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High birefringence lateral difluoro phenyl tolane liquid
crystals
Qiong Song a , Sebastian Gauza a , Haiqing Xianyu a , Shin Tson Wu a , Yung-Ming Liao b , Chin-Yen Chang b & Chain-Shu Hsu b
a
College of Optics and Photonics, University of Central Florida , Orlando, FL, 32816, USA
b
Department of Applied Chemistry , National Chiao Tung University , Hsinchu, Taiwan Published online: 11 Feb 2010.
To cite this article: Qiong Song , Sebastian Gauza , Haiqing Xianyu , Shin Tson Wu , Yung-Ming Liao , Chin-Yen Chang & Chain-Shu Hsu (2010) High birefringence lateral difluoro phenyl tolane liquid crystals, Liquid Crystals, 37:2, 139-147, DOI: 10.1080/02678290903419079
To link to this article: http://dx.doi.org/10.1080/02678290903419079
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High birefringence lateral difluoro phenyl tolane liquid crystals
Qiong Songa, Sebastian Gauzaa, Haiqing Xianyua, Shin Tson Wua*, Yung-Ming Liaob, Chin-Yen Changb and Chain-Shu Hsub
a
College of Optics and Photonics, University of Central Florida, Orlando, FL 32816, USA;bDepartment of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan
(Received 24 September 2009; final form 15 October 2009)
Four laterally difluorinated phenyl tolane liquid crystals were synthesised and their physical properties evaluated. These compounds exhibit a fairly small heat fusion enthalpy (H , 12.56 kJ mol-1), which is favourable for mixture formulations. For comparison, four lateral difluoro cyclohexane tolane homologues are also studied. We doped 10 wt% of each compound into a commercial negative mixture N1 and measured the birefringence, visco-elastic coefficient and figure of merit. Birefringence varies very little between homologues, but the visco-elastic coefficient increases as alkyl chain length increases. The refractive indices of these guest–host mixtures at 1550 nm are also measured. Keywords: high birefringence; negative dielectric anisotropy; liquid crystals
1. Introduction
High birefringence (n) and low-viscosity nematic liquid crystals (LCs) are useful electro-optic media for laser beam steering, telecommunications, reflective displays and adaptive lenses [1–5]. In the past decade, considerable attention has been paid to the synthesis of tolane-based LCs because of their high birefrin-gence [6, 7]. Several molecular structures with high nvalues, e.g. diphenyl-diacetylene [8–10], biphenyl-tolane [11–14], bisbiphenyl-tolane [15–18], naphthalene biphenyl-tolanes [19], naphthyl-bistolanes [20] and thiophenylacetylene [21, 22], have been investigated. However, three major shortcomings of these highly conjugated LC compounds are found: (1) high melting point; (2) high viscosity; and (3) inadequate photo- and thermal-stabilities [23]. For instance, if the temperature is increased to above 270C an irreversible polymerisa-tion process would occur, as marked by a colour change to dark brown. Many efforts have been taken to lower the melting temperature [24–30].
In this paper, we report the physical properties of following lateral difluoro compounds, Structures (I) and (II):
The tolane unit, comprising of two phenyl rings linked by a carbon–carbon triple bond, forms the main part of the rigid core of the molecules and defines the principal molecular axis [31]. The unsaturated phe-nyl ring is rich in p-electrons. Thus, these rings are particularly desirable for elongating p-electron conju-gation through the rod-like molecule and increasing the polarisability along the principal molecular axis. The triple bond is an effective p-electron acceptor. In addi-tion, its contribution to viscosity is not so significant. The oxygen atom in the alkoxy chain elongates the p-electron conjugation, but it also increases the visco-elastic coefficient and melting temperature.
In each structure, four homologues are selected in order to study their physical properties. These com-pounds have a relatively high melting point. Therefore, it is inconvenient to measure their proper-ties at elevated temperatures. Instead, we doped 10 wt% of each compound into a commercial negative dielectric anisotropy (e) mixture and measured the dielectric anisotropy, birefringence, visco-elastic coef-ficient and figure-of-merit (FoM) of the guest–host mixtures. For a compound with a high melting point or large heat fusion enthalpy, it is important to make certain whether the guest is dissolved into the host completely. Otherwise, the measured results will not be accurate. We usually leave the prepared guest–host mixtures for one night to see if there is any precipita-tion. In order to improve the solubility of high bire-fringence compounds, we choose a host mixture with a low melting point.
In order to measure the birefringence more accu-rately, we also used an Abbe refractrometer to mea-sure the individual refractive indices (neand no). The
*Corresponding author. Email: [email protected]
F F CnH2n+1 OCmH2m+1 (PPTP(2,3F)-nOm) F F CnH2n+1 OCmH2m+1 (CPTP(2,3F)-nOm)
ISSN 0267-8292 print/ISSN 1366-5855 online #2010 Taylor & Francis
DOI: 10.1080/02678290903419079 http://www.informaworld.com
measured wavelength dependent neand noresults fit well with the extended Cauchy model [32, 33].
Finally, we used HyperChem (v.7) single personal computer (PC) software to simulate the dipole moment and mean polarisability of these compounds. The results were used to compare with the extrapo-lated dielectric anisotropy and birefringence of these compounds. Since the simulation is based on a single molecule calculated ‘in vacuo’ to its possibly lowest energy gradient, which suggests most possible config-uration of the molecule, the result can qualitatively explain the observed differences between phenyl tolane and cyclohexane tolane, but it does not agree exactly with each homologue in the same category.
2. Compound synthesis
Both series of PPTP(2,3F)-nOm and CPTP(2,3F)-nOm were prepared by Cadiot-Chodkiewicz coupling of 1-(alkoxyl-2,3-difluorophenyl) acetylene with 4-alkyl-4’-iodobiphenyl and 4-alkylcyclohexyl-1-iodo-benzene, respectively [34]. The intermediate com-pounds 1-(alkoxyl-2,3-difluorophenyl)acetylene, 4-alkyl-4’-iodobiphenyl and 4-alkylcyclohexyl-1-iodo-benzene were prepared according to the synthetic pro-cedures reported by the National Chiao Tung University (NCTU) group [34, 35]. All of the synthe-sised LC compounds were purified several times by column chromatography until their purities (checked by high-performance liquid chromatography (HPLC)) were higher than 99.0%.
2.1 Synthesis of CPTP(2,3F)-4O2
Compound 4-butylcyclohexyl-1-iodobenzene (1.709 g, 4.99 mmol), Pd(PPh3)2Cl2(0.116 g, 0.076 mmol), triphenylphosphine (0.105 g, 0.40 mmol), CuI (0.019 g, 0.100 mmol) and dry triethylamine (50 mL) were mixed and stirred at room temperature for 30 min under nitrogen. A solution of 1-(4-ethoxyl-2,3-difluor-ophenyl) acetylene (1 g, 4.99 mmol) dissolved in 10 mL of triethylamime was added dropwise and the mixture stirred at 60C for 24 h. After cooling to room tem-perature, the mixture was filtered and the filtrate con-centrated in vacuo to remove the triethylamine. The crude product was dissolved in diethyl ether and extracted with aqueous ammonium chloride solution. The organic phase was then washed with saturated aqueous NaCl and dried over MgSO4. The crude pro-duct isolated by evaporating the solvent was purified by column chromatography using ethyl acetate/n-hex-ane¼ 1/20 as the eluant to give a white solid; yield 0.47 g (24%); purity 99.5%.1H nuclear magnetic resonance (NMR) (CDCl3, trimethylsilyl (TMS), 300 MHz),
7.38–7.36 (d, 2H, J ¼ 9 Hz, aromatic protons), 7.15–7.05 (m, 3H, aromatic protons), 6.63–6.57 (m, 1H, aromatic protons), 4.09–4.02 (q, 2H, J¼ 6.6 Hz, –OCH2CH3), 2.43–0.76 (m, 19H, –C4H9 and cyclo-hexyl C6H10); 13CNMR (CDCl3, TMS, 75 MHz): 153.59, 153.43, 150.25, 148.88, 143.14, 139.86, 131.78, 127.26, 127.21, 127.14, 120.35, 109.29, 106.16, 105.98, 98.82, 94.25, 81.25, 65.61, 44.83, 37.48, 37.30, 34.35, 33.73, 29.45, 23.24, 14.89, 14.37.; mass spectroscopy (MS) m/z (Mþ) 396; Calculated for C26H30OF2: C, 78.76; H, 7.63. found: C, 78.77; H, 7.91.
2.2 Synthesis of PPTP(2,3F)-4O3
Compound 4-butyl-4’-iodobiphenyl (3.765 g, 11.2 mmol), Pd(PPh3)2Cl2 (1.6 g, 2.2 mmol), triphenylpho-sphine (0.6 g, 2.2 mmol), CuI (0.4 g, 2.2 mmol) and dry triethylamine (60 mL) were mixed and stirred at room temperature for 30 min under nitrogen. A solution of 1-(4-ethoxyl-2,3-difluorophenyl) acetylene (3 g, 11.2 mmol) dissolved in 20 mL of triethylamime was added dropwise and the mixture stirred at 60C for 24 h. After cooling to room temperature, the mixture was filtered and the filtrate concentrated in vacuo to remove the triethylamine. The crude product was dissolved in diethyl ether and extracted with aqueous ammonium chloride solution. The organic phase was then washed with saturated aqueous NaCl and dried over MgSO4. The crude product isolated by evaporating the solvent was purified by column chromatography using n-hexane as the eluant to give a white solid; yield 2.62 g (60%); purity 99.3%. 1H NMR(CDCl3, TMS, 300 MHz): 7.57–7.49 (m, 6H, aromatic protons), 7.26–7.24 (d, 2H, J¼ 6 Hz, aromatic protons), 7.21–7.15 (m, 1H, aromatic proton), 6.72–6.66 (m, 1H, aromatic proton), 4.04–3.99 (t, 2H, J¼ 6.6 Hz, –Ph–OCH2–CH2–CH3), 2.66–2.13 (t, 2H, J¼ 6 Hz, –Ph–CH2–CH2–CH2–CH3), 1.91–1.79 (m, 2H, –Ph–OCH2–CH2 –CH3), 1.67–1.57 (m, 2H, –Ph–CH2–CH2–CH2–CH3), 1.46–1.31 (m, 2H, –Ph–CH2–CH2–CH2 C–H3), 1.09–1.04 (t, 3H, J ¼7 Hz, –Ph–OCH2–CH2–CH3), 0.95–0.90 (t, 3H, J¼7Hz, –Ph–CH2–CH2–CH2–CH3).; 13C NMR(CDCl3, TMS, 75 MHz): 153.3, 150.9, 142.8, 141.9, 137.4, 132.9, 128.9, 127.1, 126.8, 121.4, 109.0, 103.1, 92.5, 66.2, 35.3, 33.6, 29.7, 22.4, 14.0.; high-resolution mass spectroscopy (HRMS) of C27H26OF2: calculated: 404.1952, found: 404.1953.
3. Phase transitions
Differential Scanning Calorimetry (DSC, TA Instrument Model Q-100) was used to determine the phase transition temperatures. The results were 140 Q. Song et al.
obtained from 3–6 mg samples in the heating and cooling cycles at a scanning rate of 2C min–1
. According to the Schroder–Van Laar equation, both low melting temperature and low heat fusion enthalpy play equally important roles in reducing the melting temperature of a eutectic mixture. A low melting tem-perature and a high clearing point are critical for practical device applications. Normally, the desirable nematic range of a LC mixture is from-40 to þ85C. As shown in Table 1, Compound PPTP (2,3F)-4O3 has a low enthalpy (,12.56 kJ mol-1), which is favour-able for making a eutectic mixture. However, its melt-ing point is still relatively high (,110C), which on the other hand limits its solubility to a mixture. The homologue PPTP(2,3F)-6O4 exhibits a smectic phase because the tendency to form a smectic phase increases as the alkyl chain length increases. To suppress the smectic phase and reduce the melting point, we replace the left phenyl ring with a cyclohexane ring, as Structure (II) shows. From Table 1, we find that Structure (II) has a much lower melting point than Structure (I). The lower melting point is mainly attrib-uted to the non-planar structure of cyclohexane ring, in addition to that caused by the lateral difluoro groups. Because of the lower melting point, the solu-bility of Structure (II) is better than that of Structure (I), except that its n is lower.
4. Electro-optical properties
The Abbe refractrometer is a useful instrument for measuring the refractive indices of LC materials. However, to characterise other physical properties like birefringence, visco-elastic coefficient, and FoM, we used electro-optic method [36–38]. Birefringence was obtained through measuring the phase retarda-tion of 7 mm homeotropic cells (pretilt angle ,87) using a He–Ne laser (l ¼ 633 nm) [36]. At a given temperature, the phase retardation is related to cell gap d, birefringence n, and wavelength l as
¼ 2pdn=l: ð1Þ
For a vertically aligned cell, each compound’s perfor-mance was compared based on the FoM defined as [38] FoM¼ K33ðnÞ2=1; ð2Þ where K33 is the bend elastic constant and 1is the rotational viscosity. Temperature has a great influence on the LC device performance. As the temperature increases, birefringence, dielectric anisotropy, elastic constants and viscosity all decrease, but at different rates. It is important to know the exact behaviour of the guest–host mixture throughout the entire opera-tion temperature range. Therefore, in the following sections we report each parameter under different temperatures.
4.1 Dielectric anisotropy
From mean-field theory [39], the dielectric constants of a LC compound are governed by the dipole moment and its relative position with respect to the principal molecular axis. Negative dielectric anisotropy (e , 0) originates from the lateral difluoro group. The dielec-tric constants of a LC affect the operating voltage. The threshold voltage Vthof a homeotropic cell is related to the dielectric anisotropy and bend elastic constant K33 as follows: Vth¼ p ffiffiffiffiffiffiffiffiffiffi K33 eoe r : ð3Þ
Thus, low-threshold voltage can be obtained by either enhancing the dielectric anisotropy, reducing the elas-tic constant, or a combination of both.
The dielectric anisotropy of Structures (I) and (II) was measured based on 10 wt% guest doped in N1 host and the results extrapolated. An inductance, capa-citance, resistance (LCR) Hitester (Model HIOKI
Table 1. Phase transitions of the PPTP and CPTP homologues. MW: Molecular weight, K: Crystalline, Sm: Smectic, and N: Nematic phase. H: Heat fusion enthalpy.
Compounds MW (g) K!Sm (C) Sm!N (C) N!I (C) H(kJ mol-1) PPTP(2,3F)-4O3 404 109.4 256.7 13.25 PPTP(2,3F)-5O4 432 110.2 202.7 12.51 PPTP(2,3F)-6O3 432 103.4 212.0 13.13 PPTP(2,3F)-6O4 446 105.4 129.1 200.4 10.26 CPTP(2,3F)-3O2 380 84.0 228.0 25.12 CPTP(2,3F)-4O2 396 70.3 211.4 26.89 CPTP(2,3F)-5O2 408 73.0 217.0 27.21 CPTP(2,3F)-6O2 424 73.8 199.2 30.09
3532-50) was used to measure the capacitance of the homogeneous and homeotropic cells with or without LC mixtures. Dielectric anisotropy represents the dif-ference between the parallel and the perpendicular com-ponents of the dielectric constant. The measured results are shown in Table 2. The 10 wt% PPTP(2,3F)-4O3 has a solubility problem in N1, so we choose MLC-6608 as the host. The lateral difluoro PPTP and CPTP series have dielectric anisotropy of about-4 and -6, respec-tively. Previously, a high birefringence biphenyl tolane with e ,-8 was reported [12]. Our value is roughly one half of this, because Structures (I) and (II) have only one pair of difluoro groups, while the reported compound has two pairs. The tolane structure is pretty much co-planar, as a result, more lateral dipoles would lead to a larger negative e.
4.2 Birefringence
The temperature dependent birefringence of a LC can be described as follows:
n¼ noS; ð4Þ
S ð1 T=TcÞb; ð5Þ where nois the birefringence at T ¼ 0 K, S is the order parameter, b is a material constant, and Tcis the clearing temperature of the LC. no and b were obtained by fitting the experimental data using Equations (4) and (5).
Figures 1(a) and (b) shows the temperature depen-dent birefringence for the N1 host doped with four homologues of Structures (I) and (II), respectively. In the whole temperature range from room temperature to Tc, the PPTP(2,3F) series shows a ,17% higher birefringence than N1, while the CPTP(2,3F) series is ,13%. This is because both Structures (I) and (II) have a longer conjugation than the host mixture.
PPTP(2,3F)-4O3 has a solubility problem in N1, so we do not include its data in the figures.
4.3 Visco-elastic coefficient
The rotational viscosity is dependent on the activation energy, the molecular moment of inertia (including molecular shape and mass) and the temperature [37]. Thus, a linearly conjugated LC is favoured for its large optical anisotropy and relatively low rotational viscosity. From Figure 2, we can see that even if we only dope 10 wt% of each compound in N1, the alkyl chain effect on the visco-elastic coefficient is very obvious: the longer the alkyl chain, the larger the visco-elastic coefficient [40]. In the PPTP series, the homologues 5O4, -6O3 and -6O4 cause the visco-elastic coefficient of N1 to increase by 40.5%, 44.6%, and 54.5%, respectively. For the CPTP series, the homologues -3O2, -4O2, -5O2
Table 2. Extrapolated e of the PPTP and CPTP homologues. eat 22C PPTP(2,3F)-4O3* -4.06 PPTP(2,3F)-5O4 -4.37 PPTP(2,3F)-6O3 -4.00 PPTP(2,3F)-6O4 -3.13 CPTP(2,3F)-4O2 -6.25 CPTP(2,3F)-6O2 -4.05 CPTP(2,3F)-3O2 -6.35 CPTP(2,3F)-5O2 -5.65
*The value of PPTP (2,3F)-4O3 is extrapo-lated from 10 wt% doped in MLC-6608.
Figure 1. Measured birefringence of N1 host and four (a) PPTP and (b) CPTP homologues in N1 host. The dots are experimental data and the lines are fitting results.
142 Q. Song et al.
and -6O2 boost the visco-elastic coefficient of N1 by 13.9%, 27.8%, 29.0% and 44.7%, respectively.
4.4 Figure of merit
Using Equation (2) and knowing that K33, S 2
and 1,S expðE=kTÞ, the FoM can be expressed as fol-lows [38]:
FoM¼ an2oð1 T=TcÞ3bexpðE=kTÞ: ð6Þ In Equation (6), a is the proportionality constant, k is the Boltzmann constant and E is the activation energy of molecular rotation. The FoM is commonly used to compare the performance of a LC compound or mixture because it is independent of the cell gap employed.
Figure 3(a) and (b) shows the temperature depen-dent FoM of the PPTP and CPTP compounds doped in the N1 host. Birefringence varies very little within the same homologues series, while the visco-elastic coeffi-cient becomes larger as the alkyl chain length increases, so the FoM is mainly governed by the visco-elastic coefficient. PPTP(2,3F)-5O4 and PPTP(2,3F)-6O3 have a similar FoM, which is higher than that of N1, while PPTP(2,3F)-6O4 improves the FoM of N1 only at high temperatures. Among the CPTP(2,3F) series, CPTP(2,3F)-3O2 has the largest FoM because it has the lowest visco-elastic coefficient, followed by CPTP (2,3F)-4O2 and 5O2: and CPTP(2,3F)-6O2 has the smallest FoM. As shown in Figures 1 and 2, the alkyl chain length makes a more pronounced effect on the visco-elastic coefficient than on
Figure 2. Visco-elastic coefficient of four (a) PPTP and (b) CPTP compounds doped in N1 host. The dots are experimental data and the lines are fitting results.
Figure 3. Measured FoM of N1 and (a) three PPTP homologues and (b) four CPTP homologues in the N1 host. The dots are experimental data and the lines are fitting results.
birefringence for the same rigid core structure com-pared. Thus, a shorter alkyl chain is preferred from a high FoM standpoint. However, the trade-off is the higher melting point.
5. Birefringence measurements
We used a multi-wavelength Abbe refractrometer (Atago: DR-M4) to measure the LC refractive indices at six wavelengths (l¼ 450, 486, 546, 589, 633 and 656 nm) by changing the colour filters. We used a 0.294% hexadecyletri-methyle-ammonium bromide (HMAB)þ methanol solution to align the LC molecules perpen-dicular to the surfaces of the prisms. The LC refractive indices (neand no) decrease as the wavelength increases, and then saturate in the infrared (IR) region. It is found that the LC birefringence drops ,10–20% as the wave-length increases from the visible to the IR.
For the compound with a high melting point, it is inconvenient to measure its physical properties at ele-vated temperatures. Instead, we extrapolate its prop-erties by doping 10 wt% of each compound to a negative e LC host, designated as N1. The physical properties of N1 are listed in Table 3. N1 has a modest birefringence, a fairly large negative dielectric aniso-tropy (e , -5) and a low melting point (,-30C), which helps to improve the solubility of the guest compounds. The extrapolated birefringence values are listed in Table 4.
The n of a LC compound is mainly determined by the electron conjugation, differential oscillator strength and order parameter. If we use the single-band model, the birefringence dispersion of a uniaxial LC can be expressed as [41]
n¼ GðTÞ l 2l2
l2 l2; ð7Þ where G is a proportionality constant, which is related to the molecular packing density, order parameter (S) and the differential oscillator strength, and l* is the mean resonance wavelength, which depends on the molecular conjugation. In the same LC homologues, molecules with a shorter side chain may exhibit a larger n owing to the conformational effect. This equation explains the reason why the comparison of birefringence for different LC materials has to be made at the same reduced temperature and same wavelength.
Figure 4 shows the measured absorption spectrum of PPTP (2,3F)-4O3 in a cyclohexane solvent. The LC concentration is 2 10-4mol l-1 and the cell gap of the ultraviolet (UV) quartz cuvette is 1 mm. There are two absorption bands centred at l2, 200 nm and l3, 310 nm. From the three-band model [42], the long l3 leads to a high birefringence. We also measured the clearing point of 10 wt% doped N1; the clearing points are very close to Tc¼ 100C. So, the possible reason for the birefringence difference among the homolo-gues is from the molar ratio difference and the alkyl chain effect.
Table 3. Physical properties of N1 host.
K!N (C) Clearing point (C) Viscosity (mm2 s-1, 20C) n(589 nm, 20C) e(1 kHz, 25C) ,-30 85 35 0.134 -5.1
Table 4. Extrapolated n of the PPTP and CPTP compounds at l ¼ 633 nm and 25C. n N1 0.1293 PPTP(2,3F)-4O3* 0.3818 PPTP(2,3F)-6O3 0.3633 PPTP(2,3F)-6O4 0.3478 PPTP(2,3F)-5O4 0.3590 CPTP(2,3F)-3O2 0.2988 CPTP(2,3F)-4O2 0.2763 CPTP(2,3F)-5O2 0.2863 CPTP(2,3F)-6O2 0.2676
*The value of PPTP (2,3F)-4O3 is extrapolated from 8 wt% doped in MLC-6608.
Figure 4. The measured UV absorption spectrum of PPTP(2,3F)-4O3. The compound is dissolved in cyclohexane with 2 10-4mol l-1concentration and 1 mm cell gap.
144 Q. Song et al.
Within the same homologues, compound PPTP(2,3F)-4O3 has the highest birefringence (n , 0.382) and PPTP(2,3F)-6O4 has the smallest birefrin-gence (,0.348). The birefrinbirefrin-gence difference is 9.8%. The molecular weight difference between these two homologues is 10.4%, which is very close. Next, we com-pare PPTP(2,3F)-5O4 with PPTP(2,3F)-6O3 in which the alkyl chain length and molecular weight are the same. Their birefringence difference is only 1.2%, which is within the experimental error. The birefringence differ-ence between PPTP(2,3F)-6O4 and PPTP(2,3F)-5O4 is 3.2%, which is also consistent. Next, let us look at the CPTP series. The extrapolated birefringence is around 0.28; a small variation among the homologues is found.
The wavelength effect on LC refractive indices is important for the design of direct-view displays and photonic devices. Wavelength-dependent refractive
indices of N1 and N1 doped with 10 wt% PPTP and CPTP compounds at 25C are shown in Figures 5 and 6, respectively. The filled circles, triangles and squares represent the experimental data for neand noof each mixture. The dashed lines are fittings by using the extended three-coefficient Cauchy model [32, 33] as follows: ne;o¼ Ae;0þ Be;o l2 þ Ce;o l4 : ð8Þ
The fitting parameters are listed in Table 5. Some no values are beyond the range of our Abbe refractrom-eter, thus, we have only measured at four wavelengths. Therefore, its fitting is not as accurate as ne. From the fitting parameters, we can estimate the birefringence in the IR region. As Table 5 shows, the average
Figure 5. Wavelength dependent (a) neand (b) noof four
PPTP(2,3F) homologues in the N1 host. The dots are measured data and the dashed lines are fittings with Equation (8).
Figure 6. Wavelength dependent (a) neand (b) noof four
CPTP(2,3F) homologues in the N1 host. The dots are measured data and the dashed lines are fittings with Equation (8).
birefringence at l¼ 1.55 mm of 10 wt% Structures (I) and (II) in the N1 host is 0.144 and 0.125, respectively. Compared to the birefringence of N1 at 1.55 mm, the ,10 wt % PPTP and CPTP guests enhance the n by 26.9% and 10.3%, respectively.
6. Molecular modelling of phenyl tolane compounds We used HyperChem (v.7) single PC software to cal-culate the dipole moment and the mean polarisability of phenyl/cyclohexane tolane compounds. Table 6 lists the simulation results. PPTP (2,3F)-4O3 contains two laterally substituted fluoro groups into the (2,3) phe-nyl ring’s positions. The electronegativity of fluorine is known to be 4 and will always pull the electrons away from the atom, which has a lower electronegativity value. The large dipole moment calculated for phe-nyl/cyclohexane is a natural consequence of the elec-tron’s pull effect. The classical Lorentz–Lorenz equation [43] correlates the refractive index of an iso-tropic media with molecular polarisability in the opti-cal frequencies. We opti-calculated the mean polarisability of the eight compounds studied and found without surprise that compounds with a longer p-electron con-jugation show a higher polarisability and birefrin-gence [44, 45].
7. Conclusion
We have synthesised and studied two series of high birefringence negative e LC compounds: (2,3) lateral difluoro PPTP and CPTP. These compounds exhibit a high birefringence and modest negative dielectric ani-sotropy. Their visco-elastic coefficient and FoM is dependent on the alkyl chain length. These negative LC compounds are intended to enhance the birefrin-gence of dual-frequency LC mixtures for IR applications.
Acknowledgement
The authors are indebted to the Air Force Office of Scientific Research (AFOSR) for financial support under Contract No. FA95550-09-1-0170.
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n ne no (l¼ 1.55 mm) Ae Be Ce Ao Bo Co N1 0.1134 1.5787 0.01013 0.00043 1.4684 0.00293 0.00023 10.25% PPTP(FF)-5O4þN1 0.1443 1.6009 0.00966 0.00115 1.4647 0.00584 0.00005 9.7% PPTP(FF)-6O3þN1 0.1414 1.6078 0.00540 0.00176 1.4653 0.00593 0.00005 9.93% PPTP(FF)-6O4þN1 0.1454 1.6035 0.00667 0.00156 1.4641 0.00534 0.00023 10.2% CPTP(FF)-3O2þN1 0.1376 1.5991 0.00759 0.00114 1.4645 0.00509 0.00016 10% CPTP(FF)-4O2þN1 0.1223 1.5937 0.00928 0.00087 1.4679 0.00304 0.00045 10% CPTP(FF)-5O2þN1 0.1279 1.5973 0.00755 0.00108 1.4704 0.00092 0.00089 10.2% CPTP(FF)-6O2þN1 0.1326 1.5984 0.00559 0.00137 1.4673 0.00240 0.00071
Table 6. Simulated dipole moment and molecular polarisability of the eight phenyl tolane compounds studied.
Dipole moment (Debye) Mean polarisability (a.u.) PPTP(2,3F)-4O3 3.718 287.8 PPTP(2,3F)-5O4 3.696 304.4 PPTP(2,3F)-6O3 3.715 304.4 PPTP(2,3F)-6O4 3.784 312.9 CPTP(2,3F)-4O2 3.814 262.3 CPTP(2,3F)-6O2 3.819 278.4 CPTP(2,3F)-3O2 3.815 254.1 CPTP(2,3F)-5O2 3.819 270.4 146 Q. Song et al.
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