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Synthesis of trans, trans-4-(2-Fluoro-3-isothiocyanato-4-propylphenyl)-4-alkylbicyclohexyl for Liquid Crystal Display
Synthesis of trans, trans-4-(2-Fluoro-3-isothiocyanato-4-propylphenyl)-4-alkylbicyclohexyl for Liquid Crystal Display
Journal of the Korean Chemical Society. 2004. Oct, 48(5): 499-503
Copyright © 2004, The Korean Chemical Society
  • Received : September 06, 2004
  • Published : October 20, 2004
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선희 이
성훈 김

Abstract
We designed and synthesized compounds which have negative dielectric anisotropy (N type) for VALCD mode. Due to the large dipole moments toward the minor axis of molecule and anisotropy of polarizability, they can be used as N type LC. F atom and NCS group and propyl group were introduced as substituents on bicyclohexylbenzene core unit and alkyl chains (propyl~heptyl) were introduced on terminal part. All of the synthesized compounds showed negative dielectric anisotropy as expected.
Keywords
INTRODUCTION
For the latest LCD techniques, it is necessary to enhance the screen quality, viewing angle of broad width and high speed switching time and so on. The VA (vertical alignment) mode is one of the solutions to satisfy these conditions. Compounds with negative dielectric constant (Δε<0) is suitable for VA display mode. 1 - 4 In order to develop a nematic mesophase material which has a negative dielectric anisotropy, we designed and synthesized compounds with NCS group and F atom on bicyclohexylbenzene core unit and arrange those substituents toward the vertical to long axis of molecule. It is reported that liquid crystals with NCS group have low viscosity, nematic phase of wider range and the high stability to heat, 5 - 10 therefore, NCS group is regarded as an ideal substituent for nematic mesophase compound. Also, adjacent F atom increases the dipole moment and chemical stability of NCS group, 11 and decrease the viscosity. With this background, trans, trans -4-(2-fluoro-3-isothiocyanato-4-propylphenyl)-4’-alkylbicyclohexyls are designed and prepared as target materials for LCD. Alkyl groups (C3~C7) were introduced as alkyl chain of terminal group.
Synthetic procedures for intermediates and target compounds are described in 1 , 2 , 3 .
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Synthetic scheme for 4alkylbicyclohexyl-4-one (6).
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Synthetic scheme for N,N-dibenzyl-2-fluoro-6-propylaniline (7).
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Synthetic scheme for trans,trans-4-(2-fluoro-3-isothiocyanato-4-propylphenyl)-4-alkylbicyclohexyls (11).
RESULT AND DISCUSSION
Dielectric anisotropy (Δε) depends on the structure of the molecules and can be calculated according to the equation of Maier & Meier. 12
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ε o =8.85×10 −12
N V =number of molecules per unit volume
F, h =constants of the internal field
S =degree of order parameter
Δa=anisotrophy of polarizability, Δa>0
k =Boltzmann constant
β=angle between the molecular long axis and the dipole moment μ
In order for the designed molecules to have negative dielectric anisotropies, β values have to be more than 55°. So we calculated the β value with PM3 force field in MOPAC 2002 before synthesis. ( 1 ) Because β values are calculated to be much larger than 55°, it is expected that they will have negative dielectric anisotropy.
Also, we predicted the propyl group effect 13 which was based on the energy difference between cis and trans isomers. We optimized the each structure at B3LYP/6-311G+(2d, p) level using in Gaussian 03W. 14 The results are shown in the following 2 .
Calculated β values of designed compounds
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Calculated β values of designed compounds
Calculation of the heat of formations in 2-fluoroisothiocyanatobenzene conformers
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(1 Hartree=627.5095 kcal/mol)
The ΔH f value of the structure with a propyl group is larger than the structure without propyl group. It means that propyl group has large effect on negative dielectric anisotropy. So, the designed molecules are expected to have negative dielectric anisotropy and liquid crystal mesophase.
We measured the physical properties of synthesized compounds and the results are shown in the 3 and 4 . All of the synthesized compounds exhibited phase transition directly to isotropic state at low temperature without showing liquid crystal mesophase, probably because two possible conformers from interaction of -NCS and adjacent fluorine atoms reduce the vertical dipole moment at molecular long axis. Although the compounds did not exhibit the liquid crystal mesophase, have negative dielectric anisotropy as predicted.
The V sat (V 90 ) value of the synthesized compounds bring about to larger than host liquid crystals value but V th (V 10 ) value has improved about 0.2V~0.3V. The synthesized compoundsthey did not exhibit LC mesophase, however, they exhibited sufficient negative dielectric anisotropy. Also V th (V 10 ) for the mixture of synthesized compounds are enhanced a little in comparison with host LC.
Dielectric anisotropy
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(C3~C7: propyl~heptyl) Δε was measured by mixing with Merck’s MJ961213
Vthvalues
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Vth values
In conclusion, the synthesized compounds are expected to be used as mixing LC with other liquid crystal. The usage of these compounds are under investigation.
EXPERIMENTAL
1 H NMR, 13 C NMR and 19 F NMR spectra are obtained using Bruker Avance 400 spectrometer and are reported in ppm from TMS on the δ scale. We used commercially available bromoanisole and 2-fluoroaniline as starting material.
trans-4-(3-Dibenzylamino-2-fluoro-4-propylphenyl)-4’-hexylbicyclohexyl-4-ol (8). A solution of n -BuLi in hexane 2.5M (14.4 mL, 36 mmol) was added to a stirred mixture of compound ( 7 ) (10 g, 30 mmol) and TMEDA (6 mL, 36 mmol) in THF (60 mL) at −78℃ under argon. The mixture was stirred at −60℃ for 12 h and then the solution of compound ( 6 ) (7.9 g, 30 mmol) in THF (25 mL) was slowly added to it at −78℃. After addition, the mixture was allowed to rt and quenching with NH 4 Cl and extracted into ethyl acetate, washed with water and satd NaCl solution, dried over anhydrous MgSO 4 . The solvent was evaporated in vacuo , purified by a short column chromatography (hexane: ethyl acetate=10:1) then used for dehydration.
trans-Dibenzyl-[2-fluoro-3-(4-hexylbicyclohexyl-3-en-4-yl)-6-propylphenyl]amine (9). A solution of compound ( 8 ) in benzene (30 mL) and a little p-TsOH was refluxed for 12 h. The reaction mixture was extracted into ethyl acetate, washed with water and dried over anhydrous MgSO 4 . The solvent was evaporated in vacuo to give crude ( 9 ), it purified by column chromatography (hexane: ethyl acetate: 10:1) to give a solid ( 9 ) (9.8 g, 17mmol). 1 H NMR (400 MHz, CDCl 3 ): δ 0.87 (9H, m), 0.97 (1H, m), 1.17 (2H, m), 1.27 (15H, m), 1.81 (6H, m), 2.49 (4H, m), 4.11 (4H, m), 4.11 (4H, s), 5.87 (1H, s), 6.75 (1H, d), 6.90 (1H, t), 7.21 (10H, m) ; 13 C NMR (100 MHz, CDCl 3 ): δ 14.56, 14.72, 22.90, 23.14, 27.11, 27.42, 30.14 (d), 30.20, 30.39, 30.65, 32.02, 32.40, 33.16, 34.00, 37.97, 38.38, 39.23, 43.09, 58.27 (d), 120.88, 124.21, 126.31 (d), 127.29, 127.81 (d), 128.43, 129.59, 134.10, 136.41 (d), 139.77, 142.77 (d), 160.50 (d).
trans,trans-2-Fluoro-3-(4-hexylbicyclohexyl-4-yl)-6-propylphenylamine (10). The reaction mixture of compound ( 9 ) (9.8 g, 17 mmol) and palladium on activated carbon (0.01 g) in methanol/THF (20 ml/10 ml) was maintained for 6 h under H 2 stream of 60 psi. After confirmate of complete reaction by 1 H NMR the Pd/C was filtered off and the residue was evaporated in vacuo to give ( 10 ) (6.5 g, 16.2 mmol). The pure white solid trans-( 10 ) (3.1 g, 7.7 mmol) was obtained by reverse column chromatography (methanol). 1 H NMR (400 MHz, CDCl 3 ): δ 1 H NMR (400 MHz, CDCl 3 ): δ 0.84 (6H, m), 0.90 (5H, m), 1.03 (6H, m), 1.15 (8H, m), 1.31 (1H, m), 1.64 (1H, m), 1.80 (10H, m), 2.44 (2H, t), 2.74(1H, td), 3.61 (2H, s), 6.54 (1H, t), 6.75 (1H, d); 13 C NMR (100 MHz, CDCl 3 ): δ 14.34, 14.35, 22.08, 22.95, 26.92, 30.33, 30.57, 32.50, 33.08, 33.10, 33.90, 37.73, 38.18, 42.21, 43.70, 115.89 (d), 124.13 (d), 126.46 (d), 131.82 (d), 132.27 (d), 149.96 (d).
trans, trans-4-(2-Fluoro-3-isothiocyanato-4-propylphenyl)-4-hexylbicyclohexyl (11). To a solution of trans -( 10 ) (3.1 g, 7.7 mmol) in chloroform (20 mL) cooled at 0℃ were added a solution of K 2 CO 3 (1.6 g, 11.6 mmol), in water (15 ml). The mixture was treated dropwise with thiophosgene (0.7 ml, 9.2 mmol). The reaction mixture was allowed to stir at room temperature for 4 h. The mixture was extracted into chloroform, washed with water and dried over anhydrous MgSO 4 . The solvent was evaporated in vacuo to give crude ( 11 ). Then the crude ( 11 ) was purified by column chromatography (hexane) to give a white solid ( 11 ) (3.2 g, 7.2mmol). 1 H (400 MHz, CDCl 3 ): δ 0.88 (6H, m), 0.97 (4H, m), 0.98 (2H, m), 1.15 (6H, m), 1.27 (7H, m), 1.46 (2H, m), 1.62 (2H, m), 1.80 (8H, m), 2.62 (2H, t), 2.75 (1H, td), 6.88 (1H, d), 7.02 (1H, t); 13 C NMR (100 MHz, CDCl 3 ): δ 14.13, 14.50, 23.11, 23.74, 27.07, 30.48, 30.56, 32.35, 32.64, 33.42, 34.01, 34.26, 37.70, 37.86, 38.31, 43.26, 43.76, 102.99, 119.25 (d), 124.84 (d), 125.86 (d), 133.12 (d), 137.91, 156.89 (d).
Acknowledgements
This work was performed by Advanced Backbone IT technology development project supported by Ministry of Information & Communication in Republic of Korea.
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Gaussian 03W Gaussian, Inc Pittsburgh, PA