Super High Birefringence Isothiocyanato Biphenyl-Bistolane Liquid Crystals

Transcription

1 Japanese Journal of Applied Physics Vol. 43, No. 11A, 004, pp #004 The Japan Society of Applied Physics Super High Birefringence Isothiocyanato Biphenyl-Bistolane Liquid Crystals Sebastian GAUZA, Chien-Hui WEN, Shin-Tson WU, Narayanasamy JANARTHANAN 1 and Chain-Shu HSU 1 College of Optics & Photonics/CREOL, University of Central Florida, Orlando, FL 3816, USA 1 Applied Chemistry Department, National Chiao Tung University, Hsinchu 30050, Taiwan, R.O.C. (Received April 7, 004; accepted August 5, 004; published November 10, 004) We have designed, synthesized, and evaluated the physical properties of some high birefringence (n) isothiocyanato biphenyl-bistolane liquid crystals. These compounds exhibit n 0:7{0:8 at room temperature and wavelength ¼ 633 nm. Laterally substituted short alkyl chains and fluorine atom eliminate smectic phase and lower the melting temperature. The moderate melting temperature and very high clearing temperature make those compounds attractive for eutectic mixture formulation. A eutectic mixture based on those compounds was formulated and its physical properties evaluated. The simulated absorption spectra agree reasonably well with the measured results. [DOI: /JJAP ] KEYWORDS: high birefringence, isothiocyanato bistolanes, liquid crystals, eutectic mixtures, molecular modeling 1. Introduction High birefringence (n) liquid crystals (LCs) support a wide field of different applications such as flat panel display, telecommunication, and tunable-focus lens. For these devices, high n improves response time through cell gap (d) reduction. This advantage is especially important for laser beam steering 1) and variable optical attenuator ) as these devices are normally operated in the near infrared region ( ¼ 1:55 mm). In the long wavelength region, the LC dn should increase in order to achieve the required phase change, which depends on the specific LC alignment. High birefringence helps to reduce the cell gap so that the fast response time can still be maintained. To achieve high birefringence, linearly conjugated molecules are the preferred candidates. Conjugation length can be extended by multiple bonds or unsaturated rings in the rigid core. Three problems associated with highly conjugated LC compounds are high melting temperature, increased viscosity and reduced UV stability. To overcome high melting temperature, we could form eutectic mixtures. The increased viscosity is inherent to all the highly conjugated compounds. However, isothiocyanato () compounds are less viscous than the cyano (CN) ones except that the compounds tend to exhibit smectic phases. 3) To avoid UV instability, these high birefringence compounds are more suitable for infrared applications. Several high birefringence molecular structures, such as diphenyl-diacetylene, 4,5) bistolane, 6,7) naphthalene tolanes, 8) and thiophenyl-diacetylene 9 11) have been studied. The birefringence of these compounds is in the range. The diacetylene compounds do not have adequate UV and thermal stabilities 1) so that their application is limited. In this paper, we present some isothiocyanato biphenylbistolane compounds with extrapolated birefringence 0:7{0:8 at ¼ 633 nm and T ¼ 3 C. We call these compounds as super high birefringence (SHB) materials. To lower melting temperature, we tried different lateral substitutions, such as alkyl and fluoro groups. Based on these compounds and some tolanes, we formulated a eutectic mixture with T mp < 0 C, clearing point T c 180 C, and n 0:6 at room temperature. address: swu@mail.ucf.edu Experimental Several measurement techniques were used to measure the physical properties of the single compounds and mixtures. A Differential Scanning Calorimetry (DSC, TA Instrument Model Q-100) was used to determine the phase transition temperatures. Results were obtained from 3 6 mg samples in the heating and cooling cycles with rate C/min. The UV absorption spectra of these SHB single compounds were measured by using a dual channel Cary 500 UV/VIS/IR spectrophotometer. To avoid detector saturation, the LC samples were dissolved in cyclohexane solutions with 10 4 molar concentration. Standard quartz semimicrocells with 10-mm thickness were used in the sample and reference channels of the spectrophotometer. The electro-optic properties of the LC compounds and mixtures were measured using an 8 mm homogenous cell with ITO (indium tin oxide) electrodes coated in the inner sides of the glass substrates. A thin polyimide layer was overcoated on ITO and buffed in anti-parallel directions to produce a small pretilt angle ( ). A linearly polarized He-Ne laser with ¼ 633 nm and two laser diodes ( ¼ 53 nm and ¼ 1550 nm) were used as the light sources for the electro-optic measurements. The phase retardation () of the homogeneous cells was measured by the LabVIEW system. The LC birefringence (n) at wavelength and temperature T is obtained by measuring the phase retardation () from the following equation: 13) dnðv; T;Þ ðv; T;Þ¼ ð1þ In addition to the mentioned three laser wavelengths, we also measured the wavelength-dependent birefringence using a spectrophotometer. Results are used to fit the single band birefringence dispersion model: 14) n ¼ G ðþ where G ¼ gnzsð fk f? Þ; g is the proportionality constant, N is the molecular packaging density, Z is the effective number of participating electrons ( and ) S is the order parameter, (fk f? ) is the differential oscillator strength, and is the mean electronic transition wavelength. Molecular modeling was done by running Austin Model I (AM1) and Ab-Initio calculations in order to estimate the

2 Jpn. J. Appl. Phys., Vol. 43, No. 11A (004) S. GAUZA et al Table I. Abbreviations, molecular structures, phase transition temperatures and extrapolated birefringence of the high birefringence compounds. The n values are extrapolated from 10% guest-host systems in E63 host. T ¼ 3 C and ¼ 633 nm. Abbreviation Structure n 1 C5-PPTP(3Et)TP- SHB Cr (10.08) N 41.3 (0.38) Iso F C5-PPTP(3Et)TP(3F)- SHB Cr (10.8) N 41.4 (0.) Iso 3 C5-PPTP(3Et)TP(3Et)- SHB Cr I 44.0 (3.80) Cr II 80.3 (8.90) N 5.6 (.09) Iso C H 3 4 C5-PPTP(3Et)TP(Me)- SHB Cr (4.9) N 40.9 (0.9) Iso 5 C5O-NTP(3Et)P- SHB-05 O Cr 10.0 (16.14) N (0.08) Iso S 6 C-PTP(Et)TPSP-CN SHB-06 CN Cr 15.9 N Iso molecular conformations, polarizability, -electron conjugation, and total length of the single molecules. The electronic absorption spectra were calculated by the ZIN- DO/S method (modified Intermediate Neglect of Differential Overlap INDO algorithms). Results agree reasonably well with the experimental data. 3. Results and Discussions 3.1 Single compounds The structures and phase transition temperatures for the new isothiocyanato biphenyl-bistolanes are listed in Table I. Even with lateral substitutions, the melting temperatures are still relatively high. Therefore, we have to formulate eutectic mixtures in order to lower the melting temperature. All the listed compounds exhibit a pure nematic phase within the whole mesomorphic region. Moreover, the smectic phases which normally appear in the -based biphenyl tolanes or tolanes are completely suppressed. Such an unusual behavior is believed due to the laterally substituted ethyl chain(s) and/or the single fluorine atom. The lateral substitutions increase the molecular separation and break the molecular symmetry so that the smectic phase is difficult to form. Although lateral substitutions lower the melting temperature, they also decrease the birefringence and increase the viscosity. The synthesis procedures of these compounds will be published elsewhere later. The right column in Table I lists the measured birefringence of the SHB compounds. Due to high melting temperatures, we used the guest-host method to extrapolate their birefringence at T ¼ 3 C. During experiments, 10 wt% of the guest compound was mixed in the host E63 (Merck) mixture. The measurements were conducted using a He Ne laser at ¼ 633 nm. From Table I, the extrapolated birefringence of compound SHB-06 reaches This is by far the highest birefringence that we have ever found. The lateral substitutions affect the LC birefringence significantly. Compound SHB-03 has the lowest birefringence among the compounds we studied due

3 7636 Jpn. J. Appl. Phys., Vol. 43, No. 11A (004) S. GAUZA et al. to its two laterally substituted ethyl chains. A shorter lateral alkyl chain would lead to a higher birefringence due to the molecular packing density effect as shown in eq. (). The n of the compounds SHB-01 and SHB-04 is 0.73 and 0.67, respectively. From the push-pull effect viewpoint, replacing a lateral alkyl chain with a fluorine atom represents a different molecular system. Alkyl group has a larger molecular size than fluorine atom. As a result, its contribution to lowering melting temperature is more significant. We used molecular simulation program to calculate the - electron conjugation length (Le), total molecular length (Lt), and the ratio between these two (Le/Lt) for the SHB compounds we investigated. The Le/Lt ratio is essential for optimizing the molecular structures with high birefringence and low viscosity. Although a completely conjugated structure (Le=Lt ¼ 1) is effective for achieving high birefringence, such compounds may not have a desirable nematic phase. On the other hand, the compounds with a similar or same core structure but with a more flexible alkyl chain and/or an additional cyclohexane ring are more likely to exhibit a nematic phase. However, their viscosity would be increased because of the increased molecular mass, the moment of inertia, as well as the molecular volume. 13) Under such a circumstance, the Le/Lt ratio is decreased. This type of comparison is based on the assumption that the laterally substituted groups remain the same. Detailed results for each of the presented compound are listed in Tables II and III. Figure 1 plots the simulated orbital clouds, -electron conjugation length, and total molecular length of compound SHB-05 as an example. The computed molecular polarizability can be used to find the total tensor of the Table II. The -electron conjugation length (Le), total molecular length (Lt) and ratio of Le/Lt in comparison with the measured birefringence of the single SHB compounds. Name -electron Total length Lt Ratio conjugation length ( A) Le/Lt Le ( A) n SHB SHB SHB SHB SHB SHB Fig. 1. The calculated molecular properties for SHB-05. (a) Orbital clouds, (b) -electron conjugation length, and (c) total length of the molecule. The measured lengths are between two open circles on the molecular scheme. polarizability. In our molecular system, we assume that the principal molecular axis is along the x-axis and two much shorter molecular axes are in the orthogonal directions. 15) Under such a circumstance, the polarizability tensor with respect to the principal molecular axes has a diagonal form: 0 1 ¼ xx yy zz The polarizability is dependent on the molecular structure and shape. The laterally substituted atoms would pull out the -electrons from the main core of the molecule so that the C A ð3þ Table III. The calculated polarizability tensor of some elongated structures without lateral substitution. Polarizability (a.u.) Structure -electron conjugation length-le ( A) Total length-lt ( A) Ratio Le/Lt XX YY ZZ CN

4 Jpn. J. Appl. Phys., Vol. 43, No. 11A (004) S. GAUZA et al values of yy and zz become larger. The polarizability anisotropy, i.e., the difference between xx and yy or zz becomes smaller. The classical Lorentz Lorenz equation 16) correlates the refractive index of an isotropic media with molecular polarizability in the optical frequencies. Vuks made a bold assumption that the internal field in a liquid crystal is the same in all directions and gave a semi-empirical equation correlating the refractive indices with the molecular polarizabilities for anisotropic media: 17) n e,o 1 hn iþ ¼ 4 3 N e,o: ð4þ In eq. (4), n e and n o are refractive indices of the extraordinary ray and ordinary ray, respectively, N is the number of molecules per unit volume, e,o is the molecular polarizability, e ¼ xx and hn i is defined as: hn i¼ðn e þ n o Þ=3: ð5þ The average values of the polarizability () for our SHB compounds are calculated and results are listed in Table III. For a uniaxial LC, the average polarizability components can be expressed in terms of the order parameter for the long molecular axis S xx ¼ S, and the difference between the order parameters for the short molecular axes (S yy, S zz )asd¼ S yy S zz. Hence, h k i¼þ 3 S xx ð yy þ zz Þ þ Dð yy zz Þ ð6þ h? i¼ 1 3 S xx ð yy þ zz Þ þ Dð yy zz Þ Where the subscripts k and? refer to the parallel and perpendicular directions to the extraordinary refractive index. 18) The polarizability anisotropy of our SHB compounds is much larger as compared to the molecules like 5CB or isothiocyanato tolanes. 19) This is due to the elongated structures by combining carbon carbon triple bonds and biphenyl unit together with isothiocyanato polar terminal group. We calculated the polarization anisotropy tensors for our SHB compounds step-by-step in order to evaluate the polarizability of each individual segment. Figure shows the measured absorption spectra of four SHB compounds we studied. Due to the extended -electron Absorbance [a.u.] SHB-0 SHB-04 SHB-05 SHB Wavelength [nm] Fig.. The measured electronic absorption spectrum of four single SHB compounds. The LCs are dissolved in the cyclohexane solutions at 10 4 molar concentration. Quartz cell thickness is 10 mm. ð7þ Fig. 3. The measured (squares) and calculated (bars) electronic absorption spectra of SHB-05. Calculations are done by HyperChem v.7 with a semiempirical ZINDO/S method. conjugation, the absorption tail extends to ¼ 390 nm. That means, all of those compounds have a strong absorption in the UV region and should not be considered as suitable materials for the applications that requires UV curing process. The calculated UV absorption spectrum for SHB- 05 compound is shown in Fig. 3. The calculated results agree with experiment reasonably well in the nm region. However, in the shorter UV region, the calculated oscillator strengths of the n! transitions are weaker than those measured experimentally. This is typical for molecules containing heteroatom. Here n is the highest filled orbital in the ground state (essentially atomic orbital) and is the lowest unfilled orbital in case of the molecules containing heteroatoms. Besides transition intensity, the calculated transition wavelengths overlap well with the measured UV absorption spectra for SHB-05. All the high birefringence compounds are more suitable for infrared applications, such as laser beam steering systems and telecommunication, where UV stability is not a big concern. 3. Eutectic mixture Using the abovementioned SHB compounds and some short-chain isothiocyanato tolanes, 19) we formulated a eutectic mixture, designated as UCF-SHB. It is a pure nematic with melting point (measured by DSC) at 8 C and clearing point at 186 C. The birefringence was measured at several discrete wavelengths at T 3 C. Results are shown in Fig. 4. Also included in Fig. 4 for comparison is Merck s high birefringence LC mixture BL038. The solid lines in Fig. 4 are fittings using eq. (). From Fig. 4, the UCF-SHB mixture has a much higher birefringence than BL038. At ¼ 53 nm, the birefringence of UCF-SHB reaches n ¼ 0:65. The mixture has a lower n than that of the individual SHB compound we employed because the mixture contains some tolanes which have a lower birefringence (n 0:5). The visco-elastic coefficient ( 1 =K 11 ) of UCF-SHB was measured to be 60 ms/mm at T 3 C. This high viscosity results from the long molecular size, large moment of inertia, and lateral alkyl substitutions. As the temperature

5 7638 Jpn. J. Appl. Phys., Vol. 43, No. 11A (004) S. GAUZA et al. Birefringence UCF-SHB BL Wavelength [nm] Fig. 4. The wavelength dependent birefringence of UCF-SHB mixture and Merck s BL038. Squares are experimental data and lines are fitting using eq. (). increases, 1 =K 11 decreases rapidly. The visco-elastic coefficient is reduced by 8 and 13, respectively, at T ¼ 40 C and 70 C. Within this temperature range, birefringence only decreases slightly because of the high clearing temperature (T c 186 C). 4. Conclusions We have evaluated the physical properties of some high birefringence laterally substituted biphenyl-bistolane isothiocyanates LC compounds. The extrapolated birefringence of some of these compounds is greater than 0.7. A eutectic mixture consisting of these compounds and some tolanes was developed. The mixture exhibits a wide nematic range from 8 to 186 C. Its birefringence reaches 0.65 at ¼ 53 nm and T ¼ 3 C. Due to the long absorption tail, the UV stability of these compounds is a concern. Thus, they are more suitable for infrared applications. Acknowledgment This work is supported by DARPA BOSS program under Contract No. DAAD ) P. F. McManamon, T. A. Dorschner, D. L. Corkum, L. Friedman, D. S. Hobbs, M. Holz, S. Liberman, H. Q. Nguyen, D. P. Resler, R. C. Sharp and E. A. Watson: Proc. IEEE 84 (1996) 68. ) Y. Lu, F. Du, Y. H. Lin and S. T. Wu: Opt. Exp. 1 (004) 11. 3) A. Spadło, R. Dąbrowski, M. Filipowicz, Z. Stolarz, J. Przedmojski, S. Gauza, Y. H. Fan and S. T. Wu: Liq. Cryst. 30 (003) ) S. T. Wu, J. D. Margerum, B. Meng, L. R. Dalton, C. S. Hsu and S. H. Lung: Appl. Phys. Lett. 61 (199) ) S. T. Wu, M. Neubert, S. S. Keast, D. G. Abdallah, S. N. Lee, M. E. Walsh and T. A. Dorschner: Appl. Phys. Lett. 77 (000) ) S. T. Wu, C. S. Hsu and K. F. Shyu: Appl. Phys. Lett. 74 (1999) ) S. T. Wu, C. S. Hsu, Y. Y. Chuang and H. B. Cheng: Jpn. J. Appl. Phys. 39 (000) L38. 8) A. J. Seed, K. J. Toyne, J. W. Goodby and M. Hird: J. Mater. Chem. 10 (000) ) C. Sekine, N. Konya, M. Minai and K. Fujisawa: Liq. Cryst. 8 (001) ) C. Sekine, M. Ishitobi, K. Iwakura, M. Minai and K. Fujisawa: Liq. Cryst. 9 (00) ) C. Sekine, N. Konya, M. Minai and K. Fujisawa: Liq. Cryst. 8 (001) ) P. T. Lin, S. T. Wu, C. Y. Chang and C. S. Hsu: Mol. Cryst. Liq. Cryst. 411 (004) ) I. C. Khoo and S. T. Wu: Optics and Nonlinear Optics of Liquid Crystals (World Scientific, Singapore, 1993) Chap.. 14) S. T. Wu: Phys. Rev. A 33 (1986) ) S. J. Clark, G. J. Ackland and J. Crain: Europhys. Lett. 44 (1998) ) M. Born and E. Wolf: Principles of Optics (Pergamon Press, New York, 1980). 17) M. F. Vuks: Opt. Spektrosk. 0 (1966) ) D. A. Dunmur, A. Fukuda and G. R. Luckhurst: Physical Properties of Liquid Crystals: Nematics (INSPEC, London, 001) Chap ) S. Gauza, H. Wang, C. H. Wen, S. T. Wu, A. J. Seed and R. Dąbrowski: Jpn. J. Appl. Phys. 4 (003) 3463.