UV-resistant dopant 3HBBTfV has been successfully designed and synthesized, which was well mixed with LC mixture A (as a host) to develop LC mixture C and exhibited a high stability under UV exposure. By adding the UV-resistant dopant 3HBBTfV, LC mixture C possessed the highest dielectric anisotropy Δε and the lowest viscosity η, and thus to reveal the best EO performance among all LC mixtures A, B, and C. Compared with analogous LC mixture B containing a commercial component 3HBEBTf, this investigation showed that more stable VHR values of LC mixture C were still sustained after the UV irradiation. Therefore, dopant 3HBBTfV (acting as an UV stabilizer) provides a reliable UV stability and better EO performance in contrast to the commercial component 3HBEBTf. These EO performance results of LC mixture C containing the newly developed UV-resistant dopant 3HBBTfV may have significant contributions to the LC researches and applications, especially for the LC mixtures required for the UV process (such as the photo-alignment under UV).