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LIQIUD CRYSTALLINE MATERIALS WITH HIGH BIREFRINGENCE
? 2005 ?.
*; J. Dziaduszek*; S. Urban**; E. Scibior*; S. Gauza***; S. T. Wu*** A. Spadlo*; R. Dabrowski , *** Military *** E-mail.: *** Institute *** College
University of Technology, Institute of Chemistry, Warsaw, Poland email@example.com ′w, Poland of Physics, Jagiellonian University, Krako of Optics/CREOL, University of Central Florida, Orlando, USA
The mesogenic, optical and dielectric properties of the new mixtures of nematic liquid crystals with high birefringence on the base of isothiocyanatotolanes have been investigated and examples of multicomponent mixtures with low viscosity and optical anisotropy ?n ≈ 0.4 are presented.
Introduction High birefringence nematic liquid crystals are used to build quickly operating displays as well as being used for steering laser beams, colour filters, broad band reflectors, polymer dispersed liquid crystals and in holography . The birefringence, dielectric anisotropy, viscosity and phase transition temperatures are the most important properties of liquid crystalline materials (LCs) deciding on their quality and practical applications. The usefulness of LCs well describe parameter called Figure-of-Merit – FoM = K11?n2/γ (where: K11 – the splay elastic constant, ?n – birefringence, γ – rotational viscosity) [2, 3]. Isothiocyanatotolanes, among liquid crystal structure, should exhibit high birefringence, moderate viscosity and good thermal, chemical and photochemical stability [4, 5]. Recently we have been investigating some high birefringence LC isothiocyanatotolane derivatives with the following common formula (1):
We described the synthetic route and mesomorphic properties of unsubstituted compounds of formula (1) (X1 = X2 = X3 = H) . This type of structure shows only the highly ordered phases SmK and SmE in a broad temperature range. The substitution of the benzene rings by fluorine atom or chlorine atom or methyl group is necessary to destroy smectic phases in isothiocyanatotolanes. Lateral substitution totally cancels the smectic phase for the two ring compounds and greatly enhances the nematic character of the three ring compounds . The new type of nematic liquid crystals with high birefringence, recently developed in our laboratory, will be described with special attention given to fluoro and chloro substituted isothiocyanatotolanes, with following formula:
(1) where Z is an alkyl (CnH2n +1), alkoxy (CnH2n + 1O), alkylphenyl group ( C n H 2n + 1 Ph ) or alkyl cyclohexyl (CnH 2n + 1C6H10– ), A is a single bond or –CH2CH2– group and one of the lateral substituents X1, X2 or X3 is F, Cl or CH3 group.
Mesomorphic properties of novel isothiocyanatotolanes The phase transition temperatures and heat fusion of enthalpies of prepared compounds were measured using a differential scanning calorimeter (SETARAM 141) at 2 K/min temperature scanning rate. The
“??òè÷??êèé ?ó?íà?”, ò?ì 72, ? 9, 2005
Z C 2H 5 C 3H 7 C 4H 9 C5H11 C7H15 C 2H 5O C 4H 9O C5H11 C 4H 9O C5H11O
Mesophase, °C Cr 71.3 Iso Cr 73.9 Iso Cr1 38.4 Cr2 40.50 (N 17.6) Iso Cr 49 (N 40.6) Iso Cr 41.7 (N 44.4) Iso
?H, kJ/mol 6.30 5.82 2.91 4.10 6.67 7.39 8.53 5.35 6.6 6.0
Table 1. Phase transition temperatures and enthalpies for substituted isothiocyanatotolanes (see formula (1)*).
1,88 1 1,82 2
F Cl Cl
Cr 103.2 (N 93.6) Iso Cr 96.7 (N 89.9) Iso Cr 55.4 Iso Cr 81.6 (N 31)** Iso Cr 75.2 Iso
12 TN–1 – T,°C
Fig. 1. Temperature dependent extraordinary refractive index of the alkyl and alkoxy NCS tolanes studied. 1 – 4OTOLCl0, 2 – 5TOLCl0, 3 – 5TOLF0, 4 – 4OTOLF0. 1,62
X1 = X2 = H; A is single bond. Virtual transition estimated from clearing point of solutions ** in 6CHBT.
2 1 3 4
1,59 1,56 1,53
investigated compounds, and their phase transition and enthalpies are given below in Table 1. The 4′-alkyl-4-isothiocyanatotolanes with short alkyl chain does not exhibit mesomorphic properties. The monotropic nematic phase is observed for longer alkyl chains. The butyl and heptyl members have low melting point and melting enthalpy (see Table 1), so may be used as base components of mixtures. The fluorosubstituted 4′-alkoxy-4-isothiocyanatotolanes listed in Table 1 are monotropic nematics, but their melting point and enthalpy are rather high. Chlorosubstituted two ring isothiocyanatotolanes are listed also in Table 1. The isothiocyanatotolanes with alkyl chain have lower melting point than alkoxy analogues. A virtual nematic phase maybe estimated in the case of the butoxy derivative. Stability of nematic phase was estimated from point of solution in 6 CHBT. The birefringence of novel isothiocyanatotolanes The optical anisotropy and refractive indices of the alkyl and alkoxy fluoro- and chloro-substituted tolanes were measured by the refractometric method . Optical indices and birefringence of the presented compounds were calculated from the measurements of the 20 wt. % solutions in 6CHBT. Results are plotted in Figs. 1–3. Experimental points are joined for more convinient seeing. Table 2 shows the extrapolated birefringence and optical indices of some isothiocyanatotolanes (at temperatures 20 °C below the nematic-isotropic transition). Alkyl derivative have both higher refractive indices and birefringence than the alkoxy derivatives analogous. Chloro-derivatives show much higher value of the or“??òè÷??êèé ?ó?íà?”, ò?ì 72, ? 9, 2005
12 6 0 TN–1 – T,°C Fig. 2. Temperature dependent ordinary refractive index of the alkyl and alkoxy NCS tolanes studied. 1 – 4OTOLCl0, 2 – 5TOLCl0, 3 – 5TOLF0, 4 – 4OTOLF0. 1,62
1,59 1,56 1,53
12 TN–1 – T,°C
Fig. 3. Temperature dependent birefringence of the alkyl and alkoxy NCS tolanes studied. 1 – 4OTOLCl0, 2 – 5TOLCl0, 3 – 5TOLF0, 4 – 4OTOLF0.
dinary index, than fluoro analogues. Therefore their birefringence is smaller than fluoroderivatives in some cases (5TOLCL0) (see Table 2). Multicomponent mixtures In the real case for application the only mixtures can be use. We developed three multicomponent eutectic mixtures with high birefringence and low viscosity. Compounds listed in Table 2 were chosen as a base for the compositions. The mesomorphic and electrooptical properties are listed in the Table 3. To measure birefringence and viscoelastic coefficient the phase retar11
Table 2. The extrapolated values of birefringence and refractive indices at temperature 20 °C below the nematic-isotropic transition
Compound no ne ?n
dation and relaxation time was used . The measurements were conducted at the temperature of 23 °C and wavelength of 633 nm. The liquid crystal mixture was filled into 8 ?m planar cell with the ITO orienting layers rubbed in opposite directions. The dielectric anisotropy was measured by using APT III tester (Displaytech Inc.). Mixtures 1658 and 1659 exhibit excellent mesomorphic properties. The nematic phase exist in wide temperature range. Especially very low value of the melting point temperature let as consider those mixtures for commercial applications. Mixture 1610 has higher melting point temperature and cannot be used at the temperatures much below room temperature. All of mixtures listed below show very high birefringence in comparison to the commercially available materials (BL038 – ?n = 0.28 at λ = 589 nm and T = 20 °C (from Merck)). The viscoelastic coefficient measured for our mixtures has a low value. It makes them more attractive as a switching speed will be faster than any other high birefringence nematic based mixtures. The values are 3X lower than for cyano based compositions. The birefringence and viscosity of the LC mixtures affect
Table 3. Physical properties of multicomponent mixtures
Property Nematic range, °C Birefringence k33/k11 ε⊥ ε|| ? k ?n 2 ? FoM ? 11 ? ? γ ? γ1/k11 1610 10–80 0.36 — 5.1 9.4 10.8 11.8 1658 –20…110 0.37 1.82 — — 10.1 14.1 1659 –52…94 0.38 2.11 — — 11.9 12.1
the switching speed of the LC based devices. Higher birefringence let us reduce the cell gap than the switching time is greatly reduced. The lower viscosity is always preferable. The Figure-of-Merit takes into one account both, the birefringence and the viscoelastic coefficient in order to show the electrooptical performance of LC mixtures. Our mixtures exhibit the FoM values at the level of 10–12 at room temperature. Based on the reference values for BL038 of 2.7, this makes this type of mixture very attractive. The dielectric and elastic properties are typical for this class of the LC compounds. The further work to improve the birefringence value and nematic range will be performed. Conclusion The new type of the nematic liquid crystals compounds is described. The results from many different methods proofs that both of the homologues series are very attractive for mixture formulation. 4′-alkyl-3-fluoro-4-isothiocyanatotolanes are more convenient as mixture components than 4′-alkoxy analogue because they have higher birefringence and also lower viscosity due to a lack of oxygen atom in the flexible terminal chain. Finaly figure of merit is lower. The temperature range of the nematic phase is very wide in case of our new mixtures to enable variety of applications. The results listed at this paper shows that alkylisothiocyanatotolanes are very attractive as they posses high birefringence and low viscosity at the same time. Acknowledgements The work was carried under PBS 701 and project UCF Purchase Order ? 33893.
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